Ocean Drilling Program Initial Reports Volume 117lution is perpendicular to the trend of the basin and shows the bounding basement blocks and the synclinal-shaped sediment fill. B

13. SITE 7251

Shipboard Scientific Party2

HOLE 725A

Date occupied: 22 September 1987

Date departed: 22 September 1987

Time on hole: 4 hr, 30 min

Position: 18°29.200'N, 57°42.030'E

Water depth (sea level; corrected m, echo-sounding): 311.5

Water depth (rig floor; corrected m, echo-sounding): 322.0

Bottom felt (m, drill pipe): 323.1

Penetration (m): 4.5

Number of cores: 1

Total length of cored section (m): 4.5

Total core recovered (m): 4.5

Core recovery (%): 100%

Oldest sediment coredDepth sub-bottom (m): 4.5Nature: nannofossil foraminifer sandy silt

HOLE 725B




Position: 18°29.200'N, 57°42.030'E





Number of cores: 10



Core recovery (%): 11.9

Oldest sediment coredDepth sub-bottom (m): 93.8Nature: foraminifer-rich calcitic sandy siltAge: PleistoceneMeasured velocity (km/s): 1.6

HOLE 725C




1 Prell, W. L., Niitsuma, N., et al., 1989. Proc. ODP, Init. Repts., 117: Col-lege Station, TX (Ocean Drilling Program).

2 Shipboard Scientific Party is as given in List of Participants preceding thecontents.

Position: 18°29.200'N, 57°42.030'E





Number of cores: 17



Core recovery (%): 60.6

Oldest sediment coredDepth sub-bottom (m): 162.8Nature: nannofossil-rich calcitic sand, silt, and clayAge: PleistoceneMeasured velocity (km/s): 1.6

Principal results: Site 725 is the shallow landward end point of a depthtransect on the Oman continental margin. Our intention at Site 725was to trace the fluctuations in the upper boundary of the oxygen-minimum zone through time and to evaluate these fluctuations withregard to changes in the monsoon, in tectonic evolution of the mar-gin, and in sea level.

Findings at Site 725 include:

1. Organic- and opal-rich clayey silt alternates with nonlami-nated nannofossil ooze and calcitic silty clay. Laminated intervalsare confined to the upper part of the lower Pleistocene (lithologicUnit II) and are missing in both the lower (lithologic Unit III) andupper (lithologic Unit I) parts of the section.

2. The sedimentation rate averages about 120 m/m.y. over thetop 120 m.

3. Opal occurrences are confined to laminated intervals and sedi-ments having high organic-carbon concentrations of lithologic UnitsII and III.

4. Cold-water nannoplankton occurrences in the deepest part ofthe cored section.

5. Persistent occurrence of interstitial sulfate, high alkalinity,and dramatic core expansion suggest replenishment of the interstitialsulfate pool from a deep source comparable to that of Site 723.

6. Elevated ethane and propane gas concentrations with depthimply migration of thermogenic hydrocarbons from deep sources.

Site 725 constitutes a section of Pleistocene age that shows pro-nounced changes in the depositional environment, which are tenta-tively attributed to extension and intensification of the mid-wateroxygen-minimum zone in the early Pleistocene. These changes wereobserved at other margin sites and are probably contemporaneousand of a regional rather than local extent.

BACKGROUND AND OBJECTIVESSite 725 is located at 18°29.200'N and 57°42.030'E on the

continental margin of Oman in a water depth of 311 m near thelandward edge of the upper slope basin. The location of Site725 is shown in Figures 1 and 2, and its structural and deposi-tional setting is shown in Figure 3. Site 725 is located in thetransitional zone between the continental shelf and the slope ba-sin. The site lies near a faulted block that is thought to be partof an ophiolitic basement and that forms the eastern boundaryof the slope basin. Seismic reflection profiles show eastward-dipping layers that both onlap and drape the underlying base-

419

SITE 725

726 725 724

T Y Y

1 -

Q.

Q

2 -

3 -

723 727 730T T T

728

y729

yOxygen (mL/L)1 2 3 4

Oxygen-minimumzone (OMZ)

Tertiary to Holocenesediments

Ophiolite

Figure 1. Structure of the Oman margin and the oxygen-minimum zone. The schematic drawing of the Oman margin shows the seriesof basement ophiolite blocks and the sedimentary basins between them. The concentration of oxygen in the water column (RC2704,unpubl. data) defines the depth range of the oxygen-minimum zone and where it impinges on the margin. The schematic location ofSite 725 is indicated.

ment structures. Although the reflectors are generally conform-able, some faulting and erosion is indicated by offset and con-verging reflectors. Site 725 was situated upslope of the originalsite location to avoid some of the subsurface structure. Prelimi-nary correlation of seismic reflectors within the basin indicatedthat average accumulation rates should be high at Site 725, al-though winnowing of shelf-edge sediments might be possible.Site 725 was selected to provide the shallow end-member for adepth transect that spans the depth of the oxygen-minimumzone. The site was expected to provide a high-resolution sedi-mentary record of the variations of monsoonal upwelling and ofthe oxygen-minimum zone during the Pliocene-Pleistocene.

Specific objectives for drilling at Site 725 were:

1. To obtain a high-resolution record of the sediments asso-ciated with the near-coastal zone of the monsoonal upwellingsystem so as to establish changes in timing and intensity of themonsoon.

2. To provide the shallow end-member for a depth transectof sediments that will be used to examine the organic-rich sedi-mentary facies of the margin and to establish their relationshipto the oxygen-minimum zone and its spatial variation throughtime.

3. To document the diagenetic processes associated with theoxygen-minimum zone and organic-rich sediments.

4. To search for evidence of changes in the structure of theintermediate water masses of the Arabian Sea during the Plio-cene-Pleistocene.

OPERATIONSThe JOIDES Resolution arrived at Site 725 in a dynamic po-

sitioning mode by 0230 hr on 22 September 1987. This site,which previously had been surveyed jointly with Site 724, is lo-cated in a water depth of 311 m at 18°29.200'N and 57°42.030'E(see Fig. 4, "Operations" section, Site 724 chapter). Upon ar-rival, a shallow-water beacon was lowered on a taut wire, and by0430 hr Core 117-725A-1H had been shot. The core barrelparted while shooting Core 117-725A-2H, and Hole 725A hadto be abandoned. Recovery of Core 117-725A-1H (0-4.5 metersbelow seafloor [mbsf]) was 100% (Table 1).

Without offset, Hole 725B was spudded at 0600 hr on Sep-tember 22 in extended-core-barrel (XCB) coring mode. Ten XCBcores recovered only 11.9% of the cored interval, which wasdominated by sand and silt from 0 to 93.8 mbsf. After Core117-725B-10X had been retrieved, coring in Hole 725B was sus-pended and the drill string pulled. Hole 725C was spudded at1415 hr on 22 September, 20 m northeast of Hole 725B, after bitmodification.

Three advanced piston corer (APC) cores recovered 100% ofthe interval from 0 to 28.5 mbsf until an overpull of 80,000 1bmade XCB coring necessary. Cores 117-725C-4X through -17X(28.5-162.8 mbsf) showed a much improved recovery, after bitmodification, of 52.3% in Hole 725C. Overall recovery in Hole725C was 60.6%. After reaching total depth, the hole was dis-placed with weighted mud. After clearing the mud line and re-trieving the beacon, the ship was under way to Site 726 by 2400hr on 22 September 1987.

420

SITE 725

19°N

18C

17C

Arabia

Ras Sharbithat 72£ 727

730

720 _

56°E 57° 58C 60° 61'

B18°40'

18°30'

18°20'

_l Mi M M I Ml 1 M I M I I F I I I L

M M M M57°30' 57°40' 57°50' 58°00 58°10'

Figure 2. A. Bathymetry of the Oman margin and the location of Site 725. B. The detailed location of Site 725 andthe seismic profiles shown in Figure 3. Bathymetry and seismic data are from the site survey (RC2704, 1986).

421

I"0700 0711 0730 0749 0800

C2704 Line 4

Figure 3. A. Single channel seismic (SCS) reflection profiles showing thestructural and depositional setting of Site 725. Line 4 of JOIDES Reso-lution is perpendicular to the trend of the basin and shows the boundingbasement blocks and the synclinal-shaped sediment fill. B. RC2704Line 4 is along the strike of the basin and shows the thick sediments.

LITHOSTRATIGRAPHY

Lithologic UnitsSite 725 is located on the shelf/slope transition of the Oman

margin. The recovered sedimentary sequence ranges from Pleis-tocene to Holocene in age. Coring of Hole 725A was terminatedafter the first core because of core barrel failure. Because ofpoor recovery at Hole 725 B, lithologic description and interpre-tation are based predominantly on sediments recovered fromHole 725C.

Sediments recovered at Site 725 are dominated by calciticmarly, nannofossil ooze, but also include calcitic marly calcare-ous ooze; interbedded clayey silt and laminated diatomaceousmud; and calcitic sand, silt, or clay. Detrital calcite is commonto abundant in all these lithologies and may be derived from ei-ther an inorganic or a reworked biogenic source. The lithologicsection has been divided into three units using visual core de-scriptions, compositional analyses of smear slides, and carbon-ate and organic carbon contents as distinguishing parameters.Lithologic Units I and III are compositionally similar; both are

dominated by biogenic calcareous components and character-ized by bioturbation. Lithologic Unit II contains laminated dia-tomaceous deposits with interbedded detrital intervals. Thesesediments contain less inorganic calcite and biogenic carbonatethan those in lithologic Unit I. The characteristics and occur-rences of these units are summarized in Table 2 and Figure 4.

Unit I (Depth: Hole 725A, 0-4.5 mbsf; Hole 725B, 0-93.8 mbsf;Hole 725C, 0-120.0 mbsf; Age: Pleistocene to Holocene)Cores 117-725A-1H, 117-725B-1X through 117-725B-10X, and117-725C-1H through 117-725C-13X.

The composition of lithologic Unit I changes slightly down-core, from calcitic marly calcareous ooze and marly foraminifernannofossil ooze near the surface to calcitic, marly nannofossilooze and nannofossil-rich calcitic silty clay at its base. Colors inthis unit gradationally darken downcore, from olive (5Y 4/3)and olive gray (5Y 4/2) near the surface, to olive (5Y 4/3), olivegray (5Y 4/2), and dark greenish gray (10Y 5/2) at intermediatelevels, to olive (5Y 4/3), olive gray (5Y 4/2), dark olive gray (5Y4/1 and 5Y 3/2), and very dark gray (5Y 3/1) lower in the sec-tion.

Unit I is slightly to moderately bioturbated except in Core117-725C-11X, where faint laminations are locally developed.

From smear-slide analyses, lithologic Unit I contains 15% to55% inorganic calcite, 5% to 20% quartz, 10% to 45% clays,5% to 50% nannofossils, and 2% to 20% foraminifers (includ-ing abundant benthics; see "Biostratigraphy" section, this chap-ter). Feldspars, mica, dolomite, and accessory minerals are pres-ent in abundances of 1% to 5%, while phosphate concretionsare locally present and glauconite occurs sparsely throughout.Volcanic glass is notably absent in Unit I, and biogenic siliceouscomponents are present only in trace abundances at the base ofthe unit (Core 117-725C-13X). These compositional ranges indi-cate that terrigenous material is generally more abundant thanbiogenic constituents in Unit I and that the terrigenous compo-nents become more dominant downcore. In addition, clay con-tent increases and the abundance of silt-size quartz decreasesdowncore in Unit I, suggesting that Unit I generally becomesmore fine-grained downsection.

Unit II (Depth: 120.0-145.4 mbsf; Age: Pleistocene)Cores 117-725C-13X through 117-725C-16X.

Lithologic Unit II is composed of interbedded calcitic sand-silt-clay, olive gray (5Y 4/2), and laminated diatomaceous beds.These laminated beds contain diatomaceous calcareous clayeysilt; diatomaceous calcitic marly nannofossil ooze (olive [5Y 4/3], olive gray [5Y 4/2], and dark olive gray [5Y 3/2]); diatoma-ceous clayey silt (dark olive gray [5Y 3/2]); and diatomaceousmud (olive [5Y 5/3]). Smear-slide analyses indicate that thedarker diatomaceous laminae contain 10% to 30% diatoms,2% to 5% sponge spicules, 10% to 35% nannofossils, 1% to5% foraminifers, 25% to 35% clays, 15% to 25% inorganic cal-cite, and 3% to 20% quartz. The light-colored, diatomaceouslaminations have significantly different compositions, however,and average 55% diatoms, 30% clay, 3% inorganic calcite, and2% to 5% each of quartz, nannofossils, and foraminifers. Thecalcitic sand-silt-clay intervals are essentially barren of biogeniccomponents and contain 20% to 50% inorganic calcite, 10% to45% clay, 20% to 30% quartz, and 0% to 5% siliceous biogenicmaterial.

The character of the laminations within the diatomaceousintervals changes in Unit II from more diffuse laminations on ascale of 1 cm in Core 117-725C-14X (129-133 mbsf) to verywell-defined, millimeter-scale lamination in Core 117-725C-15X(136-140 mbsf). Often, diatom assemblages within the lighter-colored diatomaceous laminae are nearly monospecific. Thecontact between the uppermost laminated zone and the overly-

422

SITE 725

Table 1. Coring summary at Site 725.

Coreno.

117-725 A- 1H117-725B-1X

2X3X4X5X6X7X8X9X10X

117-725C-1H2H3H4X5X6X7X8X9X10X11X12X13X14X15X16X17X

Date(Sept.1987)

22222222222222222222222222222222222222222222222222222222

Time

0540070007500910101510501130120512351255132014451515154016201650171517401805182518501945193519552015203020552115

Table 2. Lithologic summary at Site 725.

Unit Description

I Foraminifer calcitic sandy silts tocalcitic marly calcareous oozeand calcitic marly nannofossilooze to nannofossil-rich sand-silt-clay.

II Interbedded and laminated diatoma-ceous clayey silt, diatomaceousmud, and calcitic marly nanno-fossil ooze, calcitic sand silt clay.

Ill Nannofossil-rich calcitic sand-silt-clayto calcitic marly nannofossil ooze

Depth(mbsf)

0-4.50-7.5

7.5-17.117.1-26.626.6-36.236.2-45.745.7-55.355.3-64.964.9-74.574.5-84.184.1-93.8

0-9.09.0-18.6

18.6-28.528.5-38.038.0-47.547.5-57.057.0-66.566.5-76.176.1-85.785.7-95.395.3-104.9

104.9-114.5114.5-124.1124.1-133.8133.8-143.5143.5-153.2153.2-162.8

Facies

Cored(m)

4.57.59.69.59.69.59.69.69.69.69.79.09.69.99.59.59.59.59.69.69.69.69.69.69.79.79.79.6

Hemipelagic and calciticdetrital facies

Recovered Recovery(m)

4.530.220.360.622.952.351.032.080.060.780.779.009.669.914.011.005.940.494.186.270.057.066.413.929.667.733.719.76

Depth(mbsf)

0-4.50-93.8

(%)

100.02.93.86.5

30.724.710.721.60.68.17.9

100.0100.0100.042.210.562.5

5.243.565.30.5

73.566.840.899.679.738.2

101.0

Core

1HIX to 10X

0-120.0 H to 13X

Laminated and detrital facies 120.0-145.

Hemipelagic and calciticdetrital facies

4 13H to 16H

145.4-162.8

Hole

ABC

C

C

ing unlaminated diatomaceous deposits of Unit II is sharp andlies beneath a basal foraminiferal sand.

Unit III (Depth: 145.4-162.8; Age: Pleistocene)Cores 117-725C-16X to 117-725C-17X.

Lithologic Unit III is similar to lithologic Unit I in composi-tion and appearance. It is dominated by calcitic marly nanno-fossil ooze, interbedded with olive (5Y 4/3), olive gray (5Y 4/2),and dark olive gray (5Y 3/2) intervals. Nannofossil-rich calciticsand-silt-clay is present as a minor lithology, in olive gray (5Y 4/2) and dark olive gray (5Y 3/2) intervals 20 to 75 cm thick. Fora-minifers and dispersed shell debris are common in Unit III, andthe entire unit is slightly bioturbated.

The marly nannofossil oozes contain 25% to 45% nannofos-sils, 25% to 35% inorganic calcite, 20% to 25% clay, 5% to11% quartz, and 2% to 5% each of foraminifers and accessoryminerals. The sand-silt-clay intervals contain an average of 25%quartz, 25% clay, 35% inorganic calcite, 10% biogenic carbon-ate, and 5% accessory minerals.

Discussion

The sedimentary sequence at Site 725 was recovered from ashallow water depth on the Oman margin. For this reason, thedepositional record preserved at Site 725 will provide valuablecomparisons to correlative deposits from deeper-water environ-ments elsewhere on the margin. The subdivision of the sequenceat Site 725 into three lithologic units emphasizes variations interrigenous and biogenic sediment abundances, as well as dif-ferences in depositional and preservational style (i.e., laminatedvs. bioturbated intervals). The variations in abundances of sedi-mentary components are probably related to fluctuating rates ofterrigenous sediment supply and marine planktonic productiv-ity, which may have varied in response to climatic fluctuationsduring the Pleistocene.

At the base of Hole 725C, the bioturbated nannofossil-richcalcitic sand-silt-clays and calcitic marly nannofossil oozes ofUnit III contain an important signature of marine planktonicinflux (i.e., foraminifers, coccolithophores, and marine organiccarbon). Despite the relatively high abundances of terrigenouscomponents (generally greater than 50%), organic carbon con-

423

SITE 725

Hole 725B

Diatom ooze

Nannofossil ooze

Hole 725C

Parallel laminae(not to scale)

Lithology

E T ^ V Π Foraminiferal ooze

Figure 4. Lithologic summary, Site 725. Lithology is inferred between recovered intervals. Note:No lithologic summary for 725A (recovered only I core).

tents near 3% (Table 3 and Fig. 5) are characteristic of lithologicUnit III. The elevated level of organic carbon in lithologic UnitIII can be attributed to either an increase in supply, which is re-lated to primary productivity in surface water, or enhanced pres-ervation. Changes in the amount of organic carbon preserved inmarine sediments have been attributed to variations in sedimen-tation rate, bottom-water oxygen contents, and redox potentialin the near-surface sediment (see Emerson, 1985, and referencestherein). Any of these could be factors at Site 725.

A significant contribution of biogenic silica at Site 725 isfirst evident in lithologic Unit II, which contains diatomaceouslaminated intervals. The light layers in these laminated zonescontain approximately 60% diatoms, while the dark layers con-tain lower, but still significant, abundances of biogenic silica orbiogenic carbonate. The biogenic content of the darker laminaeis lower because they are either diluted by increased detrital sup-ply or because biogenic supply was lower, perhaps from de-creased productivity. In the oldest laminated zone, the laminaeare thin (millimeter-scale) and well developed. These character-istics, together with the high organic carbon contents (Table 3)and absence of bioturbation in this interval, indicate that condi-tions at the sediment/water interface were unfavorable for ben-thic fauna at this time. In addition, laminae reflect repeatedshort-term variations in sediment supply. Such variations mayoccur annually or seasonally (Kemper and Zimmerle, 1980).

Repeated short-term (seasonal or annual?) variations in sedi-ment supply are apparent in both the relative importances of bi-ogenic and detrital components and the nature of the detritalconstituents. Smear-slide analysis indicates that the lighter lami-nae contain only fine-grained (predominantly clay-sized) detri-tal material, in addition to the higher biogenic abundances de-scribed earlier. The darker laminae, however, contain a widerrange of detrital grain sizes, including minor amounts of sand.Quantitative grain-size and compositional analyses of the terrig-enous components in the light and dark laminae will be neededto determine sediment origin and mode of transport.

In lithologic Unit I overlying the laminated deposits of UnitII, the abundance of biogenic calcareous components increasesupsection, while widespread bioturbation indicates a return to en-

vironmental conditions more favorable for benthic fauna. Thetransition to a biogenic calcareous sediment assemblage (vs. cal-careous and siliceous) is completed by approximately 70 to 80mbsf, with only marly calcareous nannofossil and foraminiferoozes occurring above that level. This transition is also evidentin measurements of sediment density, organic carbon content,and carbonate content (Fig. 5 and Tables 3 and 7).

Although biogenic carbonate abundances increase upsectionin Unit I, terrigenous abundances remain high. Inorganic cal-cite is a major detrital constituent, and its importance is at leastpartly responsible for the high carbonate abundances through-out lithologic Unit I. The origin of the inorganic calcite is notentirely clear, because it does not always co-vary with otherphases that are demonstrably terrigenous (e.g., inorganic calciteincreases, while quartz and clays decrease significantly). Oneexplanation for this difference is that the sources and/or thetransport processes of these phases are different. However, onepossible source is reworking from shelly carbonate sands nowforming on the shelf. Reworked foraminifers and aragoniticlenses derived from weathered shells were recognized while de-scribing the Site 725 cores. When crushed, this material closelyresembles the "inorganic calcite" observed in smear slides. Shore-based studies will investigate the sources of this calcite furtherby determining the relationship between calcite and palygor-skite. Palygorskite is a known indicator of eolian supply fromthe Arabian Peninsula (Kolla et al., 1981, Caulet et al., inpress), so that co-variation in inorganic calcite and palygorskitewill support an eolian interpretation for the inorganic calcite.

In summary, lithologic Units I and III are compositionallysimilar in biogenic and terrigenous material. A major differencebetween the two units, however, is the higher organic carboncontent of Unit III, which suggests that productivity and/orbottom-water conditions were different during the two periodsof deposition. These differences may have developed because ofchanges in surface or deep-water circulation or they may haveresulted from bathymetric changes produced by movement ofSite 725.

The presence of upwelling flora in the laminated intervalsand the increase in high organic carbon values (2%-3.5%) in

424

SITE 725

Table 3. Carbonate and organic carbon abundances at Site 725.

Hole, core, section,interval (cm)

117-725C-1H-2, 90-92725A-1H-3, 0-1725C-1H-5, 90-92725C-2H-2, 60-62725C-2H-4, 60-62725C-2H-7, 30-32725C-3H-2, 100-102725C-3H-4, 24-26725C-3H-4, 119-120725C-3H-6, 100-102725C-4H-3, 50-52725B-5X-2, 0-1725C-6X-2, 86-88725C-6X-2, 119-120725C-6X-4, 86-88725C-8X-3, 29-31725C-9X-3. 119-120725C-9X-4, 27-29725C-11X-2, 24-26725C-11X-5, 8-10725C-12X-2, 19-21725C-12X-3, 108-110725C-12X-3, 119-120725C-13X-3, 34-36725C-14X-2, 21-23725C-14X-4, 26-28725C-14X-6, 14-16725C-15X-2, 85-87725C-15X-4, 80-82725C-15X-4, 119-120725C-16X-3, 4-6725C-17X-2, 25-27725C-17X-4, 106-108725C-17X-5, 119-120725C-17X-6, 128-130

Depth(mbsf)

2.403.006.90

11.1014.1018.3021.1023.3424.2927.1032.0037.7049.8650.1952.8669.7980.2980.8797.04

101.38106.59108.98109.09117.84125.81128.86131.74136.15139.10139.49146.54154.95158.76160.39161.98

Totalcarbon

(%)

6.957.157.176.867.086.766.326.266.837.397.007.047.468.687.117.166.939.197.847.18

12.328.987.79

10.7211.689.477.919.347.417.41

10.5412.126.878.589.12

Inorganiccarbon

(%)

6.456.676.426.216.036.465.865.756.496.726.586.826.437.166.236.566.187.825.506.558.737.606.948.138.335.994.487.264.254.506.638.363.255.994.96

Organiccarbon

(%)

0.500.480.750.651.050.300.460.510.340.670.420.221.031.520.880.600.751.372.340.633.591.380.852.593.353.483.432.083.162.913.913.763.622.594.16

CaCO3

(%)

53.755.653.551.750.253.848.847.954.156.054.856.853.659.651.954.651.565.145.854.672.763.357.867.769.449.937.360.535.437.555.269.627.149.941.3

lithologic Unit II indicate a major depositional change with re-spect to Units I and III, which supports the hypothesis that up-welling activity was different during the early Pleistocene thantoday. This may be interpreted in term of fluctuations in thestrength of the monsoonal winds (Prell and Streeter, 1982). Forthat reason, shore-based research will consider both the produc-tivity signal of the biogenic components and the source andtransport record of the eolian components to describe sedimen-tation on the Oman margin in a more comprehensive manner.

BIOSTRATIGRAPHY

IntroductionBoth planktonic and benthic foraminiferal fauna as well as

nannofossils and radiolarians were studied at Site 725 to estab-lish biostratigraphic zonations. Another objective was to inves-tigate fluctuations in the upper boundary of the oxygen-mini-mum zone and possible changes in upwelling intensity.

The entire recovered section is referred to the Pleistocene.Planktonic and benthic foraminifers are abundant and well pre-served throughout the section; nannofossils are abundant butare poorly preserved. Radiolarians are absent in the upper partof the sequence (Fig. 6). A plot of faunal datum levels vs. depthbelow the seafloor is presented in Figure 7; for a detailed list ofthese data points, see Table 4.

Plantonic ForaminifersPlanktonic foraminifers were studied in Hole 725C. The

core-catcher samples contain highly diverse, well-preserved fau-nas throughout the entire sequence, except for Sample 117-725C-15X, CC (143.5 mbsf). This sample yields abundant, ro-

bust Globorotalia tumida tumida and common Neogloboquad-rina dutertrei specimens, whereas thin-walled species are presentonly in low numbers. We believe this distribution of species isrelated to dissolution processes. The benthic foraminifers alsoare poorly preserved in this sample.

The presence of Globigerinella calida calida in Samples 117-725C-1H, CC through 117-725C-8X, CC (9.0-76.1 mbsf) marksZone N23. Globorotalia truncatulinoides specimens are rare tocommon in the upper part of the section but disappear belowSample 117-725C-12X, CC (114.5 mbsf). However, some smallspecimens are again present in the lowermost sample (117-725C-17X, CC; 162.8 mbsf). Thus, the entire sequence is referred tothe Pleistocene. The occurrence of Globigerinoides fistulosus inthe same sample shows that the basal part of the Pleistocenewas drilled, as the last appearance of this species has been datedat 1.6 Ma (Berggren et al., 1985).

Benthic ForaminifersThe benthic foraminiferal fauna was studied at Site 725 in

the core-catcher samples of Hole 725C. Benthic foraminifersare abundant throughout the sequence, with a maximum abun-dance of about 140,000 specimens per 10 cm3 of sediment inSample 117-725C-1H, CC (9.0 mbsf). Preservation is good tomoderate but deteriorates downhole. Diversity is high, whichcan be expected in water this shallow (311 m).

The benthic foraminiferal fauna of Site 725 is relatively uni-form and displays no significant changes during the short timeinterval recovered by the drilled sequence. However, one can rec-ognize two slightly different assemblages. The first assemblagecontains species that are abundant throughout the sequence:Bolivina ordinaria, Cassidulina carinata, Cibicidoides wueller-

425

I ' I ' I • I

140

40 60 80 0 2 4CaC03 <%) Organic carbon (%)

Figure 5. Carbonate and organic carbon profiles, Site 725.

100 —

150 —

Co

re

1

2

π4

5

6

7

89

10

1 1

12

13

14

15

16

17

Ep

oc

hen

eo

loc

i

o

α>δoo

α>

Calcareousnannofossils

NN21

NN20

NN19

Radioiarians

Barren

j

f

Planktonicforaminifers

N23

N22

0.5 1.0Age (Ma)

1.5

Figure 6. Correlation of planktonic microfossil zones in Hole 725C.

Figure 7. Age-depth plot for Hole 725C. For detailed list of events, seeTable 4.

storfi, Hyalinea balthica, and Uvigerina peregrina. Additionalspecies that occur only in this assemblage are Ammonia becca-rii, Lenticulina calcer, and Trifarina angulosa. The other assem-blage contains Florilus spp., Globobulimina spp., Lenticulinacf. falcifer, and Uvigerina dirupta, in addition to those speciesof the first assemblage. Assemblage 1 occurs in Sections 117-725-1H, CC through -9X, CC (9.0-85.7 mbsf), whereas the sec-ond assemblage occurs in Sample 117-725-10X, CC and down-hole to the bottom of the recovered sequence (95.3-162.8 mbsf)•

The restriction of Ammonia beccarii to the upper 85 m mayindicate a mid-Pleistocene uplifting at this site. The appearanceof assemblage 2 coincides with an increase in the amount ofphosphorite found and with a general decrease in the total num-ber of specimens per volume.

Several large lenticuliids and nodosariids were found at Site725; the diameter of the former often exceeds 5 mm.

Calcareous Nannofossils

Hole 725A

Because of drilling equipment failure, only one core was ob-tained from Hole 725A. The core-catcher material of this core(Sample 117-725A-1H, CC; 4.5 mbsf) contains Emiliania hux-leyi and was assigned to Zone NN21 (upper Pleistocene to Hol-ocene).

Holes 725B and 725C

At Hole 725B, penetration was 93.8 m; however, because ofbottom-hole conditions, recovery of sediments was poor. Thenannofossils recovered from this hole are abundant, but the as-semblages are marked by low species diversity, which may be aresult of comparatively low temperatures caused by upwelling.Preservation is generally poor. Sediments down to 17.2 mbsfcontain E. huxleyi and belong to Zone NN21 (Holocene to up-per Pleistocene). Below this zone, sediments down to 45.7 mbsfcan be placed in Zone NN20 (Pleistocene) based on the absenceof E. huxleyi and Pseudoemiliania lacunosa. The remaining se-quence from 45.7 to 93.8 mbsf may belong to Zone NN19(Pleistocene), although the abundances of P. lacunosa rangefrom barren to common.

Nannofossil assemblages in Hole 725C are similar to those inHole 725B (Fig. 8). Based on the same evidence as the previoushole, sediments down to 18.6 mbsf, underlying sediments downto 47.5 mbsf, and the remaining sediments down to 162.8 mbsf

426

SITE 725

Table 4. Stratigraphic list of faunal events for Hole 725C.

Event

B Emilianiahuxleyi

T Pseudoemilianialacunosa

T Reticulofenestrasp. A

B Gephyrocapsaparallela

T Gephyrocapsa"large"*5

T Helicosphaerasellii

Corelevel

3H, 118-1193H, CC6X-2, 118-1196X-3, 118-11911X-3, 118-11911X-5, 93-9411X-5, 93-9411X, CC13X, CC14X-1, 118-11916X, CC17X-1, 129-130

Depth(mbsf)

25.7828.5050.1851.6899.48

102.23102.23104.90124.10125.28153.20154.49

Age(Ma)

0.19

0.49

0.82

a0.89

a1.10

Sourceof age

3

3

3

4

4

Notes

No good published age; eventappears to be diachronous.

Note: T = upper limit of event and B = lower limit. Sources of ages are: 3 = oxygen isotope data forSite 723 (N. Niitsuma, unpubl. data); 4 = Takayama and Sato, 1987.

a North Atlantic datab Long axis greater than 6 µm.

Hole725B

Hole725C

50 —

100 —

150 —

NN21

NN20

NN19

NN21

NN20

NN19

Figure 8. Correlation of calcareous nannofossil zones in Holes 725Band 725C.

were assigned to Zones NN21, NN20, and NN19, respectively.The nannofossil datums recognized in Hole 725 C are tabulatedin Table 4.

Additional Pleistocene calcareous nannofossil datums (see"Explanatory Notes" section, this volume, for explanation ofdatums) are recognized in Hole 725B as follows:

Datum 1 UncertainDatum 2 17.1-26.6 mbsf in Core 117-725B-3XDatum 3 45.7-55.3 mbsf in Core 117-725B-6XDatum 4 UncertainDatum 5 84.1-93.8 mbsf in Core 117-725B-10XDatum 6 84.1-93.8 mbsf in Core 117-725B-10X

Rare reworked specimens were found in a few cores through-out Holes 725B and 725C. These include the Cretaceous speciesMicrorhabdulus decoratus and Discoaster multiradiatus fromthe Paleocene or the Eocene.

RadiolariansSections 117-725B-1H, CC through 117-725B-10X, CC (7.5-

93.8 mbsf) and 117-725C-11X, CC to 117-725C-17X, CC (104.9-

162.8 mbsf), which together form a continuous sequence, wereexamined for radiolarians. The upper 10 cores are barren. How-ever, samples examined below that level have significant radio-larian fauna along with diatoms and sponge spicules. The pres-ence of Anthocyrtidium angulare in Samples 117-725C-12X,CC (114.5 mbsf) and 117-725C-14X, CC (133.8 mbsf) placesthese samples, at least, in the Anthocyrtidium angulare Zone.The occurrence of Lamprocyrtis neoheteroporos in Samples 117-725C-16X, CC (153.2 mbsf) and 117-725C-17X, CC (162.8 mbsf)suggests that these samples probably lie within the A. angulareZone as well. The fauna at this site is more diverse than at Site724, although there are still few tropical zonal marker species.

Paleoenvironmental ImplicationsMany Helicosphaera specimens, which seem to prefer up-

welling areas (Perch-Nielsen, 1985), as well as Coccolithus pela-gicus, a cold-water species, were found in the lowermost core-catcher sample in Hole 725C (162.8 mbsf).

The presence of Ammonia beccarii in the upper part of thesequence indicates that this site has been at or above its modernwater depth since approximately 0.7 Ma. The absence of thesame species, together with a significantly different benthic fo-raminiferal fauna in the lower part of the sequence, might indi-cate a water depth exceeding 350 m before this date.

PALEOMAGNETISM

IntroductionSite 725 is located at 18°29.20'N, 57°42.03'E in water depth

of about 310 m on the Oman margin. Holes 725A, 725B, and725C were cored to depths of 4.5, 93.8, and 162.8 mbsf, respec-tively. The lithology consisted mainly of marly nannofossil ooze.

MagnetostratigraphyAs at Sites 723 and 724, natural remanent magnetization

(NRM) intensities at Site 725 also were low, generally less than 1mA/m (arithmetic mean = 0.7 mA/m) after alternating field(AF) demagnetization at 5 to 10 m l Figure 9 depicts the com-bined discrete sample declination, inclination, and intensity datafor Holes 725A, 725B, and 725C. Points having circular stan-dard deviation values greater than 40° were excluded. Unfortu-nately, this (rather liberal) filtering procedure did not leave usenough points to permit detailed correlations with the polaritytime scale. Reference to the nannofossil stratigraphy (see "Bio-stratigraphy" section, this chapter) suggests that the shallowand reversed inclination values at 79 mbsf may correspond tothe Chron Cl/Clr (Brunhes/Matuyama Chronozone) bound-ary, while the zone of normal inclinations at 100 and 121 mbsf

427

SITE 725

20

40

60

Ė

£ 80

100

120

140

160 —

_

• —

1 ' 1

_

-al

^

180 3 6 0 - 9 0 -60 -30 0 30 60 90 I 0 " 2 10"1 1Declination (degrees) Inclination (degrees) Intensity (mA/m)

10

Figure 9. Declination, inclination, and intensity of discrete samples from Holes 725A, 725B, and 725C after AFdemagnetization at 5-10 m l

may be equivalent to Subchron Clr-1 (Jaramillo Subchrono-zone). Shore-based study of samples collected (but not mea-sured) on board the ship may enable us to make more confidentcorrelations with the polarity time scale.

Figure 10 shows a histogram of inclination values (after fil-tering) grouped into 10° classes. Inclination values of the nor-mally magnetized specimens agree with the expected geocentricaxial dipole value for the site latitude (33.8°). Values for the re-versely magnetized specimens appear significantly shallower thanexpected, which suggests the presence of a normal overprintthat was not completely removed by AF demagnetization.

Magnetic Susceptibility

The volume magnetic susceptibility of Site 725 sedimentswas measured using the Bartington Instruments whole-core sus-ceptibility sensor at the 0.1 sensitivity and low-frequency (0.47kHz) settings. Intervals of 10 cm were measured at Hole 725C

only because of the poor recovery in Holes 725A and 725B. Gasexpansion was most severe in Cores 117-725C-16X and -17X;thus, to avoid gaps, archive halves were measured and data weremultiplied by two during processing. Susceptibility data forHole 725C are shown in Figure 11. Poor recovery below Core117-725 C-4H (~32 mbsf) limited the usefulness of the suscepti-bility data for intersite correlations. Some correlations withSites 723 and 724 are possible for the first four cores of Hole725C, however.

While the gaseous nature of Hole 725C sediment precludedmeasuring susceptibility continuously, the self-extrusion of Hole725C sediments during core-splitting provided copious amountsof material that could not be curated, which we found suitablefor magnetic extractions. Approximately 0.25 kg of wet sedi-ment obtained from the top of Section 117-725C-5H-1 (-38mbsf) was dispersed in ~ 1 L of Calgon solution. The slurry wascontinuously agitated while two bar magnets (sheathed in para-

428

SITE 725

18

fβ

14

I 1 2

&10

a

1 6

I 'Site 725 Site 725

7XΔ-90 -60 -30 0 30 60

inclination (degreesHfiitβred samples)90

Figure 10. Inclination values grouped into 10° classes. Arrows indicatethe expected geocentric axial dipole value for the site latitude.

140 —

160O 31.4 62.8 94.2 125.7 157.1 188.5

K ( I 0 " 6 S1)Figure 11. Whole-core magnetic susceptibility record for Hole 725C.

film) were positioned at the top and bottom levels of the beaker.The magnets were removed after several minutes, and the adher-ing magnetic residue was washed into a separate beaker eachtime. This process was repeated approximately 15 times for each

of two aliquots of sediment. This extraction procedure was thenapplied to the primary extracts for purification. The efficiencyof the extraction is unknown. Visual inspection indicated thatthe extracted grains were quite coarse (medium silt), which sug-gests that the finer grains contributing to the NRM either werenot abundant or were not extracted.

The initiative for extracting magnetic particles came fromour desire to observe the grain sizes, shapes, and textures thatcontributed to susceptibility and remanence. The slight down-hole decrease in NRM intensity from 0 to —100 mbsf (Fig. 9)suggests that fine-grained ferrimagnetic particles may be dis-solving or altering to nonmagnetic phases. The final purifiedextraction was divided into highly magnetic and less magneticfractions by passing a bar magnet under the beaker containingthe purified extraction and drawing the most magnetic particlesto one side. These grains were removed by pipette, dried, andmounted on an SEM sample stub for observation at high mag-nification. Photographs of the observed grains are shown inFigures 12 through 14.

Observation of the grains indicated that they were remark-ably uniform in size (fine to coarse silt) and were generally wellrounded and well preserved. Uniformity of size may have been aconsequence of the extraction procedure. The degree of grainroundness was surprising in that an extract from margin sedi-ment might be expected to be more angular. However, the round-ness is consistent with an eolian mode of deposition, as sug-gested by the XRD data presented in the "Paleomagnetism"section of the Site 722 chapter.

Some indication of grain pitting and dissolution can be seenin many of the photographs. Striking examples of this can beseen in Figure 13 and in the upper right quadrant of Figure 14.Extensive pitting was apparent for about 10% of the grains,while moderate pitting included perhaps another 10%. Gener-ally, however, the preservation of the grains was very good. Tobetter understand the history of magnetic mineral deposition,we intend to conduct more thorough shore-based magnetic andmorphometric analyses of magnetic extracts from Leg 117 sedi-ments.

ACCUMULATION RATES

Sedimentation rates for the top 100 m (lithologic Unit I) ofSite 725 are calculated between nannofossil datums with agesbased on oxygen isotope stratigraphy (Table 4; Niitsuma, un-published data). The mean rate for the last million years rangesfrom 79 to 152 m/m.y. (Table 5, Fig. 15), with an overall averageof 124 m/m.y. If we include two additional nannofossil datumsrecognized at Site 725, with ages based on North Atlantic data(Table 4), the mean rate is 114 m/m.y. for the last 1.1 m.y. (Fig.15). These values are typical of hemipelagic sedimentation inthe upper slope basin. No reliable stratigraphic markers werefound below 125 mbsf, therefore, sedimentation rate are un-available for lithologic Units II and III.

The mass accumulation of calcium carbonate, organic car-bon, and noncarbonate sediment components were calculatedfrom average values between nannofossil datums (Table 6, Fig.16). Based on the homogeneous sediment character in lithologicUnit I, this large variation in accumulation rate is unexpectedand may be attributed to uncertainties in the age model. Post-cruise studies that obtain a more precise age model for Site 725will be necessary to verify these rate changes.

PHYSICAL PROPERTIES

Introduction

Physical properties measured on discrete samples of sedi-ment recovered from Site 725 included only index properties(wet-bulk density, porosity, water content, and grain density).The sediments recovered were not sufficiently indurated to allow

429

Figure 12. Scanning electron microscope photograph of magnetic con-centrates from Site 725 sediments.


compressional-wave velocities to be measured with the Hamil-ton Frame Velocimeter. The coarse-grained noncohesive sedi-ments common in the upper part of this site were not suitablefor measuring shear strengths with the Wykeham-Farrance vaneshear device. The properties that were determined for discretesamples from Holes 725A and 725B, and Hole 725C are listedin Tables 6 and 7, respectively. Wet-bulk density was measuredon all whole-round core sections that were at least 80 cm long us-ing the GRAPE logging system. Acquisition of compressional-wave velocity data using the P-wave logger was successful onlyon the cores recovered using the APC coring system. All tech-niques and equipment used are described in the "ExplanatoryNotes" (this volume).

Index Properties

Lithologic Unit I

Lithologic Unit I (0-121.0 mbsf) is made up of several sedi-ment types that, on the basis of visual description, were not di-vided into facies or subunits. However, index properties of thesediments in lithologic Unit I do show two distinct trends (Fig.17) that are related to minor changes in sediment composition

and a decrease in grain size downsection. From the seafloor to70 mbsf, lithologic Unit I is a coarse-grained unit that consistsprimarily of foraminifer-rich, calcitic, sandy silt and calcitic,marly, nannofossil ooze, throughout which the percentages ofcalcium carbonate (average of 53%) and organic carbon (< 1%)are nearly constant with depth (see "Lithostratigraphy" section,this chapter, Fig. 15). At 70 mbsf, average values of porosity andwater content decrease gradually with depth from 51% to 49%and 30% to 28%, respectively. Wet-bulk density reflects thedownsection decrease in water content, which increases from1.87 g/cm3 near the seafloor to 1.93 g/cm3 at 70 mbsf. Varia-tions in the values of porosity, water content, and wet-bulk den-sity may be partly attributed to local variation in sediment com-position and fabric, but may also be a result of the inevitabledisturbance during core recovery of unconsolidated sands andsilts. The low porosity and density gradients determined hereare typical of coarse-grained sediments, which decrease volumeslowly during the early stages of compaction (Meade, 1966). Be-cause of the strong possibility of disturbance, however, the po-rosities, water contents, and bulk densities reported here shouldbe considered as lower and upper bounds, respectively, on thein-situ values.

430

SITE 725


In the lower part of lithologic Unit I (75-121 mbsf), thephysical property trends change in response to the increasingabundance of terrigenous clay, as well as to gradual increases inboth calcium carbonate and organic carbon throughout the sec-tion (see "Lithostratigraphy" section, Fig. 15). Although nodistinct sedimentologic boundary was identified, physical prop-erties change abruptly near the top of Core 117-725C-9X (76mbsf), where porosity and water content increase to 66% and42%, respectively. Between 76 and 121 mbsf, porosity and watercontent decrease gradually and uniformly to 64% and 40%, re-spectively. One deviation from this trend occurs in a sampletaken from a calcitic silty clay, Sample 117-725C-12X-3, 108-110 cm, which shows inexplicably low porosity and water con-tent coupled with high wet-bulk density. Grain density generallydecreases from 2.60 to 2.55 g/cm3 over this interval in whichdiatoms become increasingly abundant with depth and nanno-fossils replace a portion of the terrigenous material (see "Litho-stratigraphy" section, this chapter; Fig. 17). Wet-bulk densityshows considerable variation over this interval (1.62-1.83 g/cm3) and averages 1.68 g/cm3.

Lithologic Unit IIWithin the laminated layers of calcitic marly nannofossil

ooze and diatomaceous clayey silt and mud of lithologic Unit II(121.0-145.4 mbsf) porosity, water content, and wet-bulk den-sity are relatively constant, with the exception of one sampletaken from one of the diatomaceous layers in Sample 117-725C-14X-6, 14-16 cm. The highest porosity (71%) and water content(50%) measured at Site 725 were measured in this diatomaceousinterval. Porosity and water content average 63% and 39%, re-spectively, for the entire unit. Wet-bulk density averages 1.66 g/cm3 through lithologic Unit II as a whole and decreases to 1.44g/cm3 within the diatomaceous mud. Grain densities continueto decrease downsection, with a minimum value of 2.31 g/cm3

in the diatom-rich layer of Core 117-725C-14X.

Lithologic Unit III

Within lithologic Unit III (145.4-162.8 mbsf), index proper-ties resume the downsection trends established in the base oflithologically similar Unit I. Porosity and water content at thebase of this unit average 59% and 37%, respectively. Wet-bulkdensities and grain densities measure 1.66 and 2.59 g/cm3, re-spectively.

GRAPE and P-Wave LogsData collected by the GRAPE logging system were of good

quality in Hole 725C, in part because of the relatively low gascontent of the section (Figs. 18 and 19). The major gaps in therecords correspond to intervals of no recovery, rather than areasof low data quality. The general pattern of density variation dis-played on the GRAPE records follows that of the wet-bulk den-sities determined for discrete samples. In the uppermost 70 m ofHole 725C, GRAPE records show relatively high density (1.9 g/cm3) that varies less than 0.2 g/cm3. As with the discrete sam-ples, density decreases and variability of the values increasesabruptly near 80 mbsf. A close correspondence between theGRAPE logs and discrete samples continues downsection tolithologic Unit III, where the quality of the GRAPE data de-grades as the diameter of the core decreases, producing a gapbetween the sediment and core liner.

Good quality P-wave logs were obtained in Cores 117-725C-1H through -4H. The high, near-surface values, 1600 to 1700m/s, reflect the low densities of the coarse-grained sediment.The P-wave record for Cores 117-725C-5X through -1IX is ofpoor quality because of spotty recovery, but does reflect the de-crease in density displayed in the GRAPE records. At depths be-low 100 mbsf, incomplete filling of the core liners resulted in alack of coupling between sediment and liner that prevented thetransmission of P-wave signals.

Patterns of cyclic variation on a scale of several meters dis-played on the records from Sites 721, 722, 723, and 724 are notwell defined at Site 725 because of the low variability in bothdensity and velocity in the surficial sediments.

INORGANIC GEOCHEMISTRY

IntroductionEight interstitial water samples were collected at Site 725 by

squeezing: one each from Holes 724A and 724B and the remain-der from Hole 724C. No in-situ samples were collected fromthis site. All analytical results are listed in Tables 8 and 9 andpresented in Figures 20 and 21. These profiles are compiledfrom data from all three holes.

Salinity, Chloride, and pHThe concentration profiles of salinity, chloride, and pH are

shown in Figure 20. There is a loss of salinity with depth (~ 2 g/

431

SITE 725

Table 5. Sedimentation and accumulation rate data for Site 725.

Depthinterval(mbsf)

0-27.127.1-50.950.9-100.9

Agerange(m.y.)

0-0.190.19-0.490.49-0.82

CaCO3

52.555.854.6

St*0.570.321.21

Dry-bulkdensity

(× g/cm3)

1.411.541.19

Sed.rate

(× m/m.y.)

142.679.3

151.5

CaCO3ace. rate

(g/cm2/k.y.)

10.566.819.84

Non-CaCO3ace. rate

(g/cm2/k.y.)

9.555.408.18

Corgacc. rate

(mg/cm2/k.y.)

114.639.1

218.1

100

150

152 m/m.y.

114 m/m.y.

0.5Age (m.y.)

1 .0

Figure 15. Age-depth plot of stratigraphic datums listed in Table 4. Thefilled and open boxes are the upper and lower depths of each datumlevel, respectively. Indicated sedimentation rates (solid lines) are calcu-lated between reliable nannofossil datum levels. The dashed line repre-sents the mean sedimentation rate for the last 1.1 m.y. at Site 725, basedon a best fit to all stratigraphic data.

kg over 160 m) of the same magnitude as occurs at Site 724.This decrease may also be attributed to depletion of sulfate andmagnesium ions that are involved in microbial degradation anddiagenetic reactions, respectively.

The chloride profile at Site 725 displays similar structure tothat seen at Site 724, where a broad maximum was observed be-tween 20 and 100 mbsf. However, at Site 725 the chloride maxi-mum extends over a greater depth zone and reaches higher con-centrations (+14 mmol/L relative to the topmost sample, com-pared to a ~ 10 mmol/L increase at Site 724). We believe thatthis chloride maximum may represent burial of late Pleistoceneseawater that was more saline than modern bottom water. Sub-sequent diffusion is gradually smoothing this signal. It is notclear why the chloride maximum is seen only at Sites 724 and725.

pH values measured at this site are biased by sample degas-sing. However, Site 725 has a slightly higher pH than Site 724,although this may result from the lower depressurization ofthese samples (Site 725 was one-half as deep as Site 724).

Sulfate and AlkalinitySulfate depletion occurs over the upper 110 m of the cored

section (Fig. 20), giving a gradient of 0.25 mmol/L/m.

This is one-half the gradient of Site 724, despite the higher rateof sedimentation at this site (see "Biostratigraphy" section, thischapter). Below ~ 110 mbsf, sulfate is constant at ~ 1 mmol/L,similar to the distribution seen at Site 724.

In contrast to sulfate, the alkalinity increase observed at thissite is more than three times that at Site 724. Approximately 30mmol/L of titration alkalinity was observed at the point wherethe sulfate concentration approaches zero, compared with —10mmol/L at the previous site. Although an alkalinity deficit(about 20 mmol/L) still occurs at this site, the absolute differ-ence between the profiles at the base of the sulfate reductionzones at the two sites may be attributed to less carbonate precip-itation in the rapidly accumulating sandy terrigenous sedimentof Site 725. Faster sedimentation would cause concentrationgradients for alkalinity, magnesium, and calcium to become lesssteep, which in turn would reduce the downward flux of the al-kaline earths, and thus alkalinity, to reaction sites at depth. Theincrease in alkalinity below 110 mbsf is probably from ammoniaproduction during methanogenesis (see below).

Calcium and MagnesiumBoth calcium and magnesium concentrations (Fig. 20) de-

crease with depth, although calcium displays a subsurface maxi-mum at ~ 50 mbsf. The profiles indicate that little reaction isoccurring in these sediments, while diffusion plays a major rolein determining the gradients observed (especially for magne-sium). The low magnesium/calcium ratio, low alkalinity deficit,and high accumulation rate of the sediment conspire to limitdolomite diagenesis in the upper 150 m of this site.

Ammonia, Phosphate, Silica, and Dissolved OrganicCarbon (DOC)

The distribution of these constituents is shown in Figure 20.Ammonia concentrations increase down the length of the coredsection, with a gradient (-0.13 mmol/L/m) similar to that ob-served at Site 724.

Phosphate concentrations are low and variable (perhaps dueto analytical difficulties). The low values are most likely causedby the phosphatization of biogenic skeletal debris and/or apa-tite precipitation on calcite tests.

Dissolved silica increases rapidly at 40 to 50 mbsf from — 300µmol/L to — 1200 µmol/L, which is near the saturation limit ofopal-CT at this water depth. This rapid increase suggests thatbiogenic silica should be observed in the sediment in greateramounts from 50 mbsf. In a similar fashion, DOC (Table 9 andFig. 20) increases over the same depth zone, suggesting thatgreater amounts of biogenic matter occur within the sedimentcolumn below 50 mbsf. Total organic carbon (see "OrganicGeochemistry" section, this chapter) and sediment smear-slidedescriptions (see "Lithostratigraphy" section, this chapter) indi-cate relatively constant organic matter concentrations of ~ 1.0wt% over the top 80 m of Site 725, but increasing (to 3.5 wt%)and variable concentrations of organic matter and diatoms be-low. The interstitial water geochemistry presented here broadlyconfirms these observations.

432

SITE 725

Table 6. Physical properties summary for Holes 725A and 725B.

Core, section,interval (cm)

117-725A-1H-1, 100-102725A-1H-3, 100-102725B-5X-1, 100-102725B-6X, CC 15-17725B-7X-1, 130-132

Depth(mbsf)

1.004.00

37.2046.4956.60

Wet-bulkdensity(g/cm3)

1.8831.8441.9702.039

Porosity(%)

49.451.946.940.4

Watercontent

(%)

26.928.824.420.3

Graindensity(g/cm3)

2.6152.6422.6812.6082.643

Dry-bulkdensity(g/cm3)

1.3761.3121.4901.625

50Sedimentation rate (m/m.y.)

100 150 200 250

20 —

C 40m

60 —

80 —

100

1 1 '

\N

S\

- \N

NS

1

1 '

NS

NS

N

1

—

1117 9

Accumulation rate (g/cm2/k.y.)Figure 16. Sedimentation rate (m/m.y.; solid line), calcium carbonateaccumulation rate (g/cm2/k.y.; filled circles), and noncorbonate accu-mulation rate (g/cm2/k.y.; open circles) vs. depth (mbsf) at Site 725.Accumulation rates are plotted at the midpoint of the respective depthintervals.

ORGANIC GEOCHEMISTRY

Abundance and Character of Organic MatterThe headspace residues as well as the physical properties resi-

dues of 35 samples (mainly from Hole 725C) were analyzed fortheir calcium carbonate and organic carbon content; the resultsare given in Table 3 and plotted vs. depth in Figure 5. Organiccarbon and calcium carbonate values are fairly constant in theupper 80 mbsf of lithologic Unit I, and organic carbon averagesonly 0.65%, a low value when compared to previous sites drilledon the Oman margin. The entire lithologic Unit I down to 120mbsf (see "Lithostratigraphy" section, this chapter), which con-sists of foraminifer-rich, calcitic, calcareous ooze and sandysilt, has an average value of 1.0%. Organic carbon concentra-tions vary widely in the lower 40 m of Unit I and reach high val-ues of 3.6%, which are characteristic for the underlying unit oflaminated and diatomaceous sediments that comprise lithologicUnit II (120.0-145.4 mbsf). We believe that the facies and com-position of Unit II indicate suboxic deposition during some partof the lower Pleistocene. Average organic carbon concentrationsare 3.06%. Below Unit II, sediments have the same aspect asthose in lithologic Unit I and consist of nannofossil-rich calciticsand, silt, and clay that averages 3.61% organic carbon. Sparsepyrolysis results (Table 10) indicate that samples from lithologic

Unit I with low organic carbon values have low hydrogen index(HI) values, an indication of relatively refractory organic mat-ter. Two samples from Units II and HI show high values that ap-pear to be related to the higher organic carbon content. Cur-rently, we are not certain whether influx determines the relativefreshness of the organic matter, or whether preservational sig-nals prevail (Fig. 21).

Hydrocarbon GasesBecause of the very shallow water depth (311 m), safety con-

siderations made a stringent hydrocarbon monitoring programnecessary at Site 725. Gas pockets occurred only in the last fourcores of Hole 725 C; results of analyses of the vacutainer sam-ples are given in Table 11. Methane is the most abundant gas,but considerable amounts of thermogenic ethane and propanewere detected in gas pockets of Core 117-725C-17X (Table 11).Headspace samples were taken at regular intervals in Hole 725C;results appear in Table 12. Again, a steady increase of the ther-mogenic gases, ethane and propane, was observed with depth.

SUMMARY AND CONCLUSIONSSite 725 marks a shallow landward end point of a depth tran-

sect designed to recover sediments from a water depth range of300 to 1500 m on the Oman continental margin. This transectbrackets the vertical extent of a pronounced mid-water oxygen-minimum zone. Site 725 is located on the landward flank of theprominent slope basin that had been drilled at Sites 723 and724. The goal for Site 725 was to trace the fluctuations in theupper boundary of the oxygen-minimum zone through time andto evaluate these fluctuations with regard to changes in themonsoonal upwelling, eolian transport, and eustatic sea level.Continuity of the sedimentary section and the benthic foramini-fer assemblages were expected to reveal any differential tectonicmovement of the basement blocks and the slope basins.

Some of the major findings at Site 725 are summarized inFigure 22 and include the following:

1. Identification of the alternation of millimeter-scale lami-nated organic- and opal-rich clayey silt with nonlaminated nan-nofossil ooze and calcitic silty clay. The laminated intervals areconfined to a brief interval in the lower Pleistocene.

2. The average sedimentation rate is about 120 m/m.y.3. Opal occurrences are confined to sediments with high or-

ganic carbon concentrations and overlap the laminated inter-vals.

4. Cold-water nannoplankton occurrences at the base of thesection and abundant but low-diversity assemblages throughoutthe overlying section.

5. Persistent occurrence of interstitial sulfate, high alkalini-ties, and dramatic core expansion, which indicate replenishmentof the interstitial sulfate pool at depth, comparable to Site 723.

6. A maximum in the chloride concentration at about 80mbsf (-0.7 Ma), which was also seen at Site 724, suggests thatmid-Pleistocene bottom water on the upper margin was signifi-cantly more saline than it is today.

433

SITE 725

Table 7. Physical properties summary for Hole 725C.


117-725C-1H-2, 90-921H-6, 90-922H-2, 60-622H-4, 60-622H-7, 30-323H-2, 100-1023H-4, 24-263H-6, 100-1024X-3, 50-526X-2, 86-886X-4, 86-888X-3, 29-319X-4, 27-2911X-2, 24-2611X-5, 8-1012X-2, 19-2112X-3, 108-11013X-3, 34-3614X-2, 21-2314X-4, 26-2814X-6, 14-1615X-2, 85-8715X-4, 80-8216X-3, 4-617X-2, 25-2717X-4, 106-10817X-6, 128-130

Depth(mbsf)

2.408.40

11.1014.1018.3021.1023.3427.1032.0049.8652.8669.7980.8797.04

101.38106.59108.98117.84125.81128.86131.74136.15139.10146.54154.95158.76161.98

Wet-bulkdensity(g/cm3)

1.9151.8421.9531.8151.9241.9561.9561.9361.9981.810.840.935

1.622.622.838.586.802.631.623.670.444.737.604.673.653.664

1.663

Porosity(%)

50.853.645.751.746.247.547.646.845.052.751.947.266.564.752.566.866.663.065.561.171.059.964.660.259.458.260.8

Watercontent

(%)

27.229.824.029.224.624.924.924.823.129.828.925.042.040.929.343.237.839.641.337.550.435.341.336.936.835.937.4

Graindensity(g/cm3)

2.7242.7492.6352.6472.6652.6732.6392.6562.7142.6772.6172.6642.5922.5562.6312.5132.6332.5242.6212.5132.3092.6622.4842.4762.4362.4992.506

Dry-bulkdensity(g/cm3)

.395

.2931.484.286.450.469.468.457.537.271.308.452

0.9410.9591.3000.9011.1200.9850.9521.0440.7161.1230.9421.0561.0441.0681.040

7. Elevated ethane and propane gas concentrations with depthimply migration of thermogenic hydrocarbons from deep sources.

The section penetrated at Site 725 ranges from the Holoceneto the lower Pleistocene and has been subdivided into three lith-ologic units (Fig. 22).

Lithologic Unit I (0-120 mbsf) comprises foraminifer-rich,calcitic, sandy silt to calcitic, marly, calcareous ooze; calcitic,marly, nannofossil ooze; and nannofossil-rich sand, silt, or clay.This lithology is coarser and has more detrital carbonate thansimilar sediments at Sites 723 and 724. It is considered to be ahemipelagic facies dominated by influx of detrital calcite, eoliansilts and clays, and biogenic components from coastal upwellingand modified by slight winnowing or nondeposition of fine-grained material. Organic carbon content is less than 1% overthe upper 80 m and varies between 1% and 2% in the lower partof the unit.

Lithologic Unit II (120-145.4 mbsf) is composed of diato-maceous clayey silt and diatomaceous mud intercalated withcalcitic marly nannofossil ooze and calcitic sand, silt, and clay(Fig. 25). Diatom-rich beds form laminae on less than a centi-meter scale and are dominated by frustrules of two diatom spe-cies. The unit is believed to indicate anoxic depositional condi-tions. Organic carbon content ranges between 2% and 3% inthis laminated unity.

Lithologic Unit III (145-162.8 mbsp is similar to Unit I andis composed of nannofossil-rich, calcitic sand, silt, and clay andcalcitic, marly, nannofossil ooze. All units were found to con-tain less than 2% authigenic dolomite and traces of apatite andfish scales. Organic carbon content averages 3%.

Like previous sites on the margin, Site 725 contains partiallyoverlapping intervals of cold-water nannoplankton species anda robust, radiolarian fauna dominated by Spumellarians (Fig.22). In contrast to the other sites, these occurrences are confinedto the lower Pleistocene. However, the opal-rich sediments coin-cide with the laminated facies, as observed at other sites.

Mass accumulation rates obtained with a preliminary agemodel are depicted in Figure 22. These rates may vary by almosta factor of two downhole. However, some of this variation maybe attributed to uncertainties in the age and depth of biostrati-graphic datums.

Comparison of Site 725 with Site 723 demonstrates that theratio of organic matter accumulation to clastic and biogenic ac-cumulation is higher at Site 723 in the center of the basin thanat Site 725. This pattern of accumulation may indicate a persist-ent spatial inhomogeneity of primary production and preserva-tion, or an influx of allochthonous organic-carbon-rich mate-rial.

Even though dolomite was encountered in trace amountsthroughout the cores, the extent of dolomitization was insig-nificant when compared to the pronounced dolomitization ob-served at Site 723. Interstitial water profiles at Site 725 show anincrease in chloride concentrations with depth. Sulfate ions arenever depleted at depth, even though increasing ammonia andalkalinity concentrations imply ongoing sulfate reduction at depth(Fig. 22). Thus, a replenishment of the interstitial sulfate poolby ascending water may be implied. The persistence of sulfatereduction is in accord with relatively low methane concentra-tions in gas pockets. Cores more than 100 mbsf show increasing(up to 90 µL/L) ethane and propane concentrations that are be-lieved to indicate thermal hydrocarbon generation at depth.

Site 725 constitutes a complete section of Pleistocene agethat shows significant changes in the Oceanographic and deposi-tional environment, such as the frequency of upwelling indica-tors, the accumulation of carbon, and the preservation of lami-nated facies. Similar associations were observed at Sites 723 and724, and these associations seem roughly contemporaneous. Ifthese facies prove to be synchronous, they will provide con-straints on the regional vs. local origin and possible extent ofthe oxygen-minimum zone. A preliminary interpretation of theheterogeneity observed in organic matter abundance and preser-vation of primary sediment structures (i.e., the laminations) is

434

SITE 725

Lith.unit

• D PorosityIQwater content

20 —

40 —

aE 8 0 -

f100 —

120 —

140 —

160 —IS>×

162.8

I I I

>

\

i %••D O

v»•

-• t

• .

1 Λ

AΔ

Δ

A

A

>vr-r-1 .4 2.81 .8 2.2 0 40 80 2.2 2.4 2.6

Bulk density Porosity i%) Grain density(g/cm3) Water content {%) (g/cm3)

Figure 17. Index properties (bulk density, porosity, water content, and grain density) measured on dis-crete samples from Holes 725A, 725B, and 725C. Hole 725A and 725B = open symbols; Hole 725C= closed symbols.

that the mid-water oxygen-minimum zone expanded, perphapsin response to enhanced productivity, and was more stable dur-ing the early Pleistocene.

REFERENCES

Berggren, W. A., Kent, D. V., and Van Couvering, J. A., 1985. The Ne-ogene: Part 2, Neogene geochronology and chronostratigraphy. InSnelling, N. J. (Ed.), The Chronology of the Geological Record.Geol. Soc. Mem. (London), 10:211-260.

Caulet, J. P., Debrabant, P., and Fieux, M., in press. Dynamique desmasses d'eau océanique et sedimentation quaternaire sur la margede 1'Afrique de l'Est et dans le bassin de Somalie. Resultats prélimi-naires de la mission MD44 INDUSOM du Marion-Dufresnes. CRAS.

Emerson, S., 1985. Organic carbon preservation in marine sediments.In Sundquist, E. T, and Broecker, W. S. (Eds.), The Carbon Cycleand Atmospheric CO2: Natural Variations Archean to Present. Geo-phys. Monogr., 3:78-87.

Kemper, E., and Zimmerle, W., 1980. Facies pattern of a Cretaceous/Tertiary subtropical upwelling system (Great Syrian Desert) and anAptian/Albian boreal upwelling system (NW Germany). In Thiede,

J., and Suess, E. (Eds.), Coastal Upwelling (Pt. B): New York (Ple-num Press), 29-72.

Kolla, V., Kostecki, J. A., Robinson, F., and Biscaye, P. E., 1981. Dis-tributions and origins of clay minerals and quartz in surface sedi-ments of the Arabian Sea. J. Sediment. Petrol., 51:563-569.

Meade, R. H., 1966. Factors influencing the early stages of compactionof clays and sands-review. J. Sediment. Petrol., 36:149-165.

Perch-Nielsen, K., 1985. Cenzoic calcareous nannofossils. In Bolli, H.,Saunders, J., and Perch-Nielsen, K. (Eds.), Plankton Stratigraphy:Cambridge (Cambridge Univ. Press), 427-554.

Prell, W. L., and Streeter, H. F., 1982. Temporal and spatial patterns ofmonsoonal upwelling along Arabia: a modern analogue for the in-terpretation of Quaternary SST anomalies. J. Mar. Res., 40:143-155.

Takayama, T., and Sato, T., 1987. Coccolith biostratigraphy of theNorth Atlantic Ocean, Deep Sea Drilling Project Leg 94. In Ruddi-man, W. F., Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94,Pt. 2: Washington (U.S. Govt. Printing Office), 651-702.

Ms 117A-112

435

SITE 725

0 r0 i-

1 6 0

1 .6 1 .8 2 .0GRAPE density (g/cm3)

2.2 1001450

Figure 18. GRAPE wet-bulk density (solid line) and wet-bulk densityfrom discrete samples (diamonds) for Hole 725C. GRAPE profile isbased on 10-cm-block averages of the data.

1550 1650/"-wave velocity (m/s)

1750

Figure 19. Compressional-wave velocity as measured by the P-wave log-ger in Cores 117-725C-1H through Section 117-725C-11X-2.

Table 8. Summary of interstitial-water geochemical data at Site 725.


117-725A-1H-2, 145-150725B-5X-1, 145-150725C-3H-4, 145-150725C-6X-2, 145-150725C-9X-3, 145-150725C-12X-3, 145-150725C-15X-4, 145-150725C-17X-5, 145-150

Depth(mbsf)

2.9537.6524.5550.4580.55

109.35139.75160.65

Vol.(mL)

3030423940434538

pH

7.708.158.378.298.057.968.327.95

Alk.(mmol/L)

3.674.774.66

10.5617.3127.1632.2033.93

Sal.(g/kg)

35.635.635.535.436.034.834.234.2

Mg(mmol/L)

54.3246.1146.9543.6041.9031.1227.4122.13

Ca(mmol/L)

10.6313.0311.9612.0013.498.757.345.39

Cl(mmol/L)

576583584587592583569565

SO4

(mmol/L)

27.221.321.515.513.50.81.10.8

PO4

(µmol/L)

2.81.61.26.43.65.12.02.0

NH 4

(mmol/L)

0.651.401.023.104.539.15

16.4721.34

SiO2

(µmol/L)

183310248914

1272119512621135

Mg/Ca(µmol/L)

5.113.543.933.633.113.563.734.11

Table 9. Dissolved organic carbon concentra-tions at Site 725.

DissolvedCore, section, Depth organic carboninterval (cm) (mbsf) (a.u.)

117-725A-1H-2,725C-3H-4,725B-5X-1,725C-6X-2,725C-9X-3,725C-12C-3725C-15X-4725C-17X-5

145-150145-150145-150145-150145-150

, 145-150, 145-150, 145-150

2.9537.6524.5550.4580.55

109.35139.75160.65

0.0970.4550.6251.0850.8911.1871.0310.995

Note: a.u. = absorbance units.

436

I ' I kl FI ~ P i ' • I ' I ' I ' I

20

40

60

S 80

8-Q

100

120

140

180 —; i t i • π4 8 12Calcium(mmoi/L)

20 40 60Magnesium

(mmol/L)

" I •

2 4 6 32 34 36 560 580 600 7.5Mg2*/Ca2* Salinity Chloride

(g/kg) <mmol/U

7.0pH

8.5 0 10 20Ammonia(mmol/L)

0 4 8 0Phosphate

<µπtol/L)

800Silica

(µmol/L)

0 20 40Alkalinity(mmol/L)

0 15Sulfate

(mmol/L)

30

Figure 20. Profiles of interstitial water concentrations for Site 725, compiled from data from three holes.

SITE 725

0.5 1 .0DOC (a.uJ

Figure 21. Dissolved organic carbon (DOC) profile for Site 725, com-piled from data from three holes (expressed as absorbance units).

Table 10. Results of Rock-Eval pyrolysis of headspace residues from Holes 725A, 725B, and 725C.


117-725A-1H-1, 0-3725C-3H-4, 119-120725C-6X-2, 119-120725C-9X-3, 119-120725C-12X-3, 119-120725C-15X-4, 119-120725C-17X-5, 119-120

Depth(mbsf)

3.0024.2950.1980.39

109.09139.49160.39

TÅmax( ° Q

413412416418415403400

Si

0.170.040.310.310.453.612.18

s2

0.700.252.021.491.89

10.378.64

s3

1.461.041.501.161.252.582.54

S 2 /S 3

0.470.241.341.281.514.013.40

TOC a

0.450.180.820.450.502.642.21

TOC b

0.480.341.520.750.852.912.59

HI

14673

133199222356333

OI

304305

991551478998

Note: HI = hydrogen index and OI = oxygen index; the hydrogen and oxygen indexes were calculated on the ba-sis of the TOCb values. For a detailed description of parameters, see "Explanatory Notes," chapter (this vol-ume).

a TOC values are measured by Rock-Eval pyrolysis.TOC values measured by difference method.

438

Table 11. Concentrations of methane (Cj), ethane (C2), and pro-pane (C3) measured in gas pockets from Hole 725C.


Depth(mbsf) (ppm) (ppm) (ppm) CjA^

117-725C-14X-6, 2815X-3, 1416X-1, 14517X-6, 30

131.88136.94144.95161.00

412,173255,992142,217799,315

1347430

295

1911

552

3,0763,4594,7412,709

SITE 725

Table 12. Concentrations of methane (C^, ethane (C2), and propane(C3) per liter of wet sediment in samples from Holes 725A, 725B, and725C.


117-725A-1H-3, 0-1725B-5X-2, 0-1725C-3H-4, 119-120725C-6X-2, 119-120725C-9X-3, 119-120725C-12X-3, 119-120725C-15X-4, 119-120725C-17X-5, 119-120

Depth(mbsf)

3.0037.7024.2950.1980.29

109.09139.49160.39

(µL/L)

342113895

30452,02354.57562,010

c2(µL/L)

26

182948

c3(µL/L)

32238

C

211

4851

,890,882,292

Note: Values are expressed in µm (volume gas/volume wet sediment).

ic

8-Q

4 0

βo

8 0

1 0 0

1 2 0

1 4 0

1 6 0

l>o 1

wßor.r 1

•I•1BMryuid

i i1

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"1

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f

—

unh

m

?

ε

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f

Time

0.19 ( 2 )

0.49 (5)

0.82 ©0.89 (β)

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Age

i00

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Φ

0.

Plank -tonic

foram-inifers

•>

pre

se

i

•

i

$β

c

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Φ>

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JO

i

0

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Q

P al eo environment

Radio-larians

Barren

Fewtropicalmarkerspecies

diverse

Benthicforam-inifers

<*•waE10CO

top

c

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ow

er

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it>l

«0to0a.

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l

00

Nanno-fossils

0

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Coldwater

Alkalinity(mmol/L)

0 20 40

• ' A| '111

\ CI "11

AAlk \

1 1 i

A -

\ 11 •

• \ I "\ \1 1

t A—\ '\ /\ 1 "\ ;vIA* .i T

/

/ 1/ \

" / 1"_^ I1 T

1

-' 1 ~1

*, 1 .1-

Q11I f a t fiQ U I I Q I β

(mmol/L)

0 15 301

1

#I1-1/

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Sedimen-tβt ion

rate(m/m.y.)

0 100 2001 1

\\

1

Oß

/

K1 \

\ >1\11 V-_

# CarbonateO Nor>carbonate

I . 1 r162.8mTotal depth

5 6 0 580 600Chloride(mmol/L)

5 7 9 11Accumulation

rate(g/cm2/k.y.)

Figure 22. Summary chart outlining preliminary shipboard findings at Site 725. Numbers in circles: nannofossil marker horizons.

439

Documents

Ocean Drilling Program Initial Reports Volume 117lution is perpendicular to the trend of the basin and shows the bounding basement blocks and the synclinal-shaped sediment fill. B