Geo-Marine Letters (1999) 18 : 292d296 ( Springer-Verlag 1999

N. C. Slowey ' A. C. Neumann ' S. J. Burns

Submarine outcrop and acoustic expression of lithified sediment layersin northwest Providence Channel, Bahamas

Received: 30 June 1998 / Revision received: 20 January 1999

Abstract Visual observations of the wall of GreatBahama Canyon indicate that ledges produced by di!er-ential submarine erosion occur at depths like re#ectors onhigh-resolution seismic pro"les, suggesting lithologicchanges produce acoustic impedance contrasts and there-fore re#ectors. Quaternary-aged sediments in a core fromLittle Bahama Bank exhibit changes in lithology (and pre-sumably acoustic impedance) associated with glacial-to-interglacial transitions, which also correspond in depth toseismic re#ectors. This supports the concept that re#ectorson high-resolution seismic pro"les of Bahamian periplat-form ooze correspond directly to changes in lithology andmay be associated with climate/sea level #uctuations.

Introduction

The history of Quaternary glacial cycles is recorded in theuppermost section of undisturbed pelagic carbonate sedi-ments. High-frequency seismic pro"les of these sedimentstypically display closely spaced subbottom re#ectors(Mayer 1979a,b, 1980; Damuth 1980; Embly and Johnson1980; Mullins et al. 1980). In the equatorial Paci"c, thispattern has been described as an interference composite ofre#ections o! many small layers too close together to bediscerned as discrete re#ectors (Mayer 1979b, 1980). Theselayer contacts re#ect sound because they vary in saturatedbulk density as a result of changes in weight percentcalcium carbonate (Mayer 1979a, 1980). In pelagic

N. C. Slowey (|)Dept. of Oceanography, Texas A&M University, College Station,TX 77843-3146, USA

A. C. NeumannDept. of Marine Science, University of North Carolina, Chapel Hill,NC 27599-3300, USA

S. J. BurnsGeological Institute, University of Bern, Baltzerstrasse 1,CH-3012 Bern, Switzerland

sediments such as these and in the periplatform sedimentsof the Bahamas, variations in calcium carbonate depos-ition and diagenesis correlate closely with Quaternaryclimate/sea level cycles (Droxler et al. 1983; Boardmanet al. 1986; Slowey et al. 1989). Two studies have suggestedthat changes in the lithologic character of Bahamianperiplatform ooze can be correlated one-to-one with re-#ectors (Slowey et al. 1989; Wilber et al. 1990). If so, thenseismic stratigraphy may provide a powerful tool for un-derstanding variations of climatic and paleoceanographicconditions with time and their in#uence on the depositionof sediments about the Bahama Banks.

The evidence that re#ectors on high-resolution seismicpro"les of Bahamian periplatform ooze correspond totransitions from soft to semi- or fully lithi"ed periplatformooze and that these transitions are associated withchanges in climate/sea level is limited. Slowey et al. (1989)examined the geologic and geoacoustic properties of onecore of sediment and a seismic pro"le collected from thecentral portion of Northwest Providence Channel. Wilberet al. (1990) interpreted seismic pro"les from the westernmargin of Great Bahama Bank, but had no core data toverify their suggestions.

This paper presents outcrop evidence in support of theconcept that re#ectors can indeed correspond directly tochanges in sediment lithology associated with climate/sealevel #uctuations. First, we compare the stratigraphic andlithologic character of sediments from the southwesternmargin of Little Bahama Bank and the occurrence ofseismic re#ectors. Next, we present observations of sub-marine outcrops made from the submersible DS< Alvinand consider the apparent relations between changes insediment lithology and re#ectors.

Changes in core lithology correlate with seismic reflectors

Separating Little and Great Bahama Banks, the North-west Providence Channel forms part of an open seaway

Fig. 1 The locations of piston core 36 and DS< Alvin dive inNorthwest Providence Channel, Bahamas

that connects the Atlantic Ocean and the Straits ofFlorida. Although varied depositional regimes exist with-in the channel (Mullins and Neumann 1979; Mullins et al.1980), high-resolution seismic pro"les and sediment coreevidence indicate that the southwestern margin of LittleBahama Bank and a #at-topped inter#uve between can-yon branches are covered by a generally undisturbeddrape of periplatform ooze (Mullins et al. 1980; Burns andNeumann 1987; Slowey and Curry 1992).

Both bulk sediment 14C ages and foraminiferal oxygenisotopic values indicate that sediments in piston core 36(recovered during R/< Cape Hatteras cruise CH0182,Fig. 1 shows core site) were deposited during the Holo-cene and last glacial maximum (Burns and Neumann1987; Slowey and Curry 1992). Assuming that #uctuationsof the aragonite content of the sediments generally corres-pond to climatic cycles (e.g., Droxler et al. 1983), thepresence of cyclic variations in the aragonite content ofcore 36 suggest that the core contains sediment depositedduring the last two major glacial-interglacial cycles, thatis, marine oxygen isotopic stages 1}6 (Burns andNeumann 1987). Glacial sediments are partially lithi"edand thus more dewatered and rigid than interglacial sedi-ments (Fig. 2); the shallowest layer has a 14C age of&16,000 years (Burns and Neumann 1987), which corres-ponds to the last glacial maximum (isotopic stage 2). Thedowncore pro"le of aragonite content indicates that thelayer at about 4 m depth in the core corresponds to thepenultimate glaciation (aragonite content minimum asso-ciated with isotopic stage 6). Assuming sediment depos-ition was continuous through time, the partially lithi"edlayer at about 8 m depth in the core probably correspondsto the next older glaciation (isotopic stage 8). Partiallylithi"ed glacial sediments have been found in other coresfrom the southwestern margin of Little Bahama Bank(Slowey and Curry, unpublished data).

Fig. 2 The depths of partially lithi"ed layers, the downcore pro"le ofthe weight percentage of aragonite relative to the weight of aragoniteand calcite, and the position of oxygen isotope stage boundaries incore 36, compared with the 3.5-kHz seismic data collected while thecore was being recovered. The seismic data are scaled assuming thevelocity of sound in the sediments is 1500 m/s

The partially lithi"ed layers in core 36 range in hard-ness from soft chalk to fairly well lithi"ed limestone (scaleof Gealy 1971) and in thickness from 2 to 20 cm; theuppermost 1 to 2 cm of each layer is the most lithi"ed.Examination of thin sections of the lithi"ed layers indi-cates that they are mud-supported foram pteropod bio-micrites with pteropod shell moldic porosity.

Presumably, sound re#ects from the upper surfaces ofthe lithi"ed layers because they have greater acousticimpedance (velocity]bulk density) than the overlying,softer, "ne-grained ooze (e.g., Slowey et al. 1989). Velocityand density increase in the partially lithi"ed layers be-cause associated cementation reduces porosity (dewaterssediment as mineral material replaces water in the intersti-ces) and, in a sense, greatly increases grain size by forminggrain couplets and groups. Further, increased sedimentrigidity due to cementation causes increased velocity.

Figure 2 clearly illustrates how the depths of lithi"edlayers compare with the occurrence of re#ectors on the3.5-kHz seismic pro"le recorded while core 36 was beingrecovered. The seismic pro"le was collected using an EDO248A tranceiver, the hull-mounted 3.5-kHz transducerarray on the R/<Cape Hatteras, a Raytheon CESP-III (toprocess returned signals), and an EPC graphics recorder.A scanned image of the graphics recorder output has beenscaled relative to depth by assuming that the averagecompressional wave velocity for late Quaternary periplat-from ooze is about 1500 m/sec (Slowey et al. 1989). Severalprominent acoustic re#ectors are present on this 3.5-kHzpro"le. The re#ector associated with the sea#oor

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Fig. 3a, b Photographs of canyon wall in Northwest ProvidenceChannel taken from DS< Alvin. a Detail of a cli%et of beddedlimestone at about 940 m water depth. The bed in the center of thepicture is approximately 20 cm thick. The dark color of the exposedlimestone surface is an iron}manganese coating. b Soft, &&punky''limestone outcroping on ledges between cli!s of more consolidatedmaterial. Photograph taken at the base of cli! like that shown in a

sediment/water interface is broad and extends across thedepth interval, which corresponds to the shallowest lith-i"ed layer. It is therefore di$cult to evaluate if this re#ec-tor results, in part, from a change of acoustic impedanceassociated with the transition between the relativelycoarse, partially lithi"ed sediments of stage 2 and theoverlying "ner-grained, softer sediments of stage 1 (oneclue that it might is that the thickness of this re#ector isgreater than the thickness of some underlying re#ectors).There is a clear correspondence between the next twosubbottom re#ectors and the presence of partially lithi"edlayers.

Comparison of seismic profile and Alvin observations

Observations from the submersible DS< Alvin appear tosupport the correspondence between transitions fromsofter to more lithi"ed periplatform ooze and acousticre#ectors. The #oor of Northwest Providence Channel iscut along its axis by a branch of the Great BahamaCanyon (Andrews et al. 1970). From its mouth at theAtlantic end of the seaway, a branch of the canyon runs tothe northwestern end of the channel where it forms twoheads. An undisturbed drape of periplatform carbonateooze is accumulating on the isolated, #at topographichigh between the canyon branches (Boardman et al. 1986;Slowey et al. 1989). Regional multichannel seismic strati-

graphic correlation to well-log data suggests thesesediments fall in the general age range of Oligocene toHolocene (Sheridan et al. 1981). Along the edges of thetopographic high, the steep cli!s that form the canyonwalls have faces extending hundreds of meters. The Alvinmade several dives along these cli!s to investigate pro-cesses involved in canyon formation. These dives provideda unique opportunity to inspect vertical exposures of thelayers of periplatform ooze and consider their relationshipto the pattern of re#ectors observed on 3.5-kHz high-resolution seismic pro"les.

During a dive up the cli!s where the two canyonsbranches meet (Fig. 1), the Alvin traversed a steplike seriesof three short, steep, rocky cli%ets 3}5 m high (visualestimate) of manganese}iron-stained, thick beds of con-solidated limestone. Between the cli%ets were slopingledges, several meters wide, composed of less consolidated,chalky material that exhibited an irregular pock-markedoutcrop surface. No samples were obtained (Fig. 3). Thedi!erential submarine weathering of sediment layers ex-posed along the cli%et faces re#ects di!erences in thelithologic and thus acoustic properties of the sediment.

On a 3.5-kHz seismic pro"le of the site obtained duringan earlier cruise aboard the R/< Cape Hatteras (Fig. 4),the cli!s appeared to truncate three major seismic re#ec-tors located at approximately the same depths as theledges that were observed from Alvin. Although a certaincorrelation between the transition from the more lithi"edcli! sediment to softer sediment just above the ledges and

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Fig. 4 a 3.5-kHz seismic re#ection pro"le of the canyon wall inNorthwest Providence Channel. b Interpretive drawing of the can-yon wall based upon visual observations from the DS< Alvin. Theledges discussed in the text are identi"ed as A, B, and C

the re#ectors is not possible, we speculate that the samemajor changes in lithology that resulted in ledges viadi!erential submarine erosion could produce sharp acous-tic impedance contrasts and therefore re#ectors such asthose on the seismic pro"le. The spacing of the ledges/impedance contrasts is su$cient that they could correlateone-to-one with re#ectors. However, in a manner analog-ous to the equatorial Paci"c sediments described byMayer (1979a,b 1980), the impedance contrasts and dis-tances separating the thin layers observed between the

ledges would be small enough that individual re#ectionsfrom them could not be discerned seismically.

Summary

The lithologic and stratigraphic properties of core 36 andvisual observations made from the submersible DS<Alvinhave been compared with the patterns of re#ectors onhigh-resolution seismic pro"les. The results are consistentwith the concept that re#ectors on seismic pro"les ofBahamian periplatform ooze correspond to changes inlithology and geoacoustic properties associated withQuaternary climate/sea level #uctuations (Slowey et al.1989; Wilber et al. 1990). Taken together with previouswork, these results also indicate that the occurrence ofsequences of horizontal layers of soft and partially lithi"edooze is widespread throughout marginal sediments ofthe northern Bahamas. Seismic methods should allowrapid, widespread collection of data from which to modelthree-dimensionally the regional sedimentation patternthroughout the Quaternary.

Acknowledgments This research bene"ted from our discussions withL. A. Mayer and our colleagues who participated with us on theDS< Alvin dive series (particularly P. A. Baker and B. D'Argennio),and the comments of the reviewers. The research was supported byNSF grants to A. C. Neumann and N. C. Slowey.

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