Palaeogeography, Palaeoclimatology, Palaeoecology BART IWAI_2012.pdfPhilip J. Bart a,⁎, Masao Iwai b a Department of Geology and Geophysics, Louisiana State University, Howe Russell

Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

PALAEO-05845; No of Pages 10

Contents lists available at ScienceDirect

Palaeogeography, Palaeoclimatology, Palaeoecology

j ourna l homepage: www.e lsev ie r.com/ locate /pa laeo

The overdeepening hypothesis: How erosional modification of the marine-scapeduring the early Pliocene altered glacial dynamics on the AntarcticPeninsula's Pacific margin

Philip J. Bart a,⁎, Masao Iwai b

a Department of Geology and Geophysics, Louisiana State University, Howe Russell Complex E235, Baton Rouge, La, USAb Department of Natural Environmental Science Kochi University, Akebono-Cho 2-5-1, Kochi 7808520, Japan

⁎ Corresponding author. Tel.: +1 225 578 3109; fax:E-mail addresses: [email protected] (P.J. Bart), iwaim@k

0031-0182/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.palaeo.2011.06.010

Please cite this article as: Bart, P.J., Iwai, MPliocene altered glacial dynamics on the Anta

a b s t r a c t
a r t i c l e i n f o
Article history:Received 7 February 2011Received in revised form 4 June 2011Accepted 10 June 2011Available online xxxx

Keywords:Antarctic PeninsulaEarly PlioceneOverdeepeningWarm water intrusionGrounding eventODP Leg 178

A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that thePacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepenedshelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval oferosion begun at 5.2 Ma. The erosion was caused by high frequency advances of a super-inflated AntarcticPeninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence ofabundant moisture delivered to the region as the Polar Front migrated southward. By 5.12 Ma, ice streamsincised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelfwas transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on theupper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banksbeveled. By 4.25 Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have beeninfrequent. We propose that the reduced frequency of grounding events signaled the transition to a modernforedeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the earlyPliocene because overdeepening led to accelerated heat exchange between the ocean and APIS in two ways.Firstly, the overdeepened shelf required that a larger area of the grounded ice sheet's marine terminus be incontact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelfedge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell inthe region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS todeflate on the mainland, which further decreased the possibility that grounded ice could advance on theoverdeepened shelf.

+1 225 578 2302.ochi-u.ac.jp (M. Iwai).

l rights reserved.

., The overdeepening hypothesis: How erosionrctic Peninsula..., Palaeogeogr. Palaeoclimatol. P

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Water depth at the Antarctic shelf edge averages 500 m, which isfour times deeper than that on low latitude continental margins. Thegreat depths are a consequence of glacial erosion and isostaticadjustment. Today, the deep water shelves are marked by broadbanks and troughs that extend from the inner shelf to the shelf edge.Morphologic evidence in sea floor troughs, including trough-parallel,megascale glacial lineations, demonstrates that these broad featureswere occupied by ice streams — zones of fast flowing ice (Anderson,1999; Canals et al., 2000; Dowdeswell et al., 2004; Heroy andAnderson, 2005). Antarctic shelves also have an unusual foredeepenedor reversed-grade profile, meaning that maximum water depths arefound on the inner shelf.

Shelf depth and morphology affects ice sheet mass balance in themarine environment in three important ways because it controls1) the area of the ice sheet in contact with the ocean 2) the flux of icethat can be exported to the marine environment and 3) the flux ofwarm water that can intrude onto the shelf. The latter item isextremely important because recent models indicate that oceantemperature probably is the dominant factor influencing advanceand retreat of grounded ice in the marine environment (Pollard andDeConto, 2009). This model prediction suggests that an ice sheet'smarinemarginwill not advancewhen ocean temperatures on the shelfare elevated even if all other controlling factors (e.g., sea level,atmospheric temperature, and precipitation rate) are set to aprescribed full-glacial state. This is significant because, if so, then ourability to use composite oxygen isotope data to help interpret glacialhistory from glacial marine successions is highly dependent uponwhether or not the volume and frequency of warm water intrusionschanged through geologic time as the shelf morphology shifted fromshallow to overdeepened (e.g., Naish et al., 2009). The evolution ofshelf depth and morphology is poorly constrained (Anderson and

al modification of the marine-scape during the earlyalaeoecol. (2011), doi:10.1016/j.palaeo.2011.06.010

http://dx.doi.org/10.1016/j.palaeo.2011.06.010

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2 P.J. Bart, M. Iwai / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

Bartek, 1992; Anderson, 1999). However, today's outer shelves aroundAntarctica are deep enough that relatively-warm circumpolar deepwater currents upwelling in the Southern Ocean impinge on themargin. In places, these warm waters are directed landward alongforedeepened troughs to bathe andmelt the Antarctic Ice Sheet whereit has a marine terminus (MacAyeal, 1984; Rignot and Jacobs, 2002;Jacobs, 2004). Elsewhere around the continent, freezing conditions atthe sea surface, brine exclusion and mixing on the deep shelvesproduce dense Antarctic BottomWater that cascades down the slopesand supplies the World Ocean's abyss with oxygenated waters.Increased ventilation of the deep sea has been linked to increasedlevels of CO2 primarily via the Southern Ocean (Skinner et al., 2010).Understanding the evolution of Antarctica's unique deep water shelfmay thus offer the possibility to obtain a long-term perspective andinsight as to how shelf depth and morphology might be linked toAntarctic Ice Sheet dynamics, warm water intrusion and globalthermohaline circulation.

Our study primarily is concerned with relationships between shelfdepth and glacial dynamics on the Antarctic Peninsula's Pacific margin(Fig. 1). Our overdeepening hypothesis is as follows. Overdeepeningof the outer shelf permitted frequent and voluminous warm waterintrusion, which caused major retreat of marine-based ice from theouter shelf and significantly reduced the frequency of AntarcticPeninsula Ice Sheet (APIS) advance and retreat on the outercontinental shelf. If the hypothesis is correct, then the frequency ofgrounding events on the outer shelf should be demonstrably lowerafter the margin is overdeepened. The specific objective of ourongoing study of this hypothesis was to evaluate the following threequestions. 1) When, how and why did today's overdeepenedmorphology of the Antarctic Peninsula's Pacific margin evolve? 2) Isoverdeepeningmanifest in continental margin stratal patterns? 3) Didoverdeepening of the peninsula's Pacific margin coincide with achange in APIS dynamics, i.e., reduction in the frequency of majoradvances of grounded ice to the outer shelf?

Fig. 1. Deep banks and troughs were carved by ice flowing from the mainland peninsula to tshelf. Black squares show ODP Leg 178 sites. The rectilinear lines show seismic data used in thtext. Bathymetric data are from Rebesco et al. (1998).

Please cite this article as: Bart, P.J., Iwai, M., The overdeepening hypothePliocene altered glacial dynamics on the Antarctic Peninsula..., Palaeogeogr

2. Methods

We confined our investigation to the Antarctic Peninsula's Pacificmargin because a regional seismic stratigraphic framework and glacialhistory interpretation have already been constructed from a regionalgrid of seismic data (Bart and Anderson, 1995) and because this glacialhistory data can be directly correlated to sedimentologic data (Eyles etal., 2001), chronologic data (Iwai and Winter, 2002; Iwai et al., 2002;Winter and Iwai, 2002) and diatom-based environmental change datafrom IODP Leg 178 drill sites (Fig. 1). In our study, we used twodiatoms, Paralia sulcata and Stephanophyxis spp. to deduce waterdepth changes, shallow and deep respectively. The actual water depthindicated by the presence of P. sulcata cannot be determined but P.sulcata has long been known to be common on high latitudecontinental shelves (e.g., Sancetta, 1982). P. sulcata is also known tobe common in relatively low salinity coastal waters (e.g., McQuoid andNordberg, 2003). In the Japan Sea, P. sulcata is used as a proxy ofcontinental mixed waters from the East China Sea (Tanimura, 1989;Tanimura et al., 2002). Stephanopyxis spp. is interpreted as an indicatorof deeper water conditions based on data presented by Sancetta(1982) in Bering Sea.

Our chronology for water depth changes on the shelf stratigraphyis based on published diatom biozones on the continental shelf atIODP Leg 178 Site 1097 (e.g., Iwai andWinter, 2002), the age ranges ofwhich represent the synthesis of paleomagnetic and all otherchronologic data from IODP Leg 178 sites on the continental rise(Acton et al., 2002). The Thalassiosira inura biozone is of particularinterest to our study. T. inura was assigned an age range of 4.46–4.89 Ma by Gersonde and Burckle (1990) for the Weddell Sea sector.We deem the T. inura biozone ages from Leg 178 on the AntarcticPeninsula (Winter and Iwai, 2002) to be more meaningful than theages reported by Gersonde and Burckle (1990) which were generatedfor the Weddell Sea area. Based on data presented by Winter and Iwai(2002; their Table 2) we assign the T. inura biozone a range of 4.25 to

he shelf edge. The diagonal hatching shows the limit of exposed basement on the inneris study. Bold lines show seismic profiles from Fig. 2 and other profiles referenced in the

sis: How erosional modification of the marine-scape during the early. Palaeoclimatol. Palaeoecol. (2011), doi:10.1016/j.palaeo.2011.06.010

image of Fig.�1


3P.J. Bart, M. Iwai / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

5.55 Ma. In addition, data from Leg 178 permit the T. inura biozone tobe subdivided into subzones a and b. The subzone boundary is basedon the first appearance of Thalasiosira complicatawhich is assigned anage of 5.12 Ma. These ages are consistent with the paleomagnetic ageconstraints on diatom biozones at Leg 178 continental rise sites(Acton et al., 2002). The seismic-based glacial history interpretationfrom Bart and Anderson (1995) was correlated to chronologic andlithologic control on the outer shelf at Site 1097 to determine ifoverdeepening influenced the frequency of APIS advance and retreatin the marine environment. The conversion of two-way travel timesfrom seismic reflections interpreted as glacial unconformities to drillcore depthwas based on a linear interpolation of the available velocitycontrol at Site 1097 (Barker and Camerlenghi, 2002). We alsoevaluated abundance changes of two other diatoms (Thalassiothrixantarctica and Thalassionema nitzschioides s.l.) at IODP Leg 178 Site1095 on the peninsula's continental rise (Fig. 1) to assess thepossibility that translations of the Polar Front may have causedchanges in glacial dynamics that were unrelated to water depthchanges on the outer shelf. Thalassiothrix antarctica and T. nitzschioidess.l. are a common component of diatom laminated ooze. Kemp et al.(2000) suggested that the ooze laminations are caused by rapiddeposition at oceanic frontal systems. Thalassionema nitzschioides iscosmopolitan but is almost absent today in the Southern Ocean (cf.Zielinski and Gersonde, 2002; Crosta et al., 2005).

3. Results

3.1. Diatom-based assessment of water depth changes on the outer shelfat ODP Site 1097

Neither P. sulcata nor Stephanopyxis spp. is present in the basal-mostsection of Site 1097 but P. sulcata– the shallow water indicator – ispresent up section within Actinocyclus ingens v. ovalis (Fig. 2). The firstsignificant occurrence of P. sulcata is in core segment 46. This biozoneindicates that the stratigraphic interval corresponds to the latestMiocene with a maximum geologic time range between 6.27 and7.94 Ma. The last significant occurrence of P. sulcata is in core segment32, but P. sulcata is present in core segment 27 (Fig. 2). Furtherupsection, Stephanopyxis spp., the deep water indicator, becomes thedominant species. The first significant occurrence of Stephanopyxis spp.is in core segment 25 but Stephanopyxis spp. is rare in core segment 32.The up-core transition to dominance by Stephanopyxis spp. is withinT. inura (subzone a), which has an age range of 5.12 Ma to 5.55 Ma (Iwaiand Winter, 2002). Still further upsection, an abrupt doubling ofStephanopyxis spp. is estimated to have occurred at 4.25 Ma based on itscoincidence with the top of the T. inura (subzone b) which has an agerange of 5.12 Ma to 4.25 Ma. Paralia sulcata is present in core segments8 and 9 but the upper-most section at Site 1097 is barren. Thischronology for biostratigraphic changes for Paralia sulcata and Stepha-nopyxis spp. on the shelf at Site 1097 is also observed on the continentalrise at Site 1095 (Fig. 3). Thefirst occurrence of Stephanopyxis spp. at Site1095 is estimated to be at 5.2 Ma based on the higher resolutionchronology for this deep water site.

3.2. Diatom-based assessment of Polar Front migrations from continentalrise Site 1095

Thalassiothrix antarctica and Thalassionema nitzschioides s.l. areabsent from upper Miocene strata in the basal-most sections of Site1095 on the continental rise (Fig. 3). Upcore, abundances of bothdiatoms increase within a thin interval of diatom biozone T. oestrupiihere estimated to correspond to a brief time at 5.7 Ma. A second intervalof increased abundance begins at 5.2 Ma and continues to 4.7 Mawithindiatom biozone T inura (subzones a and b). A third peak in abundancesbegins near the top of T. inura (subzone b) and continues into the


N. interfrigidaria biozone. This third abundance peak is here estimated torange from 4.4 Ma to 3.7 Ma.

3.3. Regional seismic stratigraphic framework

Regional seismic data reveal that eroded basement and forearc basinstrata are exposed along the foredeepened inner shelf of the AntarcticPeninsula (Figs. 1 and 4). A basinward thickening wedge of outer shelfstrata exists seaward of a forearc syncline and discontinuous middleshelf basement high. Within the wedge, several stratal packages havebeen recognized based on the presence or absence of upper slopeprogradation (Larter and Barker, 1989: Bart and Anderson, 1995).Within the packages, seismic reflections that exhibit cross-cuttingstratal patterns are interpreted as glacial unconformities eroded by theadvance of grounded ice. The unconformities boundunits of thicknessesthat vary in strike and dip but are mostly free of internal reflections.Within some units, reflections exhibit slight basinward dip thatterminates by downlap onto an underlying unconformity. Downlapindicates that these reflections are depositional surfaces. On this basis,the units are interpreted to represent till delta deposition at thedownstream end of grounded ice in a manner similar to that describedby Alley et al. (1989).

We utilize the stratigraphy described by Bart and Anderson (1995;1996) because they defined numerous glacial unconformities. Theseunconformities were correlated over a broad area with regionalstrike-line data on the outer shelf. The oldest and youngest strata,Packages 3 and 1 respectively, contain many discrete unconformity-bound glacial units that aggrade the shelf, meaning that till deltasediment primarily is contained in continental shelf topsets. In otherwords, a relatively minor amount of till delta sediment was recycledand re-deposited in an upper slope depocenter (Fig. 4).We follow Bartand Anderson (1995, 1996) who proposed that individual unitsexhibiting less than 2–3 km of upper slope progradation beyond thepre-existing shelf edge be defined as aggrading-shelf units. Incontrast, units in Package 2, the middle package of the outer shelfwedge, exhibiting more than 2–3 km of upper slope progradationwere defined as prograding-shelf units. Package 2 units typicallyexhibit point-sourced progradational form (Bart and Anderson, 1996),i.e., large volumes of sediment are contained in upper slope foresets.The largest magnitudes of upper slope progradation occurred atMarguerite Trough, themajor outlet for grounded ice on themainlandpeninsula (Figs. 1 and 4). These distinct point-sourced upper slopeunits are called trough mouth fans (TMFs) because they are literallysediment fans deposited on the upper slope at the mouth of glacialtroughs. Other investigators propose that prograding wedges on theAntarctic Peninsula are not TMFs because they do not occur at themouths of glacial troughs (Rebesco et al., 1998; Amblas et al., 2006).The significance of TMFs for glacial history interpretation remainsuncertain (Larter and Barker, 1989; Cooper et al., 1991; Kuvaas andLeitchenkov, 1992; Bart and Anderson, 1995; Rebesco et al., 2006).

3.4. Seismic-based glacial history data for the Antarctic Peninsula'sPacific margin

In addition to the evidence of changing architectural styles, regionalgrids of high resolution seismic profiles permit correlation of individualseismic reflections interpreted as glacial unconformities on the outershelf. Each unconformity is interpreted to represent an interval ofsubglacial erosion by ice flow in contact with the seafloor. The ice sheetadvance that eroded the unconformity is referred to as a groundingevent. Here, we review the seismic evidence of ice sheet grounding-event history within the context of aggrading and prograding stratalpackages and sub-package units, and within the context of thechronostratigraphic levels at which water depth changes are inferredto have occurred based on diatoms P. sulcata and Stephanopyxis spp. atSite 1097 (Fig. 2). Individual grounding-event unconformities are



coresegm

ent

10R

5R

1R

15R

25R

30R

35R

40R

45R

50R

20R

depth(m

b sf)

400

436.6

100

200

300

0

unit 3.9

unit 3.10

unit 3.11

unit 3.13

unit 3.14

unit 2.9

unit 2.8

unit 2.2

unit 1.2

unit 1.1

bubblepulse

unit 3.12

glaciomarine

glaciomarine

glaciomarine

glaciomarine

glaciomarine

glaciomarine

glaciomarineglaciomarine

glaciomarine

glaciomarine

subglacial (?)

subglacial (?)

subglacial

subglacial (?)

subglacial

subglacial (?)

subglacial (?)

subglacial

subglacialsubglacialsubglacialsubglacial

glaciomarine

subglacial

subglacial

glaciomarine

reflectors

1.3

1.1

1.2

2.3

2.9

2.10

3.10

3.11

3.12

3.13

3.14

seismic

uni ts

msec

bsf

lithologicinterpretation

of core@Site 1097

Package

1P

a ckag e2

Package

3Pack ages

seismicstratigraphy(this study)

5 rock clasts1 rock clast3 rock clasts

subglacial

subglacial

subglacialsubglacialsubglacialsubglacialsubglacial

subglacial

subglacial

subglacial

11 rock clasts

4 rock clasts

6 rock clasts

7 rock clasts

11 rock clasts

4 rock clasts

1 rock clast

lithostratigraphy(Eyles et al., 2001)

95

411

48

80

128

155

180

210

237

252

291

352

0

biozones

ag erange

(Ma)

T.i nura

4 .25 -5 .12

5.12-

6.27-7.94

samples

dia toma bund.

diatom biostratigraphy( IwaiandWinter , 2002)

P. sulcata

shallow-waterneritic

Stephano-pyxis spp.

relativelydeep-water

outer shelf water depth proxy

5.2*sza

N. reinholdiiT. oestrupii

szb

-5.55

T. lentiginosa

A.ingens

v .o valis

Stage

4S

tage3

Stage

1S

tag e(F ig. 5 )

Stg 2

Abundance key

barren

barren

barren

abundantcommonfewrarepresent

Fig. 2. Litho-, bio- and chronostratigraphic data at Site 1097 correlated to seismic stratigraphic results shown in Fig. 2. P. sulcata is replaced by Stehapnopyxis spp. within Package 3.Doubling of Stephanopyxis spp. coincides with base of Package 1. The width of the horizontal black boxes in the lithologic column corresponds to the recovery for that core segment.The diagonal line pattern corresponds to barren zones.


assigned numbers, e.g., 3.14. The first part of the number corresponds tothe Package in which the unconformity is found. The second part of thenumber corresponds to the relative position of the unconformitywithinthe Package. For example, unconformity 3.14 is the 14th unconformityfrom the top of Package 3 (Fig. 4). The glacial interpretation of thesecross-cutting seismic reflections is confirmed by correlation toglaciogenic sediments at Site 1097 (Eyles et al., 2001; Bart et al.,


2005). A key point is that regional seismic stratigraphy provides themost complete composite view of grounding event unconformitiespreserved on the outer shelf as opposed to that which would otherwisebe determined from a single dip-oriented transect of the outer shelf. Forexample, dip-oriented profile 88–4 (Fig. 4A) shows unconformities 3.10overlain by unconformity 2.10 whereas strike-oriented profiles (e.g.,88-B, Fig. 4B) shows the regional view of cross-cutting relationships


image of Fig.�2


(x106 #/g)

Par

ali a

sulc

ata

Tha

lass

ione

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nitz

s chi

oide

ss.

l.

Tha

lass

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anta

r cti

ca

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han o

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3.7

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5.7

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st.

late

earl

yPlio

cene

late

Mio

cene C3A

C4

C5

C4A

Diatom Zone(modified from

Harwood &Maruyama, 1992)

T. lentiginosa

T. kolbeiT. vulnifica

T. insigna- T. vulnifica

N. interfrigidaria

N. barronii

T. oestrupii

Ac. ingensvar. ovalis

T. torokina

Ac. kennettii

N. reinholdii

a

bT. inura

Ac. ingens

C3B

C1n

r

nr

n

r

n

r

n

r

n

r

n

r

C2

C2A

C3

8

6

2

4

(Cande

andK

ent,1995)

Barren|

Rare

N. kerguelensis

Polar Front south Site 1095shallow-wateroutershelf

5.2

6.7

7.4

20 1 30 1 0 1

3.0

Chron

Tim

e

Epoch

0 1

deep-wateroutershelf

6.27

7.94

5.55

5.12

4.25

Fig. 3. Bio- and chronosratigraphic data at Site 1095 showing P. sulcata is replaced by Stephanopyxis spp. at an estimated time of 5.2 Ma, and peak abundances of Thalassiothrixantarctica and Thalassionema nitzschioides s.l. suggesting that the Polar Front migrated south of Site 1095 on three occasions in the late Miocene and early Pliocene. Data fromHarwood and Maruyama (1992) and Cande and Kent (1995).


revealing the existence of unconformities 3.1 through 3.9 along thestrike of the outer shelf.

Within the basal part of Package 3, five grounding eventsrepresented by unconformities 3.14 to 3.10 occurred before the firstsignificant appearance of Stephanopyxis spp., the deeperwater diatom,at Site 1097 (i.e., while the continental shelf presumably was shallow,see Section 4.1). The base of the upper Miocene biozone Actinocyclusingens var. ovaliswas not reached at Site 1097 (Fig. 2). Thus, the lower-most two grounding event unconformities (i.e., 3.14 and 3.13) are onlyconstrained to have occurred after 7.94 but prior to 6.27 Ma. Thisequates to an average minimum frequency of 1.2 grounding event onthe outer shelf per 1 M.y or a minimum average grounding-eventreoccurrence of 835 k.y. (Table 1). The remaining three groundingevents, represented by unconformities 3.12, 3.11, and 3.10, areconstrained to have occurred between the time represented by thetop of A. ingens var. ovalis and the base of T. inura (subzone a), i.e.,between 6.27 and 5.55 Ma. This equates to an average minimumfrequency of 4.2 grounding events on the outer shelf per 1 M.y or aminimum grounding-event reoccurrence of 240 k.y. (Table 1). Furtherupsection, cross-cutting and stratigraphic superposition from regionalstrike-line seismic data (Fig. 4b) requires that at least ten groundingevents (represented by unconformities 3.9–3.1 and unconformity2.10) occurred during the time represented by T. inura (subzone a)which ranges from 5.12 to 5.55 Ma. This equates to an averageminimum frequency 23.2 grounding events on the outer shelf per1 M.y or a minimum reoccurrence of 43 k.y. (Table 1). These tengrounding events occurred after the first significant appearance ofStephanophyxis spp. (i.e., the deeper water indicator, Sections 4.1 and4.2).


Based on cross-cutting relationships in Package 2 (Figs. 2 and 4),nine discrete ice sheet grounding events (represented by unconfor-mities 2.9 to 2.1) occurred during the time represented by T. inura(subzone a), i.e., between 5.12 and 4.25 Ma. This equates to 10.3grounding events on the outer shelf per 1 M.y or a minimumreoccurrence of 96 k.y. (Table 1).

The abrupt doubling of Stephanopyxis spp. coincides with thecessation of Package 2 TMF construction (Fig. 2). Only three shelf-widegrounding events (represented by unconformities 1.3–1.1) areseismically resolved within Package 1 (Bart and Anderson, 1995)(Fig. 4). Package 1 is constrained to have occurred during the timesince the end of T. inura (subzone b), i.e., since 4.25 Ma. This equates toan average minimum frequency minimum of 0.7 grounding events onthe outer shelf per 1 M.y. for Package 1 or aminimumgrounding-eventreoccurrence of 1,416 k.y. (Table 1).

4. Discussion

4.1. Timing of overdeepening: transition to modern-like conditions at4.25 Ma

The occurrence of P. sulcata within upper Miocene section (Fig. 2)suggests that the margin was shallow during at least part of the latestMiocene from 6.27 to 5.55 Ma (Table 1). The upsection transition fromdominance by P. sulcata to dominance by Stephanopyxis spp. at Site1097 (Fig. 3) suggests that the Antarctic Peninsula outer shelfunderwent a transition from shallower to deeper water depths duringthe T. inura (subzone a) biozone (5.55 to 5.12 Ma), i.e., during the earlyPliocene (Table 1). Based on the continued presence of Stephanopyxis


image of Fig.�3


wwater bottom multiiple

Package 1Package 1

package 3package 3Package 3

sea floor bubble pulse

shelf edge

upperslopeforeset

88-B(Fig.2B)

NW

Marguerite Trough

0 105

kilometersV.E. = 40:1

187.5

337.5

487.5

637.5

787.5

937.5

1237.5

1087.5

250

450

650

850

1050

1250

1450

1650

3.123.11

3.15

3.163.18

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SE

A

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2.10 2.92.32.22.1

1.2

Package 2Package 2Package 2

TMF

1.1

ODPSite 1097

middle shelf basement highmiddle shelf basement highmiddle shelf basement high

eroded forearc basineroded forearc basineroded forearc basin

exposed basement of foredeepened inner shelf

outer shelf wedgeupper slope

middle shelf bank

forearc basin syncline exposed basement on

foredeepened inner shelf

water botttom multiiple

water bottom multiple

3.9

3.3

3.5

3.17

water botttom multiple

water bottom multiple

sea floor

88-4(Fig. 2A)

Marguerite Trough

NESW450

650

850

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14500 105

kilometersV.E. = 40:1

B

bubble pulse1.2

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2.8

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2.62.3

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1.1

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3.12

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mid shelf basement high

two-

way

trav

el ti

me

(mse

c)tw

o-w

ay tr

avel

tim

e (m

sec)

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er d

epth

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met

ers)

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er d

epth

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ers)

337.5

487.5

637.5

787.5

937.5

1087.5

continental shelf topset

outer shelf bank outer shelf bank

paleo-trough paleo-bank

Fig. 4. A) Dip-oriented profile 88–4 showing deeply scoured basement rock on the inner shelf, a middle shelf bank and an outer continental shelf wedge. ODP Site 1097 penetrates the upper part of the wedge. The shaded area highlightsPackage 2 Trough Mouth Fans. B) Strike-oriented seismic profile 88-B crossing Site 1097. Reflections show that cross cutting is pervasive on the outer shelf.

6P.J.Bart,M

.Iwai

/Palaeogeography,Palaeoclim

atology,Palaeoecologyxxx

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Table 1Antarctic Peninsula Ice Sheet grounding events on the pacific margin outer shelf interpreted from the regional seismic stratigraphy with respect to available biozone age control atIODP Site 1097 on the continental shelf.

Seismicpackage

Number of ice-sheet advances(preserved unconformities)

Diatom biozoneat Site 1097

Estimated time range(Ma)

Biozone durations(Ma)

Minimum number ofgrounding events/M.y.

Min. inferredgrounding eventcyclicity

Inferred waterdepth on outershelf

1 3 (1.1–1.3) T. lentiginosa–N.barronii

0–4.25 4.25 0.7 1,416 ka Overdeepenedforedeepened

2 9 (2.1–2.9) T. inura szb 4.25–5.12 0.87 10.3 96 ka Deepeningforedeepened

2, 3 10 (2.10, 3.1–3.9) T. inura sza 5.12–5.55 0.43 23.2 43 ka Deepening3 3 (3.10–3.12) T. oestrupii–N.

reinholdi5.55–6.27 0.72 4.2 240 ka Shallow

3 2 (3.13,3.14) A. ingens var. ovalis 6.27–7.94 1.67 1.2 835 ka Shallow


spp. within T. inura subzone a and subzone b (Fig. 2), we infer that thedeeper water conditions continued until 4.25 Ma, i.e., in the earliestPliocene (Table 1). The abrupt increase of Stephanopyxis spp. above thetop of T. inura subzone b (Fig. 2) suggests that deep water conditionsbecame widespread at 4.25 Ma (Table 1). Since the upper 60 m of Site1097 is barren (Fig. 2), we cannot estimate whether or not significantwater depth changes occurred within the time span represented bythis section.

Unfortunately, only a few polarity zones can be discerned in thepaleomagnetic data from the continental shelf sites (Acton et al.,2002). Thus, these paleomagnetic data yield no independent ageconstraints for the shelf successions.Moreover, the recovery at all shelfsites is extremely low (e.g., Fig. 2). Nonetheless, the biostratigraphicevidence for the early Pliocene T. inura biozone at Site 1097 on the shelf(Winter and Iwai, 2002) is consistentwith the synthesis of all availablebiostratigraphic and paleomagnetic data from Leg 178 sites on thecontinental rise (Acton et al., 2002). The early Pliocene biozoneassignment at Site 1097 is also consistent with regional correlation ofseismic units on the outer shelf to age control at Sites 1100, 1101 and1103 (Bart et al., 2005). Diatoms are relatively abundant within thestratigraphic section of interest to water depth changes at Site 1097(Iwai and Winter, 2002) and the strata includes glacial marinesediments interpreted to be deposited beyond the limits of groundedice (Eyles et al., 2001). We cannot estimate water depth changesbetween core segments 31 thru 28 because this zone is barren.However, the first and last occurrences of P. sulcata and Stephanopyxisspp. at Site 1097 are not related to the biostratigraphic range of eitherspecies because both are long ranging and extant.

Based on the higher resolution chronostratigraphy on thecontinental rise at Site 1095 (Acton et al., 2002; Iwai et al., 2002),we infer that the initial transition to a deeper water outer shelf begancirca 5.2 Ma. The first significant occurrence of Stephanopyxis spp. atSite 1095 is taken to reflect basinward transport of shelf-derivedsediment via mass wasting of upper slope strata that were originallyeroded from the shelf and deposited at the shelf edge by grounded ice.

An early Pliocene evolution of overdeepening is perhaps somewhatexpected considering that eustatic levelswere high but also somewhatunusual in that significant glaciation probably existed on the AntarcticPeninsula since the Eocene (Birkenmajer, 1991) and a permanent icesheet existed on East Antarctica since themiddleMiocene (Shackletonand Kennett, 1975; Kennett, 1977). An early study of water depthconditions on the outer shelf, foram-based evidence fromDSDP Leg 28Site 270 suggested that eastern Ross Sea overdeepened abruptly in theOligocene (Leckie and Webb, 1983) presumably in association withmajor ice volume buildup (Bartek et al., 1991). In more recent studies,DeSantis et al. (1999) used seismic stratigraphic and backstrippingresults to conclude that eastern Ross Sea outer shelf shoaled since theOligocene when deep basins filled with glacial marine sediment in theearly–middle Miocene before returning to an overdeepened-shelfconfiguration in the late Miocene (DeSantis et al., 1995). The Ross Sea


results from DeSantis et al. (1999) are important because it suggeststhat Antarctic shelves may have undergone multiple cycles ofdiachronous overdeepening and shoaling long after the initialglaciation of the continent. Correlation of seismic stratigraphic datato lithologic/chronologic control at IODP Leg 188 drill sites on thePrydz Bay continental shelf suggests that this sector of the EastAntarctic margin also underwent an the early Pliocene transition to adeeper shelf (O'Brien et al., 2007).

4.2. Cause of early Pliocene Overdeepening: frequent erosion by a superinflated Antarctic Peninsula Ice Sheet

No acceleration in thermal subsidence or uplift is expected at Site1097 for the early Pliocene since this sector of the margin has beenpassive since the early Miocene (DeLong et al., 1978; Barker, 1982).Global ice volumewas low formost of the early Pliocene (Shackleton etal., 1995). In the Antarctic, lower Pliocene diatomite units drilled atAND-1B demonstrate that large areas of the West Antarctic interiorwere ice free (Naish et al., 2009). Hambrey et al. (1991) likewiseconcluded that Prydz Bay was ice free during some parts of thePliocene. Although early Pliocene eustatic levels were high relative totoday, theywere not highwith respect to eustatic levels existing in thelate Miocene (Haq et al., 1987). We therefore exclude the possibilitythat eustatic highstand was the major factor contributing to increasedwater depth on the Antarctic Peninsula's Pacific margin. Moreover,despite thewarmer early Pliocene climates, themagnitudes of shifts tolarger δ18O values in the early Pliocene (Kennett and Hoddell, 1993;Shackleton et al., 1995) are sufficiently large to easily accommodatemultiple full-glacial expansions of the relatively small APIS (Bart,2001). Indeed, grounded ice advances to the outer shelf probablyoccurred in all three major sectors of Antarctica during the earlyPliocene (Bart et al., 1999; Bart, 2001). Thus, both significantexpansion and contraction of grounded ice occurred in the earlyPliocene. The early Pliocene transition to deeperwater on theAntarcticPeninsula could not have been associated with first advance ofgrounded ice with ice streams because paleo-troughs are observedwithin the lower part of Package 3 (Bart et al., 2005; 2007; Larter,2007), which is dated to be late Miocene in age (Fig. 4B). Instead, weinfer that deepening of the Antarctic Peninsula begun at 5.2 Ma wascaused by frequent advances of a super-inflated APIS as opposed to athin, low elevation grounded ice sheet that eroded a new deeperequilibrium bathymetric profile. Sedimentation rates on the abyssalplain and continental rise were relatively high during the earlyPliocene (Hollister et al., 1976; Tucholke et al., 1976; Barker andCamerlenghi, 2002). Several large drifts have been described in detailfrom seismic surveys (Rebesco et al., 1996, 1997). Sedimentologiccriteria from Site 1095 on the continental rise have been used by someto support the view that grounded ice existed on the adjacentcontinental shelf during the early Pliocene (Hepp et al., 2006). Wepropose that frequent advances of a super-inflated APIS were




associatedwithwarmmoist airmassesmoving southward as the PolarFront shifted toward themargin (Fig. 3). The southward translations ofwarmth also affected the IndianOcean sector during the early Pliocene(Whitehead and Bohaty, 2003). Numericalmodels have long predictedthat slightly warmer than present climates would increase precipita-tion sufficiently to cause ice sheets to expand (Oerlemans, 1982;Huybrechts, 1994). Recent models also predict that a super-inflatedglacial configuration would be created if precipitation remainedelevated during a glacial cycle (Pollard and DeConto, 2009). On acontinental scale, the relative closeness of the Polar Front to theAntarctic Peninsula's Pacific margin (Orsi et al., 1995) perhapsenhanced the delivery of moisture to this region whereas in othersectors, a similar magnitude of southern shift may have keep the PolarFront relatively distance from the adjacent margin.

4.3. Stratal manifestation of overdeepening: modern overdeepenedconditions indicated by upsection transition from progradation toaggradation

The occurrence of the shallow to deeper water transition withinPackage 3 aggrading-shelf units (Fig. 2) indicates that the onset ofdeeper water conditions at 5.2 Ma is not uniquely manifested as astratal stacking-pattern change. The near constancy of Stephanopyxisspp. abundance within aggrading-shelf units from the upper part ofPackage 3 and within prograding-shelf units of Package 2 (Fig. 2) alsoshows that the onset of TMF deposition was not associated with amajor additional increase or decrease in water depth on the outershelf.1 Larter (2007) proposed that the onset of TMF depositionoccurred at Unconformity 3.13, i.e., within A. ingens v. ovalis biozone(i.e., between 6.12 and 7.94 Ma). Some upper slope progradation isnoted within Package 3 but in our view, the major transition occurredhigher in the section, at the base of Package 2, which is within T. inurasubzone a (Fig. 2). It was originally believed that TMF depositioncorresponded to an initial advance of ice sheets across the continentalshelf to the shelf edge (Larter and Barker, 1989; Cooper et al., 1991;Kuvaas and Leitchenkov, 1992). However, subglacial and glaciomarinesediments found in Packages 3, 2 and 1 (Eyles et al., 2001)demonstrated that the Antarctic Peninsula outer shelf has experi-enced dynamic glacial conditions well before, during and after theconstruction of TMFs (Barker and Camerlenghi, 2002; Bart et al.,2005). Alternately, TMFs may have been constructed during discretelong-duration grounding events (Bart and Anderson, 1995; 1996) oreustatic lowstands. This is not possible given that all of Package 2 wasdeposited within a relatively short time interval. Rebesco et al. (2006,2007) refer to a major unconformity marking a change in themargin'sstratal architecture that they proposed was associated with globalcooling at 2.9 Ma. The unconformity corresponds to boundarybetween two sequences, S1 and S2, that were originally defined byLarter and Barker (1989). Bart and Anderson (1995; their Fig. 13)showed that the S2-S1 boundary is within Package 2. In other words,the stratal change that Rebesco et al. (2006, 2007) attributed toclimatic cooling does not apply to the onset of TMF developmentdiscussed here.

The point-sourced nature of Package 2 units suggests that sedimentwas delivered to the outer shelf/upper slope through cross-shelfglacial troughs (Bart and Anderson, 1996). The lack of significantalong-slope shifts for Package 2 units supports the view that icestreams had incised foredeepened troughs on the inner shelf. Theabrupt increased abundance of Stephanopyxis spp. and its coincidencewith the cessation of Package 2 TMF deposition suggest that deeperwater depths became widespread after 4.25 Ma perhaps via widening

1 Our time-depth conversion places the first occurrence of Stephanophyxis spp.~20 m below Package 2 TMFs. If our time-depth conversion is inaccurate, then theonset of deeper water may actually correspond to the TMF deposition.


of troughs and bevelling of banks. Thus, the transition to a modernoverdeepened shelf is manifest as a change from prograding-shelfunits to aggrading-shelf units. The stratal shift to Package 1 aggradingshelf units is also consistent with the view that excess accommodation(i.e., space available for sediment and/or grounded ice) existed at thistime. We cannot categorize water depth changes for the upper-mostpart of Package 1 because the interval is barren (Fig. 2). At most, minorshoaling of ~100 m may have occurred via deposition of Package 1aggrading-shelf units based on the average composite thickness ofPackage 1 units.

4.4. Effect of early Pliocene overdeepening on glacial dynamics: reducedgrounding-event frequency on the outer shelf after modern-likeoverdeepened shelf developed at 4.25 Ma

If the overdeepening hypothesis as presented in the introduction iscorrect, then the transition froma shallowwater to a deeperwater outershelf should coincidewithadecrease in grounding-event frequency. Thesmall number of upper Miocene unconformities may simply indicatethat advances of APIS on the shallowwater shelfwere infrequent duringthe lateMiocene. Alternately, given the shallowwater conditionsduringthe late Miocene, erosional amalgamationmay have been severe on thelowaccommodationouter shelf. Thehigher frequency groundingeventspreserved within the overlying T. inura subzones a and b (Table 1) areconsistent with the view of orbital forcing at obliquity (i.e., 41 k.y.) andprecessional (i.e., 100 k.y.) frequencies, respectively. If the firstappearance of Stephanopyxis spp. at Site 1097 (Fig. 2) is strictly takento be at 5.2 Ma (Fig. 3), then ten grounding-event unconformities (3.9–3.1 and 2.10)were erodedbetween5.2 and 5.12 Ma, i.e., at anultra-highfrequency. Given that the Polar Front first migrated southward at5.2 Ma, we infer that these frequent grounding events in the earlyPliocene were a consequence of the atmospheric warming (Oerlemans,1982; Huybrechts, 1994; Pollard and DeConto, 2009) as opposed tobeing a consequence of a transition to the deeper water outer shelf.

The small number of unconformities within Package 1 indicatesthat the transition to modern overdeepened and foredeepenedconditions at 4.25 Ma was associated with a major reduction in theouter shelf grounding events. This change in glacial dynamicsoccurred at a time when the Polar Front was still south of Site 1095(Fig. 3). Some amalgamation is likely but we exclude the possibilitythat the stratigraphic evidence of many additional Package 1grounding events, that might be expected from composite δ18Orecords (Zachos et al., 2001; Lisiecki and Raymo, 2005) wereamalgamated since there is an overall decrease in sedimentationrates on the continental rise and abyssal plain for this timeframe(Hollister et al., 1976; Tucholke et al., 1976; Barker and Camerlenghi,2002). Various criteria have been utilized to extract grounding-eventhistory from sedimentologic evidence on continental rise sites aroundAntarctica (Pudsey, 2000; Hillenbrand and Ehrmann, 2002, 2005;Junttila et al., 2005; Iorio et al., 2005; Hepp et al., 2006) but questionsremain as to whether this evidence from the rise can be uniquelyrelated to ice sheet grounding as opposed to oceanographic influences(Shipboard Scientific Party, 2002; Bart et al., 2005).

essation of outer shelf grounding events in the Prydz Bay sectoroccurred prior to 1.0 Ma (Passchier et al., 2003; Cooper and O'Brien,2004; Grutzner et al., 2005; O'Brien et al., 2007). Cessation of outershelf grounding events in Prydz Bay has been partly attributed tooverdeepening and to a precipitation deficit as inflation of the EastAntarctic Ice Sheet caused low pressure cells to permanently migratebasinward, which thereafter decreased snowfall accumulation overthe EAIS periphery (O'Brien et al., 2007). A precipitation deficit shouldnot have been a major contributing factor to reduced groundingevents on the peninsula because the Polar Front remained south ofSite 1095 until 3.7 Ma (Fig. 3). We propose that significant warmwater intrusion occurred beginning at 4.25 Ma as the outer shelf waserosionally lowered past a critical threshold depth where after



Stage 4

modern foredeepenedand overdeepen shelf

CPDWx

xx

foredeepened deep water shelf

Stage 3

TMF

Stephanophyxis spp.

x

xx

deep water shelf

Stephanophyxis spp.

P. sulcata

Stage 2

xxx

shallow water shelf

1097

P. sulcata

Stage 1

xxx

cross-cutting troughs

max. trough depth

basement rock

CPDW

t

DCBA

Fig. 5. Four stage conceptual model showing overdeepening of the Antarctic Peninsula's Pacific margin. A) In the late Miocene, ice advanced and retreated on a shallow shelf.B) Southward migration of the Polar Front at 5.2 Ma caused frequent advances of a super-inflated APIS, which eroded a deeper water marine-scape. C) Progressive incision of icestreams incised foredeepened troughs on the inner shelf. The eroded sediment was deposited in TMFs on the upper slope. D) TMF development ceased at 4.25 Ma becauseprogressive additional erosion of the outer shelf lowered the shelf edge sill sufficiently to permit significant warm water intrusion.


circumpolar deep warm waters upwelling in the region was directedonto the overdeepened shelf. In this scenario, near perennial warmwater intrusion at 4.25 Ma accelerated melting at the APIS marineterminus (Rignot and Jacobs, 2002; Smethie and Jacobs, 2005).Perhaps the early Pliocene history of super-interglacial retreatsdeduced from lower Pliocene diatomite units at AND-1B wasassociated with warm water intrusions (Naish et al., 2009) relatedto a preceding interval of late Miocene overdeepening (DeSantis et al.,1999). On the Antarctic Peninsula, we infer that the resultant negativemass balance at the APIS'smarine terminus probably caused the ice onthe mainland to deflate. Some recent studies suggest that somesectors of the Antarcticmay have experienced a long-term lowering ofthe ice sheet since the early Pliocene as opposed to presumed post-LGM time frame (e.g., Lilly et al., 2010). If our overdeepeninghypothesis is correct, some nunataks on the Antarctic Peninsulashould have been exposed since ~4.0 Ma.

5. Conclusion

We propose a four stage conceptual model to explain ouroverdeepening hypothesis for the evolution of the Antarctic Peninsu-la's overdeepened Pacific margin (Fig. 5). During Stage 1, ice sheetwaxed and waned across a shallow outer continental shelf. In Stage 2,southward migration of the Polar Front delivered abundant warmmoist air masses to the region, which led to the establishment of asuper-inflated APIS on the Pacific margin. Frequent advances of thehigher elevation APIS to the outer shelf eroded a deeper marine-scapebeginning at 5.2 Ma. In Stage 3, ice streams incised foredeepenedtroughs into basement on the inner shelf. Thematerial excavated fromthe shelf was deposited in TMFs on the upper slope. By Stage 4, amodern overdeepened and foredeepened shelf profile was estab-lished. We infer that as the shelf depth increased past a criticalthreshold, warm water intrusion on the shelf from upwellingcircumpolar deep water became frequent and voluminous. Perennialwarm water intrusion caused a major reduction in outer shelfgrounding events and an overall deflation of the APIS on themainland.Aggradational stacking of till sheets deposited on the outer shelfduring Package 1 grounding events could have produced only minorshoaling (~100 m) of the overdeepened outer shelf.

Acknowledgment

This research was funded by the US National Science FoundationOffice of Polar Programs. Seismic grid PD88 was acquired by JohnAnderson. Seismic grid NBP02 was acquired by Phil Bart.


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http://dx.doi.org/10.1130/G23422C.1

http://dx.doi.org/10.1029/2004PA001071

http://dx.doi.org/10.1029/2002PA000829


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Palaeogeography, Palaeoclimatology, Palaeoecology BART IWAI_2012.pdfPhilip J. Bart a,⁎, Masao Iwai b a Department of Geology and Geophysics, Louisiana State University, Howe Russell