The Overdeepening Hypothesis: How Erosional Modification of the Marine

PALAEO-05845; No of Pages 10 Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx

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Palaeogeography, Palaeoclimatology, Palaeoecology

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The overdeepening hypothesis: How erosional modiﬁcation of the marine-scape during the early Pliocene altered glacial dynamics on the Antarctic Peninsula'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, USA b Department of Natural Environmental Science Kochi University, Akebono-Cho 2-5-1, Kochi 7808520, Japan article info abstract

Article history: A new synthesis of diatom assemblage data from Ocean Drilling Program (ODP) Leg 178 suggests that the Received 7 February 2011 Pacific margin of the Antarctic Peninsula underwent a transition from a shallow shelf to an overdeepened Received in revised form 4 June 2011 shelf in the early Pliocene. This modification of the marine-scape was due to a relatively brief interval of Accepted 10 June 2011 erosion begun at 5.2 Ma. The erosion was caused by high frequency advances of a super-inflated Antarctic Available online xxxx Peninsula Ice Sheet (APIS). The frequent advances of the higher elevation ice sheet were a consequence of abundant moisture delivered to the region as the Polar Front migrated southward. By 5.12 Ma, ice streams Keywords: Antarctic Peninsula incised foredeepened glacial troughs into basement on the inner shelf. Sediment eroded from the inner shelf Early Pliocene was transported through cross-shelf troughs and deposited in large trough-mouth-fan depocenters on the Overdeepening upper slope. Overdeepened shelf conditions became widespread as troughs widened and intra-trough banks Warm water intrusion beveled. By 4.25 Ma, trough-mouth-fan construction ceased and subsequent advances of the APIS have been Grounding event infrequent. We propose that the reduced frequency of grounding events signaled the transition to a modern ODP Leg 178 foredeepened and overdeepened shelf. We hypothesize that a new glacial dynamic emerged in the early Pliocene 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 in contact with the ocean. Secondly, erosional deepening of the outer shelf was equivalent to lowering a shelf edge sill that permitted frequent and voluminous intrusion of warm circumpolar deep waters that upwell in the region. The resultant accelerated melting at the APIS marine terminus, caused the super-inflated APIS to deflate on the mainland, which further decreased the possibility that grounded ice could advance on the overdeepened shelf. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Shelf depth and morphology affects ice sheet mass balance in the marine environment in three important ways because it controls Water depth at the Antarctic shelf edge averages 500 m, which is 1) the area of the ice sheet in contact with the ocean 2) the flux of ice four times deeper than that on low latitude continental margins. The that can be exported to the marine environment and 3) the flux of great depths are a consequence of glacial erosion and isostatic warm water that can intrude onto the shelf. The latter item is adjustment. Today, the deep water shelves are marked by broad extremely important because recent models indicate that ocean banks and troughs that extend from the inner shelf to the shelf edge. temperature probably is the dominant factor influencing advance Morphologic evidence in sea floor troughs, including trough-parallel, and retreat of grounded ice in the marine environment (Pollard and megascale glacial lineations, demonstrates that these broad features DeConto, 2009). This model prediction suggests that an ice sheet's were occupied by ice streams — zones of fast flowing ice (Anderson, marine margin will not advance when ocean temperatures on the shelf 1999; Canals et al., 2000; Dowdeswell et al., 2004; Heroy and are elevated even if all other controlling factors (e.g., sea level, Anderson, 2005). Antarctic shelves also have an unusual foredeepened atmospheric temperature, and precipitation rate) are set to a or reversed-grade profile, meaning that maximum water depths are prescribed full-glacial state. This is significant because, if so, then our found on the inner shelf. ability to use composite oxygen isotope data to help interpret glacial history from glacial marine successions is highly dependent upon whether or not the volume and frequency of warm water intrusions changed through geologic time as the shelf morphology shifted from ⁎ Corresponding author. Tel.: +1 225 578 3109; fax: +1 225 578 2302. shallow to overdeepened (e.g., Naish et al., 2009). The evolution of E-mail addresses: [email protected] (P.J. Bart), [email protected] (M. Iwai). shelf depth and morphology is poorly constrained (Anderson and

Bartek, 1992; Anderson, 1999). However, today's outer shelves around 2. Methods Antarctica are deep enough that relatively-warm circumpolar deep water currents upwelling in the Southern Ocean impinge on the We confined our investigation to the Antarctic Peninsula's Pacific margin. In places, these warm waters are directed landward along margin because a regional seismic stratigraphic framework and glacial foredeepened troughs to bathe and melt the Antarctic Ice Sheet where history interpretation have already been constructed from a regional it has a marine terminus (MacAyeal, 1984; Rignot and Jacobs, 2002; grid of seismic data (Bart and Anderson, 1995) and because this glacial Jacobs, 2004). Elsewhere around the continent, freezing conditions at history data can be directly correlated to sedimentologic data (Eyles et the sea surface, brine exclusion and mixing on the deep shelves al., 2001), chronologic data (Iwai and Winter, 2002; Iwai et al., 2002; produce dense Antarctic Bottom Water that cascades down the slopes Winter and Iwai, 2002) and diatom-based environmental change data and supplies the World Ocean's abyss with oxygenated waters. from IODP Leg 178 drill sites (Fig. 1). In our study, we used two Increased ventilation of the deep sea has been linked to increased diatoms, Paralia sulcata and Stephanophyxis spp. to deduce water levels of CO2 primarily via the Southern Ocean (Skinner et al., 2010). depth changes, shallow and deep respectively. The actual water depth Understanding the evolution of Antarctica's unique deep water shelf indicated by the presence of P. sulcata cannot be determined but P. may thus offer the possibility to obtain a long-term perspective and sulcata has long been known to be common on high latitude insight as to how shelf depth and morphology might be linked to continental shelves (e.g., Sancetta, 1982). P. sulcata is also known to Antarctic Ice Sheet dynamics, warm water intrusion and global be common in relatively low salinity coastal waters (e.g., McQuoid and thermohaline circulation. Nordberg, 2003). In the Japan Sea, P. sulcata is used as a proxy of Our study primarily is concerned with relationships between shelf continental mixed waters from the East China Sea (Tanimura, 1989; depth and glacial dynamics on the Antarctic Peninsula's Pacific margin Tanimura et al., 2002). Stephanopyxis spp. is interpreted as an indicator (Fig. 1). Our overdeepening hypothesis is as follows. Overdeepening of deeper water conditions based on data presented by Sancetta of the outer shelf permitted frequent and voluminous warm water (1982) in Bering Sea. intrusion, which caused major retreat of marine-based ice from the Our chronology for water depth changes on the shelf stratigraphy outer shelf and significantly reduced the frequency of Antarctic is based on published diatom biozones on the continental shelf at Peninsula Ice Sheet (APIS) advance and retreat on the outer IODP Leg 178 Site 1097 (e.g., Iwai and Winter, 2002), the age ranges of continental shelf. If the hypothesis is correct, then the frequency of which represent the synthesis of paleomagnetic and all other grounding events on the outer shelf should be demonstrably lower chronologic data from IODP Leg 178 sites on the continental rise after the margin is overdeepened. The specific objective of our (Acton et al., 2002). The Thalassiosira inura biozone is of particular ongoing study of this hypothesis was to evaluate the following three interest to our study. T. inura was assigned an age range of 4.46– questions. 1) When, how and why did today's overdeepened 4.89 Ma by Gersonde and Burckle (1990) for the Weddell Sea sector. morphology of the Antarctic Peninsula's Pacific margin evolve? 2) Is We deem the T. inura biozone ages from Leg 178 on the Antarctic overdeepening manifest in continental margin stratal patterns? 3) Did Peninsula (Winter and Iwai, 2002) to be more meaningful than the overdeepening of the peninsula's Pacific margin coincide with a ages reported by Gersonde and Burckle (1990) which were generated change in APIS dynamics, i.e., reduction in the frequency of major for the Weddell Sea area. Based on data presented by Winter and Iwai advances of grounded ice to the outer shelf? (2002; their Table 2) we assign the T. inura biozone a range of 4.25 to

Fig. 1. Deep banks and troughs were carved by ice flowing from the mainland peninsula to the shelf edge. The diagonal hatching shows the limit of exposed basement on the inner shelf. Black squares show ODP Leg 178 sites. The rectilinear lines show seismic data used in this study. Bold lines show seismic profiles from Fig. 2 and other profiles referenced in the text. Bathymetric data are from Rebesco et al. (1998).

5.55 Ma. In addition, data from Leg 178 permit the T. inura biozone to N. interfrigidaria biozone. This third abundance peak is here estimated to be subdivided into subzones a and b. The subzone boundary is based range from 4.4 Ma to 3.7 Ma. on the first appearance of Thalasiosira complicata which is assigned an age of 5.12 Ma. These ages are consistent with the paleomagnetic age 3.3. Regional seismic stratigraphic framework constraints on diatom biozones at Leg 178 continental rise sites (Acton et al., 2002). The seismic-based glacial history interpretation Regional seismic data reveal that eroded basement and forearc basin from Bart and Anderson (1995) was correlated to chronologic and strata are exposed along the foredeepened inner shelf of the Antarctic lithologic control on the outer shelf at Site 1097 to determine if Peninsula (Figs. 1 and 4). A basinward thickening wedge of outer shelf overdeepening influenced the frequency of APIS advance and retreat strata exists seaward of a forearc syncline and discontinuous middle in the marine environment. The conversion of two-way travel times shelf basement high. Within the wedge, several stratal packages have from seismic reflections interpreted as glacial unconformities to drill been recognized based on the presence or absence of upper slope core depth was based on a linear interpolation of the available velocity progradation (Larter and Barker, 1989: Bart and Anderson, 1995). control at Site 1097 (Barker and Camerlenghi, 2002). We also Within the packages, seismic reflections that exhibit cross-cutting evaluated abundance changes of two other diatoms (Thalassiothrix stratal patterns are interpreted as glacial unconformities eroded by the antarctica and Thalassionema nitzschioides s.l.) at IODP Leg 178 Site advance of grounded ice. The unconformities bound units of thicknesses 1095 on the peninsula's continental rise (Fig. 1) to assess the that vary in strike and dip but are mostly free of internal reflections. possibility that translations of the Polar Front may have caused Within some units, reflections exhibit slight basinward dip that changes in glacial dynamics that were unrelated to water depth terminates by downlap onto an underlying unconformity. Downlap changes on the outer shelf. Thalassiothrix antarctica and T. nitzschioides indicates that these reflections are depositional surfaces. On this basis, s.l. are a common component of diatom laminated ooze. Kemp et al. the units are interpreted to represent till delta deposition at the (2000) suggested that the ooze laminations are caused by rapid downstream end of grounded ice in a manner similar to that described deposition at oceanic frontal systems. Thalassionema nitzschioides is by Alley et al. (1989). cosmopolitan but is almost absent today in the Southern Ocean (cf. We utilize the stratigraphy described by Bart and Anderson (1995; Zielinski and Gersonde, 2002; Crosta et al., 2005). 1996) because they defined numerous glacial unconformities. These unconformities were correlated over a broad area with regional strike-line data on the outer shelf. The oldest and youngest strata, 3. Results Packages 3 and 1 respectively, contain many discrete unconformity- bound glacial units that aggrade the shelf, meaning that till delta 3.1. Diatom-based assessment of water depth changes on the outer shelf sediment primarily is contained in continental shelf topsets. In other at ODP Site 1097 words, a relatively minor amount of till delta sediment was recycled and re-deposited in an upper slope depocenter (Fig. 4). We follow Bart Neither P. sulcata nor Stephanopyxis spp. is present in the basal-most and Anderson (1995, 1996) who proposed that individual units section of Site 1097 but P. sulcata– the shallow water indicator – is exhibiting less than 2–3 km of upper slope progradation beyond the present up section within Actinocyclus ingens v. ovalis (Fig. 2). The first pre-existing shelf edge be defined as aggrading-shelf units. In significant occurrence of P. sulcata is in core segment 46. This biozone contrast, units in Package 2, the middle package of the outer shelf indicates that the stratigraphic interval corresponds to the latest wedge, exhibiting more than 2–3 km of upper slope progradation Miocene with a maximum geologic time range between 6.27 and were defined as prograding-shelf units. Package 2 units typically 7.94 Ma. The last significant occurrence of P. sulcata is in core segment exhibit point-sourced progradational form (Bart and Anderson, 1996), 32, but P. sulcata is present in core segment 27 (Fig. 2). Further i.e., large volumes of sediment are contained in upper slope foresets. upsection, Stephanopyxis spp., the deep water indicator, becomes the The largest magnitudes of upper slope progradation occurred at dominant species. The first significant occurrence of Stephanopyxis spp. Marguerite Trough, the major outlet for grounded ice on the mainland is in core segment 25 but Stephanopyxis spp. is rare in core segment 32. peninsula (Figs. 1 and 4). These distinct point-sourced upper slope The up-core transition to dominance by Stephanopyxis spp. is within units are called trough mouth fans (TMFs) because they are literally T. inura (subzone a), which has an age range of 5.12 Ma to 5.55 Ma (Iwai sediment fans deposited on the upper slope at the mouth of glacial and Winter, 2002). Still further upsection, an abrupt doubling of troughs. Other investigators propose that prograding wedges on the Stephanopyxis spp. is estimated to have occurred at 4.25 Ma based on its Antarctic Peninsula are not TMFs because they do not occur at the coincidence with the top of the T. inura (subzone b) which has an age mouths of glacial troughs (Rebesco et al., 1998; Amblas et al., 2006). range of 5.12 Ma to 4.25 Ma. Paralia sulcata is present in core segments The significance of TMFs for glacial history interpretation remains 8 and 9 but the upper-most section at Site 1097 is barren. This uncertain (Larter and Barker, 1989; Cooper et al., 1991; Kuvaas and chronology for biostratigraphic changes for Paralia sulcata and Stepha- Leitchenkov, 1992; Bart and Anderson, 1995; Rebesco et al., 2006). nopyxis spp. on the shelf at Site 1097 is also observed on the continental rise at Site 1095 (Fig. 3). The first occurrence of Stephanopyxis spp. at Site 3.4. Seismic-based glacial history data for the Antarctic Peninsula's 1095 is estimated to be at 5.2 Ma based on the higher resolution Pacific margin chronology for this deep water site. In addition to the evidence of changing architectural styles, regional 3.2. Diatom-based assessment of Polar Front migrations from continental grids of high resolution seismic profiles permit correlation of individual rise Site 1095 seismic reflections interpreted as glacial unconformities on the outer shelf. Each unconformity is interpreted to represent an interval of Thalassiothrix antarctica and Thalassionema nitzschioides s.l. are subglacial erosion by ice flow in contact with the seafloor. The ice sheet absent from upper Miocene strata in the basal-most sections of Site advance that eroded the unconformity is referred to as a grounding 1095 on the continental rise (Fig. 3). Upcore, abundances of both event. Here, we review the seismic evidence of ice sheet grounding- diatoms increase within a thin interval of diatom biozone T. oestrupii event history within the context of aggrading and prograding stratal here estimated to correspond to a brief time at 5.7 Ma. A second interval packages and sub-package units, and within the context of the of increased abundance begins at 5.2 Ma and continues to 4.7 Ma within chronostratigraphic levels at which water depth changes are inferred diatom biozone Tinura(subzones a and b). A third peak in abundances to have occurred based on diatoms P. sulcata and Stephanopyxis spp. at begins near the top of T. inura (subzone b) and continues into the Site 1097 (Fig. 2). Individual grounding-event unconformities are

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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 to 2005). A key point is that regional seismic stratigraphy provides the the Package in which the unconformity is found. The second part of the most complete composite view of grounding event unconformities number corresponds to the relative position of the unconformity within preserved on the outer shelf as opposed to that which would otherwise the Package. For example, unconformity 3.14 is the 14th unconformity be determined from a single dip-oriented transect of the outer shelf. For from the top of Package 3 (Fig. 4). The glacial interpretation of these example, dip-oriented profile 88–4(Fig. 4A) shows unconformities 3.10 cross-cutting seismic reflectionsisconfirmed by correlation to overlain by unconformity 2.10 whereas strike-oriented profiles (e.g., glaciogenic sediments at Site 1097 (Eyles et al., 2001; Bart et al., 88-B, Fig. 4B) shows the regional view of cross-cutting relationships

eand Maruyama, 1992) (x106 #/g) 0101012 3 01 Ma n T. lentiginosa C1 Ac. ingens r Barren Pleist. N. kerguelensis | 2 n T. kolbei C2 r T. vulnifica Rare e T. insigna lat 3.0 n - T. vulnifica C2A N. interfrigidaria 3.7 r 4 N. barronii 4.25 Pliocene 4.4 n T. inura b 4.7

early C3 5.12 a 5.2 5.2 r T. oestrupii 5.55 5.7 6 n N. reinholdii C3A 6.27 r 6.7 C3B Ac. ingens ctica var. ovalis r n 7.4 8 7.94 spp. s.l.

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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 Thalassiothrix antarctica 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 from Harwood and Maruyama (1992) and Cande and Kent (1995). revealing the existence of unconformities 3.1 through 3.9 along the Based on cross-cutting relationships in Package 2 (Figs. 2 and 4), strike of the outer shelf. nine discrete ice sheet grounding events (represented by unconfor- Within the basal part of Package 3, five grounding events mities 2.9 to 2.1) occurred during the time represented by T. inura represented by unconformities 3.14 to 3.10 occurred before the first (subzone a), i.e., between 5.12 and 4.25 Ma. This equates to 10.3 significant appearance of Stephanopyxis spp., the deeper water diatom, grounding events on the outer shelf per 1 M.y or a minimum at Site 1097 (i.e., while the continental shelf presumably was shallow, reoccurrence of 96 k.y. (Table 1). see Section 4.1). The base of the upper Miocene biozone Actinocyclus The abrupt doubling of Stephanopyxis spp. coincides with the ingens var. ovalis was not reached at Site 1097 (Fig. 2). Thus, the lower- cessation of Package 2 TMF construction (Fig. 2). Only three shelf-wide most two grounding event unconformities (i.e., 3.14 and 3.13) are only grounding events (represented by unconformities 1.3–1.1) are constrained to have occurred after 7.94 but prior to 6.27 Ma. This seismically resolved within Package 1 (Bart and Anderson, 1995) equates to an average minimum frequency of 1.2 grounding event on (Fig. 4). Package 1 is constrained to have occurred during the time the outer shelf per 1 M.y or a minimum average grounding-event since the end of T. inura (subzone b), i.e., since 4.25 Ma. This equates to reoccurrence of 835 k.y. (Table 1). The remaining three grounding an average minimum frequency minimum of 0.7 grounding events on events, represented by unconformities 3.12, 3.11, and 3.10, are the outer shelf per 1 M.y. for Package 1 or a minimum grounding-event constrained to have occurred between the time represented by the reoccurrence of 1,416 k.y. (Table 1). top 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 minimum 4. Discussion frequency of 4.2 grounding events on the outer shelf per 1 M.y or a minimum grounding-event reoccurrence of 240 k.y. (Table 1). Further 4.1. Timing of overdeepening: transition to modern-like conditions at upsection, cross-cutting and stratigraphic superposition from regional 4.25 Ma strike-line seismic data (Fig. 4b) requires that at least ten grounding events (represented by unconformities 3.9–3.1 and unconformity The occurrence of P. sulcata within upper Miocene section (Fig. 2) 2.10) occurred during the time represented by T. inura (subzone a) suggests that the margin was shallow during at least part of the latest which ranges from 5.12 to 5.55 Ma. This equates to an average Miocene from 6.27 to 5.55 Ma (Table 1). The upsection transition from minimum frequency 23.2 grounding events on the outer shelf per dominance by P. sulcata to dominance by Stephanopyxis spp. at Site 1 M.y or a minimum reoccurrence of 43 k.y. (Table 1). These ten 1097 (Fig. 3) suggests that the Antarctic Peninsula outer shelf grounding events occurred after the first significant appearance of underwent a transition from shallower to deeper water depths during Stephanophyxis spp. (i.e., the deeper water indicator, Sections 4.1 and the T. inura (subzone a) biozone (5.55 to 5.12 Ma), i.e., during the early 4.2). Pliocene (Table 1). Based on the continued presence of Stephanopyxis

6 P.J. Bart, M. Iwai / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx water depth (in meters) (in depth water 187.5 337.5 487.5 637.5 787.5 937.5 1087.5 1237.5 NW upper slope foreset slope upper 2 TMF TMF TMF

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w ODP w (Fig.2B) Site 1097 2.3 3.13 3.18 2.9 2.1 outer shelf bank 3.10 3.14 3.15 3.16 2.3 bubble pulse 2.10 3.10 3.12 3.11 sea floor Marguerite Trough Marguerite continental shelf topset outer shelf wedge 3.14 3.15 3.18 88-4 ODP (Fig. 2A) paleo-bank

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le 88-B crossing Site 1097. Re d e ﬁ d o r e eroded forearc basin eroded forearc basin 2.3 forearc basin syncline 3.12 2.2 3.11 2.5 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 p – 2.6 outer shelf bank 10 le 88 1.1 ﬁ 10 5 1.2 5 2.9 2.7 0 kilometers kilometers V.E. = 40:1 V.E. V.E. = 40:1 V.E. exposed basement of foredeepened inner shelf 0 exposed basement on 2.8 SE foredeepened inner shelf SW A) Dip-oriented pro

250 450 650 850 450 650 850

1050 1250 1050 1250 1450 1450 1650 two-way travel time (msec) time travel two-way two-way travel time (msec) time travel two-way A B Package 2 Trough Mouth Fans. B) Strike-oriented seismic pro Fig. 4.

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

Seismic Number of ice-sheet advances Diatom biozone Estimated time range Biozone durations Minimum number of Min. inferred Inferred water package (preserved unconformities) at Site 1097 (Ma) (Ma) grounding events/M.y. grounding event depth on outer cyclicity shelf

1 3 (1.1–1.3) T. lentiginosa–N. 0–4.25 4.25 0.7 1,416 ka Overdeepened barronii foredeepened 2 9 (2.1–2.9) T. inura szb 4.25–5.12 0.87 10.3 96 ka Deepening foredeepened 2, 3 10 (2.10, 3.1–3.9) T. inura sza 5.12–5.55 0.43 23.2 43 ka Deepening 3 3 (3.10–3.12) T. oestrupii–N. 5.55–6.27 0.72 4.2 240 ka Shallow reinholdi 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 the results from DeSantis et al. (1999) are important because it suggests deeper water conditions continued until 4.25 Ma, i.e., in the earliest that Antarctic shelves may have undergone multiple cycles of Pliocene (Table 1). The abrupt increase of Stephanopyxis spp. above the diachronous overdeepening and shoaling long after the initial top of T. inura subzone b (Fig. 2) suggests that deep water conditions glaciation of the continent. Correlation of seismic stratigraphic data became widespread at 4.25 Ma (Table 1). Since the upper 60 m of Site to lithologic/chronologic control at IODP Leg 188 drill sites on the 1097 is barren (Fig. 2), we cannot estimate whether or not significant Prydz Bay continental shelf suggests that this sector of the East water depth changes occurred within the time span represented by Antarctic margin also underwent an the early Pliocene transition to a this section. deeper shelf (O'Brien et al., 2007). Unfortunately, only a few polarity zones can be discerned in the paleomagnetic data from the continental shelf sites (Acton et al., 4.2. Cause of early Pliocene Overdeepening: frequent erosion by a super 2002). Thus, these paleomagnetic data yield no independent age inflated Antarctic Peninsula Ice Sheet constraints for the shelf successions. Moreover, the recovery at all shelf sites is extremely low (e.g., Fig. 2). Nonetheless, the biostratigraphic No acceleration in thermal subsidence or uplift is expected at Site evidence for the early Pliocene T. inura biozone at Site 1097 on the shelf 1097 for the early Pliocene since this sector of the margin has been (Winter and Iwai, 2002) is consistent with the synthesis of all available passive since the early Miocene (DeLong et al., 1978; Barker, 1982). biostratigraphic and paleomagnetic data from Leg 178 sites on the Global ice volume was low for most of the early Pliocene (Shackleton et continental rise (Acton et al., 2002). The early Pliocene biozone al., 1995). In the Antarctic, lower Pliocene diatomite units drilled at assignment at Site 1097 is also consistent with regional correlation of AND-1B demonstrate that large areas of the West Antarctic interior seismic units on the outer shelf to age control at Sites 1100, 1101 and were ice free (Naish et al., 2009). Hambrey et al. (1991) likewise 1103 (Bart et al., 2005). Diatoms are relatively abundant within the concluded that Prydz Bay was ice free during some parts of the stratigraphic section of interest to water depth changes at Site 1097 Pliocene. Although early Pliocene eustatic levels were high relative to (Iwai and Winter, 2002) and the strata includes glacial marine today, they were not high with respect to eustatic levels existing in the sediments interpreted to be deposited beyond the limits of grounded late Miocene (Haq et al., 1987). We therefore exclude the possibility ice (Eyles et al., 2001). We cannot estimate water depth changes that eustatic highstand was the major factor contributing to increased between core segments 31 thru 28 because this zone is barren. water depth on the Antarctic Peninsula's Pacific margin. Moreover, However, the first and last occurrences of P. sulcata and Stephanopyxis despite the warmer early Pliocene climates, the magnitudes of shifts to spp. at Site 1097 are not related to the biostratigraphic range of either larger δ18O values in the early Pliocene (Kennett and Hoddell, 1993; species because both are long ranging and extant. Shackleton et al., 1995) are sufficiently large to easily accommodate Based on the higher resolution chronostratigraphy on the multiple full-glacial expansions of the relatively small APIS (Bart, continental rise at Site 1095 (Acton et al., 2002; Iwai et al., 2002), 2001). Indeed, grounded ice advances to the outer shelf probably we infer that the initial transition to a deeper water outer shelf began occurred in all three major sectors of Antarctica during the early circa 5.2 Ma. The first significant occurrence of Stephanopyxis spp. at Pliocene (Bart et al., 1999; Bart, 2001). Thus, both significant Site 1095 is taken to reflect basinward transport of shelf-derived expansion and contraction of grounded ice occurred in the early sediment via mass wasting of upper slope strata that were originally Pliocene. The early Pliocene transition to deeper water on the Antarctic eroded from the shelf and deposited at the shelf edge by grounded ice. Peninsula could not have been associated with first advance of An early Pliocene evolution of overdeepening is perhaps somewhat grounded ice with ice streams because paleo-troughs are observed expected considering that eustatic levels were high but also somewhat within the lower part of Package 3 (Bart et al., 2005; 2007; Larter, unusual in that significant glaciation probably existed on the Antarctic 2007), which is dated to be late Miocene in age (Fig. 4B). Instead, we Peninsula since the Eocene (Birkenmajer, 1991) and a permanent ice infer that deepening of the Antarctic Peninsula begun at 5.2 Ma was sheet existed on East Antarctica since the middle Miocene (Shackleton caused by frequent advances of a super-inflated APIS as opposed to a and Kennett, 1975; Kennett, 1977). An early study of water depth thin, low elevation grounded ice sheet that eroded a new deeper conditions on the outer shelf, foram-based evidence from DSDP Leg 28 equilibrium bathymetric profile. Sedimentation rates on the abyssal Site 270 suggested that eastern Ross Sea overdeepened abruptly in the plain and continental rise were relatively high during the early Oligocene (Leckie and Webb, 1983) presumably in association with Pliocene (Hollister et al., 1976; Tucholke et al., 1976; Barker and major ice volume buildup (Bartek et al., 1991). In more recent studies, Camerlenghi, 2002). Several large drifts have been described in detail DeSantis et al. (1999) used seismic stratigraphic and backstripping from seismic surveys (Rebesco et al., 1996, 1997). Sedimentologic results to conclude that eastern Ross Sea outer shelf shoaled since the criteria from Site 1095 on the continental rise have been used by some Oligocene when deep basins filled with glacial marine sediment in the to support the view that grounded ice existed on the adjacent early–middle Miocene before returning to an overdeepened-shelf continental shelf during the early Pliocene (Hepp et al., 2006). We configuration in the late Miocene (DeSantis et al., 1995). The Ross Sea propose that frequent advances of a super-inflated APIS were

Please cite this article as: Bart, P.J., Iwai, M., The overdeepening hypothesis: How erosional modification of the marine-scape during the early Pliocene altered glacial dynamics on the Antarctic Peninsula..., Palaeogeogr. Palaeoclimatol. Palaeoecol. (2011), doi:10.1016/j.palaeo.2011.06.010 8 P.J. Bart, M. Iwai / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx associated with warm moist air masses moving southward as the Polar of troughs and bevelling of banks. Thus, the transition to a modern Front shifted toward the margin (Fig. 3). The southward translations of overdeepened shelf is manifest as a change from prograding-shelf warmth also affected the Indian Ocean sector during the early Pliocene units to aggrading-shelf units. The stratal shift to Package 1 aggrading (Whitehead and Bohaty, 2003). Numerical models have long predicted shelf units is also consistent with the view that excess accommodation that slightly warmer than present climates would increase precipita- (i.e., space available for sediment and/or grounded ice) existed at this tion sufficiently to cause ice sheets to expand (Oerlemans, 1982; time. We cannot categorize water depth changes for the upper-most Huybrechts, 1994). Recent models also predict that a super-inflated part of Package 1 because the interval is barren (Fig. 2). At most, minor glacial configuration would be created if precipitation remained shoaling of ~100 m may have occurred via deposition of Package 1 elevated during a glacial cycle (Pollard and DeConto, 2009). On a aggrading-shelf units based on the average composite thickness of continental scale, the relative closeness of the Polar Front to the Package 1 units. Antarctic Peninsula's Pacific margin (Orsi et al., 1995)perhaps enhanced the delivery of moisture to this region whereas in other 4.4. Effect of early Pliocene overdeepening on glacial dynamics: reduced sectors, a similar magnitude of southern shift may have keep the Polar grounding-event frequency on the outer shelf after modern-like Front relatively distance from the adjacent margin. overdeepened shelf developed at 4.25 Ma

If the overdeepening hypothesis as presented in the introduction is 4.3. Stratal manifestation of overdeepening: modern overdeepened correct, then the transition from a shallow water to a deeper water outer conditions indicated by upsection transition from progradation to shelf should coincide with a decrease in grounding-event frequency. The aggradation small number of upper Miocene unconformities may simply indicate that advances of APIS on the shallow water shelf were infrequent during The occurrence of the shallow to deeper water transition within the late Miocene. Alternately, given the shallow water conditions during Package 3 aggrading-shelf units (Fig. 2) indicates that the onset of the late Miocene, erosional amalgamation may have been severe on the deeper water conditions at 5.2 Ma is not uniquely manifested as a low accommodation outer shelf. The higher frequency grounding events stratal stacking-pattern change. The near constancy of Stephanopyxis preserved within the overlying T. inura subzones a and b (Table 1)are spp. abundance within aggrading-shelf units from the upper part of consistent with the view of orbital forcing at obliquity (i.e., 41 k.y.) and Package 3 and within prograding-shelf units of Package 2 (Fig. 2) also precessional (i.e., 100 k.y.) frequencies, respectively. If the first shows that the onset of TMF deposition was not associated with a appearance of Stephanopyxis spp. at Site 1097 (Fig. 2)isstrictlytaken major additional increase or decrease in water depth on the outer to be at 5.2 Ma (Fig. 3), then ten grounding-event unconformities (3.9– 1 shelf. Larter (2007) proposed that the onset of TMF deposition 3.1 and 2.10) were eroded between 5.2 and 5.12 Ma, i.e., at an ultra-high occurred at Unconformity 3.13, i.e., within A. ingens v. ovalis biozone frequency. Given that the Polar Front first migrated southward at (i.e., between 6.12 and 7.94 Ma). Some upper slope progradation is 5.2 Ma, we infer that these frequent grounding events in the early noted within Package 3 but in our view, the major transition occurred Pliocene were a consequence of the atmospheric warming (Oerlemans, higher in the section, at the base of Package 2, which is within T. inura 1982; Huybrechts, 1994; Pollard and DeConto, 2009)asopposedto subzone a (Fig. 2). It was originally believed that TMF deposition being a consequence of a transition to the deeper water outer shelf. corresponded to an initial advance of ice sheets across the continental The small number of unconformities within Package 1 indicates shelf to the shelf edge (Larter and Barker, 1989; Cooper et al., 1991; that the transition to modern overdeepened and foredeepened Kuvaas and Leitchenkov, 1992). However, subglacial and glaciomarine conditions at 4.25 Ma was associated with a major reduction in the sediments found in Packages 3, 2 and 1 (Eyles et al., 2001) outer shelf grounding events. This change in glacial dynamics demonstrated that the Antarctic Peninsula outer shelf has experi- occurred at a time when the Polar Front was still south of Site 1095 enced dynamic glacial conditions well before, during and after the (Fig. 3). Some amalgamation is likely but we exclude the possibility construction of TMFs (Barker and Camerlenghi, 2002; Bart et al., that the stratigraphic evidence of many additional Package 1 2005). Alternately, TMFs may have been constructed during discrete grounding events, that might be expected from composite δ18O long-duration grounding events (Bart and Anderson, 1995; 1996)or records (Zachos et al., 2001; Lisiecki and Raymo, 2005) were eustatic lowstands. This is not possible given that all of Package 2 was amalgamated since there is an overall decrease in sedimentation deposited within a relatively short time interval. Rebesco et al. (2006, rates on the continental rise and abyssal plain for this timeframe 2007) refer to a major unconformity marking a change in the margin's (Hollister et al., 1976; Tucholke et al., 1976; Barker and Camerlenghi, stratal architecture that they proposed was associated with global 2002). Various criteria have been utilized to extract grounding-event cooling at 2.9 Ma. The unconformity corresponds to boundary history from sedimentologic evidence on continental rise sites around between two sequences, S1 and S2, that were originally defined by Antarctica (Pudsey, 2000; Hillenbrand and Ehrmann, 2002, 2005; Larter and Barker (1989). Bart and Anderson (1995; their Fig. 13) Junttila et al., 2005; Iorio et al., 2005; Hepp et al., 2006) but questions showed that the S2-S1 boundary is within Package 2. In other words, remain as to whether this evidence from the rise can be uniquely the stratal change that Rebesco et al. (2006, 2007) attributed to related to ice sheet grounding as opposed to oceanographic influences climatic cooling does not apply to the onset of TMF development (Shipboard Scientific Party, 2002; Bart et al., 2005). discussed here. essation of outer shelf grounding events in the Prydz Bay sector The point-sourced nature of Package 2 units suggests that sediment occurred prior to 1.0 Ma (Passchier et al., 2003; Cooper and O'Brien, was delivered to the outer shelf/upper slope through cross-shelf 2004; Grutzner et al., 2005; O'Brien et al., 2007). Cessation of outer glacial troughs (Bart and Anderson, 1996). The lack of significant shelf grounding events in Prydz Bay has been partly attributed to along-slope shifts for Package 2 units supports the view that ice overdeepening and to a precipitation deficit as inflation of the East streams had incised foredeepened troughs on the inner shelf. The Antarctic Ice Sheet caused low pressure cells to permanently migrate abrupt increased abundance of Stephanopyxis spp. and its coincidence basinward, which thereafter decreased snowfall accumulation over with the cessation of Package 2 TMF deposition suggest that deeper the EAIS periphery (O'Brien et al., 2007). A precipitation deficit should water depths became widespread after 4.25 Ma perhaps via widening not have been a major contributing factor to reduced grounding events on the peninsula because the Polar Front remained south of Site 1095 until 3.7 Ma (Fig. 3). We propose that significant warm 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 the water intrusion occurred beginning at 4.25 Ma as the outer shelf was onset of deeper water may actually correspond to the TMF deposition. erosionally lowered past a critical threshold depth where after

A B C D shallow water shelf deep water shelf P. sulcata P. sulcata foredeepened deep water shelf modern foredeepened Stephanophyxis spp. Stephanophyxis spp. and overdeepen shelf x x basement rock x x x 1097 x t x max. trough depth x C x TMF P C D P W cross-cutting troughs D x W x Stage 1 x Stage 2 Stage 3 Stage 4

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 ice streams 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 because progressive additional erosion of the outer shelf lowered the shelf edge sill sufficiently to permit significant warm water intrusion.

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