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The Overdeepening Hypothesis: How Erosional Modification of the Marine PALAEO-05845; No of Pages 10 Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx Contents lists available at ScienceDirect Palaeogeography, Palaeoclimatology, Palaeoecology journal homepage: www.elsevier.com/locate/palaeo 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 0031-0182/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2011.06.010 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 2 P.J. Bart, M. Iwai / Palaeogeography, Palaeoclimatology, Palaeoecology xxx (2011) xxx–xxx 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.
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