My research focuses on understanding the Cenozoic history of cryosphere from the perspective of the outer continental shelf seismic stratigraphy.  The major emphasis of this work has been to constrain the time, frequency, duration and locations of past ice sheet dynamics within the context of possible forcing mechanisms.  Finding good age control has been a major challenge for the Antarctic margin stratigraphy.  My group has used diatom biostratigraphy, radiocarbon dating, and sediment flux calculations to improve the overall understanding of Antarctic Ice Sheet dynamics. 

Work within the last year has produced what I feel are my four best publications.  Bart and Iwai (2012) describe the Overdeepening Hypothesis, which proposes that the Pacific-margin of the Antarctic Peninsula reached its current depth and foredeepened configuration during the early Pliocene.  We provide detailed seismic correlations to IODP Site 1097 for age control.  Within this age framework, we present data and detailed correlations to diatom assemblage data, generated by Masao Iwai, to infer that the margin experienced a major change from a shallow to a deep-water shelf configuration.  Our detailed understanding of major ice sheet advance and retreat based on regional seismic correlations and lithologic control at IODP sites on the shelf and continental rise permit us to conclude that since the margin overdeepened, there have been very few ice sheet advances to the outer shelf.  We propose that overdeepening of the margin was equivalent to lowering a sill across which the relatively warm water upwelling on the Pacific margin could be directed landward.  The warm water intrusion has since predisposed the ice sheet to maintaining an interglacial configuration.  In other words, there have been far fewer advances of the Antarctic Peninsula Ice Sheet since the early Pliocene than would be otherwise inferred from proxy evidence.  My group is in the process of evaluating whether this scenario applies to the entire continent. 

Bart and Cone (2012) presented the largest multibeam image of a Grounding Zone Wedge (GZW) on the Antarctic shelf. GZWs are seismically-resolvable subaqueous terminal moraines of the West Antarctic Ice Sheet when it was larger than present and grounded on what is now an open-water continental shelf.  Several of these GZWs are stacked in an overall backstepping fashion in troughs of the eastern Ross Sea continental shelf.  This situation provides an interesting opportunity to constrain the recent retreat history of the ice sheet for this sector of Antarctica.  The existence of these wedges has been known for some time.  Bart and Cone (2012) used the high-quality image of the GZW to obtain piston and kasten cores in places they deemed most likely to contain in situ carbonate.  Using a new strategy, they isolated small forams from the foreset surface of the GZW.  They obtained radiocarbon dates on the forams they isolated to constrain the date of the grounding event.  In other words, they dated the last time the ice sheet was grounded on the middle shelf.  The dates suggest the ice grounded on the outer shelf at 27.5 14C ky BP, earlier than generally assumed.  Many will find these dates suspect but the data is highly congruent with data provided by other strategies. 

As a companion to these studies, Bart and Owolana (2012) presented a detailed evaluation of sediment flux to a GZW. The regional seismic framework suggests the GZW we mapped is the third pause that the West Antarctic Ice Sheet experienced during its overall retreat from the outer shelf position it occupied during the last glacial maximum.  Dating these events is extremely relevant but complicated because carbonate material is rare, and it is difficult to distinguish in situ from reworked carbonate or acid insoluble organics.  Our recent work included the acquisition of the multibeam, seismic and core data to map the limits of the 3rd GZW at its middle shelf position.  We calculated a mapped volume and devised a new scheme to infer the sediment flux that must have existed when the ice was grounded on the middle shelf.  With the inferred sediment flux and GZW sediment volume, we estimated the duration of time that would be required to deposit the GZW.  The reviewers were highly impressed with our approach and we feel confident that our strategy can be used to determine grounding-event durations where ever GZWs are found.  This breakthrough is important because the recent history of grounding-line stability, i.e., distinct pauses in the retreat, and the subsequent instability, i.e., accelerate retreat to an interior position, will help us better appreciate how long the current stability will last, before (presumably) the next major retreat.  

Bart et al. (2012) utilized diatom biostratigraphy to constrain the ages of till sheet deposition in western Ross Sea.  This sector of the margin received drainage from the East Antarctic Ice Sheet, the largest land-based ice sheet on Antarctica.  The regional seismic stratigraphic framework allowed us to use piston cores to sample the upper part of a unit that is deeply buried in most parts of the study area.  The coring results show that an open-marine sediment unit overlies a seismically-resolved unconformity surface.  The diatom assemblage data indicates that the unconformity was eroded prior to 2.0 My BP.  Our detailed correlation to drill data at the MIS AND-1B site permits us to make a detailed correlation between the outer shelf stratigraphy and an inner shelf site 600 kilometers to the south.  These correlations are important to establish because no single site can be used to infer the maximum and minimum translation distances for ice-sheet fluctuations on the shelf.   

I was the lead PI for an IODP proposal to eastern Ross Sea continental shelf and upper slope.  At the moment, the lead collaborator Laura De Santis is leading the proposal through the revision process.  We are excited about the possibility to obtain long drill sections and thus ground truth many of the concepts and hypothesis we and others have made concerning the history of West Antarctic Ice Sheet fluctuations for the Miocene to present time frame. 

To broaden our horizon, I have begun a new initiative with Dr. Sam Bentley.  We have identified an excavation site within the Baton Rouge city limits.  The excavation is unearthed several in situ cypress tree stumps.  The land surface on which the trees grew is about 4 meters below land level.  We have already obtained radiocarbon dates for the stumps and are excited about the insight the dates provide.  These data are interesting because they provide a good estimate of upper Mississippi River delta flood plain aggradation.  These data are being prepared for publication.