Reconstruction of Koettlitz Glacier, Southern McMurdo Sound, Antarctica, During the Last Glacial Maximum and Termination

Accurate reconstructions of the Antarctic Ice Sheet (AIS) are important for evaluating past, present, and future sea-level change. Insight into future changes of the AIS and its tolerances to various climate variables can come from investigation of its past behavior. During the last glacial maximum (LGM), ice grounded in the Ross Sea, reaching close to the continental shelf edge. One hypothesis is that this event was caused largely by changing sea level that led to widespread grounding of floating portions of the ice sheet. This grounding buttressed the inflowing East Antarctic outlet glaciers and caused thickening on the lower reaches of these glaciers; interior ice remained the same or even thinned because of reduced accumulation. The Holocene was characterized by rapid recession of marine portions and possible thickening of interior ice and growth of local glaciers in response to accumulation increase. In contrast, an alternate hypothesis is that expansion of grounded Ross Sea ice was due to growth of local glaciers and East Antarctic outlets. These glaciers are thought to have receded to their present positions in the Holocene despite relatively high accumulation. These hypotheses have very different implications for the future of the ice sheet under global warming. Koettlitz Glacier, a large local glacier, flows from the Royal Society Range into McMurdo Sound (78°S, 163°E) and is ideal for testing these two hypotheses. Competing hypotheses as to how this glacier behaved during the LGM range from minor recession to significant expansion. Today, Koettlitz Glacier blocks the mouth of ice-free Pyramid Trough. However, based on surficial mapping, I infer that grounded Ross Sea ice blocked the valley mouth at the LGM. Radiocarbon dates of subfossil lacustrine algae from a lake dammed in Pyramid Trough by the Ross Sea ice date to 11-23 ka, suggesting the ice dam existed throughout that time period. The stratigraphic position and geometry of moraines indicates that Koettlitz Glacier was smaller at the LGM than it is at present. A single radiocarbon age suggests Koettlitz Glacier has advanced within the last 3 ka. Altogether, existing data suggest that Koettlitz Glacier, and by inference other local glaciers in the region, retreated during the LGM and advanced in the Holocene, probably because of fluctuations in accumulation. My work favors the first hypothesis of growth of local glaciers and at least terrestrial portions of the ice sheet during times of high accumulation, which correspond to warm periods in the Antarctic. In contrast, marine-based areas of the ice sheet, such as in the Ross Sea, appear to have advanced during the LGM and retreated in the Holocene, likely in response to changing sea level. This bimodal response of the ice sheet to climate change has implications for future ice-sheet behavior and implies that the future of the ice sheet will depend on the interaction between accumulation-caused thickening and retreat due to marine instabilities

Advances in Sample Preparation at the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS): Investigation of Carbonate Secondary Standards

The development of robust sample preparation techniques for ocean science research has been a hallmark of NOSAMS since its inception. Improvements to our standard methods include reducing the minimum size of the samples we can analyze, building modular graphite reactors of different sizes that we can swap in and out depending on our sample stream, and modifying our carbonate acidification methods to improve handling of the smaller samples we now receive. A relatively new instrument, the Ramped PyrOx, which allows the separation of organic matter into thermal fractions, has attracted much interest as a research and development tool. We will also discuss our progress on incorporating a Picarro isotope analyzer into our sample preparation options

Analysis of Sediment Traps in Linnévatnet, Svalbard for Reconstruction of Annual Sediment Flux and Lacustrine Processes

The warming trend of the 20th century has caused significant environmental changes to cryosphere in the arctic regions. Because the arctic plays an important role in the global climate system, it is necessary to understand how glaciers have responded to climate variation in the past in order to predict how they will react to warming in the future. Linnévatnet is a high arctic glacier-fed lake in Svalbard that serves as an excellent environmental observatory for glacial-fluvial and lake processes. Recent research on laminated lake sediments in Linnévatnet has yielded annual records of past climate that extends back at least 1,000 years. The laminae are comprised of annual couplets consisting of distinct coarse summer and fine winter layers. Analysis of sediment traps in the lake provides a calibration for interpreting the long term sediment record linking modern watershed and climatic processes to annual sediment yield. Since 2003, arrays of sediment traps, temperature loggers and other environmental instrumentation have been deployed in a network of six moorings in Linnévatnet at depths ranging from 10 to 35 meters. The moorings are recovered annually in late summer and receiving tubes on the traps are collected and instrumentation downloaded before redeployment. In the laboratory, the receiving tubes are split, and visual stratigraphy are logged. Magnetic susceptibility was measured at 0.5 cm intervals in the split cores and the sediment was subsampled in continuous 0.25 cm slices for grain size analysis. The other halves of the cores were subsampled for mineralogical analysis using the X-Ray Diffraction method. In the sediment traps, the spring/summer melt season and significant rain events are represented in the sediment by distinct coarse layers. In 2013-14, the first coarse (13 microns) sediment pulse seen in the traps, comprising 57% of the annual vertical accumulation, was deposited on August 15-16, 2013, coinciding with a heavy rain storm. The timing of this event was constrained by an intervalometer sediment trap and time lapse photography. Subsequent events appear as both finer (10 microns) and coarser (15 microns) laminae in the traps and are associated with rain storms later in the fall. The fine winter layer (3-5 microns) overlies the fall events and reflects quiet winter sedimentation. The beginning of the nival melt season (May 30-July 8 2014) is observed as a finer-gained (8 microns) layer, above which the coarsest (19 microns) grain sediment pulse was deposited July 9-21 2014, coinciding with peak snowmelt discharge. Sediment stratigraphy and grain size trends in 2013-2014 were compiled with sediment trap analyses back to 2004 to form a composite record to compare with lake-bottom deposition as reflected in sediment cores collected in the area