Rebekka A Larson

Sea level rise in Tampa Bay

Understanding relative sea level (RSL) rise during periods of rapid climatic change is critical for evaluating modern sea level rise given the vulnerability of Antarctic ice shelves to collapse [Hodgson et al, 2006], the retreat of the worlds glaciers [Oerlemans, 2005], and mass balance trends of the Greenland ice sheet [Rignot and Kanagaratnam, 2006]. The first-order pattern of global sea level rise following the Last Glacial Maximum (LGM, ∼21,000 years ago) is well established from coral [Fairbanks, 1989], continental shelf [Hanebuth et al, 2000], and other records [Pirazzoli, 2000] and has been integrated into a global ICE-5G model of glacio-isostatic adjustment (GIA) [Peltier, 2004]. However, uncertainty introduced by paleo water depth of sea level indicators, radiocarbon chronology (i.e., reservoir corrections for marine shell dates), postglacial isostatic adjustment, and other processes affecting vertical position of former shorelines produces scatter in RSL curves, limiting our knowledge of sea level rise during periods of rapid glacial decay. One example of this limitation is the Gulf of Mexico/Florida region where, despite decades of study, RSL curves produce two conflicting patterns: those showing progressive submergence with a decelerating rate during the past 5000 years [Scholl et al, 1969] and those showing high sea level during the middle of the Holocene [Blum et al, 2001; Balsillie and Donoghue, 2004], where the Holocene represents a geologic epoch that extends from about 10,000 years ago to present times. This discrepancy is emblematic of the uncertainty surrounding Holocene sea level and ice volume history in general.

Sediment Core Extrusion Method at Millimeter Resolution Using a Calibrated, Threaded-rod

Aquatic sediment core subsampling is commonly performed at cm or half-cm resolution. Depending on the sedimentation rate and depositional environment, this resolution provides records at the annual to decadal scale, at best. An extrusion method, using a calibrated, threaded-rod is presented here, which allows for millimeter-scale subsampling of aquatic sediment cores of varying diameters. Millimeter scale subsampling allows for sub-annual to monthly analysis of the sedimentary record, an order of magnitude higher than typical sampling schemes. The extruder consists of a 2 m aluminum frame and base, two core tube clamps, a threaded-rod, and a 1 m piston. The sediment core is placed above the piston and clamped to the frame. An acrylic sampling collar is affixed to the upper 5 cm of the core tube and provides a platform from which to extract sub-samples. The piston is rotated around the threaded-rod at calibrated intervals and gently pushes the sediment out the top of the core tube. The sediment is then isolated into the sampling collar and placed into an appropriate sampling vessel (e.g., jar or bag). This method also preserves the unconsolidated samples (i.e., high pore water content) at the surface, providing a consistent sampling volume. This mm scale extrusion method was applied to cores collected in the northern Gulf of Mexico following the Deepwater Horizon submarine oil release. Evidence suggests that it is necessary to sample at the mm scale to fully characterize events that occur on the monthly time-scale for continental slope sediments.

Linking Land & Sea: Watershed Evaluation and Mineralogical Distribution of Sediments in Eastern St. John, USVI

Abstract Tropical islands such as St. John in the U.S. Virgin Islands are naturally susceptible to terrigenous (land-based) sediment erosion due to their high-relief slopes, fast weathering rates, and intense precipitation events. Nearshore ecosystems that exist near these islands tend to thrive in static conditions, and are especially stressed by increases in terrigenous input. In the last few decades, island development and population have increased dramatically in some areas of St. John. We conducted a detailed characterization of watersheds and their sediments from ‘source to sink’ in eastern St. John. To accomplish this we combined field observations and sampling with a digital elevation model. Our research was focused on several morphologically similar embayments in eastern St. John; three impacted by anthropogenic development (Coral Harbor, Johnson Bay, and Sanders Bay) and an adjacent, virtually undeveloped bay within the Virgin Islands National Park and Virgin Islands Coral Reef National Monument (Otter Creek). We found a large disparity in upslope watershed size between Otter Creek and Coral Harbor: Otter Creek (0.09 km2) is ∼73× smaller than Coral Harbor (6.54 km2). As expected, watersheds transport terrigenous volcaniclastic sediments directly to the marine environment where shallow-water marine carbonates precipitate. Terrigenous volcaniclastic sediments persist furthest from the source in the basin of the largest watershed with the most development (Coral Harbor), and decay closest to the source in the basin of the smallest watershed with the least development (Otter Creek). Due to large disparities in watershed size, further research is required in order to determine the relative contribution of development on the distribution of terrigenous sediments.