No description available.

No description available.

No description available.

No description available.

No description available.

No description available.

Abstract Multiple proxy analysis of lake sediment records are crucial for understanding changes in environmental and climate conditions over historical and geological time. Most recently, the use of biomarker proxies coupled with sedimentological investigations provides a new approach for gaining insight into the lake processes that capture information about past climate change. This approach is applied here to better understand the paleoclimate record from Lake El’gygytgyn in Western Beringia. Multiple organic geochemical compound concentrations were measure as proxies for both aquatic and terrestrial biological productivity. Measurements of n-alkane (plant leaf waxes) as well as concentrations of the compounds arborinol (marker for trees), dinosterol (dinoflagellates), and long chain (C28 – C32) 1,15 n-alkyl diols (eustigmatophyte algae) demonstrate warming conditions around Lake El’gygytgyn during MIS 9 and MIS 11, especially when compared to diatom production and palynological investigations from Melles et al. (2012). These time periods illustrate the presence of extensive forest cover as well as elevated concentrations of all aquatic biomarkers analyzed, corroborating their “super interglacial” designation. Analysis of branched glycerol dialkyl glycerol tetraethers, a relatively new proxy used to estimate mean annual temperatures and soil pH, was applied also suggesting warming conditions during MIS 9 and MIS 11, although further calibration techniques are needed to accurately estimate temperature changes. Sedimentological results include the analysis of bulk mineralogy, clay mineralogy, iron oxide, and color measurements for the same MIS 8 through MIS 12 interval. The hue color parameter, measured from high resolution core scans, suggests a link to global climate records, with green sediments reflective of cold intervals and red sediments indicative of warmer climate conditions. Validation of the color record was done in part by analyzing the clay mineralogy and the abundances of clay minerals. These data show that clay deposition dominates interglacial periods. Moreover the clay polytypes can be linked to bedrock weathering. Bulk mineralogy measurements allow for the reconstruction of synthetic color spectra which link mineralogy to sediment color. Overprinted on the mineralogical color signal is red color staining from iron oxide minerals, formed within the catchment during wet intervals when increasing amounts of eroded Fe – bearing silicate minerals are available for oxidation. If true, interpretation of the hue record then suggests hue is a proxy for wet/dry conditions within the lake, and when paired with the biomarker analysis shows significant warmer and wetter conditions during MIS 9 and 11. However, the hue record also demonstrates notable variability outside of these two interglacial periods, not recognized by other proxies, are not currently well understood. Overall, the multi-proxy results from this work can be further applied to the longer temporal scale of the Lake El’gygytgyn sediment core, and potentially elucidate climate changes deeper into the Pleistocene, and even into the Pliocene portions of the sediment record.

As Earth’s climate changes, polar environments experience a disproportionate share of extreme shifts. Because the Ross Sea shelf has the highest annual productivity of any Antarctic continental shelf, this region is of particular interest when striving to characterize current and future changes in Antarctic systems. However, understanding of mesoscale variability of biogeochemical patterns in the Ross Sea and how this variability affects assemblage dynamics is incomplete. Furthermore, it is unknown how the Ross Sea may respond to projected warming, reduced summer sea ice concentrations, and shallower mixed layers during the next century. to investigate these dynamics and explore their consequences over the next century, high-resolution glider observations were analyzed and used in conjunction with a one-dimensional, data-assimilative biogeochemical-modeling framework. An analysis of glider observations from two latitudinal sections in the Ross Sea characterized mesoscale variability associated with the phytoplankton bloom and highlighted potential mechanisms driving change in the assemblage. In particular, an observed increase in the ratio of carbon to chlorophyll (C:Chl) suggested a marked transition from a phytoplankton assemblage dominated by Phaeocystis antarctica- to one dominated by diatoms. The expected control of phytoplankton variability by Modified Circumpolar Deep Water and mixed layer depth were shown to be insignificant relative to the effects of wind and sea surface temperature on the temporal/spatial scales measured by the glider. Additional glider measurements were used to force the Model of Ecosystem Dynamics, nutrient Utilisation, Sequestration and Acidification, which was adapted for use in the Ross Sea (MEDUSA-RS) to include both solitary and colonial forms of Phaeocystis antarctica. The impacts of climate-induced changes on Ross Sea phytoplankton were investigated with MEDUSA-RS using projections of physical drivers for mid- and late-21st century, and these experiments indicated increases of primary productivity and carbon export flux. Additional scenario experiments demonstrated that earlier availability of low light due to reduction of sea ice early in the growing season was the primary driver of simulated productivity increases over the next century; shallower mixed layer depths additionally contributed to changes of phytoplankton composition and export. Glider data were assimilated into MEDUSA-RS using the Marine Model Optimization Testbed (MarMOT) to optimize eight phytoplankton model parameters. Assimilation experiments that used different data subsets suggest that assimilating observations at the surface alone, as are typically available from remote-sensing platforms, may underestimate carbon export to depth and overestimate primary production. Experiments assimilating observations characteristic of a cruise-based sampling frequency produced a wide range of solutions, depending on which days were sampled, suggesting the potential for large errors in productivity and export. Finally, assimilating data from different spatial areas resulted in less variation of optimal solutions than assimilating data from different time periods in the bloom progression; these temporal differences are primarily driven by decreasing colonial P. antarctica growth rates, increasing colonial P. antarctica C:Chl, and faster sinking of colonies as the bloom progresses from the accumulation stage through dissipation. Overall, this dissertation research demonstrates the value of using bio-optical glider observations in conjunction with modeling to characterize phytoplankton dynamics in a remote marine ecosystem. High-resolution glider data are better able to resolve mesoscale physical-biological relationships, which are typically not discernible from lower frequency data, but it can be difficult to identify mechanistic relationships from in situ measurements alone. In addition, biogeochemical models can be used to extend insights gained by empirical observation, but application is often limited by the quantity and type of in situ data appropriate for evaluation and forcing. The use of gliders for facilitating development and operation of a lower trophic level model demonstrated the effectiveness of a synthetic approach that partly overcomes the individual limitations of these otherwise distinct approaches. Finally, the combination of these approaches is especially useful for gaining a better understanding of ecosystem dynamics in regions similar to the Ross Sea that are undergoing substantive climate-induced changes and where harsh conditions make other means of access difficult.

A study of the organic matter (OM) sources and biogeochemical and physicochemical sinks was undertaken in the York River estuary, Virginia. The reactivity of dissolved organic carbon (DOC) was enhanced from ∼25--68% by the combined effects of exposure to natural sunlight and bacterial decomposition. In contrast, sunlight exposure decreased the bioreactivity of DOC in the higher salinity lower York by a factor of five. The combined effects of photochemical and bacterial processing were found to modify both the bioavailability and metabolic fate of OM (e.g. respiration vs. biomass). Stable isotopic (delta13C, delta15N) and radiocarbon (Delta14C) values of bacterial nucleic acids were used to estimate the sources and ages of OM assimilated by bacteria in the York and Hudson River estuaries. Bacterial production in freshwater regions of the York was fueled by OM of young, terrigenous origin which accounted for 42--89% of OM assimilated. The remainder (11--58%) of OM assimilated was derived from freshwater algae. In the mid-salinity York, bacterial production was supported by phytoplankton-derived OM in the spring and summer (93--100%) and marsh-derived OM in the fall (73--100%). Isotopic values of bacteria in the lower York suggested production was supported by phytoplankton-derived OM (86--100%) in July and November and algal and marine-like OM (50--69%) in October. In contrast to the young (10--20 yr) OM assimilated by bacteria in the York, production in the Hudson River was subsidized by old (∼1200 BP) terrigenous OM. Higher C:N ratios, lower delta13C and delta 15N values and depletions of total lipid and lipid compound classes in high molecular weight dissolved organic matter (HMW DOM (≥3kDa)) relative to particulate organic matter (POM), suggested differences in the reactivity and cycling of these two OM fractions. Within the dissolved pool, polyunsaturated fatty acids (FA) were a strong predictor of DOC decomposition in bioassays. FA and sterol distributions suggest that POM is derived from phytoplankton/zooplankton sources, while HMW DOM has a bacterial and vascular plant signature. Thus, the physical form of OM (particulate vs dissolved) may affect both the distribution and biogeochemical processing of OM such that terrigenous DOM may be exported, while POM is retained within the estuary.

The sources and fates of carbon in a tidal freshwater marsh (Sweet Hall marsh; Pamunkey River, Virginia) were determined to understand the role that these marshes play with respect to estuarine carbon cycling. A carbon gas flux model, based on measured carbon dioxide and methane fluxes, was developed to calculate annual rates of macrophyte and microalgal photosynthesis and community and belowground respiration. Because carbon fluxes out of marsh sediments may underestimate true belowground respiration if sediment-produced gases are transported through plant tissues, gross nitrogen mineralization was used as a proxy for belowground carbon respiration. Annual community respiration exceeded gross photosynthesis, suggesting an allochthonous input of organic carbon to the marsh. Sediment deposition during tidal flooding was measured as a potential exogenous carbon source. Short term deposition rates (biweekly to monthly) were spatially and temporally variable, with highest rates measured near a tidal creek during summer. Annual deposition on the marsh was sufficient to balance relative sea level rise and measured respiration rates. Sediment inventories of 7Be indicated that spatial patterns of sedimentation were not due to sediment redistribution within the marsh. Accretion rates calculated from 137Cs (decadal scale) and 14C (centuries to millennia) were substantially less than annual deposition rates. The concentration and isotopic composition of dissolved and particulate inorganic and organic carbon (DIC, DOC, POC) were measured in a marsh creek which drained the study site. Seasonal isotopic variations in DIC were explained by marsh porewater drainage and decomposition of marsh-derived carbon. A model linking DIC concentrations and water transport showed that DIC export from tidal marshes could explain a significant portion of excess DIC production in the adjacent estuary. Isotopic mixing models indicated seasonal variability in the importance of phytoplankton as a source of DOC and POC although there was no evidence for a net flux of these materials between the marsh and estuary. Annually, the marsh carbon budget was closely balanced, with sources exceeding sinks by approximately 5 percent. This similarity suggests that those processes which were not quantified (e.g. consumption by marsh and riverine fauna) were quantitatively unimportant with respect to the entire marsh carbon budget.