The paleolacustrine significance of sedimentary nitrogen isotopes: A comparative study of late Holocene records in two lakes on the eastern Qinghai-Tibet Plateau.

The bulk nitrogen isotope composition of lacustrine sediments can be a useful proxy of past climatic and environmental changes. We previously reconstructed paleoenvironmental histories recorded in sediments of Lake Ximencuo and Lake Ngoring, both located on the eastern Qinghai-Tibetan Plateau (QTP), using a suite of biogeochemical paleoclimate proxies that included the sedimentary δ15Ntot values. We have revisited the different patterns of δ15Ntot variations in the two lakes and now conclude that their isotopic trends over the last millennium have been controlled by multiple factors. Regional temperature evidently is one potential factor affecting the sedimentary δ15Ntot values, indicating that the lake ecosystems respond to regional temperature changes on a sub-millennial timescale but in indirect and lake-specific ways. The processes involved in the sedimentary δ15Ntot changes appear to be more strongly influenced by the shapes of lake basins and associated hydrologic characteristics that control the origins of nitrogen-containing compounds in the lakes. To help understand the dynamics of nitrogen cycling and nitrogen isotope records in the QTP lakes, we identified two patterns - a terrestrial nitrogen-controlled pattern (TNCP) that is observed in deeper, steep-walled glacial-basin lakes and an aquatic nitrogen-controlled pattern (ANCP) that is observed in shallower, tectonic-basin lakes. We also considered the influences of the "amount effect" and the "temperature effect" on sedimentary δ15Ntot values and their possible operative mechanisms in these montane lakes. We postulate that both patterns are applicable to the QTP lakes, including both glacial and tectonic lakes, and probably to lakes in other regions that have also not experienced significant human disturbance.

Significance of different n-alkane biomarker distributions in four same-age peat sequences around the edges of a small maar lake in China.

A decade-resolution study of peat cores from four different locations around Yuanchi Lake, a small shallow maar lake in the Changbai Mountains of northeastern China, has established that peat deposition around this lake amplified at ca. 1800 CE with accumulation rates that differ among the four closely spaced sites. Comparisons of the n-alkane distributions of typical plants and the distributions in the peat cores indicate that the differences in the n-alkane contents at the four sites around the lake are consequences of differences in the peat-forming plant communities that have developed on the lake edges. These floral differences likely resulted from different littoral water depths from small but significant variations in bottom topography around the lake, compounded by progressive infilling of the edges at different rates as peat accumulated. Moreover, several n-alkane-inferred variations in peat accumulation rates from 1800 to 1950 CE are common to the four sites and appear to reflect local cold and dry periods. We infer that these periods are associated with nearby volcanic eruptions in the Changbai Mountains and possibly to distal eruptions in the tropical Pacific. Since 1950 CE, decreases in peat accumulations around the lake may result from enhanced peat decomposition as the water table declined in response to a warm and dry climate and to anthropogenic impacts on the lake catchment.

Particle Fluxes and Bulk Geochemical Characterization of the Cabo Frio Upwelling System in Southeastern Brazil: Sediment Trap Experiments between Spring 2010 and Summer 2012.

Physical and biogeochemical processes in continental shelves act synergistically in both transporting and transforming suspended material, and ocean dynamics control the dispersion of particles by the coastal zone and their subsequent mixing and dilution within the shelf area constrained by oceanic boundary currents, followed by their gradual settling in a complex sedimentary scenario. One of these regions is the Cabo Frio Upwelling System located in a significantly productive area of Southeastern Brazil, under the control of the nutrient-poor western boundary Brazil Current but also with a wind-driven coastal upwelling zone, inducing cold-water intrusions of South Atlantic Central Water on the shelf. To understand these synergic interactions among physical and biogeochemical processes in the Cabo Frio shelf, a series of four experiments with a total of 98 discrete samples using sediment traps was performed from November 2010 to March 2012, located on the 145 m isobath on the edge of the continental shelf. The results showed that lateral transport might be relevant in some cases, especially in deep layers, although no clear seasonal cycle was detected. Two main physical-geochemical coupling scenarios were identified: singular downwelling events that can enhance particles fluxes and are potentially related to the Brazil Current oscillations; and events of significant fluxes related to the intrusion of the 18°C isotherm in the euphotic zone. The particulate matter settling in the Cabo Frio shelf area seems to belong to multiple marine and terrestrial sources, in which both Paraiba do Sul River and Guanabara Bay could be potential land-sources, although the particulate material might subject intense transformation (diagenesis) during its trajectory to the shelf edge.