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Carbon geochemistry, lithology, and pollen from the TL05 Core and TL12 Outcrop, Tulare Lake, CA: lake environmental conditions
Palynological, geophysical, and geochemical proxy data from TL05-4A/B core and TL12-1B trench localities of Tulare Lake, California provide a record of regional paleoclimate and paleolake-levels for most of the past 19,000 years. Hydrologic balance modeling has shown that water levels in Tulare Lake in the San Joaquin Valley are highly responsive to climatic and hydrologic changes in the Sierra Nevada Mountains, the principal source of the discharge that feeds the lake. Previous studies of paleolake levels on Tulare Lake sediments have utilized magnetic susceptibility, grain size analysis, organic carbon-nitrogen (C/N) ratios, total inorganic carbon, and total organic carbon as paleolake-level and paleoclimate reconstruction proxies. This study contributes to previous studies on Tulare Lake with pollen as an additional proxy. This study examines Tulare Lake sediments in two locations at 2 cm and 10 cm intervals, from TL05 core and TL12 trench samples, respectively, for palynological species changes. Pollen abundance per cubic centimeter of Tulare Lake sediments is sparse compared to other lakes in the western United States, which may be attributed to poor preservation and changes in lake geochemistry, particularly fluctuating organic C/N ratios. Nevertheless, a change in algae and aquatic pollen reflects a shifting lacustrine environment between brackish marsh, deep-water marsh, and shallow water marsh and terrestrial pollen species reflect transitions between arid, humid, cool, and warm regional climates. This study demonstrates that sediment intervals with a low C/N ratio are accompanied by low terrestrial pollen counts and high algal counts and vice-versa as predicted in the paleolimnological literature. Accordingly, low C/N values suggest lacustrine environments with relatively low input of terrestrial pollen via high discharge events. Low lake conditions are characterized by the presence of the pelagic algae Botryococcus braunii, found in eutrophic lake conditions, while higher lakes are characterized by the presence of Pediastrum species, found in mesotrophic lake conditions. On the whole, the lake-level and regional climate conditions indicated by the palynological analyses of this study confirm results of previous studies based on mapping of trench exposures and geochemical and geophysical proxy data from cores.
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