Pollen, Geochemisty, And Paleoclimatic Implications Of Tulare Lake Sediments, San Joaquin Valley, CA Over The Past 19 Ka

Abstract

Palynological, geophysical, and geochemical proxy data from TL05-4B and TL12-1B 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. We examine Tulare Lake sediments in two locations at 10 cm intervals 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. An overall decrease in pine (Pinus) and increase in narrow leaf cattail (Typha angustifolia) and pelagic algae are observed over the past 5900 years. This is consistent with data indicating an overall increase in C/N and decrease in total inorganic carbon in that these observations collectively suggest a warm, dry period followed by increasingly wetter conditions. An increase in Pinus and decrease in littoral pollen and pelagic algae since 1780 years BP suggests the return of overall drier conditions. Furthermore, 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 generally agree with previous studies based on mapping of trench exposures and geochemical and geophysical proxy data from cores.

AAPG Datapages/Search and Discovery © 2014 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Bakersfield, California, April 27-30, 2014