N-ALKANES AS BIOMARKERS FOR HOLOCENE PALEOCLIMATE IN THE TULARE LAKE CATCHMENT, CALIFORNIA, USA

With water security for California’s $42.6 billion agricultural industry a major concern, detailed paleohydrologic models for California lakes are in high demand. Tulare Lake, a quasi-terminal lake which has a capacity of 10M km³ (~8M acre-ft), has the potential to constrain such models. Leaf waxes from terrestrial runoff, preserved as long chain alkanes in lake sediments, have been shown to reveal changes in surrounding vegetation. Cores from Tulare Lake were sampled and extracted for long chain n-alkanes (C₂₀ – C₃₆), and submitted to GC-MS analysis to determine concentrations. The distributions of these alkanes are used to construct qualitative trends of runoff and relative contribution of grasses vs trees and shrubs over the past 20k years.

A ratio of the relative values of alkane concentrations corresponding to certain odd numbered chains [P_aq=(C₂₃+C₂₅)/(C₂₃+C₂₅+C₂₉+C₃₁)] has been used in other studies to estimate the relative contribution of aquatic vs terrestrial plants. For the Tulare sediments, this ratio is almost always well above 0.3 which suggests terrestrial input into a dominantly aquatic assemblage, a result consistent with C/N data from a prior study on Tulare Lake. The Carbon Preference Index (CPI), reflecting the relative abundance of odd/even chain alkanes, and, as such, an indicator of terrestrial input, has a mean value between 1.0 and 1.5 also suggesting abundant aquatic composition of the lacustrine organics though with a slightly more terrigenous composition.

Relative abundance of longer chains, C₃₁ vs C₂₉ and C₂₇ is used to represent change in the contribution of grasses vs. woody angiosperms to terrestrial carbon input. This method can reveal changes, with an increase in C₃₁/(C₂₇+C₂₉+C₃₁) reflecting an increase in grasses over woody angiosperms. Since grasses make up a majority of C4 species, this is likely to correlate strongly with the C3/C4 data when δ¹³C and δD analyses are completed. Initial results show systematic variations in grass abundance over time including abundant grasses during the early Holocene lake highstand observed in lake records throughout central and southern California.