BULK ORGANIC CARBON ISOTOPE RATIOS FROM STONEMAN LAKE AZ AS A PROXY FOR GLACIAL-INTERGLACIAL CYCLES DURING THE QUATERNARY

Stoneman Lake is an internally drained basin in central Arizona roughly 80 km south of Flagstaff, AZ. The lake was drilled in two places, Core 1A (72 m) and core 1B (36 m), roughly 100 meters apart near the basin depocenter. Initial dating suggests that the probable age range of the cores is 1.2 to 1.5 million years. The δ¹³C values from bulk organic carbon in lacustrine sediments can indicate the proportion of C3 plants vs C4 plants in the watershed. More C4 plants imply warmer temperatures and warm season precipitation (interglacial climate) whereas more C3 plants imply colder temperatures and predominantly winter precipitation (glacial climate). We hypothesize that δ¹³C data obtained from the upper half of core A from Stoneman Lake can represent a proxy for glacial-interglacial cycles. C3 plants generally have δ¹³C values ranging from -24 to -34‰ and C4 plants have values varying from -6 to -19‰. Therefore, more negative δ¹³C values in bulk organics reflect more C3 plants and more positive δ¹³C values reflect more C4 plants in the watershed. Our results show more positive δ¹³C values (-22 to -24‰) in the Holocene section and more negative δ¹³C values (-24 to -27‰) during the late glacial period. The C:N ratios range from about 11 to 16, indicative of a mixture of land plants and lake algae contributions to the lacustrine sediment organics. C:N ratios vary systematically with depth with higher values during the glacial period suggesting a reduction in lake algae productivity at that time. Additional analyses back to the previous interglacial (MIS 5) will show the entire last glacial cycle trend in watershed vegetation. Further analysis of the top half of core 1A may explain the rapid variability seen in the top of core B as well as provide a record of the glacial-interglacial cycle.