UNDERSTANDING SPELEOTHEM δ13C VARIABILITY USING 14C AT MOANING CAVE, CALIFORNIA

The majority of speleothem paleoclimate studies focus on δ¹⁸O records as proxies of variability in the isotopic composition of rainfall. However, many other proxies (trace elements, Sr isotopes, growth rates) can shed light on climate variability. While multi-proxy studies allow the investigation of a range of environmental processes, including water-soil-rock interactions and shifts in vegetation amount and type, they can also highlight the complexity and variability of cave systems. For example, speleothem δ¹³C can respond to a number of complex processes that are both directly and indirectly related to climate.

Here we develop use of the dead carbon proportion (dcp), estimated from coupled ¹⁴C and ²³⁰Th/U measurements of a stalagmite taken from Moaning Cave, central Sierra Nevada foothills, CA and independently calibrated records of atmospheric radiocarbon content. The dead carbon proportion provides an additional constraint on how water-rock interaction, degassing and prior calcite precipitation have affected the stalagmite δ¹³C record, and by extension, the trace element and ⁸⁷Sr/⁸⁶Sr records. This, in turn, allows us to constrain the degree to which shifts in Moaning Cave stalagmite δ¹³C compositions record external environmental changes such as variation in climate-driven vegetation density and soil productivity above the cave. Our results demonstrate that degassing and prior calcite precipitation can exert a significant control on stalagmite δ¹³C values, as up to 90% of the HCO₃^- reservoir can be lost before precipitation of stalagmite calcite. This work has implications for the interpretation of speleothem proxy records, especially from caves in arid and semi-arid environments.