Extreme sub-centennial Dead Carbon Fraction variability in a predominantly hydroclimate-driven environment

Tremendous progress has been made in the research on mechanisms modulating the dead carbon fraction (DCF) in speleothems, but the causes for high spatial and temporal variability of DCF in different climate zones are still not understood. We have chosen a predominantly hydroclimate-driven environment in the tropics to assess the variability of the DCF and to attempt to reveal the key governing processes. Our high-resolution ¹⁴C record obtained from a ²³⁰Th/U-dated stalagmite from Larga Cave, Puerto Rico, covers large parts of the last glacial between 41 ka and 12 ka before present (BP), as well as the past 400 years BP in the Holocene. Moreover, hydroclimate variability is resolved through trace metal ratios (e.g. Mg/Ca) and stable oxygen and carbon isotope records (δ¹⁸O and δ¹³C).

The stable isotopes and Mg/Ca time series exhibit a relatively stable mean state indicative of a generally constant hydroclimate (wet vs. dry) and a superimposed correlated variability on the millennial scale. On the contrary, the DCF values are extremely variable on a large range, from 20% to more than 50%. The DCF data indicates that the carbon cycling switches on a sub-centennial time-scale between three apparent modes: a maximum host rock dominated level around 50%, a more soil carbon driven mode around 25%, and an intermediate state in the range of 35%. Notably, this high variability seems to be absent in the Holocene growth period. However, DCF data of another stalagmite from the same cave at a significantly more ventilated location suggest relatively stable DCF values of 15-25 % throughout the glacial period.

With our multi-tracer record from Larga Cave, Puerto Rico, we will discuss the possible dominant processes causing the oscillation between the aforementioned distinct modes of carbon cycling. The comparison to records from the same cave and other locations in the area will disentangle local karst processes and hydroclimate influences on both spatial and temporal scales.