Speleothem biomarker evidence for a negative terrestrial feedback on climate during Holocene warm periods

Understanding how terrestrial carbon storage feeds back on warm climate states is critical for improving global warming projections. Soils may act as a positive feedback on climate if warming increases soil carbon decomposition rates. Conversely, if increases in net primary production (NPP) exceed increases in decomposition, the climate feedback will be negative. Here we utilize the first palaeoclimatic application of compound-specific δ¹³C measurements on n-fatty acid biomarkers (extracted from a stalagmite from central China) to constrain the response of catchment terrestrial carbon cycle feedbacks during warmer phases of the Holocene. We resolve proportional increases in C₃ plants in the catchment area during these warmer/wetter intervals. Moreover, we infer that heterotrophic soil respiration was highly substrate selective, indicating that NPP outpaced decomposition and the catchment behaved as a carbon sink (mediated and enhanced by changes in the relative proportion of C₃ vs C₄ plants). Thus, we provide palaeoclimate evidence that subtropical soils in a warmer/wetter climate acted as a sink for organic carbon, and thus as a negative climate feedback, during warmer climatic phases.