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

•The first compound specific δ13C analysis of fatty acids from a stalagmite.•Proportional increases in C3 plants during warmer/wetter Holocene intervals.•Soil respiration is more substrate selective during warmer/wetter Holocene intervals.•Primary production outpaces soil respiration during warmer/wetter Holocene intervals.•Subtropical soils act as a negative feedback in a warmer/wetter climate.