Does δ13C carb of the Chinese loess indicate past C 3/C 4 abundance? A review of research on stable carbon isotopes of the Chinese loess

Abstract

Current research on stable carbon isotopes of the Chinese loess is reviewed. The stable carbon isotopic composition of soil organic matter (SOM) ( δ 13C org) of paleosols formed during interglacials is more positive than that within loess accumulated during glacials. Spatially, it becomes progressively more negative from the eastern Chinese Loess Plateau (CLP) to the western. However, stable carbon isotopic composition of pedogenic carbonates ( δ 13C carb) from the same sequence produces an opposing trend, both temporally and spatially, and variation differs from expectations under conditions of equilibrium. This indicates that δ 13C carb in the Chinese loess cannot be used to reconstruct the relative proportion of C 3/C 4 plants in past ecological communities. Furthermore, use of δ 13C carb to reconstruct the expansion of C 4 plants in older Tertiary red clay deposits is considered unreliable. We believe δ 13C carb may act as an indirect indicator of vegetation density, following on changes in moisture supply controlled by the intensity of the Asian Summer Monsoon, but conclude that full understanding of the paleoclimatic significance of δ 13C carb in the Chinese loess and underlying Tertiary red clay requires better understanding of modern isotopic processes.