Millennial-scale Variation in East Asian Summer Monsoon Front Position Deduced from Provenance Changes in Yangtze River Delta Sediments

Abstract:

Spatio-temporal variability of East Asian summer monsoon (EASM) precipitation on millennial timescale during the Holocene is of major interest to climatologists. Recent studies on the variability of EASM precipitation during the Holocene focus on its high resolution reconstruction using stalagmite’s δ¹⁸O record. However, stalagmite’s δ¹⁸O does not necessarily reflect EASM precipitation intensity and information on spatial precipitation pattern is difficult to obtain.

Yangtze River drainage occupies major part of South China and lies oblique to EASM front direction. Consequently, when EASM front penetrated deep(shallow) into China, more precipitation occurred in the upper(lower) reaches. Because over 96% of the sediment discharge from Yangtze is transported as suspension, it is possible to estimate relative contribution of precipitation between the lower and upper reaches from the provenance of suspended particles.

Here we use ESR intensity of quartz in fine silt fraction from Yangtze River Delta core to detect changes in heavy precipitation area within Yangtze River drainage. ESR intensity reflects the average age of source rocks. Since Mesozoic rocks are widely exposed in the upper reaches whereas Proterozoic rocks are exposed in the lower reaches, it is possible to detect relative contribution of suspended sediments from the upper vs. lower reaches. Our study on ESR intensity on modern Yangtze River sediments suggests that we can distinguish the suspended sediments from the upper vs. lower reaches, and that their mixing ratio reflects the relative ratio of the water from the two areas. We will present the result and demonstrate its implication.