A Quantitative 80 ka-Long Rainfall Record for the Chinese Loess Plateau Using 10Be and Magnetic Susceptibility in Loess

We demonstrate a new method to derive quantitative paleo-rainfall estimates for continental interior regions using 10Be in loess. We apply this technique to derive an 80,000 year long rainfall record from the Chinese Loess Plateau. The key steps in this technique are to account for variations in 10Be flux that are due to magnetic field modulation of 10Be production rate, and to account for 10Be derived from recycled dust. Aside from these two factors, variations in 10Be flux are chiefly due to changes in wet precipitation rate. We calibrate the 10Be flux dependence on rainfall rate using 7Be concentrations in modern rainfall as a proxy. In three different 7Be vs. precipitation amount studies on three continents a similar linear relationship is found. Using the average slope in these studies we convert from 7Be to 10Be using the known 10Be/7Be flux ratio in modern tropospheric precipitation. The precipitation record we derive with this method is well correlated with the speleothem delta(18)O records from Dongge and Hulu caves in S.E. China, which are widely regarded as a robust record of Asian Monsoon intensity. Our record indicates extremely low rainfall for the LGM and marine isotope stage 4 (MIS4), with precipitation rising gradually out of MIS4 to near modern levels during MIS3. After circa 35 ka BP precipitation began a long decline culminating in a 5 ka long minima during the LGM ending at circa 20 ka BP. Precipitation then increased stepwise in dramatic fashion at ~20 ka, and again at 14.3 ka. Precipitation was constant or decreased slightly between 13-11ka, followed by another abrupt precipitation increase culminating in a Holocene maximum at 9 ka BP. Precipitation then dropped rapidly to a Holocene minimum at 5.8 ka BP followed by a modest rise to 4 ka then gentle decay and another small rise to the present during the late Holocene. Two large precipitation increases are observed in our record which are roughly coeval with melt-water pulses 1 & 2 suggesting a coupling between high and low latitude climate dynamics during the deglacial. The low frequency component of our precipitation record resembles summer (JJA) solar insolation curve for 30°N, except during MIS3, when it more strongly resembles the insolation differential between 30°N and 30°S. This can be understood in terms of two of the chief elements of monsoon forcing, land/sea temperature differential, and cross-hemispheric moisture transport linked to atmospheric pressure gradient forces associated with the Mascarene High over the SW subtropical Indian ocean.