A new regional, mid-Holocene palaeoprecipitation signal of the Asian Summer Monsoon

The Dongge Cave speleothem delta O-18 record, which lies in the Pearl River basin (China), has been interpreted as recording a regional decline in Asian Summer Monsoon precipitation over the last 6.5 ka. The same overall trend is seen in the bulk sedimentary organic delta C-13(org) record from a core in the Pearl River Estuary. However, the two records differ in detail and the regional nature of the Dongge palaeoprecipitation signal has therefore been questioned. Our study re-evaluates both records by constructing, for the same estuarine core, biomarker and compound-specific delta C-13 records, which have better constrained terrestrial and marine end members than delta C-13(org), providing additional insights into the evolution of the Asian Summer Monsoon. The Branched Isoprenoidal Tetraether (BIT) index reflects the ratio of soil versus marine organic matter. The BIT record from the estuarine core co-varies with the Dongge Cave delta O-18 record suggesting the two share a common control which is likely to be driven by regional climate. By contrast, the sterols, n-alcohols and n-fatty acid ratios show the same overall trend as Dongge, but parallel the delta C-13(org) re cord's variability between 6.5 and 2 ka indicating a partial decoupling between soil and land-plant organic matter fluxes in the Pearl River Basin. There is clear divergence between the biomarker and C-13(org) records from 2 ka to present. Analysis of the leaf wax delta C-13 suggests that this results from an abrupt change in vegetation probably resulting from local, anthropogenic cultivation two thousand years ago. The basin scale of these estuarine records equates to up to 15 grid cells in typical Earth System Models used for simulating global climate. This permits comparison of Palaeoclimate Model Intercomparison Project simulations of the mid-Holocene with spatially equivalent data relating to the Summer Asian Monsoon, for the first time. (C) 2013 Elsevier Ltd. All rights reserved.