Investigating the long-term palaeoclimatic controls on the δD and δ18O of precipitation during the Holocene in the Indian and East Asian monsoonal regions.

This paper aims to achieve an improved understanding of the long-term change trends of precipitation δD and δ 18 O values (δD p and δ 18 O p ) in the Asian monsoonal region and their relationship with the corresponding humidity trends during the Holocene. To do this we first review the observed modern spatial pattern of summer precipitation distribution in the East Asian summer monsoon (EASM) region under different EASM intensities, and the relationship between modern observed δ 18 O p values and corresponding precipitation amounts on monthly and inter-annual timescales in the EASM and Indian summer monsoon (ISM) regions. Second, we compare Holocene lacustrine and marine compound-specific hydrogen isotopic records of n -alkanes/ n -alkanoic acid (δD n ), lacustrine authigenic carbonate and cave stalagmite oxygen isotopic records (δ 18 O c and δ 18 O s ) from the Asian monsoonal region, all of which are closely related to δD p and δ 18 O p variations. The results demonstrate that in both the ISM and EASM regions, all of these isotopic records exhibit roughly similar long-term characteristics, i.e. they were all more negative during the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.; B.P. means before present, present = 1950 AD), and then became more positive towards the late-Holocene. Third, we compare representative paleo-humidity records from the Asian monsoonal region; the results confirm that, in the ISM region, a humid interval occurred in the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.) and subsequently the climate became more arid towards the late-Holocene. This indicates an enhanced ISM during the early-Holocene and early mid-Holocene (ca. 11–6 ka B.P.), and an ISM of decreasing intensity towards the late-Holocene. On a Holocene orbital scale, both δ 18 O p and δD p appear to be controlled by an “amount effect” in the ISM region, consistent with the region's inter-annual modern δ 18 O p data. This evidence indicates that both δ 18 O p and δD p paleo-records are significantly related to paleo-humidity in the ISM region. In contrast, Holocene humidity variations in the EASM region exhibit clear spatial differences: a humid mid-Holocene interval (ca. 8–3 ka B.P.) occurred in southern and northern China, but an arid interval from ca. 7–3 ka B.P. occurred in central China, in the middle reaches of the Yangtze River. Based on precipitation distribution patterns under different EASM intensities in the EASM region over the past few decades, we conclude that EASM intensity was enhanced during the mid-Holocene (ca. 8–3 ka B.P.). Relative to the ISM intensity, the response of EASM intensity to summer insolation was relatively slow. In the EASM region the relationship between climate and δ 18 O p and δD p is more complex, consistent with analyses of regional inter-annual modern δ 18 O p data. This evidence demonstrates that both δ 18 O p and δD p paleo-records cannot be used directly as paleo-humidity (i.e. precipitation amount or EASM intensity) indicators in the EASM region. Further comparison and analyses demonstrate that the coupled variations in west–east Equatorial Pacific temperature gradients and the West Pacific subtropical high (WPSH) played an important role in determining EASM intensity during the Holocene. [ABSTRACT FROM AUTHOR]