Abstract
The loess stratigraphic boundary at the Pleistocene/Holocene transition defined by the magnetic susceptibility (MS) has previously been assumed to be synchronous with the Marine Isotope Stage (MIS) 2/1 boundary, and approximately time-synchronous at different sections across the Chinese Loess Plateau (CLP). However, although this assumption has been used as a basis for proxy-age model of Chinese loess deposits, it has rarely been tested by using absolute dating methods. In this study, we applied a single-aliquot regenerative-dose (SAR) protocol to the 45–63 μm quartz grain-size fraction to derive luminescence ages for the last glacial and Holocene sections of three loess sections on a transect from southeast to northwest across the CLP. Based on the 33 closely spaced optically stimulated luminescence (OSL) samples from the three sections, OSL chronologies were established using a polynomial curve fit at each section. Based on the OSL chronology, the timing of the Pleistocene/Holocene boundary, as defined by rapid changes in MS values, is dated at ~10.5 ka, 8.5 ka and 7.5 ka in the Yaoxian section, Jingchuan and Huanxian sections respectively. These results are clearly inconsistent with the MIS 2/1 boundary age of 12.05 ka, and therefore we conclude that the automatic correlation of the Pleistocene/Holocene transition, as inferred from the MS record, with the MIS 2/1 boundary is incorrect. The results clearly demonstrate that the marked changes in MS along the southeast to northwest transect are time-transgressive among the different sites, with the timing of significant paleosol development as indicated by the MS record being delayed by 3–4 ka in the northwest compared to the southeast. Our results suggest that this asynchronous paleosol development during the last deglacial was caused by the delayed arrival of the summer monsoon in the northwest CLP compared to the southeast.
Highlights
The loess-paleosol sequences of the Chinese Loess Plateau (CLP) provide a continuous, highresolution record of climate change throughout the Quaternary period, unparalleled in other terrestrial sediment archives [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
The typical optically stimulated luminescence (OSL) decay curve can be decomposed into three components: the OSL signal is dominated by fast component, of which the fast component contributes about 95% in the first 0.64s of the 40s stimulation (Fig 3)
The stratigraphic boundary defined by the major change in magnetic susceptibility (MS) across the Pleistocene/Holocene boundary has long been assumed to be synchronous in different sections across the CLP, and many proxy-age models use this assumption in establishing time series of climate proxies for Chinese loess [14]
Summary
The loess-paleosol sequences of the Chinese Loess Plateau (CLP) provide a continuous, highresolution record of climate change throughout the Quaternary period, unparalleled in other terrestrial sediment archives [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. Proxy climate data recorded in loess sequences, such as the magnetic susceptibility and the sediment grain size distribution, correlate well with the orbitally-tuned marine oxygen isotope record and exhibit clear glacial and interglacial cycles [1,5]. There are two important assumptions made with regard to the stratigraphic boundaries, or age control points, used in developing such proxy age models for Chinese loess deposits. (ii) The second assumption is that changes in the loess proxy records are isochronous in potentially widely separated sections on the CLP [5]. If this assumption is correct sites can be correlated and chronologies established across the entire CLP. On a finer time scale, recent investigations have highlighted age discrepancies based on different dating method between these age models and those derived from absolute dating techniques [17,18,19,20,21,22,23], indicating that these assumptions may not always be correct
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