Magnetic susceptibility and grain size records of Bole loess section in the northern piedmont of Tianshan Mountains and their implications for paleoclimatic changes
Background, aim, and scope Loess-paleosol sequences provide key aeolian archives for Quaternary climate changes. Loess as an aeolian sediment are closely related to climate changes during mobilization, transportation and deposition process. Grain size (GS) and magnetic susceptibility (MS) of loess sediments are regarded as important paleoclimatic proxies in the Chinese Loess Plateau. Loess sediments are also extensively distributed in westerly-dominated Tianshan Mountains area. However, their paleoclimate proxies are not fully understood by now. In this paper, we selected a loess section from north piedmont of Tianshan Mountains to investigate the paleoclimatic significances of these proxies. This section provides an important archive for climatic and environmental change history in the arid Central Asia. Materials and methods Bole loess section has a thickness of 12.7 m. A total of 635 samples were collected at 2 cm intervals for GS and MS analyses. GS distribution was analyzed using a Malvern 2000 laser instrument, and calculated for 100 grain size classes within a measuring range of 0.02 2000 mum. MS was measured with a Bartington MS2 meter both low- (0.47 kHz) and high- (4.7 kHz) frequency magnetic susceptibility (chilf and chihf, respectively). All measurements were completed at the State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences. Seventeen age samples were measured in the Xi'an AMS facility, and all radiocarbon dates were calibrated to calendar years. The visual curve matching method is used to establish age-depth model of this section, due to deficiencies and shortcomings of 14C age. The grain size-standard deviation method here has been employed to extract the environmental sensitive components. Results The results show that (1) Variations in chilf range from 38.80*10~(-8) m~3 · kg~(-1) to 84.70*10~(-8) m~3 · kg~(-1) with an average of 59.52*10~(-8) m~3·kg~(-1). Frequency-dependent magnetic susceptibility (chifd) and frequency-dependent magnetic susceptibility (chifd%) yield consistently low values throughout the BL section; (2) The BL loess is dominated by fine silt-sized particles (21.69% 69.65%, with an average of 44.58%). Coarse silt-sized particles vary from 17.95% to 40.60%, averaging 31.68%. Sand fraction component account for 6.78% 40.50%, averaging of 17.33%. BL loess contains minor clay-sized particles (1.66% 14.69%, 6.41%); (3) The 14C ages almost stop increasing by ca. 25 cal ka BP at ~1 m depth). (4) Two sensitive components (<31.7 mum and 31.7 282.5 mum) were distinguished. Discussion Variations in chifd and chifd% throughout the BL section suggest weak pedogenesis, and MS is more strongly influenced by allogenetic magnetic minerals than pedogenesis. Therefore, wind strength may control on the MS enhancement of this section. The BL loess GS is dominated by silt-sized particles, suggesting a typical aeolian sediment. Two sensitive components represent suspension after dust storm and proximal deposit component mainly transported by dust storms, respectively. We compare the 31.7 282.5 mum sensitive component to delta~(18)O records from Greenland ice core and the July solar insolation at 65°N based the age-depth model, variations in grain size of this section indicate good correlations to the global climate change especially millennial scale climate events during MIS3. Conclusions The BL loess is a typical aeolian sediment, which experiences very weak pedogenesis. MS variabilities are controlled by wind strength, and wind vigor model can apply to explain the MS enhancement of the BL loess.