Geochemical evidence of windblown origin of the Late Cenozoic lacustrine sediments in Beijing and implications for weathering and climate change Hanchao Jiang (蒋汉朝), Gaoxuan Guo (郭高轩), Xiangmin Cai (蔡向民), Jessica Thompson, Hongyan Xu (徐红艳), Ning Zhong (钟宁) The origin of lacustrine sediments in the middle latitudes of Asia remains controversial. In this study, we conducted major and trace element analyses on 86 lacustrine samples from the X5 core in Beijing to determine their origin and provenance.The results indicate that their abundances all varied in relatively narrow ranges, similar to those of the samples in the Chinese Loess Plateau (CLP). A high correlation of major and trace element abundances exists between our lacustrine samples and the CLP samples. UCC normalized major and trace element abundances also have a similar pattern between the lacustrine and the CLP samples, with the exception of a few elements with high solubility, such as Mg. In the ternary diagrams of A-CN-K, La-Th-Sc and Zr/10-Th-Sc, and the correlation maps of major/trace elements and their ratios, four groups of the X5 lacustrine samples with different sedimentation stages (3.16-2.7 Ma, 2.7-1.8 Ma, 1.8-0.9 Ma and 0.9-0 Ma) overlap well with each other and with the CLP samples, indicating that the X5 lacustrine samples are likely of windblown origin and possibly shared common, unchanged dust source regions with the CLP since the Late Pliocene. This interpretation is supported by the overlap of the lacustrine samples from Beijing with the CLP samples in the correlation maps of Ms vs. sorting, Ms vs. skewness, Ms vs. kurtosis, and kurtosis vs. skewness. Analysis of the frequency and cumulative frequency curves indicates that the X5 lacustrine sediments were dominantly transported by the winter monsoon and the westerly circulation. Given more scattered distributions of different elemental ratios and grain size parameters of the X5 core lacustrine sediments compared to those of the CLP samples, local dust particles must have contributed occasionally, which deserves further investigation in the future. Several chemical ratios, such as Rb/Sr, Na 2 O/Al 2 O 3 , CaO/Al 2 O 3 , chemical index of alteration (CIA), were calculated for the lacustrine samples in Beijing. Comparison of these ratios’ variations with those of the sea level of the South China Sea and the global benthic δ 18 O record indicates that Na 2 O/Al 2 O 3 and CIA can be regarded as sensitive indicators of chemical weathering of lacustrinedust sediments in the mid-latitudes of Northern Hemisphere. Palaeo2016-Jiang et al..pdf
~4 Ma coarsening of sediments from Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH glaciation Hanchao Jiang (蒋汉朝), Xue Mao (毛雪), Hongyan Xu (徐红艳), Jessica Thompson, Xiaolin Ma (马小林) Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3-4 Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3-4 Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain-size, sea level and benthic δ 18 O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~4 Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0 Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~4 Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2 Ma. Notably, absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1 Ma and the gradual decrease in coarse-grained peaks since 2.1 Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8-4.0 Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 Ma. A persistent and steady cooling during 4-3 Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75 Ma ago. Palaeo2010-Jiang et al.pdf
标签: sediments
