High resolution elemental records in core MD05-2903 from the northern South China Sea were obtained from XRF scanning and grain size and mineral component analyses.The K/Ti curve reveals climate changes since MIS 3 on a millennial scale,representing one of the best such records so far from deep-sea sediments of the South China Sea.When compared our K/Ti record with other climate records from high latitude ice cores and from East Asia stalagmites,variations in the K/Ti record show some typical saw-shaped features of rapid climate changes in the Northern Hemisphere.Grain size analysis reveals a close relationship between variations in K/Ti and in the fine end-member grain size.XRD analysis confirms that high K/Ti layers were dominated by K-rich weathering minerals,especially illite,while low K/Ti layers contain low illite but high abundance of Ti-rich heavy minerals.Therefore,the K/Ti record reflects changes in the input and deposition of fine weathering minerals and heavy minerals in the northern South China Sea region during stadials and interstadials.These elemental changes were largely controlled by variations in rainfall,erosion,and fluvial transportation induced by East Asia Monsoon.
As the key link connecting the earth’s spheres,continental weathering plays an important role in regulating the global biogeochemical cycle and long-term climate change.Siliciclastic sediments derived from large river basins can record continental weathering and erosion signals,and are thus widely used to investigate weathering processes.However,sediment grain size,hydrodynamic sorting and sedimentary recycling complicate the interpretation of sediment weathering proxies.This study presents elemental and lithium isotope compositions of estuarine surface sediments(SS)and suspended particulate matters(SPM)collected from the Changjiang(Yangtze River)Estuary.Based on a simple mass balance model,the proportions of different end-members(i.e.,igneous rocks,modern weathering products and inherited weathering products)in sediments were quantitatively calculated and thus the silicate weathering process can be estimated.Overall,the sediments in the Changjiang Estuary are mainly eroded from un-weathered rock fragments(>60%),while modern weathering products account for less than 40%.The fine-grained SPM contain more shale components(52–66%),and the modern weathering products account for 21–40%.Comparatively,the coarse-grained surface sediments contain more un-weathered igneous rock fragments(63–84%)and less modern weathering products(only 4–18%).The comparison ofδ^(7)Li values with the weathering proxy(Chemical Index of Alteration,CIA)suggests that sediment weathering intensity declines with increasing proportion of un-weathered igneous rock fragments.Additionally,the occurrence of inherited weathering products(i.e.,shale)in modern sediments makes it a challenge to simply use CIA andδ^(7)Li as indicators of weathering intensity.This study confirms that fine-grained particles are more suitable for tracing contemporary weathering process,albeit with the influence of sedimentary recycling.Lithium isotopes combining with the mass balance model can quantitatively constrain the continental weathering processes in large
Fang CAOShouye YANGChengfan YANGYulong GUOLei BIYuze LI
Suspended particulate matter samples were collected monthly for more than 2 years in Nanjing,China to examine seasonal changes in the Sr-Nd isotopic compositions of the lower Changjiang River(CR).The results indicate that the 87Sr/86Sr ratios of the samples ranges from 0.725352 to 0.738128,and the values ofεNd(0)ranges from-10.55 to-12.29.The Sr-Nd isotopic compositions show distinct seasonal variations.The samples had lower 87Sr/86Sr ratios and higherε Nd(0)values during the flood season than the dry seasons.The seasonal variations primarily reflect the controls of provenance rocks and erosion in different sub-catchments.The relative decrease in 87Sr/86Sr ratios and the increase inε Nd(0)values during the flood season may reflect an increasing in the mechanical erosion rate in the upper basin and the contribution of more sediment from the upper reaches.The end member values of 87Sr/86Sr andε Nd(0)of the samples were 0.728254 and?11.26,respectively.
MAO ChangPingCHEN JunYUAN XuYinYANG ZhongFangJI JunFeng
This study examines the effects of hydrologic sorting and mixing of sources on the Sr-Nd isotopic compositions of suspended sediments at different water depths. The samples were collected from three layers (surface, middle and bottom) at Datong hydrological station of Yangtze River during the flood season of 2010. Our results show that, 87 Sr/ 86 Sr values decrease from surface to bottom, ranging from 0.730332 to 0.720857. ε Nd (0) values range from 14.75 to 10.09, with surface sediments being the most negative. The isotope composition at the middle layer can best represent the mean isotopic composition of the total suspended sediments transported by a river. It is believed that the stratification of Sr-Nd isotope is attributable to mixing of sediments from different sources due to hydrological sorting. Sediments from the upper stream are found to be coarser, and tend to contribute more to the lower water column. Although Sr-Nd isotope is a well acknowledged tool to trace sediment provenance, the current study suggests that the grain size of the samples and the sampling locations should be taken into consideration when applying this method to provenance study.
LUO ChaoZHENG HongBoWU WeiHuaWANG PingCHEN YuLiangWEI XiaoChun
The reconstruction of paleofloods in the Holocene has become one research highlight for the present global change study.The core sediments from one newly-emerged bar in the lower Changjiang River(Yangtze River)mainstream were collected for grain size and organic elemental measurements,with aim to reconstruct the flood events over the past 150 years.Major grain size parameters such as mean grain size,probability cumulative curve and C-M diagram of the core sediments clearly indicate the flood event deposition.Furthermore,the TOC/TN ratios in the sediments can indicate flood events considering that during the flash floods,strong surface erosion in the upper and lower reaches of the Changjiang River can transport a large amount of undecomposed plant debris and organic components with relatively low C/N ratios into the lower mainstream.Based on 210Pb dating and sedimentary geochemical results,the research profile recorded several large floods happened from 1850 to 1954,which agrees well with the historical documents and hydrological observations.Interesting to note that the flood events since the 1960s cannot be distinctly recognized on the basis of sediment grain size and organic elemental compositions of the profile,which mainly reflects the intensive human activities over the last fifty years,especially condense dam construction,have significantly changed the characters of suspended sediment into the lower mainstream.
