Increasing terrigenous sediment supply from Taiwan to the southern Okinawa Trough over the last 3000 years evidenced by Sr[sbnd]Nd isotopes and geochemistry

Abstract

High-resolution sedimentary records reveal the patterns of weathering and erosion in response to climate change, environmental variability, and human activity. Here, we present major and trace element data, SrNd isotopic compositions, and mass accumulation rates (MARs) for the siliciclastic sediment fraction of a relatively high-resolution sedimentary archive from the southern Okinawa Trough, East China Sea. We reveal the sedimentary source-to-sink processes and the physical erosion and chemical weathering history of Taiwan. The SrNd isotopic data indicate that sediments were mainly sourced from northeastern and western Taiwan rivers and thus, the sedimentary records reflect the weathering and erosion conditions in this source region. Volcanic rocks, aeolian dust, and terrigenous sediment from rivers in mainland China and eastern Taiwan have not significantly contributed to the deposits in the southern Okinawa Trough since ca. 3000 cal yr BP. Multiple indices, including the chemical index of alteration (CIA), K/Al, αAlK, Ti/Na, and K/Na, are used to identify variations in the intensity of regional-scale chemical weathering. The strength of the sediment input is estimated using the MAR of the siliciclastic (MARsiliciclastic) and the MAR of Ti (MARTi). The results suggest that the bedrock of small mountainous river systems in Taiwan has experienced relatively stable and moderate chemical alteration and that the terrigenous sediment supply from Taiwan to the southern Okinawa Trough has progressively increased since ca. 3000 cal yr BP. A comparison of these data with climate and human activity indicates that the increased input of terrigenous sediment was closely related to an intensification of rainfall in Taiwan over the last 3000 years. After ca. 1500 cal yr BP, a sustained increase in the terrigenous sediment input likely to be attributed to the combined effects of intensified heavy rainfall and human activity, occurred. Intense physical erosion in Taiwan's river basins has exerted a long-term constraint on chemical weathering intensity. The lack of evident link between silicate weathering and climate forcing in these catchments suggests that paleoclimate reconstruction based on sedimentary geochemical records offshore of Taiwan rivers requires careful consideration.