CLIMATIC AND GEOMORPHIC CONTROLS ON ISOTOPE GEOCHEMICAL SIGNATURES OF FLUVIAL SEDIMENTARY ARCHIVES IN GAOPING SOURCE-TO-SINK SYSTEM, SOUTHWESTERN TAIWAN

Fluvial sediments are important archives of paleoenvironments, but variations in sediment production and transport processes greatly influence sediment geochemistry and resultant interpretations of ancient conditions. Sedimentary records of orogenic belt in tectonically-active tropical regions characterized by high precipitation rates, rapid erosion, and short sediment residence times may be particularly sensitive to these surface processes. To examine climatic and geomorphic controls on isotope geochemical signatures of fluvial sedimentary archives, we present the leaf-wax n-alkanes hydrogen and carbon isotope (δD_n-alkane and δ¹³C_n-alkane) records from two sediment cores collected in onshore and offshore of the Gaoping river-submarine canyon system in southwestern Taiwan. These records span the last ~26 kyr and provide the information about the temporal and spatial differences in the geochemical signal within a source-to-sink system. The results show that the records of the offshore core which is distal to the orogeny reflects predominantly the environmental variation due to climate change through the glacial-interglacial transition. From Last Glacial Maximum (LGM) to present, the δ¹³C_n-alkane of the offshore core decreased by ~1.5-2.0‰ and the δD_n-alkane increased by ~10-15‰. These variations reflect the biologic responses to global atmospheric δ¹³C_atm and pCO₂ increase and precipitation δD_precip changes associated with regional warming due to deglaciation. The onshore core records have captured the same environmental signal variation through time, but the δ¹³C_n-alkane are systematically larger and δD_n-alkane smaller than the offshore core, which may be because it is proximal to the orogeny and has higher catchment integration elevation. ­­Our findings demonstrate that in tropical mountain system, isotopic variations in sedimentary records will most strongly reflect changes to catchment integration of sediments and organic materials, highlighting the critical role that depositional setting plays in controlling geochemical records of paleoenvironment.