MOUNTAINS, RIVERS AND CARBON: CATCHMENT INTEGRATION OF ORGANIC MATTER AND ISOTOPIC RECORDS OF PALEOELEVATION

Organic biomarker-based proxies have proven to be a practical approach for understanding a diverse range of complex geological systems – more recently, these proxies are increasingly utilized for paleoaltimetry studies. One of the fundamental challenges of using biomarkers to understand landscape evolution is the uncertainty in the connection between the geochemical signatures of organic matter and surface processes. Conventional approaches typically depend on specific environmental constraints (e.g., arid climate, orographic topography, depositional basin). Here, we utilize a novel approach for reconstructing topography from the catchment-integrated signal of organic molecular biomarkers in Taiwan – a tropical to subtropical region characterized by some of the highest global rates of exhumation, erosion, precipitation, and carbon export. We analyzed the geochemical signature of leaf wax n-alkanes from in-situ soil samples and exported low-order river sediments in 19 Taiwan catchments. Soil samples were collected along a vertical transect representing a catchment elevation gradient, and corresponding river sediments were collected at elevations comparable to those of the soil samples. The isotopic signatures of hydrogen and carbon in plant-derived n-alkanes from these samples (δ²H_n-alkane and δ¹³C_n-alkane, respectively) were shown to reflect the integration of organic matter transported from upstream sources. The data show a significant biomarker isotope gradient (~10-15‰ δ²H_n-alkane) as a function of elevation as well as variable offset between soil and river sediment pairs (~10-25‰ in δ²H_n-alkane). Additionally, remote sensing approaches used to obtain catchment soil production functions and biomass for the integrated catchment signals indicate that the offset in isotopic composition differs between catchments along the orogen – this is interpreted as a function of terrestrial organic matter retention times in catchments with different hypsometry. Ultimately, these results support catchment integration of biomarker isotope signatures and highlight implications for isotope reconstruction of past topography in a variety of environments.