Connecting climate change to coastal evolution: Impact of sub-millenial- scale precipitation variability on fluvial sediment discharge

While millennial- scale variations in climate forcing drives changes in terrestrial processes, which are directly linked to fluvial sediment loads (e.g., weathering and erosion), the impact of decadal- to centennial- scale climate fluctuations on downstream coastal sedimentation patterns and landscape evolution remains unclear. Specifically, the connection between long-term (decades or more) precipitation seasonality and sediment export from river systems has not been established. This study examines the Tijucas strandplain (Southern Brazil) to determine if sub-millennial-scale fluctuations in precipitation at river systems have a detectable influence on the coastal landscape. A 5-km strandplain, formed over the last 5800 years through the rapid reworking of sediment discharged from the proximal Tijucas River in a regime of falling sea level, encompasses nearly 70 distinct transitions between shore-parallel sand- and mud- dominated facies. An overall shift from sand- to mud- dominance is due to a long-term reduction in wave energy caused by bay shoaling. Bulk and terrestrial vascular plant wax fatty acid stable hydrogen (δD) and carbon (δ13C) isotopic measurements from sediments from select sandy and muddy ridges across the plain reveal that these two sedimentological units are geochemically distinct. Furthermore, waxes from sediments deposited during periods of sandy progradation were, on average, >10‰ more enriched in deuterium than those from mud-dominated periods, indicating that these sedimentary units reflect different climatic conditions within the river drainage basin at the time of deposition. Comparison of plant wax isotopic signatures of river and beach sediments during the current period of mud-dominated progradation, reveals a close correlation with earlier periods of mud deposition within the Tijucas Strandplain. Thus, decadal- to centennial- scale sedimentologic transitions within the plain are interpreted to reflect climate-driven changes in mud export rates, as a product of modifications in river basin vegetation and soil formation as well as erosional processes.