Sediment Mixing in the Holocene and Late Wisconsin Mississippi Source-To System Using Detrital Zircons

U-Pb geochronology of detrital zircons (DZ) is a robust tool used to elucidate linkages between tectonics, climate, drainage configurations, and sediment provenance. The late Pleistocene to modern Mississippi drainage basin has experienced modifications by glacial ice and recent anthropogenic modifications, making it an ideal natural laboratory for sedimentary system response to forcings over different timescales. DZs in the lower Mississippi Valley, delta, and deep-sea fan reflect time-integrated (103-4 yrs) pre-anthropogenic contributions from large tributary catchments. Here we present results of DZ mixture modeling using modern sediment samples from tributaries as parent components, and samples from the modern lower Mississippi River and delta, and late Wisconsin deep-sea fan (marine isotope stage 2; DSDP 96 cores) as daughter mixtures. Mixture modeling of modern daughter samples shows broad agreement between modeled relative contributions and measured suspended sediment loads (1976-2009). Differences between model results and historical records can best be explained by anthropogenic sediment impoundment within tributaries to the lower Mississippi. Modeling of late Wisconsin deep-sea daughter samples indicates the Missouri and Upper Mississippi river basins supplied >95% of the sediment to the late Wisconsin deep-sea fan. We interpret these results in terms of increased sediment supply during the last glacial maximum, and increased transport capacity related to deglacial melt-water floods. We suggest sediment composition in large rivers responds to icehouse climate change at timescales of 103-4 years, in contrast to indications that anthropogenic river modifications can alter sediment flux nearly instantaneously. Furthermore, adjustments in lower Mississippi River, delta, and deep-sea fan mixtures occur at timescales an order-of-magnitude less than Milankovitch-scale climate changes, or the equilibrium response time of the Mississippi River itself, indicating rapid environmental signal propagation and preservation within the Mississippi source-to-sink system.