Climatic versus Anthropogenic Influences on Sediment Delivery to the Gulf of Mexico Marginal Sea since 5000 y BP

We present a compilation of provenance data derived from the lower reaches of the Mississippi river showing how the source of sediment supplied to the lower reaches has changed through time in the recent geologic past. We integrate data from a late Holocene point bar, its associated oxbow lake (False River) and the channel plug that infilled since ~500 y BP, as well as another oxbow located upstream at Lake St John. Another finer grained sediment record was derived from coring close to the Mississippi south of New Orleans. The sediments were analysed for an array of major elements, Sr and Nd isotopes, as well as detrital zircon U-Pb geochronology. Grain size is a critical factor in controlling the provenance because suspended sediment is transported rapidly through the river compared to coarse-grained material which travels more slowly as bedload. The radiogenic isotope signature of the fine-grained sediment shows a long-term shift since 4.5 ka towards more radiogenic signatures indicative of more erosion from ancient continental crust, likely the Appalachians and Mid Continent rather than the Rocky Mountain foreland, although this remains the dominant source of material supplied to the Gulf of Mexico. While some of this shift may be anthropogenic, the trend suggests long-term drying of the continental interior and reduced erosion of the foreland. Nonetheless, sand-silt sized zircon U-Pb ages indicate that between 1600 and ~1920 CE flood sediments were dominated by supply from the Missouri River, which is largely sourced from the Rocky Mountain foreland. From 500 BCE until 1600 CE supply was more skewed to the Upper Mississippi and Red River, and with some input from the Arkansas River, also derived from the west. Coarse grained sediments deposited in the lower reaches during the last 10 years show a high degree of variability which we interpret to reflect reduced sediment buffering driven by the inability of the lower reaches to meander and recycle flood sediments in the way expected prior to the installation of levees. The modern tributaries all carry sediment that is much more altered than was true in the recent geological past and reflects heightened soil erosion driven by agriculture. The modern Mississippi is a poor analogue for the natural state of the river when compared to ancient geological deposits.