Climatic and anthropogenic controls on late Holocene sediment transport to the Gulf of Mexico by the Mississippi River

Abstract

Models of large alluviated rivers suggest that erosional signals from the headwaters are not transported to the marine depocenter over many timescales because of extensive sediment buffering and recycling in flood plains. We present here a new integrated Late Holocene sedimentary record of the Mississippi River, synthesizing earlier analyses and new material from oxbow lakes, filled channel plugs and a continuous core from the delta to reconstruct a detailed 3000-year record of sediment compositions in the lower reaches. As well as major element data and new detrital zircon U-Pb dating since 860 y BP, our study presents a new basin-wide Sr and Nd isotope record. We show that weathering proxies are controlled by grain size, with little evidence for a long-term trend in chemical weathering in the last 3000 years. ⁸⁷Sr/⁸⁶Sr, but not ε_Nd values are linked to grain size and the degree of chemical alteration, with coarser material generally lower in ⁸⁷Sr/⁸⁶Sr compared to fine sediment.

There is a long-term trend towards more erosion of ancient crust shown in suspended sediment, with greater flux from the Superior Province via the Upper Mississippi, increasing after 2000 y BP, when the climate dried, and humans adopted a more sedentary rather than hunter-gatherer lifestyle. This contrast with the sandy sediment that shows less erosion from the Trans-Hudson, Superior Province and Appalachian until ∼400 years ago. Another change is noted in both muddy and sandy sediment after ∼400 years ago, close to the start of the Little Ice Age, a time of colder and drier climate, when there was a gradual decrease in flux from the Rocky Mountain foreland basin via the Missouri River. The Mississippi River is not fully buffered on centennial scales prior to the installation of man-made levees. Short-term changes in zircon U-Pb populations indicate pulses of sediment supply to the lower reaches, likely related to floods. Maximum sediment supply from the Missouri River occurred at the Last Glacial Maximum and in the recent past (∼10 years).

A drying climate after 1000 years ago increased sediment delivery from the Appalachians, Trans-Hudson and Yavapai terranes by enhancing stream incision while reducing reworking of moraines eroded from the Superior Province. After ∼400 years ago human settlement of the Rocky Mountain foreland enhanced erosion from that region. Modern Mississippi sediment supply is heavily anthropogenically disrupted and thus makes a poor analog for older sediments deposited in the Gulf of Mexico.