Reconstructing terrestrial organic matter input into the northern Gulf of Mexico and its relation to marine productivity

Abstract

Global river systems pay a vital role in the transportation of organic matter (OM) between the terrestrial and the marine realm, and as such are a vital component to the carbon cycle. River systems have formed a vital role for humanity, and as such have been influenced by human activity. These anthropogenic changes can influence the amount and composition of OM. This project aims to reconstruct the relative amount and composition of fluvially discharged organic carbon (OC) by the Mississippi River and how this may have influenced marine productivity. The project uses a Multi-core (MC), 49cm in length ranging from the years 1963-2018, and Piston-core (PC), ranging from the approximate age of 1620-1963 taken at 100m water depth in front of the Mississippi River mouth in the northern Gulf of Mexico (GOM). We will use both bulk analysis (TOC and δ13C) as well as lipid biomarker analysis, including GDGTs, n-alkanes, alkenones and long chain diols. Initial results from the TOC and δ13C signature indicate that this site has seen a shift from a primarily marine sourced OC to more terrestrially sourced OC. The δ13C signature is becoming increasingly negative, with values ranging from -22.0‰ in the deepest part of the core to -23.2‰ at the top of the core, as well as the TOC content, with values increasing from 1.13% in the deepest part of the PC to 1.71% in the MC. Particularly since approximately the 1870’s this shift can be clearly noticeable. Other biomarker and proxy data suggest that the M100 site is under intermediate influence from riverine input, this is reflected in the %C32 1,15-diol, which follows a similar trend to the TOC and δ13C record, generally increasing towards 2018, from 25% in 1820s to 35% in 2018. The high concentration of n-alkanes throughout the MC and PC suggests there is a substantial input of terrestrial higher plants to this location. Contrastingly, the BIT values remain low throughout 1820- 2018, peaking at 0.31, and show a decreasing trend through the core. This would suggest an increase in marine sourced OC. However, the increasing concentration of Crenarchaeol is likely a cause for the low BIT values. The increase in concentration of Crenarchaeol and alkenones, from 5.39μg/g TOC to 17.64μg/g TOC and from 5.11μg/g TOC to 20.77μg/g TOC respectively. A likely cause of this increase in marine primary productivity links to the increase in TOC and a more negative δ13C signature. River engineering methods have likely caused a disconnect between rivers and their floodplains, which causes fresh OC to be transported directly to the river mouth. Fresh terrestrial OC may form an additional food source to marine primary producers, which is enhancing primary productivity, and further enhancing the development of the coastal hypoxia zone in the GOM.