Early diagenesis in the hypoxic zone of the Northern Gulf of Mexico : a disconnection between water column and sediment organic matter reactivity ?
|Temporal extent||2017-07-15 -2017-08-16|
|Author(s)||Owings Shannon2, Rabouille Christophe1, Lansard Bruno1, Bombled Bruno1, Brethous Laurie1, Metzger Edouard3, Eitel Eryn2, Beckler Jordon4, Taillefer Martial2, Richirt Julien3, Boever Anthony2|
|Affiliation(s)||1 : Laboratoire des Sciences du Climat et de l’Environnement, LSCE/IPSL,CEA-CNRS-UVSQ-Université Paris Saclay, 91198 Gif-sur-Yvette, France
2 : School of Earth and Atmospheric Sciences; Georgia Institute of Technology, GA 30332-0340 Atlanta, USA
3 : Laboratoire de Planétologie et Géodynamique, LPG-BIAF, Faculté des Sciences, 2 boulevard Lavoisier, 49045 Angers, France
4 : Florida Atlantic University, Harbor Branch Oceanographic Institute, Boca Raton, FL, USA
|Keyword(s)||Hypoxia, Oceanography, Gulf of mexico, acidified zone, organic matter, recycling, sediment|
Hypoxia and associated acidification is a growing concern for ecosystems and biogeochemical cycles in the coastal zone. The northern Gulf of Mexico (nGoM) has been experiencing large seasonal hypoxia for decades linked to the eutrophication of the continental shelf by the Mississippi River nutrient discharge. Sediments play a key role in maintaining hypoxic and acidified bottom waters, but the precise understanding of their role is still progressing. During the summer 2017 which hit a record in hypoxic surface area in the nGoM, we investigated 4 sediment stations located on the continental shelf under the influence of the Mississippi-Atchafalaya River System which are differentially influenced by the river input and seasonal hypoxia. Using a coupling between amperometric, potentiometric and voltammetric microprofiling with high-resolution Diffusion Equilibration in Thin-films (DET) profiles, and porewater analysis, we investigated the diagenetic processes in sediments under normoxic, hypoxic and nearly anoxic bottom waters. In addition, we used a time-series of bottom-water dissolved oxygen from May to November 2017 which indicated intense O2 consumption in bottom waters related to organic carbon recycling. At the sediment-water interface, we found that oxygen consumption linked to organic matter recycling was large with diffusive oxygen uptake (DOU) of 8 and 14 mmol m-2 y-1, except when oxygen concentration is near anoxia (5 mmol m-2 y-1). Deeper in the sediment, except at the station located near the river outlet, downcore sulfate decrease in pore waters was limited, with little increase of alkalinity, dissolved inorganic carbon (DIC), ammonium and phosphate suggesting that low oxygen conditions did not favor anoxic diagenesis as could be anticipated. We attributed the low intensity of anoxic diagenesis to a limitation in organic substrate supply possibly linked to the reduction of bioturbation during the hypoxic springs and summer.
|Acknowledgments||We thank the captain and crew of the RV Savannah for their help with the operations at sea. We acknowledge the help of L. Brethous, B.P. Fields, E. Buckley and A. Stancil for the work at sea and further analysis in the laboratory. DIC and Alkalinity measurements in the water column were performed at SNAPO-CO2 in Paris (France). Figure 7 was created using R software (R Core Team, 2017). Funding was provided by INSU-EC2CO/LEFE MissRhoDia project to the French group (LSCE and BIAF), NSF to the Georgia Tech group (OCE-1438648), National Academies of Science, Engineering, and Medicine Gulf Research Program (Early Career Grant 2000007281) to J. Beckler, and the Chateaubriand Fellowship of the Office for Science & Technology of the Embassy of France in the United States awarded to S. Owings, support from the U. S National Oceanic and Atmospheric Administration (NA16OAR4320199) and the National Science Foundation (OCE-1559312) to N.N. Rabalais|