Benthic fluxes from hypoxia-influenced Gulf of Mexico sediments: Impact on bottom water acidification

William M. Berelson, James McManus, Silke Severmann, Nick Rollins

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Abstract

Benthic fluxes are reported from 8 sites on the shelf and slope of the Gulf of Mexico (GoM) in the region near the Mississippi River delta. Benthic landers, with the capability to incubate 3 chambers simultaneously, were deployed in August 2011 to establish the rate of phosphate, nitrate, ammonium, dissolved Si, Dissolved Inorganic Carbon (DIC) and alkalinity concentration change vs. incubation time (7–50 h) thereby providing flux estimates for these parameters. Stations located within 350 km of the Mississippi River mouth have nutrient and DIC fluxes that vary by a factor of 10 but show no longitudinal trends. Chambers deployed in low oxygen bottom water (<4 μM) do not have different flux values compared to chambers deployed in settings with higher bottom water oxygen (160 μM). At some stations, ammonium shows a change in flux with incubation time, yet no other species shows this change; hence neither a change in transport mechanism or reaction rate can explain the non-steady state behavior of ammonium. A positive correlation (r 2 = 0.91) between the fluxes of dissolved Si and DIC imply a linkage between the diagenesis of biogenic Si and organic C throughout this region. The efflux of DIC and alkalinity in a ratio > 1 drives GoM bottom waters to a lower calcite and aragonite saturation state. Carbon isotopes of the DIC were measured on chamber waters and the calculated δ 13 C of the DIC source at 4 stations was similar, between −14 to −17‰ compared to typical δ 13 C values of bulk organic carbon in the GoM of −22‰. At a fifth station, the DIC added was calculated to have an isotope value of −31‰. Dissolved inorganic carbon isotope compositions that are significantly heavier or lighter than particulate organic carbon are unexpected and may be explained by a combination of inputs: the oxidation of average marine organic carbon and calcium carbonate dissolution, methanogenesis, and/or from oxidation of terrestrial lignin fractions.

Original languageEnglish (US)
Pages (from-to)94-106
Number of pages13
JournalMarine Chemistry
Volume209
DOIs
Publication statusPublished - Feb 20 2019

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All Science Journal Classification (ASJC) codes

  • Oceanography
  • Chemistry(all)
  • Environmental Chemistry
  • Water Science and Technology

Keywords

  • Acidification
  • Hypoxia
  • Mississippi
  • Sediments

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