Nitrogen cycling in the deep sedimentary biosphere; nitrate isotopes in pore waters underlying the oligotrophic North Atlantic

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doi: 10.5194/bg-12-7483-2015
Author(s): Wankel, S. D.; Buchwald, C.; Ziebis, W.; Wenk, C. B.; Lehmann, M. F.
Author Affiliation(s): Primary:
Woods Hole Oceanographic Institution, Department of Marine Chemistry and Geochemistry, Woods Hole, MA, United States
Other:
University of Southern California, United States
University of Basel, Switzerland
Volume Title: Biogeosciences
Source: Biogeosciences, 12(24), p.7483-7502. Publisher: Copernicus GmbH on behalf of the European Union, Katlenburg-Lindau, International. ISSN: 1726-4170
Note: In English. 114 refs.; illus., incl. 2 tables
Summary: Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite limited organic matter delivery from overlying water, the extent and modes of nitrogen transformations have not been widely investigated. Here we use the N and oxygen (O) isotopic composition of porewater nitrate (NO3-) from a site in the oligotrophic North Atlantic (Integrated Ocean Drilling Program - IODP) to determine the extent and magnitude of microbial nitrate production (via nitrification) and consumption (via denitrification). We find that NO3- accumulates far above bottom seawater concentrations (∼ 21 µM) throughout the sediment column (up to ∼ 50 µM) down to the oceanic basement as deep as 90 m b.s.f. (below sea floor), reflecting the predominance of aerobic nitrification/remineralization within the deep marine sediments. Large changes in the δ15N and δ18O of nitrate, however, reveal variable influence of nitrate respiration across the three sites. We use an inverse porewater diffusion-reaction model, constrained by the N and O isotope systematics of nitrification and denitrification and the porewater NO3- isotopic composition, to estimate rates of nitrification and denitrification throughout the sediment column. Results indicate variability of reaction rates across and within the three boreholes that are generally consistent with the differential distribution of dissolved oxygen at this site, though not necessarily with the canonical view of how redox thresholds separate nitrate regeneration from dissimilative consumption spatially. That is, we provide stable isotopic evidence for expanded zones of co-occurring nitrification and denitrification. The isotope biogeochemical modeling also yielded estimates for the δ15N and δ18O of newly produced nitrate (δ15NNTR (NTR, referring to nitrification) and δ18ONTR), as well as the isotope effect for denitrification (15εDNF) (DNF, referring to denitrification), parameters with high relevance to global ocean models of N cycling. Estimated values of δ15NNTR were generally lower than previously reported δ15N values for sinking particulate organic nitrogen in this region. We suggest that these values may be, in part, related to sedimentary N2 fixation and remineralization of the newly fixed organic N. Values of δ18ONTR generally ranged between -2.8 and 0.0 per mil, consistent with recent estimates based on lab cultures of nitrifying bacteria. Notably, some δ18ONTR values were elevated, suggesting incorporation of 18O-enriched dissolved oxygen during nitrification, and possibly indicating a tight coupling of NH4+ and NO2- oxidation in this metabolically sluggish environment. Our findings indicate that the production of organic matter by in situ autotrophy (e.g., nitrification, nitrogen fixation) supplies a large fraction of the biomass and organic substrate for heterotrophy in these sediments, supplementing the small organic-matter pool derived from the overlying euphotic zone. This work sheds new light on an active nitrogen cycle operating, despite exceedingly low carbon inputs, in the deep sedimentary biosphere.
Year of Publication: 2015
Research Program: IODP Integrated Ocean Drilling Program
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 07 Marine Geology and Oceanography; Atlantic Ocean; Biochemistry; Biosphere; Chemical composition; Chemical reactions; Concentration; Cores; Deep-sea environment; Denitrification; Dissolved oxygen; Eh; Expedition 336; Fixation; Geochemical cycle; Geochemistry; IODP Site U1382; IODP Site U1383; IODP Site U1384; Integrated Ocean Drilling Program; Isotope ratios; Isotopes; Marine environment; Marine sediments; Microorganisms; Mid-Atlantic Ridge; N-15/N-14; Nitrate ion; Nitrification; Nitrite ion; Nitrogen; Nitrogen cycle; North Atlantic; O-18/O-16; Ocean Drilling Program; Oligotrophic environment; Organic compounds; Oxygen; Pore water; Productivity; Reactivity; Respiration; Sea water; Sediments; Sensitivity analysis; Solutes; Stable isotopes; World ocean
Coordinates: N224843 N224843 W0460521 W0460521
N224808 N224808 W0460310 W0460310
N224521 N224521 W0460453 W0460453
Record ID: 2019022265
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Copernicus Gesellschaft, Katlenburg-Lindau, Germany