Nitrogen cycling in the sediments of Santa Barbara basin and Eastern Subtropical North Pacific: Nitrogen isotopes, diagenesis and possible chemosymbiosis between two lithotrophs (Thioploca and Anammox)—“riding on a glider”

“…Altabet et al, 1999;Freudenthal et al, 2001;Lehman et al, 2002). This increase is likely due to isotopic fractionations during NH 4 + release and partial oxidation in pore waters (Moebius, 2013;Prokopenko et al, 2006). In contrast, diagenesis under anoxic conditions, or under oxic conditions with high sedimentation rates, only imparts small isotopic fractionations of <1‰, making bulk sedimentary nitrogen slightly lighter (Altabet et al, 1999;Lehman et al, 2002;Thunell et al, 2004).…”

Section: Preservation Of Nitrogen Isotopes In the Rock Recordmentioning

confidence: 96%

The evolution of Earth's biogeochemical nitrogen cycle

Stüeken

Kipp

Koehler

et al. 2016

Earth-Science Reviews

328

287

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Nitrogen is an essential nutrient for all life on Earth and it acts as a major control on biological productivity in the modern ocean. Accurate reconstructions of the evolution of life over the course of the last four billion years therefore demand a better understanding of nitrogen bioavailability and speciation through time. The biogeochemical nitrogen cycle has evidently been closely tied to the redox state of the ocean and atmosphere. Multiple lines of evidence indicate that the Earth"s surface has passed in a non-linear fashion from an anoxic state in the Hadean to an oxic state in the later Phanerozoic. It is therefore likely that the nitrogen cycle has changed markedly over time, with potentially severe implications for the productivity and evolution of the biosphere. Here we compile nitrogen isotope data from the literature and review our current understanding of the evolution of the nitrogen cycle, with particular emphasis on the Precambrian. Combined with recent work on redox conditions, trace metal availability, sulfur and iron cycling on the early Earth, we then use the nitrogen isotope record as a platform to test existing and new hypotheses about biogeochemical pathways that may have controlled nitrogen availability through time. Among other things, we conclude that (a) abiotic nitrogen sources were likely insufficient to sustain a large biosphere, thus favoring an early origin of biological N 2 fixation, (b) evidence of nitrate in the Neoarchean and Paleoproterozoic confirm current views of increasing surface oxygen levels at those times, (c) abundant ferrous iron and sulfide in the midPrecambrian ocean may have affected the speciation and size of the fixed nitrogen reservoir, and (d) nitrate availability alone was not a major driver of eukaryotic evolution.

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“…After the death of organisms, organic-N is converted to ammonium (NH 4 + ) through ammonification with little fractionation (ε = ␦ 15 N reactant − ␦ 15 N product < 3‰) (Prokopenko et al, 2006;Sigman et al, 2009). This ammonium is then rapidly oxidized to nitrite (NO 2 − ) and subsequently to nitrate (NO 3 − ) through nitrification with negligible fractionation under modern oxic conditions or with a significant fractionation under oxygen-deficient conditions (Sigman et al, 2009;Wang et al, 2013).…”

Section: The Marine Nitrogen Cyclementioning

confidence: 99%

“…During denitrification, oxidized N species (NO 3 − and NO 2 − ) are reduced to gaseous forms of N such as nitrous oxide (N 2 O) or N 2 with substantial isotopic fractionation (c. 20‰ to 30‰) in the water column, but with negligible fractionation in the sediments (Sigman et al, 2009). The anammox process is conducted by strictly anaerobic chemoautotrophic bacteria with ammonium (NH 4 + ) being oxidized to N 2 by nitrite (NO 2 − ) (Kuypers et al, 2003); isotopic fractionation during this process remains unclear but has been inferred to be ∼25‰ (Prokopenko et al, 2006). During both water-column denitrification and anammox processes, 14 NO 3 − is preferentially used and so the heavier isotope 15 N will become relatively enriched in the nitrate pool (Sigman et al, 2009).…”

Section: The Marine Nitrogen Cyclementioning

confidence: 99%