Dataset Information

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

ABSTRACT: While chromium stable isotopes (?53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and ?53Cr is still lacking. Here we present total dissolved ?53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elevated NCP, but not in lower productivity waters. Observed Cr deficits correspond well with calculated Cr export derived from NCP and Cr:C ratios of natural phytoplankton and marine particulates. Chromium (III) concentrations are stable over the diel cycle yet correlate with NCP, with maxima found in highly productive surface waters but not in lower productivity waters, indicating biological control on Cr (III). The relationship between Cr (III) and ?53Cr suggests that ?53Cr distributions may be controlled by the removal of isotopically light Cr (III) at an isotopic enrichment factor (?53Cr) of -1.08‰ ± 0.25 relative to total dissolved ?53Cr, in agreement with the global ?53Cr-Cr fractionation factor (-0.82‰ ± 0.05). No perturbation to ?53Cr, Cr, or Cr (III) is observed in oxygen-depleted waters (~10 ?mol/kg), suggesting no strong control by O2 availability, in agreement with other recent studies. Therefore, we propose that biological productivity is the primary control on Cr and ?53Cr in the modern ocean. Consequently, ?53Cr records in marine sediments may not faithfully record oxygen availability in the Late Quaternary. Instead, our data demonstrate that ?53Cr records may be a useful tracer for biological productivity.

SUBMITTER: Janssen DJ

PROVIDER: S-EPMC7375040 | biostudies-literature | 2020 Jan

REPOSITORIES: biostudies-literature

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Publications

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

Janssen David J DJ Rickli Jörg J Quay Paul D PD White Angelicque E AE Nasemann Philipp P Jaccard Samuel L SL

Global biogeochemical cycles 20200116 1

While chromium stable isotopes (δ53Cr) have received significant attention for their utility as a tracer of oxygen availability in the distant geological past, a mechanistic understanding of modern oceanic controls on Cr and δ53Cr is still lacking. Here we present total dissolved δ53Cr, concentrations of Cr (III) and total dissolved Cr, and net community productivity (NCP) from the North Pacific. Chromium concentrations show surface depletions in waters with elev ...[more]

PMID: 32713990

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Project description:Oxygen fugacity in metal-bearing systems controls some fundamental aspects of the geochemistry of the early Earth, such as the FeO and siderophile trace element content of the mantle, volatile species that influence atmospheric composition, and conditions for organic compounds synthesis. Redox and metal-silicate equilibria in the early Earth are sensitive to oxygen fugacity (fO(2)), yet are poorly constrained in modeling and experimentation. High pressure and temperature experimentation and modeling in metal-silicate systems usually employs an approximation approach for estimating fO(2) that is based on the ratio of Fe and FeO [called "?IW (ratio)" hereafter]. We present a new approach that utilizes free energy and activity modeling of the equilibrium: Fe + SiO(2) + O(2) = Fe(2)SiO(4) to calculate absolute fO(2) and relative to the iron-wüstite (IW) buffer at pressure and temperature [?IW (P,T)]. This equilibrium is considered across a wide range of pressures and temperatures, including up to the liquidus temperature of peridotite (4,000 K at 50 GPa). Application of ?IW (ratio) to metal-silicate experiments can be three or four orders of magnitude different from ?IW (P,T) values calculated using free energy and activity modeling. We will also use this approach to consider the variation in oxygen fugacity in a magma ocean scenario for various thermal structures for the early Earth: hot liquidus gradient, 100?°C below the liquidus, hot and cool adiabatic gradients, and a cool subsolidus adiabat. The results are used to assess the effect of increasing P and T, changing silicate composition during accretion, and related to current models for accretion and core formation in the Earth. The fO(2) in a deep magma ocean scenario may become lower relative to the IW buffer at hotter and deeper conditions, which could include metal entrainment scenarios. Therefore, fO(2) may evolve from high to low fO(2) during Earth (and other differentiated bodies) accretion. Any modeling of core formation and metal-silicate equilibrium should take these effects into account.

Dataset Information

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

Publications

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean.

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