A Cenozoic record of seawater Mg isotopes in well-preserved fossil corals

Gothmann, Anne M.; Stolarski, Jarosław; Adkins, Jess F.; Higgins, John A.

doi:10.1130/g39418.1

Cited by 40 publications

(24 citation statements)

References 24 publications

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“…)−3.5 (N.P. )8–10, 13, 16, 17Ion adsorption onto detrital clays0.1 (0–0.2)−0.8314, 15The steady-state budget includes the inputs of magnesium to the ocean and output processes of magnesium from the ocean, with the corresponding isotopic delta values of the sources or sinks. Values are those given in the respective references for each input/output process, with ranges or standard deviations given, where available, in parentheses (N.P.…”

Section: Discussionmentioning

confidence: 99%

“…Because these processes are also major drivers of long-term carbon dynamics and affect many other element cycles in the ocean, paleo-oceanographic reconstructions of the magnesium cycle can provide information about long-term climate and element cycling in the ocean. Fluctuations in Mg/Ca ratios of biogenic sedimentary carbonates reflect oscillations between “hot-house” and “ice-house” conditions, and the evolution of magnesium isotope ratios in the ocean have been interpreted as recording changes in silicate weathering on land and in the oceanic crust 10,12,17,18 . Models that use paleo-oceanographic reconstructions of proxies such as Mg/Ca or δ 26 Mg rely on knowledge of the modern-day magnesium cycle to use as a benchmark for quantification of the changes in the past.…”

Section: Introductionmentioning

confidence: 99%

“…These isotopic variations reflect the balance between formation of isotopically-light carbonates and isotopically-heavy aluminosilicates in the sediment column. Measured fractionations of magnesium isotopes in marine pore waters are large enough to significantly affect the oceanic magnesium isotope ratio over time periods of millions of years through exchange with the overlying ocean 10,11,34 . With authigenic mineral formation reactions fractionating the pore water isotopes, the ratio of the diffusional gradients of the individual isotopes will vary from their concentration ratio in the overlying ocean, and thus the diffusion results in removal of magnesium with a different isotopic ratio than the overlying oceanic source.…”

Section: Introductionmentioning

confidence: 99%

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The role of marine sediment diagenesis in the modern oceanic magnesium cycle

2019

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The oceanic magnesium cycle is largely controlled by continental weathering and marine authigenic mineral formation, which are intimately linked to long-term climate. Uncertainties in the magnesium cycle propagate into other chemical budgets, and into interpretations of paleo-oceanographic reconstructions of seawater δ26Mg and Mg/Ca ratios. Here, we produce a detailed global map of the flux of dissolved magnesium from the ocean into deeper marine sediments (greater than ∼1 meter below seafloor), and quantify the global flux and associated isotopic fractionation. We find that this flux accounts for 15–20% of the output of magnesium from the ocean, with a flux-weighted fractionation factor of ∼0.9994 acting to increase the magnesium isotopic ratio in the ocean. Our analysis provides the best constraints to date on the sources and sinks that define the oceanic magnesium cycle, including new constraints on the output flux of magnesium and isotopic fractionation during low-temperature ridge flank hydrothermal circulation.

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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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The role of marine sediment diagenesis in the modern oceanic magnesium cycle

2019

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“…Such a relationship could have interesting effects on the oceanic Mg isotope mass balance if the removal of Mg through hydrothermal interactions is not fully quantitative. Although it is debated whether the modern oceans are in concentration or isotopic equilibrium for Mg (Holland, 2005;Tipper et al, 2006b;Pogge von Strandmann et al, 2014;Higgins and Schrag, 2015;Gothmann et al, 2017;Voigt et al, 2020), if we assume for simplicity's sake that they are, the mass balance can be inverted to determine the effect of isotope fractionation caused by a not fully quantitative Mg removal associated with the Rayleigh fractionation observed in our Myvatn samples. We use the original basic steady-state oceanic Mg mass balance of Tipper et al (2006b):…”

Section: Hydrothermal and Cold Groundwatersmentioning

confidence: 99%

Hydrothermal and Cold Spring Water and Primary Productivity Effects on Magnesium Isotopes: Lake Myvatn, Iceland

et al. 2020

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is one of the most biologically productive lakes in the northern hemisphere, despite seasonal ice cover. Hydrothermal and groundwater springs make up the dominant source to this lake, and we investigate their Mg isotope ratio to assess the effect of mid-ocean ridge hydrothermal springs, which are the primary modern sink of seawater magnesium. We also examine a time series in the only outflow from this lake, the Laxa River, to assess the effects of seasonal primary productivity on Mg isotopes. In the hydrothermal waters, there is a clear distinction between cold waters (largely unfractionated from primary basalt) and relatively hot waters, which exhibit over 1 fractionation, with consequences for the oceanic mass balance if the hydrothermal removal of Mg is not fully quantitative. The outflow Mg isotopes are similar to basalts (δ 26 Mg = −0.2 to −0.3) during winter but reach a peak of ∼0 in August. This fractionation corresponds to calcite precipitation during summer in Lake Myvatn, preferentially taking up light Mg isotopes and driving the residual waters isotopically heavy as observed, meaning that overall the lake is a CO 2 sink.

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“…Therefore, the factors controlling Mg isotope fractionation during calcification of biogenic HMC need to be constrained and LBF is appropriate because of (1) availability of asexually reproduced individuals and (2) sufficient growth under constant chemical and physical condition in a laboratory (Maeda et al, ). Together with records of Mg isotopes and Mg/Ca of past seawater may be used to determine the importance of Mg‐bearing silicate versus carbonate fluxes associated with weathering and burial, which has influence on regulating seawater chemistry (Gothmann et al, ; Higgins & Schrag, ; Pogge von Strandmann et al, ). Although an underlying assumption for a Mg paleoproxy is that Mg 2+ substitutes ideally for Ca 2+ in the calcite lattice, the chemical environment of Mg in LBF tests has not yet been confirmed.…”

Section: Introductionmentioning

confidence: 99%

Magnesium Isotopic Composition of Tests of Large Benthic Foraminifers: Implications for Biomineralization

Maeda

Yoshimura

Araoka

et al. 2019

Geochem Geophys Geosyst

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The Mg isotopic composition in biogenic CaCO3 is expected to reflect oceanic Mg cycles, yet its practical application for paleoenvironmental reconstruction is complicated by the influences of seawater temperature and biologically mediated isotope fractionation. In this study, we investigated the relationship between water temperature, growth rate, and the Mg isotope ratio (δ26Mg) of the biogenic high‐Mg calcite that forms the tests of large benthic foraminifers. We cultured asexually reproduced large benthic foraminifers (Amphisorus kudakajimensis and Calcarina gaudichaudii) at 21–30 °C. We measured Mg/Ca ratios, chemical Mg speciation, and δ26Mg for both species. Mg/Ca ratios showed significant temperature dependence for both species, while the δ26Mg values for different populations of C. gaudichaudii exhibited increasing discrimination of heavy isotope with increasing temperature. X‐ray absorption of near‐edge structure spectra of the foraminiferal tests showed Mg speciation to be consistent with that of inorganic calcite, indicating ideal substitution of Mg for Ca in the crystal lattice. The δ26Mg values for both species were close to those of inorganic calcite and other marine organisms that precipitate high‐Mg calcite. These results suggest that precipitation of high‐Mg calcite tests for both species is similar to that of inorganic calcite and that Mg isotope fractionation for both species is only slightly influenced by individual physiological processes related to growth temperature. The ∆26Mgcarb‐sol values of multiple field samples fell within the ∆26Mgcarb‐sol range for inorganic calcite, providing preliminary evidence that the effects of intraspecific differences in δ26Mg values can be averaged out by careful choice of samples for analysis.

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A Cenozoic record of seawater Mg isotopes in well-preserved fossil corals

Cited by 40 publications

References 24 publications

The role of marine sediment diagenesis in the modern oceanic magnesium cycle

The role of marine sediment diagenesis in the modern oceanic magnesium cycle

Hydrothermal and Cold Spring Water and Primary Productivity Effects on Magnesium Isotopes: Lake Myvatn, Iceland

Magnesium Isotopic Composition of Tests of Large Benthic Foraminifers: Implications for Biomineralization

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