Tertiary seawater chemistry; implications from primary fluid inclusions in marine halite

Zimmermann, Heide

doi:10.2475/ajs.300.10.723

Cited by 78 publications

(37 citation statements)

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“…6e). Overall, these models yield somewhat lower Mg than amounts inferred from ridge axis carbonate veins (Coggon et al, 2010;Horita et al, 2002;Zimmermann, 2000), which tend to suggest a more gradual slope (Fig. 6e).…”

Section: A Cenozoic Seawater Reconstruction Based On Foraminiferal Camentioning

confidence: 73%

“…Given its residence time of ∼ 10 Myr, this strongly suggests that Mg in the modern oceans is not in steady state (Vance et al, 2009), but is evolving rapidly to more negative δ 26 Mg values. The lack of steady state is supported by data inferred from fluid inclusions in halite, as well as from Sr and Mg isotope ratios in pore fluids, which suggest that the Mg concentration in seawater over the last 40 Ma has been increasing, with a gradient that is becoming steeper towards the present (Holland and Zimmermann, 2000;Zimmermann, 2000;Fantle and DePaolo, 2006;Higgins and Schrag, 2012).…”

Section: A Cenozoic Seawater Reconstruction Based On Foraminiferal Camentioning

confidence: 80%

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Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

2014

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Abstract. Magnesium is an element critically involved in the carbon cycle, because weathering of Ca-Mg silicates removes atmospheric CO 2 into rivers, and formation of Ca-Mg carbonates in the oceans removes carbon from the oceanatmosphere system. Hence the Mg cycle holds the potential to provide valuable insights into Cenozoic climate-system history, and the shift during this time from a greenhouse to icehouse state. We present Mg isotope ratios for the past 40 Myr using planktic foraminifers as an archive. Modern foraminifera, which discriminate against elemental and isotopically heavy Mg during calcification, show no correlation between the Mg isotope composition (δ 26 Mg, relative to DSM-3) and temperature, Mg / Ca or other parameters such as carbonate saturation ( CO 3 ). However, inter-species isotopic differences imply that only well-calibrated single species should be used for reconstruction of past seawater. Seawater δ 26 Mg inferred from the foraminiferal record decreased from ∼ 0 ‰ at 15 Ma, to −0.83 ‰ at the present day, which coincides with increases in seawater lithium and oxygen isotope ratios. It strongly suggests that neither Mg concentrations nor isotope ratios are at steady state in modern oceans, given its ∼ 10 Myr residence time. From these data, we have developed a dynamic box model to understand and constrain changes in Mg sources to the oceans (rivers) and Mg sinks (dolomitisation and hydrothermal alteration). Our estimates of seawater Mg concentrations through time are similar to those independently determined by pore waters and fluid inclusions. Modelling suggests that dolomite formation and the riverine Mg flux are the primary controls on the δ 26 Mg of seawater, while hydrothermal Mg removal and the δ 26 Mg of rivers are more minor controls. Using Mg riverine flux and isotope ratios inferred from the 87 Sr / 86 Sr record, the modelled Mg removal by dolomite formation shows minima in the Oligocene and at the present day (with decreasing trends from 15 Ma), both coinciding with rapid decreases in global temperatures.

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Section: A Cenozoic Seawater Reconstruction Based On Foraminiferal Camentioning

confidence: 73%

Section: A Cenozoic Seawater Reconstruction Based On Foraminiferal Camentioning

confidence: 80%

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

2014

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“…Using a general Mg/Catemperature relationship, this change in Mg/Ca would imply a temperature increase since 25 Ma of more than 2 • C. However, it is not only [Ca 2+ ] that has changed over time, but also the Mg/Ca of seawater. The secular variation of seawater Mg/Ca is well constrained through different approaches, which include Mg/Ca analyses of fluid inclusions in halite (Horita et al, 2002;Lowenstein et al, 2001;Zimmermann, 2000), fossil echinoderms (Dickson, 2002), and calcium carbonate veins from mid-ocean ridge flanks (Coggon et al, 2010). All these studies revealed remarkably similar results, showing that Mg/Ca sw increased since the Cretaceous from slightly more than 1 to 5.2 at present.…”

Section: Conclusion and Implications For Paleoreconstructionsmentioning

confidence: 95%

Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state

et al. 2010

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Abstract.We investigated the effect of the calcium concentration in seawater and thereby the calcite saturation state ( ) on the magnesium and strontium incorporation into benthic foraminiferal calcite under laboratory conditions. For this purpose individuals of the shallow-water species Heterostegina depressa (precipitating high-Mg calcite, symbiont-bearing) and Ammonia tepida (low-Mg calcite, symbiont-barren) were cultured in media under a range of [Ca 2+ ], but similar Mg/Ca ratios. Trace element/Ca ratios of newly formed calcite were analysed with Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and normalized to the seawater elemental composition using the equation D TE =(TE/Ca calcite )/(TE/Ca seawater ). The culturing study shows that D Mg of A. tepida significantly decreases with increasing at a gradient of −4.3×10 −5 per unit. The D Sr value of A. tepida does not change with , suggesting that fossil Sr/Ca in this species may be a potential tool to reconstruct past variations in seawater Sr/Ca. Conversely, D Mg of H. depressa shows only a minor decrease with increasing , while D Sr increases considerably with at a gradient of 0.009 per unit. The different responses to seawater chemistry of the two species may be explained by a difference in the calcification pathway that is, at the same time, responsible for the variation in the total Mg incorporation between the two species. Since the Mg/Ca ratio in H. depressa is 50-100 times higher than that of A. tepida, it is suggested that the latter exhibits a mechanism that deCorrespondence to: M. Raitzsch (raitzsch@uni-bremen.de) creases the Mg/Ca ratio of the calcification fluid, while the high-Mg calcite forming species may not have this physiological tool. If the dependency of Mg incorporation on seawater [Ca 2+ ] is also valid for deep-sea benthic foraminifera typically used for paleostudies, the higher Ca concentrations in the past may potentially bias temperature reconstructions to a considerable degree. For instance, 25 Myr ago Mg/Ca ratios in A. tepida would have been 0.2 mmol/mol lower than today, due to the 1.5 times higher [Ca 2+ ] of seawater, which in turn would lead to a temperature underestimation of more than 2 • C.

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“…All fluxes used to achieve steady state are scaled within the range of previously published estimates (see reference list). The initial sulfate concentration is 27 mmol L −1 which is in the range of estimates based on fluid inclusions in halite for late Miocene/Pliocene (Horita et al, 2002;Zimmermann, 2000;Brennan et al, 2013 (e.g., Nameroff et al, 2004;Francois et al, 1997;Dean et al, 1997). On the other hand, while redox proxies support decreased oxygen levels in some parts of the deep glacial ocean (François et al, 1997;Thomson et al, 1990;Mangini et al, 2001;Dean et al, 1997), other areas, specifically continental margins, show the opposite trend (i.e., higher oxygen levels, Ganeshram et al, 2002).…”

Section: Model Forcingmentioning

confidence: 71%

Pleistocene sediment offloading and the global sulfur cycle

2015

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Abstract. Quaternary sea level fluctuations have greatly affected the sediment budgets of the continental shelves. Previous studies suggested that this caused a considerable increase in the net loss of shelf sediments. Since sediment accumulation and erosion are closely tied to the formation and re-oxidation of pyrite, we use a high-resolution record of sulfur isotope ratios ( 34 S / 32 S) of marine sulfate to evaluate the implications of the so-called "shelf sediment offloading" on the global sulfur cycle. Modeling of our δ 34 S record suggests that erosion during sea level lowstands was only partly compensated by increased sedimentation during times of rising sea level and sea level highstands. Furthermore, our data suggests that shelf systems reached a new equilibrium state about 700 ka, which considerably slowed or terminated shelf sediment offloading.

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Tertiary seawater chemistry; implications from primary fluid inclusions in marine halite

Cited by 78 publications

References 0 publications

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes

Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state

Pleistocene sediment offloading and the global sulfur cycle

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