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| Titel |
Modern and Cenozoic records of seawater magnesium from foraminiferal Mg isotopes |
| VerfasserIn |
P. A. E. Pogge von Strandmann, J. Forshaw, D. N. Schmidt |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 11, no. 18 ; Nr. 11, no. 18 (2014-09-25), S.5155-5168 |
| Datensatznummer |
250117609
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| Publikation (Nr.) |
 copernicus.org/bg-11-5155-2014.pdf |
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| Zusammenfassung |
| Magnesium is an element critically involved in the carbon cycle, because
weathering of Ca-Mg silicates removes atmospheric CO2 into rivers, and
formation of Ca-Mg carbonates in the oceans removes carbon from the
ocean-atmosphere 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
(δ26Mg, relative to DSM-3) and temperature, Mg / Ca or other
parameters such as carbonate saturation (ΔCO3). However,
inter-species isotopic differences imply that only well-calibrated single
species should be used for reconstruction of past seawater. Seawater δ26Mg 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 δ26Mg
of seawater, while hydrothermal Mg removal and the δ26Mg of
rivers are more minor controls. Using Mg riverine flux and isotope ratios
inferred from the 87Sr / 86Sr 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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