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| Titel |
Incorporation of Mg and Sr in calcite of cultured benthic foraminifera: impact of calcium concentration and associated calcite saturation state |
| VerfasserIn |
M. Raitzsch, A. Dueñas-Bohórquez, G.-J. Reichart, L. J. Nooijer, T. Bickert |
| 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 ; 7, no. 3 ; Nr. 7, no. 3 (2010-03-04), S.869-881 |
| Datensatznummer |
250004581
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| Publikation (Nr.) |
 copernicus.org/bg-7-869-2010.pdf |
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| Zusammenfassung |
| We investigated the effect of the calcium concentration in seawater and
thereby the calcite saturation state (Ω) 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 [Ca2+], 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 DTE=(TE/Cacalcite)/(TE/Caseawater). The culturing
study shows that DMg of A. tepida significantly decreases with
increasing Ω at a gradient of −4.3×10−5 per Ω unit. The
DSr value of A. tepida does not change with Ω,
suggesting that fossil Sr/Ca in this species may be a potential tool to
reconstruct past variations in seawater Sr/Ca. Conversely, DMg of
H. depressa shows only a minor decrease with increasing Ω,
while DSr increases considerably with Ω at a gradient of 0.009
per Ω 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 decreases 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 [Ca2+] 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 [Ca2+] of
seawater, which in turn would lead to a temperature underestimation of more
than 2 °C. |
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