Publication Date:
2015-04-13
Description:
Abstract. Mg= Ca ratios in foraminiferal tests are routinely
used as paleotemperature proxies, but on long timescales,
they also hold the potential to reconstruct past seawater
Mg= Ca. The impact of both temperature and seawater
Mg= Ca on Mg incorporation in Foraminifera has been quantified
by a number of studies. The underlying mechanism responsible
for Mg incorporation in foraminiferal calcite and
its sensitivity to environmental conditions, however, has not
been fully identified. A recently published biomineralization
model (Nehrke et al., 2013) proposes a combination of transmembrane
transport and seawater leakage or vacuolization to
link calcite Mg= Ca to seawater Mg= Ca and explains interspecies
variability in Mg= Ca ratios. To test the assumptions
of this model, we conducted a culture study in which seawater
Mg= Ca was manipulated by varying [Ca2C] and keeping
[Mg2C] constant. Foraminiferal growth rates, test thickness
and calcite Mg= Ca of newly formed chambers were analyzed.
Results showed optimum growth rates and test thickness
at Mg= Ca closest to that of ambient seawater. Calcite
Mg= Ca is positively correlated to seawater Mg= Ca, indicating
that it is not absolute seawater [Ca2C] and [Mg2C]
but their ratio that controls Mg= Ca in tests. These results
demonstrate that the calcification process cannot be based
only on seawater vacuolization, supporting the mixing model
proposed by Nehrke et al. (2013). Here, however, we suggest
transmembrane transport fractionation that is not as strong as
suggested by Nehrke et al. (2013).
Repository Name:
EPIC Alfred Wegener Institut
Type:
Article
,
isiRev
Format:
application/pdf
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