Abstract
Abstract. The Mg/Ca ratio of Foraminifera calcium carbonate tests is used as proxy for seawater temperature and widely applied to reconstruct global paleo-climatic changes. However, the mechanisms involved in the carbonate biomineralization process are poorly understood. The current paradigm holds that calcium ions for the test are supplied primarily by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy observations of a membrane-impermeable fluorescent marker in the extant benthic species Ammonia aomoriensis with dynamic 44Ca-labeling and NanoSIMS isotopic imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primarily via trans-membrane transport, but that a small component of passively transported (e.g., by endocytosis) seawater to the site of calcification plays a key role in defining the trace-element composition of the test. Our model accounts for the full range of Mg/Ca and Sr/Ca observed for benthic Foraminifera tests and predicts the effect of changing seawater Mg/Ca ratio. This places foram-based paleoclimatology into a strong conceptual framework.
Highlights
Calcium carbonate tests formed by unicellular Foraminifera are present in marine sedimentary records since the Ordovician (∼ 290 million years ago) (Martin, 1995; Schallreuter, 1983)
We infer that seawater endocytosis and subsequent Ca storage is not likely to be the primary source of Ca for chamber formation for A. aomoriensis
The discrimination of a typical Ca channel against Mg is much stronger than against Sr, because the surface charge density of Mg is different from that of Ca and Sr, which are similar (Allen and Sanders, 1994). This is clearly reflected in the trace-element composition of coccolithophores, which are extremely depleted in Mg, with Mg / Ca ratios in the range 0.06–0.2 mmol mol−1 (i.e., about 4–5 orders of magnitude lower than the seawater Mg / Ca ratio of ∼ 5200 mmol mol−1 (Fig. 3))
Summary
Calcium carbonate tests (shells) formed by unicellular Foraminifera are present in marine sedimentary records since the Ordovician (∼ 290 million years ago) (Martin, 1995; Schallreuter, 1983). With different species adapted to specific environmental conditions, their relative taxonomic abundances as well as the elemental (e.g., Mg / Ca) and isotopic (e.g., δ18O) composition of their tests are frequently used to reconstruct global climate change associated with, for example, glacial–interglacial cycles (Elderfield and Ganssen, 2000) and mass extinctions (Kiessling et al, 2008). The use of Foraminifera tests as paleo-environmental archives is complicated by biological processes, which cause their chemical and isotopic compositions to be significantly different from Ca carbonates precipitated inorganically under the same environmental conditions. These differences are referred to as the “vital effect” (Urey et al, 1951). One fundamental question in this regard is how the constituents of the tests, especially the dominant cation Ca2+ and the minor/trace elements Mg and Sr, are transported from the ambient seawater to the site of calcification
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