Abstract
Abstract. Culturing studies and empirically based core top calibrations have been used to infer that elemental ratios in benthic foraminifera can be used as proxies to reconstruct past variations in bottom water temperature and saturation state (Δ [CO32−]). However the mechanisms linking elemental ratios to these parameters are poorly constrained. Here, we explore the environmental parameters influencing the incorporation of B, Li, Sr and Mg in Oridorsalis umbonatus in early Cenozoic sediments from Ocean Drilling Program Site 1209. We investigate the influence of middle Eocene variations in intermediate water Δ [CO32−] using relationships developed from core top samples. The fidelity of bottom water Δ[CO32−] reconstructions based on single element ratios is assessed by comparing the X/Ca-based reconstructions to each other and to carbon cycle proxy records (benthic foraminifera δ13C, organic carbon content, foraminifera dissolution indices), and a seawater δ18O reconstruction for Site 1209. Discrepancies in the reconstructed Δ[CO32−] values based on these different metal ratios suggest that there are still gaps in our understanding of the parameters influencing X/Ca and demonstrate that caution is required when interpreting palaeo-reconstructions that are derived from a single elemental ratio. The downcore record of O. umbonatus Mg/Ca does not exhibit any similarities with the Li/Ca, B/Ca and Sr/Ca records, suggesting that the environmental parameters influencing Mg/Ca may be different for this species, consistent with temperature as the strongest control on this elemental ratio. This hypothesis is supported by the coefficients of multiple linear regression models on published Mg/Ca data. An incomplete understanding of the controls on elemental incorporation into benthic foraminifera hinders our ability to confidently quantify changes in saturation state using single X/Ca reconstructions over a range of timescales.
Highlights
The structure of the Mg/Ca record for Site 1209 is consistently different to the Li/Ca, B/Ca and Sr/Ca records during the middle Eocene, further suggesting that it is temperature, rather than other environmental parameters, that is the dominant influence on O. umbonatus Mg/Ca
Bottom water [CO23−] may influence X/Ca; discrepancies in the nature and magnitude of downcore X/Ca records at Site 1209 suggest that either empirically derived core top regression models are not appropriate for the early Cenozoic and/or X/Ca records are influenced by secondary parameters
The O. umbonatus Mg/Ca record for Site 1209 is consistently different to the Li/Ca, B/Ca and Sr/Ca records suggesting that bottom water [CO23−] is not the dominant control on Mg/Ca ratios for this species
Summary
Model simulations and proxy-based reconstructions hypothesise that early Cenozoic glaciation was closely linked to variations in the global carbon cycle (DeConto and Pollard, 2003; Coxall et al, 2005; Tripati et al, 2005, 2008; Riesselman et al, 2007; DeConto et al, 2008; Merico et al, 2008; Pearson et al, 2009; Spofforth et al, 2010; Coxall and Wilson, 2011). In order to better define the relationship between global climate, glaciation and the carbon cycle, it is necessary to have proxies for seawater carbonate system parameters. Such proxies are useful because on timescales greater than 104 yr, but less than 106–108 yr, the oceanic inventory and distribution of carbon is thought in part to be responsible for regulating atmospheric pCO2.
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