Exploring the controls on element ratios in middle Eocene samples of the benthic foraminifera <i>Oridorsalis umbonatus</i>

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 ( [CO ] 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 δ 13 C, organic carbon content, foraminifera dissolution indices), and a seawater δ 18 O reconstruction for Site 1209. Discrepancies in the reconstructed [CO 2− 3 ] 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.

Global Extent of Early Eocene Hyperthermal Events: A New Pacific Benthic Foraminiferal Isotope Record From Shatsky Rise (ODP Site 1209)

“…However, because the carbon and oxygen isotope values of benthic species are known to be offset due to species‐specific metabolic variation in isotopic fractionation and isotopically distinct microenvironments (see Katz et al, , for detailed discussion), the isotope values had to be adjusted. This was achieved by comparing our data along with existing isotope data for Site 1209 (Dawber & Tripati, , ; Dutton et al, ; Figure ). Species‐specific adjustments (i.e., corrections) were estimated based on paired analyses of 128 samples (Figure ) and applied so all data could be stacked into a single record.…”

“…Paired isotope analysis of N. truempyi and O. umbonatus for (left) δ 18 O and (right) δ 13 C. Paleocene, gray; Eocene, black. The linear regression line (red) for δ 18 O and δ 13 C paired analysis including data from Dutton et al (), Dawber and Tripati (), and Dawber and Tripati () to establish the isotopic adjustment (i.e., correction) factor for benthic foraminifera from Site 1209. See Table for details on correction factors used in this study.…”

“…The Site 1209 benthic stable isotope samples in this study span standard planktonic foraminiferal Zones P4c to P11 according to the zonal scheme of Berggren et al (1995) (for details see Petrizzo, 2005). All picked benthic specimens showed good preservation and were of glassy appearance in wet condition; optical (2011), and Dawber and Tripati (2012) to establish the isotopic adjustment (i.e., correction) factor for benthic foraminifera from Site 1209. See Table 1 for details on correction factors used in this study.…”

“…A lone low-resolution benthic stable isotope record was produced for the Paleocene to middle Eocene at Site 1209 from 63 to 42 Ma (Dutton et al, 2005). A more detailed benthic isotope record from 45 to 37 Ma was produced at Site 1209, which could not be consistently correlated to other middle Eocene records (Dawber & Tripati, 2012), because enhanced lysocline shoaling in this period condensed parts of the Site 1209 record (Hancock & Dickens, 2005;Petrizzo, 2005). An accurate depiction of the early to middle Eocene climate and carbon cycle variability on orbital time scales requires a consistent high-resolution benthic stable isotope record from the Pacific, one that can be unequivocally correlated to high resolution Atlantic records.…”