Abstract
Abstract. Past ocean temperatures and salinities can be approximated from combined stable oxygen isotopes (δ18O) and Mg ∕ Ca measurements in fossil foraminiferal tests with varying success. To further refine this approach, we collected living planktic foraminifers by net sampling and pumping of sea surface water from the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analyses of δ18O and Mg ∕ Ca in eight living planktic species (Globigerinoides sacculifer, Orbulina universa, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii, Globorotalia ungulata, Globorotalia truncatulinoides and Globorotalia tumida) were compared to measured in situ properties of the ambient seawater (temperature, salinity and δ18Oseawater) and fossil tests of underlying surface sediments. “Vital effects” such as symbiont activity and test growth cause δ18O disequilibria with respect to the ambient seawater and a large scatter in foraminiferal Mg ∕ Ca. Overall, ocean temperature is the most prominent environmental influence on δ18Ocalcite and Mg ∕ Ca. Enrichment of the heavier 18O isotope in living specimens below the mixed layer and in fossil tests is clearly related to lowered in situ temperatures and gametogenic calcification. Mg ∕ Ca-based temperature estimates of G. sacculifer indicate seasonal maximum accumulation rates on the seafloor in early spring (March) at Caribbean stations and later in the year (May) in the Florida Straits, related to the respective mixed layer temperatures of ∼26 ∘C. Notably, G. sacculifer reveals a weak positive linear relationship between foraminiferal derived δ18Oseawater estimates and both measured in situ δ18Oseawater and salinity. Our results affirm the applicability of existing δ18O and Mg ∕ Ca calibrations for the reconstruction of past ocean temperatures and δ18Oseawater reflecting salinity due to the convincing accordance of proxy data in both living and fossil foraminifers, and in situ environmental parameters. Large vital effects and seasonally varying proxy signals, however, need to be taken into account.
Highlights
Calcite tests of planktic foraminifers are precipitated from the surrounding seawater and their stable oxygen isotope compositions (δ18Ocalcite) and Mg / Ca ratios are established proxies to reconstruct past ocean conditions (e.g. Erez and Luz, 1983; Nürnberg et al, 2000)
Earlier studies showed that δ18Ocalcite reveals an offset to the equilibrium of the seawater caused by environmental factors and/or biological controlled processes, so-called vital effects (Weiner and Dove, 2003) as influencing factors (Spero and Lea, 1993; Spero et al, 1997; Bemis et al, 1998; Bijma et al, 1999)
The red vertical line indicates the critical [CO23−] value of 21.3 μmol kg−1, below which selective Mg2+ ion removal starts (Regenberg et al, 2014); the black dashed line marks the calcite saturation horizon (CSH), which is defined as 0 μmol kg−1 and represents the top of the lysocline at ∼ 4600 m water depth; the brown dashed line indicates the maximum plankton tow sampling depth
Summary
Calcite tests of planktic foraminifers are precipitated from the surrounding seawater and their stable oxygen isotope compositions (δ18Ocalcite) and Mg / Ca ratios are established proxies to reconstruct past ocean conditions (e.g. Erez and Luz, 1983; Nürnberg et al, 2000). Calcite tests of planktic foraminifers are precipitated from the surrounding seawater and their stable oxygen isotope compositions (δ18Ocalcite) and Mg / Ca ratios are established proxies to reconstruct past ocean conditions The δ18Ocalcite signature depends on the ambient seawater temperatures and oxygen isotopic compositions (δ18Oseawater) the planktic organism is thriving in. Their relationship was defined in several δ18O paleotemperature equations Earlier studies showed that δ18Ocalcite reveals an offset to the equilibrium of the seawater caused by environmental factors (e.g. salinity, carbonate ion concentration [CO23−], ocean pH) and/or biological controlled processes, so-called vital effects (Weiner and Dove, 2003) (e.g. symbionts photosynthesis, respiration) as influencing factors (Spero and Lea, 1993; Spero et al, 1997; Bemis et al, 1998; Bijma et al, 1999).
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