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Abstract Neogloboquadrina pachyderma is the dominant species of planktonic foraminifera found in polar waters and is therefore invaluable for paleoceanographic studies of the high latitudes. However, the geochemistry of this species is complicated due to the development of a thick calcite crust in its final growth stage and at greater depths within the water column. We analyzed the in situ Mg/Ca and δ18O in discrete calcite zones using laser ablation‐inductively coupled plasma‐mass spectrometry, electron probe microanalysis, and secondary ion mass spectrometry within modern N. pachyderma shells from the highly dynamic Fram Strait and the seasonally isothermal/isohaline Irminger Sea. Here we compare shell geochemistry to the measured temperature, salinity, and δ18Osw in which the shells calcified to better understand the controls on N. pachyderma geochemical heterogeneity. We present a relationship between Mg/Ca and temperature in N. pachyderma lamellar calcite that is significantly different than published equations for shells that contained both crust and lamellar calcite. We also document highly variable secondary ion mass spectrometry δ18O results (up to a 3.3‰ range in single shells) on plankton tow samples which we hypothesize is due to the granular texture of shell walls. Finally, we document that the δ18O of the crust and lamellar calcite of N. pachyderma from an isothermal/isohaline environment are indistinguishable from each other, indicating that shifts in N. pachyderma δ18O are primarily controlled by changes in environmental temperature and/or salinity rather than differences in the sensitivities of the two calcite types to environmental conditions. 

Stable oxygen isotopic ratios in corals (δ¹⁸O_coral) are commonly utilized to reconstruct climate variability beyond the limit of instrumental observations. These measurements provide constraints on past seawater temperature, due to the thermodynamics of isotopic fractionation, but also past salinity, as both salinity and seawater δ¹⁸O (δ¹⁸O_sw) are similarly affected by precipitation/evaporation, advection, and other processes. We use historical observations, isotope‐enabled model simulations, and the PAGES Iso2k database to assess the potential of δ¹⁸O_coralto provide information on past salinity. Using ‘‘pseudocorals’’ to represent δ¹⁸O_coralas a function of observed or simulated temperature and salinity/δ¹⁸O_sw, we find that δ¹⁸O_swcontributes up to 89% of δ¹⁸O_coralvariability in the Western Pacific Warm Pool. Although uncertainty in the δ¹⁸O_sw‐salinity relationship influences the inferred salinity variability, corals from these sites could provide valuable δ¹⁸O_swreconstructions. Coordinated in situ monitoring of salinity and δ¹⁸O_swis vital for improving estimates of hydroclimatic change.

 

Paper was published in Earth System Science Data Discussions on Feb 5, 2020.