Sodium incorporation in foraminiferal calcite: An evaluation of the Na/Ca salinity proxy and evidence for multiple Na-bearing phases

The ratio of sodium to calcium (Na/Ca) in foraminiferal calcite has been proposed as a proxy for salinity, yet relatively little is known about the incorporation of sodium into the shells of foraminifera. Ongoing debates include the location of Na in the calcite crystal lattice, the possibility that at least some Na might be complexed with organics, and the influence of spines/spine bases. We present new Na/Ca measure-ments, determined using both solution and laser ablation ICP-MS, of the planktonic foraminifera Globigerinoides ruber (white) from plankton tows and sediment traps spanning a wide salinity range (32.5-40.7 salinity units), laboratory cultures under varying carbonate chemistry, and globally-distributed core-top samples. Our results show that Na/Ca in recently living foraminifera measured by laser ablation ICP-MS is elevated by up to 5 mmol/mol (-85%) relative to the same samples measured by solution ICP-MS (the same comparison for Mg/Ca shows excellent agreement between the tech-niques). Na/Ca in recently living foraminifera measured by laser ablation ICP-MS displays a significant relationship with salinity above-36 salinity units with a slope of-0.7 mmol/mol/salinity unit; however, only a weak relationship is observed between salinity and Na/Ca measured by solution ICP-MS. We pro-pose that Na is incorporated in at least two discrete phases; a primary phase within the CaCO3 mineral, and a (or likely multiple) secondary phase(s). Possibilities for these secondary phases include residual metastable CaCO3, fluid inclusions, high Na/Ca spine bases, and organics. These secondary phases con-tribute to spatially-resolved analyses (i.e. laser ablation ICP-MS) of recently living foraminifera but are removed by crushing/oxidative cleaning for solution ICP-MS, and during early diagenesis, as evidenced by the agreement between laser analysis of coretop samples and Na/Ca measured by solution. The amount of one of these secondary phases, or the amount of Na within this phase, appears to vary as a function of salinity, and is likely the principal driver of the previously observed steep Na/Ca-salinity rela-tionship in recently living foraminifera analysed by laser ablation. Overall, we find salinity, temperature, carbonate chemistry, and bottom water saturation state (Xcalcite) all have a significant but relatively weak effect on Na/Ca in the primary calcite phase. As such, Na/Ca in planktonic foraminifera recovered from sediment cores is unlikely to find widespread utility as a salinity proxy.(c) 2023 Elsevier Ltd. All rights reserved.

Automated summary

The incorporation of sodium into the shells of foraminifera is still not well understood, and ongoing debates surround its location in the calcite crystal lattice and its potential complexation with organics. This study presents new measurements of sodium to calcium ratio (Na/Ca) in planktonic foraminifera using different analytical techniques, revealing elevated Na/Ca values measured by laser ablation ICP-MS compared to solution ICP-MS. The results suggest that Na is incorporated in at least two phases, with one primary phase within the CaCO3 mineral and one or multiple secondary phases likely contributing to the observed Na/Ca-salinity relationship.