Abstract
Abstract. Sea surface salinity is one of the most important parameters to reconstruct in paleoclimatology, reflecting amongst other things the hydrological cycle, paleodensity, ice volume, and regional and global circulation of water masses. Recent culture studies and a Red Sea field study revealed a significant positive relation between salinity and Na incorporation within benthic and planktonic foraminiferal shells. However, these studies reported varying partitioning of Na between and within the same species. The latter could be associated with ontogenetic variations, most likely spine loss. Varying Na concentrations were observed in different parts of foraminiferal shells, with spines and regions close to the primary organic sheet being especially enriched in Na. In this study, we unravel the Na composition of different components of the planktonic foraminiferal shell wall using electron probe micro-analysis (EPMA) and solution ICP-MS. A model is presented to interpret EPMA data for spines and spine bases to quantitatively assess differences in composition and contribution to whole-shell Na∕Ca signals. The same model can also be applied to other spatial inhomogeneities observed in foraminiferal shell chemistry, like elemental (e.g., Mg, Na, S) banding and/or hotspots. The relative contribution of shell carbonate, organic linings, spines and spine bases to whole-shell Na chemistry is considered quantitatively. This study shows that whereas the high Na areas may be susceptible to taphonomic alterations, the Na chemistry of the shell itself seems relatively robust. Comparing both shell and spine Na∕Ca values with salinity shows that shell chemistry records salinity, albeit with a very modest slope.
Highlights
Salinity is one of the most popular parameters to reconstruct in paleoceanography, driving, together with temperature, the thermohaline circulation and reflecting regional hydrological cycling
Even though Na is considered as a conservative element in seawater, recent culture studies and a Red Sea field study reveal a significant positive relation between salinity and Na incorporation within benthic (Wit et al, 2013; Geerken et al, 2018) and planktonic (Allen et al, 2016; Mezger et al, 2016; Bertlich et al, 2018) foraminiferal shells
Cultured T. sacculifer specimens were collected at 3–8 m water depth 1.6–3.2 km off the south coast of Curaçao and off the west coast of Barbados, after which they were grown in filtered seawater with salinities ranging from 26 to 45 (Nürnberg et al, 1996; Bijma et al, 1990; Bertlich et al, 2018)
Summary
Salinity is one of the most popular parameters to reconstruct in paleoceanography, driving, together with temperature, the thermohaline circulation and reflecting regional hydrological cycling. Even though Na is considered as a conservative element in seawater, recent culture studies and a Red Sea field study reveal a significant positive relation between salinity and Na incorporation within benthic (Wit et al, 2013; Geerken et al, 2018) and planktonic (Allen et al, 2016; Mezger et al, 2016; Bertlich et al, 2018) foraminiferal shells. This relation between salinity and Na incorporation, potentially related to an increase in the Na+/Ca2+ activity ratio with salinity, is observed for foraminiferal calcite (Allen et al, 2016; Mezger et al, 2016; Wit et al, 2013), and for barnacles and Atlantic oyster shells (Rucker and Valentine, 1961; Gordon et al, 1970) and inorganically precipitated calcium carbonate (Kitano et al, 1975; Ishikawa and Ichikuni, 1984)
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