The effect of salinity on the biogeochemistry of the coccolithophores with implications for coccolith-based isotopic proxies

Abstract

Reconstruction of sea surface temperatures from the oxygen isotope composition (δ 18 O) of calcite biominerals synthesised in the mesopelagic zone of the oceans requires knowledge of the δ 18 O of seawater and constraints on the magnitude of biological 18 O/ 16 O fractionation (the so-called vital effect). In the palaeoceanography community, seawater δ 18 O and salinity are unduly treated as a common parameter owing to their strong co-variation both geographically and in the geological register. If the former parameter has arguably no notable influence on the biogeochemistry of marine calcifiers, 10 salinity potentially does. However how salinity per se and the effect of osmotic adjustment can modulate the biogeochemistry, and in turn, the expression of the vital effect in calcite biomineral such as the coccoliths remains undocumented. In this culture-based study of coccolithophores (Haptophyta) belonging to the Noelaerhabdaceae family, we kept temperature and seawater δ 18 O constant, and measured basic physiological parameters (growth rate and cell size), and the isotope composition ( 18 O/ 16 O and 13 C/ 12 C) of coccoliths grown under a range of salinity comprised between 29% and 1539%. The overarching aim of this biogeochemical work has a geological finality and aims to refine the accuracy of palaeotemperature estimates using fossil coccoliths. We found that, although entailing large physiological changes in coccolithophores, salinity does not modulate biological fractionation in the oxygen isotope system. This is a contrasting observation with previous in vitro manipulations of temperature and carbonate chemistry that led to substantial changes in expression of the vital effect. As such, salinity is not a complicating factor to derive temperatures from coccolith-bearing 20 pelagic sequences deposited during periods of change in ice volume, especially at the highest latitudes, or in coastal regions.By contrast, the carbon isotope composition of the coccoliths is influenced by a growth rate-mediated control of salinity with implications for deriving productivity indices from pelagic carbonate.

Controls Over Oxygen Isotope Fractionation in the Waters and Bivalves (Arthritica helmsi) of an Estuarine Lagoon System

“…In addition to the potential biological effects, recent research has investigated kinetic effects on the equilibrium oxygen isotope fractionation between organic and inorganic carbonates and water (Daëron et al., 2019; Watkins et al., 2013). The effects of pH, growth rate and salinity on the carbonate‐water fractionation factor have been identified in several studies (Dietzel et al., 2009; Kim et al., 2007; Zeebe, 1999) but also excluded by others (Hermoso & Lecasble, 2018; Kim et al., 2006). While there is no general scientific consensus, it is clear that the influence of biological and kinetic factors on oxygen isotope fractionation must be considered.…”

Towards the use of the coccolith vital effects in palaeoceanography: A field investigation during the middle Miocene in the SW Pacific Ocean

“…Since the early work by Dudley et al (1986), the origin of the isotopic vital effects in coccoliths has remained elusive. However, recent experimental and modelling work has enabled significant advances in our understanding of the causes underlying biological fractionation (Bolton and Stoll, 2013;Hermoso, 2014;2015;2016;Hermoso et al, 2015;2016a;Hermoso and Lecasble, 2018;Holtz et al, 2015;McClelland et al, 2017;Rickaby et al, 2010;Ziveri et al, 2012). Specifically, it has emerged from these culture studies that the availability of ambient dissolved inorganic carbon (DIC) for the cells can significantly influence and modulate coccolith calcite δ 18 O and δ 13 C values, beyond expectations from ambient seawater composition and temperature alone.…”