Abstract
Abstract. By recreating a range of geologically relevant concentrations of dissolved inorganic carbon (DIC) in the laboratory, we demonstrate that the magnitude of the vital effects in both carbon and oxygen isotopes of coccolith calcite of multiple species relates to ambient DIC concentration. Under high DIC levels, all the examined coccoliths exhibit significantly reduced isotopic offsets from inorganic calcite compared to the substantial vital effects expressed at low (preindustrial and present-day) DIC concentrations. The supply of carbon to the cell exerts a primary control on biological fractionation in coccolith calcite via the modulation of coccolithophore growth rate, cell size and carbon utilisation by photosynthesis and calcification, altogether accounting for the observed interspecific differences between coccolith species. These laboratory observations support the recent hypothesis from field observations that the appearance of interspecific vital effect in coccolithophores coincides with the long-term Neogene decline of atmospheric CO2 concentrations and bring further valuable constraints by demonstrating a convergence of all examined species towards inorganic values at high pCO2 regimes. This study provides palaeoceanographers with a biogeochemical framework that can be utilised to further develop the use of calcareous nannofossils in palaeoceanography to derive sea surface temperature and pCO2 levels, especially during periods of relatively elevated pCO2 concentrations, as they prevailed during most of the Meso-Cenozoic.
Highlights
The quest to generate reliable and accurate palaeoenvironmental reconstructions is hindered by uncertainties in our current proxies from the sedimentary archive
At the highest dissolved inorganic carbon (DIC) concentrations, it appears that the averages between the two duplicates show coccolith calcite δ13Cc values for these two species indistinguishable from that of the inorganic reference; vital effects vanish at high DIC
This positive evolution is linear for C. leptoporus (r2 = 0.83) and C. pelagicus (r2 = 0.85), for the latter largest species the 2 mmol kg−1 datapoints depart from the 4–12 mmol kg−1 linear trend with substantial low δ13C values
Summary
The quest to generate reliable and accurate palaeoenvironmental reconstructions is hindered by uncertainties in our current proxies from the sedimentary archive. In the case of the foraminifera, corals and coccoliths, the foremost carbonate producers in the marine realm, there has been a considerable number of studies during which living organisms were cultured in strictly controlled environmental conditions and their biominerals measured for a range of isotopic systems to generate empirical proxy calibrations (Erez and Luz, 1982; Dudley et al, 1986; Spero et al, 1997; Bemis et al, 1998; Ziveri et al, 2003; Tripati et al, 2010; Rickaby et al, 2010; Rollion-Bard et al, 2011; Grauel et al, 2013; Hermoso et al, 2014; Minoletti et al, 2014; Hermoso, 2015) Another important aim in palaeoceanography is to determine whether the physiology-induced fractionation for a given taxon was constant through time from an evolutionary perspective, and over shorter time intervals comprising large climatic fluctuations, in turn inducing an environmentally driven modulation of the vital effect (Hermoso, 2014). In the absence of more reliable information, the Uniformitarianism principle – by which, the processes that were operating in the geological past still exist today, and vice versa – is commonly applied for elucidating vital effects and reconstructing primary oceanographic signals
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