Calcification intensity in planktonic Foraminifera reflects ambient conditions irrespective of environmental stress

Abstract

Planktonic Foraminifera are important marine calcifiers, and the ongoing change in the oceanic carbon system makes it essential to understand the influence of environmental factors on the biomineralization of their shells. The amount of calcite deposited by planktonic Foraminifera during calcification has been hypothesized to reflect a range of environmental factors. However, it has never been assessed whether their calcification only passively responds to the conditions of the ambient seawater or whether it reflects changes in resource allocation due to physiological stress. To disentangle these two end-member scenarios, an experiment is required where the two processes are separated. A natural analogue to such an experiment occurred during the deposition of the Mediterranean sapropels, where large changes in surface water composition and stratification at the onset of the sapropel deposition were decoupled from local extinctions of planktonic Foraminifera species. We took advantage of this natural experiment and investigated the reaction of calcification intensity, expressed as mean area density (MAD), of four species of planktonic Foraminifera to changing conditions during the onset of Sapropel S5 (126–121 ka) in a sediment core from the Levantine Basin. We observed a significant relationship between MAD and surface water properties, as reflected by stable isotopes in the calcite of Foraminifera shells, but we failed to observe any reaction of calcification intensity on ecological stress during times of decreasing abundance culminating in local extinction. The reaction of calcification intensity to surface water perturbation at the onset of the sapropel was observed only in surface-dwelling species, but all species calcified more strongly prior to the sapropel deposition and less strongly within the sapropel than at similar conditions during the present-day. These results indicate that the high-salinity environment of the glacial Mediterranean Sea prior to sapropel deposition induced a more intense calcification, whereas the freshwater injection to the surface waters associated with sapropel deposition inhibited calcification. The results are robust to changes in carbonate preservation and collectively imply that changes in normalized shell weight in planktonic Foraminifera should reflect mainly abiotic forcing

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…Our results show that for a given species, SNW is not statistically different regarding the hydrography but that SNW varies significantly between the dominant species N. pachyderma, T. quinqueloba and G. bulloides. This suggests that ecological conditions other than the carbon chemistry of ambient seawater at long (Weinkauf et al, 2013) and short time scale (de Villiers, 2004;Marshall et al, 2013), and species physiological characteristics and metabolism might be responsible for the three-fold SNW increase between T. quinqueloba and G. bulloides. This has potentially important implications for the carbon pumps because it implies that planktic foraminifer community composition together with the magnitude of the numerical flux (number of individuals) plays a role in the foraminifer-mediated PIC flux.…”

Size-dependent response of foraminiferal calcification to seawater carbonate chemistry

“…For size-normalisation of shell weight measurements, we follow the "area density" (ρ A ) approach of Marshall et al (2013), as used elsewhere (Weinkauf et al, 2013(Weinkauf et al, , 2016Marshall et al, 2015;Osborne et al, 2016). From each core-top sample, individual specimens (n = 8-26) of G. ruber sensu stricto and sensu lato were picked from discrete sieve size fractions, imaged (umbilical side up), and weighed, and their cross-sectional or silhouette area determined using Macnification (Orbicule Inc., v2.0).…”

“…Our modelling approach shows that there is in fact a strong effect of test size on SNW. Modelled area density (ρ A , a commonly used SNW metric Marshall et al, 2013Marshall et al, , 2015Osborne et al, 2016;Weinkauf et al, 2013Weinkauf et al, , 2016) is shown in Fig. 6a as a function of test diameter.…”

“…We therefore urge caution before attributing open-ocean SNW patterns exclusively to either primary or secondary processes. In sites where carbonate saturation at the site of deposition is high, it is probably reasonable to attribute SNW changes to surface conditions (as in Barker and Elderfield, 2002;Marshall et al, 2013;Weinkauf et al, 2013;Osborne et al, 2016, etc.). At depths approaching the lysocline, our data support the earlier uses of SNW as a proxy for the deep sea carbonate system (e.g.…”

“…However, shell size can vary within a studied sieve range (Beer et al, 2010a). Many later studies circumvented this issue by directly measuring the major axis length (Barker and Elderfield, 2002;Aldridge et al, 2012;Beer et al, 2010a) or cross-sectional area (Beer et al, 2010b;Marshall et al, 2013Marshall et al, , 2015Osborne et al, 2016;Weinkauf et al, 2013Weinkauf et al, , 2016 of each individual within a sampled population. However, as discussed by Weinkauf et al (2016), an assumption common to most shell-weight studies is that SNW metrics themselves do not vary as a function of size -which is unlikely to be true.…”