Calcification of the planktonic foraminifera Globigerina bulloides and carbonate ion concentration: Results from the Santa Barbara Basin

Abstract

Planktonic foraminiferal calcification intensity, reflected by shell wall thickness, has been hypothesized to covary with the carbonate chemistry of seawater. Here we use both sediment trap and box core samples from the Santa Barbara Basin to evaluate the relationship between the calcification intensity of the planktonic foraminifera species Globigerina bulloides, measured by area density (µg/µm2), and the carbonate ion concentration of seawater ([CO32−]). We also evaluate the influence of both temperature and nutrient concentration ([PO43−]) on foraminiferal calcification and growth. The presence of two G. bulloides morphospecies with systematically different calcification properties and offset stable isotopic compositions was identified within sampling populations using distinguishing morphometric characteristics. The calcification temperature and by extension calcification depth of the more abundant “normal” G. bulloides morphospecies was determined using δ18O temperature estimates. Calcification depths vary seasonally with upwelling and were used to select the appropriate [CO32−], temperature, and [PO43−] depth measurements for comparison with area density. Seasonal upwelling in the study region also results in collinearity between independent variables complicating a straightforward statistical analysis. To address this issue, we use additional statistical diagnostics and a down core record to disentangle the respective roles of each parameter on G. bulloides calcification. Our results indicate that [CO32−] is the primary variable controlling calcification intensity while temperature influences shell size. We report a modern calibration for the normal G. bulloides morphospecies that can be used in down core studies of well‐preserved sediments to estimate past [CO32−].

Size-dependent response of foraminiferal calcification to seawater carbonate chemistry

“…This response is of the same magnitude as the area density response to carbonate system changes reported in other studies (e.g. Marshall et al, 2013;Osborne et al, 2016). Higher overall ρ A observed by Marshall et al (2013) for T. sacculifer in larger size fractions may constitute Figure 6.…”

“…ρ A was calculated as mass (in µg) over silhouette area (in mm 2 ). As in Marshall et al (2013) and Osborne et al (2016), the mean ρ A and silhouette area in each core-top sample was then used in multivariate statistical analysis.…”

“…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.…”

“…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).…”

“…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.…”