Planktonic foraminiferal area density as a proxy for carbonate ion concentration: A calibration study using the Cariaco Basin ocean time series

Abstract

AbstractBiweekly sediment trap samples and concurrent hydrographic measurements collected between March 2005 and October 2008 from the Cariaco Basin, Venezuela, are used to assess the relationship between [CO32−] and the area densities (ρA) of two species of planktonic foraminifera (Globigerinoides ruber (pink) and Globigerinoides sacculifer). Calcification temperatures were calculated for each sample using species‐appropriate oxygen isotope (δ18O) temperature equations that were then compared to monthly temperature profiles taken at the study site in order to determine calcification depth. Ambient [CO32−] was determined for these calcification depths using alkalinity, pH, temperature, salinity, and nutrient concentration measurements taken during monthly hydrographic cruises. The ρA, which is representative of calcification efficiency, is determined by dividing individual foraminiferal shell weights (±0.43 µg) by their associated silhouette areas and taking the sample average. The results of this study show a strong correlation between ρA and ambient [CO32−] for both G. ruber and G. sacculifer (R2 = 0.89 and 0.86, respectively), confirming that [CO32−] has a pronounced effect on the calcification of these species. Though the ρA for both species reveal a highly significant (p < 0.001) relationship with ambient [CO32−], linear regression reveals that the extent to which [CO32−] influences foraminiferal calcification is species specific. Hierarchical regression analyses indicate that other environmental parameters (temperature and [PO43−]) do not confound the use of G. ruber and G. sacculifer ρA as a predictor for [CO32−]. This study suggests that G. ruber and G. sacculifer ρA can be used as reliable proxies for past surface ocean [CO32−].