Varying results using the shell weights of different planktonic foraminiferal species as a proxy for changing carbonate ion concentrations in the Cariaco Basin, Venezuela

Ocean acidification, due in part to increased atmospheric CO2, is thought to reduce calcification rates of planktonic foraminifera. A positive linear relationship has been found between foraminiferal shell weight and the carbonate ion concentrations [CO32-] of the waters in which they calcify (Barker and Elderfield, 2002). Such a relationship provides a useful proxy for determining past changes in ocean carbonate chemistry. The Cariaco Basin, Venezuela (10°30' N, 65°31 W) is an ideal study area to determine the relationship between seawater [CO32-] and planktonic foraminiferal shell weight as the region is characterized by the seasonal upwelling of low pH, low temperature, and low carbonate ion concentrated waters. In this study we use biweekly sediment trap samples and concurrent hydrographic measurements collected between January 2005 and May 2010 in the Cariaco Basin to assess the relationship between [CO32-] and shell weight of three planktonic foraminiferal species, Globigerinoides ruber, Orbulina universa and Globigerinoides sacculifer. Measured salinity, pH, sea surface temperature (SST) and total alkalinity (TA) were used to estimate pCO2 and [CO32-]. Size-normalized shells of G. ruber, O. universa and G. sacculifer were picked (> 10 per sample), photographed for size analysis and weighed. Globigerinoides ruber shell weights varied from 10.4 μg to 17.7 μg, with an average weight of 14.62 μg from 2005 to 2007, with concurrent surface water [CO32-] ranging from ~220-280 μmol/kg. Orbulina universa shell weights ranged from 35.4 μg to 67.8 μg with an average shell weight of 50.1 μg. Preliminary data reveal a strong positive linear relationship between foraminiferal shell weight and [CO32-] for G. ruber (R2 = 0.52, 99% confidence level; p = 0.007) but no significant relationship for O. universa (R2 = 0.05, ; p = 0.4). The initial results suggest that the foraminiferal shell weight changes of certain planktonic foraminiferal species may reflect changes in the carbonate system better than others.