Abstract
Planktonic foraminifera are calcifying protists used extensively as proxies in reconstructing paleoenvironmental conditions during calcification, as the ambient seawater's isotopic signatures are recorded in their tests. Test compositions of oxygen and carbon isotopes reflect the seawater temperature and carbonate ion concentration present during calcification, respectively. Globigerinoides ruber (white) is a highly successful planktonic, symbiont-bearing species, dwelling in tropical, subtropical, and transitional waters worldwide. Its vast distribution and shallow calcification depth (0-50 m) facilitate its use for the global analysis of surface mixed layer depth chemistry. Some studies comparing the species' morphotypes (sensu stricto and sensu lato) have identified variations in shell isotopic composition, suggesting potential differences in habitat depth among morphotypes and stressing a need to separate the morphotypes for analysis. However, studies from other regions have contradicted these results, asserting that depending upon the location and hydrographic dynamics at that study site, separation of morphotypes is not necessary. Previous studies have compared the isotopic composition of morphotype G. ruber (w) in the Pacific Ocean, China Sea, Indian Ocean, and Gulf of Mexico. The chromotype Globigerinoides ruber (pink), which lacks morphotypes, has been examined in the Sargasso Sea. To make best use of G. ruber (w) as a global proxy in climate reconstructions, it is imperative to understand the species' life habits in all environments and impacts on shell composition of changes in ambient water chemistry. The study uses preserved shells of Sargasso Sea G. ruber (w) from 1998-2010 that were collected by the Oceanic Flux Program (OFP) sediment trap time-series. Samples were analyzed for morphology, shell areal density, and stable carbon and oxygen isotope composition. Bermuda Atlantic Time-Series (BATS) hydrographic data were used to determine concurrent ambient water chemistry. Use of OFP samples and BATS data allows us to determine if G. ruber (w) shell chemistry significantly records surface [CO₃²⁻] and establishes if the morphotypes accurately (1) reflect seasonal variability of stable isotopes in the Sargasso Sea, (2) record surface carbonate ion concentration [CO₃²⁻], and (3) if there is a significant relationship between [CO₃²⁻] and shell density.