Environmental and biological controls on size-specific δ13C and δ18O in recent planktonic foraminifera

As living organisms, planktonic foraminifera are not passive tracers of the environment. Their test geochemistry - arguably the single most important resource for paleoceanographic research - reflects the combined signal of environmental, biological and preservational processes. For most species, comparisons of test stable isotopic composition within and among taxa provide the primary means for disentangling the relative influences of these different processes. Here we test the foundations of our paleoceanographic interpretations with the first quantitative comparison of the determinants of carbon and oxygen isotopic variation across multiple ocean basins, studies, and species by re-analyzing size-specific data collated from the literature. We find clear evidence of species-specific biological effects (i.e., vital effects), as the intercepts of size-specific carbon and oxygen isotopic compositions differs significantly amongst species. Trends in body size and isotopic composition, particularly in dinoflagellate bearing taxa, suggest that much of the size-dependent isotopic variation observed in death assemblages (i.e., core-tops and sediments) relates to factors influencing the maximum size obtained by adults rather than ontogeny. The presence and type of photosymbiont hosted (dinoflagellate, chrysophyte or none) was a major factor affecting species- and size-specific δ18O values. In contrast, size-related trends in δ13C values were driven more by depth habitat (mixed layer, thermocline, sub-thermocline) than symbiont ecology. On this broad geographic and oceanographic scale, ocean basin and biome had a significant effect on the isotopic composition of individuals. Our analysis and its model-averaged predictions provide a quantitative basis for interpreting size-specific isotopic variation in 22-species of modern macroperforate planktonic foraminifera. We conclude by highlighting existing data gaps and outstanding questions of the relative influence of environmental, preservational and biological processes on variation in the test geochemistry of planktonic foraminifera.