BEYOND SYMBIOSIS: WHAT DOES THE δ13C:SIZE RELATIONSHIP OF PLANKTONIC FORAMINIFERA REALLY INDICATE?

The evolutionary history of symbiosis in planktonic foraminifera is largely inferred from stable isotope data, with high δ¹³C and a positive δ¹³C:size relationship being interpreted as indicators of photosymbiosis. Likewise, changes in these isotopic signatures are hypothesized to represent changes in the habitat or symbiont biology of the foraminifera, such as symbiont bleaching in response to rapid ocean warming. However, recent work has shown how these isotopic signatures can be affected by a range of factors, including morphological and behavioral variables such as the presence of spines and associated symbiont streaming behaviors. We review lesser-known controls on the δ¹³C signatures of planktonic foraminifera. Using a new chemical model of vital effects, we further show how changes in pH and CO₂ diffusion in the foraminifera’s microenvironment and within the calcifying fluid can produce anomalous δ¹³C signatures and can explain previously enigmatic observations such as the “metabolic” vital effect, the presence of symbiont-like δ¹³C signatures in some asymbiotic foraminifera, and the negative δ¹³C:size relationships observed in many Cretaceous foraminifera.