Environmental controls on planktonic foraminiferal diversity in ancient and modern oceans

Abstract

Planktonic foraminifera are unicellular zooplankton, whose calcium carbonate ‘shells’, wide geographic distributions and very large population sizes combine to give them perhaps the best fossil record over the last 66 Ma of any group. Site-level assemblage diversity can be estimated comparably in the Recent and through geological time. In this thesis I model the environmental correlates of coretop (Recent) planktonic foraminiferal diversity (Chapter 2), with the aim of applying the model to the Eocene. Present-day diversity patterns are shaped by a richer combination of factors than suggested by previous work. I assess the potential of several non-biological biases to distort diversity patterns (Chapter 3). Functional and evolutionary diversity are less prone to bias than are species richness and evenness, while water depth has little impact on diversity in sites deeper than 500m. Asexuality has been suggested as an adaptation in low diversity environments. I used NanoCT scans of proloculi to test whether Neogloboquadrina pachyderma, the dominant species in polar waters, contains an asexual morph (Chapter 4), finding no support for this hypothesis. Having dealt with potential sources of bias, I use models from Chapter 2 to predict diversity in another time period, the Eocene (Chapter 5), based on current understanding of Eocene environments. The latitudinal gradient of species richness developed through the Eocene in both planktonic foraminifera and coccolithophores. Predicted and observed diversity fit well in the late – but not the early – Eocene. My analyses support two explanations for the early-Eocene mismatch. First, early Eocene climate model estimates of environment differ from the proxy records (which fit the fossil data better). Second, the intercorrelations among facets of diversity have changed through time (Chapter 6). Despite our limited understanding of some aspects of their biology, planktonic foraminifera have much to offer as a model system for macroevolution.