Ocean acidification has wide ranging implications for the ocean-earth-climate system. The pace of this perturbation and its impact on biogeochemical cycles can be constrained through records of past ocean acidification events. To generate these records, however, the paleoceanographic community needs accurate proxies for the marine carbonate system.
In planktic foraminifera, a widely used archive, empirical calibrations show that test boron/calcium, boron isotopes, and uranium/calcium each vary with pH. Despite the utility of these and other proxies, it is clear that proxy behavior is impacted by a host of environmental and biological factors. Work over the last decade, initiated by Steve Eggins, Aleksey Sadekov, and co-workers, has identified a major component of this proxy variability: skeletal composition changes dramatically between day and night in planktic forams, even when individuals are cultured with all conditions other than light kept constant. This unexplained variability dwarfs typical environmental sensitivity and can complicate the interpretation of marine records.
As part of a comprehensive focus on foram biomineralization, we are mapping the response of multiple proxies to diunrnal variability, quantifying the role of ion transport in foraminifera, and testing if crystal growth rate effects can explain micron-scale compositional variability. To measure the diurnal variability of paleo pH proxies we developed a new method that can un-mix day calcite from night calcite in dissolved foram samples. This alternative to microanalysis yields high-precision measurements of the pH proxies B/Ca and U/Ca, as well as a host of additional diagnostic tracers including Mg/Ca, Sr/Ca, δ44Ca, Li/Ca, and Ba/Ca.
We are using correlations between these proxies to identify the main factors controlling foram composition while simultaneously mapping the effects of these processes onto pH-proxies. Then, following a strategy developed in deep-sea coral, we plan to use this mechanistic foundation together with multiple proxy measurements to undo the diurnal effect and correct proxy variability – improving records of paleo ocean acidification events.