How confident can we be in climate reconstructions based on foraminifer shell chemistry?

Date & time

1–2pm 23 February 2017

Location

Jaeger Seminar Room (1st Floor, Jaeger 1, RSES, Mills Road, ANU)

Speakers

Kate Holland (RSES)

Event series

Contacts

 Jon Pownall
 0435314031

The geochemistry of planktic foraminifer shells are among our most important proxies for reconstructing past ocean conditions and climate. These proxies are based on empirical correlations between shell trace element and stable isotope compositions and ocean chemistry and state variables. If we are to be confident in our ability to evaluate past ocean conditions using these proxies we need to be certain of causes of geochemical variation in foraminiferal shell calcite. I have cultured Orbulina universa, a model planktic foraminifer, to investigate how the widely used Mg/Ca-seawater temperature and B/Ca-seawater carbonate chemistry proxies record the variables, they are thought to record. My investigations have examined the behavior of these proxies over linked temporal and spatial scales ranging from diurnal micron scale variations within individual shells, through to the effects of seawater chemistry variation over millions of years on bulk shell populations. I show how seawater carbonate system changes in the foraminiferal microenvironment, due to respiration, photosynthesis and calcification induce B/Ca and U/Ca and probably also cause Mg/Ca banding within shells. I attempt to isolate specific carbonate system controls on bulk shell B/Ca chemistry and find shell growth kinetics are unlikely to account for bulk shell B/Ca compositional variation. Finally, I show how the sensitivity of the Mg/Ca seawater thermometer change with seawater chemistry, and how existing Mg/Ca thermometers underestimate sea surface temperature variations across major climate transitions in the past (e.g. the Paleocene-Eocene Thermal Maximum)

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