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The CAMECA IMS-1280 Secondary Ion Mass Spectrometer housed in the WiscSIMS lab, Department of Geoscience, University of Wisconsin-Madison. (Image credit: P. Kuhl)
"Conventional" Analytical Approaches
"Conventional" Analytical Approaches
Fossil shell of the species M. velascoensis (~56 Ma). The blade-shape muricae are diagenetic precipitates and bias paleoclimate records towards colder water temperatures when the whole shell is analyzed by conventional approaches. In situ analytical approaches can exclude these unwanted phases from analysis.
Most of our knowledge of the Earth's climate during the past ~70 million years is deduced from minute (typically less than 0.5 mm diameter) foraminifera shells. These microfossils, which are ubiquitous in sea floor sediments, record the environmental conditions that prevailed during their lifetime in the chemical and isotopic composition of their calcite shells. Since more than 60 years, scientist analyze these shells to reconstruct the Earth's past climate. While foraminiferal shells faithfully record the Earth's past climate over tens of millions of years, their preservation is typically degrading with increasing age and burial depth, which may also modify their original isotopic and chemical composition. My research group focuses on the application of new analytical approaches to analyse small domains within foraminifera shells that are less affected by alteration than the remaining material, thereby further improving the veracity of paleoclimate records.
The image to the left shows a ~56 Ma foraminifera shell featuring large diagenetic overgrowths ('blade-shaped' muricae) that were formed in the sediment column at much cooler temperatures. These diagenetic phases, which cannot be separated in conventional analytical approaches, bias the resulting paleoclimate record towards cooler temperatures.
In situ analytical approaches
In situ analytical approaches
In situ ("in its original place") technologies allow to analyse material at its original location within the sample. Thus, the sample can be carefully investigated using various imaging techniques to identify the most suitable domains for analyses. For δ18O and δ13C analyses by Secondary Ion Mass Spectrometry (SIMS) and the measurement of Mg/Ca and Sr/Ca ratios by Electron Probe Microanalyser (EPMA), foraminifera shells are embedded in epoxy and polished to midsection. This preparation technique exposes minute domains encapsulated within the chamber wall that were not in direct contact with pore-water in the sediment column and may be better preserved than the remaining shell.
For in situ analysis, shells are embedded in epoxy resin and ground to midsection. The most suitable (best-preserved) domains are identified by SEM imaging, and subsequently analyzed by SIMS or EPMA.
About 130 foraminifera shells embedded in a 1-inch gold-coated epoxy mount.
SEM image of ~200 foraminifera shells polished to midsection. Each shell was imaged by SEM to locate suitable domains for analysis.
Polished cross section of M. velascoensis. The diagenetic muricae-blades are clearly visible. A suitable domain for in situ analysis, away from the diagenetic muricae-blades, is highlighted.
SEM (backscattered electron) image of a foraminifera chamber wall in cross section. The black ellipse is a 10 µm-diameter SIMS analysis pit for δ18O. The texture visible in the image provides preservational information: Better-preserved domains typically feature a microgranular texture, similar to that observed in modern life-collected or cultured shells, whereas the diagenetic precipitates on the outer chamber wall are composed from much larger crystals.
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