Determining and improving the analytical blank for radiocarbon analyses of small foraminifera samples using MICADAS and the gas interface system

Foraminifera isolated from deep-sea sediments are among the most common materials in AMS radiocarbon analysis. These results are used to determine accurate age models for sediment sequences as well as to detect changes in deep-sea ventilation. Often, only small numbers of (monospecific) foraminifera shells can be isolated, in particular when studying benthic species in sediments from the polar regions. Therefore, these samples are often analyzed as CO₂ gas using MICADAS instruments, and the method can typically be used for samples of up to around 40 ka in age. For reliable results, an accurate determination and minimization of processing blanks is required.

Processing blanks for foraminifera samples may in part derive from acid hydrolysis of the carbonates. It has, however, been shown that contamination of the carbonate fossils, mainly from atmospheric CO₂ adsorbed on the porous surfaces of foraminifera, is the largest source of blank found in foraminifera samples. The removal of such contamination has been attempted by various leaching methods, which come at the risk of introducing additional contaminations. Alternatively, blank correction of AMS results may be achieved using fossil foraminifera from ancient deposits much beyond the range of the radiocarbon method.

Here we report results of a systematic test comparing the F¹⁴C levels obtained for fossil (>130 ka) and sub-modern monospecific planktic and benthic foraminifera samples using different blank correction approaches. Specifically, we compare leaching with dilute hydrochloric acid, blank correction relative to a leached and an un-leached fossil foraminifera standard, and blank correction relative to the IAEA-C1 certified carbonate standard.