Testing Three new Mg/Ca-Temperature Calibrations for Arctic, Benthic Foraminifera: A High Resolution Study of the Last two Millennia.

Since the mid-1990s foraminiferal Mg/Ca studies have been growing more important for paleo-seawater reconstructions. A distinct advantage of the Mg/Ca method is the opportunity to reconstruct temperature and salinity of seawater from the same medium, by paired measurements of Mg/Ca and δ 18O. Calibrations of Mg/Ca ratio against temperature are reasonably well-constrained for temperatures above 10° C but are less well developed for cooler temperatures, thus limiting the methods applicability to high-latitude studies.

We have developed preliminary calibrations for three common, arctic, benthic foraminifera (Melonis barleeanus, Cassidulina neoteretis, and Islandiella norcrossi/helenae) by using modern, sediment-surface samples from the Iceland and Greenland margins (spatial bottom-temperature gradient of 0-9° C). We use a set of surface grab-samples collected from the Iceland margin (cruise B997) and the Greenland margin (cruise BS1191). All samples were stained with Rose Bengal upon collection. Both stained and unstained individuals were used due to a limited number of stained individuals in the samples. Before analysis each Mg/Ca sample (60-100 individuals) went through rigorous cleaning (Boyle and Keigwin, 1985/1986; Boyle and Rosenthal,1996).

Of the three species, M. barleeanus showed the greatest temperature sensitivity and was fitted with an exponential relationship between Mg/Ca and temperature, while both C. neoteretis and I. norcrossi/helenae showed a lower temperature sensitivity and were fitted with a linear relationship in the range of 0-7° C.

To test our new calibrations we have reconstructed a 2000-yr temperature record from a well studied and dated core (MD99-2269) from the NW-Iceland shelf. Where available all three benthic species were analyzed. We compare our reconstructions to carbonate content, oxygen isotope, diatom and coccolith temperature and environmental reconstructions from the same core. During this interval in the core we expect to see climate signals from the Little Ice Age and the Medieval Warm Period.