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New calibrations of the Mg/Ca-paleothermometer and their testing by the reconstruction of Late Pliocene paleoceanographic and paleoclimatic changes

Final Activity Report Summary - PLIO-CLIMATE (New calibrations of the Mg/Ca-paleothermometer and their testing by the reconstruction of Late Pliocene paleoceanographic and paleoclimatic changes)

Understanding the evolution of ocean temperature, ocean salinity and ice-sheet dynamics, as an active component of Earth's climate system, is fundamental to assess and quantify changes in ocean circulation and climate. However, to understand past changes in climate and oceanography it is essential to determine the past record of temperature and salinity of ocean surface and deep waters. Here, the development of the Mg/Ca paleothermometer has the potential to record relatively small changes in water temperature, and, through paired measurements of Mg/Ca and d18O on the same shells, adjust for the temperature-dependency of d18O and to isolate salinity and ice-volume effects. There are, however, still uncertainties in the applicability of this proxy, i.e. species-specific vital effects or intra-specific ontogenetic variability. Therefore, the Marie Curie project consisted of two parts:
(1) assessment of the applicability of Mg/Ca in planktic foraminifera and,
(2) use of Mg/Ca in a study to reconstruct paleoceanographic changes in the Late Pliocene North Atlantic.

For the first part of the project, we have generated Mg/Ca data for the most abundant planktic foraminiferal species of two core top samples from the North Atlantic Ocean on a wide range of narrow test-size fractions to show the development of Mg/Ca ratios for all post-juvenile ontogenetic stages of various species. Our data show that Mg/Ca changes significantly with test size but different in single species. These inter-specific differences are related to ecology and depth habitat. Furthermore, plankton tow samples were used to analyse the influence of water depth on Mg/Ca ratios in the North Atlantic and Arabian Sea in order to detect possible dissolution during sinking of the tests. These data show no dissolution effect on Mg/Ca in the Arabian Sea but strong effects in the North Atlantic, most reasonably explained by local oceanographic changes and the population dynamic behaviour of individual foraminiferal species. The combination of data sets from water column and surface sediments displays the effect of ecology, ontogeny, and taphonomy on the Mg/Ca ratio of planktic foraminifera. This ultimately leads to a better understanding of the Mg/Ca proxy and a more reliable use of Mg/Ca paleothermometry in paleoceanography.

For the second part of the project we have generated high-resolution planktic and benthic foraminiferal stable isotope (d18O, d13C) and Mg/Ca records from the Late Pliocene (MIS 96-M2) sediments from the North Atlantic (IODP Site 1313). Combining these data sets enables us to estimate temperature changes and ice-volume/sea-level changes on millennial time-scales. Our high-resolution records (~400 years resolution) show a strong imprint of a 1,500-year cycle, most likely comparable to Dansgaard-Oeschger cycles of the Pleistocene. We can demonstrate that millennial-scale climate variability prevails regardless of (inter)glacial state, and that this variability is not significantly amplified during the major Late Pliocene glacials and their associated sea level lowstands. This implies that a mechanism external to the climate system has paced millennial-scale variability throughout the Plio-Pleistocene.