Modelling Changes of the Paleogene Ca Budget Using Benthic Foraminifera

Abstract:

Understanding the earth´s climate as well as the oceanic chemical and isotopic evolution in the past is one of the main aims in earth science. Ca as one of the major elements in the ocean is especially important. Its variation in concentration are controlled by different factors including the CO₂ concentration of the atmosphere, continental weathering and Ca carbonate sedimentation. We used samples from IODP Exp. 320/321 to establish a δ^44/40Ca paleo-seawater record between 45 and 25 Ma and model changes in the Ca budget through time. Our results show differences in the Eocene and Oligocene Ca isotope record of benthic foraminifers. The δ^44/40Ca values during the Eocene are relatively constant with no significant fluctuations during phases of large short term CCD fluctuations^[1]. The Oligocene is characterized by sediments with uniformly high carbonate content and increasing δ^44/40Ca towards the late Oligocene. Past seawater δ^44/40Ca values (Fig. 1) were calculated from the measured benthic foraminifer record applying the calibration for Gyroidinoides spp.^[2]. The Ca budget during the Eocene is relatively constant and not affected by short term CCD fluctuations, indicating that they are too small to alter the isotopic Ca budget. The Oligocene, in contrast is characterized by a general increase in δ^44/40Ca seawater values and a continuously deep CCD^[1]. This is consistent with a massive long term (>1Ma) CaCO₃ deposition and decreasing Ca concentration in the ocean water. To examine the preservation (dissolution and recrystallization) of the foraminifer test through time, we studied additionally the changes in the crystallographic orientations trough time by Electron Backscatter Diffraction (EBSD) analysis and Raman spectroscopy. As a final step we use our δ^44/40Ca seawater record to run a combined Ca and C model showing the effect of Ca weathering input, carbonate remobilization and dolomitization on the Ca and carbonate system of seawater ^[1].

^[1]Pälike H., Mitchell W. Lyle, Hiroshi Nishi, Isabella Raffi, Andy Ridgwell, Kusali Gamage, Adam Klaus et al. (2012): A Cenozoic record of the equatorial Pacific carbonate compensation depth. Nature 488, 609–614

^[2]Gussone N. & Filipsson H.L. (2010): Calcium isotope ratios in calcitic tests of benthic Foraminifers. Earth and Planetary Science Letters 290, 108–117