TY - JOUR
T1 - Towards an understanding of the Ca isotopic signal related to ocean acidification and alkalinity overshoots in the rock record
AU - Fantle, Matthew S.
AU - Ridgwell, Andy
N1 - Funding Information:
The CrunchTope work could not have been completed without the sage advice, and considerable patience, of Dr. J. Druhan. MSF also thanks Ben Barnes for extremely helpful, and wide-ranging, discussions, and for sharing his opinion and specific suggestions for revisions of this manuscript. The authors thank one anonymous reviewer, Andy Jacobson, Guest Editor Elizabeth Griffith, and Editor Don Porcelli for helpful comments and advice on revisions. A. Ridgwell acknowledges funding from the European Research Council as part of the “PALEOGENiE” project ( ERC-2013-CoG-617313 ) as well as from the Heising Simons Foundation ( 2015-145 ) and the National Science Foundation ( 1658024 ).
Publisher Copyright:
© 2020 Elsevier B.V.
PY - 2020/8/5
Y1 - 2020/8/5
N2 - In this contribution, we explore the idea that the Ca isotope proxy has utility as an indicator of carbonate authigenesis (i.e., post-depositional precipitation of CaCO3 within the sedimentary package). Given the strong contrast in isotopic fractionation factor between the formational and diagenetic environments, Ca isotopes have the potential to fingerprint carbonate authigenesis when it occurs close to the seawater-sediment interface. We demonstrate that Ca isotopes are particularly applicable to exploring ocean acidification events, and potentially ocean anoxic events, and focus our attention on ocean acidification related to the Paleocene-Eocene Thermal Maximum (PETM). We present three scenarios that vary in the magnitude and duration of the carbon fluxes that are simulated using an Earth System model of intermediate complexity (cGENIE), and we use the cGENIE output to constrain the upper boundary conditions of 1-D reactive transport models of authigenesis and recrystallization in the sedimentary section. Along with simple mixing calculations, the models inform our exploration of the hypothesis that authigenic carbonate induced by a saturation state overshoot during the PETM explains recently published Ca isotope records, and perhaps bulk carbonate records over Ocean Anoxic Event (OAE) 2. Our simulations suggest that fractionation factor variability does not explain the PETM δ44Ca records, and we propose a δ44Ca-CaCO3 space framework to assist with the elucidation of authigenic additions over time scales that are short relative to the residence time of Ca in the ocean (~1 Ma). Ultimately, we find that the ‘authigenic zone’ generated in the sedimentary column may be influenced by alkalinity overshoots or redox state; the CaCO3 produced in this zone can overprint temporal signals with depth-dependent signals that reflect lithology and sedimentation rate and need not be spatially uniform, even when driven by a global event. Ultimately, we demonstrate the utility of Ca isotopes for exploring short time scale climatic events and a quantitative framework to guide interpretations.
AB - In this contribution, we explore the idea that the Ca isotope proxy has utility as an indicator of carbonate authigenesis (i.e., post-depositional precipitation of CaCO3 within the sedimentary package). Given the strong contrast in isotopic fractionation factor between the formational and diagenetic environments, Ca isotopes have the potential to fingerprint carbonate authigenesis when it occurs close to the seawater-sediment interface. We demonstrate that Ca isotopes are particularly applicable to exploring ocean acidification events, and potentially ocean anoxic events, and focus our attention on ocean acidification related to the Paleocene-Eocene Thermal Maximum (PETM). We present three scenarios that vary in the magnitude and duration of the carbon fluxes that are simulated using an Earth System model of intermediate complexity (cGENIE), and we use the cGENIE output to constrain the upper boundary conditions of 1-D reactive transport models of authigenesis and recrystallization in the sedimentary section. Along with simple mixing calculations, the models inform our exploration of the hypothesis that authigenic carbonate induced by a saturation state overshoot during the PETM explains recently published Ca isotope records, and perhaps bulk carbonate records over Ocean Anoxic Event (OAE) 2. Our simulations suggest that fractionation factor variability does not explain the PETM δ44Ca records, and we propose a δ44Ca-CaCO3 space framework to assist with the elucidation of authigenic additions over time scales that are short relative to the residence time of Ca in the ocean (~1 Ma). Ultimately, we find that the ‘authigenic zone’ generated in the sedimentary column may be influenced by alkalinity overshoots or redox state; the CaCO3 produced in this zone can overprint temporal signals with depth-dependent signals that reflect lithology and sedimentation rate and need not be spatially uniform, even when driven by a global event. Ultimately, we demonstrate the utility of Ca isotopes for exploring short time scale climatic events and a quantitative framework to guide interpretations.
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U2 - 10.1016/j.chemgeo.2020.119672
DO - 10.1016/j.chemgeo.2020.119672
M3 - Article
AN - SCOPUS:85085273335
SN - 0009-2541
VL - 547
JO - Chemical Geology
JF - Chemical Geology
M1 - 119672
ER -
