TY - JOUR
T1 - A model of Phanerozoic cycles of carbon and calcium in the global ocean
T2 - Evaluation and constraints on ocean chemistry and input fluxes
AU - Locklair, Robert E.
AU - Lerman, Abraham
N1 - Funding Information:
This work was supported by NSF grants OAR-0002889 and EAR-02-23509, and the Arthur L. Howland Fund of the Department of Geological Sciences, Northwestern University. We thank Robert A. Berner (Yale University) and Klaus Wallmann (Leibniz-Institut für Meereswissenschaften, Universität Kiel) whose critical reviews helped improve the paper, Fred T. Mackenzie (University of Hawaii) and Bruce H. Wilkinson (University of Michigan) for reviews and helpful comments on an earlier version of this manuscript, Richard E. Zeebe (University of Hawaii) for making available a paper in press, and Robert Berner for making available unpublished data. [LW]
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.
PY - 2005/4/15
Y1 - 2005/4/15
N2 - The relationships between the global carbon cycle and paleo-climates on short and long time scales have been based on studies of accumulation rate of the two main components of the sedimentary carbon reservoir, organic carbon and carbonate carbon. Variations in the rate and proportion of carbonate burial through Phanerozoic time have been attributed to the effects of tectonics on eustasy, atmospheric CO2 concentration, MOR (Mid-Ocean Ridge) hydrothermal flux, and weathering and riverine flux. This study addresses the history of variations in the state of the surface ocean and its degree of saturation with respect to calcite and aragonite, based on a geochemical model that considers the Phanerozoic atmospheric PCO2 and surface ocean temperature reconstructions as the main forcings on the system. The results show that, using near-present-day values of ocean salinity and alkalinity, the Early Paleozoic and Middle Mesozoic oceans are calculated to be undersaturated (or nearly undersaturated) with respect to CaCO3. For the near-present-day values of supersaturation (ω=ICP/Ksp) of 3-5 with respect to calcite, paleo-alkalinity of ocean water would have been up to 2.5 times greater than at present, although the pH values of surface ocean water would have been somewhat lower than the present values. This alkalinity factor is consistent with a higher calcium concentration (up to ×2.5) due to increased circulation at ocean spreading-zones and also higher salinity (up to ×1.5) attributed by other authors to segments of the geologic past. Our model results indicate that although PCO2 was a contributing factor to shifts between calcite and aragonite saturation of seawater, additional changes in alkalinity were needed to maintain supersaturation at the level of 3-5, comparable to the present. Continental weathering of crystalline and older carbonate rocks, in addition to MOR (Mid-Ocean Ridge) circulation, was likely an important mechanism for maintaining supersaturation of surface ocean water, particularly during times of increased carbonate storage.
AB - The relationships between the global carbon cycle and paleo-climates on short and long time scales have been based on studies of accumulation rate of the two main components of the sedimentary carbon reservoir, organic carbon and carbonate carbon. Variations in the rate and proportion of carbonate burial through Phanerozoic time have been attributed to the effects of tectonics on eustasy, atmospheric CO2 concentration, MOR (Mid-Ocean Ridge) hydrothermal flux, and weathering and riverine flux. This study addresses the history of variations in the state of the surface ocean and its degree of saturation with respect to calcite and aragonite, based on a geochemical model that considers the Phanerozoic atmospheric PCO2 and surface ocean temperature reconstructions as the main forcings on the system. The results show that, using near-present-day values of ocean salinity and alkalinity, the Early Paleozoic and Middle Mesozoic oceans are calculated to be undersaturated (or nearly undersaturated) with respect to CaCO3. For the near-present-day values of supersaturation (ω=ICP/Ksp) of 3-5 with respect to calcite, paleo-alkalinity of ocean water would have been up to 2.5 times greater than at present, although the pH values of surface ocean water would have been somewhat lower than the present values. This alkalinity factor is consistent with a higher calcium concentration (up to ×2.5) due to increased circulation at ocean spreading-zones and also higher salinity (up to ×1.5) attributed by other authors to segments of the geologic past. Our model results indicate that although PCO2 was a contributing factor to shifts between calcite and aragonite saturation of seawater, additional changes in alkalinity were needed to maintain supersaturation at the level of 3-5, comparable to the present. Continental weathering of crystalline and older carbonate rocks, in addition to MOR (Mid-Ocean Ridge) circulation, was likely an important mechanism for maintaining supersaturation of surface ocean water, particularly during times of increased carbonate storage.
KW - Alkalinity
KW - Calcium carbonate
KW - Carbon cycle
KW - Carbonate minerals saturation
KW - Phanerozoic
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UR - http://www.scopus.com/inward/citedby.url?scp=15344342554&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2004.12.010
DO - 10.1016/j.chemgeo.2004.12.010
M3 - Article
AN - SCOPUS:15344342554
VL - 217
SP - 113
EP - 126
JO - Chemical Geology
JF - Chemical Geology
SN - 0009-2541
IS - 1-2
ER -