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A 20 million year record of planktic foraminiferal B/Ca ratios: Systematics and uncertainties in pCO_2 reconstructions

Tripati, Aradhna K. and Roberts, Christopher D. and Eagle, Robert A. and Li, Gaojun (2011) A 20 million year record of planktic foraminiferal B/Ca ratios: Systematics and uncertainties in pCO_2 reconstructions. Geochimica et Cosmochimica Acta, 75 (10). pp. 2582-2610. ISSN 0016-7037.

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We use new and published data representing a 20 million long record to discuss the systematics of interpreting planktic foraminiferal B/Ca ratios. B/Ca-based reconstructions of seawater carbonate chemistry and atmospheric pCO_2 assume that the incorporation of boron into foraminiferal tests can be empirically described by an apparent partition coefficient, K_D = [B/Ca_(CaCO_3)]/[B(OH)¯_4]/[HCO¯_3]_seawater (Hemming and Hanson, 1992). It has also been proposed that there is a species-specific relationship between K_D and temperature (Yu et al., 2007). As we discuss, although these relationships may be robust, there remain significant uncertainties over the controls on boron incorporation into foraminifera. It is difficult to be certain that the empirically defined correlation between temperature and K_D is not simply a result of covariance of temperature and other hydrographic variables in the ocean, including carbonate system parameters. There is also some evidence that K_D may be affected by solution [HCO¯_3]/[CO^(2-)_3] ratios (i.e., pH), or by [CO^(2-)_3. In addition, the theoretical basis for the definition of KD and for a temperature control on K_D is of debate. We also discuss the sensitivity of pCO_2 reconstructions to different K_D–temperature calibrations and seawater B/Ca. If a K_D–temperature calibration is estimated using ice core pCO_2 values between 0 and 200 ka, B/Ca ratios can be used to reasonably approximate atmospheric pCO_2 between 200 and 800 ka; however, the absolute values of pCO_2 calculated are sensitive to the choice of K_D–temperature relationship. For older time periods, the absolute values of pCO_2 are also dependent on the evolution of seawater B concentrations. However, we find that over the last 20 Ma, reconstructed changes in declining pCO_2 across the Mid-Pleistocene Transition, Pliocene glacial intensification, and the Middle Miocene Climate Transition are supported by the B/Ca record even if a constant coretop K_D is used, or different K_D–temperature calibrations and models of seawater B evolution are applied to the data. The inferred influence of temperature on K_D from coretop data therefore cannot itself explain the structure of a published pCO_2 reconstruction (Tripati et al., 2009). We conclude the raw B/Ca data supports a coupling between pCO_2 and climate over the past 20 Ma. Finally, we explore possible implications of B/Ca-based pCO_2 estimates for the interpretation of other marine pCO_2 proxies.

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Additional Information:© 2011 Elsevier Ltd. Received 1 December 2009; accepted 11 January 2011. Associate editor: Robert H. Byrne. Available online 21 January 2011. We would like to express our appreciation to three anonymous reviewers, the associate editor Robert Byrne, Jay Black, Ed Boyle, Caroline Dawber, Andrew Dickson, John Eiler, Harry Elderfield, David Hodell, Miriam Kastner, Dan Lunt, Tim Naish, Edwin Schauble, and Danny Sigman for their comments on this work, which substantially improved the manuscript. We also would like to thank Jeannie Booth, Emma Khadun, Oliver Shorttle, Lavaniya Thanalasundaram, and Aaron Bufe for invaluable assistance with sample preparation; Linda Booth, Jason Day, and Mervyn Greaves for technical assistance; Lorraine Lisiecki for assistance with the age model; and Katherine Allen, Simon Crowhurst, Alex Gagnon, Baerbel Hönisch, Seth John, Nele Meckler, Ben Passey, Nivedita Thiagarajan, and Jimin Yu for discussions. Support for this project was provided by NERC, Magdalene College, and the UCLA Division of Physical Sciences to A.T. Samples for this study were obtained from the Godwin Laboratory sample archives and the Ocean Drilling Program.
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Magdalene College UNSPECIFIED
UCLA Division of Physical SciencesUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20110817-095151817
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Official Citation:Aradhna K. Tripati, Christopher D. Roberts, Robert A. Eagle, Gaojun Li, A 20 million year record of planktic foraminiferal B/Ca ratios: Systematics and uncertainties in pCO2 reconstructions, Geochimica et Cosmochimica Acta, Volume 75, Issue 10, 15 May 2011, Pages 2582-2610, ISSN 0016-7037, DOI: 10.1016/j.gca.2011.01.018.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24906
Deposited By: Jason Perez
Deposited On:18 Aug 2011 14:38
Last Modified:03 Oct 2019 03:00

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