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^(40)K–^(40)Ca isotopic constraints on the oceanic calcium cycle

Caro, G. and Papanastassiou, D. A. and Wasserburg, G. J. (2010) ^(40)K–^(40)Ca isotopic constraints on the oceanic calcium cycle. Earth and Planetary Science Letters, 296 (1-2). pp. 124-132. ISSN 0012-821X.

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The contributions of crustal silicates to the oceanic calcium cycle are investigated using high-precision ^(40)K–^(40)Ca measurements in Archean and Proterozoic carbonates, river waters, and a series of terrestrial and extraterrestrial mafic/ultramafic samples. Using a multidynamic data collection scheme with strict controls on instrumental mass fractionation, we show that a reproducibility of 0.35 ε-units (2σ) is obtained for the ^(40)Ca/^(44)Ca ratio using the ThermoFinnigan Triton. This represents an improvement by a factor of 3 compared with previous generations of data. Well-defined excesses of ^(40)Ca from ^(40)K decay were found in river waters draining the Himalayas, but not in the Mississippi and Columbia Rivers. All marine carbonate samples ranging in age from Archean to recent show no discernable effects of ^(40)K decay to within the limit of ± 0.35 ε-units (2σ) in ^(40)Ca/^(44)Ca. Our results, therefore, indicate that the Ca isotopic composition of seawater has remained constant and indistinguishable from that of the mantle for the past 3.5 Ga, despite the influx of radiogenic calcium delivered by weathering of the high K/Ca components of the continental crust. Thus, over most of geologic time, the contributions of the high K/Ca sources from the continents are far below the contributions of hydrothermal sources. Mass balance constraints indicate that unless the present-day contribution of continental silicates is much less than 8% of the river Ca flux, the total hydrothermal flux of calcium must exceed the input from high-temperature vents at ridge axes by at least one order of magnitude. The present-day oceanic mass balance requires a high input of calcium [(4–15) × 10^(12) mol Ca/yr] at low-temperature hydrothermal sites.

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Wasserburg, G. J.0000-0002-7957-8029
Additional Information:© 2010 Elsevier B.V. Received 8 November 2009; revised 3 May 2010; accepted 3 May 2010. Editor: R.W. Carlson. Available online 1 June 2010. We thank J. W. Schopf, P. F. Hoffman, P.-H. Blard, and the Oregon State University Marine Geology Repository and Martin Fisk, who generously provided the carbonate and basalt samples for this study. We thank Henry Ngo for his assistance and advice during development of the analytical techniques. We also thank N. Vigier, and L. Reisberg for fruitful discussions and informal reviews of the manuscript. D. A. Papanastassiou was supported by JPL RTD; the laboratory is supported by a NASA Cosmochemistry RTOP. G. J. Wasserburg acknowledges support by a NASA Cosmochemistry RTOP to J. Nuth, at GSFC, and by the Epsilon Foundation.
Funding AgencyGrant Number
Epsilon FoundationUNSPECIFIED
Goddard Space Flight CenterUNSPECIFIED
Subject Keywords:calcium cycle; K-40-Ca-40; ocean chemistry
Issue or Number:1-2
Record Number:CaltechAUTHORS:20100806-080351974
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:19308
Deposited By: Tony Diaz
Deposited On:06 Aug 2010 16:45
Last Modified:09 Mar 2020 13:19

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