Sediment size fractionation and focusing in the equatorial Pacific: Effect on ²³⁰Th normalization and paleoflux measurements Public Deposited

http://ir.library.oregonstate.edu/concern/articles/nz806141w

This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Geophysical Union and published by John Wiley & Sons, Inc. It can be found at:  http://agupubs.onlinelibrary.wiley.com/agu/journal/10.1002/%28ISSN%291944-9186/

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  • Sediment size fractionation and sediment focusing in the Equatorial Pacific: Effect on 230Th normalization and paleoflux measurements
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  • We use flux, dissolution, and excess ²³⁰Th data from the Joint Global Ocean Flux Study and Manganese Nodule Project equatorial Pacific study Site C to assess the extent of sediment focusing in the equatorial Pacific. Measured mass accumulation rates (MAR) from sediment cores were compared to reconstructed MAR by multiplying the particulate rain caught in sediment traps by the ²³⁰Th focusing factor and subtracting measured dissolution. CaCO₃ MAR is severely overestimated when the ²³⁰Th focusing factor correction is large but is estimated correctly when the focusing factor is small. In contrast, Al fluxes in the sediment fine fraction are well matched when the focusing correction is used. Since CaCO₃ is primarily a coarse sediment component, we propose that there is significant sorting of fine and coarse sediments during lateral sediment transport by weak currents. Because CaCO₃ does not move with ²³⁰Th, normalization typically overcorrects the CaCO₃ MAR; and because CaCO₃ is 80% of the total sediment, ²³⁰Th normalization overestimates lateral sediment flux. Fluxes of ²³⁰Th in particulate rain caught in sediment traps agree with the water column production-sorption model, except within 500 m of the bottom. Near the bottom, ²³⁰Th flux measurements are as much as 3 times higher than model predictions. There is also evidence for lateral near-bottom ²³⁰Th transport in the bottom nepheloid layer since ²³⁰Th fluxes caught by near-bottom sediment traps are higher than predicted by resuspension of surface sediments alone. Resuspension and nepheloid layer transport under weak currents need to be better understood in order to use ²³⁰Th within a quantitative model of lateral sediment transport.
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  • Lyle, M., Marcantonio, F., Moore, W. S., Murray, R. W., Huh, C. A., Finney, B. P., ... & Mix, A. C. (2014). Sediment size fractionation and focusing in the equatorial Pacific: Effect on 230Th normalization and paleoflux measurements. Paleoceanography, 29(7), 747-763. doi:10.1002/2014PA002616
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