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

2014; American Geophysical Union; Volume: 29; Issue: 7 Linguagem: Inglês

10.1002/2014pa002616

1944-9186

Mitchell W Lyle, Franco Marcantonio, Willard S. Moore, Richard W. Murray, Chih‐An Huh, Bruce P. Finney, David Murray, Alan C Mix,

Paleontology and Stratigraphy of Fossils

Abstract We use flux, dissolution, and excess 230 Th 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 230 Th focusing factor and subtracting measured dissolution. CaCO 3 MAR is severely overestimated when the 230 Th 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 CaCO 3 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 CaCO 3 does not move with 230 Th, normalization typically overcorrects the CaCO 3 MAR; and because CaCO 3 is 80% of the total sediment, 230 Th normalization overestimates lateral sediment flux. Fluxes of 230 Th 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, 230 Th flux measurements are as much as 3 times higher than model predictions. There is also evidence for lateral near‐bottom 230 Th transport in the bottom nepheloid layer since 230 Th 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 230 Th within a quantitative model of lateral sediment transport.