Comment on ‘‘Do geochemical estimates of sediment focusing pass the sediment test in the equatorial Pacific?’’ by M. Lyle et al

[1] Accurately estimating the vertical flux of material reaching the seafloor from the overlying surface waters is essential for the paleoceanographic reconstruction of a wide variety of oceanic processes. Two approaches are currently being used. One consists of estimating mass accumulation rates (MAR) between dated horizons as the product of linear sedimentation rates, sediment dry bulk densities, and concentrations. One pitfall with this approach is that sediments can be redistributed on the seafloor by bottom currents, and their accumulation may not necessarily reflect the true vertical rain rate originating from the overlying water column. To address this problem, the method of Th normalization was developed [Bacon, 1984]. This method is based on the assumption that the rapid scavenging of Th produced in the water column by decay of dissolved uranium results in its flux to the seafloor always being close to its known rate of production. To the extent that this assumption is correct, scavenged Th can be used as a reference to estimate the settling flux of other sedimentary constituents and to correct for sediment redistribution on the seafloor [Henderson and Anderson, 2003; Francois et al., 2004]. [2] MAR calculation and Th normalization sometimes give widely divergent results. This is particularly true in the equatorial Pacific, where MARs often indicate higher fluxes during the last glacial period [Lyle et al., 2002], while Th normalization suggests unchanged or even lower glacial fluxes [Marcantonio et al., 2001; Loubere et al., 2004]. The proponents of Th normalization argue that changes in MAR mainly reflect changes in sediment focusing and do not record actual changes in vertical flux from the overlying water. In a recent paper, Lyle et al. [2005] dispute this interpretation and contend that Th normalization grossly overestimates sediment focusing in the equatorial Pacific. They argue that lateral transport of Th in the water column is much larger than generally appreciated. We disagree with this view and in the following we point to shortcomings in their arguments.