Productivity control of fine particle transport to equatorial Pacific sediment

Abstract

Abstract. Accumulation rates of 3He (from cosmic dust), 23øTh (produced in the water column), barite (produced in the water column during decay of organic matter), and Fe and Ti (arriving with wind-borne dust) all are positively correlated in an equatorial Pacific core (TT013-PC72; 01.1 øN, 139.4øW; water depth 4298 m). These accumulation rates are also positively correlated with the accumulation rates of noncarbonate material. They are not significantly correlated to the mass accumulation rate of carbonate, which makes up the bulk of the sediment. The fluctuations in accumulation rates of these various components from different sources thus must result from variations in some process within the oceans and not from variations in their original sources. Sediment focusing by oceanic bottom currents has been proposed as this process [Marcantonio et al., 1996]. We argue that the variations in the accumulation rates of all these components are dominantly linked to changes in productivity and particle scavenging (3He, 23øTh, Fe, Ti) by fresh phytoplankton detritus (which delivers Ba upon its decay) in the equatorial Pacific upwelling region. We speculate that as equatorial Pacific productivity is a major component of global oceanic productivity, its variations over time might be reflected in variations in atmospheric levels of methanesulfonic acid (an atmospheric reaction product of dimethyl sulfide, which is produced by oceanic phytoplankton) and recorded in Antarctic ice cores. We evaluate published data on an equatorial Pacific core and suggest a mechanism (marine productivity and particle scavenging) by which all these different data sets can be explained in an internally consistent manner. We add a 945

Modeling the particle flux effect on distribution of 230Th in the equatorial Pacific

“…There has been a good deal of controversy between these two approaches: the proponents of 230 Th normalization argue that differences between 230 Th‐based and age‐model‐based MARs simply reflect changes in sediment focusing or age model errors, and that age‐model‐based MARs often do not accurately record variations in vertical flux from the overlying water (see Francois et al [2007] for discussion). The opponents of 230 Th normalization argue that 230 Th is preferentially removed in areas of high particle flux, such as at the equatorial Pacific [ Thomas et al , 2000; Lyle et al , 2005]. Removing 230 Th from the water column reduces the concentrations of dissolved 230 Th in areas of high particle flux.…”

“…We define this as the “particle flux effect”; this is sometimes referred to as “boundary scavenging” but we prefer the term “particle flux effect” to distinguish this process from additional processes which happen at ocean margins (see section 2.2 for discussion). It has been suggested that the particle flux effect leads to the high values in 230 Th flux observed at the equatorial Pacific and that such observations are not a result of sediment focusing [ Thomas et al , 2000; Lyle et al , 2005]. Previous modeling work has concluded that the particle flux effect may account for increases in 230 Th flux to the sediment of up to 30% compared to the 230 Th input from the overlying water column (equivalent to a focusing factor of 1.3) [ Henderson et al , 1999].…”

Chemical Tracers of Particle Transport

“…Upon correcting the barite results for sediment focusing, the record shows little evidence for glacial-interglacial changes in productivity of the central equatorial Pacific Ocean. Thomas et al (2000) suggested that variable scavenging intensity might have biased the 230 Th-normalized barite fluxes in equatorial Pacific sediments. Marcantonio et al (2001) refuted this argument by showing that 10 Be/ 230 Th ratios, which are sensitive to changes in scavenging intensity, were relatively constant throughout the period studied, further supporting the view that there has been little glacial-interglacial change in particle flux.…”

GEOTRACES – An international study of the global marine biogeochemical cycles of trace elements and their isotopes

“…This method relies on the assumption that the rapid scavenging and removal of dissolved 230 Th from seawater causes its flux to the sediments to be everywhere approximately equal to its known rate of production by radioactive decay of dissolved 234 U. Fluxes of other sedimentary constituents can then be estimated by normalising to the known flux of 230 Th. This method holds great promise for improving paleoflux reconstructions, but questions have been raised regarding the underlying principle of a constant scavenging flux of 230 Th (Thomas et al, 2000;Lyle et al, 2005). This assumption is an approximation only and its accuracy still needs to be evaluated systematically.…”