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

Abstract

[1] The normalization of sediment accumulation rates by 230 Th is increasingly used to constrain particle rain rates because 230 Th is produced at a known rate in the water column and removed relatively quickly to the sediment. Several recent contributions have questioned this approach and suggested that the preferential removal of 230 Th in areas of high particle flux, the ''particle flux effect,'' introduces a systematic bias in the 230 Th normalization technique. We use a circulation model that includes a description of particle scavenging to show that the particle flux effect cannot explain observations of high 230 Th accumulation in equatorial Pacific sediments (relative to 230 Th production in the water column). We further consider the possible variation of particle rain rates over time. We find only a minimal sensitivity in the particle flux effect due to increases in the particle rain rate at the equatorial Pacific by a factor of up to 10. This situation exists because the residence time of 230 Th in the water column is too short to permit significant lateral transport with reasonable isopycnal mixing coefficients. We conclude that the increased rates of 230 Th accumulation found in equatorial Pacific sediments deposited during the Last Glacial Maximum cannot have been caused by the particle flux effect.

How well can we quantify dust deposition to the ocean?

“…Confidence in these approaches rests on the fact that the one-dimensional mass budget of 230 Th is known to ±30-40%, as demonstrated in global models [64], in regional models [94] and by applying empirical calculations using dissolved 230 Th data from the ETNA [65]. 230 Th-normalized fluxes of lithogenic material obtained from surface sediments are influenced by two factors that cause overestimation of the historical dust flux.…”

Timescales of lateral sediment transport in the Panama Basin as revealed by radiocarbon ages of alkenones, total organic carbon and foraminifera

“…Contrary results were obtained in studies using constant flux proxies Loubere, 1999;Loubere, 2000;Loubere et al, 2003;Loubere et al, 2004) and faunal assemblages (Loubere, 1999;Martinez et al, 2006) implying reduced glacial productivity and enhanced sediment focusing. In light of this paradox, the applicability of the 230 Th method in the in the Panama Basin is controversial (Broecker, 2008;Francois et al, 2007;Kienast et al, 2007;Lyle et al, 2005;Lyle et al, 2007;Siddall et al, 2008).…”

“…Nevertheless, the applicability of 230 Th method in the Equatorial Pacific and particularly in the Panama Basin remains a subject of debate (Broecker, 2008;Francois et al, 2007;Kienast et al, 2007;Lyle et al, 2005;Lyle et al, 2007;Siddall et al, 2008).…”

Evaluating Cenozoic equatorial sediment deposition anomalies for potential paleoceanographic and Pacific plate motion applications

“…If that explanation is correct, it also implies a link to the Coriolis effect and hence to the geographic equator. As the high Ψ−values occur over only a limited range of latitude and appear centered on the equator Siddall et al 2008;Figure 3a), any physical movement of particles to the equator may be expected to have reinforced the effect of enhanced surface productivity. The geochemical proxies of surface productivity (Murray et al, 2000a(Murray et al, , 2000bPayton et al, 1996) also seem to be somewhat more sharply peaked at the equator than fluxes derived from sediment traps (Figure 2c; Dymond and Lyle, 1994;Honjo et al, 1995), potentially a result of some physical movement of particles.…”

“…This interpretation of the 230 Th data has been controversial, however, with some researchers arguing that Ψ−values are also affected by grain--size effects (preferential absorption of Th onto clay particles meaning that the Ψ--values represent mainly transport of fines), local sediment redistribution (winnowing over a hill will reduce deposition rates and conversely preferential accumulation in a valley will increase deposition rates), leakage of 230 Th from seabed sediment by expelled pore waters and a particle flux effect whereby rapid stripping of 230 Th by sinking particles is compensated by isopycnal diffusion of water--borne 230 Th from outside the equatorial zone (Broecker 2008;Lyle et al 2005;Lyle et al 2007;Siddall et al 2008; Thomas et al 2000). These effects were re--evaluated in Mitchell and Huthnance (2013).…”