230Th and 231Pa on GEOTRACES GA03, the U.S. GEOTRACES North Atlantic transect, and implications for modern and paleoceanographic chemical fluxes

Abstract

29The long-lived uranium decay products 230

How well can we quantify dust deposition to the ocean?

“…Dissolved 230 Th is removed from the ocean by reversible scavenging to settling particles [63] with a residence time that is sufficiently short (20-40 years in the deep sea) that the tracer experiences relatively little net lateral redistribution between its production in the water column and its burial in the underlying sediments. One modelling study concluded that the removal flux of 230 Th to the sediments is within ±30% of its production rate in the overlying water column throughout approximately 70% of the ocean [64], a finding subsequently shown to apply in our study area [65]. Consequently, the short residence time of dissolved Th allows its mass budget to be applied in a one-dimensional framework with relatively small uncertainty.…”

“…Bioturbation of surface sediments filters the record of sediment accumulation over time scales of centuries (near the continents) to millennia (in the open ocean), so the average fluxes derived from surface sediments include dust supplied by rare large events whose signal in the water column dissipated long ago. Sedimentation on the continental slope and rise may also incorporate episodic input of material that was reworked from sites upslope or on the nearby continental shelf [95], as evidenced by nepheloid layers observed along the African margin [65,68]. Partially offsetting these effects, the preferential scavenging of dissolved 230 Th in the particlerich water column near the coast of Mauritania, combined with lateral mixing, generates a net flux of dissolved 230 Th towards the margin.…”

“…The 230 Th-normalized flux of lithogenic particles is relatively constant between approximately 1000 and 3000 m at each station ( figure 7). Below approximately 3000 m, at some stations, the lithogenic flux again increases with depth, generally at sites where profiles of beam transmission indicate the presence of resuspended sediments [65]. Consequently, here we use the average lithogenic flux between 900 and 3000 m for each station to represent the flux of dust derived using this strategy (figure 6b, red circles).…”

“…Partially offsetting these effects, the preferential scavenging of dissolved 230 Th in the particlerich water column near the coast of Mauritania, combined with lateral mixing, generates a net flux of dissolved 230 Th towards the margin. Within the region between Mauritania and Cape Verde Islands, this boundary scavenging effect is estimated to cause the 230 Th-normalization approach to underestimate the actual vertical flux of particles by 30-40% [65].…”

“…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.…”