How well can we quantify dust deposition to the ocean?

Abstract

One contribution of 20 to a discussion meeting issue 'Biological and climatic impacts of ocean trace element chemistry'. Deposition of continental mineral aerosols (dust) in the Eastern Tropical North Atlantic Ocean, between the coast of Africa and the Mid-Atlantic Ridge, was estimated using several strategies based on the measurement of aerosols, trace metals dissolved in seawater, particulate material filtered from the water column, particles collected by sediment traps and sediments. Most of the data used in this synthesis involve samples collected during US GEOTRACES expeditions in 2010 and 2011, although some results from the literature are also used. Dust deposition generated by a global model serves as a reference against which the results from each observational strategy are compared. Observation-based dust fluxes disagree with one another by as much as two orders of magnitude, although most of the methods produce results that are consistent with the reference model to within a factor of 5. The large range of estimates indicates that further work is needed to reduce uncertainties associated with each method before it can be applied routinely to map dust deposition to the ocean. Calculated dust deposition using observational strategies thought to have the smallest uncertainties is lower than the reference model by a factor of 2-5, suggesting that the model may overestimate dust deposition in our study area.This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'.

230Th Normalization: New Insights on an Essential Tool for Quantifying Sedimentary Fluxes in the Modern and Quaternary Ocean

“…Provided that the aggregation and disaggregation of small and large particles are in equilibrium (Bacon et al, ), fluxes determined on suspended particles will still be valid. Results from a limited number of measurements using large (>51 μm) particles indicate an offset from results obtained using the smaller size class of only a few tens of percent (Anderson et al, ; Pavia, Anderson, Lam, et al, ); however, additional measurements of 230 Th on larger sinking particles will ultimately be needed to shed light on the particle dynamics involved in Th removal.…”

“…This bias can be illustrated by calculating the 230 Th‐normalized flux of lithogenic particles, traced using 232 Th. In regions of the ocean far from continents where the lithogenic material in the water column is supplied mainly as dust, the 230 Th‐normalized flux of particulate 232 Th is expected to be uniform throughout the water column (Anderson et al, ). This expectation is clearly violated within the nepheloid layers of the NW Atlantic Ocean, where 230 Th‐normalized fluxes of particulate 232 Th increase through the nepheloid layer by an order of magnitude at Station 8 and by about a factor of 40 at Station 10 (Figure e).…”

“…This approach is equivalent to the application of 230 Th normalization to sediments, except that particles collected from the water column by filters are used instead (Hirose, ). Thanks to advances in analytical capabilities under GEOTRACES, particulate 230 Th normalization in filter samples has now been used to generate flux profiles of trace elements (Hayes et al, ), particulate organic carbon (Hayes et al, ; Pavia, Anderson, Lam, et al, ), and lithogenic dust (Anderson et al, ).…”

An investigation of basin-scale controls on upper ocean export and remineralization

“…These flux and residence time estimates will help to close trace metal budgets for the Pacific Ocean and we anticipate that these data will aid in the improvement of current GBMs used to model dissolve and particulate trace metal distributions, especially in the Pacific OMZ, where GBMs struggle to reproduce in-situ distributions (van Hulten et al, 2017). The 234 Th-method for determining the residence time of trace metals in the surface ocean provides a useful alternative to dust-based estimates, where observation-based fluxes have been shown to disagree by two orders of magnitude in some cases (Anderson et al, 2016).…”

Dissolved scandium, yttrium, and lanthanum in the surface waters of the North Atlantic: Potential use as an indicator of scavenging intensity

“…Anderson et al . [] elected to consider only the dry deposition velocity since dry deposition dominates in this region north of the Intertropical Convergence Zone [ Baker et al ., ; Powell et al ., ]; considering only dry deposition leads to slightly higher surface dissolved residence time estimates (Table ). These estimates indicate that, as anticipated, dissolved La has a longer residence time than dissolved Sc along this transect.…”