Size-fractionated major particle composition and concentrations from the US GEOTRACES North Atlantic Zonal Transect

Abstract

Opal Biogenic silica POC Ballast Dust Lithogenic material a b s t r a c tThe concentration and the major phase composition (particulate organic matter, CaCO 3 , opal, lithogenic matter, and iron and manganese oxyhydroxides) of marine particles is thought to determine the scavenging removal of particle-reactive TEIs. Particles are also the vector for transferring carbon from the atmosphere to the deep ocean via the biological carbon pump, and their composition may determine the efficiency and strength of this transfer. Here, we present the first full ocean depth section of sizefractionated (1-51 mm, 4 51 mm) suspended particulate matter (SPM) concentration and major phase composition from the US GEOTRACES North Atlantic Zonal Transect between Woods Hole, MA and Lisbon, Portugal conducted in 2010 and 2011. Several major particle features are notable in the section: intense benthic nepheloid layers were observed in the western North American margin with concentrations of SPM of up to 1648 mg/L, two to three orders of magnitude higher than surrounding waters, that were dominated by lithogenic material. A more moderate benthic nepheloid layer was also observed in the eastern Mauritanian margin (44 mg/L) that had a lower lithogenic content and, notably, significant concentrations of iron and manganese oxyhydroxides (2.5% each). An intermediate nepheloid layer reaching 102 mg/L, an order of magnitude above surrounding waters, was observed associated with the Mediterranean Outflow. Finally, there was a factor of two enhancement in SPM at the TAG hydrothermal plume due almost entirely to the addition of iron oxyhydroxides from the hydrothermal vent. We observe correlations between POC and CaCO 3 in large (4 51 mm) particles in the upper 2000 m, but not deeper than 2000 m, and no correlations between POC and CaCO 3 at any depth in small ( o51 mm) particles. There were also no correlations between POC and lithogenic material in large particles. Overall, there were very large uncertainties associated with all regression coefficients for mineral ballast ("carrying coefficients"), suggesting that mineral ballast was not a strong predictor for POC in this section.

How well can we quantify dust deposition to the ocean?

“…Rapid scavenging of dissolved Al in this region is consistent with elevated Al/Ti ratios in particles throughout the water column at stations nearest the African margin [68]. Thus, the rate of scavenging and removal of dissolved Al from surface waters where high particle abundances [67,68] reflect the combined effects of elevated dust flux and biological productivity [57] may be much greater than assumed in calculating the results shown in figure 3. On the other hand, the residence time of dissolved Al in the western portion of our study area may be as large as several decades (figure 9), long enough for substantial redistribution by surface currents.…”

“…This approach can also be applied to particles collected by in situ filtration, the method used to collect the samples analysed for this study [67,68]. Particles convey scavenged Th to the sediments, so the amount of adsorbed 230 Th on particles filtered from a given depth reflects the integrated production of dissolved 230 Th in the overlying water column.…”

“…Profiles of particulate 230 Th concentration measured using samples from GA03 [66] collected by in situ filtration [67,68] thus offer a novel approach to estimate dust flux. Given the simple mass budget for 230 Th in the ocean, where production of dissolved 230 Th by 234 U decay (P Th , 433 µBq m −3 yr −1 ) is balanced by scavenging loss onto settling particles, one expects the concentration of unsupported (i.e.…”

“…Settling rates can be as high as 200-300 m day −1 [70]. A few samples of the greater than 51 µm particle size class collected at the GA03 stations [67,68] were also analysed for Th isotopes to investigate the sensitivity of the inferred dust flux to particle size. Only a small percentage of the total collection of greater than 51 µm particle samples could be analysed for Th because it was necessary to process an entire sample to measure Th with good precision, [82] used an algorithm based on grain size to distinguish between hemipelagic sources and dust.…”

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with realistic particle dynamics (NEMO–ProThorP 0.1)

“…Pa have been measured (Hayes et al, 2015b), but also their adsorbed forms (Hayes et al, 2015b) and biogenic and lithogenic particle concentrations in two size classes (Lam et al, 2015).…”