Dust deposition to the surface waters of the western and central North Pacific inferred from surface water dissolved aluminum concentrations

Abstract

[1] Dissolved Al was determined on $3500 surface water samples collected in the NW Pacific during the 2002 Intergovernmental Oceanographic Commission (IOC) Contaminant Baseline survey. In addition, dissolved Al was determined on samples collected at 9 vertical stations occupied along the cruise track. Surface water Al distributions, when converted to annual mineral dust deposition (Measures and Brown, 1996), imply extremely low depositions (mean values 0.3 g m À2 yr À1 ) to surface waters of the subarctic gyre despite the atmospheric transport of large amounts of mineral dust across this region from the desert regions of China. The calculated deposition values are also much lower than the 1-10 g m À2 yr À1 predicted for this region by the GESAMP model (Duce et al., 1991). The lowest mineral dust depositions correspond to the High Nutrient Low Chlorophyll (HNLC) region of the NW Pacific, which implies that the biogeochemical status of this region is the result of low atmospheric micronutrient input, similar to other HNLC regions. In the subtropical gyre the Al-derived dust deposition (mean values 0.1-0.6 g m À2 yr À1 ) agrees well with the (<1 g m À2 yr À1 ) predictions of the GESAMP model. Enriched concentrations of dissolved Al in the coastal waters near Kauai, from fluvial sources, indicate the potential role of remote high islands in adding reactive trace elements to the nearby surface waters. In the vertical profiles, elevated dissolved Al concentrations (4-8 nM) were found to correspond to the potential density of the western Pacific ''Subtropical Mode'' waters which form in the recirculation zone of the Kuroshio.

Eolian dust input to the Subarctic North Pacific

“…Sedimentary dust flux data compiled in DIRTMAP3 are generally higher than our reconstructions, possibly as a result of the inability to geochemically remove volcanic material. Dust fluxes based on dissolved aluminum concentrations in seawater in the North Pacific (Measures et al, 2005), with a limited spatial overlap with the INOPEX cruise track, appear to yield dust fluxes that are lower by about an order of magnitude than results presented here. The discrepancy may indicate uncertainties in the estimate of the Al residence time in this region (Hayes et al, 2013).…”

“…To test this hypothesis under modern conditions and in the past, we need reliable high-resolution dust records that are presently not available. The recent DIRTMAP3 compilation (Maher and Kohfeld, 2009, and references therein) with the addition of data based on surface water concentrations of dissolved aluminum (Measures et al, 2005) and 230 Th-normalized 232 Th fluxes (Kohfeld and Chase, 2011) show considerable variability in the Holocene spatial pattern of dust input to the SNP, varying between 0.1 and 21.4 g/m 2 /yr without a clear geographical pattern. Therefore, most studies rely on dust model predictions.…”

Quantifying lithogenic inputs to the North Pacific Ocean using the long-lived thorium isotopes

“…The "mid-depth-minimum" profile shape of the 232 Th profiles of SO202-32 through -44 is similar to that of Al in the North Pacific (Measures et al, 2005;Orians and Bruland, 1986). As Orians and Bruland (1986) pointed out for Al, we conclude that there are two sources of Th to the ocean: one shallow due to dust dissolution and one deep associated with sediment dissolution/resuspension; and one removal mechanism, scavenging throughout the water column.…”

“…Dissolved Al has also been used to estimate dust deposition in the North Pacific (Measures et al, 2005) in a similar way to that described for Th, although for Al, a residence time, in addition to a fractional solubility, must be assumed. These Al results from the subarctic suggest very low dust fluxes (< 0.3 g m −2 yr −1 ), even lower than model-derived estimates.…”

“…In particular, in the western North Pacific between 30-45 • N, mixed layer depths can often exceed 200 m (Ohno et al, 2009;Oka et al, 2007 (Bingham, 1992;Hanawa and Suga, 1995), known to be enriched in Al (Measures et al, 2005), outcrops or forms south of the Kuroshio Extension (generally south of 36 • N, Fig. 1) (Chen, 2008;Oka et al, 2007).…”