Quantifying the Impact of Atmospheric Deposition on the Biogeochemistry of Fe and Al in the Upper Ocean

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ATMOSPHERIC DEPOSITION: THE OCEAN PERSPECTIVE
To collect samples for TM analysis within the time frame allotted on CLIVAR cruises, a TM clean rosette-based system instrumented with a conductivitytemperature-depth (CTD) sensor and fluorometer that could collect 12 samples within the upper 1,000 m of the water column was developed (Measures et al., 2008a).This system was the prototype of the 24-bottle sampling system that is now being used by the US-GEOTRACES program (Cutter and Bruland, 2012).The decision to sample the upper 1,000 m frequently rather than obtaining full-depth water column profiles less frequently on CLIVAR cruises was motivated by the desire to maintain high-resolution cruise (Measures et al., 2008b;Barrett et al., 2012) (Buck et al., 2010).An advantage to the DI water leaching treatment is that the leach solutions can be easily and directly analyzed for most of the GEOTRACES "key parameter" trace elements and isotopes (TEIs) using HR-ICP-MS (High Resolution Inductively Coupled Plasma Mass Spectrometry; SCOR Working Group, 2006).Trace element analyses are more difficult in a seawater matrix.
Replicate aerosol samples collected on North Atlantic (A16N) and Pacific (P02 and P16) CLIVAR sections were extracted with a rapid flow-through procedure using either filtered surface seawater or ultrapure deionized water (Buck et al., 2006).Comparison of the Once the A16S cruise is completed in the South Atlantic in January 2014, we will be able to assemble a global view of trace elements in the main thermocline that can be used to constrain oceanic Fe models and improve our understanding of the fractional solubility of aerosol TMs.These data will be valuable in future process studies and in timeseries endeavors.Fe data plotted with a linear least-squares regression trend line and 68% confidence limits.Aerosol Fe fractional solubility was 15% ± 8% (DI water) and 9% ± 5% (SW) in the Atlantic and 10% ± 5% (DI water) and 8% ± 9% (SW) in the Pacific.Figure from Buck et al. (2013) Quantifying the Impact of Atmospheric Deposition on the Biogeochemistry of Fe and Al in the Upper Ocean B Y M A X I M E M .G R A N D , C L I F T O N S .B U C K , W I L L I A M M .L A N D I N G , C H R I S T O P H E R I .M E A S U R E S , M A R I K O H AT TA , W I L L I A M T. H I S C O C K , M AT T H E W B R O W N , A N D J O S E P H A .R E S I N G A Decade of Collaboration with the US CLIVAR-CO 2 Repeat Hydrography Program Oceanography | Vol. 27, No. 1 62 OVERVIEW The aerosol deposition of continental material and its partial dissolution in the surface ocean exerts an important control on the distribution of iron and other potentially limiting trace metal (TM) micronutrients in the open ocean.This dust deposition has implications for the regulation of global climate through the coupling of biolimiting TM cycles, marine productivity, and the global carbon cycle.Thus, it is important to determine the locations of dust deposition in the open ocean and to quantify the magnitude and subsequent dissolution of the dust.At present, there are too few dust deposition estimates and solubility measurements in the open ocean to adequately constrain this key source term in global biogeochemical models.While early sampling efforts were invaluable in highlighting the importance of TMs in regulating nutrient cycling and phytoplankton productivity in vast ocean regions, they lacked the spatial resolution and global coverage required to constrain model simulations and identify features that illuminate the processes controlling TM distributions.Starting in 2003, the US Climate Variability and Predictability (CLIVAR)-CO 2 Repeat Hydrography Program provided an hour of ship time for dedicated TM sampling at every degree of latitude or longitude along selected cruise tracks.The principal goals for CLIVAR TM sampling were to produce a high-resolution dissolved iron (Fe) and aluminum (Al) data set with global coverage to better understand upper-ocean Fe biogeochemistry, determine patterns of atmospheric dust deposition based on surface dissolved Al levels, and improve our estimates of the fractional solubility of aerosol TMs using a dedicated shipboard aerosol sampling program.This paper describes recent advances in our understanding of dust deposition and the solubility of aerosol material resulting from 10 years of collaborative work under the US CLIVAR-CO 2 Repeat Hydrography Program.
coverage and the fact that most TM input and cycling occurs within the upper 1,000 m.The first deployment of the system in 2003 during the CLIVAR A16N scheme when unanticipated biogeochemical features were encountered.Patterns of dust deposition in the openocean can be estimated by considering the surface ocean as an integrated long-term dust collector(Measures and Brown, 1996).The essence of this approach is that once dust is deposited to the ocean surface, the dissolution of lithogenic elements leaves behind a

Figure 1
Figure 1 shows a contour plot of dissolved Al within the mixed layer of every station of the 10 CLIVAR cruise tracks sampled to date.In the North Atlantic, maxima in dissolved and particulate Al unequivocally reflect the deposition of Saharan dust (Measures et al., 2008b; Barrett et al., 2012).The data also highlight inputs of Asian and Australian dust propagating eastward in the northwestern and southwestern Pacific, respectively (Figure 1).In the Southern Ocean, the section running poleward of South Africa hints at a possible long-range dust signal emanating from Patagonia and South Africa near 45°S and 60°S.The possibility of Patagonian and southern African dust signals

Figure 1 .
Figure 1.Contoured surface dissolved Al from all stations sampled on CLIVAR cruises with dedicated trace metal (TM) sampling.Black dots represent TM stations.The dashed gray line shows the planned A16S cruise track to be completed in early 2014.