Text01
Estimation of the atmospheric deposition of soluble cobalt along the GEOTRACES-A02 section
1) Estimation of the dust deposition
The dust deposition was inferred from the concentrations of dissolved aluminium measured in seawater along the
section, following a modified version of the MADCOW model (de Jong et al., 2007).
Model:
Dust deposition rate (g.m-2.year-1) = DAl * z * M Al * (Ab. Al * S Al * τ Al)-1 Equation 1
τ Al = DAl * z * (Scav. Al)-1
Equation 2
In steady state condition dP Al/ dt = 0; Scav. Al = Dust dissolution; Scav. Al can be rewritten:
Scav. Al = FPOC * (PAl:POC) * S Al
Equation 3
Conditions:
-
Assuming dissolved (DAl) and particulate (PAl) aluminium are at steady state in the mixed layer, and
thus that their stocks are constant.
Assuming the dissolution of dust is the only source of DAl in the mixed layer * and scavenging on
particles its only sink; and that the sum of both is equal to zero.
*except in the subantarctic waters where input of 6 nmol.m-2.yr-1 is attributed to lateral advection using DAl gradient between 40°S and 50°S (Grad 40°S→50°S
DAl = 4 10-12 mol.m-3.m-1), an advection of 30cm.s-1 in this area for the upper 100 meters [Vivier and Provost, 1999] and an area of 0.5 1012 m-2
Parameters:
DAl : mean concentration of dissolved aluminium measured in the mixed layer (in mol.m -3; ts01) [R. Middag,
unpublished data]
z : depth of the mixed layer derived from CTD data (in m; ts01)
M Al : molar mass of aluminium (27 g.mol-1)
Ab. Al : crustal abundance of aluminium (0.077 g.g-1; Rudnick and Gao [2003])
S Al : solubility of aluminium from dust (3 to 15% Baker et al. [2013]; ts01)
τ Al : residence time of aluminium in the mixed layer calculated using Equation 2 (in year; 0.03 - 3.6 years,
supplement 2).
Scav. Al : scavenging of dissolved aluminium on settling particles calculated using Equation 3 (in mol.m -2.year1
; ts01)
FPOC : exported flux of organic carbon on settling particles (Owens [2013] in the southern section; Puigcorbé et
al., [in prep.] in the northern section)
PAl:POC : Aluminium to organic carbon ratio in particles used were determined in particles collected of the
eastern Atlantic (Al:POC ~ 1mmol/mol) and the Mediterranean Sea (Al:POC ~ 10mmol/mol) for low and
intensive dust input areas respectively [Dulaquais et al., unpublished data]. Data used are in accordance with
Sherrel and Boyle [1992] dataset in the Sargasso Sea area
2)
Estimation of the atmospheric deposition rate of soluble cobalt
The deposition of soluble cobalt from dust was inferred from the dust deposition rate (Equation 1) and
the abundance of cobalt in dusts.
Model
FCoatm.. = Dust deposition rate * [Co]dust
F SCo = FCoatm. * S Co
Equation 4
Equation 5
Parameters:
[Co]dust : mean concentration of cobalt reported in aerosols of the West Atlantic (in g.g -1;17 10-6 - 170 10-6 g.g-1;
[Gaiero et al., 2003; Reid et al., 2003; Rudnick and Gao, 2003; Baker et al., 2007; Trapp et al. 2010; Xia & Gao,
2010; Shelley et al., 2012; Rachel Shelley personal communication]; ts01)
FCoatm. : Atmospheric deposition flux of cobalt calculated using Equation 4 (in mol.m -2.d-1 using the molar mass
of Co (59 g.mol-1) and the number of days per year (1 year = 365.25 d); ts01)
SCo : Cobalt fractional solubility of atmospheric particles in seawater. Without data available at date it is
assumed that S Al = S Co (supplement 2). Values used are in the range of reported data [Gaiero et al., 2003;
Baker et al., 2007; Turoczy et al., 2010; Shelley et al., 2012]
F SCo. : dust deposition rate of soluble cobalt calculated using Equation 5 (in mol.m -2.d-1; ts01)
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