Aluminium in an ocean general circulation model compared with the West Atlantic Geotraces cruises

Abstract

International audienceA model of aluminium has been developed and implemented in an Ocean General Circulation Model (NEMO-PISCES). In the model, aluminium enters the ocean by means of dust deposition. The internal oceanic processes are described by advection, mixing and reversible scavenging. The model has been evaluated against a number of selected high-quality datasets covering much of the world ocean, especially those from the West Atlantic Geotraces cruises of 2010 and 2011. Generally, the model results are in fair agreement with the observations. However, the model does not describe well the vertical distribution of dissolved Al in the North Atlantic Ocean. The model may require changes in the physical forcing and the vertical dependence of the sinking velocity of biogenic silica to account for other discrepancies. To explore the model behaviour, sensitivity experiments have been performed, in which we changed the key parameters of the scavenging process as well as the input of aluminium into the ocean. This resulted in a better understanding of aluminium in the ocean, and it is now clear which parameter has what effect on the dissolved aluminium distribution and which processes might be missing in the model, among which boundary scavenging and biological incorporation of aluminium into diatoms

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

“…However, a recent comparison of thirteen Fe-incorporating GBMs found that 'all models struggle to reproduce many aspects of the observed spatial patterns' (Tagliabue et al, 2016). For other metals, such as Mn and Al, good agreement between observed and modeled values have been achieved in select parts of the global ocean (van Hulten et al, 2017(van Hulten et al, , 2014(van Hulten et al, , 2013. However, each GBM still struggles to replicate certain processes (e.g.…”

How well can we quantify dust deposition to the ocean?

“…In regions where removal of dissolved metals by scavenging occurs at a much faster rate than transport by advection, ocean circulation is expected to contribute little to the overall uncertainty in dust deposition calculated from dissolved metal inventories [45,88]. In highly advective regimes, such as the eastern half of our study area under the influence of the North Equatorial Current [75], the dissolved metal distribution will be spatially smeared to a degree that may be quantifiable as biogeochemical modelling of dissolved trace element cycles improves.…”

“…A critical factor limiting the use of dissolved Al inventories to quantify the supply of dust is the uncertainty of the dissolved Al residence time, including its sensitivity to spatial and temporal variability of biological productivity as well as the abundance and composition of particles that scavenge Al from the water column [45,53,88,89]. To illustrate this point, we calculate the residence time of dissolved Al in surface waters that would be implied by applying an assumed aerosol settling rate of 1000 m day −1 to the surface aerosol Al concentration measured along section GA03 ( §3a) and multiplying by each set of measured aerosol Al solubility results ( §3c).…”

“…Another major unknown that limits model-derived estimates of dissolved Al residence time is whether dissolved Al is scavenged primarily by diatoms [88] or if it is removed more uniformly by sorption to all particulate phases [89]. Additional study of the scavenging behaviour of Al is warranted to explore its sensitivity to the abundance and composition of particles and thereby improve the algorithms used to derive dust fluxes from measured dissolved Al inventories.…”

“PRISTINE”, a new high volume sampler for ultraclean sampling of trace metals and isotopes

“…We closed all 24 PRISTINE samplers at the same depth near the bottom at 1015 m. The concentrations of DAl were similar in all 24 bottles with an average concentration of 17.41 ± 0.22 nM (Figure 7a). The concentrations of DFe were very similar in 22 out of 24 bottles with an average concentration of 1.029 ± 0.045 nM (bottles 2 & 18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 , Al (Bruland et al, 2014;Middag et al, 2015;van Hulten et al, 2013), Hg (Lamborg et al, 2014), Co (Dulaquais et al, 2014) and the organic complexation of Fe . This confirms that the intended ultraclean performance of the PRISTINE samplers is excellent.…”