Ocean Dust Deposition Rates Constrained in a Data‐Assimilation Model of the Marine Aluminum Cycle

Abstract

The formation of organic matter in the surface ocean and its sinking and remineralization through the water column lead to a net carbon (C) transfer to depth that is known as the biological carbon pump (Ducklow, 2001). Changes in strength of the biological pump can repartition carbon dioxide (CO 2 ) between the atmosphere and ocean (Passow & Carlson, 2012), and have likely contributed to major climatic shifts through Earth's history, including the Pleistocene glacial-interglacial cycles (Martinez-Garcia et al., 2014). Production of organic matter by marine phytoplankton requires a suite of nutrient elements, each with their own unique biogeochemical cycles, which can limit primary production and regulate biological C storage in the ocean (Moore et al., 2013).Dust deposition on the surface ocean is an important source of trace metal micronutrients like iron (Fe), which serves as a cofactor in various algal pigments and enzymes (Morel et al., 1991). Dissolved Fe (dFe) scarcity directly limits phytoplankton growth and prevents the utilization of the major nutrients nitrate (NO 3 ) and phosphate (PO 4 ) at high latitudes (Moore et al., 2013), and limits the growth of nitrogen-fixation planktons that add new NO 3 to the system in low-latitude systems where NO 3 is the primary limiting nutrient (Moore et al., 2009). Therefore, Fe availability is an important factor regulating C export to depth in most ocean regions (Moore et al., 2013). There are three major external sources of Fe to the global ocean: (a) aeolian transport of dust particles, mostly from desert regions (Jickells et al., 2005); (b) release from continental margin sediments (Elrod et al., 2004); (c) injection of metal-rich hydrothermal fluids along mid-ocean ridges (Tagliabue, 2014). However, the magnitude of these sources and their relative importance in fueling phytoplankton growth remains highly uncertain (Tagliabue et al., 2016), which limits our understanding of the biological pump's sensitivity to perturbation of the marine Fe budget. Because dust is the only process that delivers Fe directly to the euphotic zone in the open ocean, it is often considered the most important Fe source to phytoplankton communities, suggesting that changing dust fluxes can drive global climate change through the biological pump (Martin, 1990).

Phytoplankton community response to episodic wet and dry aerosol deposition in the subtropical North Atlantic

“…Although this calculation is subject to significant uncertainties associated with residence time and fractional solubility (Measures and Brown 1996;Barraqueta et al 2019), which will subsequently propagate to deposition flux estimates, this range is generally consistent with previous observations in the region (Fig. 1a; Albani et al 2014) and results from other models (Han et al 2008;Xu and Weber 2021). Notably, two significant increases in model-derived deposition fluxes were found on 17 September (1.6 mg m À2 d À1 ) and on 24 September to 25 September (2.1 and 3.0 mg m À2 d À1 , respectively).…”

Low Nitrogen Fixation Related to Shallow Nitracline Across the Eastern Indian Ocean

“…No significant correlation was found between nitrogen fixation and aerosol iron supply (Table 2). Furthermore, nitrogen fixation was low even in the BOB (Figure 7), which receives high dust deposition from the continents, as confirmed by previous studies (Garcia, Baer, et al, 2018;Grand, Measures, Hatta, Hiscock, et al, 2015;Grand, Measures, Hatta, Morton, et al, 2015;Mahowald et al, 2005;Xu & Weber, 2021), implying that low nitrogen fixation in the EIO was not attributed to iron availability. This is different from the general assumption that lower nitrogen fixation rates are related to low iron supply, as represented by the eastern South Pacific Ocean and central South Atlantic Ocean (Großkopf et al, 2012;Knapp et al, 2016;Wang et al, 2019).…”

“…This study assumed dust deposition as a primary iron source (Jickells et al., 2005), since the upwelling of iron supply does not sufficiently support phytoplankton growth in the Indian Ocean (Vu & Sohrin, 2013). The aerosol supply rates of soluble Fe were extracted from the published model results of Xu and Weber (2021). This model was based on a combination of the GEOTRACES 2017 archive of intercalibrated dissolved aluminum (dAl) observations with a data‐assimilation global ocean model.…”

“…This model was based on a combination of the GEOTRACES 2017 archive of intercalibrated dissolved aluminum (dAl) observations with a data-assimilation global ocean model. It should be noted that the aerosol iron supply estimated in this study (Xu & Weber, 2021) represents the annual trend and may involve uncertainty due to the seasonal monsoon in the EIO. However, the model output agreed well with the surface dissolved iron (dFe) concentration during the cruise from St. 1 to 10 (r = 0.92, Pearson's correlation; Ikhsani, 2022), confirming the validity of the output as a proxy for the aerosol iron supply.…”

Global Patterns of Surface Ocean Dissolved Organic Matter Stoichiometry

“…We find that modeled dust deposition rates increase and NPQ-corrected φ sat decreases from East to West across the Pacific Ocean (Figure 7). Gradients in both metrics suggest that phytoplankton experience less iron stress in the Western than Eastern Pacific Ocean, consistent with observations that iron limits 2), DON:DOP concentration ratios (dashed red line, same data as in Figure 2), surface ocean P* (black line), water column denitrification rates (Wang et al, 2019) (blue line), NPQ-corrected φ sat (green line), and dust deposition rates from 12 different model outputs (Xu & Weber, 2021) (gray lines). Shadings reflect the 95% confidence interval.…”