Distribution and cycling of dissolved aluminium in the Arabian Sea and the Western Equatorial Indian Ocean

Highlights

• Surface dAl distribution shows a strong east-west gradient in the Arabian Sea.

• Water mass advection plays an important role in controlling the surface dAl distribution in the studied regions.

• Surface dAl scavenging timescales in the Arabian Sea between the summer and fall seasons are very short: days to months.

• The intrusion of high-salinity water masses influences the thermocline distribution of dAl in the Arabian Sea.

Abstract

Dissolved aluminium (dAl) distribution has been studied over the full vertical water column profiles along the GEOTRACES-India (GI) transect, GI-05, in the Arabian Sea (AS) and the Western Equatorial Indian Ocean (W-Eq.IO) during the fall inter-monsoon period. Surface dAl distribution in the AS demonstrates an east-west gradient, i.e., elevated dAl (6.5–21.6 nM) close to the Indian coastal region and low dAl (1.5–3.6 nM) along the western boundary of the AS. Rapid surface dAl removal due to relatively high biological productivity and a decrease in atmospheric dust deposition during the fall inter-monsoon result in low surface dAl levels in the western AS. Mass balance for surface dAl variation in the AS reveals very short scavenging removal timescales (0.01–0.47 yr) between the mid-summer and fall inter-monsoon period. Further, dAl input/dilution due to surface water advection is found to play an important role in controlling the surface dAl variation, varying between 190 and 300% of the dust-supported dAl input in different regions of the AS. These results have important implications for the use of surface dAl as a proxy of dust deposition in the AS. In the W-Eq.IO, a relative increase observed in the surface dAl concentrations, compared to the western AS and the central equatorial region, suggests a local dAl input, presumably, due to the dust influx from the Somali coast. Probable mechanism for this could be dust input from Somalia to the coastal western equatorial region and subsequent advection of the dAl enriched coastal waters to the offshore sampling sites, facilitated by mesoscale eddies.

The intrusions of the high salinity water masses (the Arabian Sea High Salinity Water and the Persian Gulf Water) in the thermocline depths (~75–300 m) are observed to carry the dAl-rich signal of their formation regions to the open AS. This dAl enrichment in thermocline waters is, however, mostly restricted to the northern and north-western AS during the study period. Correlated dissolved Al and Fe maxima observed in the deep water over the Murray Ridge in the northern AS suggest Al and Fe release from reactive clay minerals (e.g., illite), found abundantly in the sediments deposited over the ridge. Further, elevated dAl levels were also seen near the Laxmi-Panikkar-Palitana ridge system (~10.0 nM) and the Carlsberg Ridge (~4.5 nM) in the AS and the W-Eq.IO, respectively.

Introduction

Dissolved aluminium (dAl) distributions in surface water have been extensively used as a proxy for the atmospheric mineral dust input to the open ocean (Measures and Vink, 1999; Measures et al., 2005; Measures et al., 2015; Grand et al., 2015a). The rationale behind exploiting surface dAl to estimate mineral dust deposition flux to the ocean include (1) high abundance (~8% by weight; McLennan, 2001) and largely invariant composition of Al in the upper continental crust, a predominant source of mineral aerosols over the open ocean regions (Mahowald et al., 2005 and references therein), (2) short removal timescales of dAl in the surface ocean (~few weeks to 4 years, Orians and Bruland, 1986), (3) insensitivity of parent dAl species (i.e., Al(OH)₄⁻and Al(OH)₃⁰) to redox changes in seawater (Bruland et al., 2014 and references therein; Measures and Vink, 2000), and (4) no (known) role of Al in active biological processes. Constraints on eolian dust deposition over the marine environment are crucial to estimate the atmospheric input of bio-essential metals (e.g., Fe, Mn, P) to the ocean, which set major controls on the abundance and diversity of the marine phytoplankton in the open oceanic regions (Jickells et al., 2005; Sunda, 2012 and references therein). Apart from natural dust sources, land-based anthropogenic activities (e.g., coal combustion, fuel-oil combustion, ore smelting, commercial constructions, etc.) may lead to fine aerosol emissions, which could potentially be transported (by wind) over large distances in the marine region (Hashimoto et al., 1992; Kaufman et al., 2005; Kedia and Ramachandran, 2008; Philip et al., 2017). The relative impact of these anthropogenic emissions on surface dAl would, however, strongly depend on: (1) fractional contribution of anthropogenic aerosol to the total dust input and (2) dominant processes contributing to the anthropogenic emissions, which control the metal composition of the anthropogenically emitted dust (Hashimoto et al., 1992). Although, the anthropogenic aerosol component over the global ocean is estimated to be significant (~20%, Kaufman et al., 2005), its impact on the surface dAl concentrations, upon deposition, is still uncertain and crustal-derived mineral dust is considered the dominant external Al source to the ocean waters (van Hulten et al., 2013; van Hulten et al., 2014).

In several open ocean regions, surface dAl-based estimates of dust deposition fluxes are found to correlate well with that determined using shipboard aerosol sampling and composite aerosol models (Grand et al., 2015a; Measures and Vink, 2000). Close to the continental margins, however, surface dAl distribution may have additional (other than dust deposition) controls due to significant input from fluvial discharge and/or resuspension of the shelf sediments (Menzel Barraqueta et al., 2018; Middag et al., 2012; Middag et al., 2015; Singh et al., 2020). Moreover, in the dynamic marine system, the impact of water mass advection on the surface dAl distribution may not be neglected a-priori (Baker et al., 2016). The relative importance of advection on modulating the surface dAl distribution may be understood by comparing the timescales of dAl removal and surface water advection in a given oceanic region (Baker et al., 2016; van Hulten et al., 2013). Particle scavenging processes predominantly control the output flux of dAl from the seawater, where passive adsorption of Al onto the biogenic particulates and their subsequent removal out of the water column is particularly important (Middag et al., 2015; Moran and Moore, 1988b; Orians and Bruland, 1986). Consequently, residence time of surface dAl with respect to particle scavenging is influenced by the biological productivity and export of biogenic particles out of the upper water column (Orians and Bruland, 1986). In summary, it is essential to understand and decouple the potential impacts of the processes mentioned above (other than atmospheric dust deposition) on the surface dAl distribution prior to using it as a tracer of continental mineral dust input over the ocean.

The Arabian Sea (hereafter, AS) is characterized as one of the highest dust receiving regions in the global ocean (Jickells et al., 2005; Mahowald et al., 2005). Dust emission, and its subsequent transport, from various neighboring arid (e.g., southern Arabian Peninsula, Makran Basin, Sistan Basin) and semi-arid (Thar Desert) continental regions contribute to the high net annual dust deposition (5.4–16.7 g m⁻² yr⁻¹, Schüßler et al., 2005 and references and therein) estimated over the AS. However, significant seasonal and spatial variability in dust concentrations in the surface-level aerosols over the AS and deposition is observed (Chester et al., 1985; Kumar et al., 2008; Kumar et al., 2012; Pease et al., 1998; Siefert et al., 1999; Tindale and Pease, 1999). This variability arises in response to the marked seasonal changes in wind stress over the AS and surrounding continents, which influences the dust production and its transport from different neighboring continental sources (Schott and McCreary, 2001; Tindale and Pease, 1999). Total atmospheric mineral dust deposition to the surface ocean may include both wet and dry deposition. Relative importance of wet mineral dust deposition depends on the precipitation patterns (both spatial and temporal), size distribution of aerosols and transportation altitude (Jickells et al., 2005; Jickells et al., 2016; Kadko et al., 2020; Li et al., 2008). There are no direct observational studies comparing the wet and dry mineral dust deposition over the open AS. However, both direct and satellite data-based investigations (Suresh et al., 2021a; Ramaswamy et al., 2017) show that during the summer monsoon long-range transport of mineral dust from the Arabian Peninsula, north-east Africa and the Thar Desert and its subsequent washout via precipitation (wet deposition) is the dominant pathway of dust deposition (~3 times higher than dry deposition) at the eastern AS coastal region. The eastern AS (east of ~65°E) receives the majority of the annual precipitation occurring in the AS (Behara et al., 2019; Kumar and Prasad, 1997; Shetye et al., 1994) with annual mean of ~1.0–1.5 m yr⁻¹; therefore, wet deposition of mineral dust could possibly influence the surface dAl distribution in the eastern AS. Comparatively, the western AS (west of ~65°E) receive, relatively, insignificant rainfall (<0.1–0.4 m yr-1, Behara et al., 2019; Kumar and Prasad, 1997; Shetye et al., 1994) throughout the year.

Significant spatio-temporal changes in surface water dAl concentrations in the AS are observed (Measures and Vink, 1999; Schüßler et al., 2005). Seasonal variations in dust deposition flux and biological production (Barber et al., 2001; Kumar et al., 2000) may play an important role in controlling the net balance between the input and output Al flux and result in significant temporal changes in surface water dAl concentrations in the AS. On the other hand, advective dAl input/dilution in a given region depends on the gradient of dAl and velocity of ocean current. The impact of advection on surface dAl also becomes important to study given that spatial variations in surface dAl and current (Schott et al., 2009; Wyrtki, 1973) are observed in the AS. In this study, we attempt to quantify the input and removal of surface dAl through these processes (dust deposition, particle scavenging and advection) to understand their relative role in controlling the surface dAl distribution in different regions of the AS.

Most of the labile Al from the eolian dust deposited over the ocean is released in the surface mixed layer (Maring and Duce, 1987). In the sub-surface depths, water mass advection and reversible scavenging of Al from suspended (near continental slope, ridges and abyssal region) and sinking particulates become more important in controlling the dAl distribution (Grand et al., 2015b; Middag et al., 2015; Singh et al., 2020; van Hulten et al., 2013). The impact of water mass advection on sub-surface dAl distribution is observed in different oceanic regions (Measures et al., 2015; Middag et al., 2015; Singh et al., 2020; Grand et al., 2015b). In the AS, the thermocline water mass structure (200–900 m) is influenced by the outflow of high salinity water masses from the neighboring marginal seas (the Red Sea, the Persian Gulf; Shenoi et al., 1993). These shallow marginal basins receive high eolian dust input from the nearby deserts (e.g., the Rub-Al-Khali and the Nubian Desert), affecting the trace element distribution in their water column (Chase et al., 2011; Measures and Vink, 1999; Shriadah et al., 2004). These sub-surface waters in the AS could entrain into the surface layer under the strong upwelling conditions that develop during the summer and winter monsoon periods (Wyrtki, 1973). The probable impact of the advection of waters from the marginal seas on the dAl and other lithogenic trace metals (e.g., Fe, Mn) distribution in the AS is, therefore, important to evaluate.

Earlier studies (Measures and Vink, 1999; Narvekar and Singbal, 1993; Schüßler et al., 2005; Thi Dieu and Sohrin, 2013; Upadhyay and Sen, 1994) had investigated the surface dAl concentrations in the AS. However, these studies were mostly restricted to the summer and winter monsoon periods, with minimal spatial coverage during the intermonsoon seasons. Also, full vertical depth dAl profiles in the AS are limited (Narvekar and Singbal, 1993; Schüßler et al., 2005; Thi Dieu and Sohrin, 2013; Upadhyay and Sen, 1994). In this study, we measured full vertical water column dAl profiles across the AS during the fall inter-monsoon period (late September-mid October) to better understand the (1) annual surface dAl cycling in response to the seasonal variations in dust deposition over the AS and (2) processes controlling the dAl distribution in the thermocline and deeper waters of the AS. This is the first report on surface dAl distribution in the AS during the fall intermonsoon period and, therefore, adds to the previous understanding of surface dAl annual cycling in the Sea (Measures and Vink, 1999; Schüßler et al., 2005; Thi Dieu and Sohrin, 2013). This study also reports the first comprehensive investigation of dAl cycling over the full vertical water column in the western AS and the Western Equatorial Indian Ocean (hereafter, W-Eq.IO).