Rare earth element distributions in the Arabian Sea reveal the influence of redox processes within the oxygen deficient zone
Introduction
Understanding processes occurring in modern oxygen deficient zones (ODZs) is important as ODZs are expected to increase in intensity and magnitude due to climate change (Stramma et al., 2008). The Arabian Sea contains the most dynamic modern oxygen deficient zone (ODZ) as it is driven by intense monsoon cycling, complex water mass interactions, and multiple inputs of micro and macronutrients. While fluvial inputs are relatively limited due to increased hydraulic engineering and irrigation (Kravtsova et al., 2009), dust flux to the Arabian Sea persists year-round, mainly originating from the Omani Coast (Pease et al., 1998), and δ3He values of >14% in deep seawater (2000–3000 m) is evidence of hydrothermal input to this basin (Takahata et al., 2018). Of particular relevance to this work are three high salinity water masses: the Arabian Sea High-Salinity Water (ASHSW) mass, the Persian Gulf Water (PGW) mass, and the Red Sea Water (RSW) mass, which form due to excess evaporation over precipitation (Kumar and Prasad, 1999), and have unique end member salinity signatures of 36.9, 36.1, and 35.7 psu, respectively (Goswami et al., 2014 and references therein). In particular, the PGW seems to be closely related to the upper boundary of the ODZ and may enhance the pycnocline at the upper boundary of the ODZ, particularly in the north Arabian Sea (Prasad et al., 2001). Further south, the influence of the PGW is manifested largely to the west (Acharya and Panigrahi, 2016). Four major seasons have been identified in this region: Spring Intermonsoon, Southwest (SW) Monsoon, Fall Intermonsoon, and Northeast (NE) Monsoon (Schott et al., 1990), with high rates of primary production during the NE and SW monsoons (Nair et al., 1989).
ODZs can have particularly interesting chemistry due to the reductive dissolution of transition metal oxides under low oxygen and use of alternative terminal electron acceptors, such as nitrate. This allows for accumulation of metals usually found in the dissolved form due to seawater thermodynamics, such as dissolved iron (dFe), dFe(II), and dissolved manganese (dMn), and nutrient species like nitrite (NO2; Moffett et al., 2007; Vedamati et al., 2014; Vedamati et al., 2015). Thus, it has been proposed that ODZs can serve as a source of Fe to the interior ocean (Scholz et al., 2016), and it is likely that ODZs can be a source of dMn as well. In the Arabian Sea ODZ, Moffett et al. (2015) saw both a plume of dFe and dFe(II) in the ODZ waters corresponding to the Secondary Nitrite Maximum (SNM) while other groups have seen a similar pattern of heightened dMn (Vu and Sohrin, 2013; Lewis and Luther III, 2000; Saager et al., 1989). German and Elderfield (1990) have reported the REE data for one station within the Arabian Sea ODZ. Here, we report the first ever basin-wide transect of REEs in the Arabian Sea ODZ and the implications for inputs to the basin and trace metal cycling.
Major inputs of rare earth elements (REEs) to the ocean include rivers, aeolian dust deposition (Greaves et al., 1999), and hydrothermal vents (Elderfield, 1988), with the dominant source thought to be riverine input from weathered continental rocks (Piper, 1974). Most REEs exist in the +3 oxidation state (de Baar et al., 1988), whereas Cerium (Ce) can exist in the +4 state due to redox cycling and Europium (Eu) can exist in the +2 state under elevated temperature and pressure or strong reducing environments (Sverjensky, 1984; Zheng et al., 2016). Due to systematic differences in their coordination chemistry in seawater associated with the lanthanide contraction and minor influence of selective biological cycling when compared to transition metals, REEs form a predictable distribution pattern with characteristic anomalies in seawater when normalized to Post Archean Australian Shale (PAAS; Tostevin et al., 2016; Elderfield et al., 1990). In seawater there is a steady increase from light REEs (LREEs) to heavy REEs (HREEs), with the exception of Ce, due to preferential scavenging of LREEs by carbonate complexes (Byrne and Kim, 1990; Zhong and Mucci, 1995). Eu anomalies can be evidence of a high temperature hydrothermal input (Sverjensky, 1984) as iron oxides in vent fluids are efficient scavengers of Eu (Olivarez and Owen, 1991). Deviations from a typical REE pattern can be used as a tracer for trace element input and cycling in seawater (Behrens et al., 2016). Cerium is strongly depleted relative to the other REE in seawater because of microbially-mediated scavenging by Mn oxidizing bacteria (Moffett, 1990).
Section snippets
Sample collection
Samples were collected aboard the R/V Roger Revelle (RR0708) during the SW monsoon season, from August to September 2007 (Fig. 1) using 5 L Teflon coated external spring Niskin-type bottles (Ocean Test Equipment) with a trace metal clean rosette (Sea-Bird Electronics). Samples were filtered via 0.2 μm filters (Nuclepore), stored in acid-washed low-density polyethylene (LDPE) bottles, and acidified in the laboratory to pH ~ 1.7 via 1 mL of hydrochloric acid (Optima, Fisher Scientific) per 500 mL
Results
Temperature, salinity (practical salinity units, PSU), oxygen and nitrite for this transect are shown in Fig. 2a–d.The data reveal 4 separate regions within the transect: stations 22 and eastward are within the true ODZ, as defined by elevated nitrite and negligible oxygen concentrations, stations 20 and 21 are within a transition zone, stations 11/18 (same location re-occupied on a separate date) exhibit low oxygen and no nitrite accumulation, and stations 15 and 16 are in well oxygenated
Aeolian input into the Arabian Sea
As seen in Fig. 3a, the surface trend shows heightened abundance of LREEs and lower fractionation between the LREEs and HREEs, relative to deeper samples While this pattern can be attributed to input from terrigenous sediments, another potential source is dust deposition. Dust can be an important source of Nd and other LREEs (e.g. Spivack and Wasserburg, 1988). Satellite imagery reflects seasonal periods of high dust (Sirocko and Sarnthein, 1989) and sediment cores from the Arabian Sea suggest
Conclusions
The Arabian Sea is one of the most dynamic marginal seas due to the complex water mass interactions and seasonal changes. With many potential inputs, REEs can aid in constraining the sources of biologically important transition metals. The coupling of the Ce/Ce* to elevated dMn in a plume coming off part of the Indian shelf is strong evidence for a shelf-to-basin shuttle that has been proposed in other ODZs. However, deviations from this relationship are being influenced by inputs from the oxic
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank the captain and crew of the R/V Revelle and Xiaopeng Bian, Paulina Pinedo-Gonzalez, and Kenneth Bolster for assistance in method development. This manuscript benefitted from perceptive feedback from Christopher German on an early draft and two anonymous reviewers. This research was supported by the US National Science Foundation Grant OCE1636332.
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