Abstract
The Asian summer monsoon affects the lives of billions of people. With the aim of identifying geochemical tracers for the monsoon related freshwater input from the major rivers draining into the Bay of Bengal and Andaman Sea we have taken surface seawater samples from various locations up and down the Andaman Islands during 2011. Importantly, in some locations samples have been taken in March, July and November, covering most of a seasonal cycle and different monsoon phases. Samples were collected from the side of small wooden boats or while swimming and were filtered within a few hours at 0.45 or 0.22 microns using the vacuum produced by a water jet or a hand operated peristaltic pump. Filtered and unfiltered samples were acidified to < pH 2 and analysed for Y and the REEs with an automated online preconcentration ICP-MS technique [1]. The local input of REEs from streams and sediment rich areas such as mangrove environments is clearly identified by middle REE enrichments in the shale normalised patterns of some samples. These middle REE bulges accompany large increases in dissolved REE concentrations at some locations, especially for the July samples obtained during the peak monsoon season with frequent storms. Y/Ho fractionation aslo occurs during the local input of dissolved REEs with affected samples having lower Y/Ho ratios. Conversly, some samples, in particular those taken after heavy rainfall in March, show strong REE scavenging accompanied by the prefferential removal of dissolved light REEs and higher Y/Ho ratios. The time series at a location away from local input sources shows remarkably similar REE patterns and concentraions in March and July. Then in October-November, following the peak in monsoon river discharge, the dissolved REE concentration increases by almost a factor of 2. The notable exception to this seasonal pattern is the Ce anomally which is around 0.3 in March and November but 0.6 in July, implying less oxidative removal of Ce(IV) during the peak summer monsoon rains. With the exception of elevated dissolved Ce concentrations, the North Pacific Deep Water normalised REE patterns are similar to those reported for offshore samples from the Bay of Bengal and Andaman Sea [2]. These seawater normalised patterns are distinctive having a middle REE enriched arc with similar light and heavy REE values suggesting the input from large rivers in the region is traceable using seawater REE chemistry. [1] Hathorne et al. (2012), Online preconcentration ICP-MS analysis of rare earth elements in seawater, Geochem. Geophys. Geosyst., 13, Q01020, doi:10.1029/2011GC003907. [2] Amakawa et al. (2000), Geochim. Cosmochim. Acta 64, 1715-1727.
Highlights
The rare earth elements (REEs) have been recognized as a means to detect trace metal input from rivers to coastal waters since the pioneering work of Elderfield and colleagues in the 1980s (Hoyle et al, 1984; Goldstein and Jacobsen, 1988b; Sholkovitz and Elderfield, 1988; Elderfield et al, 1990)
Rapid removal following an exceptional rainfall event in March 2011 left the remaining Nd with a markedly radiogenic isotope composition, and as a result of this, the dissolved Nd isotope composition varies by some 7 εNd units across the islands
Changes in the yttrium and rare earth element (YREE) and Nd isotope composition within 1 year are observed at three fixed locations, confirming recent suggestions that short-term temporal variations occur in the Bay of Bengal (BoB)
Summary
The rare earth elements (REEs) have been recognized as a means to detect trace metal input from rivers to coastal waters since the pioneering work of Elderfield and colleagues in the 1980s (Hoyle et al, 1984; Goldstein and Jacobsen, 1988b; Sholkovitz and Elderfield, 1988; Elderfield et al, 1990). The importance of the colloidal pool is most obvious when river water first meets salty water in the estuary and saltinduced coagulation removes most, if not all, of the REEs contained in the colloidal pool (e.g., Sholkovitz and Szymczak, 2000). This results in a range of estuarine Nd removal from 97 to 40% (Rousseau et al, 2015 and the references therein; Adebayo et al, 2018) and exerts a fundamental control on the REE signal of river input and the REE budget of the oceans. Organic matterbound REE behaves nearly conservatively, whereas waters rich in inorganic colloids quickly lose their REE load as salinity increases (Merschel et al, 2017a)
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