Assessing the utility of barium isotopes to trace Eurasian riverine freshwater inputs to the Arctic Ocean
Introduction
The surface of the Arctic Ocean contains a large amount of freshwater, which stratifies the water column controlling circulation, sea-ice formation, and biological productivity (Carmack et al., 2016). The export of freshwater stored in the Arctic Ocean to the North Atlantic can also influence global meridional overturning circulation (Aagaard and Carmack, 1989). The storage, distribution and export of Arctic Ocean freshwater have varied over the past century, with particularly pronounced changes during recent decades, which have important implications for both regional and global climate (Rabe et al., 2014; Proshutinsk et al., 2015; Haine et al., 2015; Alkire et al., 2017). River inputs are the main source of Arctic Ocean freshwater, contributing 4200 ± 420 km3 yr−1, along with net precipitation (2200 ± 220 km3 yr−1) and the inflow of low salinity Pacific seawater through the Bering Strait (2640 ± 100 km3 yr−1) (Haine et al., 2015). Freshwater distribution within the Arctic Ocean is further controlled by the seasonal formation and melting of sea-ice (e.g. Rosén et al., 2015), and changes in wind driven circulation (Proshutinski and Johnson, 1997; Steele and Boyd, 1998; Johnson and Polyakov, 2001; Morison et al., 2012).
Unraveling the roles of the multitude of freshwater sources that govern Arctic Ocean freshwater distributions requires the use of multiple geochemical tracers. Oxygen isotopes and nutrients (phosphate and nitrate) are routinely used to quantify ice melt/formation, Pacific inflow and meteoric (river discharge and net precipitation) contributions to Arctic Ocean freshwater inventories (e.g. Yamamoto-Kawai et al., 2008; Rosén et al., 2015). Dissolved Ba concentrations have further been used to distinguish between freshwater inputs from Eurasian versus North American rivers (Guay and Falkner, 1997; Macdonald et al., 1999; Guay et al., 2001; Taylor et al., 2003; Dodd et al., 2009; Guay et al., 2009; Yamamoto-Kawai et al., 2010; Roeske et al., 2012; Charette et al., 2020). The use of this tracer has helped to reveal the role of riverine freshwater transport pathways for controlling key features of Arctic Ocean hydrography. Notably, that Eurasian riverine freshwater is an important component of the large freshwater inventory stored in the Beaufort Gyre (Guay et al., 2009), and of freshwater exported to the North Atlantic via the Fram Strait (Taylor et al., 2003; Dodd et al., 2009). By contrast, this tracer suggests freshwater inputs from the major North American river, the Mackenzie, do not make significant contributions to freshwater inventories in the central Arctic basins, and are instead are predominantly exported via the Canadian Arctic Archipelago (Guay et al., 2009).
The utility of dissolved Ba to trace Arctic Ocean riverine freshwater relies upon a difference in Ba concentration between freshwater inputs from major Eurasian and North American rivers (Guay and Falkner, 1998). This approach assumes that the Ba concentrations of these freshwater inputs are constant and that Ba behaves conservatively within the Arctic Ocean. Both of these assumptions, however, are questionable. In particular, dissolved Ba does not behave conservatively in the ocean, being removed from the upper water column and regenerated at depth in response to the production and export of organic matter (e.g. Jacquet et al., 2016). Non-conservative depletions and enrichments of dissolved Ba have been documented in the Barents Sea, Laptev Sea, Chukchi Sea, Gulf of Amundsen and to the north of Svalbard (Guay and Falkner, 1997; Abrahamsen et al., 2009; Thomas et al., 2011; Roeske et al., 2012; Hendry et al., 2018), regions that represent important transport pathways of major riverine freshwater and seawater sources to/from the central Arctic Ocean basins. Barium inputs from continental margins, via submarine groundwater discharge and/or diagenetic release from sediments (e.g. Mayfield et al., 2021) could further impact the use of dissolve Ba as a conservative tracer of Arctic Ocean freshwater sources. Both biological productivity and trace metal inputs from Arctic margins have increased in recent decades in response to declining sea-ice cover and permafrost thawing (Arrigo and van Dijken, 2015; Williams and Carmack, 2015; Kipp et al., 2018). With these on-going changes the utility of dissolved Ba to trace riverine freshwater transport within the Arctic Ocean will become increasingly questionable without additional constraints (Abrahamsen et al., 2009).
Stable barium isotope variations are a new tool that can provide additional insights into marine Ba sources and the processes controlling marine Ba cycling. The cycling of Ba between the surface and deep ocean, linked to the biological carbon pump, is associated with significant Ba isotope fractionation resulting in coupled changes in dissolved Ba concentration and isotope composition (Horner et al., 2015; Bates et al., 2017; Hsieh and Henderson, 2017; Bridgestock et al., 2018; Geyman et al., 2019; Cao et al., 2020a; Cao et al., 2020b). Riverine and submarine groundwater discharge inputs could potentially result in mixing relationships between dissolved Ba concentrations and isotope compositions that are distinct from those resulting from these non-conservative marine processes (Hsieh and Henderson, 2017; Mayfield et al., 2021; Bridgestock et al., 2021). These systematics could be exploited to resolve the impact of biogeochemical cycling versus conservative mixing on Arctic Ocean dissolved Ba distributions. In turn this would improve the application of Ba to reliably trace riverine freshwater transport pathways within central Arctic Ocean basins. The utility of such an approach depends on the specific Ba concentration-isotope composition relationships produced by riverine freshwater-seawater mixing and non-conservative Ba cycling in the Arctic Ocean. To assess this, dissolved Ba concentration and isotope compositions for surface seawaters from the Siberian Shelf and the Bering Sea/Strait are presented. These data constrain the Ba isotope composition of major Eurasian river inputs and Pacific water inflow and document the impact of non-conservative processes on mixing relationships between Arctic Ocean water sources.
Section snippets
Study area and samples
The Siberian shelves, notably the Kara, Laptev and East Siberian Seas, receive ~60% of the total riverine freshwater inputs to the Arctic Ocean (Dickson et al., 2007), ~80% of which is supplied by the rivers Yenisey (620 km3 yr−1), Lena (520 km3 yr−1) and Ob (390 km3 yr−1) (Milliman and Farnsworth, 2011) (Fig. 1). The rivers Yenisey and Ob discharge freshwater into the Kara Sea, which is transported either northwards into the Eurasian basin, mixing with seawater of Atlantic origin, or eastwards
Analytical techniques
The Ba isotope compositions of waters were determined using thermal ionization mass spectrometry (TIMS; TRITON instrument, Thermo Scientific), and a previously described double spike technique to correct for instrumental mass bias (Hsieh and Henderson, 2017). Sample aliquots of 50 ml were accurately weighed and equilibrated with a known quantity of a 135Ba137Ba double spike solution. The Lena River water sample was evaporated to dryness and refluxed in 15 M HNO3 to digest any organic material.
Results
Dissolved Ba concentrations are highest proximal to the Lena delta (209 nmol kg−1) at salinity 2.7, and generally decrease with increasing salinity to 38–70 nmol kg−1 (Fig. 2, Table 1). Two distinct relationships between dissolved Ba concentration and salinity are observed (Fig. 3a,b). Samples from the Lena River freshwater plume extending eastward into the East Siberian Sea, along with the sample proximal to the Lena delta, define a negative correlation (r2 = 0.82) with salinity, with a
Constraints on the Ba concentration and δ138/134Ba of major Eurasian river inputs to the Arctic Ocean
The Yenisey (620 km3 yr−1), Lena (520 km3 yr−1) and Ob (390 km3 yr−1) are the three largest rivers in terms of supplying freshwater to the Arctic Ocean, representing ~80% of total Eurasian river discharge (Dickson et al., 2007; Milliman and Farnsworth, 2011) (Fig. 1). To estimate the Ba concentrations and δ138/134Ba of freshwater input from these rivers, mixing relationships between these variables and salinity are extrapolated to salinity 0 (Fig. 3, Fig. 4, Fig. 5; Supplementary Information).
Conclusions
Mixing between freshwater inputs from major Eurasian rivers, the Yenisey, Lena and Ob, and Atlantic and Pacific derived seawater are traced by relationships between salinity, Ba concentration and δ138/134Ba. These water sources are constrained to feature δ138/134Ba of 0.23 ± 0.04‰ (average Eurasian river freshwater), 0.53 ± 0.06‰ (Atlantic-derived seawater) and 0.55 ± 0.03‰ (Pacific-derived seawater). Non-conservative processes are inferred to modify these mixing relationships along the Laptev
Author contributions
L. B. conceived and designed the study, with input from Y-T. H., D. P. and G. M. H. Sample collection was conducted by D. P. and P. S. A. Data was produced by J. N., L. B., Y-T. H. and P. H. The manuscript was written by L. B. with input from all of the authors.
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
Robert F. Anderson and Martin Q. Fleisher (Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, USA) are thanked for providing seawater samples from the Bering Sea/Strait collected during the GEOTRACES GN01 section cruise. We also thank the captains, crews, Igor Semiletov and cruise participants of the H/V Yacob Smirnitskyi during the International Siberian Shelf Study (ISSS-08; August to September 2008), and the USCGC Healy during the GEOTRACES GN01 section cruise
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