Inorganic riverine freshwater tracers and water mixing identifiers in southeastern Hudson Bay

Abstract

This thesis was designed to investigate the potential use of barium (Ba) concentrations as well as strontium (Sr) concentrations and Sr isotope ratios as freshwater tracers in southeastern Hudson Bay with an emphasis on the Belcher Islands. Ba concentrations ranged from 40 to 672 nM with higher concentrations observed near river mouths in the winters of 2015-2017. This contrasts with Sr concentrations where the higher levels were found offshore (up to 105 µM). Both surface profiles and depth profiles of Ba concentrations in conjunction with coloured/chromophoric dissolved organic matter (CDOM) absorption coefficient and salinity showed clear signs of freshwater mixing in the southeastern side of the Belcher Islands sourced from nearby rivers. A three-endmember Ba mixing model was applied to determine fractions of freshwater, seawater, and sea ice melt at various sites. Based on the fraction of freshwater determined using oxygen isotope ratio, La Grande River was identified as the major source of freshwater to the Belcher Islands whereas the influence of the Great Whale River was found to be only locally important. Sr concentrations were successfully employed to identify mixing between freshwater and seawater when used in conjunction with CDOM absorption coefficients (a[355]) and salinity. 87/86Sr ratios (ranging from 0.7092 in open ocean to 0.7100-0.7340 near river mouth) were also used in a two-endmember model to assess mixing processes. The predicted 87/86Sr ratios were within 0.002% of the observed ratios in the Nelson, Winisk and Great Whale estuaries. Although the two-member model was less robust around the Belcher Islands, it found that the Great Whale river was not the main contributor in the region, confirming the Ba results. Together these results showed that both Sr and Ba can be used in a complementary fashion to other freshwater tracers (salinity, a[355], δ18O) and make for great tools in modelling freshwater fractions. However, due to their low sensitivity of freshwater detection, they should not be used as a standalone tracer. These tracers used in a complementary fashion can suggest which rivers have the greatest impact to the freshwater environment and ultimately need to be more seriously preserved.