Inorganic carbon cycling and biogeochemical processes in an Arctic inland sea (Hudson Bay)

William J Burt,Helmuth Thomas,Leah Pengelly,Tim N Papakyriakou,Lisa A Miller,Mats A Granskog

doi:10.5194/bg-13-4659-2016

Abstract

Abstract. The distributions of carbonate system parameters in Hudson Bay, which not only receives nearly one-third of Canada's river discharge but is also subject to annual cycles of sea-ice formation and melt, indicate that the timing and magnitude of freshwater inputs play an important role in carbon biogeochemistry and acidification in this unique Arctic ecosystem. This study uses basin-wide measurements of dissolved inorganic carbon (DIC) and total alkalinity (TA), as well as stable isotope tracers (δ18O and δ13CDIC), to provide a detailed assessment of carbon cycling processes within the bay. Surface distributions of carbonate parameters reveal the particular importance of freshwater inputs in the southern portion of the bay. Based on TA, we surmise that the deep waters in the Hudson Bay are largely of Pacific origin. Riverine TA end-members vary significantly both regionally and with small changes in near-surface depths, highlighting the importance of careful surface water sampling in highly stratified waters. In an along-shore transect, large increases in subsurface DIC are accompanied by equivalent decreases in δ13CDIC with no discernable change in TA, indicating a respiratory DIC production on the order of 100 µmol kg−1 DIC during deep water circulation around the bay.

Highlights

The Arctic Ocean is vulnerable to “ocean acidification”, because changes in pH and carbonate ion concentration in response to a given uptake of atmospheric CO2 are more pronounced in cold, low-alkalinity waters compared to warm waters with high alkalinity
The dilution of carbonate parameters by freshwater is more pronounced for sea-ice melt (DIC and total alkalinity (TA) 300–600 μmol kg−1; e.g., Miller et al, 2011) compared to river outflow, and distributions of dissolved inorganic carbon (DIC), TA, and Ar in the southern Hudson Bay likely reflect the variable impacts of river runoff and sea-ice melt on the carbonate system
Surface distributions reveal the importance of variable freshwater inputs from runoff and sea-ice melt in altering the carbonate system, especially in the southern part of Hudson Bay

Summary

Introduction

The Arctic Ocean is vulnerable to “ocean acidification” (defined as the combined results of decreasing pH and increasing calcium carbonate solubility), because changes in pH and carbonate ion concentration in response to a given uptake of atmospheric CO2 are more pronounced in cold, low-alkalinity waters compared to warm waters with high alkalinity. Inflowing Pacific water with naturally high CO2 concentrations, as well as dilution from sea-ice melt and river waters, exacerbates the so-called “vulnerability” of polar waters (AMAP, 2013; Shadwick et al, 2013). The Hudson Bay system receives nearly one-third of Canada’s river discharge, and Hudson Bay itself goes from complete ice cover in winter to open water in summer, culminating in an annual freshwater yield from river runoff and sea-ice melt that is more than double that of the Arctic Ocean (Granskog et al, 2011).

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Conclusion