Using 226 Ra and 228 Ra isotopes to distinguish water mass distribution in the Canadian Arctic Archipelago

<p>As a shelf-dominated basin, the Arctic Ocean and its biogeochemistry are heavily influenced by continental and riverine sources. Radium isotopes (<span class="inline-formula">²²⁶Ra</span>, <span class="inline-formula">²²⁸Ra</span>, <span class="inline-formula">²²⁴Ra</span>, and <span class="inline-formula">²²³Ra</span>), are transferred from the sediments to seawater, making them ideal tracers of sediment–water exchange processes and ocean mixing. As the two long-lived isotopes of the radium quartet, <span class="inline-formula">²²⁶Ra</span> and <span class="inline-formula">²²⁸Ra</span> (<span class="inline-formula">²²⁶Ra</span> with a <span class="inline-formula"><i>t</i>_1∕2</span> of 1600 years and <span class="inline-formula">²²⁸Ra</span> with a <span class="inline-formula"><i>t</i>_1∕2</span> of 5.8 years) can provide insight into the water mass compositions, distribution patterns, as well as mixing processes and their associated timescales throughout the Canadian Arctic Archipelago (CAA). The wide range of <span class="inline-formula">²²⁶Ra</span> and <span class="inline-formula">²²⁸Ra</span> activities, as well as of the <span class="inline-formula">²²⁸Ra∕²²⁶Ra</span>, measured in water samples collected during the 2015 GEOTRACES cruise, complemented by additional chemical tracers – dissolved inorganic carbon (DIC), total alkalinity (AT), barium (Ba), and the stable oxygen isotope composition of water (<span class="inline-formula"><i>δ</i>¹⁸O</span>) – highlight the dominant biogeochemical, hydrographic, and bathymetric features of the CAA. Bathymetric features, such as the continental shelf and shallow coastal sills, are critical in modulating circulation patterns within the CAA, including the bulk flow of Pacific waters and the inhibited eastward flow of denser Atlantic waters through the CAA. Using a principal component analysis, we unravel the dominant mechanisms and apparent water mass end-members that shape the tracer distributions. We identify two distinct water masses located above and below the upper halocline layer throughout the CAA and distinctly differentiate surface waters in the eastern and western CAA. Furthermore, we highlight water exchange across 80<span class="inline-formula">^∘</span>&thinsp;W, inferring a draw of Atlantic water (originating from Baffin Bay) into the CAA. This underscores the presence of an Atlantic water “U-turn” located at Barrow Strait, where the same water mass is seen along the northernmost edge at 80<span class="inline-formula">^∘</span>&thinsp;W as well as along the southeasternmost confines of Lancaster Sound. Overall, this study provides a stepping stone for future research initiatives within the Canadian Arctic Archipelago, revealing how quantifying disparities in the distributions of radioactive tracers can provide valuable information on water mass distributions, flow patterns, and mixing within vulnerable areas such as the CAA.</p>

Abstract licence: CC BY

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