Physical and biogeochemical controls on the variability in surface pH and calcium carbonate saturation states in the Atlantic sectors of the Arctic and Southern Oceans

Eithne Tynan,Jennifer S Clarke,Matthew P Humphreys,Mariana Ribas-Ribas,Mario Esposito,Victoire M.C Rérolle,C Schlosser,Sally E Thorpe,Toby Tyrrell,Eric P Achterberg

doi:10.1016/j.dsr2.2016.01.001

Abstract

Polar oceans are particularly vulnerable to ocean acidification due to their low temperatures and reduced buffering capacity, and are expected to experience extensive low pH conditions and reduced carbonate mineral saturations states (Ω) in the near future. However, the impact of anthropogenic CO2 on pH and Ω will vary regionally between and across the Arctic and Southern Oceans. Here we investigate the carbonate chemistry in the Atlantic sector of two polar oceans, the Nordic Seas and Barents Sea in the Arctic Ocean, and the Scotia and Weddell Seas in the Southern Ocean, to determine the physical and biogeochemical processes that control surface pH and Ω. High-resolution observations showed large gradients in surface pH (0.10–0.30) and aragonite saturation state (Ωar) (0.2–1.0) over small spatial scales, and these were particularly strong in sea-ice covered areas (up to 0.45 in pH and 2.0 in Ωar). In the Arctic, sea-ice melt facilitated bloom initiation in light-limited and iron replete (dFe>0.2nM) regions, such as the Fram Strait, resulting in high pH (8.45) and Ωar (3.0) along the sea-ice edge. In contrast, accumulation of dissolved inorganic carbon derived from organic carbon mineralisation under the ice resulted in low pH (8.05) and Ωar (1.1) in areas where thick ice persisted. In the Southern Ocean, sea-ice retreat resulted in bloom formation only where terrestrial inputs supplied sufficient iron (dFe>0.2nM), such as in the vicinity of the South Sandwich Islands where enhanced pH (8.3) and Ωar (2.3) were primarily due to biological production. In contrast, in the adjacent Weddell Sea, weak biological uptake of CO2 due to low iron concentrations (dFe<0.2nM) resulted in low pH (8.1) and Ωar (1.6). The large spatial variability in both polar oceans highlights the need for spatially resolved surface data of carbonate chemistry variables but also nutrients (including iron) in order to accurately elucidate the large gradients experienced by marine organisms and to understand their response to increased CO2 in the future.

Highlights

Arctic, sea-ice melt facilitated bloom initiation in light-limited and iron replete regions, such as the Fram Strait, resulting in high pH (8.45) and Ωar (3.0) along the sea-ice edge
Pteropods living in polar waters will be most affected by a decrease in aragonite saturation state (Comeau et al, 2012), and live shell dissolution has been reported in the Southern Ocean (Bednaršek et al, 2012)
Horizontal gradients in surface pH and Ωar in the Arctic and Southern Ocean were due to the prevalence of different physical and biogeochemical processes in each ocean (Fig. 13)

Summary

Introduction

Sea-ice melt facilitated bloom initiation in light-limited and iron replete (dFe4 0.2 nM) regions, such as the Fram Strait, resulting in high pH (8.45) and Ωar (3.0) along the sea-ice edge. During summer months polar surface waters have typically higher calcium carbonate saturations states due to intense primary productivity, as shown in the Arctic shelf seas including the Chukchi Sea (Bates et al, 2009), the Amundsen Gulf (Chierici et al, 2011) and the Barents Sea Opening (Tynan et al, 2014), and in western (MattsdotterBjörk et al, 2014) and eastern Antarctica (Roden et al, 2013). This results in more favourable pH and Ω conditions for organisms during summer. Glacial melt (Evans et al, 2014) and river runoff (Mathis et al, 2011) were found to drive low saturation states in the subpolar Pacific

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