Review on ocean acidification and carbon cycling in the Arctic Ocean

Abstract

The uptake of anthropogenic CO2 by the ocean decreases seawater pH and carbonate mineral aragonite saturation state ( Ω arag), a process known as Ocean Acidification (OA). This could be detrimental to marine organisms and ecosystems. It is well known that the Arctic Ocean is a Bellwether for prospective changes in the global ocean acidification. Over the past two decades, global warming and climate change have caused rapid changes in the Arctic, especially in the western Arctic Ocean. These changes include rapid sea ice retreat and increases in sea surface temperatures, Pacific water inflow, freshwater storage, primary production, and surface CO2 concentrations. Projected climate change processes are thought to amplify OA in the Arctic Ocean, and absorption of CO2 and carbon burial in the arctic ocean makes it more rapid and significant in response to climate change than other basins. Based on the advantages of high-level China-US cooperation over the past decade, thorough and complete work of data collection, sorting and analysis for the 7 Chinese Arctic Research Expedition (CHINARE-Arc) cruises from 1999 to 2016 with the icebreaker R/V Xuelong, and a supplement of other US database collected during 1994−2011 which have provided opportunities for Chinese and U.S. scientists to investigate the status and changes of the Arctic Ocean. In this review, we first summarize the climate and environmental change in the arctic over the past 20 years. These changes have important implications for the carbon cycle and ocean acidification. With respect to carbon cycle, we summarize influences of rapidly changing sea-ice and growing Pacific inflow on the absorption of atmospheric CO2 in the Chukchi Sea, the response of deep-ocean carbon sequestration (continental shelf pump) to the changing environment, and to summarize a hypothesis of low-low-high variation trend in sea surface water p CO2 along a decreasing ice-cover gradient across three different zones in the western Arctic Ocean under a rapid sea ice melting scenario in the summer. In particular, high resolution survey of sea surface CO2 level (high p CO2) were reported across the Canada Basin, showing a great increase over earlier observations. Contrary to the current prediction, the Arctic Ocean basin would not become a large atmospheric CO2 sink under ice-free conditions. In addition, we review the rate and scale of ocean acidification in the western Arctic Ocean and evaluate links to environmental and climate change over the past two decades. Our new research indicates that, between the 1990s and 2010, acidified waters has increased in the upper 250 m from 5% to 31% of the total area north of 70°N. The Arctic Ocean is the first open ocean where we see such a large-scale increase in acidification, more than two times faster than that observed in the Pacific or Atlantic oceans. Tracer data and model simulations suggest that increased Pacific Winter Water (PWW), driven by retreating sea ice and anomalous circulation patterns in the summer season, is primarily responsible for this OA expansion. Finally, the future changes of carbon cycling and ocean acidification in the context of climate change are outlined.