It is not just about the ice: a geochemical perspective on the changing Arctic Ocean

J Environ Stud Sci (2015) 5:288–301 DOI 10.1007/s13412-015-0302-4 It is not just about the ice: a geochemical perspective on the changing Arctic Ocean R. W. Macdonald 1,2 & Z. A. Kuzyk 2 & S. C. Johannessen 1 Published online: 16 July 2015 # The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Much concern has accompanied the dramatic decrease in area covered by permanent pack ice in the Arctic Ocean during the past two decades. Ice is undeniably the most obvious feature distinguishing the Arctic Ocean, and its loss seizes public and scientific attention like no other tipping point. Beneath that challenging ice surface lies an ocean that is strongly affected by other less-visible factors that also have a large say in how change will occur in this ocean. Especially important to the Arctic Ocean is its connection to the surrounding land, which feeds it fresh water and organic carbon, and the large shelves and enclosed geography that accentuate the importance of these external factors. Like the sea ice, land is changing rapidly due to widespread thawing of permafrost. For the three global risks that have been deeply thought about recently in the context of Arctic Ocean ecosystems (i.e. contaminants, warming, ocean acidification), the Arctic appears to be exceptionally sensitive, sufficiently so that it has been termed a bellwether for each. Here, we examine how the lessvisible factors (fresh water, organic carbon cycling) affect the Arctic’s reception of risk and its potential to export risk to the rest of the globe. We conclude that there needs to be a better coordinated effort to collect time series for the terrestrial * R. W. Macdonald Z. A. Kuzyk S. C. Johannessen 1 Department of Fisheries and Oceans, Institute of Ocean Sciences, PO Box 6000, Sidney, BC, Canada V8L 4B2 2 Centre for Earth Observation Science and Department of Geological Sciences, University of Manitoba, Winnipeg, MB, Canada R3T 2 N2 components cycling within the Arctic Ocean such that we can understand what is happening to the marine components. Keywords Arctic Ocean . Change . Fresh water . Organic carbon Introduction There is no doubt that the Arctic Ocean is undergoing change. If you ask the public or arctic scientists what constitutes that change, both groups would most likely answer first that the sea ice is disappearing. Once that is agreed upon, the discussion of the significance of vanishing ice provokes differing views. For the public, it is the uncertain future faced by polar bears and other charismatic animals; for people who live in the north, it is threats to culture, health, food security and travel; and for scientists, it is feedbacks that affect not only the function of the Arctic but also the potential for Arctic change to impact global systems. All of these topics have merit and urgency. Numerous articles in the popular press and the scientific literature during the past two decades have focussed, almost obsessively, on the ongoing decline in the Arctic’s sea ice. Although most of these articles present the loss of ice as a disaster unfolding, some propose that more open water provides opportunities for exploration, exploitation and transport and, with these, challenges to sovereignty. These notions, which tie the loss of ice to sustainable development of renewable and non-renewable Arctic resources (i.e. economics), technological advancement, management and governance, have clinched Arctic sea-ice loss as a topic of immediate regional, national and international relevance. The obvious visibility of sea ice, which is the face of the Arctic Ocean, deflects attention away from other features of J Environ Stud Sci (2015) 5:288–301 this ocean that are not as visible, but are assuredly as critical for understanding the changes faced by this region. A biogeochemist would find the Arctic Ocean unique among world oceans, with unique vulnerabilities, whether or not there was any sea ice. In this paper, we will take the position that other factors—fresh water runoff and terrigenous organic carbon—play equally critical roles on the Arctic Ocean stage. The approach that we will follow here, therefore, will be first to discuss the salient oceanographic features that make the Arctic Ocean what it is, and from there discuss the biogeochemical changes faced by this ocean. Fresh water—the significance of runoff, precipitation and ice melt in the Arctic Ocean To an ocean scientist, the hydrological cycle begins at the estuary, and nowhere is this more true than in the Arctic Ocean, which itself may be viewed as a grand estuary (McClelland et al. 2012). Because it receives only 11 cm year−1 of precipitation, the Arctic Ocean qualifies as a desert. Nevertheless, the 2000 km3 year−1 of direct precipitation is augmented by a further ~3300 km3 year−1 of fresh water that flows into the ocean’s margin from four large rivers (Lena, Ob, Yenisei, Mackenzie) and numerous smaller ones (Figs. 1 and 2). As this fresh water circulates and passes through the Arctic Ocean, it produces a low density surface layer (<50 m thick) often referred to as the polar mixed layer (PML) (Fig. 3). The Arctic Ocean is also a conduit through which ~2500 km3 year−1 excess fresh water from the Pacific Ocean is able to return to the Atlantic, thus maintaining a longterm balance in the global fresh water cycle (e.g. see Wijffels et al. 1992). Not all the seawater has the same salinity or density, and waters of different densities do not mix easily. Consequently, various water masses enter the Arctic Ocean at different depths, then find their place among the layered, or stratified, waters (Fig. 2). Pacific water that enters through Bering Strait is less dense than that which comes into the Arctic Ocean from the Atlantic side. However, some of this Pacific water becomes modified by processes shortly after it enters the Arctic Ocean, becoming slightly more dense by cooling and the addition of brine from ice production over the Chukchi Sea in winter. These changes force it below the Arctic Ocean’s surface layer (PML) where it forms a layer of cold salty water termed a halocline because salt content increases with depth (Fig. 3, Pacific Halocline (50–300 m)). Over on the Atlantic side, the densest (saltiest) water enters the Arctic Ocean. Seasonal cycling of the surface water, which includes mixing of sea-ice melt, cooling and the addition of brine to this Atlantic water in the Barents Sea (Rudels 2015), also causes surface water to subduct under the Arctic Ocean surface layer forming the Atlantic Halocline (Fig. 2), further reinforcing salt stratification. These large-scale processes lead 289 to a robust salinity stratification throughout the Arctic Ocean where each layer is progressively denser with depth (Fig. 3). Stratification is the single most important control for how the Arctic Ocean can respond to forcing from inside or outside the Arctic. It takes time for fresh water to pass (...truncated)