Award: OCE-1435708

Lately, chemical oceanographers have been studying trace elements like iron that are beneficial to life and others that are the opposite. Arsenic is famously toxic to people, but it is also toxic to microscopic marine plants, the phytoplankton. This occurs because arsenic is almost chemically identical to the essential element phosphorus and when the concentration of dissolved phosphorus is the same as or lower than arsenic, it can affect the growth of marine phytoplankton. If these abundant phytoplankton are not growing, they cannot take up carbon dioxide, which then means more of this greenhouse gas in the atmosphere, and the whole planet is adversely affected. To make this a bit more complicated, it turns out that the toxicity of arsenic to phytoplankton depends on its chemical form – arsenate or As+5 is just like the nutrient form of phosphorus, phosphate, and hence is toxic. However, arsenite or As+3 is not toxic at all, nor are organic forms of arsenic. Interestingly, we have found that some marine phytoplankton can change the chemical form of arsenate to detoxify it; they convert it to arsenite and methyl arsenic. If a trace metal like iron is always good and arsenic is always bad, are there elements that can switch, good at one level and toxic at another? Indeed, the trace element selenium is one of these – essential for many enzymes at one concentration and chemical form, but then toxic when it is too high or the wrong chemical form. Thus, plants in the ocean are affected by not only light and nutrients, but also trace elements like iron, arsenic, and selenium. Interestingly, coal has high arsenic and selenium levels, and studies in the Pacific and Atlantic Oceans show that coal combustion enriches these elements in the oceans. But, what about the high latitude Arctic Ocean, what are arsenic and selenium doing in this rapidly changing ocean? It turns out that they have never been studied here, but there are two compelling reasons why we should: first, it is changing fast and in this century it could be more like the Atlantic or Pacific Oceans – are arsenic and selenium cycled differently in the present Arctic and will it change in a new Arctic? Secondly, there is evidence that fossil fuel combustion in Asia is enriching at least selenium in the Bering Sea, so could this increase the concentration of this element in the current and future Arctic? These and many other questions were addressed in the 2015 US GEOTRACES Expedition to the Arctic Ocean aboard the US Coast Guard Cutter Healy, our only research ice breaker capable of transiting all the way to the North Pole. We left Dutch Harbor, Alaska on 9 August 2015, made it to the North Pole on 6 September, and returned to Dutch Harbor on 11 October; the route is shown in Figure 1. Although we sampled the water column from top to bottom, surface water data illustrate how selenium and arsenic behave differently in the current Arctic (Figure 2). First, essential phosphate concentrations were pretty uniform across the more productive, ice-free shelves to the ice-covered deep basins. Not shown is nitrate, but it was at very low to non-detectable concentrations, implying that it was bio-limiting and allowed phosphate to remain elevated. These higher phosphate concentrations then minimize arsenate uptake and toxicity, although arsenate does vary along the transect. Nevertheless, the detoxification products like arsenite had unusually low concentrations except for monomethyl As (Fig. 2). Surface concentrations of arsenate were approximately 15% higher than other ocean basins, so if the future Arctic has higher phytoplankton growth and lowers phosphate, the toxicity of arsenic may increase as in other oceans. The concentration of total selenium only varied slightly along the 6000 km transect (Fig. 2), but unlike other oceans, its concentration was about 25% higher. Further, the most bioavailable form of selenium, selenite (SeIV), was elevated in surface waters, even on the more productive shelves. While we cannot rule out slow biological uptake to explain selenium?s higher levels, data for selenium in atmospheric aerosols taken during the cruise show the highest concentrations in the western (return) leg of the expedition (Figure 3) and could deposit into the Arctic. To establish the source of atmospheric selenium, either dust or emissions from fossil fuel combustion, we normalized the Se data to aluminum that only comes from soils; this gives the enrichment factor (EF). If the selenium is soil-derived, the EF is one, but the addition of gaseous selenium from coal combustion raises it. The EF values in Fig. 3 are 5,000 and above, documenting a likely pollution source for selenium that deposits into the Arctic Ocean. If the Arctic has less sea ice in the near future, this selenium, and other elements, could be directly transferred into the water column. Last Modified: 04/23/2019 Submitted by: Gregory A Cutter