Ironing out the Arctic carbon cycle
In sunlit waters or dark soils, evidence from my group suggests that the conversion of DOM to CO2 in the Arctic may be controlled by a close linkage of iron and carbon chemistry (Ward & Cory 2016), (Page et al. 2014), and (Page et al. 2013). For example, analysis of photochemical conversion of DOM to CO2 across several hundred streams, rivers, thaw ponds, and lakes in the Arctic over the past 6 years has revealed that the best predictor for photochemical conversion of DOM to CO2 was high vs. low iron waters. Our work suggests that the explanation for this pattern is an interaction of DOM and Fe that may enhance photochemical reactions that convert DOM to CO2 (Ward & Cory 2016). We are currently working with scientists at EMSL to identify organic compounds associated with iron, which is at the forefront of geo- and environmental chemistry due to the chemical complexity of DOM, a mixture of thousands of organic molecules each with unknown affinity to bind with iron and alter iron chemistry and reactivity.
In addition to a role for iron in sunlit surface waters, iron may also play a role in DOM degradation in dark soils. Reduced (ferrous) iron is made by microbes in waterlogged soils or sediments, when microbes run out of oxygen to breathe. There are high concentrations of ferrous in waterlogged soils over the summer in the Arctic. When this iron-rich water gets flushed with air, the ferrous iron interacts with oxygen to make very reactive radicals and oxidants (i.e., reactive oxygen species, called ROS). ROS such as hydroxyl radical can interact with and breakdown DOM, converting DOM to CO2 or to more labile organic moieties that bacteria can turn into CO2. We showed for the first time that ROS are present in soils, i.e., dark hydroxyl radical was found in all soil waters of the dominant vegetation of the low Arctic (Page et al. 2013). As we expected, this dark hydroxyl radical was formed in proportion to the amount of reduced iron present. Our first approximation suggested that dark ROS could yield similar amounts of CO2 as produced from bacterial oxidation of DOM in surface waters of the Arctic.