CAREER: Quantifying the Extent and Biogeochemical Impact of Modern Ferruginous Lakes

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Many small, deep lakes formed during the melting of continental ice sheets that covered much of the northern hemisphere before 10,000 years ago. It is hard for oxygen to mix into small, deep lakes, which makes them “microbial worlds”, as most aquatic animals can’t live without oxygen. This project will test whether such small lakes that lack oxygen also contain abundant iron, and whether that iron is supplied from the surrounding glacial deposits. With no oxygen, and abundant iron, lake microbes can produce methane out of sinking algal or plant material. Road salt applications near lakes might also cause lakes that did mix to stop mixing, making conditions better for methane-producing microbes. This work will give a better idea how lake shape, microbes, and water chemistry contribute to lake emissions of methane, which is an important but not well-understood contributor to natural greenhouse gas emissions to the atmosphere. The chemistry of these lakes is also similar to the oceans before 500 million years ago, before animals appeared, and so we can use them to help understand if oceans were producing methane in the past. Undergraduate and graduate students will collect field data from lakes in a rural and urban area in Minnesota (MN) as part of class and research expeditions. The field methods, findings, and data will be shared through traditional (publications and conferences) and non-traditional (YouTube videos, Wikipedia pages) outlets. Workshops in Minneapolis, MN will engage diverse youth, including residents of a historically African American neighborhood adjacent to an urban study lake. Workshops in Itasca State Park, MN, the site of the rural lakes, will engage the visiting public and residents of surrounding counties and nearby Native American population centers.There are only 22 ferruginous meromictic lakes worldwide identified in the literature, but do more exist? Most of the recognized ferruginous meromictic lakes are postglacial, with groundwater or peatlands a hypothesized, but unverified source of iron. More than a dozen postglacial lakes in two field areas in Minnesota (MN) will be evaluated for meromixis and ferruginous conditions, giving insight into how land use changes enhance meromixis. Detailed iron budgets will be constructed for two lakes to identify and quantify iron sources. Many ferruginous meromictic lakes have a subsurface chlorophyll a maximum layer at the iron-oxygen redoxcline, and the contribution of this layer to primary productivity and oxygen production will be quantified in two lakes. Furthermore, rates of methanogenesis, methane fluxes, and the contribution of methane to the organic carbon degradation will be quantified in two lakes to better constrain carbon fluxes under ferruginous conditions. The results of these activities will better delineate 1) the extent of meromictic ferruginous lakes, 2) fluxes of water and elements between surface and groundwater reservoirs, 3) and carbon cycling under ferruginous conditions, which predominated in Earth's past oceans and influenced the oxidation of Earth's surface.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.