LINKING POST-GLACIAL (HYDRO)GEOLOGY TO REDOX GEOCHEMISTRY AND ECOLOGY OF MIDWESTERN LAKES

The late Eville Gorham’s influential work looked holistically at the elemental inventory of lakes and wetlands, then by using mass-balance accounting and statistics to infer mechanisms, linked geochemical cycles to ecological processes. His words are a great reminder that hypothesis-driven research is rooted with observations: "My kind of science is based on chance and serendipity. Something will turn up.”[1] This shared scientific process unites limnologists from both the ecological and the geological sides of the spectrum.

As a geochemist interested in the pervasive redox-stratified oceans of Earth’s past, I have been drawn to study some seemingly quirky upper-Midwest lakes: those that are meromictic with > mM concentrations of dissolved iron in anoxic bottom waters (“ferruginous” conditions to geochemists). Such conditions arise from strong redox gradients within a lake, which in turn result from favorable morphometric parameters, an active carbon cycle, and a supply of solutes. After studying several of these lakes in the Midwest, accessing state-available data, and digging deep into the limnological literature, I have begun to wonder if some of these features are not quirky, but instead common among our lake-rich states. These observations have led me to hypothesize that ferruginous and/or meromictic systems may be an expected result of (hydro)geological features of post-glacial landscapes. I will discuss predictions and tests of this hypothesis, as well as whether data collected so far supports this hypothesis.

While ferruginous lakes are of interest to a geochemist, an ecologist might ask, does iron matter? If time permits, I will discuss a few examples of how this redox-active element and nutrient influences the ecology (encompassing phytoplankton and microbes) within ferruginous lakes, and therefore in-lake cycling of carbon. Hypotheses and data arising from these observations bring me back full circle, and enrich our understanding of how the marine carbon cycling might have operated in its redox-stratified past.

[1] Bright, E. (2020) “Prominent acid rain researcher Eville Gorham dies.” Minnesota Public Radio News, 15 January.