Two lakes in east-central Minnesota, in close proximity and on the same substrate, show divergent carbon storage histories, possibly linked to climate change or vegetation shifts in the catchment. Green Lake, eutrophic and hydrologically open, has sediments rich in calcium carbonate (inorganic carbon, "IC"; 30-70% by weight low-Mg calcite) throughout the Holocene. Spectacle Lake, 1 km away, has no surface outflow. It shows a rapid transition from 20% carbonate to high organic carbon ("OC") sediments (up to 64% organic matter with <1% carbonate) at a depth of 8m below the sediment surface, possibly associated with the mid-Holocene dry period. Green Lake carbonate remains high until a precipitous drop at the time of basin clearance and residential shoreline development in the late 19th century. This pair of lakes represents two parallel natural experiments which show differential response to the same climate and vegetation changes. My study proposes a detailed comparative analysis of the sediments across four transitions: Late Glacial to Holocene, mid-Holocene, late 19th century settlement, and late 20th century remediation. Through comparison of the rate and style of natural and anthropogenic transitions, I seek to clarify the controls on OC and IC preservation and accumulation in the lacustrine environment, and test the idea that small biogeochemical shifts elicit dramatic responses in lake systems.
Lake sediments are significant sinks for carbon both from the landscape and from within the lake (Dean & Gorham 1998); at the same time they are often net sources of carbon to the atmosphere (Cole et al 1994). Recent studies by Aller (1994), Ahlgren et al (1997), Wachniew and Rozanski (1997), Hodell and Schelske (1998), and Hodell et al (1998) have examined and modeled aquatic carbon cycling in terms of biological and chemical processes; however, less attention has been paid to geological aspects, inorganic carbon components (e.g. calcite), and changes in the lake carbon system over time. My study focuses on threshold transitions between IC and OC dominance in order to resolve the relative impacts of production and preservation, biotic changes, chemical and hydrologic shifts, and nutrient loading. The inverse relation of IC and OC holds true for many Minnesota lakes (Dean 1999a), so this detailed study provides a basis for a regional comparative analysis of lower analytical complexity.
Lakes respond rapidly to environmental changes, so their sediments provide a high-resolution integrative record of productivity, chemistry, climate, and human impacts in the lake and its watershed. The inherent sensitivity of small lakes suggests that the abrupt divergence observed in Green and Spectacle results from a relatively minor shift in some chemical parameter in one or both of the lakes. An understanding of such threshold behavior is vital in light of human impacts on lakes and increasing atmospheric CO2. Algal and microbial photosynthesis in surface waters encourages carbonate precipitation by drawing down dissolved CO2 and providing a nucleation site (Hodell et al 1998); oxidation of sinking organic biomass may deplete bottom water dissolved O2 and increase CO2, promoting preservation of organic matter and dissolution of carbonates (Dean 1999a). The respective magnitudes of these fluxes, and the related interactions between in-lake processes, determine whether the individual lake is a source or a sink for C, and on a global scale, whether lakes will buffer the atmosphere or exacerbate the anthropogenic rise in carbon gases. My calculations give a conservative annual total C burial, at its maximum in the mid-1950s, of 2.6 x 105 kg C in Green Lake (200 g C m-2 y-1). Though surprisingly large, this figure is in keeping with the average rate in Minnesota lakes over the past 4000 years calculated by Dean and Gorham (1998) of 81 g C m-2 y-1.
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