Hypolimnetic dominance of epilimnetic dissolved inorganic carbon (DIC) δ13C and its relationship to isotopic covariance in the sedimentary record from a meromictic urban lake (Lake McCarrons, Minnesota, USA)
Abstract
Water column stratification is an influential process in the lacustrine carbon cycle. In meromictic lakes with persistently anoxic bottom waters, the hypolimnetic processing and allocation of C may be more important than epilimnetic productivity in determining δ13C values of dissolved inorganic carbon (DIC), and by extension the δ13C values of organic and carbonate sedimentary components. In addition, hypolimnetic processes may be central to reconciling seemingly contradictory records from authigenic carbonates and autochthonous organic matter. Resolution of the biogeochemical and physical processes involved in determining mean δ13CDIC values and the direction and relative magnitude of δ13CDIC anomalies is critical to understanding the widely used sedimentary records of anoxic, meromictic lakes. It is likely that methanogenesis, advection via thermocline erosion, and ebullition of methane gas are key processes in the evolution of epilimnetic δ13CDIC values over time. Lake McCarrons (Minneapolis-St. Paul metropolitan area, population 3.4x106) provides an intriguing field setting for examining the effects of hypolimnetic processes on the lacustrine carbon cycle as represented in both the modern system and paleorecord. An isotope mass-balance model illuminates the observed systematic relationship between carbon and oxygen isotopic values of authigenic calcite (the poorly understood "isotopic covariance" of Talbot and others), and changes in the phasing of oscillations in the two records following the establishment of persistent hypolimnetic anoxia and stratification in the early 20th century. The model parameterizes simplified lake mixing, carbon allocation, and isotope fractionation processes, allowing examination of the sensitivity of sediment isotope trends in meromictic systems to changes in these limnologic characteristics, as well as tentative generalizations to other meromictic lake systems.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.B21E0304M
- Keywords:
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- 0404 BIOGEOSCIENCES / Anoxic and hypoxic environments;
- 0428 BIOGEOSCIENCES / Carbon cycling;
- 0454 BIOGEOSCIENCES / Isotopic composition and chemistry;
- 0458 BIOGEOSCIENCES / Limnology