Interpreting the History of Lake Anoxia Using Iron and Sulfur Geochemistry

Details Interpreting the History of Lake Anoxia Using Iron and Sulfur Geochemistry Interpreting the History of Lake Anoxia Using Iron and Sulfur Geochemistry

May 2004

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004agusmgc42a..05k&link_type=abstract

American Geophysical Union, Spring Meeting 2004, abstract #GC42A-05

Mathematics

Logic

1030 Geochemical Cycles (0330)

Scientific paper

Over the last thousand years, anthropogenic activities, such as land cultivation and atmospheric pollution, have increased the flux of growth-limiting nutrients to several North American lakes. The most common effect of this enhanced nutrient supply is a dramatic increase in the abundance of aquatic plants and algae. When these organisms die, the organic carbon in their remains falls through the water column and is oxidized by dissolved oxygen. Thus, eutrophic lakes are characterized by seasonally or permanently anoxic bottom waters because the rate of organic carbon oxidation exceeds the rate at which oxygen is replenished. The depletion of water column oxygen adversely affects lake ecosystems by decreasing water quality and by altering the community structure of fish and algae populations. In this study we examine the history of lake anoxia in two North American lakes, Half-Moon Lake in Michigan and Crawford Lake in Ontario. Sediment freeze cores and water column samples were taken from each site and both lakes contained well-preserved varved sediments. The sediments were analyzed for FeH (HCl-extractable iron), AVS (acid-volatile sulfur), CRS (chromium-reducible sulfur), d34S(CRS), CaCO3, CH2O, C/N, and d13C (CaCO3). Water samples were analyzed for pH, O2, cations, anions, and d34S(SO4). Today, Half-Moon Lake is seasonal anoxic while Crawford Lake has not overturned in the past ~15 years. Geochemical and biological data indicate that both lakes have experienced cultural eutrophication events in the 1800-1900s related to European-style agricultural practices. In addition, Crawford Lake experienced an earlier eutrophication episode around 1325 A.D. related to Iroquoian settlement of the area. Each eutrophication event showed an increase in the mass accumulation rate of pyritic sulfur, suggesting that sediments were exposed to longer durations of low-O2, H2S-rich waters during periods of cultural eutrophication. The geochemical parameter DOP (degree of pyritization) is defined as pyrite Fe/(pyrite Fe + FeH) and quantifies the fraction of iron that is converted into pyrite. DOP values from Half-Moon Lake are low (~0.1) prior to European settlement and increase to high values (>0.7) after European settlement. In Crawford Lake, our preliminary data indicate that the Iroquoian horizon has lower DOP values than the European horizon. This indicates that the water column was less sulfidic (more oxygen-rich) during the 1300s than in the 1800-1900s. These results show that Fe and S geochemistry can be used to document the history of lake anoxia.

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