Sedimentary geolipid records of historical changes in the watersheds and productivities of Lakes Ontario and Erie

The organic matter contents of sediments deposited in Lakes Erie and Ontario contribute to the record of changes in the lake watersheds and aquatic ecosystems which have resulted from European settlement and cultural eutrophication of these systems over the past two centuries. Compositions of n-alkanes and n-alkanoic acids extracted from lake sediments track the clearing of the original natural forests and the appearance of agriculture in the watershed areas beginning about 1820. Aquatic productivity increased as runoff of soil nutrients increased. Cultural eutrophication in the 1950s is recorded in increases in organic C and in n-C,, alkane concentrations. Diagenetic effects overprint source changes as shorter chain length and unsaturated lipid components are preferentially removed from the sedimentary record. The geolipid fraction of sedimentary organic matter is a combination of the original, biologically synthesized lipid materials and diagenetically derived materials. Geolipid biomarker compounds and their distributions have proved useful in studies of the histories of organic matter accumulations in lake sediments. Differences in aquatic productivity are recorded in the types and amounts of geolipids incorporated into lake sediments and by their degree of diagenetic reworking (Cranwell 1978; Kawamura and Ishiwatari 1985). Straight-chain acids and alcohols, in particular, seem to be especially sensitive to preservational conditions (Meyers et al. 19843; Wiinsche et al. 1988; Ho and Meyers 1994) and hence signal changes in depositional environments. Hydrocarbon molecules are more resistant to such factors and thereby provide better indications of the original geolipid sources (e.g. Giger et al. 1980; Prahl and Carpenter 1984; Ho and Meyers 1994). The combined distributions of sedimentary alkanes, fatty acids, alcohols, and sterols have enabled reconstructions of postglacial histories of organic matter deposition for Cam Loch, Scotland (Cranwell 1977), Heart Lake, New York (Meyers et al. 1984a), and Voua de la Motte, France (Wiinsche et al. 1988). These lacustrine depositional histories include indications of changes in watershed vegetation, in rates of aquatic productivity, and in amounts of organic matter preservation in bottom sediments. Geolipid studies of sediment cores from Lake Huron, one of the Laurentian Great Lakes, have similarly revealed evidence of paleodepositional changes (Meyers et al. 1980; Meyers and Takeuchi 198 1). Of particular prominence are the changes associated with conversion of the lake watershed from forested land to farm land and with more recent cultural eutrophication. Lake Huron is the second largest of the Laurentian Great Lakes and is generally considered to be impacted relatively less than other Great Lakes, except Lake Superior, by these human changes. We report here the recent geolipid depositional records of sediment cores from Lakes Ontario and Erie. We obtained our sediment samples with a 40-cm box corer (50-cm square) at locations having different sedimentation rate;3 so that accumulation of geolipids under different conditions could be studied. Stations E30 and G32 in the Rochester basin of Lake Ontario (Fig. 1) were sampled in 198 1. Station LS in the eastern basin and station G 16 in the central basin of Lake Erie (Fig. 1) were sampled in 1982. The Lake Ontario cores are hand-collected subcores that were sectioned vertically by hydraulic extrusion into l-cm intervals to 20 cm and 2-cm intervals from 20 to 40 cm on-board ship immediately after collection. The Lake Erie cores were similarly collected and sectioned. Sections were frozen on-board ship in solvent-cleaned glass jars, then freeze-dried within 3 months of collection. The data presented for each site come from a single box core. Companion ,subcores from the box cores were dated by the 210Pb methods used by Robbins and coworkers on Great Lakes cores (e.g. Robbins and Edgington 1975). The excess 210Pb contents of the sediments were applied to a steady-state mixing model, and sediment accumulation rates were determined (Eisenreich et al. 1989; Schelske et al. !. 988). Linear and mass accumulation rates of sediments dlztermined by J. A. Robbins (pers. comm.) for the four locations are summarized in Table 1. Approximate deposition dates have been assigned to each section of the cores using the mass sedimentation rates determined on companion subcores and the cumulative dry weights of the present cores. Data are presented relative to deposition dates rather than core depths to facilitate discussion and comparisons among the sites. Smearing of 137Cs peaks in cores from both lakes is evidence of sediment mixing from bioturbation and indicates that the bottom waters have remained oxygenated, at least for most of each year. This mixing integrates the accumulation of a single year over a multiyear depth. The temporal .resolution of near-surface sediments from station E30 is 11 yr and that at station G32 is 13 yr (Eisenreich et al. 1989). The resolution is finer where sedimentation rates are greater and where periods of sum-