Master of Science
Longstaffe, Fred J
The St. Agatha kettle lake deposits, ON, Canada, contain a thick section of endogenic marl and coeval shelly fauna. Marl accumulation occurred from ~13,500 to ~8,200 cal yrs BP. This study examines controls on marl and shell δ44/42Ca, element ratios (Mg, Fe, and Ba), δ18O, δ13C, and 87Sr/86Sr ratios of marl calcite, shell aragonite, groundwater, and bedrock as proxies for paleolacustrine conditions. Results for Interval A (532.0 to 475.0cm) indicates a cooler, wetter climate, lower primary lake productivity and higher lake levels. Interval B (475.0 to 345.0cm) marks the onset of a closed lake system, warming climate, and perhaps Ca-limitation on marl calcite δ44/42Ca. Data for Interval C (345.0 to 192.0cm) suggest a reduction in lake depth, increased lakewater evaporation and greater lake productivity which continues into Interval D (192.0 to 128.0cm) before marl precipitation ceases. Marl calcite δ44/42Ca is likely controlled by non-equilibrium processes in this freshwater system.
Summary for Lay Audience
The St. Agatha kettle lake deposits, ON, Canada, contain a thick section of marl and mollusc shells. Marl is made up of calcite, while mollusc shells are mainly aragonite. Both occur in the sequentially deposited St. Agatha kettle lake deposits. Marl is a white to off-white calcium carbonate (CaCO3)-rich sediment commonly found in shallow lakes that contain high concentrations of calcium (Ca) and bicarbonate (HCO3-). The marl at St. Agatha formed and accumulated as sequential laminae that have been dated from ~13,500 to ~8,200 years ago. This interval spans the change from the Pleistocene to Holocene Epochs, which was a period of great climate warming. The marl and shells should have recorded these changing conditions in their chemistry. We analyzed magnesium (Mg), iron (Fe), and barium (Ba) contents, and calcium (Ca)-, oxygen (O)-, carbon (C)-, and strontium (Sr)-isotope compositions of this marl and associated shells, using samples taken from cores throughout the lake sediments. A particular focus was placed on the Ca-isotope composition of the marl and shells. Very little is known about how Ca-isotope compositions change in marl lakes, and much could be learned by comparing its behaviour with the much better understood Sr-, O- and C-isotope tracers of water sources, climate and environment in such settings. Collectively, these data provide information about the environmental and climatic changes recorded in this kettle lake over this significant period of climate change. The changes deduced from the geochemical measurements can be subdivided into four Intervals (A to D). From youngest to oldest, Interval A (located at 532.0 to 475.0cm depth) was a time of cooler, wetter conditions, higher lake levels, and lower primary lake organic productivity. Interval B (475.0 to 345.0cm) marked the beginning of a closed lake system (no outflow of water except through evaporation), warming climate, and a reduction in Ca-availability in lakewater. Interval C (345.0 to 192.0cm) was a time of progressively shallower lake level, increased evaporation and greater lake productivity which continued into Interval D (192.0 to 128.0cm) before marl formation ceased with the complete infilling and death of the lake.
Ferguson, Emma-Dawn, "Calcium Isotopes in the Saint Agatha Kettle Lake Deposits of Southern Ontario" (2019). Electronic Thesis and Dissertation Repository. 6293.
Available for download on Tuesday, August 31, 2021