Stromatolites Record Changing Primary Productivity in Perennially Ice-Covered Lake Joyce, McMurdo Dry Valleys, Antarctica

Calcite-rich dm-scale columnar stromatolites grew in perennially ice-covered Lake Joyce, Antarctica, and their calcite contains carbon isotopic records of microbial activity during recent lake level rise. Stromatolite growth initiated in water depths < 10 m based on the presence of bubble molds in calcite, and growth ended prior to 2009 when stromatolites were at 20-22 m water depth and received insufficient irradiance for net photosynthetic growth. Some calcite layers in the stromatolites contain cyanobacterial microfossils as well as sediment laminae, which indicate that this calcite precipitated at the stromatolite surface in association with photosynthesizing communities. The innermost stromatolite layers have variable δ13C values ranging from 3.9 to 9.6‰ in coeval calcite. Regions such as topographic highs and parts of stromatolites growing into open water have both thicker calcite layers and δ13C values that are 0.3 to 1.0‰ higher than other areas. Outer stromatolite layers have a smaller range of δ13Ccalcite values spanning 1.3‰. Variations in carbon isotopes can be attributed to photosynthetic effects. Photosynthetic shallow modern mats in Lake Joyce have pH up to 0.4 units higher than the water column, and pH increases and decreases with irradiance through diurnal cycles. Irradiance also varies laterally; light transmission through the Lake Joyce ice cover varied over 500% laterally in 2014. If the modern mats reflect conditions present during early stromatolite growth, high photosynthetic rates likely enhanced calcite precipitation and produced a photosynthetic δ13C signature in stromatolitic calcite. Variability in innermost stromatolite δ13C values is consistent with different rates of photosynthesis due to laterally variable light transmission through the ice. With lake level rise, incident irradiance decreased and became more uniform, leading to more consistent δ13Ccalcite values. Lake Joyce stromatolites thus record the effects of changing irradiance on photosynthetic signatures.