Constraining the formation conditions of modern pisoids at Ore Lake, Michigan

Abstract

Large concentrically laminated carbonate grains (here referred to as pisoids) have been observed sporadically throughout the geological record and in modern environments. Explanations for how these grains form have varied widely in different settings, although microbial effects are often involved. In Ore Lake, a ~1 km2 flow-through lake in southeast Michigan, one to four centimeter oblong calcite pisoids are observed in both lake bottom shallows and mounded as a small spit near the primary outflow. In section mm-scale light and more porous along with and dark and more dense concentric laminations are apparent. Here we use field observations, petrography, water chemistry, and stable isotopes to understand their formation. Measurements of pisoid calcite δ18O and lake water δ18O indicate that precipitation occurs in waters between roughly 19 – 28°C. These warm temperatures imply that pisoid growth happens almost entirely within the summer, contrary to prior work that suggested wintertime precipitation was important. Pisoid δ18O values largely overlap with coexisting lake bivalve values, suggesting that pisoid precipitation is in equilibrium. In contrast, pisoid δ13C is as much as 8‰ more positive than bivalve δ13C due to photosynthetic effects. We propose that the laminations in these pisoids arise from different rates of formation within the warm months, rather than large seasonal differences. A decline in lake alkalinity beginning in late spring likely coincides with more rapid growth, with slower growth mediated by cyanobacteria continuing through the summer. This range of observations enables the use of Ore Lake as a potential model for understanding pisoid formation throughout the geological record.