I study oolites, a certain type of limestone formed by spontaneous precipitation of calcium carbonate in shallow-marine seas. Oolites are composed of millimeter-sized spherical grains with concentric laminae of calcium carbonate, called ooids. In this project, I am particularly interested in oolites formed during the Hirnantian (latest Ordovician). The Hirnantian Stage is characterized by profound changes in the marine environment and in Earth's climate, which culminates in an ice-age and one of the largest extinction events in Earth's history. The presence of oolites in the world’s shallow-marine seas increases markedly during this time interval. Why so? Precambrian oolites are common, but the rock-type is far rarer in strata from the Phanerozoic, probably as a result of the development and diversification of multicellular, shell-bearing biota that provided a new calcium carbonate sink. It is likely that the significant increase in ooid formation during the late Ordovician can be related to the major mass extinction event at that time and associated to the reduction in shell-bearing organisms in the oceans and a major climate change, but the relationships are unclear. I will try to understand the mechanisms behind oolite genesis, its significance and relation to changes in climate, oceanography and marine ecology. I will also apply studies of recent ooid formation to understand the processes.