Spherules from three impact layers ranging in age from 3.24 Ga to 2.49 Ga display petrologic and size diversity. All of these layers represent the distal debris from asteroid impacts in the Archean and early Proterozoic. We examine the petrologic control on the size of these spherules. Though all three spherule layers have different diagenetic histories, some textural properties are consistent in all of the layers and can be used to infer original mineralogy. Spherules that are uniform in composition, with the exception of fine-grained minerals around the rims, are inferred to have been altered from an original glassy composition and are the largest spherules. Spherules that are heterogeneous and contain either pseudomorphs of olivine or contain Ni-chromite are the smallest spherules in all sections. Spherules with plagioclase pseudomorphs tend to be intermediate in size. The larger the impact, the more pronounced is the size segregation of these spherule types. Spherules in the older S3 layer from the Barberton greenstone belt are significantly larger and display a wider range of sizes, and these differences in sizes are related to their petrologic type compared to the younger Dales Gorge and Paraburdoo layers from the Hamersley Basin. The S3 layer also tends to have the largest aggregate bed thickness and Ir content, consistent with this bed resulting from a larger impactor. The Dales Gorge spherule layer contains nonspherical particles, indicative of a ballistic melt, and it is therefore used as a point of comparison with the vapor plume condensates of the S3 and Paraburdoo layers. The Dales Gorge spherules are similar in size to those of the Paraburdoo layer; however, they have a significantly higher percentage of largely glassy spherules, and only ~21% have crystalline pseudomorphs.

This review summarizes and assesses a series of papers presented at the Geological Society of America Annual Meeting in 2011 on the changing composition of Earth's early atmosphere. This is a developing field, with differing views, due largely to the facts of an incomplete rock record and negligible preservation of the gaseous components of the atmosphere. Nevertheless, there are constraints, available through geological proxies in the form of chemical sedimentary rocks that reflect the composition of the hydrosphere, and because the two are directly linked, the atmosphere. A review of the geological constraints on atmospheric conditions is presented for early Earth, from its formation at 4.56 Ga up to 1.8 Ga, followed by a developing model that links changing atmosphere/hydrosphere conditions and biosphere evolution to changes in planetary tectonics, including the evolving supercontinent cycle.