ABSTRACT Large bolide impacts in the Phanerozoic produced global change identifiable in the postimpact sediments. Aside from a few isolated examples, however, evidence of postimpact change associated with Precambrian impacts is sparse. This study used the Neoarchean Paraburdoo spherule layer as a case study to search for impact-induced change in the sediments above the spherule layer. We found possible minor sedimentary changes that may have been due to either a disturbance by bottom currents or changing diagenetic conditions. Contrary to the trends found with several post–Great Oxidation Event large bolide impacts, we found no evidence of shifts in tectonic regime, sediment weathering and deposition, or paleoenvironment induced by the Paraburdoo spherule layer impact, for which the impactor is estimated to have been approximately three times larger than the Cretaceous–Paleogene bolide. This lack of a clear signal of climatic shift may be due to one or more mechanisms. Either the Paraburdoo spherule layer’s deposition in several-hundred-meter-deep water within the Hamersley Basin of Western Australia was too deep to accumulate and record observable changes, or the Neoarchean’s high-CO 2 atmospheric composition acted as a threshold below which the introduction of more impact-produced gases would not have produced the expected climatic and weathering changes. We also report minor traces of elevated iron and arsenic concentrations in the sediments immediately above the Paraburdoo spherule layer, consistent with trends observed above other distal impact deposits, as well as distinctive layers of hematite nodules bracketing the spherule layer. These geochemical changes may record ocean overturn of the Neoarchean stratified water column, which brought slightly oxygenated waters to depth, consistent with the observation of tsunami deposits in shallower impact deposits and/or heating of the global oceans by tens to hundreds of degrees Celsius in the wake of the Paraburdoo spherule layer impact. Either or both of these mechanisms in addition to impact-induced shallow-water ocean evaporation may also have caused a massive die-off of microbes, which also would have produced a postimpact increase in iron and arsenic concentrations.

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.

Intercontinental correlation of distal Archean impact ejecta layers can be used to help create a global time-stratigraphic framework for early Earth events. For example, an impact spherule layer in the Neoarchean Monteville Formation (Griqualand West Basin, South Africa) may be correlated with layers in one or more formations in Western Australia. To help assess the degree to which diagenetic alteration would hinder such correlations, we performed a petrographic study of spherules in the Monteville layer. Most of the spherules in the Monteville layer have botryoidal rims composed of radial-fibrous K-feldspar, but compaction and replacement have greatly altered their appearance and mineralogy. Moreover, the Monteville spherule layer consists of three main subunits, and spherule compaction varies between subunits as well as across the Griqualand West region. Compaction is about three times greater in a medial spherule-rich subunit as compared to a basal subunit rich in large intraclasts, resulting in better preservation of the shapes of melt particles in the latter. However, spherule rims have comparable numbers of fractures in both subunits, indicating the melt particles were fractured prior to compaction. Some spherules contain mica ribbons with a septarian geometry. Fracturing via rapid thermal quenching could help explain all of these features. If hot spherules possessing crystalline rims were thermally shocked when they hit the ocean, fractures would have the observed geometries and provide pathways for fluid infiltration and local replacement of glass by mica. Although heavily distorted, impact spherules in the Monteville layer are very similar to those in the Hesta occurrence of the Neoarchean Jeerinah spherule layer of the Hamersley Basin, even showing similar diagenetic histories. In this instance, diagenetic alteration may actually help rather than hinder intercontinental correlation of impact spherule layers.

Geological and geochemical characteristics of banded iron formations (BIFs) suggest that they formed by mixing locally (or regionally) discharged submarine hydrothermal fluids with local seawater, rather than by upwelling deep ocean water. Submarine hydrothermal fluids typically evolved from local seawater by acquiring heat, metals, and sulfur during deep circulation through a variety of rocks (e.g., volcanics, evaporites) in greenstone terranes that developed under a variety of tectonic settings. In general, when the fluids were heated above ∼350 °C, they may have produced Cu- and Zn-rich volcanogenic massive sulfide deposits (VMSDs), whereas those heated less than ∼200 °C were generally poor in H 2 S and heavy metals, except Fe, and may have subsequently produced BIFs. Depending on the salinity contrast between discharging hydrothermal fluids (evolved seawater) and local seawater, hydrothermal fluids may (1) mix rapidly with local seawater to form smoker-type BIFs or (2) create a metal- and silica-rich brine pool, mix slowly with the overlying water body, and form brine pool-type BIFs. BIFs associated with VMSDs and volcanic rocks generally belong to smoker-type BIFs; many formed at seawater depths >2.5 km. Large BIFs, including the 2.6–2.4 Ga BIFs in the Hamersley Basin, Australia, the 2.5 Ga Kuruman IF in South Africa, and the 1.87 Ga BIFs in the Lake Superior region, United States-Canada, belong to brine pool-type BIFs. The Hamersley Basin and possibly other large BIF-hosting basins were probably land-locked seas (like the Black Sea) where river waters diluted the surface water zone and the underlying water bodies were anoxic. During the accumulation of a BIF sequence, the dominant Fe mineralogy frequently changed from ferric (hydr)oxides (oxide BIFs) to siderite (carbonate BIFs) and to pyrite (sulfide BIFs). Such changes were probably caused by changes in the relative amounts of dissolved O 2 (DO), ΣCO 3 2− , and ΣS 2− in local seawater. From the Fe 2+ -O 2 mass balance calculations for the formation of iron oxides in smoker-type BIFs, and the relationship between the atmospheric pO 2 and oceanic O 2 depth profile, we conclude that the atmosphere and oceans have been fully oxygenated since ca. 3.8 Ga, except in local anoxic basins. Thermodynamic analyses of the formational conditions of siderite and analyses of the carbon isotopic composition of siderite associated with major BIFs suggest that the pre–1.8 Ga atmosphere was CO 2 -rich (pCO 2 >100 PAL) and CH 4 -poor (pCH 4 ≈ 10 ppm); therefore, CO 2 , rather than CH 4 , was the major greenhouse gas throughout geologic history. After a decline of hydrothermal fluid flux, BIF-hosting basins generally became euxinic (H 2 S-rich) because of the increased activity of sulfate-reducing bacteria (SRB) and SO 4 2− -rich seawater, and thereby accumulated organic carbon-rich and pyrite-rich black shales (sulfide-type BIFs). The SO 4 2− contents and SRB activity in the oceans have been essentially the same since ca. 3.8 Ga. The Archean oceans were most likely poor in both Fe 2+ and silica, much like modern oceans. Our study also suggests that diverse communities of organisms, including cyanobacteria, SRB, methanogens, methanotrophs, and eukaryotes, evolved very early in Earth's history, probably by the time the oldest BIFs (ca. 3.8 Ga) formed. BIFs have been found in rocks of all geologic age. Therefore, they cannot be indicators of an anoxic atmosphere and/or anoxic oceans as suggested by many previous researchers. Instead, BIFs indicate that the atmosphere and ocean chemistry have been regulated at present compositions (except pCO 2 ) through geologic history by interactions with the biosphere. The general trend of declining size and abundance of BIFs with geologic time reflects the cooling history of Earth's interior.