Impact spherules from Karelia, Russia: possible ejecta from the 2.02 Ga Vredefort impact event

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was <2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic

The REE-composition and petrography of apatite in 2Ga Zaonega Formation, Russia: the environmental setting for phosphogenesis

The first significant P-rich deposits appear in the global rock record during the Paleoproterozoic around 2 Ga, however the specific triggers that led to apatite precipitation are still under debate. The ca. 2 Ga Zaonega Formation, Karelia, Russia contains P-rich intervals in its upper part with abundantly occurring apatite. These apatites have been studied for their Rare Earth Element (REE) composition using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) with an emphasis on the environmental condition during phosphogenesis. Petrographic observations by scanning electron microscopy (SEM) integrated with LA-ICP-MS results allow recognition of variously recrystallized apatite, and distinction of best-preserved diagenetic apatite which presumably records syn-depositional REE characteristics. Diagenetic apatite exhibits moderately negative Ce anomalies that indicate an at least partially oxygenated water column. A variable but typically positive Eu anomaly is consistent with geologic evidence suggesting intensive magmatic activity and hydrothermal influence during apatite precipitation. We conclude that phosphatic sediments in the Paleoproterozoic Zaonega Formation record phosphogenesis in a vent/seep influenced setting that experienced fluctuating redox conditions at the sediment–water interface

Impact spherules from Karelia, Russia: Possible ejecta from the 2.02 Ga Vredefort impact event

Spherule beds of possible impact origin have been discovered in two drill cores in the Paleoproterozoic Zaonega Formation, Karelia, northwest Russia. Spherules are found within the dolomite matrix of in-situ brecciated sedimentary dolostones. Spherules are millimeter size and generally round, although teardrop and dumbbell morphologies are present. Spherules contain up to 0.75 ppb Ir, with a Ru/Ir of 2, indicating a mixing of target rocks with a minor chondritic component. The age of the Zaonega Formation is constrained between limits of 1.975 ± 0.024 Ga (Sm-Nd) and 1.980 ± 0.057 Ga (Pb-Pb), and 2.050 Ga (Re-Os), which brackets the age of the 2020 Ma Vredefort impact structure in South Africa, and suggests that the spherule beds could represent ejecta from that event. If the link is confirmed, the size of the spherules and thickness of the beds suggest that the distance from the impact site was <2500 km, thereby constraining the paleogeographic distance between the Fennoscandian Shield and Kaapvaal craton during the late Paleoproterozoic

The onega basin

The main geological and stratigraphic features of the Onega Basin are discussed in Chap. 4.3. Given here is a brief geological outline to provide a scientific context and background information for the FAR-DEEP implemented in this area

Potential influence of sulphur bacteria on Palaeoproterozoic phosphogenesis

All known forms of life require phosphorus, and biological processes strongly influence the global phosphorus cycle. Although the record of life on Earth extends back to 3.8 billion years ago and the advent of biological phosphate processing can be tracked to at least 3.5 billion years ago,the earliest known P-rich deposits appeared only 2 billion years ago. The onset of P deposition has been attributed to the rise of atmospheric oxygen 2.4–2.3 billion years ago and the related profound biogeochemical shifts which increased the riverine input of phosphate to the ocean and boosted biological productivity and phosphogenesis. However, the P-rich deposits post-date the rise of oxygen by about 300 million years. Here we use microfabric, trace element and carbon isotope analyses to assess the environmental setting and redox conditions of the 2-billion-year-old P-rich deposits of the vent- or seep-influenced Zaonega Formation, northwest Russia. We identify phosphatized microorganism fossils that resemble modern methanotrophic archaea and sulphur-oxidizing bacteria, analogous to organisms found in modern seep settings and upwelling zones with a sharp redoxcline. We therefore propose that the P-rich deposits in the Zaonega Formation were formed by phosphogenesis mediated by sulphur bacteria, similar to modern site, and by the precipitation of calcium phosphate minerals on microbial templates during early diagenesis