Eero J. Hanski

Emergence of an aerobic biosphere during the Archean-Proterozoic transition: Challenges of future research

The earth system experienced a series of fundamental upheavals throughout the Archean-Paleoproterozoic transition (ca. 2500–2000 Ma). Most important were the establishment of an oxygen-rich atmosphere and the emergence of an aerobic biosphere. Fennoscandia provides a fairly complete record of the hallmark events of that transition: widespread igneous activity, its association with a possible upper-mantle oxidizing event, the global Huronian glaciation, a rise in atmospheric oxygen, the protracted and large-magnitude Lomagundi-Jatuli carbon isotope excursion, a substantial increase in the seawater sulfate reservoir, changes in the sulfur and phosphorus cycles, a radical modification in recycling of organic matter, and the Shunga Event—the accumulation of unprecedented organic-matter–rich sediments and the oldest known inferred generation of significant petroleum. Current research efforts are focused on providing an accurate temporal framework for these events and linking them into a coherent story of earth system evolution.

Reading the Archive of Earth’s Oxygenation

Part V FAR-DEEP Core Archive and Database.- Part VI FAR-DEEP Core Descriptions and Rock Atlas.

6.2 The Pechenga Greenstone Belt

Geology and stratigraphy of the Pechenga Greenstone Belt is described in detail in Chap. 4.2. The brief geological outline presented here provides a scientific context and background information for the FAR-DEEP implemented in this area.

Enhanced accumulation of organic matter: the Shunga event

A number of sedimentary formations deposited globally around 2.0 Ga ago are characterised by high abundances of organic carbon. These formations often contain occurrences of highly concentrated, matured organic material representing metamorphosed oil, now pyrobitumen. Apart from their common names pyrobitumen or anthraxolite, different terminology has been used for these rocks within the pertinent literature, including shungite, thucolite, or Precambrian “coal”. Given their long and frequently complex geologic history, these sedimentary formations exhibit a variable and sometimes substantial degree of metamorphic (thermal) overprint. Consequently, many of them show undisputable signs of thermal mobilisation, migration and likely loss of hydrocarbons/bitumen. This includes the so-called shungite rocks on the Fennoscandian Shield.

The Tulomozero Formation: FAR-DEEP Holes 10A and 10B

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.

Mantle source of the 2.44–2.50-Ga mantle plume-related magmatism in the Fennoscandian Shield: evidence from Os, Nd, and Sr isotope compositions of the Monchepluton and Kemi intrusions

Significant PGE and Cr mineralization occurs in a number of 2.44–2.50-Ga mafic layered intrusions located across the Karelian and Kola cratons. The intrusions have been interpreted to be related to mantle plume activity. Most of the intrusions have negative εNd values of about −1 to −2 and slightly radiogenic initial Sr isotope compositions of about 0.702 to 0.703. One potential explanation is crustal contamination of a magma derived from a mantle plume, but another possibility is that the magma was derived from metasomatized sub-continental lithospheric mantle. Samples from the upper chromitite layers of the Kemi intrusion and most samples from the previously studied Koitelainen and Akanvaara intrusions have supra-chondritic γOs values indicating some crustal contamination, which may have contributed to the formation of chromitites in these intrusions. Chromite separates from the main ore zone of the Kemi and Monchepluton intrusions show nearly chondritic γOs, similar to the coeval Vetreny belt komatiites. We suggest that the Os isotope composition of the primitive magma was not significantly changed by crustal contamination due to a high Os content of the magma and a low Os content of the contaminant. Modeling suggests that the Os and Nd isotope compositions of the Monchepluton and Kemi intrusions cannot be explained by assuming a magma source in the sub-continental lithospheric mantle with sub-chondritic γOs. A better match for the isotope data would be a plume mantle source with chondritic Re/Os and Os isotope composition, followed by crustal contamination.

The Imandra/Varzuga greenstone belt

The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period when the Earth System experienced a series of fundamental upheavals. Among them, the most important was the establishment of an oxygen-rich atmosphere (sometimes referred to as the Great Oxidation Event) and the emergence of an aerobic biosphere. Associated with this, either incidentally or causally, was a cascade of other prominent, global-scale events that considerably modified Earth’s surface environments, either temporarily or permanently; these are reviewed in Parts 1 and 8 in full, and detailed in Part 7. Briefly mentioned here, these include: the severe and global climatic event known as the Huronian glaciation; an unprecedented perturbation of the global carbon cycle, the large-magnitude Lomagundi-Jatuli positive excursion of δ13Ccarb, lasted over 160 Ma; radical changes in the phosphorus and sulphur cycles resulting in accumulation of the first-known massive sulphates and sedimentary phosphates; a radical modification in recycling of organic matter leading to the emergence of a new 13C-depleted carbon reservoir in the form of carbonate concretions; and an unprecedented accumulation of organic-rich sediments and formation of the earliest supergiant petroleum deposits.

Mantle hydration and the role of water in the generation of large igneous provinces

The genesis of large igneous provinces (LIP) is controlled by multiple factors including anomalous mantle temperatures, the presence of fusible fertile components and volatiles in the mantle source, and the extent of decompression. The lack of a comprehensive examination of all these factors in one specific LIP makes the mantle plume model debatable. Here, we report estimates of the water content in picrites from the Emeishan LIP in southwestern China. Although these picrites display an island arc-like H2O content (up to 3.4 by weight percent), the trace element characteristics do not support a subduction zone setting but point to a hydrous reservoir in the deep mantle. Combining with previous studies, we propose that hydrous and hot plumes occasionally appeared in the Phanerozoic era to produce continental LIPs (e.g., Tarim, Siberian Trap, Karoo). The wide sampling of hydrous reservoirs in the deep mantle by mantle plumes thus indicates that the Earth’s interior is largely hydrated.The genesis of large igneous provinces (LIPs) remains controversial. Here, the authors examine the water contents of picrites from the Emeishan LIP and find that despite high water contents, the elevated temperature and trace elements suggest a mantle plume from a hydrous deep reservoir rather than subduction zone related.

7.4 An Apparent Oxidation of the Upper Mantle versus Regional Deep Oxidation of Terrestrial Surfaces in the Fennoscandian Shield

Part of the Palaeoproterozoic Karelian igneous and sedimentary rocks of the Fennoscandian Shield were erupted and deposited during the “Great Oxidation Event” (GOE). The drillcores collected for the Fennoscandia Arctic Russia – Drilling Early Earth Project (FAR-DEEP) allow detailed geological and geochemical sampling through this very dynamic time in geologic history. One of the unusual characteristics of the Palaeoproterozoic volcanic rocks in the eastern part of the Fennoscandian Shield is the presence of highly oxidised lava flows (Fig. 7.45), suggestive of a link to the GOE, either cause or effect. The most oxidised volcanic rocks are found in the Jatulian system deposited within the time interval of 2.3–2.06 Ga (see Fig. 7.46). The age and sampling structure of the FAR-DEEP cores permit the testing and assessment of two competing hypotheses for the origin of the highly oxidised volcanic rocks of the Fennoscandian Shield: an apparent increase in the oxidation state of the upper mantle from which the lavas were erupted, or subsequent deep oxidative weathering of the lavas as a result of the GOE, or the combined effect of both. The rocks sampled by the FAR-DEEP cores allow the comparison of primary and secondary mineralogical and diagenetic details, which may not be present in outcrop. In addition to investigating the origin of the highly oxidised rocks, other questions can be addressed because of the exquisite preservation of the rocks sampled in the FAR-DEEP cores. Specifically, are there discernable physical and/or chemical differences in weathering profiles developed on lava flows before and after the GOE, and can palaeo-water tables in the shield be identified through the use of redox proxies? The FAR-DEEP cores also sample igneous rocks erupted during the proposed magmatic activity shutdown/slowdown between 2.45 and 2.2 Ga (Condie et al. 2009). Overall, the FAR-DEEP cores are conducive to detailed geochemical analysis and potential insight into a poorly understood interval in Earth’s history.

Polisarka sedimentary formation: FAR-DEEP hole 3A

The Late Archaean-Early Palaeoproterozoic transition (2500–2000 Ma) represents a hallmark period when the Earth System experienced a series of fundamental upheavals. Among them, the most important was the establishment of an oxygen-rich atmosphere (sometimes referred to as the Great Oxidation Event) and the emergence of an aerobic biosphere. Associated with this, either incidentally or causally, was a cascade of other prominent, global-scale events that considerably modified Earth’s surface environments, either temporarily or permanently; these are reviewed in Parts 1 and 8 in full, and detailed in Part 7. Briefly mentioned here, these include: the severe and global climatic event known as the Huronian glaciation; an unprecedented perturbation of the global carbon cycle, the large-magnitude Lomagundi-Jatuli positive excursion of δ13Ccarb, lasted over 160 Ma; radical changes in the phosphorus and sulphur cycles resulting in accumulation of the first-known massive sulphates and sedimentary phosphates; a radical modification in recycling of organic matter leading to the emergence of a new 13C-depleted carbon reservoir in the form of carbonate concretions; and an unprecedented accumulation of organic-rich sediments and formation of the earliest supergiant petroleum deposits.