Archaean Research Papers - Academia.edu (original) (raw)

The study of microbial fossils involves a broad array of disciplines and covers a vast diversity of topics, of which we review a select few, summarizing the state of the art. Microbes are found as body fossils preserved in different modes... more

The study of microbial fossils involves a broad array of disciplines and covers a vast diversity of topics, of which we review a select few, summarizing the state of the art. Microbes are found as body fossils preserved in different modes and have also produced recognizable structures in the rock record (microbialites, microborings). Study of the microbial fossil record and controversies arising from it have provided the impetus for the assembly and refining of powerful sets of criteria for recognition of bona fide microbial fossils. Different types of fossil evidence concur in demonstrating that microbial life was present in the Archean, close to 3.5 billion years ago. Early eukaryotes also fall within the microbial realm and criteria developed for their recognition date the oldest unequivocal evidence close to 2.0 billion years ago (Paleoproterozoic), but Archean microfossils >3 billion years old are strong contenders for earliest eukaryotes. In another dimension of their contribution to the fossil record, microbes play ubiquitous roles in fossil preservation, from facilitating authigenic mineralization to replicating soft tissue with extracellular polymeric substances, forming biofilms that inhibit decay of biological material, or stabilizing sediment interfaces. Finally, studies of the microbial fossil record are relevant to profound, perennial questions that have puzzled humanity and science—they provide the only direct window onto the beginnings and early evolution of life; and the methods and criteria developed for recognizing ancient, inconspicuous traces of life have yielded an approach directly applicable to the search for traces of life on other worlds.

This study presents bulk-rock major, trace, and platinum-group element data, as well as mineral chemistry for peridotites which form large enclaves (up to 500 × 1000 m) within Mesoarchaean orthogneisses of the Akia terrane in the... more

This study presents bulk-rock major, trace, and platinum-group element data, as well as mineral chemistry for peridotites which form large enclaves (up to 500 × 1000 m) within Mesoarchaean orthogneisses of the Akia terrane in the Fiskefjord region, southern West Greenland. The largest peridotite body, known as Seqi, contains highly fosteritic olivine with a median Mg# of 92.6 and hosts extensive layers of chromitite, which can be traced for tens of metres with thicknesses of up to 30 cm. Thinner (<100 m thick), but extensive (up to 2000 m long) peridotite sheets are associated with coarse norite and orthopyroxenite with distinct cumulate textures in the Amikoq complex, located a few tens of kilometres south of Seqi. Intercalated amphibolites of tholeiitic basaltic composition show complementary geochemical evolution to the peridotites, consistent with igneous crystal fractionation trends. The U-shaped trace element patterns of the peridotites may either reflect the parental melt composition from which these olivine-rich rocks were derived, or alternatively this feature may be the result of melt-rock interaction. Overall, we interpret the Fiskefjord region peridotites to have formed as ultramafic cumulates derived from Archaean high-Mg, low Ca/Al magmas, although their geodynamic setting remains to be established.

1. Literatur/Quellen..................................................................................1 2. Was ist Erdgeschichte..........................................................................2 3.... more

1. Literatur/Quellen..................................................................................1
2. Was ist Erdgeschichte..........................................................................2
3. Altersmessungen..................................................................................3
4. Ur-Atmosphäre.....................................................................................5
5. Präkambrium: Ediacara Fauna.............................................................5
6. Archaische Grünstein Gürtel................................................................6
7. Entwicklung der Tektonik....................................................................7
8. Ende Prä-Kambrium.............................................................................8
9. Der Übergang vom Präkambrium zum Kambrium...............................8
10. Kambrium.............................................................................................9
11. Kambrische Fossil-Lagerstätten.......................................................10
12. Ende des Ordoviziums.......................................................................11
13. Die Bildung von Laurentia und dem Old Red Kontinent...................12
14. Devon..................................................................................................13

We report whole-rock elemental and Sm–Nd isotope geochemical data from mafic–ultramafic supracrustal rocks from the Nunatak 1390 area in southern West Greenland. Additionally, we report the metamorphic temperature history for these rocks... more

Zircons formed at 3.08; 3.17-3.19 and 3.23 Ga were found in metaterrigeneous rocks that occur between horizons of iron-bearing quartzites on the Chertomlyk volcanogenic iron ore deposit. The youngest detrital zircon crystallized at 3.08... more

Zircons formed at 3.08; 3.17-3.19 and 3.23 Ga were found in metaterrigeneous rocks that occur between horizons of iron-bearing quartzites on the Chertomlyk volcanogenic iron ore deposit. The youngest detrital zircon crystallized at 3.08 Ga signifies the maximal age of sedimentation. The 3.17-3.19 Ga zircons resemble those found in the early tonalite-trondhjemite-granite series in this area, whereas those crystallized at 3.23 Ga belong probably to the rocks of the basement on which Mesoarchaean greenstone belts of the Middle-Dnieper terrain were formed.

As part of the International Continental Scientific Drilling Program's Fennoscandian Arctic Russia–Drilling Early Earth Project (ICDP FAR-DEEP), Palaeoproterozoic diamictic and associated rocks were targeted and recovered in Hole 3A on... more

As part of the International Continental Scientific Drilling Program's Fennoscandian Arctic Russia–Drilling Early Earth Project (ICDP FAR-DEEP), Palaeoproterozoic diamictic and associated rocks were targeted and recovered in Hole 3A on the Kola Peninsula of NW Russia. In addition to the diamictites, carbonate sedimentary rocks and volcanic ash layers (all metamorphosed to greenschist grade) were encountered. Sedimentology and geochemistry suggest deposition of the diamictites in an open-marine aragonite-precipitating environment. Sampling of the core and of outcrops from the same geographical area yielded a number of zircons for analyses, the majority of which were inherited. However a tuff at 20.01 m core depth yielded zircons dated at 2434 ± 1.2 Ma (± 6.6 Myr including decay constant uncertainties) that we interpret as a magmatic age. These data, combined with dates from underlying intrusions, indicate deposition of the Polisarka Sedimentary Formation diamictites and underlying carbonates during an interval of time from ca. 2430 to 2440 Ma. The carbonate rocks, which likely originally included aragonitic limestones, were deposited mostly in a deep-water setting (i.e, at least below storm wave base) and occur below the diamictite. They record two inorganic carbon δ13C excursions, from values of ca. 0‰ to minima of ca. -5.4‰, as the contact with the overlying diamictite is approached. The older excursion occurs about 9 m below the base of the diamictic units and the younger one at 1 m below. Throughout that interval, Mg/Ca ratios correlate strongly with δ13C (n = 38, r = 0.85), and combined with petrographic observations, this indicates that the first (stratigraphically lower) excursion was modified by secondary alteration and the second is recorded in resedimented dolostone clasts. It is tempting to speculate that these dolostone clasts weredeposited in penecontemporaneous shallow-marine waters, and that their low δ13C values might reflect input of oxidised atmospheric methane to the ocean surface (and therefore the cause of the glaciation); the dolostones were subsequently resedimented into the deeper marine settings. However this must be left as a hypothesis to be tested when further age-constrained contemporaneous pre-glacial carbonate sections are found.

A detailed sedimentological and chronostratigraphic analysis of the Umberatana Group in the northern Adelaide Geosyncline has uncovered a depositional history involving the rapid progradation (at least 20 km) of a giant reef complex (up... more

A detailed sedimentological and chronostratigraphic analysis of the Umberatana Group in the northern Adelaide Geosyncline has uncovered a depositional history involving the rapid progradation (at least 20 km) of a giant reef complex (up to 1.1 km relief) during mid-Cryogenian interglacial times. The reef complex, which occurs in the Balcanoona Formation, displays facies similar to Phanerozoic reefs. These include a basal forereef (slope) facies, overlain by a reef-margin facies (consisting of both stromatolitic and non-stromatolitic frameworks), and an upper backreef (platform) facies consisting of shallow-water peloidal and oolitic carbonate. The thickening of the reef complex in a basinward direction, and the distribution of the key facies are consistent with the progradation of the platform into deep water. Progradation was contemporaneous with deposition of the upper Tapley Hill Formation and had largely ceased after a major margin failure event. Following this event, reef growth continued for a short time before becoming extinct, possibly as a result of global climatic cooling and/or eustatic sea-level fall.

The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic... more

The Johannesburg Dome, located in the central part of the Kaapvaal Craton, constitutes one of the key areas to better understand the Archaean crustal evolution of this part of the craton. The dome comprises a variety of Archaean granitic rocks intruded into mafic–ultramafic greenstone remnants. This study presents new precise U–Pb single zircon dating for seven different granitoid samples and

A new trilobite species, Schopfaspis? graciai, from the middle Cambrian of Spain is the first member of Alokistocaridae reported from west Gondwana. A cladistic analysis of this trilobite and other Gondwanan trilobites of possible... more

A new trilobite species, Schopfaspis? graciai, from the middle Cambrian of Spain is the first member of Alokistocaridae reported from west Gondwana. A cladistic analysis of this trilobite and other Gondwanan trilobites of possible alokistocarid affinities (Schopfaspis granulosa, Chelidonocephalus anatolicus, Derikaspis toluni, Kounamkites multiformis) suggests that this family can be divided into two subfamilies: Alokistocarinae and Altiocculinae. Schopfaspis? graciai nov. sp. and Schopfaspis granulosa are assigned to the subfamily Altiocculinae laying in a more basal position than Altiocculus species. The cladistic analysis also demonstrates a possible relationship of Chelidonocephalus anatolicus and Derikaspis toluni with Alokistocaridae. Enrolment is analysed in this new trilobite describing novelties in the ventral surface of the cephalon which allowed interlocking of the trunk and cephalon in a discoidal enrolment-type.

The late paleoarhean (3297±22 Ma) age of tonalite gneisses was at first identified on West-Azov block. They are characterized by a positive value of 􀀀εNd(3300) = 0,3÷1,8, that they are juvenile source. With the new geochronological data,... more

The late paleoarhean (3297±22 Ma) age of tonalite gneisses was at first identified on West-Azov block. They are characterized by a positive value of 􀀀εNd(3300) = 0,3÷1,8, that they are juvenile
source. With the new geochronological data, in the western part of Azov megablock three stages of formation of tonalite of ancient crust 􀀀 (3,67±0,05); (3,5±0,005) and (3,3±0,007) Ga was established, during which they formed the nucleus ancient sialic continental crust of Ukrainian Shield, allocated in the Azov and Dniester-Bug megablocks.