A diverse and exquisitely preserved organic-walled microfossil assemblage from the Meso–Neoproterozoic Mbuji-Mayi Supergroup (Democratic Republic of Congo) and implications for Proterozoic biostratigraphy (original) (raw)
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Precambrian Research, 2017
The well-preserved Meso-Neoproterozoic shallow marine succession of the Atar/El Mreïti Group, in the Taoudeni Basin, Mauritania, offers a unique opportunity to investigate the mid-Proterozoic eukaryotic record in Western Africa. Previous investigations focused on stromatolites, biomarkers, chemostratigraphy and palaeoredox conditions. However, only a very modest diversity of organic-walled microfossils (acritarchs) has been documented. Here, we present a new, exquisitely well-preserved and morphologically diverse assemblage of organic-walled microfossils from three cores drilled through the Atar/El Mreïti Group. A total of 48 distinct entities including 11 unambiguous eukaryotes (ornamented and process-bearing acritarchs), and 37 taxonomically unresolved taxa (including 9 possible eukaryotes, 6 probable prokaryotes, and 22 other prokaryotic or eukaryotic taxa) were observed. Black shales preserve locally abundant fragments of organic-rich laminae interpreted as benthic microbial mats. We also document one of the oldest records of Leiosphaeridia kulgunica, a species showing a circular opening interpreted as a sophisticated circular excystment structure (a pylome), and one of the oldest records of Trachyhystrichosphaera aimika and T. botula, two distinctive process-bearing acritarchs present in welldated 1.1 Ga formations at the base of the succession. The general assemblage composition and the presence of three possible index fossils (A. tetragonala, S. segmentata and T. aimika) support a late Mesoproterozoic to early Neoproterozoic (Tonian) age for the Atar/El Mreïti Group, consistent with published lithostratigraphy, chemostratigraphy and geochronology. This study provides the first evidence for a moderately diverse eukaryotic life, at least 1.1 billion years ago in Western Africa. Comparison with coeval worldwide assemblages indicates that a broadly similar microbial biosphere inhabited (generally redox-stratified) oceans, placing better time constraints on early eukaryote palaeogeography and biostratigraphy.
Precambrian Research, 2017
Reconstructing the spatial distribution of early eukaryotes in palaeoenvironments through Proterozoic sedimentary basins provides important information about their palaeocology and taphonomic conditions. Here, we combine the geological context and a reconstruction of palaeoenvironmental redox conditions (using iron speciation) with quantitative analysis of microfossil assemblages (eukaryotes and incertae sedis), to provide the first palaeoecological model for the Atar/El Mreïti Group of the Taoudeni Basin. Our model suggests that in the late Mesoproterozoic-early Neoproterozoic, the availability of both molecular oxygen and 2 nutrients controlled eukaryotic diversity, higher in oxic shallow marginal marine environments, while coccoidal colonies and benthic microbial mats dominated respectively in anoxic iron-rich and euxinic waters during marine highstands or away from shore where eukaryotes are lower or absent.
Palaeogeography, Palaeoclimatology, Palaeoecology, 2013
The late Mesoproterozoic-middle Neoproterozoic period (ca. 1300 Ma-800 Ma) heralded extraordinary climatic and biological changes related to the tectonic changes that resulted in the assembly (~1.0 Ga) and the break-up of Rodinia (880 Ma-850 Ma). In the Democratic Republic of Congo, these changes are recorded in the Mbuji-Mayi Supergroup which was deposited in the SE-NW trending siliciclastic-carbonate failed-rift Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. New LA-ICP-MS U-Pb laser ablation data on detrital zircon grains retrieved from the lower arenaceous-pelitic sequence (BI group) together with C and Sr isotopic data on carbonates from the upper dolomitic-pelitic sequence (BII group) and an 40 Ar/ 39 Ar age determination on a dolerite give a new depositional time frame between 1174 ± 22 Ma and ca. 800 Ma for the Mbuji-Mayi Supergroup. The upper age limit is based on the assumption that the transition between the BIIb and BIIc subgroups recorded the Bitter Springs anomaly. In terms of tectonic and paleoclimatic settings, the BII group was deposited in the eastern passive margin of the Congo Craton during warm periods interlaced with temporarily dry and wet seasons, suggesting greenhouse conditions during the fragmentation of Rodinia.
Precambrian Research, 2017
In this paper, we present new age constraints for the lower part of the Meso-Neoproterozoic sedimentary Mbuji-Mayi Supergroup (Democratic Republic of Congo, DRC). This Supergroup preserves a large diversity of organic-walled microfossils, evidencing the diversification of early eukaryotes for the first time in Central Africa. We use different methods such as in situ U-Pb geochronology by LA-ICP-MS and U-Th-Pb chemical datings by Electron Microprobe on diagenetic and detrital minerals such as xenotimes, monazites and zircons. We attempt to better constrain the provenance of the Mbuji-Mayi sediments and the minimum age of the Mbuji-Mayi Supergroup to constrain the age of the microfossils. Results with LA-ICP-MS and EMP provide new ages between 1030 and 1065 Ma for the diagenesis of the lower part of the sedimentary sequence. These results are consistent with data on biostratigraphy supporting the occurrence of worldwide changes at the Mesoproterozoic/Neoproterozoic boundary.
Journal of African Earth Sciences, 2006
Lithogeochemical, chemostratigraphic, Rb-Sr and Sm-Nd isotope as well as U-Pb single zircon age data from the principal stratigraphic units of the West Congolian Group (West Congo Supergroup) in the West Congo Belt, Democratic Republic of Congo, presented in this reconnaissance study provide new insights into regional stratigraphic correlation and sediment provenance. The oldest unit (Sansikwa Subgroup) is a continental rift basin fill that culminated in a marine glacial diamictite (Lower Mixtite Formation), which is correlated with the global 750-720 Ma Sturtian glaciation. This is followed by a post-glacial, marine carbonate-dominated succession (Haut Shiloango Subgroup), a second diamictite unit (Upper Mixtite Formation) for which a syn-Marinoan (636 Ma) age is favoured, a post-glacial carbonate succession (Schisto-Calcaire Subgroup) and finally molasse sediments that experienced orogenic deformation at c. 566 Ma, as indicated by new 40 Ar-39 Ar data. The youngest unit (Inkisi Group) is post-orogenic, derived from the West Congo Belt, and thus not part of the same supersequence. The proposed regional to global correlation of the various units is in line with progressively younger detrital zircon age spectra upsection. Overall, these age spectra indicate a Neoarchaean basement source (Congo Craton) and a Palaeoproterozoic source from the known Kimezian Supergroup to the west of the belt, which corresponds in age with the Eburnean of southern Africa. In addition, we identified a hitherto unknown late Mesoproterozoic source to the west of the belt, the age of which is comparable to the Espinhaço Supergroup of the Araçuai Belt in Brazil.
The Neoproterozoic of southwestern Africa, with emphasis on platform stratigraphy and paleontology
Precambrian Research, 1995
Neoproterozoic sediments in southwestern Africa occur in the Pan-African Kaoko, Khomas, Gariep, Vanrhynsdorp and Saldania Belts. They were deposited in response to rifting, break-up ofa supercontinent (formed at ~ 1000 Ma) and subduction and collisions of the Congo, Kalahari, Rio de la Plata and Malvinas plates. These collisions led to the formation of a Neoproterozoic/Cambrian supercontinent. This paper deals with the geology of the platform successions of the Kaoko, Khomas, Gariep and Vanrhynsdorp Belts.
Precambrian Research, 2016
Unequivocal evidence for Archean eukaryotic life has been long sought for and is a matter of lively debate. In the absence of unambiguous fossils this debate has focused on biogeochemical signatures and molecular phylogenies. Most researchers agree that fossil forms comparable with modern eukaryotic cells can be credibly identified only in Proterozoic (~1.8-1.6 Ga) and younger rocks. Herein, we report for the first time, Neoarchean mineralized tubular microfossils from ~2.8-2.7 Ga lacustrine deposits of South Africa. The exceptional preservation of these microfossils allows recognition of important morphological details in petrographic thin section and in HF-macerates that links them to modern siphonous (coenocytic) green or yellow-green microalgae (Chlorophyta and Xanthophyta). The microfossil identification is supported by Raman spectroscopic analyses, EPMA, SEM/BSE 2 and SEM/EDS microprobe analytical results, NanoSIMS elemental mapping and microtomographic sectioning of the thalli. All results point to indigenous, bona fide eukaryotic microfossils of algal affinity. These Neoarchean microalgae-like remains and their assumingly combined in vivo and early post-mortem precipitated mineral envelopes greatly improve our knowledge of early life and its habitats and may have far-reaching consequences for the studies of the evolution of life.
Fossilized eukaryotes from 2.7 billion year old shales in Tanzania
Some of the evidence for the existence of life in the Archean Eon (4.0-2.5 Ga) comes from ‘microfossils’ that are believed to be the mineralized remnants of simple prokaryotic lifeforms. They are spheroidal shaped and a few microns to tens of microns across and most likely represent early prokaryotic organisms. Here we describe spheroidal carbon structures that reach sizes of up to 540 microns from the 2.72 billion years ago (Ga) carbonaceous shales from the Lower Nyanzian Formation in the Geita Greenstone Belt of Tanzania. They are interpreted as microfossils based on their multi-chambered structures with pentagonal to hexagonal sectional geometries or floral forms, and chambers connected by regularly placed, tube-like structures that also connect inner chambers to the outside. The carbonaceous material in the walls is partly composed of disorganised kerogenous carbon with δ13C values averaging -44.6‰ that forms membranes coating a denser core of hematite enriched in P and transiti...
Bulletin of Geosciences, 2009
Late Carbonifeous to Permian core samples from two borehole sections, STRAT 1 and CKP 6, that penetrate the glacial to postglacial and coal-bearing Lower Karoo sequence, Kalahari Karoo Basin, Botswana, contain well preserved pollen and spore palynomorphs. The palynomorph assemblages comprise 165 species of spores and pollen, and few acritarchs and chlorophycean algae. This paper presents a local biozonation of three succeeding assemblage zones, recognisable in each borehole section. The assemblage zones are named: the Hamiapollenites bullaeformis Biozone, the Cyclogranisporites gondwanensis Biozone and the Platysaccus papilionis-Striatopodocarpites fusus Biozone, in ascending order of stratigraphy. Assemblages are compared and correlated with assemblages described from other Gondwana areas of Africa, Australia, Arabia, South America and Antarctica. A general analysis of taxa from the Kalahari Karoo Basin indicates a distinct similarity with assemblages from the Paraná Basin of South America. The Hamiapollenites bullaeformis Biozone is comparable with the Vittatina costabilis Interval Zone of the Paraná Basin. Assemblages from the Cyclogranisporites gondwanensis and the Platysaccus papilionis-Striatopodocarpites fusus zones are comparable with the Lueckisporites virkkiae Interval Zone. Age determination for the assemblages in the Kalahari Karoo Basin is inferred from comparison with similar assemblages from previous Gondwana studies, and ranges from the Late Carboniferous (Kasimovian-Gzhelian) to latest Early or possibly earliest Middle Permian (Late Cisularian to Early Guadalupian).
The Proterozoic Record of Eukaryotes
Proterozoic strata host evidence of global “Snowball Earth” glaciations, large perturbations to the carbon cycle, proposed changes in the redox state of oceans, the diversification of microscopic eukaryotes, and the rise of metazoans. Over the past half century, the number of fossils described from Proterozoic rocks has increased exponentially. These discoveries have occurred alongside an increased understanding of the Proterozoic Earth system and the geological context of fossil occurrences, including improved age constraints. However, the evaluation of relationships between Proterozoic environmental change and fossil diversity has been hampered by several factors, particularly lithological and taphonomic biases. Here we compile and analyze the current record of eukaryotic fossils in Proterozoic strata to assess the effect of biases and better constrain diversity through time. Our results show that mean within assemblage diversity increases through the Proterozoic Eon due to an increase in high diversity assemblages, and that this trend is robust to various external factors including lithology and paleogeographic location. In addition, assemblage composition changes dramatically through time. Most notably, robust recalcitrant taxa appear in the early Neoproterozoic Era, only to disappear by the beginning of the Ediacaran Period. Within assemblage diversity is significantly lower in the Cryogenian Period than in the preceding and following intervals, but the short duration of the nonglacial interlude and unusual depositional conditions may present additional biases. In general, large scale patterns of diversity are robust while smaller scale patterns are difficult to discern through the lens of lithological, taphonomic, and geographic variability.