Pondering the Precambrian #40

Proterozoic:

NeoProterozoic:

Palaeopascichnus jiumenensis can be used to determine the Ediacaran/Cambrian boundary.

There's a new way to determine oxygen levels during paleo time periods.  It seems to check out in the Neoproterozoic-Cambrian transition based on comparison to other methods.

A bilateralan metazoan (animal) fossil and trackway was found in China dating from the Ediacaran.

The rise of siliceous sponges helped shift the Ediacaran ecosystems to the modern Cambrian. 

The differences between the Weng'an and Kuanchuanpu biotas is largely due to how they were preserved.

Acritarchs from the Ediacaran Doushantuo Formation allow for biostratigraphic correlation with the Tanarium conoideum–Cavaspina basiconica Assemblage Zone.

There appears to have been long term denudation of the continents from the middle Ediarcaran based on Nd isotope traces in carbonates.

A new formation has been found dating from the Cryogenian with evidence of bacterial and algae life in South China.

How much hydrothermal activity was there during the Marinoan Glaciation (snowball earth episode) during the Cryogenian?

There is evidence of a large igneous province in the Tonian.

MesoProterozoic:

Horodyski moniliformis, regarded by some as a eukaryote fossil from the Calymmian, has been declared a pseudofossil.

PaleoProterozoic:

Plate tectonics have evolved over the last 2.5 billion years.

Is there evidence for the oxygen overshoot hypothesis from the Orosirian?

There was a paleocontinent named 'Atlantica' proposed.  Recent research suggests it was configured rather differently than proposed.

Evidence has arisen supporting a snowball earth scenario during the Huronian glaciation.

An asteroid impact has been IDed from the Rhyacian in Australia.

Did the supercontinental cycle begin in the PaleoProterozoic?

Archean:

There is evidence of a PaleoArchean continent.

Origin of Life:

Prebiotic amino acids can bind to lipid membranes and stabilize them.

When did eukaryotes originate?  Do we have a way to understand that?

Did the lack of nitrogenase limit early life?

Pondering the Precambrian #4

NeoProterozoic:

Ediacaran:

From Brazil, Ediacaran vase shaped fossils have been found.  is this the earliest known protist fossils yet?

How are the various frond critters of the Ediacaran related to each other?

How did the Ediacaran critter Dickinsonia grow?

A mixed Cloudinia-Corumbella-Namacalathus assemblage shows increasing ecological complexity over the course of the Ediacaran.

Cryogenian:

Did a freshwater layer exist on the world's oceans and persist after the Snowball Earth episodes like the one in the Cryogenian?

As the Snowball Earth ended, how much oxygen weathering took place and what were the biotic impacts?

Tonian:

There was a huge Andes-like mountain range on the northwest of Rodinia that may have lasted for 100 million years during the Tonian 800 million years ago.

MesoProterozoic:

Wind patterns have been inferred from dunes from Calymmian Brazil.

The diversity of Eukaryote microfossils of Calymmian China is impressive.

PaleoProterozoic:

In Statherian China, there is evidence of a sillicified microbiota from the Dahongyu Formation.

The Sudbury Impact appears to have caused long lived volcanic eruptions during the Orosirian PaleoProterozoic.  Since it was one of 150 impacts within a relatively short period, combined with this above volcanism, it should be no surprise life didn't recover to take a second stab at complexity for a billion years.

The Sudbury Basin continued to have geothermal heat during the Huronian snowball earth.

Can the Rhyacian/Siderian Glaciations (huronian snowball earth) be dated based on subglacial hydrothermal activity?

Beginning in the Siderian, ancient carbon was subsumed into the Earth's mantle.

Did anaerobic oxygenic photosynthesis (read the paper) come about in cyanobacteria prior to modern aerobic photosynthesis?

During the Siderian, Earth had a hazy, methane filled atmosphere.

Did eukaryotes arise during the Siderian?

Archean:

Lenticular organisms from South Africa are related to the Pilbara forms.

Did life arise during the EoArchean WITHOUT using phosphate?

Fossils were found from the Eoarchean 3.77 billion years ago in Quebec, Canada.

How did the crust form?

Hadean:

The Earth probably began with a solid shell for a crust, like Mars.

META:

New branches have been found in Archaea.

Iron eating, methanogen organisms probably kept the Earth warm for its first 2 billion years.

Mineral self assembly was common in the early years of the Earth.

Calymmian MesoProterozoic Nuna/Columbia Supercontinent Reconstructed

Paleogeography of the Congo/São Francisco craton at 1.5 Ga: expanding the core of Nuna supercontinent

Authors:

Salminen et al

Abstract:

The Congo/São Francisco (C/SF) craton, one of the largest cratons in Proterozoic paleogeography, has been lacking reliable paleomagnetic data for the supercontinent Nuna interval (ca. 1600-1300 Ma). Here we provide a new paleomagnetic key pole for this craton from recently dated mafic dykes in the Curaçá (1506.7±6.9 Ma) region of Brazil. The characteristic remanent magnetization (ChRM) direction D=070.6°, I=54.0° (k=22.1 and a95=13.1°) corresponds with a paleomagnetic pole at 10.1°N, 009.6°E (K=15.6, A95=15.8°), which places C/SF craton in moderate paleolatitudes at the time of remanence acquisition. Primary nature of the paleomagnetic remanence is supported by a baked-contact test. A similar ChRM direction was obtained for four Mesoproterozoic mafic intrusions in Chapada Diamantina region. The new pole, only from Curaçá, for C/SF allows us to reconstruct the extended core of the supercontinent Nuna at 1.5 Ga. Based on coeval 1.5 Ga and 1.38 Ga magmatism in Baltica, Siberia and C/SF, we favor the position where Southwest Congo is reconstructed against present South-Southeast (S-SE) Baltica. We explore two alternative 1.5 Ga reconstructions of Nuna’s core. In both of them Baltica and Laurentia are shown in the well-defined NENA (Northern Europe North America) fit, together with Siberia in a tight fit to northern Laurentia. In reconstruction option A, more traditional fit of Amazonia with Baltica is shown, modified from the geologically based SAMBA (South AMerica BAltica) model to accommodate paleomagnetic data. In this option, however, West Africa must be extricated from SAMBA because C/SF has taken its place. For reconstruction option B, Amazonia is shifted to lie adjacent to NE Laurentia and West Baltica. In both options SW Congo is reconstructed against S-SE Baltica, but in option B there is a tighter fit between them, and there is a better match with our new paleomagnetic data for C/SF. In either option, separation of C/SF from Baltica and Siberia probably occurred at 1.38 Ga, the age of pronounced mafic magmatism throughout this sector of Nuna.

Macroscopic (complex?) Eukaryotes From the Calymmian MesoProterozoic One Billion Years Before the Cambrian Explosion

Decimetre-scale multicellular eukaryotes from the 1.56-billion-year-old Gaoyuzhuang Formation in North China

Authors:

Zhu et al

Abstract:

Fossils of macroscopic eukaryotes are rarely older than the Ediacaran Period (635–541 million years (Myr)), and their interpretation remains controversial. Here, we report the discovery of macroscopic fossils from the 1,560-Myr-old Gaoyuzhuang Formation, Yanshan area, North China, that exhibit both large size and regular morphology. Preserved as carbonaceous compressions, the Gaoyuzhuang fossils have statistically regular linear to lanceolate shapes up to 30 cm long and nearly 8 cm wide, suggesting that the Gaoyuzhuang fossils record benthic multicellular eukaryotes of unprecedentedly large size. Syngenetic fragments showing closely packed ~10 μm cells arranged in a thick sheet further reinforce the interpretation. Comparisons with living thalloid organisms suggest that these organisms were photosynthetic, although their phylogenetic placement within the Eukarya remains uncertain. The new fossils provide the strongest evidence yet that multicellular eukaryotes with decimetric dimensions and a regular developmental program populated the marine biosphere at least a billion years before the Cambrian Explosion.

Microfossils from Calymmian MesoProterozoic Russia

Microfossils from the lower Mesoproterozoic Kaltasy Formation, East European Platform

Authors:

Sergeev et al

Abstract:

Basinal shales of the lower Mesoproterozoic Kaltasy Formation, sampled from three boreholes drilled into the southeastern East European Platform, Russia, contain abundant and moderately well preserved microfossils. 34 distinct entities have been identified, most assigned to simple sphaeromorphic or small filamentous taxa found widely and characterized by long stratigraphic ranges. Ornamented microfossils found in coastal successions of other lower Mesoproterozoic basins are absent, but large filamentous microfossils interpreted as possible benthic photosynthetic eukaryotes are recorded, drawing comparisons to relatively deep water shales in Siberia. In overall aspect, the Kaltasy microfossils are consistent with other broadly coeval assemblages, but they highlight the importance of environment, as well as age, in determining the distributions of remains that record the early diversification of marine eukaryotes. Rectia magna is described as a new species.

Pondering the Precambrian #3

Proterozoic:

NeoProterozoic:

The Royal Tyrrell Museum has a lecture on the Ediacaran.

There is stronger evidence for the biomarkers for sponges being present starting in the Cryogenian.

MesoProterozoic:

The shallow seawaters of the Calymmian MesoProterozoic were very hypoxic, showing evidence of having .1% of the current oxygen in seawater.

PaleoProterozoic:

There is evidence of intracontinental rifting from China during the Rhyacian Paleoproterozoic.

Was there a 'Mawson continent' during the Paleoproterozoic?

Archean:

The SETI Institute has a lecture on volcanism during the Archean.

PaleoArchean:

At least part of the trace fossils from Australia are pseudofossils (not real fossils).

EoArchean:

There is evidence of atmospheric oxygen from the EoArchean, 800 million years earlier than ever detected (or suspected) before.

Hadean:

In an almost extraordinary claim, the Siberian Craton may have bits of the crust from the Hadean!

Sufficient Oxygen in Paleoatmosphere at the Calymmian/Ectasian MesoProterozoic Boundary for Animals to Breathe

Sufficient oxygen for animal respiration 1,400 million years ago

Authors:

Zhang et al

Abstract:

The Mesoproterozoic Eon [1,600–1,000 million years ago (Ma)] is emerging as a key interval in Earth history, with a unique geochemical history that might have influenced the course of biological evolution on Earth. Indeed, although this time interval is rather poorly understood, recent chromium isotope results suggest that atmospheric oxygen levels were less than 0.1% of present levels, sufficiently low to have inhibited the evolution of animal life. In contrast, using a different approach, we explore the distribution and enrichments of redox-sensitive trace metals in the 1,400 Ma sediments of Unit 3 of the Xiamaling Formation, North China Block. Patterns of trace metal enrichments reveal oxygenated bottom waters during deposition of the sediments, and biomarker results demonstrate the presence of green sulfur bacteria in the water column. Thus, we document an ancient oxygen minimum zone. We develop a simple, yet comprehensive, model of marine carbon−oxygen cycle dynamics to show that our geochemical results are consistent with atmospheric oxygen levels greater than 4% of present-day levels. Therefore, in contrast to previous suggestions, we show that there was sufficient oxygen to fuel animal respiration long before the evolution of animals themselves.

pop sci write up here and here.

Anomalous Supply of Bioessential Molybdenum From the Orosirian PaleoProterozoic to Calymmian MesoProterozoic

Anomalous supply of bioessential molybdenum in mid-Proterozoic surface environments

Authors:

Parnell et al

Abstract:

Granites aged 1.9 Ga to 1.5 Ga exhibit molybdenite mineralization globally. Sandstones deposited during the mid-Proterozoic have a provenance dominated by 1.9 to 1.7 Ga basement. The mid-Proterozoic surface environment was, consequently, receiving detritus from the molybdenum-rich granites. Thus there was a supply of molybdenum available in terrestrial or shallow marine environments at a time when molybdenum was required to support the evolution of multicellular life.

The PaleoPosition of the Amazonian Craton in the Calymmian MesoProterozoic

Reassessment of Aguapeí (Salto do Céu) Paleomagnetic pole of the Amazonian Craton and implications for Proterozoic supercontinents

Authors:

D’Agrella-Filho et al

Abstract:

The Aguapei paleomagnetic pole obtained for mafic sills and dykes from Salto do Céu region at the western margin of the Amazonian Craton constrains its links with Baltica and Laurentia in Rodinian reconstructions. A new U-Pb age on baddeleyites at 1439 ± 4 Ma for the intrusives, constrasts strongly with a previous Ar-Ar age at 981 ± 2 Ma, with important consequences for paleogeographic reconstructions. We report new paleomagnetic and magnetic anisotropy results for sills from the Salto do Céu region and reassess the paleomagnetic data in view of the new geochronological age. A total of 155 samples were collected from thirteen new paleomagnetic sampling sites, five of them corresponding to sedimentary rocks located at the borders of the sills in an attempt to perform baked contact tests. After thermal and alternating field demagnetization, the sills provided a characteristic magnetic component at Dm = 208.2°, Im = 68.5° (N = 8, α95 = 6.4°), with a corresponding paleomagnetic pole at 46.4°S; 277.0°E (A95 = 10.2°). Directions obtained in this study are similar to those reported previously for other mafic sills and dykes in the same region. A pole integrating the new results for the sills with those of the previous Aguapeí pole is situated at 56.0°S; 278.5°E (A95 = 7.9°). This new combined Salto do Céu pole supersedes the previous Aguapeí pole. Magnetic mineralogy studies, optical and electronic microscopy indicates PSD magnetite as the main magnetic carrier in these rocks. The baked contact tests were inconclusive, but the similarity between Salto do Céu pole and other high-quality poles with ages around 1420-1430 Ma suggests they carry a primary thermoremanence of that age. The Salto do Céu and other coeval poles are compatible with a connection between Amazonia and Baltica at the Mesoproterozoic in a paleogeographic configuration slightly different from SAMBA (South America-Baltica). At the same time, the new geochronological and paleomagnetic data imply that all paleogeographic interpretations for the position of Amazonia in Rodinia based on the previously published Aguapeí pole (and now renamed as Salto do Céu pole) must be revised.

Earth's Inner Core Solidified 1.5 Billion Years ago During Calymmian MesoProterozoic

There have been many estimates for when the earth's inner core was formed, but scientists from the University of Liverpool have used new data which indicates that the Earth's inner core was formed 1 - 1.5 billion years ago as it "froze" from the surrounding molten iron outer core.

The inner core is Earth's deepest layer. It is a ball of solid iron just larger than Pluto which is surrounded by a liquid outer core. The inner core is a relatively recent addition to our planet and establishing when it was formed is a topic of vigorous scientific debate with estimates ranging from 0.5 billion to 2 billion years ago

In a new study published in Nature, researchers from the University's School of Environmental Sciences analysed magnetic records from ancient igneous rocks and found that there was a sharp increase in the strength of the Earth's magnetic field between 1 and 1.5 billion years ago.

This increased magnetic field is a likely indication of the first occurrence of solid iron at Earth's centre and the point in Earth's history at which the solid inner core first started to "freeze" out from the cooling molten outer core.

Liverpool palaeomagnetism expert and the study's lead author, Dr Andy Biggin, said: "This finding could change our understanding of the Earth's interior and its history."

link.

Evidence From Tasmania East Antarctica and Laurentia Connected in the Columbia (Nuna) Supercontinent

Mesoproterozoic Tasmania: Witness to the East Antarctica–Laurentia connection within Nuna

Authors:

Mulder et al

Abstract:

Most recent paleogeographic reconstructions of the supercontinent Nuna juxtapose the North Australian craton, Mawson continent (South Australia–East Antarctica), and Laurentia between 1.6 Ga and 1.3 Ga but differ in their relative positioning. The greater than 10-km-thick siliciclastic Rocky Cape Group of Tasmania was deposited in an opening marine basin on the margin of East Antarctica during Nuna breakup. Based on a similar detrital zircon signature and depositional age, the Rocky Cape Group has been correlated with the upper Belt-Purcell Supergroup in Laurentia, thus representing a key tie point within Nuna. Here the detrital zircon age signature of Mesoproterozoic Rocky Cape Group quartzites is investigated by comparing new detrital zircon U-Pb-Hf isotopic data to an extensive compilation of zircon isotopic data from Australia, East Antarctica, and Laurentia. Our analysis demonstrates that the Rocky Cape Group is unlikely to have been sourced from any geological terrane exposed in presentday Australia. Instead, zircon U-Pb-Hf isotopic data from basement terranes in Laurentia and East Antarctica show striking similarities to the Rocky Cape Group detrital signature. Paleocurrent data indicate that the majority of sediment in the Rocky Cape Group was sourced from Laurentia, which was to the southeast (present-day coordinates) of Tasmania, supporting a SWEAT-like (southwest United States–East Antarctica) configuration for Nuna. We suggest that rifting left a thinned continental connection between East Antarctica and Laurentia onto which the lower-middle Rocky Cape Group was deposited between 1.45 and 1.30 Ga.

Evidence of a Gradual Oxygenation of the Calymmian MesoProterozoic PaleoAtmosphere From Sulfur Isotopes

Sulfur isotope composition of carbonate-associated sulfate from the Mesoproterozoic Jixian Group, North China: Implications for the marine sulfur cycle

Authors:

Guo et al

Abstract:

The Mesoproterozoic has been traditionally viewed as a period of prolonged stability in terms of environmental and biological evolution, yet growing body of evidence suggests subtle, yet dynamic changes in marine biogeochemical cycles. In this study, we present a high-resolution analysis of carbonate-associated sulfate (CAS) from the Mesoproterozoic Jixian Group (1.6–1.4 Ga), North China. Combined with previously reported carbon isotopes from the same sections, sulfur isotope data provide insight into both oceanic chemistry and biospheric evolution during this period.

The sulfur isotopic composition of CAS in the Jixian Group displays high-amplitude variations with a total range from +3.7‰ to +51.9‰. Stratigraphic variation in δ34SCAS is most noted in the Wumishan Formation and records shifts of greater than 10‰ over stratigraphic thickness of 100–200 m, indicating a short residence time resulting from a limited oceanic sulfate reservoir. Differences in the pattern of isotopic variability between the Gaoyuzhuang and Wumishan formations suggest complex behavior of the marine sulfur cycle based on proximity of depositional environments to a local or regional chemocline, and the degree of mixing between adjacent environments. Deposition under predominantly oxic conditions favored a stable sulfate reservoir whose isotopic composition was controlled primarily by pyrite burial. By contrast, depositional environments proximal to euxinic waters were subject to rapid changes in both sulfate reservoir size and isotopic composition, resulting from growth and subsequent oxidation of a reactive hydrogen sulfide reservoir. Together, these units demonstrate the difficulty of interpreting marine sulfur cycling in a low oxygen, low sulfate world subject to a range of environmental controls.

Data also reveal a long-term positive trend in average δ34SCAS and δ13Ccarb from approximately +10‰ to ∼+20‰, and ∼−0.5‰ to +0.5‰, respectively, which is interpreted as reflecting prolonged organic carbon and pyrite burial. Burial of reduced carbon and sulfur may have resulted in a slow, but steady release of oxygen to the biosphere, ultimately driving a gradual increase in oceanic sulfate levels. Our study, combined with previous contributions from the late Mesoproterozoic, supports hypotheses of protracted biospheric oxygenation marked by a modest increase in oceanic sulfate concentrations through the Mesoproterozoic.

Digging Into Statherian PaleoProterozoic/ Calymmian MesoProterozoic Eukaryotic Fossil Valeria

A biomechanical analysis of the early eukaryotic fossil Valeria and new occurrence of organic-walled microfossils from the Paleo-Mesoproterozoic Ruyang Group

Authors:

Pang et al

Abstract:

The Paleo-Mesoproterozoic Ruyang Group of North China hosts early eukaryotic fossils such as Dictyosphaera, Shuiyousphaeridium, and Valeria, and thus offers valuable insights into the early evolution of single-celled eukaryotic life. In this paper, we report several additional forms of organic-walled microfossils from the Ruyang Group, including Plicatidium latum, Spiromorpha sp., and an unnamed form. V. lophostriata from the Ruyang Group is investigated using transmitted light microscopy, scanning electron microscopy, transmission electron microscopy, and biomechanical analysis. V. lophostriata is reconstructed as a spherical vesicle with two hemispherical halves bearing concentric striations resembling latitudinal circles. The formation of striations could be explained using the Belousov-Zhabotinsky reaction model or the Turing reaction-diffusion model. A biomechanical analysis using the thin-walled spherical pressure vessel model suggests that the concentric striations of V. lophostriata may have functioned as a mechanism to guide biologically programmed excystment through medial split. Our analysis provides essential paleontological data to better understand the functional biology and life cycles of early eukaryotes such as Valeria.

Orbital Climate Forcing at the Calymmian/Ectasian MesoProterozoic Boundary

Orbital forcing of climate 1.4 billion years ago

Authors:

Zhang et al

Abstract:

Fluctuating climate is a hallmark of Earth. As one transcends deep into Earth time, however, both the evidence for and the causes of climate change become difficult to establish. We report geochemical and sedimentological evidence for repeated, short-term climate fluctuations from the exceptionally well-preserved ∼1.4-billion-year-old Xiamaling Formation of the North China Craton. We observe two patterns of climate fluctuations: On long time scales, over what amounts to tens of millions of years, sediments of the Xiamaling Formation record changes in geochemistry consistent with long-term changes in the location of the Xiamaling relative to the position of the Intertropical Convergence Zone. On shorter time scales, and within a precisely calibrated stratigraphic framework, cyclicity in sediment geochemical dynamics is consistent with orbital control. In particular, sediment geochemical fluctuations reflect what appear to be orbitally forced changes in wind patterns and ocean circulation as they influenced rates of organic carbon flux, trace metal accumulation, and the source of detrital particles to the sediment.

Declinate in Oceanic Sulfate During Calymmian MesoProterozoic

Decline in oceanic sulfate levels during the early Mesoproterozoic

Authors:

Luo et al

Abstract:

Multiple-sulfur isotope compositions (32S, 33S, 34S and 36S) were analyzed for paired carbonate-associated sulfate (CAS) and disseminated pyrite (PY) from the ∼1.6-Ga Gaoyuzhuang Formation of the North China Craton to reconstruct the history of sulfate levels in Proterozoic oceans. The 200-m-thick study interval yielded relatively constant values for δ34SCAS (13.0 ± 1.8‰), δ34SPY (8.0 ± 2.3‰), and Δ34SCAS-PY (∼5‰), as well as relatively constant Δ33S (0 ± 0.05‰) and Δ36S (0.35 ± 0.15‰) for both CAS and pyrite. Limited variation in δ34SPY and slightly lower Δ33S of pyrite relative to CAS suggest water-column precipitation of pyrite. Limited fractionation of sulfur during microbial sulfate reduction (as documented by Δ34SCAS-PY) implies low seawater sulfate concentrations in the early Mesoproterozoic ocean. We quantitatively constrained paleo-seawater [SO42−] using a novel modeling approach based on measured values of Δ34SCAS-PY and ∂δ34SCAS/∂t(max). For the study unit, Δ34SCAS-PY is 5.4 ± 1.4‰ (n = 17), and ∂δ34SCAS/∂t(max) is 6.8-34‰ Myr−1 based on sedimentation rates of 30-150 m Myr−1. These data indicate early Mesoproterozoic seawater [SO42−] of ∼ < 0.1 to 0.35 mM (with a maximum possible concentration of 1.8 mM), a range that is lower and more tightly constrained than earlier estimates for the Mesoproterozoic. Compilation of published data suggests that low seawater sulfate concentrations began about ∼1.7 Ga and persisted until at least the mid-Mesoproterozoic (∼1.4 Ga), documenting a distinct early Mesoproterozoic perturbation in ocean chemistry that may have been related to a decline in atmospheric pO2 after Great Oxidation Event I.

Evidence of Calymmian MesoProterozoic Tidal/River Delta System From Brazil

Mesoproterozoic delta systems of the Açuruá Formation, Chapada Diamantina, Brazil

Authors:

Magalhães

Abstract:

Facies analysis, distinct depositional paleoflow directions and grain size ranges suggest that different fluvial systems fed coeval distinct, independent river-dominated and tide-dominated braid delta systems in the Mesoproterozoic Açuruá Formation. The river-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 30 m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front, distributary channels, and floodplain; (c) grain size of sandstone from delta front and distributary channels varies respectively from very fine to medium and medium to coarse; (d) lacking of wave or tide reworking suggests delta development in a relatively low energy marine basin; (e) paleocurrent readings indicate delta front sandstone bodies shifting laterally towards NE to SE directions. The tide-dominated delta system is characterized by: (a) vertical stacking of parasequences up to 25 m which are characterized by coarsening- and thickening-upward trends; (b) gradationally based vertical succession of prodelta, delta front and tide-influenced distributary channels; (c) grain size of sandstone from delta front and tide-influenced distributary channels varies respectively from very fine to coarse and fine to granular; (d) abundance of tide-influenced sedimentary structures suggest tidal dominated delta development; (e) consistent bidirectional paleocurrents with northwards ebb-tide direction and no lobe shifting. The studied braid delta systems belong to a highstand system tract and as such, they exhibit classic parasequence sets with progradational stacking pattern. The facies association variability and overall characteristics of these Mesoproteozoic delta systems are compatible with delta systems of all ages. However, contrary to typically Phanerozoic deltas – which are characterized by progradational clinoforms – the coalescence of the studied braid delta facies associations promoted fairly sheet-like geometry limited by marine flooding surface that more likely resulted from the response of deposition on shallow and wide epeiric sea.

Diverse Microbial Biota From Calymmian MesoProterozoic Siberia

Kotuikan Formation assemblage: a diverse organic-walled microbiota in the Mesoproterozoic Anabar succession, northern Siberia

Authors:

Vorob’eva et al

Abstract:

Abundant and diverse microfossils from shales of the Lower Member of the ∼1500-Ma-old Kotuikan Formation in northern Siberia document early Mesoproterozoic life along the Siberian Platform. Similar to many Meso- and Neoproterozoic microbiota, the Kotuikan assemblage is dominated by prokaryotic cyanobacteria, both filamentous (oscillatorialeans and nostocaleans, which are represented primarily by cellular trichomes and by empty sheaths) and coccoidal (chroococcaleans, including solitary and colonial specimens). However, unlike Mesoproterozoic microbiota of shallow-water settings, the Kotuikan compressed organic-walled microfossil assemblage in the open shelf facies includes diverse microscopic eukaryotes: large (up to ∼1-mm diameter) megasphaeromorph acritarchs, branched filaments that are most likely of eukaryotic algae and coccoidal monostromatic colonies of chlorococcaleans, as well as other morphologically complex microorganisms.

The Kotuikan assemblage contains numerous taxa previously recorded from Neoproterozoic successions (e.g., Elatera, Eosolena, Palaeastrum, Pterospermopsimorpha, Rugosoopsis and Navifusa), but these are long-ranging genera and no Neoproterozoic index fossils have been recorded. The microbiota is considered to be Mesoproterozoic in age because it contains a mixed assemblage of prokaryotic and eukaryotic microorganism remains including new taxa unknown in younger deposits. The Kotuikan assemblage documents the status of the marine biosphere at an early Mesoproterozoic time preceding the primary radiation of eukaryotes and representing an evolutionary stage transitional between the predominantly prokaryote-dominated Paleoproterozoic and the eukaryote-dominated Neoproterozoic microbiota.

As reported in this study, 35 taxa were identified in the Kotuikan assemblage (of which five forms are described informally) that are assigned to 18 genera of microscopic prokaryotes and eukaryotes. Two new genera and five new species are proposed: Hirudiforma lancetica gen. and sp. nov., H. simmetrica gen. and sp. nov., Lineaforma elongata gen. and sp. nov., Elatera minor sp. nov., Eosolena minuta sp. nov.

Paleo-position of the North China Craton Within the supercontinent Columbia

Paleo-position of the North China craton within the supercontinent Columbia: Constraints from new paleomagnetic results

Authors:

Xu et al

Abstract:

Several new paleomagnetic and geological studies focused on the reconstruction of the North China Craton (NCC) within the Paleo-Mesoproterozoic Columbia supercontinent. In spite of these new data, there are still widely divergent opinions regarding supercontinental reconstructions. In addition to qualitative correlations of orogenic belts, rift basin ages, stratigraphy and distribution of igneous provinces, paleomagnetic data can provide key constraints on the positioning of individual cratons on the globe. In this paper, we report a detailed paleomagnetic study on the coeval ∼1780 Ma mafic dyke swarm and Xiong’er volcanic province, which extended for more than one thousand kilometers in the central NCC. Rock magnetic studies, including thermomagnetic curves, hysteresis loops and the progressive acquisition of isothermal remanence conducted in selected samples, indicate that the dominant magnetic carriers are PSD magnetite. Stepwise thermal demagnetization isolated higher-temperature components directed to NNE/SSW with shallow inclinations from 37 sampling sites (16 sites in Yinshan area, 13 sites in Taihang area and 8 sites in Xiaoshan area). A baked contact test conducted on two Yinshan dykes intruded by a younger dyke demonstrates the magnetization in the Yinshan dykes pre-dates 1320 Ma. The existence of dual-polarity magnetizations in both Taihang and Xiong’er areas support our contention that the ChRM was acquired during the cooling of the magma. The primary origin of the ChRM is also supported by a positive fold test on the Xiong’er data, and a coherent regional test between the results from the Taihang and Xiong’er areas. Two different site-mean directions were compiled from these new results along with previous publications. The first direction, from the Taihang and Xiong’er areas, yields Declination (D)/Inclination (I) = 12.4°/−3.7° (κ = 20.5, α95 = 4.3°, N = 57 sites). The second, from the Yinshan area is at (D) 36.7°/(I)−12.4° (κ = 86.8, α95 = 2.7°, N = 32 sites). We argue that the difference is due to Mesozoic and/or Cenozoic vertical-axis rotation of the Taihang and Xiong’er areas with respect to the fixed Yinshan-Ordos basin. The corresponding paleopoles for the Yinshan dykes falls at 245.2°E/35.5°N (A95 = 2.4°). A comparison between the NCC, Laurentia, Siberia and Baltica is consistent with possible links between these four blocks in a perhaps, even larger, continent. The proximity of Australia and India to the NCC is also evaluated.

Is the Calymmian MesoProterozoic Ladoga Rift Really a Continental Rift?

Is the Proterozoic Ladoga Rift (SE Baltic Shield) a rift?

Authors:

Artemieva et al

Abstract:

The southern part of the Baltic Shield hosts a series of mafic dykes and sills of Mesoproterozoic ages, including a ca. 1.53-1.46 Ga sheet-like gabbro-dolerite sills and the Salmi plateau-basalts from the Lake Ladoga region. Based on chiefly geochemical data, the region is conventionally interpreted as an intracratonic Ladoga rift (graben). We question the validity of this geodynamic interpretation by analyzing regional geophysical data (crustal structure, heat flow, Bouguer gravity anomalies, magnetic anomalies, and mantle Vs velocities). We provide a complete list of tectonic, magmatic, and geophysical characteristics typical of continental rifts in general and demonstrate that, except for magmatic and, perhaps, some gravity signature, the Lake Ladoga region lacks any other rift features. We also compare the geophysical data from the Lake Ladoga region with similar in age Midcontinent and Valday rifts, and provide alternative explanations for Mesoproterozoic geodynamic evolution of the southern Baltic Shield. We propose that Mesoproterozoic mafic intrusions in southern Fennoscandia may be associated with a complex deformation pattern during reconfiguration of (a part of) Nuna (Columbia) supercontinent, which led to magma intrusions as a series of mafic dykes along lithosphere weakness zones and ponding of small magma pockets within the cratonic lithosphere. Consequent magma cooling and its partial transition to eclogite facies could have led to the formation of a series of basement depressions, similar to intracratonic basins of North America, while spatially heterogeneous thermo-chemical subsidence, with phase transitions locally speeded by the presence of (subduction-related) fluids, could have produced a series of faults bounding graben-like structures.

Acritarch and Macroalgae Fossils From Calymian MesoProterozoic China

Diagenetic xenotime age constraints on the Sanjiaotang Formation, Luoyu Group, southern margin of the North China Craton: Implications for regional stratigraphic correlation and early evolution of eukaryotes

Authors:

Lan et al

Abstract:

Eukaryote fossil-bearing Ruyang and Luoyu groups are extensively exposed along the southern margin of the North China Craton, but their age has long been a matter of controversy due to the absence of reliable radiometric dates. By dating detrital zircons and diagenetic xenotimes from the Sanjiaotang Formation of the Luoyu Group, this paper confirms that the Ruyang and Luoyu groups were deposited during the period of 1750–1400 Ma. The new age constraints suggest that the Sanjiaotang Formation is coeval with the Xiamaling Formation of the Jixian Group on the northern margin of the North China Craton. Correlation of the Luoyu Group with the Jixian Group is thus indicated. The Ruyang and Luoyu groups contain abundant fossils of eukaryotic organisms such as acritarch and macroalgae. Accordingly, eukaryotic proliferation extends back to the Mesoproterozoic, consistent with results of molecular phylogenetic analysis that a rapid morphological diversification of eukaryotic organisms occurred prior to 1400 Ma.