Mesoproterozoic Research Articles

Characteristics of the carbon cycle in late Mesoproterozoic: Evidence from carbon isotope composition of paired carbonate and organic matter of the Shennongjia Group in South China

The late Mesoproterozoic Era (1300–1000 Ma) was a critical period that witnessed the origin and early evolution of multicellular eukaryotes. Multiple studies have reported observations that the carbon isotope composition of carbonate (δ13Ccarb) underwent a substantial increase after ca. 1250 Ma. The high δ13Ccarb values suggest that the fraction of organic carbon burial (forg) had increased, which, consequently, might have promoted the oxidation of surface environments and the diversification of multicellular eukaryotes. However, details of the carbon cycle during this critical time interval remain elusive, which is pivotal to testing this hypothesis. In this study, we analyzed the stable carbon isotope composition of paired organic matter (δ13Corg) and carbonate, as well as the thermal characteristics of the organic matter, from the Shennongjia Group in South China, which has been constrained to an age of ∼ 1340–1150 Ma. A gradual increase in δ13Ccarb, from ∼–2‰ to >+4‰, is observed in the interval from the middle Shennongjia Group. Above which, the δ13Ccarb declines to ∼–2‰ in the uppermost. The paired δ13Corg values show an extensive range of compositions, from ∼ –22 to –32‰. Raman spectral data revealed that the organic matter in the Shennongjia Group experienced a wide range of thermal history, which might be ascribed to the involvement of detrital organic matter. We reconstructed the Δ13Ccarb-org and forg values in two different scenarios: one included all the δ13Corg data, while the other screened the δ13Corg data to minimize the possible effects of detrital organic matter. The results of these two reconstructions show similar temporal trends. Intriguingly, the Δ13Ccarb-org values are generally larger than the modern counterpart, which can be attributed to higher pCO2. The calculated forg of the Shennongjia Group is generally lower than the modern value, 20%. However, the forg experienced a substantial increase between ca. 1250 and 1190 Ma, suggesting enhanced oxidation of surface environments, which might have paved the way for the diversification of multicellular eukaryotes.

Early Prosperity of Iron Bacteria at the End of the Paleoproterozoic Era

AbstractVarious iron formation (IFs) document the interplay between the geosphere and the early biosphere during their co‐evolution. At the transition between the Paleo‐ and Mesoproterozoic eras, the oxidation of the atmosphere and ocean resulted in the waning of the typical marine deposited banded IFs. Here, we report the ∼1.7 Ga Yunmengshan IF at the southern margin of the North China Craton deposited in the intertidal to upper subtidal zone belonging to river delta of coastal environments. The combined REE + Y patterns and negative δ56Fe with positive δ53Cr values reveal terrestrial precipitation of iron oxide and an enhanced continental oxidative weathering at the end of Paleoproterozoic era due to increased atmospheric oxygen level. The oxygen level happened to have facilitated the early prosperity of microaerophilic Fe(II)‐oxidation bacteria on Earth's terrestrial surface, which is recorded by the coastal shallow water iron deposits globally characterized by their small scale and subaerial features.

Casting a vote for shifting the Statherian: Petrogenesis of 1.70 and 1.62 Ga mafic dykes in the North China Craton

The Statherian Period (1.8–1.6 Ga), currently the final period of the Paleoproterozoic Era, is named “stable, firm” for the interval following orogenesis (the Orosirian Period) that culminated in the assembly of likely Earth's first supercontinent, Columbia. Thus, the Paleoproterozoic–Mesoproterozoic boundary is supposed to demarcate increased continental “stability”, defined as a transition from orogenesis and arc magmatism to unmetamorphosed shallow-water cover sequences and intraplate magmatism. It has become apparent, however, that (i) the assembly of Columbia appears to be highly diachronous and (ii) many stable cover sequences once mapped as Mesoproterozoic appear to pre-date 1.6 Ga quite significantly. These discrepancies have led to the recent suggestion by a large group of Precambrian geologists that the Statherian Period belongs more to the Mesoproterozoic Era and thus that the end of the Paleoproterozoic Era should be redefined at ca. 1.8 Ga. North China Craton is one such craton that has pre-1.6 Ga stable sedimentary covers, but dating the base of the basins have proven difficult and thus other indicators of continental stability must be sought.Here we present new geochronological and geochemical data from two dyke swarms at 1.70 and 1.62 Ga from western Shandong Province (Luxi area) of the North China Craton that can be used to constrain the tectonic setting at this critical interval in question. Precise U-Pb SIMS analysis yields baddeleyite 207Pb/206Pb ages of 1700 ± 5 Ma (n = 9, MSWD = 0.14) and 1620 ± 4 Ma (n = 14, MSWD = 0.56). The new dated ca. 1.62 Ga Xiaobeizhuang dyke (330° trending, >15 m wide) belongs to the coeval Taishan dyke swarm. The ca. 1.70 Ga swarm, newly discovered in the North China Craton, is referred to as Ximaiyao dyke swarm (340–345° trending, 0.4–13 m wide). Paleogeographic affinities of the North China Craton with the São Francisco Craton in supercontinent Columbia are considered in light of this newly identified swarm. Given that both dyke swarms pre-date 1.6 Ga and that their geochemistry implies intraplate magmatism, our new data support the suggestion that this time is more Mesoproterozoic than Paleoproterozoic in character. These pre-1.6 Ga intraplate dyke swarms, including the 1.78 Ga, the 1.73 Ga, and later 1.70 Ga and 1.68–1.62 Ga swarms, thus provide local support from North China Craton for redefining the base of the Mesoproterozoic Era as ca. 1.8 Ga and including the Statherian Period, consistent with the ages of platform cover sequences.

Ages of the Proterozoic strata in Fanhe Basin revisited: Implications for geological records of the Great Oxidation Event in the North China Craton

The Great Oxidation Event (GOE) and Lomagundi-Jatuli Event (LJE) during Paleoprterozoic Era represent significant global environmental changes in Earth’s history and their geological records have been identified from many ancient cratons. However, reliable records of GOE and LJE are rare in the North China Craton (NCC) due to high-grade metamorphism on Paleoproterozoic strata during Orosirian Period and poor age constraints. Weakly metamorphosed to unmetamorphosed Proterozoic strata termed as Fanhe Group of around 8 km thick are well preserved in Fanhe Basin in the Archean Longgang Block in northeastern NCC and the Guanmenshan Formation from the lower part of the Fanhe Group is characterized by positive δ13Ccarb excursion of around +5‰. Since the Fanhe Group was usually considered as Meso-Neoproterozoic in ages, the reasons for positive δ13Ccarb excursions in the lower Fanhe Group are unclear. Our new LA-(MC-)ICP-MS U-Pb dating on a dolerite sill intruding into the Guanmenshan Formation yields two upper intercept 207Pb/206Pb ages of 2065 ± 20 Ma (95% confidence, MSWD = 1.4, N = 26) and 2062 ± 22 Ma (95% confidence, MSWD = 1.05, N = 22), respectively. Combined with the weighted mean 207Pb/206Pb age of the youngest detrital zircon population of the Daposhan Formation (2199 ± 19 Ma, 95% confidence, N = 8, MSWD = 0.47) and their contact relations, the deposition ages of the Daposhan, Kangzhaungzi, Guanmenshan and Tongjiajie formations from the lower part of Fanhe Group were constrained to be Rhyacian Period from ca. 2200 Ma to ca. 2060 Ma, not Mesoproterozoic Era as previously regarded. These new age constraints provide solid evidence for the existence of weakly metamorphosed stratigraphic records of GOE and LJE in lower part of Fanhe Group in the NCC. Stratigraphic correlations with other ancient cratons show that the Guanmenshan Formation in Fanhe Basin in the northeastern NCC can be correlated with the Dashiling Formation of the Hutuo Group in the central NCC, the Lucknow Formation in West Australia, the Upper Pretoria Formation in South Africa, the Gordon Lake Formation in North America and the Tulomozero and Kuetsjärvi formations in Fennoscandia, and therefore the Fanhe Group provides excellent stratigraphic records to study GOE and LJE in the NCC.

Contrasting nutrient availability between marine and brackish waters in the late Mesoproterozoic: Evidence from the Paranoá Group, Brazil.

Understanding the delayed rise of eukaryotic life on Earth is one of the most fundamental questions about biological evolution. Numerous studies have presented evidence for oxygen and nutrient limitations in seawater during the Mesoproterozoic era, indicating that open marine settings may not have been able to sustain a eukaryotic biosphere with complex, multicellular organisms. However, many of these data sets represent restricted marine basins, which may bias our view of habitability. Furthermore, it remains untested whether rivers could have supplied significant nutrient fluxes to coastal habitats. To better characterize the sources of the major nutrients nitrogen and phosphorus, we turned to the late Mesoproterozoic Paranoá Group in Brazil (~1.1Ga), which was deposited on a passive margin of the São Francisco craton. We present carbon, nitrogen and sulphur isotope data from an open shelf setting (Fazenda Funil) and from a brackish-water environment with significant riverine input (São Gabriel). Our results show that waters were well-oxygenated and nitrate was bioavailable in the open ocean setting at Fazenda Funil; the redoxcline appears to have been deeper and further offshore compared to restricted marine basins elsewhere in the Mesoproterozoic. In contrast, the brackish site at São Gabriel received only limited input of marine nitrate and sulphate. Nevertheless, previous reports of acritarchs reveal that this brackish-water setting was habitable to eukaryotic life. Paired with previously published cadmium isotope data, which can be used as a proxy for phosphorus cycling, our results suggest that complex organisms were perhaps not strictly dependent on marine nutrient supplies. Riverine influxes of P and possibly other nutrients likely rendered coastal waters perhaps equally habitable to the Mesoproterozoic open ocean. This conclusion supports the notion that eukaryotic organisms may have thrived in brackish or perhaps even freshwater environments.

Sedimentary facies of the Mesoproterozoic Srisailam Formation, Cuddapah basin, India: Implications for depositional environment and basin evolution

Sedimentary facies of the Mesoproterozoic (~1400−1327 Ma) Srisailam Formation, exposed in the Srisailam sub-basin in the northern part of the Cuddapah basin, India, were analyzed. It helped in identifying eighteen different facies that can be grouped into six facies associations (FA-I to VI) – alluvial fan/fan-delta, delta top, delta front, lacustrine, fluvial, and aeolian. Hinterland geology, integrated palaeomagnetic, geochemical and sedimentological observations, and inferred palaeohydrological condition and palaeoclimate indicate more control of tectonic regime on the evolution of the facies architecture. Analysis of vertico-lateral disposition of the identified facies suggests that the deposition of the Srisailam sediments was initiated in a number of fault-controlled isolated depocenters (riftogenic lakes). However, those isolated lakes were deep enough to allow the evolution of lacustrine and deltaic depositional regimes. Basin-wide lacustrine environment was established during subsequent rifting and regional peak subsidence (rift-climax) of the basin floor. Cessation of further basin subsidence facilitated progradation of fluvial sediments. During low-stage fluvial sediments were reworked locally by aeolian processes, and during protracted dry periods, aeolian regimes were established over considerable stretches of the Srisailam sub-basin.It is likely that the Srisailam half-graben, adjacent to the Nallamalai Fold Belt, was developed due to lithospheric doming and limited crustal stretching during a mantle upwelling event responsible for lamproite (alkaline) magmatism in the northern part of the Cuddapah basin. The evolution of the Srisailam rift-basin may be compared with the Cenozoic rift system of Europe, where rift development took place simultaneously with the orogenic activity in the Alps and Pyrenees. The style of sedimentation in the Srisailam sub-basin may be considered as a variant of the Newark-type lacustrine facies complex, and the tripartite basin filling model of half-graben lacustrine complexes may possibly be extended to the Mesoproterozoic era.

A template for an improved rock-based subdivision of the pre-Cryogenian timescale

The geological timescale before 720 Ma uses rounded absolute ages rather than specific events recorded in rocks to subdivide time. This has led increasingly to mismatches between subdivisions and the features for which they were named. Here we review the formal processes that led to the current timescale, outline rock-based concepts that could be used to subdivide pre-Cryogenian time and propose revisions. An appraisal of the Precambrian rock record confirms that purely chronostratigraphic subdivision would require only modest deviation from current chronometric boundaries, removal of which could be expedited by establishing event-based concepts and provisional, approximate ages for eon-, era- and period-level subdivisions. Our review leads to the following conclusions: (1) the current informal four-fold Archean subdivision should be simplified to a tripartite scheme, pending more detailed analysis, and (2) an improved rock-based Proterozoic Eon might comprise a Paleoproterozoic Era with three periods ( early Paleoproterozoic or Skourian , Rhyacian, Orosirian), Mesoproterozoic Era with four periods (Statherian, Calymmian, Ectasian, Stenian) and a Neoproterozoic Era with four periods (pre-Tonian or Kleisian , Tonian, Cryogenian and Ediacaran). These proposals stem from a wide community and could be used to guide future development of the pre-Cryogenian timescale by international bodies.

Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era

Black shale Mo isotope record reveals dynamic ocean redox during the Mesoproterozoic Era

Carbonate fabric diversity and environmental heterogeneity in the late Mesoproterozoic Era

AbstractTime-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantineet al.(2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.

Petrogenesis of (meta-) basalts from the North Qilian Orogenic Belt, NW China: implications for the Palaeoproterozoic–Mesoproterozoic tectonic evolution of the North Qilian Block

AbstractThe North Qilian Orogenic Belt is surrounded by the Tarim Craton to the NW and the North China Craton to the NE. The Precambrian continental crust remnants that are distributed in the North Qilian Orogenic Belt are termed the North Qilian Block (NQB), and their tectonic evolution has profound implications for the evolution of the Columbia Supercontinent. Here we present major- and trace-element and Sr–Nd–Hf isotope data for (meta-) basalts from the Beidahe Group (BDHG) and Zhulongguan Group (ZLGG) in the western North Qilian Orogenic Belt, to investigate the tectonic evolution of the NQB during the Proterozoic Eon. The protoliths of Palaeoproterozoic amphibole gneisses and plagioclase amphibolites from the BDHG are calc-alkaline series basalts. These metabasalts show island-arc-basalt affinities with variable Nd and Hf isotopes (ϵNd(t) = −5.0–0.6 and 2.7–4.3;ϵHf(t) = −14.2–2.0 and 6.9–8.8) and were generated by partial melting of the asthenospheric mantle that was metasomatized by aqueous fluid and sediment melt in a continental-arc setting. The early Mesoproterozoic ZLGG basalts show features of shoshonite-series basalts and are geochemically similar to ocean-island basalts. These basalts show variable (87Sr/86Sr)i,ϵNd(t) andϵHf(t) values of 0.70464–0.70699, −1–2.6 and −1.5–5.7, and are products of mantle plume magmatism that participated with subducted oceanic crust in an intracontinental rift setting. This study suggests that the NQB underwent tectonic evolution from palaeo-oceanic subduction to intracontinental rifting during the Palaeoproterozoic–Mesoproterozoic eras. Furthermore, the above tectonomagmatic events were in response to convergence–splitting events of the Columbia Supercontinent during the Palaeoproterozoic–Mesoproterozoic eras.

A Pulsed Oxygenation in Terminal Paleoproterozoic Ocean: Evidence From the Transition Between the Chuanlinggou and Tuanshanzi Formations, North China

AbstractThe mid‐Proterozoic (ca. 1.8–0.8 Ga) witnessed a period of stable carbon cycling and long stasis in eukaryotic evolution, which was commonly ascribed to the persistently low oxygen levels in the atmosphere‐ocean system. Recently, several pulsed marine oxygenations were identified from different continents, and presumed to have facilitated the short‐term diversification of eukaryotes in the early Mesoproterozoic Era. To test this hypothesis, an integrated study of iodine species [I/ (Ca + Mg)], paired carbon isotopes (δ13Ccarb and δ13Corg), and elemental abundances was conducted on the fossil‐bearing Tuanshanzi Formation (∼1.64–1.63 Ga), North China. The near‐zero I/(Ca + Mg) ratios (0.00 ± 0.01 μmol/mol) coupled with low δ13Corg (−30.2 ± 1.4‰) in the strata below the fossil‐bearing interval suggests anoxic conditions in shallow seawater. An increase of I/ (Ca + Mg) up to ∼1.54 μmol/mol coupled with a ∼2‰ negative shift in δ13Ccarb and a ∼11‰ positive excursion in δ13Corg in the fossil‐bearing interval point to an increased oxygen concentration. Above the fossil‐bearing interval, I/ (Ca + Mg) decrease to <0.5 μmol/mol, suggesting deoxygenation and a return to anoxic to suboxic conditions. Combined with relevant data from other mid‐Proterozoic sections of North China and Australia, these data likely indicate dynamic redox conditions in the shallow seawaters of this period, with multiple pulses in oxygenation in otherwise anoxic conditions. These pulsed oxygenations may have facilitated short‐term proliferation and diversification of early eukaryotes in shallow seawaters, but the overall low oxygen levels have impeded their continuous development and the rise to ecological dominance until late Neoproterozoic.

Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

The Mesoproterozoic era has long been considered a time of relative environmental and biological stasis. However, emerging insight suggests that this period may have been more dynamic than previously considered, both in terms of oxygenation and potential consequences for biological evolution. Nevertheless, our understanding of this immense period of time remains limited. To provide more detailed constraints on oxygenation dynamics, we report a multiproxy geochemical study of an early Mesoproterozoic (∼1600–1540 million years ago, Ma) carbonate-dominated succession from the North China craton. We include inorganic carbon isotope (δ13Ccarb), iron-speciation, and major and trace element data, in addition to molybdenum isotopic compositions (δ98/95Mo). These geochemical data support previous inferences of persistent anoxic and ferruginous deeper water conditions in the earliest Mesoproterozoic ocean, with limited oxygenation of surface waters. However, the behaviour of these redox-sensitive geochemical proxies reveals pulsed oxygenation events, with each event increasing the maximum depth of oxygenation, leading to overall progressive oxygenation of the ocean. During these pulsed oxygenation events we find the lightest Mo isotope signatures ever measured in the rock record, which we attribute to initial drawdown of isotopically light Mo in association with extensive Mn and Fe (oxyhydr)oxide precipitation, followed by diagenetic recycling. However, shallower water sediments deposited after the pulses of deeper water oxygenation more faithfully record the Mo isotopic composition of coeval seawater. For these samples, we utilise a single reservoir Mo cycling model, constrained by an updated estimate of Mesoproterozoic seawater Mo concentration, and scaled using a function associated with differential organic carbon flux between the shelf and basin. When scaled to modern rates of Mo accumulation under variable marine redox conditions, our modelling estimates suggest a minimum oxic seafloor area of ∼30% of the total seafloor area at ∼1540 Ma. It remains unclear whether the oxygenation observed across this ∼60 million year interval represents a progressive transition to a more persistently oxygenated ocean, or whether oceanic oxygen levels fluctuated considerably through the later Mesoproterozoic.

Heterogeneous oxygenation coupled with low phosphorus bio-availability delayed eukaryotic diversification in Mesoproterozoic oceans: Evidence from the ca 1.46 Ga Hongshuizhuang Formation of North China

The Mesoproterozoic Era is characterized by low atmospheric oxygen levels, pervasive ocean anoxia, and long stasis in biotic evolution. As to the ocean oxygenation patterns, timing and their potential effects on the early eukaryotic evolution, however, opinions are controversial. To further constrain the redox evolution of Mesoproterozoic oceans, an integrated study was conducted on the ~1.46 Ga Hongshuizhuang Formation of the North China Platform using multiple techniques. The results show that the black shales were largely deposited in the environments below storm-wave base in an intra-shelf basin. The concentrations of redox sensitive trace metals measured from the shales are at nearly upper continent crust level, with average Mo, U, and V enrichment factors of 6.65, 1.99, and 1.24, respectively, suggesting predominantly suboxic to weekly oxygenated conditions. Short anoxic conditions with intermittent pore-water euxinia existed only in the middle part of the formation. No framboidal pyrite was observed through the studied intervals, consistent with the low degree of pyritization (DOPT, 0.01 ± 0.02), S/Fe (0.02 ± 0.04) and Fe/Al (0.37 ± 0.10) values indicative of non-euxinic water-column conditions. Nitrogen isotope compositions show exclusively positive value (δ15N = 2.43 ± 0.54‰), invoking aerobic N cycling and a relatively stable nitrate pool in contemporary seawaters, which are also supported by the presence of low organic carbon to phosphorus (Corg/P) ratios and negative Ce anomaly. Multiple-proxy correlation across the North China Platform shows a spatio-temporal heterogeneity in redox landscape during the Hongshuizhuang deposition. Relatively constant euxinic conditions developed in northern part of the basin, which became waning and replaced by anoxic–suboxic conditions towards south. Low total organic carbon (TOC), total nitrogen (TN) and P contents in this formation point to oligotrophic water-mass in the basin. It seems that the nutrient limitation, coupled with heterogeneous oxygen concentrations, have depressed the diversification of early eukaryotes. Given that the Hongshuizhuang Formation was largely deposited under more or less oxygenated seawaters with moderate nitrate availability, the absence of eukaryotic records and their biomarkers may have been caused by nutrient limitation, especially phosphorus, in the contemporary seawater.

The Yunmengshan iron formation at the end of the Paleoproterozoic era

Precambrian iron formations (IF) carry rich information on the interplay between the geosphere and the early biospheres. Here, we report mineralogical and petrologic characterizations of the Yunmengshan IF deposited at ~1.7 Ga, the transition between Paleoproterozoic and Mesoproterozoic eras. The IF contains granular hematite with evident oncoidal structures and lacks laminated structures, which are distinct to the typical banded iron formation (BIF) or granular iron formation (GIF) deposited early in the marine environments with high silica content. Based on observations of clastic sedimentary structures around the Yunmengshan IF, including parallel bedding, cross-bedding, asymmetric ripple mark, and mud crack, we deduce that the Yunmengshan IF was mainly deposited in a supratidal to intertidal zone. Electron microscopic observations show two types of hematite euhedral crystals: laminar hematite of 200–500 nm thick and 3-5 μm in diameter and granular hematite of 200–300 nm in diameter randomly distributed in the matrix. Stilpnomelane and minnesotaite, as Fe2+-silicates in the typical Archean- and the early Paleoproterozoic BIF, are identified but with extremely low abundance. The morphology and paragenetic association of these minerals imply that the Yunmengshan IF was probably deposited in an aquatic environment with low Si concentration. These results indicate that the Yunmengshan IF represents a transitional type of iron deposition between BIF/GIF and ironstone in the geological history of the iron cycle on the surface of Earth.

The formation of marine red beds and iron cycling on the Mesoproterozoic North China Platform

Abstract Marine red beds (MRBs) are common in sedimentary records, but their genesis and environmental implications remain controversial. Genetic models proposed for MRBs variably invoke diagenetic or primary enrichments of iron, with vastly different implications for the redox state of the contemporaneous water column. The Xiamaling Formation (ca. 1.4 Ga) in the North China Platform hosts MRBs that offer insights into the iron cycling and redox conditions during the Mesoproterozoic Era. In the Xiamaling MRBs, well-preserved, nanometer-sized flaky hematite particles are randomly dispersed in the clay (illite) matrix, within the pressure shadow of rigid detrital grains. The presence of hematite flake aggregates with multiple face-to-edge (“cardhouse”) contacts indicates that the hematite particles were deposited as loosely bound, primary iron oxyhydroxide flocs. No greenalite or other ferrous iron precursor minerals have been identified in the MRBs. Early diagenetic ankerite concretions hosted in the MRBs show non-zero I/(Ca+Mg) values and positive Ce anomalies (>1.3), suggesting active redox cycling of iodine and manganese and therefore the presence of molecular oxygen in the porewater and likely in the water column during their formation. These observations support the hypothesis that iron oxyhydroxide precipitation occurred in moderately oxygenated marine waters above storm wave base (likely <100 m). Continentally sourced iron reactivated through microbial dissimilatory iron reduction, and distal hydrothermal fluids may have supplied Fe(II) for the iron oxyhydroxide precipitation. The accumulation of the Xiamaling MRBs may imply a slight increase of seawater oxygenation and the existence of long-lasting adjacent ferruginous water mass.

Role of Paleomagnetism in the Construction of Earth’s Geographic Past

Rocks have the ability to preserve magnetic information used in determining past geographic formations. The purpose of this report is to determine the past location of a site from a given data set’s magnetic information and the calculations found through their application to paleomagnetism. Magnetic information includes the rock sample’s location and concentration of trace magnetic particles which were used to find declination and inclination on site. The sample’s paleolatitude and paleolongitude are calculated using trigonometric equations that are derived using calculus. After a statistical analysis, these results are compared to the present day’s magnetic poles to determine the past location of the site. This location, along with the magnetic information, is combined to construct a past geographic formation that existed a billion years ago. This process reveals that the site currently found in southwest Namibia, was located near the coast of modern-day northwest Africa during the late Mesoproterozoic Era within a 95% certainty. When compared to past literature these results show the reliability and role of paleomagnetism, as well as the importance of collaboration across the geosciences.

Molybdenum isotope and trace metal signals in an iron-rich Mesoproterozoic ocean: A snapshot from the Vindhyan Basin, India

Fundamental questions persist regarding the redox structure and trace metal content of the Mesoproterozoic oceans. Multiple lines of evidence suggest more widespread anoxia in the deep oceans compared to today, and iron speciation indicates that anoxia was largely accompanied by dissolved ferrous iron (ferruginous conditions) rather than free sulfide (euxinia). Still, exceptions exist—euxinic conditions have been reported from some ocean margin and epeiric sea settings, and oxic conditions were reported in one deeper water environment and are also known from shallow waters. Constraining the temporal evolution of Mesoproterozoic marine redox structure is critical because it likely governed redox-sensitive trace metal availability, which in turn played a significant role in marine diazotrophy and the evolution of early eukaryotes. Here, we present a new, multi-proxy geochemical dataset from the ~1.2 Ga Bijaygarh Shale (Kaimur Group, Vindhyan Basin, India) emphasizing total organic carbon, iron speciation, and trace metal concentrations, as well as sulfur, nitrogen, and molybdenum isotopes. This unit was deposited in an open shelf setting near or just below storm wave base. Taken together, our data provide a unique snapshot of a biologically important shallow shelf setting during the Mesoproterozoic Era, which includes: 1) locally ferruginous waters below the zone of wave mixing, 2) muted enrichment of trace metals sensitive to general anoxia (e.g., chromium) and variable enrichment of trace metals sensitive to euxinia (e.g., molybdenum and, to a lesser extent, vanadium), 3) general sulfate limitation, and 4) nitrogen fixation by molybdenum-nitrogenase and a dominantly anaerobic nitrogen cycle in offshore settings. Differential patterns of trace metal enrichment are consistent with data from other basins and suggest a largely anoxic ocean with limited euxinia during the Mesoproterozoic Era. Our new molybdenum isotope data—the first such data from unambiguously marine shales deposited between 1.4 and 0.75 Ga—record values up to +1.18 ± 0.12‰ that are analogous to data from other Mesoproterozoic shale units. Ultimately, this study provides a broad, multi-proxy perspective on the redox conditions that accompanied early eukaryotic evolution.

Sedimentary environment and organic matter accumulation of Neoproterozoic black shale in the North China Craton: A case study of the Tonian Baishugou Formation in the Luonan area

With acceleration of “oil exploration near the source” in the petroleum industry, black shales (BS) play an increasingly important role in oil and gas exploration. Meso-Neoproterozoic BS are well developed in the North China Craton (NCC), China, many of which are identified as potential hydrocarbon source rocks. Although BS from the Mesoproterozoic Era has been widely investigated, few studies were focused on the Neoproterozoic BS so far. Based on petrology and geochemistry analysis, the sedimentary environment, provenance and organic matter accumulation of two BS layers in the Tonian Baishugou Formation from Sujiacun section, Luonan Area, southern NCC were investigated. It suggests that the lower BS (LB) was deposited in a shallow and non-stratified cratonic-interior sea environment with unstable dysoxic or suboxic conditions. The seawater salinity increased from brackish to marine facies conditions due to the decrease of runoff caused by climate cooling. The Upper BS (UB) was deposited in an open paleomarine environment with stable suboxic conditions and brackish salinity in continuously humid and hot paleoclimate. Both BS were derived from granitoids formed by the ca.2.5 Ga collisional-thermal events in the NCC. The OM accumulation rate of UB is higher than that of UB while LB records lower OM loss rate. On these bases, two developmental modes for the BS from the Baishugou Formation were established.

Spatiotemporal redox heterogeneity and transient marine shelf oxygenation in the Mesoproterozoic ocean

The Mesoproterozoic Era (1.6–1.0 Ga), long regarded as an interval of sluggish biotic evolution and persistently low atmospheric-oceanic oxygen levels, has become the subject of recent controversy regarding putative large-scale oxygenation events. In this study, we conducted a comprehensive investigation of redox, productivity, seawater sulfate concentrations, and hydrographic conditions for the ∼1.4–1.32-Ga Xiamaling Formation in the shallow Hougou and mid-depth Huangtugui sections in the Yanshan Basin (North China). Integrated Fe-trace metal data suggest that bottom-water redox conditions mainly underwent a three-stage secular evolution: (1) suboxic (-oxic), (2) predominantly anoxic, and (3) oxic. Integrated productivity (TOC, P/Al, and Cu-Zn-Ni enrichment factors) and depositional system (major and trace elemental enrichment patterns) data suggest varying productivity levels during deposition of the Xiamaling Formation, with high productivity fueled mainly by intense open-marine upwelling. In combination with published data from the deep Xiahuayuan and Chicheng sections, a refined stratigraphic correlation framework was developed for the Xiamaling Formation based on the basinal upwelling event and existing lithostratigraphic framework. Within this new framework, sulfur concentration and pyrite δ34S (δ34Spy) data suggest a rise in marine sulfate concentrations at ∼1.36 Ga. Furthermore, strong spatiotemporal redox heterogeneity was observed within the ∼1.4–1.32-Ga Yanshan Basin, i.e., strong bottom-water anoxia (or euxinia) in deep sections and less anoxic (even oxic) conditions in shallow sections, although all sections experienced high productivity owing to oceanic upwelling. We propose a productivity-driven shelf oxygenation model to explain this spatiotemporal redox heterogeneity within the context of eustatic changes, an upwelling event, and the rise of oceanic sulfate concentrations at ∼1.36 Ga. In this model, from initially low atmospheric oxygen levels, increasing surface productivity in shelf areas due to elevated nutrient supplies from upwelling not only intensified bottom-water anoxia through organic matter export but also stimulated expansion of the ocean-surface oxic layer through photosynthetic O2 release, contributing to the development of contrasting bottom-water redox conditions between shallow and deep sections during the upwelling event identified in this study. Our model helps reconcile conflicting observations concerning Mesoproterozoic marine redox conditions, and provides new insights into putative Mesoproterozoic oceanic oxygenation events and their relationship to the evolution of early eukaryotes.

Organically-preserved multicellular eukaryote from the early Ediacaran Nyborg Formation, Arctic Norway

Eukaryotic multicellularity originated in the Mesoproterozoic Era and evolved multiple times since, yet early multicellular fossils are scarce until the terminal Neoproterozoic and often restricted to cases of exceptional preservation. Here we describe unusual organically-preserved fossils from mudrocks, that provide support for the presence of organisms with differentiated cells (potentially an epithelial layer) in the late Neoproterozoic. Cyathinema digermulense gen. et sp. nov. from the Nyborg Formation, Vestertana Group, Digermulen Peninsula in Arctic Norway, is a new carbonaceous organ-taxon which consists of stacked tubes with cup-shaped ends. It represents parts of a larger organism (multicellular eukaryote or a colony), likely with greater preservation potential than its other elements. Arrangement of open-ended tubes invites comparison with cells of an epithelial layer present in a variety of eukaryotic clades. This tissue may have benefitted the organism in: avoiding overgrowth, limiting fouling, reproduction, or water filtration. C. digermulense shares characteristics with extant and fossil groups including red algae and their fossils, demosponge larvae and putative sponge fossils, colonial protists, and nematophytes. Regardless of its precise affinity, C. digermulense was a complex and likely benthic marine eukaryote exhibiting cellular differentiation, and a rare occurrence of early multicellularity outside of Konservat-Lagerstätten.