Authors:van der Meer et alAbstract:The eustatic sea-level curves published in the seventies and eighties have supported scientific advances in the Earth Sciences and the emergence of sequence-stratigraphy as an important hydrocarbon exploration tool. However, validity of reconstructions of eustatic sea level based on sequence stratigraphic correlations has remained controversial. Proposed sea level curves differ because of site-to-site changes in local tectonics, depositional rates, and long-wavelength dynamic topography resulting from mantle convection. In particular, the overall amplitude of global Phanerozoic long-term sea level is poorly constrained and has been estimated to vary between ~ 400 m above present-day sea level to ~ 50 m below present-day sea level. To improve estimates of past sea level, we explore an alternative methodology to estimate global sea level change. We utilise the Phanerozoic-Neoproterozoic 87Sr/86Sr record, which at first order represents the mix of inputs from continental weathering and from mantle input by volcanism. By compensating for weathering with estimates of runoff from a 3D climate model (GEOCLIMtec), a corrected 87Sr/86Sr record can be obtained that solely reflects the contribution of strontium from mantle sources. At first order, the flux of strontium from the mantle through time is due to increases and decreases in the production of oceanic crust through time. Therefore, the changing levels of mantle-derived strontium can be used as a proxy for the production of oceanic lithosphere. By applying linear oceanic plate age distributions, we compute sea level and continental flooded area curves. We find that our curves are generally within the range of previous curves built on classical approaches. A Phanerozoic first order cyclicity of ~ 250 Myr is observed that may extend into the Neoproterozoic. The low frequency (i.e., on the order of 10 to 100 My) sea level curve that we propose, while open for improvement, may be used as baseline for refined sequence-stratigraphic studies at a global and basin scale.
Showing posts with label paleooceans. Show all posts
Showing posts with label paleooceans. Show all posts
Friday, December 23, 2016
Reconstructing Sea Level Rise and Fall Since the NeoProterozoic
Friday, December 16, 2016
Evidence of a Radical PaleoOcean Chemistry Change Circa 545 Million Years Ago From Russia
Authors:Wood et alAbstract:The trigger for biomineralization of metazoans in the terminal Ediacaran, ca. 550 Ma, has been suggested to be the rise of oxygenation or an increase in seawater Ca concentration, but geochemical and fossil data have not been fully integrated to demonstrate cause and effect. Here we combine the record of macrofossils with early marine carbonate cement distribution within a relative depth framework for terminal Ediacaran to Cambrian successions on the eastern Siberian Platform, Russia, to interrogate the evolution of seawater chemistry and biotic response. Prior to ca. 545 Ma, the presence of early marine ferroan dolomite cement suggests dominantly ferruginous anoxic "aragonite-dolomite seas," with a very shallow oxic chemocline that supported mainly soft-bodied macrobiota. After ca. 545 Ma, marine cements changed to aragonite and/or high-Mg calcite, and this coincides with the appearance of widespread aragonite and high-Mg calcite skeletal metazoans, suggesting a profound change in seawater chemistry to "aragonite seas" with a deeper chemocline. By early Cambrian Stage 3, the first marine low-Mg calcite cements appear, coincident with the first low-Mg calcite metazoan skeletons, suggesting a further shift to "calcite seas". We suggest that this evolution of seawater chemistry was caused by enhanced continental denudation that increased the input of Ca into oceans so progressively lowering Mg/Ca, which, combined with more widespread oxic conditions, facilitated the rise of skeletal animals and in turn influenced the evolution of skeletal mineralogy.
Could the mid Ediacaran Shuram Event be Significantly Older Than 551 Million Years Ago?
Authors:Zhou et alAbstract:The middle Ediacaran Shuram excursion represents the most pronounced negative carbon isotopic shift in Earth history, and has been considered as evidence for a profound disturbance to the global carbon cycle and proposed as a key chemostratigraphic marker for Ediacaran stratigraphic subdivision and global correlation. Previous study has revealed a pronounced negative δ13C shift (EN3) in the upper Doushantuo Formation of South China, which has been interpreted as an equivalent of the Shuram excursion. Detailed δ13C investigation of multiple sections of the Ediacaran Doushantuo Formation around the Huangling Anticline, western Hubei Province, South China, indicates that the δ13C variation in the upper Doushantuo Formation is more complex than previously reported. In the western region of the Huangling anticline, a moderately positive δ13C excursion (here termed the Diaoyapo positive excursion) bisects two strongly negative δ13C excursions (here termed the Jiuqunao negative excursion and the younger Miaohe negative excursion), with a ∼551 Ma ash bed capping the Miaohe negative excursion. In the central region, there is only one negative δ13C excursion, whereas in the eastern region, the negative δ13C excursion is absent. The variable stratigraphic expression of these δ13C excursions is most parsimoniously interpreted as reflecting local facies variation, diagenesis, and/or a cryptic unconformity. The more complete but more complex δ13C chemostratigraphic record in the western region of Huangling Anticline implies that, depending on how this record is correlated with the Shuram excursion, the latter may be significantly older than 551 Ma if it does represent a global chemostratigraphic marker.
Using Zinc and Cadmium Isotopes to Determine Post Snowball Earth Environmental Conditions
Authors:John et alAbstract:Zinc and cadmium are important biologically cycled nutrients in the modern ocean. Recent analytical advances and sampling efforts such as GEOTRACES have led to measurements of the global distribution of Zn and Cd and their stable isotope ratios (δ66Zn and δ114Cd), providing an understanding of how biological processes cycle these elements and their isotopes in the modern ocean. This opens the door to better application of δ66Zn and δ114Cd as tracers of biogeochemical processes archived in the geological record. Here we present new measurements of [Cd] and δ114Cd on samples from the Nuccaleena cap dolostone in South Australia, which was deposited during deglaciation of the ca. 635 Ma old Marinoan snowball Earth. Our new data are combined with previously measured δ66Zn data from the same samples. The combined record of δ114Cd and δ66Zn is used to explore various hypotheses about the sources and sinks of Cd and Zn in the post-Marinoan ocean. Key features of this record are that [Zn] and [Cd] remain relatively similar throughout, that δ66Zn and δ114Cd change at the same stratigraphic level and in the same direction, and that δ66Zn changes are about four times larger than δ114Cd changes even though both quantities reflect a similar fractional mass difference. We suggest that both Zn and Cd concentration and isotopic data are consistent with a change in the sink by which Zn and Cd are removed from the oceans. We propose that no isotope fractionation is expressed during removal of Zn and Cd as sulfides, while Zn and Cd removed with biological material are about 0.6‰ and 0.2‰ lighter than deep ocean δ66Zn and δ114Cd, respectively. Many previous studies attribute the burial of isotopically light biological Zn to preferential assimilation of isotopically lighter Zn by phytoplankton. In contrast, we propose that lighter biological δ66Zn occurs mostly because isotopically heavy Zn was scavenged by organic matter, which causes dissolved δ66Zn in the surface ocean to be lighter than deep ocean δ66Zn. The organic matter originated from primary producers that thrived in the surface ocean during deglaciation. For Cd, whose isotope cycling in the modern ocean is dominated by preferential assimilation of isotopically lighter Cd by phytoplankton, much less biological fractionation is expressed. The combined analyses of δ66Zn and δ114Cd provide insight into the biogeochemical processes which occurred after the Marinoan snowball Earth glaciation, and may be a useful new tool to explore the history of life on Earth in other geological records as well.
Current molded, storm damaged, sinuous columnar stromatolites From Ectasian Mesoproterozoic China
Authors:Tosti et alAbstract:A thin (~ 50 cm) horizon of sinuous stromatolites occurs within a succession of elongate upright columns in the ~ 1.4 Ga Tieling Formation, near Jixian, China. The upright columns form aligned closely spaced ridges, separated by narrow runnels with signs of current scour. The sinuous and upright stromatolites, and their intervening matrix, were originally mainly composed of carbonate mud. In end-on view, the sinuous columns incline back and forth. Each column consists of well-defined laminae that successively rotate relative to column curvature, maintaining their orientation approximately at right-angles to the column axis. In inclined parts of the columns, laminae are typically asymmetric and the steeper side faces the direction of column inclination. We interpret this column sinuosity to be a response to changes in current-direction, with accreting laminae facing into currents that supplied sediment. We find no evidence for heliotropism (mat growth towards the sun) in these examples. Adjacent columns typically show similar shapes, bending back and forth together, but their angles of curvature and inclination can change laterally from column to column, from near vertical to 45°, over distances of 30 cm. Columns can show breakage and separation of adjacent laminae. Some of this is enhanced by compaction and stress, but the occurrence of sinuous columns on their sides or upturned, in spaces between undisturbed columns, indicates that column curvature developed during growth and that toppling of columns was syndepositional. We infer that sinuosity developed in response to changes in current direction, that column inclination reflects current strength, and that breakage and toppling was produced by strong currents. Curved and sinuous columns could reflect shoaling. This would also have exposed them to storm damage and to the effects of currents that scoured sufficient matrix to locally break and topple columns. Markedly sinuous Mesoproterozoic columns also occur in Siberia and North America, suggesting that similar conditions and processes of stromatolite formation may have been widespread at this time. Formation and preservation of the sinuous stromatolites described here required a combination of conditions that included abundant fine-grained carbonate sediment, microbial mats capable of trapping it, reduced early lithification, and absence of bioturbation.
Was Delayed Intercontinental Seas' Euxinia A Vital for Eukaryotes to Survive During the Statherian PaleoProterozoic?
Authors:Spinks et alAbstract:Increased flux of sulfate to the oceans in the aftermath of the Great Oxidation Event (GOE) ∼2.4 billion years ago (Ga) caused major changes in seawater chemistry, which eventually contributed to the cessation of iron formation deposition ∼1.8 Ga. It is generally accepted that this engendered heterogeneous stratified redox conditions, with anoxic and sulfidic (euxinic) conditions in shallow open-marine environments and anoxic ferruginous conditions in deeper environments. However, the redox evolution of intracontinental marine basins following the cessation of iron formation deposition remains poorly understood.Here, we report contrasting paleoredox conditions in two shale units of the lower McArthur Basin, northern Australia, soon after the cessation of iron formation deposition ∼1.84 Ga. Our data shows that the ∼1.78 Ga McDermott Formation was deposited in a sulfur-limited, anoxic shallow-marine environment, whereas the younger ∼1.73 Ga Wollogorang Formation was deposited in a euxinic shallow-marine environment. This implies a delay in the development of euxinia in a shallow intracontinental basin following the onset of euxinia in the open marine realm. Since bioessential metals are sequestered by pyrite deposition under euxinic conditions, protracted low-sulfidic conditions in 1.78 Ga intracontinental shallow environments could have provided vital niches for nitrogen-fixing prokaryotes and eukaryotes. Thus the ability for localized Paleoproterozoic intracontinental basins to remain non-euxinic after the onset of euxinia in shallow open-marine shelves highlights the importance of intracontinental environments to the evolution and diversification of microbial life, perhaps throughout the wider Proterozoic.
Friday, November 18, 2016
Evidence of Extensive Mercury Pollution During Permian Triassic Mass Extinction
Authors:Grasby et alAbstract:Sedimentary records from the northwest margin of Pangea and the Tethys show anomalously high Hg levels at the latest Permian extinction boundary. Background δ202Hg values are consistent with normal marine conditions but exhibit negative shifts coincident with increased Hg concentrations. Hg isotope mass-independent fractionation (Δ199Hg) trends are consistent with volcanic input in deep-water marine environments. In contrast, nearshore environments have Δ199Hg signatures consistent with enhanced soil and/or biomass input. We hypothesize that the deep-water signature represents an overall global increase in volcanic Hg input and that this isotope signature is overwhelmed in nearshore locations due to Hg from terrestrial sources. High-productivity nearshore regions may have experienced stressed marine ecosystems due to enhanced Hg loading.
Friday, October 28, 2016
The Strontium Cycle of the NeoProterozoic was Driven by Seemingly Unique Paleogeography
Authors:Goddéris et alAbstract:The period spanning from 825 to 540 Ma is characterized by major changes in the surficial Earth system. This extraordinary interval starts with the breakup of the Rodinia supercontinent and eruption of a series of large igneous provinces and ends with the assembly of Gondwana, giving rise to the Pan-African orogenies. This paleogeographic reorganization is accompanied by a global climatic cooling, including the paroxysmal Cryogenian “snowball” glacial events. The 87Sr/86Sr of seawater displays a major long-term rise over this interval that is punctuated by episodic, smaller declines and inflections. We use a coupled deep time climate-carbon numerical model to explore the complex role of tectonics and climate on this distinct evolution in seawater 87Sr/86Sr. We show that the modulation of the weathering of the erupted large igneous provinces by continental drift explains the changes in seawater 87Sr/86Sr from 800 to 635 Ma. The subsequent sharp rise in seawater 87Sr/86Sr from 635 to 580 Ma is the result of erosion of radiogenic crust exposed in the Pan-African orogens. Coeval evolution of atmospheric CO2 displays a decrease from about 80 times the pre-industrial level around 800 Ma to 5 times just before the beginning of the Phanerozoic.
How Salty Were the Seas of the Rhyacian PaleoProterozoic?
PIXE and microthermometric analyses of fluid inclusions in hydrothermal quartz from the 2.2 Ga Ongeluk Formation, South Africa: implications for ancient seawater salinity
Authors:
Saito et al
Abstract:
The Ongeluk Formation mainly consists of submarine volcanics (pillow lavas and sheet flows) composed of basaltic andesites that is between underlying and overlying glaciogenic deposits (i.e., the Makganyene diamictite and a dropstone layer at the base of the Hotazel Formation, respectively). The stratigraphic position of the Ongeluk Formation indicates that the Ongeluk volcanism occurred during a period of global glaciation. The Ongeluk volcanic rocks are host to subseafloor hydrothermal quartz deposits as drainage cavities and interpillow voids. The hydrothermal quartz contains many primary (Type 1) and secondary (Type 2) liquid-vapor fluid inclusions, as well as inclusions that are randomly distributed without a trace of secondary healed cracks (Type 3). All these fluid inclusions types were individually analyzed with microthermometry and particle-induced X-ray emission (PIXE) methods. The results show that Type 1 fluid inclusions are highly saline, whereas Type 2 fluid inclusions are relatively less saline. Type 3 fluid inclusions have bimodal peaks of salinity corresponding to those of Type 1 and Type 2 fluid inclusions and thus appear to represent mixtures between latter two inclusion types. Among the various fluid inclusion salinities of fluid inclusions, a wide range in Na/Ca values was identified in the high-salinity fluid inclusions (i.e., Type 1 and a subset of Type 3) which are thought to represent subseafloor fluids circulated by the Ongeluk submarine volcanism. The wide range in Na/Ca values can be explained by a mixing process between Na-rich and Ca-rich fluids associated with albitization of the host basaltic andesites. In the albitization process, Na is removed from the fluids and fixed by the host rocks; by contrast Ca is released from the host rocks into the fluids. PIXE analysis also showed two distinct trends (i.e., vertical and horizontal) on variation diagram of Ca/Cl versus Cu/Cl and positive correlations between Mn/Cl, Cu/Cl, Zn/Cl and Pb/Cl. These data are best explained by the presence of other mixing processes between; 1) a Na-rich, Ca- and Cu-poor fluid, 2) a Ca-rich, Na- and Cu-poor fluid, and 3) a Na- and Cu-rich, Ca-poor fluid. The Cu-, Mn-, Zn- and Pb-rich fluid likely represents a high-temperature hydrothermal fluid from a deep reaction zone in the Ongeluk subseafloor hydrothermal system. By contrast, the (1) Na- (and K-) rich, Ca- and Cu-poor endmember for the high-salinity primary inclusions is considered to represent the composition of 2.2 Ga Ongeluk seawater. We propose the estimation of 2.2 Ga Ongeluk seawater composition is > ∼2,237 mmol/kg of Na, 200–1,000 mmol/kg of K, < 135 mmol/kg of Ca, ∼3,230 mmol/kg of Cl (average values), and ∼400–500 of Cl/Br.
Evidence of Sulfur Oxidizing Bacteria Prior to the Great Oxidation Event
Authors:Czaja et alAbstract:The first 2 b.y. of Earth's history was an important time for life when microbes evolved and diversified into essentially all of the metabolic forms that now exist. Because of feedbacks between biology and the surface environment, understanding Earth's biological history can help us understand the evolution of Earth itself. The morphological and geochemical evidence for this ancient biological history is sparse but is increasing. Here we report evidence for 2.52 Ga exceptionally large, organic, smooth-walled, coccoidal microfossils preserved in a deep-water black chert in the Gamohaan Formation of the Kaapvaal craton of South Africa. These fossils occur mainly as compressed solitary coccoids that range in size from 20 to 265 μm but occasionally occur in short chains of cells. Morphologically these fossils are similar to Proterozoic and Phanerozoic acritarchs and to certain Archean fossils interpreted as possible cyanobacteria. However, their exceptionally large size, simple cell wall microstructure, and paleoecological setting, as well as multiple sulfur isotope systematics of pyrite within the unit, suggest that the Gamohaan Formation fossils were sulfur-oxidizing bacteria similar to those of the modern genus Thiomargarita, organisms that live in anoxic and sulfidic deepwater settings. These are the oldest reported fossil sulfur bacteria and reveal a diversity of life and ecosystems, previously only interpreted from geochemical proxies, just prior to the Great Oxidation Event, a time of major atmospheric evolution.
Methanotrophic Microbes in the Archean Oceans
Authors:Flannery et alAbstract:Highly 13C-depleted organic matter reported from Neoarchean formations worldwide has led to the concept of a “Global Age of Methanotrophy” (GAM) in the Neoarchean. A temporal peak in the GAM is suggested by values as low as −61‰ that are reported from rocks deposited at ∼2.7 Ga. Here we analyse previously reported values, report new field observations and isotope data, and re-evaluate the depositional settings of several units of this age. We find a statistically significant lowering of δ13Corg values in units of Neoarchean age compared to values reported from other Precambrian intervals, both older and younger, confirming the existence of the GAM. However, we also report a correlation between very low δ13Corg values and lacustrine units deposited during the Neoarchean. We hypothesize methanogenesis may have been promoted in some Neoarchean lakes due to local deficiencies of oxidants, specifically Fe3+ and SO4, relative to the Archean oceans. Lower availability of these oxidants could have limited higher energy yield metabolisms such as sulfate and iron reduction and provided an ecological niche for methanogens, ultimately resulting in the local burial of biomass highly depleted in 13C. We conclude that the exceptionally low δ13Corg values reported from formations deposited at ∼2.7 Ga could represent the prevalence of closed basin depositional environments preserved in the limited outcrop available, rather than a peak in the global age of methanotrophy at this time.
Friday, October 14, 2016
Evidence of Euxinia in Terminal Ediacaran Oceans
Authors:Cui et alAbstract:The Ediacaran Period witnessed the first appearance of macroscopic animal life in Earth's history. However, the biogeochemical context for the stratigraphic occurrence of early metazoans is largely uncertain, in part due to the dearth of integrated paleobiological and chemostratigraphic datasets. In this study, a comprehensive geochemical analysis was conducted on the fossiliferous Khatyspyt Formation in Arctic Siberia, in order to gain insights into the Ediacaran paleoenvironments. This study was designed to specifically address the relationship between paleoredox conditions and Ediacaran fossil occurrences in the Khatyspyt Formation. Our data reveal a dramatic shift in pyrite sulfur isotope compositions (δ34Spyrite) from ca. − 20‰ to ca. 55‰, and this shift is intriguingly associated with the first occurrence of Ediacara-type macrofossils at the studied section, suggesting a possible link between seawater redox conditions and distribution of early macroscopic organisms. Based on multiple lines of sedimentological and geochemical evidence, we propose that the development of oceanic euxinia — which may be widespread in the continental margins due to enhanced oxidative weathering in the terminal Ediacaran Period — may have locally prohibited the colonization of Ediacara-type organisms and resulted in low δ34Spyrite values in the lower Khatyspyt Formation. In the middle and upper Khatyspyt Formation, progressive secular transition from euxinic to non-euxinic and more habitable conditions may have allowed for the colonization of Ediacara-type and other macro-organisms.
Ediacaran Soft Bodied Fossils Were Preserved due to Silicia Rich Oceans?
Authors:Tarhan et alAbstract:The Ediacara Biota, Earth's earliest fossilized ecosystem of complex, macroscopic, multicellular organisms, occurs in terminal Ediacaran strata worldwide, yet how the fossils are preserved remains controversial. Ediacara assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and phylogenetic affinity. Many of these fossil assemblages are anactualistically preserved as casts and molds in sandstones ("Ediacara-style" preservation). Here we present evidence from the Ediacara Member of South Australia that Ediacara-style preservation was due to rapid, early-stage precipitation of silica cements, facilitated by the high silica saturation state of the oceans prior to the appearance of prolific silica biomineralizers. An early silicification model provides a coherent, mechanistic and empirically supported explanation for the widespread preservation of soft-bodied organisms of Ediacaran–early Paleozoic age as sandstone casts and molds. The prevalence of early silicification confirms that Ediacara-style fossil assemblages can provide an accurate window into life on the Ediacaran seafloor that can be used to reconstruct critical steps in the development and diversification of early animal ecosystems.
Ediacaran Oceans had a Higher Sulfate Concentration
Authors:Zhou et alAbstract:Authigenic carbonate associated with anaerobic oxidation of methane (AOM), usually via microbial sulfate reduction (MSR) or ferric iron reduction, is generally characterized by extremely low δ13C values (<− 30‰, VPDB). This has been used as one of the major diagnostic features for the recognition of hydrocarbon seep carbonate in the geological past. Previous reports on Precambrian authigenic carbonates are rare, limiting our understanding of the effects of their deposition on the Earth's carbon isotopic mass balance. In this study, mainly based on petrographic features and pronounced negative δ13C values as low as − 38.1‰, we discovered authigenic calcite cement immediately above the cap dolostone in the basal Ediacaran Doushantuo Formation in the Jiulongwan section, Yangtze Gorges area, South China. Our observations not only provide direct evidence for the involvement of AOM during carbonate precipitation in the early Ediacaran (~ 635 Ma), but also suggest that the seawater sulfate concentrations in the early Ediacaran may have been higher than previously thought.
Were There Anoxic Oceans During the Cryogenian NeoProterozoic?
Authors:Hood et alAbstract:Emerging geochemical proxies have improved our understanding of the broad-scale history of Earth's oxygenation. However, paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting. This is particularly an issue with marine carbonates, which although being potentially ideal ocean redox archives, are commonly altered during diagenesis. Here we provide new insight into the robustness of uranium isotopes (238U/235U ratios: δ238U values) as paleoredox tracers by determining texture-specific δ238U values from a well-described Cryogenian (Balcanoona) reef complex in South Australia. We found high variability in δ238U values between different carbonate components, even within a single sample. Petrographically, the best-preserved components from the Balcanoona reef are marine cements, which have a mean δ238U value of –0.23‰, essentially unfractionated from riverine inputs. These values are interpreted as reflecting a marine system with widespread anoxic and iron-rich settings. Less-well-preserved phases have δ238U values spanning almost the entire extent of the documented isotopic range. This integrated petrographic-geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case-by-case basis in future carbonate metal isotope geochemistry.
Friday, July 29, 2016
The Ediacaran Marine Habitats
Viehmann et alAbstract:The Urucum Iron and Manganese Formation (IF and MnF) in the Neoproterozoic Santa Cruz Formation, Brazil, provides unique insights into Neoproterozoic seawater and the habitat for the evolution of the Ediacaran fauna. Pure Urucum IF drill core samples, i.e. chemical sediments that are devoid of any syn- or post-depositional alteration, are reliable and robust archives of Neoproterozoic seawater proxies. They display low concentrations of rather immobile elements and shale-normalized rare earths and yttrium (REYSN) patterns similar to those of modern seawater. Positive LaSN and GdSN anomalies, enrichment of heavy relative to light REYSN, negative CeSN anomalies and super-chondritic Y/Ho ratios indicate a depositional environment dominated by open ocean water masses and oxic atmosphere-hydrosphere conditions. The REYSN patterns of manganese-rich chemical sediments from the Mn1 and Mn2 horizons, however, show significantly different REY distributions between and within these units. While REYSN patterns from the Mn2 unit are similar to those of the IF, the REY systematics of the Mn1 horizon can be subdivided into two different groups. Fe-poor group A shows rather flat REYSN patterns with large negative CeSN anomalies, while Fe-rich group B shows insignificant to positive CeSN anomalies, YbSN/PrSN ratios below unity and sub-chondritic Y/Ho ratios, suggesting that trace element distributions are controlled by the mineralogical composition rather than by sedimentation rate.Pure IF samples, reflecting Urucum seawater, yield εNd0.635Ga values between -4.56 and -4.08, and are more positive than those of the MnF (-5.52 to -4.66) and associated siliciclastic rocks (-8.35 to -7.69), and are considerably more radiogenic than the crystalline Rio Apa Basement (-13.7). While clastic sediments originated from the Amazonia Craton, the dissolved REY budget of Urucum seawater was derived from terrigenous material of the nearby Neoproterozoic Brasília Belt. There is no evidence for any REY input via high-temperature, hydrothermal fluids or for REY input from a mantle source.
Thursday, July 07, 2016
Was the Cryogenian Marinoan Glaciation REALLY a Snowball Earth?
Authors:Prave et alAbstract:The end-Cryogenian glaciation (Marinoan) is portrayed commonly as the archetype of snowball Earth, yet its duration and character remain uncertain. Here we report U-Pb zircon ages for two ash beds from widely separated localities of the Marinoan-equivalent Ghaub Formation in Namibia: 639.29 ± 0.26 Ma and 635.21 ± 0.59 Ma. These findings verify, for the first time, the key prediction of the snowball Earth hypothesis for the Marinoan glaciation, i.e., longevity, with a duration of ≥4 m.y. They also show that the nonglacial interlude of Cryogenian time spanned 20 m.y. or less and that glacigenic erosion and sedimentation, and at least intermittent open-water conditions, occurred 4 m.y. prior to termination of the Marinoan glaciation.
Tuesday, June 28, 2016
Did the Ediacaran NeoProterozoic Ocean Have a Higher Silica Content Than Present?
Insights into cyanobacterial fossilization in Ediacaran siliciclastic environments
Authors:
Newman et al
Abstract:
Ediacaran sedimentary successions are noted for the preservation of microbes and microbial textures on the surfaces of sandstones and siltstones. Although microorganisms have been preserved in coarse-grained siliciclastic sand throughout geologic history, the exceptional preservation of microbes in Ediacaran sediments suggests the potential for a unique taphonomic window. Here, we identify conditions conducive to the fossilization of filamentous cyanobacteria growing in the presence of siliciclastic sand and demonstrate that the sheaths of filamentous cyanobacteria can become coated by clay minerals within days under oxic conditions. Smooth, extensive mineral coatings develop in the presence of 5.6 to 55.6 mg/L of suspended clay and 0.1 mM or greater concentrations of dissolved silica. Thus, elevated concentrations of seawater silica and the delivery of suspended clays promote microbial preservation on sandy and silty surfaces. These factors likely facilitated microbial fossilization in coarse-grained siliciclastic sand throughout the Ediacaran Period and may have also contributed to microbial fossilization in siliciclastic deposits at other times throughout Earth’s history.
Friday, June 24, 2016
Evidence of a Microbial Dominated Ocean During the Late MesoProterozoic
Carbonate rocks and related facies with vestiges of biomarkers: Clues to redox conditions in the Mesoproterozoic ocean
Authors:
Patranabis-Deb et al
Abstract:
The Raipur Group of the Chattisgarh Basin preserves two major Late Mesoproterozoic carbonate platforms. The lower platform is about 490-m thick, separated from the upper platform (~ 670 m thick) by a 500-m thick calcareous shale. Carbonate strata cover almost 40% of the Chattisgarh Basin outcrop and represent two major platform types: a) a non-stromatolitic ramp (the Charmuria/Sarangarh Limestone) and b) a platform developed chiefly in the intertidal to shallow subtidal environment with prolific growth of stromatolites (the Chandi/Saradih Limestone). The first platform consists primarily of the black Timarlaga limestone that is locally replaced by early diagenetic dolomite. This carbonate platform experienced strong storm waves and was subsequently drowned by a major transgression, during which extensive black limestone–marl rhythmite was deposited, followed by deposition of the Gunderdehi Shale. The carbonate factory was later re-established with development of an extensive stromatolite-dominated Charmuria/Sarangarh platform that ranged from restricted embayment to open-marine conditions. Sea-level change played a major role in controlling the broad facies pattern and platform evolution. The δ13C signatures of the Chattisgarh limestones, falling within a relatively narrow range (0 to + 4‰) are typical for Upper Mesoproterozoic carbonate rocks. δ18O values, however, have a greater range (− 5.7 to − 13.3‰) indicating significant diagenetic alteration of some samples. Likely dysoxic or anoxic conditions prevailed during deposition of the black Timarlaga limestone and well-oxygenated conditions during deposition of the Gunderdehi Shale and Saradih/Chandi stromatolite. The lack of 17β,21α (moretanes) and high Tmax values suggest mature organic matter in the non-stromatolitic ramp. A paucity of diagnostic eukaryotic steroids indicates that algae were rare in the Chattisgarh Basin. A high content of hopanes supports a generally bacterially-dominated Proterozoic ocean in which various stromatolites flourished.
Wednesday, June 15, 2016
The Oceans Recovered From the Permian Mass Extinction Quickly
Reptiles rapidly invaded the seas soon after a global extinction wiped out most life on Earth, according to a new study led by University of California, Davis, researchers.
Global climate change -- likely triggered by massive volcanic eruptions -- killed off more than 95 percent of all species about 250 million years ago, at the end of the Permian period. Land reptiles colonized the ocean in just 3.35 million years at the beginning of the Triassic, a speedy recovery in geologic time, the researchers report today (June 13) in the journal Scientific Reports.
"Our results fit with the emerging view that the recovery was faster than previously thought," said study co-author Ryosuke Motani, professor of paleobiology at UC Davis' Department of Earth and Planetary Sciences.
The research was led by Wanlu Fu, now of the Laboratory of Orogenic Belt and Crustal Evolution at Peking University. Fu conducted the research while an in-residence doctoral student working with study co-author Isabel Montañez, professor of geochemistry at UC Davis. Co-authors include scientists from the University of Wisconsin and the University of Milan in Italy. The fossils and rock samples were collected from Majiashan in Chaohu, South China.
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