Carbon Isotope Ratios Across Cambrian/Ordovician Boundary

High-resolution chemostratigraphy of the Cambrian-Ordovician GSSP: Enhanced global correlation tool

Authors:

Azmy et al

Abstract:

The Green Point Formation of the Cow Head Group in western Newfoundland (Canada) represents the Global Stratotype Section and Point (GSSP) for the Cambrian–Ordovician boundary on Laurentia. The formation consists of the Martin Point (lower) and the Broom Point (upper) members, which constitute a thick (~ 170 m) deep subtidal to slope marine carbonate sequence. Preservation of the micritic carbonates of the Green Point Formation was evaluated by multiple petrographic and geochemical screening tools. The δ13C and δ18O values of near-primary micrites range from − 4.7 ‰ to + 1.7‰ (VPDB) and from − 8.7 ‰ to − 5.5 ‰ (VPDB), respectively, with no significant correlation (R2 = 0.002). Similarly, the δ13C values show no significant correlation with Mn/Sr ratios or total REE contents, which suggest that the investigated carbonates retain their near-primary δ13C signatures that can be utilized to construct a high-resolution carbon-isotope profile for the GSSP. The δ13Ccarb profile about the GSSP trends generally positive into Bed 22 of the Green Point Formation. The top of the C. intermedius Zone marks the onset of a negative δ13Ccarb shift to about 6.0 ‰ culminating in the upper part of the zone. Subsequently, the δ13Ccarb profile trends positive with a ‘double switch-back’ about the boundary point and then continues on to the highest value in the uppermost part of Bed 23. In addition, the Ce*/Ce values approximately between 0.8 and 1.0, through the GSSP suggest generally dysoxic waters before, during, and immediately after the boundary. The high-resolution chemostratigraphic results, curve, and trends covering the GSSP should facilitate correlation of the Cambrian–Ordovician boundary sections/sequences from other locations.

Ladinian Through Rhaetian Triassic Carbon and Oxygen Isotope Record From the Tethyan Ocean

A Middle–Late Triassic (Ladinian–Rhaetian) Carbon and Oxygen isotope record from the Tethyan Ocean

Authors:

Muttoni et al

Abstract:

We obtained bulk-sediment δ18O and δ13C data from biostratigraphically-constrained Tethyan marine sections at Aghia Marina (Greece), Guri Zi (Albania), Brumano and Italcementi Quarry (Italy), and revise the published chemostratigraphy of the Pizzo Mondello section (Italy). We migrated these records from the depth to the time domain using available chronostratigraphic tie points, generating Ladinian–Rhaetian δ13C and δ18O records spanning from ~ 242 to ~ 201 Ma. The δ18O record seems to be affected by diagenesis, whereas the δ13C record appears to preserve a primary signal and shows values increasing by ~ 1‰ in the Ladinian followed by a ~ 0.6‰ decrease across the Ladinian–Carnian boundary, followed by relatively constant (but oscillatory) Carnian values punctuated by a negative excursion at ~ 233 Ma in the early Carnian, a second negative excursion at ~ 229.5 Ma across the early–late Carnian boundary, and a positive excursion at ~ 227 Ma across the Carnian–Norian boundary. The Norian record is characterized by a long-term decreasing trend and a negative excursion at ~ 216 Ma. Rapid increases and decreases in δ13C have been observed in the Rhaetian, but these may be at least in part due to mixing of different sources of carbonate carbon with different δ13C values. Our Triassic δ13C record has been compared to data from the literature, and a composite δ13C record spanning the last ~ 242 Myr of Earth’s history has been generated. This composite record shows a sequence of dated δ13C trends and events that can be used for stratigraphic correlation as well as for a better understanding of the global carbon cycle in the Mesozoic–Cenozoic.

How Much did the Variscan Relief Influence the Permian climate of Mesoeurope?

Did the Variscan relief influence the Permian climate of Mesoeurope? Insights from geochemical and mineralogical proxies from Sardinia (Italy)

Authors:

Sinisi et al

Abstract:

Red beds deposits of Permian-Triassic basins of Sardinia (western Italy), where a wide segment of the Variscan orogen is exposed, were studied. The basins were selected according to their position within the structural zones of Variscan orogen, i.e. from the external to the inner sectors of the chain. Detailed mineralogical, petrographical, and chemical analyses were performed on shale and calcrete layers laying along the sedimentary sequences, to examine their compositional features and stratigraphic variation. In this regard, enrichment factors (F(e)) for major and trace elements (relative to the PAAS composition) were calculated and discussed. Several geochemical proxies were also calculated and used to assess the palaeoclimate conditions during the red beds deposition. The weathering indices (CIA and CIW) accounted for more humid conditions in the basins of the axial zone relative to the basins those of the foreland, which instead were characterized by an arid climate. However, the presence of carbonates, albeit discontinuous, and the values of calcification, salinity, and hydrolysis indices suggest that, in the axial zone, dry and hot periods also occurred. Compositional data of the sedimentary records were correlated to the Sardinia geological setting and significant information were obtained on the palaeoenvironment characterizing the south-western Mesoeurope during Permian and Triassic. Al2O3-TiO2-Zr ternary plot and Zr/Sc vs. Th/Sc diagram show that both sorting and sedimentary recycling affected the studied basins. Precisely, our data lead us to suppose that a wide regional subsidence caused both the recycling of uplifted Variscan rocks and the diagenetic K-metasomatism of Permian sediments revealed by the A-CN-K diagram. Finally, a supply from upper continental source has been suggested by the provenance proxies (such as (Gd/Yb)N and Eu/Eu*) and the La-Th-Sc plot that also reflect the paucity of mafic rocks in feeding areas.

Evidence of a River Delta From Statherian PaleoProterozoic Australia

Riverine mixing and fluvial iron formation: A new type of Precambrian biochemical sediment

Authors:

Pufahl et al

Abstract:

Precambrian iron formations are biochemical sediments that record ocean chemistry and circulation on the early Earth. The appearance of large, economically important continental margin iron formation reflects the creation of extensive continental shelves and oxygenation of the ocean-atmosphere system near the end of the Archean. Exhalative iron formation contains a record of hydrothermal vent chemistry through time. We introduce here fluvial iron formation, a new type of Fe-rich microbial-biochemical sediment that formed by mixing river discharge and seawater in coastal environments. The Paleoproterozoic Chiall Formation (ca. 1.8 Ga), Earaheedy Basin, Western Australia, contains laminated and granular hematitic iron formation in delta channel deposits. Where mixing occurred in adjacent peritidal settings, laminated iron formation and hematitic oncoids formed. Because fluvial iron formations precipitated at the interface between terrestrial and marine realms, the locus of known Fe precipitation processes is shifted landward into paleoestuarine settings and reflects Fe derived from both terrestrial weathering and coastal upwelling, providing a new window into ocean-atmosphere evolution.

Caution When Interpreting Detrital Zircon Data for Reconstructing Deep Time Supercontinents

The detrital zircon record: Supercontinents, parallel evolution - or coincidence?

Author:

Tom Andersen

Abstract:

The interpretation of detrital zircon data is facing some important and potentially damaging inconsistencies that are commonly overlooked: Combined U-Pb and Lu-Hf data from detrital zircon in sedimentary rocks have been used successfully to study global processes such as the extraction, growth and preservation of continental crust, suggesting generally similar patterns of evolution in different continents. Yet at the same time, such data are increasingly used to identify the source-rocks of individual clastic deposits, based on the implicit assumption that material from different source terranes carry distinct provenance signatures.

For example, Precambrian detrital zircon age-fractions from late Mesoproterozoic to Recent sediments in Laurentia show patterns of U-Pb ages and initial Hf isotope composition indistinguishable from that of zircon from granitoids in Fennoscandia. This is perhaps not too surprising, as Fennoscandia and Laurentia have been near neighbours in three supercontinents (Nuna, Rodinia, Pangea). Transport and homogenization of clastic material in intracontinental basins and along a common continental margin would smooth out differences, and the two neighbouring continents may have closely parallel histories of internal growth.

A stronger reason for concern is that Precambrian age fractions in detrital zircon suites from areas as distant as Australia and South Africa also show distribution patterns that cannot be distinguished with confidence from sources in Fennoscandia. These continents have not been near neighbours in post-Archaean supercontinents. Since exchange of detritus between such distant blocks is improbable, one is left with the alternative explanations of pronounced parallel evolution (perhaps related to the assembly of supercontinents in the geological past), or pure coincidence.

Whatever the explanation: Exchange, parallelism or coincidence, observations such as these challenge the main assumptions underlying the use of zircon as an indicator of sedimentary provenance: That a given age and initial Hf isotopic pattern of a population of detrital zircons can be unequivocally related to a specific first-generation source.

The Orosirian PaleoProterozoic Zaonega Formation in Russia Suggests Caution With Interpreting Paleo Isotope Ratios

Petrography and geochemistry of carbonate rocks of the Zaonega Formation, Russia: Documentation of 13C-depleted non-primary calcite

Authors:

Črne et al

Abstract:

The ca. 2.0 Ga Zaonega Formation in the Onega Basin of NW Russia represents a deep-water, mixed siliciclastic-carbonate depositional system with voluminous mafic volcanism. It is typified by extremely organic-rich rocks (TOC > 40 wt%) and represents one of the earliest known episodes of oil/asphalt generation. These rocks have been inferred to archive one of the largest negative δ13C excursions in Earth history, one that followed and/or partially overlapped with the 2.2–2.06 Ga worldwide Lomagundi-Jatuli carbonate carbon isotopic excursion to high values and thought to be linked to the Paleoproteorozoic oxygenation of Earth's surface environments.

In order to assess the post-depositional integrity of the carbonate carbon isotopic signal (δ13Ccarb) of the Zaonega rocks, we examined in detail the petrography and geochemistry of eight carbonate beds (0.3 to 0.9 m thick) from different stratigraphic levels of the formation. The range of δ13C values for a single bed can be as much as 17 ‰, with calcite being significantly depleted in 13C relative to co-existing dolomite; the 13C-depleted calcite likely formed by involvement of carbon derived from diagenetic and catagenetic alteration of organic matter possibly abetted by volcanic CO2. The presence of calcite ± talc ± phlogopite ± actinolite indicates metamorphic reaction of dolomite with quartz, or possibly K-feldspar, in the presence of water; commonly accompanied by degassing of 13C-enriched CO2, this caused further 13C depletion of newly formed calcite. The least altered dolomite is documented in central parts of thick dolostone beds with variably calcitized margins. This dolomite is considered as the earliest and possibly primary carbonate phase, potentially recording the δ13C signal of the ambient seawater. Among the studied carbonates two stratigraphic intervals contain the least-altered dolomite exhibiting δ13C values of +8 ‰ for the middle part of the formation, and −2 and −4 ‰ for the upper part. All other beds, with δ13C ranging from −19 to +3 ‰, are considered to have been variably depleted in their 13C content by post-depositional processes and therefore cannot be reliably used for assessing the carbon isotope composition of Paleoproterozoic seawater. Our results emphasise the importance of distinguishing primary versus secondary (or later) isotopic compositions in studies of carbonate rocks used for reconstruction of global environmental change.

Interesting Environmental Changes Documented in Namibian Ediacaran NeoProterozoic

Stratigraphy, Palaeontology and Geochemistry of the late Neoproterozoic Aar Member, southwest Namibia; Reflecting environmental controls on Ediacara fossil preservation during the terminal Proterozoic in African Gondwana

Authors:

Hall et al

Abstract:

Common, Ediacaran fossils are well preserved in a Late Neoproterozoic (ca. 545 Ma) shallow marine sequence, described here as the Aar Member of the Dabis Formation (Kuibis Subgroup, Nama Group), near Aus in southwest Namibia. This 31-38 m thick, shale-dominant unit records the transition from fluvial-shallow marine Kliphoek Sandstone to open marine limestone of the Mooifontein Member of the Zaris Formation, deposited on a subsiding continental margin during a major, regional transgression. Thin sandstone beds contain fossils at a number of levels throughout the Aar Member. Concentrations of Pteridinium were mostly transported in flood-derived sheets, while some Ernietta assemblages are preserved close to in-situ. Rangea has also been transported, and is mostly confined to thin sandstone lenses incised into mudstone. Limestone beds, common throughout, include at least two marker horizons that can be followed regionally and show local evidence of storm reworking. Systematic sampling and analyses of limestone reveals enrichment in both 13C and 18O higher in the section, with negative δ13C near the base rising to moderate positive values near the top. The negative-to-positive transition in δ13C values is more pronounced in the east, with all of the lower Aar Member samples consistently depleted in 13C. While this may reflect greater degrees of alteration by meteoric or dewatering fluids, the same carbonates are notably enriched in 18O relative to those at the same stratigraphic position to the west. The overall rise in 13C is attributed to greater proportional burial of organic matter and release of oxygen to surface environments, while the spatial variability is likely the result of a strong surface-to-deep carbon isotopic gradient in seawater. A number of the fossils, especially Rangea, are encrusted with jarosite, an iron-bearing sulphate mineral and common weathering product of pyrite. This observation suggests that preservation of the fossils may have resulted from the rapid encrustation of pyritic on the surface of the organisms as they decomposed and were consumed by sulphate-reducing bacteria within the sandy, near shore sediments. Insofar as pyrite formation requires iron, which is soluble and reactive in anoxic solutions, it is likely that the deeper subtidal environments lacked oxygen. In-situ pyritized forms like Ernietta may have developed the capacity to survive under episodically anoxic or sub-oxic environmental conditions, while Pteridinium and Rangea lived within an oxygenated estuarine or fluvial setting and were transported during storms to anoxic, ferruginous environments where they were exquisitely preserved

Evolution of the Lower Cambrian Oceanic Enviroment in China

Depositional environments for stratiform witherite deposits in the Lower Cambrian black shale sequence of the Yangtze Platform, southern Qinling region, SW China: Evidence from redox-sensitive trace element geochemistry

Authors:

Dao-Hui Pi, Shao-Yong Jiang, Li Luo, Jing-Hong Yang and Hong-Fei Ling

Abstracts:

Very large stratiform witherite mineralization occurs in Lower Cambrian black shales and siliceous rocks in the southern Qinling region of SW China. Two types of witherite ores are divided according to their colours, i.e., the white ores and the dark-gray ores. We analyzed a number of redox-sensitive trace elements in both the white and dark-gray ores from various witherite deposits. The redox-sensitive metal concentrations and the distribution of the two types of ores provide good evidence for their depositional redox conditions in the Early Cambrian ocean in the southern Qinling region. Our geochemical data suggest that both the dark-gray and white witherite ores may have been deposited in reducing environments, with the dark-gray ores deposited in an anoxic-euxinic environment, whereas the white ores deposited under a suboxic-anoxic condition.

Israeli Permian Triassic Boundary Marine Sediments Appear to NOT Have been Deposited in Acidic Ocean

THE PERMIAN–TRIASSIC TRANSITION IN THE CENTRAL COASTAL PLAIN OF ISRAEL (NORTH ARABIAN PLATE MARGIN), DAVID 1 BOREHOLE

Authors:

DORIT KORNGREEN, OLGA ORLOV-LABKOVSKY, OR BIALIK, and CHAIM BENJAMINI

Abstract:

A succession that includes the Permian–Triassic transition was penetrated in the David 1 Borehole (Israel), located on the Levant margin sector of the northwestern Gondwanan plate. This is a fully marine section with a complex depositional interplay of distal siliciclastic and ramp carbonate sediment. Foraminifera distribution, sedimentary microfacies, δ13C excursions and e-log (gamma-ray; resistivity) data were analyzed. The local P-T boundary is placed at the first appearance of Triassic taxa, but there is a 90 m interval of overlapping faunas where the Permian taxa are small species atypical of the underlying Permian strata, and some specimens are clearly reworked. Carbonate strata were deposited in mid- to outer-ramp environments under varying conditions of sea level, storm regime, and siliciclastic (mostly silty) influx. Sea level, tracked by facies shifts, follows a low-order cyclic pattern. Two higher orders of cyclicity are also present: a short-term oscillation in storm frequency in the local depositional basin and a high-order mode in which clastic supply alternates with clastic-free carbonates, interpreted as reflecting humid-arid climate shifts of a remote, continental source region. The last mode lies within the Milankovich band of frequencies. Negative δ13C excursions, glauconite, and pyrite-rich horizons indicate hypoxic or anoxic conditions and reflect perturbations of the global carbon system in the oceans during this time. Although lithification of the latest Permian carbonates in the transitional interval was suppressed, the state of preservation of the carbonate components is not indicative of deposition under conditions of oceanic acidification.

Proposed Mechanism for the Frasnian-Famennian Devonian Mass Extinctions: Global Conflagration

A forest fire and soil erosion event during the Late Devonian mass extinction

Authors:

1. Kunio Kaiho (a)
2. Susumu Yatsu (a)
3. Masahiro Oba (a)
4. Paul Gorjan (b)
5. Jean-Georges Casier (c)
6. Masayuki Ikeda (d)

Affiliations:

a. Institute of Geology and Paleontology, Tohoku University, Sendai 980-8578, Japan

b. Department of Earth and Planetary Science, Washington University, St. Louis MO 63130, USA

c. Department of Paleontology, Royal Belgian Institute of Natural Sciences, rue Vautier, 29, B-1000, Brussels, Belgium

d. Faculty of Science, Ehime University, Matsuyama 790-8577, Japan

Abstract:

The Late Devonian mass extinction occurred in a stepwise manner and culminated close to the Frasnian-Famennian (F-F) boundary (372 million years ago). Organic-molecular indices from marine sedimentary rocks at the Sinsin section, Belgium, indicate that the sequence of combustion of land vegetation, soil erosion, and anoxia-euxinia occurred close to this boundary. The increased concentrations of biomarkers indicating forest fire and soil erosion measured in the Sinsin section suggest that fire became widespread at this time, leading to various damaging consequences (increased runoff and oceanic anoxia) that caused marine extinctions. Magnetic susceptibility data in the Sinsin section indicate a relatively dry climate spanning the F-F boundary, which would have encouraged forest fires. The study of organic biomarkers presents several lines of evidence to link forest fire and soil erosion to the Late Devonian mass extinction.

Comments:

 More samples from other sites.  Make sure this is not local.  Too many cases of projecting a globally what may have been a local event for paleontology.

Check for platinum group metals, please.  You know this is coming anways, get ahead of the curve.

Do some experimental tests with similar plants in the same oxygen levels.  See if you get a conflagration.  Larger size, better.  Build a big terrarium, grow some devonian analog plants (not many around, but...)  Attempt to match the paleoatmospheric content.  Pull the trigger.  Wet, dry and otherwise.  If you built this right, you can do multiple studies of multiple eras.  Past and future. 

Hidden Carbon Reservoir May Exist in Terrestrial Planetary Cores

Carbon substitution for oxygen in silicates in planetary interiors

Authors:

1. Sabyasachi Sena,
2. Scarlett J. Widgeona,b,
3. Alexandra Navrotskya,b,1,
4. Gabriela Merac,
5. Amir Tavakolib,
6. Emanuel Ionescu (c)
7. Ralf Riedel (c)

Affiliations:

a. Department of Chemical Engineering and Materials Science and

b. Peter A. Rock Thermochemistry Laboratory and Nanomaterials in the Environment, Agriculture, and Technology Organized Research Unit, University of California, Davis, CA 95616; and

c. Institut für Materialwissenschaft, Technische Universität Darmstadt, D-64287 Darmstadt, Germany

Abstract:

Amorphous silicon oxycarbide polymer-derived ceramics (PDCs), synthesized from organometallic precursors, contain carbon- and silica-rich nanodomains, the latter with extensive substitution of carbon for oxygen, linking Si-centered SiOxC4-x tetrahedra. Calorimetric studies demonstrated these PDCs to be thermodynamically more stable than a mixture of SiO2, C, and silicon carbide. Here, we show by multinuclear NMR spectroscopy that substitution of C for O is also attained in PDCs with depolymerized silica-rich domains containing lithium, associated with SiOxC4-x tetrahedra with nonbridging oxygen. We suggest that significant (several percent) substitution of C for O could occur in more complex geological silicate melts/glasses in contact with graphite at moderate pressure and high temperature and may be thermodynamically far more accessible than C for Si substitution. Carbon incorporation will change the local structure and may affect physical properties, such as viscosity. Analogous carbon substitution at grain boundaries, at defect sites, or as equilibrium states in nominally acarbonaceous crystalline silicates, even if present at levels at 10–100 ppm, might form an extensive and hitherto hidden reservoir of carbon in the lower crust and mantle.

Evidence of a Large Impact in the Norian Triassic

Osmium isotope evidence for a large Late Triassic impact event

Authors:

1. Honami Sato (a)
2. Tetsuji Onoue (a)
3. Tatsuo Nozaki (b)
4. Katsuhiko Suzuki (b)

Affiliations:

a. Department of Earth and Environmental Sciences, Kagoshima University, 1-21-35 Korimoto, Kagoshima 890-0065, Japan

b. Institute for Research on Earth Evolution (IFREE), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka, Kanagawa 237-0061, Japan

Abstract:

Anomalously high platinum group element concentrations have previously been reported for Upper Triassic deep-sea sediments, which are interpreted to be derived from an extraterrestrial impact event. Here we report the osmium (Os) isotope fingerprint of an extraterrestrial impact from Upper Triassic chert successions in Japan. Os isotope data exhibit a marked negative excursion from an initial Os isotope ratio (187Os/188Osi) of ~0.477 to unradiogenic values of ~0.126 in a platinum group element-enriched claystone layer, indicating the input of meteorite-derived Os into the sediments. The timing of the Os isotope excursion coincides with both elevated Os concentrations and low Re/Os ratios. The magnitude of this negative Os isotope excursion is comparable to those found at Cretaceous–Paleogene boundary sites. These geochemical lines of evidence demonstrate that a large impactor (3.3–7.8 km in diameter) produced a global decrease in seawater 187Os/188Os ratios in the Late Triassic.

Evidence for end-Guadalupian Mass Extinction From Carbon and Sulfur Isotopic Fluctuations

Carbon and sulfur isotopic fluctuations associated with the end-Guadalupian mass extinction in South China

Authors:

1. Yan Detian (a, b)
2. Zhang Liqin (c)
3. Qiu Zhen (b)

Affiliations:

a. Key Laboratory of Tectonics and Petroleum Resources of Ministry of Education, China University of Geosciences, Wuhan 430074, China

b. Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

c. Key Laboratory of Geospace Environment and Geodesy of Ministry of Education, Wuhan University, Wuhan 430079, China

Abstract:

Concentrations of total organic matter (TOC), carbon isotopic compositions of carbonate and organic matter (δ13Ccarb, δ13Corg), and sulfur isotopic compositions of carbonate associated sulfate (δ34Ssulfate) across the Guadalupian–Lopingian (G–L) boundary were analyzed from identical samples of Tieqiao section, Laibin, Guangxi province, South China. The δ13Ccarb values show a positive excursion from −0.45‰ to the peak of 3.80‰ in the Laibin limestone member of the Maokou Formation, followed by a drastic drop to −2.60‰ in the lowest Heshan formation, then returned to about 1.58‰. Similar to the trends of the δ13Ccarb values, Δ13Ccarb–org values also show a positive excursion followed by a sharp negative shift. The onset of a major negative carbon isotope excursion postdates the end Guadalupian extinction that indicates subsequent severe disturbance of the ocean–atmosphere carbon cycle. The first biostratigraphic δ34Ssulfate values during the G–L transition exhibit a remarkable fluctuation: a dramatic negative shift followed by a rapid positive shift, ranging from 36.88‰ to −37.41‰. These sulfate isotopic records suggest that the ocean during the G–L transition was strongly stratified, forming an unstable chemocline separating oxic shallow water from anoxic/euxinic deep water. Chemocline excursions, together with subsequent rapid transgression and oceanic anoxia, were likely responsible for the massive diversity decline of the G–L biotic crisis.

Interpreting Oxygen-18 Records from the Paleocene-Eocene Thermal Maximum Can be Complicated

In situ δ18O and Mg/Ca analyses of diagenetic and planktic foraminiferal calcite preserved in a deep-sea record of the paleocene-eocene thermal maximum

Authors:

1. Reinhard Kozdon (a)

2. D. C. Kelly (a)

3. K. Kitajima (a)

4. A. Strickland (a)

5. J. H. Fournelle (a)

6. J. W. Valley (a)

Affiliation:

a. WiscSIMS, Dept. of Geoscience, University of Wisconsin, Madison, 1215 W. Dayton St., Madison,

WI 53706, USA

Abstract:

We report δ18O and minor-element (Mg/Ca, Sr/Ca) data acquired by high-resolution, in situ secondary ion mass spectrometry (SIMS) from planktic foraminiferal shells and 100–500 µm sized diagenetic crystallites recovered from a deep-sea record (ODP Site 865) of the Paleocene-Eocene thermal maximum (PETM). The δ18O of crystallites (~1.2‰ PDB) is ~4.8‰ higher than that of planktic foraminiferal calcite (−3.6‰ PDB), while crystallite Mg/Ca and Sr/Ca ratios are slightly higher and substantially lower than in planktic foraminiferal calcite, respectively. The focused stratigraphic distribution of the crystallites signals an association with PETM conditions; hence, we attribute their formation to early diagenesis initially sourced by seafloor dissolution (burndown) ensued by reprecipitation at higher carbonate saturation. The Mg/Ca ratios of the crystallites are an order of magnitude lower than those predicted by inorganic precipitation experiments, which may reflect a degree of inheritance from ‘donor’ phases of biogenic calcite that underwent solution in the sediment column. In addition, SIMS δ18O and electron microprobe Mg/Ca analyses taken within a planktic foraminiferal shell yield parallel increases along traverses that coincide with muricae-blades on the chamber wall. The parallel δ18O and Mg/Ca increases indicate a diagenetic origin for the blades, but their δ18O value (−0.5‰ PDB) is lower than that of crystallites suggesting that these two phases of diagenetic carbonate formed at different times. Finally, we posit that elevated levels of early diagenesis acted in concert with sediment mixing and carbonate dissolution to attenuate the δ18O decrease signaling PETM warming in ‘whole-shell’ records published for Site 865.

Tonian/Cryogenian Age for Original Depositional Age of Metacarbonate Rocks in Sør Rondane Mountains, East Antarctica

Late-Tonian to early-Cryogenian apparent depositional ages for metacarbonate rocks from the Sør Rondane Mountains, East Antarctica

Authors:

1. Naho Otsuji (a)
2. M. Satish-Kumar (a, b)
3. Atsushi Kamei (c)
4. Noriyoshi Tsuchiya (d)
5. Tetsuo Kawakami (e)
6. Masahiro Ishikawa (f)
7. Geoffrey H. Grantham (g)

Affiliations:

a. Graduate School of Science and Technology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan

b. Department of Geology, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181, Japan

c. Department of Geosciences, Shimane University, Matsue 690-8504, Japan

d. Graduate School of Environmental Studies, Tohoku University, Aramaki, Aoba, Sendai 980-9579, Japan

e. Department of Geology and Mineralogy, Kyoto University, Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan

f. Graduate School of Environment and Information Sciences, Yokahama National University, Tokiwadai, Hodogaya-ku, Yokohama 248-501, Japan

g. Council for Geoscience, P/Bag X112, Pretoria, South Africa

Abstract:

The Sør Rondane Mountains (SRM), located in the Neoproterozoic to Early Cambrian East African-Antarctic collisional orogen is composed of medium- to high-grade metasedimentary, metaigneous and intrusive rocks of diverse composition. Within the metasedimentary rocks, the metacarbonate rocks are considered to have deposited chemically in the so-called the “Mozambique Ocean” that separated the continental blocks that amalgamated to form Gondwana and possibly record geochemical signatures of contemporaneous seawater. Here we attempt to constrain the apparent age of sedimentation of metasedimentary sequences using strontium isotope chemostratigraphy of the least altered metacarbonate rocks. Pure metacarbonate samples, collected during the 51st Japanese Antarctic Research Expedition from different regions throughout the SRM, were selected based on careful screening using multiple geochemical parameters, such as carbon and oxygen isotopic composition and trace and rare earth element contents. We could successfully test the extent of alteration for the Sør Rondane metacarbonate samples, using Mn/Sr ratios, which show a positive correlation with 87Sr/86Sr ratios. After a rigorous geochemical screening in terms of post-depositional alterations, 18 samples were identified as least altered. These samples collectively gave regional initial 87Sr/86Sr ratios between 0.70566 and 0.70630, from Balchen, Brattnipene, Menipa and Tanngarden regions of the SRM, with the exception of Perlebandet region. These Sr isotopic ratios reflect seawater compositions of late-Tonian and early-Cryogenian age (880–850 Ma and 820–790 Ma), when compared with the evolution of Sr isotopes in the Neoproterozoic Oceans. Furthermore, these estimates are consistent with the carbon isotope chemostratigraphic curves of Neoproterozoic. The estimated apparent depositional ages of carbonate rocks in the SRM are also conformable with the reported detrital and metamorphic ages for this region. Carbonate rocks in the Perlebandet region shows low initial Sr isotope ratio (0.70482), suggesting that these rocks may have deposited earlier than other carbonate rocks in the SRM. Our results can be correlated with the chemostratigraphic depositional ages reported for carbonates from the Montepuetz Complex, Mozambique, suggesting the presence of contemporaneous platform environment on both sides of the possible suture. The finding of late-Tonian and early-Cryogenian carbonate deposition, potentially points toward a platform environment surrounding the tonalitic continental arc in the SW region of the SRM, prior to the amalgamation of Gondwana. The results obtained need to be tested with similar studies on metacarbonate sequences from the surrounding regions, which would help to resolve the processes and sequence of collision events that finally amalgamated Gondwana.

Was There a Nitrogen Nutrient Limitation in the MesoProterozoic for Eukaryotes?

A test of the nitrogen-limitation hypothesis for retarded eukaryote radiation: Nitrogen isotopes across a Mesoproterozoic basinal profile

Author:

1. Eva E. Stüeken (a)

Affiliation:

a. University of Washington, Department of Earth & Space Sciences and Astrobiology Program, Box 351310, Seattle, WA 98195, USA

Abstract:

Nitrogen limitation caused by trace metal scarcity under euxinic ocean conditions has been proposed as an explanation for the delayed radiation of eukaryotes until at least the late Mesoproterozoic. However, evidence for how the nitrogen cycle was operating during the middle Precambrian is, so far, rare. More specifically, it is unknown which steps in the biogeochemical nitrogen cycle, e.g. nitrogen fixation, nitrification or ammonification, were rate-limiting and thus controlling microbial community structures. The Mesoproterozoic Belt Supergroup in western Montana hosts a variety of facies ranging from shallow to deep water and thus offers the opportunity to address this issue. Bulk δ15N values show a clear trend from −1‰ in the deepest part of the basin to +5‰ along basin margins, which suggests that coastal areas were sufficiently oxygenated for aerobic nitrogen cycling. The total fractionation of carbon isotopes between carbonate and organic carbon (Δ13C) increases from 20‰ to 32‰ in the same direction, possibly indicating an ecological response to redox stratification and nitrogen speciation. Evidence from the Belt Supergroup is thus consistent with the idea that nutrient availability may have restricted early eukaryotic organisms to a narrow range of habitats, which thus prevented a global rise to ecological dominance until concentrations of fixed nitrogen increased in the global open ocean.

Marine Osmium Source Transitioned From Mantle in Archean to Continents in the Proterozoic

Constraining the depositional history of the Neoproterozoic Shaler Supergroup, Amundsen Basin, NW Canada: Rhenium-osmium dating of black shales from the Wynniatt and Boot Inlet Formations

Authors:

1. David van Acken (a)
2. Danielle Thomson (b)
3. Robert H. Rainbird (c)
4. Robert A. Creaser (a)

Affiliations:

a. University of Alberta, Dept. of Earth and Atmospheric Sciences, 1-26 ESB, Edmonton, AB, Canada T6G 2E3

b. Carleton University, Dept. of Earth Science, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6

c. Geological Survey of Canada, 601 Booth Street, Ottawa, ON, Canada K1A 0E8

Abstract:

New Re-Os ages from black shales of the Neoproterozoic Shaler Supergroup from the Minto Inlier, Victoria Island, Arctic Canada, constrain the late Tonian to early Cryogenian depositional history of the Amundsen Basin. The Re-Os ages of 761 ± 41 Ma and 848 ± 49 Ma for the Wynniatt Fm., and 892 ± 13 Ma for the Boot Inlet Fm., along with recently reported U-Pb ages, provide anchor points for correlation of stable isotope stratigraphy, most notably the Bitter Springs isotopic anomaly within the Wynniatt Fm. across the Tonian-Cryogenian transition in NW Canada. These data suggest that deposition of lower units of the Shaler Supergroup began earlier than previously estimated. Initial 187Os/188Os for all three sample suites is around 0.6, lower than modern seawater, but significantly higher than unradiogenic Archean/Paleoproterozoic seawater composition, supporting a Proterozoic transition from mantle-dominated Os input into the global ocean towards a crust-dominated flux.

Is Mercury a Signal for Mass Vulcanism Across the KT/K=Pg Boundary?

Mercury as a proxy for volcanic activity during extreme environmental turnover: The Cretaceous-Paleogene transition

Authors:

1. A.N. Sial (a)
2. L.D. Lacerda (b)
3. V.P. Ferreira (a)
4. R. Frei (c)
5. R.A. Marquillas (d)
6. J.A. Barbosa (e)
7. C. Gaucher (f)
8, C.C. Windmöller (g)
9, N.S. Pereira (a)

Affiliations:

a. NEG-LABISE, Department of Geology, Federal University of Pernambuco, Recife, PE, 50740-530, Brazil

b. LABOMAR, Institute of Marine Sciences, Federal University of Ceará, Fortaleza, 60165-081, Brazil

c. Institute of Geography and Geology, University of Copenhagen, Oster Volgade 10, Copenhagen, Denmark, 1350

d. Universidad de Salta, Salta, Argentina CONICET, Buenos Aires 177, 4400 Salta, Argentina

e. LAGESE, Department Geology, Federal University of Pernambuco, Recife, 50740-530, Brazil

f. Facultad de Ciencias, Universidad de La Republica, Montevideo, Uruguay

g. Department of Chemistry, Federal University of Minas Gerais, Av. Antônio Carlos 6627, Belo Horizonte, Minas Gerais, 31270-901, Brazil

Abstract:

The usually low geological background concentrations of Hg makes this trace element suitable for identifying accumulation pulses in sediments that can be tentatively related to weathering processes and thus to climatic changes. Intense volcanism has witnessed the Cretaceous‒Paleogene transition (KTB) and was, perhaps, responsible for dramatic climatic changes and decrease in biodiversity and mass extinction. We have used Hg concentrations as a proxy for volcanic activity and atmospheric Hg and CO2 buildup across the KTB at three localities. In the Salta Basin, Argentina, Hg contents display several spikes across the KTB, with a maximum value of 250 ng.g− 1. In three drill cores across the KTB in the Paraíba Basin, northeastern Brazil, Hg contents increase from the late Maastrichtian to early Danian and Hg spikes predate the KTB, perhaps, as a record of volcanic activity before (but very close to) this transition. At Stevns Klint, Denmark, Hg contents reached almost 250 ng.g− 1 within a 5 cm thick-clay layer, the Fiskeler Member (‘Fish Clay’) that comprises the KTB. Some co-variation between Hg and Al2O3 contents has been observed in all of the studied sections across the KTB, suggesting that Hg is probably adsorbed onto clays. Thermo-desorption experiments in selected samples from the Yacoraite Formation showed Hg+ 2 as the major species present, which is in agreement with a volcanic origin. Combined Hg and C-isotope chemostratigraphy may become a powerful tool for the eventual assessment of the role of volcanic activity during extreme climatic and biotic events, such as those during the KTB.

Permian Triassic Impact Araguainha Impact Did Not Cause the Mass Extinction, But Left Its Mark

Shaking a methane fizz: Seismicity from the Araguainha impact event and the Permian–Triassic global carbon isotope record

Authors:

1. E. Tohver (a)

2. P.A. Cawood (a, b)

3. C. Riccomini (c)

4. C. Lana (d)

5. R.I.F. Trindade (c)

Affiliations:

a. School of Earth & Environment, University of Western Australia, Australia

b. St. Andrews University, Scotland, United Kingdom

c. Universidade de São Paulo, Brazil

d. Universidade Federal de Ouro Preto, Brazil

Abstract:

The Late Permian and Early Triassic periods are marked by large fluctuations in the carbon isotope record, but the source(s) of the disturbance to the global carbon cycle and the link to the end–Permian mass extinction are widely debated. This contribution explores the possible isotopic effects of an impact event into the hydrocarbon-rich rocks of the Paraná-Karoo basin. Recent U–Pb and 40Ar/39Ar dating of the 40 km Araguainha impact structure of central Brazil reveals an age of 254.7±2.5 Ma (2σ error) for this event. The calculated energy (105–106 MT of TNT equivalent) released by this impact is less than threshold values of 107–108 MT TNT equivalent for global mass extinctions. Thus, the Araguainha crater is unlikely to have been the cause of the end–Permian biotic crisis. However, the combined seismic effects from the impact itself and the post-impact collapse of the 20–25 km diameter transient crater to its present 40 km diameter would result in large magnitude earthquakes (Mw 9.3–10.5) and tsunamis in the shallow marine Paraná-Karoo Basin. Slope failure and sediment liquefaction are predicted to have occurred within a 700–3000 km radius of the crater, causing large-scale release of methane from organic-rich sediments of this basin, including the oil shale horizons of the Iratí Formation. New geological evidence for seismicity in the Paraná Basin at the time of impact is presented, together with a compilation of existing carbon isotope data from the Paranáa Basin, which demonstrate a widespread pattern of disturbance consistent with the release of methane. These two datasets suggest that both seismicity and methane release took place within ca.1000 km of the impact site, with mass balance calculations suggesting ca. 1600 gigatons of methane were released into the atmosphere at this time. Methane release at this scale would have significant climate effects and would contribute to a sharp (< 1 ka) negative shift in δ13C values at the time of the impact, which should be distinguishable from the more gradual shift over 0.5 – 1 Ma caused by contemporaneous intrusion of the Siberian traps.

Sea Water Chemistry Driven by Supercontinent Cycle

Seawater chemistry driven by supercontinent assembly, breakup, and dispersal

Authors:

1. R.D. Müller (a)
2. A. Dutkiewicz (a)
3. M. Seton (a)
4. C. Gaina (b)

Affiliations:

a. EarthByte Group, School of Geosciences, University of Sydney, Sydney, NSW 2006, Australia

b. Centre for Earth Evolution and Dynamics, CoE, University of Oslo, Oslo 0316, Norway

Abstract:

Global oceans are known to have alternated between aragonite and calcite seas. These oscillations reflect changes in the Mg/Ca ratios of seawater that control biomineralization and the composition of marine carbonates, and are thought to be caused mainly by the time dependence of crustal accretion at mid-ocean ridge crests and the associated high-temperature mid-ocean ridge fluid flux. Here we use global ocean basin reconstructions to demonstrate that the fluctuations in hydrothermal ocean inputs are instead caused by the gradual growth and destruction of mid-ocean ridges and their relatively cool flanks during long-term tectonic cycles, thus linking ocean chemistry to off-ridge low-temperature hydrothermal exchange. Early Jurassic aragonite seas were a consequence of supercontinent stability and a minimum in mid-ocean ridge length and global basalt alteration. The breakup of Pangea resulted in a gradual doubling in ridge length and a 50% increase in the ridge flank area, leading to an enhanced volume of basalt to be altered. The associated increase in the total global hydrothermal fluid flux by as much as 65%, peaking at 120 Ma, led to lowered seawater Mg/Ca ratios and marine hypercalcification from 140 to 35 Ma. A return to aragonite seas with preferential aragonite and high-Mg calcite precipitation was driven by pronounced continental dispersal, leading to progressive subduction of ridges and their flanks along the Pacific rim.