37 research outputs found
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Neoproterozoic Stratigraphic Comparison of the Lesser Himalaya (India) and Yangtze Block (South China): Paleogeographic Implications
Recent studies of terminal Neoproterozoic rocks (ca. 590ā543 Ma) in the Lesser Himalaya of northwestern India and the Yangtze block (south China) reveal remarkably similar facies assemblages and carbonate platform architecture, with distinctive karstic unconformities at comparable stratigraphic levels. These similarities suggest that south China may have been located close to northwestern India during late Neoproterozoic time, an interpretation permitted by the available, yet sparse paleomagnetic data. Additional parallels in older rocks of both blocksāsimilar rift-related siliciclastic-volcanic successions overlying metamorphic basement, and comparable glaciogenic intervals of possibly Sturtian and Marinoan or Varanger ageāsuggest that this spatial relationship may have developed earlier in the Neoproterozoic. With the exception of basal Cambrian phosphorite and comparable small shelly fossils, stratigraphic contrasts between northern India and south China and increasing biogeographic affinity between south China and northwestern Australia suggest that south China may have migrated toward northwestern Australia during the Cambrian
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Stable Isotope Record of the Terminal Neoproterozoic Krol Platform in the Lesser Himalayas of Northern India
The terminal Neoproterozoic succession in the Lesser Himalaya of India, including the Infra Krol Formation and Krol Group, represent the thickest known accumulation of carbonate strata of this time period, and hence is an ideal target for chemostratigraphic and sequence stratigraphic investigation. High-resolution analyses reveal several negative Ī“13C excursions. Only one of these is related to known Neoproterozoic ice ages, and many vary in amplitude and stratigraphic position in an oblique regional transect of the platform. A combination of stratigraphic and geochemical tests, and comparison with sections elsewhere, leads to an evaluation of the relative contributions of primary and secondary signals. Two excursions are associated with abrupt facies changes associated with shallowing and accumulation of carbonate in organic-rich restricted environments, or with karstification of a subaerially exposed platform; these are currently interpreted to be diagenetic artifacts. Other negative Ī“13C excursions are recorded in open marine transgressive facies, and are interpreted to represent biogeochemical anomalies of global significance. The magnitude of these excursions, however, is complicated by their lateral inconsistency between adjacent sections. Two interpretations are proposed. One possibility is that the observed isotopic inconsistency is due to diagenesis not revealed by the application of existing geochemical criteria. A composite Ī“13C curve constructed under this assumption is broadly consistent with that documented elsewhere, and may reflect modest biogeochemical changes of global scale. An alternative interpretation is that the inconsistency of Ī“13C values relates to stratigraphic hiatus between correlated horizons, incomplete sampling, and/or lack of appropriate carbonates facies. The composite Ī“13C curve constructed under this assumption shows large-magnitude (up to 15ā°) negative Ī“13C anomalies that are taken to imply remarkable perturbations of ocean geochemistry and the episodic input of 13C-depleted alkalinity during terminal Neoproterozoic time
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Carbonate Platform Growth and Cyclicity at a Terminal Proterozoic Passive Margin, Infra Krol Formation and Krol Group, Lesser Himalaya, India
The Infra Krol Formation and overlying Krol Group constitute a thick (less than 2 km), carbonate-rich succession of terminal Proterozoic age that crops out in a series of doubly plunging synclines in the Lesser Himalaya of northern India. The rocks include 18 carbonate and siliciclastic facies, which are grouped into eight facies associations: (1) deep subtidal; (2) shallow subtidal; (3) sand shoal; (4) peritidal carbonate complex; (5) lagoonal; (6) peritidal siliciclasticācarbonate; (7) incised valley fill; and (8) karstic fill. The stromatolite-rich, peritidal complex appears to have occupied a location seaward of a broad lagoon, an arrangement reminiscent of many Phanerozoic and Proterozoic platforms. Growth of this complex was accretionary to progradational, in response to changes in siliciclastic influx from the south-eastern side of the lagoon. Metre-scale cycles tend to be laterally discontinuous, and are interpreted as mainly autogenic. Variations in the number of both sets of cycles and component metre-scale cycles across the platform may result from differential subsidence of the interpreted passive margin. Apparently non-cyclic intervals with shallow-water features may indicate facies migration that was limited compared with the dimensions of facies belts. Correlation of these facies associations in a sequence stratigraphic framework suggests that the Infra Krol Formation and Krol Group represent a north- to north-west-facing platform with a morphology that evolved from a siliciclastic ramp, to carbonate ramp, to peritidal rimmed shelf and, finally, to open shelf. This interpretation differs significantly from the published scheme of a basin centred on the Lesser Himalaya, with virtually the entire Infra KrolāKrol succession representing sedimentation in a persistent tidal-flat environment. This study provides a detailed Neoproterozoic depositional history of northern India from rift basin to passive margin, and predicts that genetically related Neoproterozoic deposits, if they are present in the High Himalaya, are composed mainly of slope/basinal facies characterized by fine-grained siliciclastic and detrital carbonate rocks, lithologically different from those of the Lesser Himalaya
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Carbon Isotope Variability across the Ediacaran Yangtze Platform in South China: Implications for a Large Surface-to-Deep Ocean Ī“13C Gradient
New isotope data obtained from relatively conformable, carbonate-rich strata of the Ediacaran Yangtze platform in South China reveal substantial Ī“13C variability. In platform sections, four negative Ī“13C anomalies with a nadir down to ā¤ā 8ā° (PDB) are present in the interval between the cap carbonate level (ā¼ 635 Ma) and the Precambrian/Cambrian boundary (ā¼ 542 Ma), while in slope and basinal sections, Ī“13C values are negative through the entire Doushantuo Formation (ā¼ 635ā551 Ma). If these Ī“13C values are close to their primary seawater signature, they imply a strong (ā„ 10ā°) surface-to-deep ocean Ī“13C gradient that is consistent with long-term deep ocean anoxia and the presence of a large dissolved organic carbon (DOC) reservoir. The two prominent negative Ī“13C excursions within the Doushantuo Formation above the cap carbonate level are associated with shoaling and local exposure of the platform. The anomalies may thus record remineralization of a large oceanic DOC pool via sulfate reduction that transferred 13C-depleted carbon from the oceanic DOC reservoir to the surface ocean during regression. Inconsistencies in Ediacaran Ī“13C profiles globally and variations in South China in particular highlight the need for further evaluation of local departures in Ī“13C from an inferred average seawater signature
Chemostratigraphic Correlations Across the First Major Trilobite Extinction and Faunal Turnovers Between Laurentia and South China
During Cambrian Stage 4 (~514āMa) the oceans were widely populated with endemic trilobites and three major faunas can be distinguished: olenellids, redlichiids, and paradoxidids. The lowerāmiddle Cambrian boundary in Laurentia was based on the first major trilobite extinction event that is known as the Olenellid Biomere boundary. However, international correlation across this boundary (the Cambrian Series 2āSeries 3 boundary) has been a challenge since the formal proposal of a four-series subdivision of the Cambrian System in 2005. Recently, the base of the international Cambrian Series 3 and of Stage 5 has been named as the base of the Miaolingian Series and Wuliuan Stage. This study provides detailed chemostratigraphy coupled with biostratigraphy and sequence stratigraphy across this critical boundary interval based on eight sections in North America and South China. Our results show robust isotopic evidence associated with major faunal turnovers across the Cambrian Series 2āSeries 3 boundary in both Laurentia and South China. While the olenellid extinction event in Laurentia and the gradual extinction of redlichiids in South China are linked by an abrupt negative carbonate carbon excursion, the first appearance datum of Oryctocephalus indicus is currently the best horizon to achieve correlation between the two regions
Perspectives on Proterozoic surface ocean redox from iodine contents in ancient and recent carbonate
Ā© The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 463 (2017): 159-170, doi:10.1016/j.epsl.2017.01.032.The Proterozoic Eon hosted the emergence and initial recorded diversification of
eukaryotes. Oxygen levels in the shallow marine settings critical to these events were lower than
todayās, although how much lower is debated. Here, we use concentrations of iodate (the oxidized
iodine species) in shallow-marine limestones and dolostones to generate the first comprehensive
record of Proterozoic near-surface marine redox conditions. The iodine proxy is sensitive to both
local oxygen availability and the relative proximity to anoxic waters. To assess the validity of
our approach, Neogene-Quaternary carbonates are used to demonstrate that diagenesis most often
decreases and is unlikely to increase carbonate-iodine contents. Despite the potential for
diagenetic loss, maximum Proterozoic carbonate iodine levels are elevated relative to those of the
Archean, particularly during the Lomagundi and Shuram carbon isotope excursions of the Paleo-
and Neoproterozoic, respectively. For the Shuram anomaly, comparisons to Neogene-Quaternary
carbonates suggest that diagenesis is not responsible for the observed iodine trends. The baseline
low iodine levels in Proterozoic carbonates, relative to the Phanerozoic, are linked to a shallow
oxic-anoxic interface. Oxygen concentrations in surface waters would have at least intermittently
been above the threshold required to support eukaryotes. However, the diagnostically low iodine data from mid-Proterozoic shallow-water carbonates, relative to those of the bracketing time
intervals, are consistent with a dynamic chemocline and anoxic waters that would have
episodically mixed upward and laterally into the shallow oceans. This redox instability may have
challenged early eukaryotic diversification and expansion, creating an evolutionary landscape
unfavorable for the emergence of animals.TL, ZL, and DH thank NSF EAR-1349252. ZL further thanks OCE-1232620. DH, ZL, and TL
acknowledge further funding from a NASA Early Career Collaboration Award. TL, AB, NP, DH,
and AK thank the NASA Astrobiology Institute. TL and NP received support from the Earth-Life
Transitions Program of the NSF. AB acknowledges support from NSF grant EAR-05-45484 and
an NSERC Discovery and Accelerator Grants. CW acknowledges support from NSFC grant
40972021
Subglacial Meltwater Supported Aerobic Marine Habitats During Snowball Earth
The Earthās most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the āSnowball Earthā glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations
Stable Isotopic Evidence for Methane Seeps in Neoproterozoic Postglacial Cap Carbonates
The Earth's most severe glaciations are thought to have occurred about 600 million years ago, in the late Neoproterozoic era. A puzzling feature of glacial deposits from this interval is that they are overlain by 1ā5-m-thick 'cap carbonates' (particulate deep-water marine carbonate rocks) associated with a prominent negative carbon isotope excursion. Cap carbonates have been controversially ascribed to the aftermath of almost complete shutdown of the ocean ecosystems for millions of years during such ice agesāthe 'snowball Earth' hypothesis. Conversely, it has also been suggested that these carbonate rocks were the result of destabilization of methane hydrates during deglaciation and concomitant flooding of continental shelves and interior basins. The most compelling criticism of the latter 'methane hydrate' hypothesis has been the apparent lack of extreme isotopic variation in cap carbonates inferred locally to be associated with methane seeps. Here we report carbon isotopic and petrographic data from a Neoproterozoic postglacial cap carbonate in south China that provide direct evidence for methane-influenced processes during deglaciation. This evidence lends strong support to the hypothesis that methane hydrate destabilization contributed to the enigmatic cap carbonate deposition and strongly negative carbon isotopic anomalies following Neoproterozoic ice ages. This explanation requires less extreme environmental disturbance than that implied by the snowball Earth hypothesis
Uranium and molybdenum isotope evidence for an episode of widespread ocean oxygenation during the late Ediacaran Period
The final publication is available at Elsevier via https://doi.org/10.1016/j.gca.2015.02.025 Ā© 2015. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/To improve estimates of the extent of ocean oxygenation during the late Ediacaran Period, we measured the U and Mo isotope compositions of euxinic (anoxic and sulfidic) organic-rich mudrocks (ORM) of Member IV, upper Doushantuo Formation, South China. The average d238U of most samples is 0.24 Ā± 0.16& (2SD; relative to standard CRM145), which is slightly higher than the average d238U of 0.02 Ā± 0.12& for restricted Black Sea (deep-water Unit I) euxinic sediments and is similar to a modeled d238U value of 0.2& for open ocean euxinic sediments in the modern well-oxygenated oceans. Because 238U is preferentially removed to euxinic sediments compared to 235U, expanded ocean anoxia will deplete seawater of 238U relative to 235U, ultimately leading to deposition of ORM with low d238U. Hence, the high d238U of Member IV ORM points to a common occurrence of extensive ocean oxygenation ca. 560 to 551 Myr ago. The Mo isotope composition of sediments deposited from strongly euxinic bottom waters ([H2S]aq >11 lM) either directly records the global seawater Mo isotope composition (if Mo removal from deep waters is quantitative) or represents a minimum value for seawater (if Mo removal is not quantitative). Near the top of Member IV, d98Mo approaches the modern seawater value of 2.34 Ā± 0.10&. High d98Mo points to widespread ocean oxygenation because the preferential removal of isotopically light Mo to sediments occurs to a greater extent in O2-rich compared to O2-deficient marine environments. However, the d98Mo value for most Member IV ORM is near 0&(relative to standard NIST SRM 3134 = 0.25&), suggesting extensive anoxia. The low d98Mo is at odds with the high Mo concentrations of Member IV ORM, which suggest a large seawater Mo inventory in well-oxygenated oceans, and the high d238U. Hence, we propose that the low d98Mo of most Member IV ORM was fractionated from contemporaneous seawater. Possible mechanisms driving this isotope fractionation include: (1) inadequate dissolved sulfide for quantitative thiomolybdate formation and capture of a seawater-like d98Mo signature in sediments or (2) delivery of isotopically light Mo to sediments via a particulate FeāMn oxyhydroxide shuttle. A compilation of Mo isotope data from euxinic ORM suggests that there were transient episodes of extensive ocean oxygenation that break up intervals of less oxygenated oceans during late Neoproterozoic and early Paleozoic time. Hence, Member IV does not capture irreversible deep ocean oxygenation. Instead, complex ocean redox variations likely marked the transition from O2-deficient Proterozoic oceans to widely oxygenated later Phanerozoic oceans.National Science Foundation
NASA Astrobiology Institute
Agouron Institute
Natural Sciences and Engineering Research Council of Canada Discovery Gran
Paired carbonate-organic carbon and nitrogen isotope variations in Lower Mississippian strata of the southern Great Basin, western United States
Ā© 2017 Elsevier B.V. The Early Mississippian K-O (Kinderhookian-Osagean) carbon isotope (Ī“13C) excursion or TICE (mid-Tournaisian carbon isotope excursion) is one of the most prominent positive Ī“13C excursions of the Phanerozoic. Recent studies raise uncertainties about the representative shape (single vs. double spikes) and magnitude of this Ī“13C excursion (3ā° to ā„ 6ā° in South China; ā„ 5.5ā° in Europe; and ā„ 7ā° in North America) and the 3ā° unidirectional increase in nitrogen isotopes across the Ī“13C excursion, which is unanticipated considering the amount of organic carbon burial required to form the Ī“13C excursion and the resultant oxygen increase and global cooling. To test if stratigraphic completeness and spatial isotope variations caused such uncertainties, we have conducted paired carbonate carbon (Ī“13Ccarb), organic carbon (Ī“13Corg) and nitrogen (Ī“15N) isotope analyses across the K-O interval in two well-exposed sections of the southern Great Basin, western United States. The two sections represent proximal shallow-water and distal deep-water depositional settings of a west-dipping carbonate ramp. In the distal ramp section where no exposure surface is present, both Ī“13Ccarb and Ī“13Corg show double spikes with peak Ī“13Ccarb values up to 7ā° and a negative shift down to 4ā° between the peaks. In the proximal shallower-water section where two karstic disconformities are observed, Ī“13Corg shows similar double spikes but Ī“13Ccarb displays only a single peak with the highest value of 5.5ā°. The missing Ī“13Ccarb spike is likely caused by diagenetic alteration below a karstic disconformity that lowered Ī“13Ccarb but not Ī“13Corg values, resulting in smaller magnitude of the Ī“13Ccarb excursion. These features suggest that the 7ā° magnitude and double spikes are more representative of the K-O Ī“13C excursion in the southern Great Basin. The smaller magnitude of the K-O Ī“13Ccarb excursion in some sections of the Great Basin and the TICE in other sections globally may have overprinted with local environmental/diagenetic signal or resulted from stratigraphic hiatus/truncation, which needs to be clarified in future research. The Ī“15N across the K-O Ī“13C excursion in the distal ramp section is decoupled from Ī“13C, with the majority of Ī“15N values around 4 Ā± 1ā° that do not show any obvious temporal trend. In contrast, Ī“15N values in the shallow-water section is coupled with the K-O Ī“13C excursion, with a 3ā° positive shift from 4ā° to 7ā° at the rising limb of the Ī“13C excursion and a negative shift from 7ā° to 1ā2ā° at the falling limb of the Ī“13C excursion. The Ī“15N trend from the distal ramp section is, in some extent, comparable with that documented from a section in South China, while the coupled Ī“13CāĪ“15N pattern in the proximal section seems better explain the potential redox change across a prominent Ī“13C excursion. Considering the sensitivity of Ī“15N to redox conditions of depositional environments, a more comprehensive Ī“15N study in a broader paleogeographic context is required to better understand the interactions between carbon and nitrogen cycles across the K-O intervalāa critical transition from the mid-Paleozoic greenhouse clime to Late Paleozoic Ice Age (LPIA)