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Paul F. Hoffman - One of the best experts on this subject based on the ideXlab platform.
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An early diagenetic deglacial origin for basal Ediacaran “cap Dolostones”
Earth and Planetary Science Letters, 2019Co-Authors: Anne-sofie C. Ahm, Adam C. Maloof, Francis A. Macdonald, Paul F. Hoffman, Christian J. Bjerrum, Uyanga Bold, Catherine V. Rose, Justin V. Strauss, John A. HigginsAbstract:Abstract The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous Dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap Dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability within cap Dolostones, both between continents, across single continental margins, and within individual stratigraphic sections has been difficult to reconcile with depositional models. Using a compilation of new calcium and magnesium isotope measurements in Marinoan cap Dolostone successions worldwide, we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid- and sediment-buffered conditions. Dolostones from the outer platform formed under fluid-buffered conditions, whereas Dolostones on the inner platform and foreslope environment formed under sediment-buffered conditions. This spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea-level rise and development of a meltwater surface ocean. Using a numerical diagenetic model to evaluate the geochemical differences between sediment- and fluid-buffered cap Dolostone units, we constrain the chemical and isotopic composition of both the dolomitizing fluid (glacial seawater [ δ 13 C ∼ 0–2‰]), the meltwater lens ( δ 13 C ∼−11‰), and the primary aragonite sediment ( δ 13 C ∼−6 to −3‰). These model end-members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal Ediacaran cap Dolostones. Our results provide a novel framework for understanding the geochemical variability of cap Dolostone units, including large excursions in carbon isotopes, and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last Snowball Earth.
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chapter 5 chemical sediments associated with neoproterozoic glaciation iron formation cap carbonate barite and phosphorite
Geological Society London Memoirs, 2011Co-Authors: Francis A. Macdonald, Paul F. Hoffman, Galen P. HalversonAbstract:Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were preci- pitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern sea- water, with a weaker hydrothermal signature than Archaean- Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap Dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap Dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the melt- down of ice-sheets was rapid, cap Dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap Dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap Dolostones, either as digitate seafloor cement associ- ated with Fe-dolomite at the top of the cap Dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like brec- cias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap Dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phos- phorite formed during a Neoproterozoic deglaciation.
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Are basal Ediacaran (635 Ma) post-glacial ?cap Dolostones? diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:A layer of shallow-water Dolostone (?cap Dolostone?) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ? 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13C by 2 3? compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal ?13C curve over time with a net decline of 4.4?. In addition, a lateral gradient of 1.0?/100 km existed from the inner to outer bank. If the flooding was rapid (< 10 kyr), as suggested by ice-melting models, the ?13C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the ?13C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ? 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
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Are basal Ediacaran (635 Ma) post-glacial “cap Dolostones” diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:Abstract A layer of shallow-water Dolostone (“cap Dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼ 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13 C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal δ 13 C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank. If the flooding was rapid ( δ 13 C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the δ 13 C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO 2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼ 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
Galen P. Halverson - One of the best experts on this subject based on the ideXlab platform.
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chapter 5 chemical sediments associated with neoproterozoic glaciation iron formation cap carbonate barite and phosphorite
Geological Society London Memoirs, 2011Co-Authors: Francis A. Macdonald, Paul F. Hoffman, Galen P. HalversonAbstract:Orthochemical sediments associated with Neoproterozoic glaciation have prominence beyond their volumetric proportions because of the insights they provide on the nature of glaciation and the records they hold of the environment in which they were preci- pitated. Synglacial Fe formations are mineralogically simple (haematite jaspilite), and their trace element spectra resemble modern sea- water, with a weaker hydrothermal signature than Archaean- Palaeoproterozoic Fe formations. Lithofacies associations implicate subglacial meltwater plumes as the agents of Fe(II) oxidation, and temporal oscillations in the plume flux as the cause of alternating Fe- and Mn-oxide deposits. Most if not all Neoproterozoic examples belong to the older Cryogenian (Sturtian) glaciation. Older and younger Cryogenian (Marinoan) cap carbonates are distinct. Only the younger have well-developed transgressive cap Dolostones, which were laid down during the rise in global mean sea level resulting from ice-sheet meltdown. Marinoan cap Dolostones have a suite of unusual sedimentary structures, indicating abnormal palaeoenvironmental conditions during their deposition. Assuming the melt- down of ice-sheets was rapid, cap Dolostones were deposited from surface waters dominated by buoyant glacial meltwater, within and beneath which microbial activity probably catalysed dolomite nucleation. Former aragonite seafloor cement (crystal fans) found in deeper water limestone above Marinoan cap Dolostones indicates carbonate oversaturation at depth, implying extreme concentrations of dissolved inorganic carbon. Barite is associated with a number of Marinoan cap Dolostones, either as digitate seafloor cement associ- ated with Fe-dolomite at the top of the cap Dolostone, or as early diagenetic void-filling cement associated with tepee or tepee-like brec- cias. Seafloor barite marks a redoxcline in the water column across which euxinic Ba-rich waters upwelled, causing simultaneous barite titration and Fe(III) reduction. Phosphatic stromatolites, shrub-like structures and coated grains are associated with a glacioisostatically induced exposure surface on a cap Dolostone in the NE of the West African craton, but this appears to be a singular occurrence of phos- phorite formed during a Neoproterozoic deglaciation.
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Are basal Ediacaran (635 Ma) post-glacial ?cap Dolostones? diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:A layer of shallow-water Dolostone (?cap Dolostone?) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ? 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13C by 2 3? compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal ?13C curve over time with a net decline of 4.4?. In addition, a lateral gradient of 1.0?/100 km existed from the inner to outer bank. If the flooding was rapid (< 10 kyr), as suggested by ice-melting models, the ?13C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the ?13C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ? 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
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Are basal Ediacaran (635 Ma) post-glacial “cap Dolostones” diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:Abstract A layer of shallow-water Dolostone (“cap Dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼ 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13 C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal δ 13 C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank. If the flooding was rapid ( δ 13 C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the δ 13 C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO 2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼ 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
John A. Higgins - One of the best experts on this subject based on the ideXlab platform.
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An early diagenetic deglacial origin for basal Ediacaran “cap Dolostones”
Earth and Planetary Science Letters, 2019Co-Authors: Anne-sofie C. Ahm, Adam C. Maloof, Francis A. Macdonald, Paul F. Hoffman, Christian J. Bjerrum, Uyanga Bold, Catherine V. Rose, Justin V. Strauss, John A. HigginsAbstract:Abstract The beginning of the Ediacaran Period (∼635 Ma) is marked by conspicuous Dolostone units that cap Marinoan glacial deposits worldwide. The extent and sedimentary characteristics of the cap Dolostones indicate that anomalous carbonate over-saturation coincided with deglacial sea-level rise and ocean warming. However, the geochemical variability within cap Dolostones, both between continents, across single continental margins, and within individual stratigraphic sections has been difficult to reconcile with depositional models. Using a compilation of new calcium and magnesium isotope measurements in Marinoan cap Dolostone successions worldwide, we show that the geochemical variability can be explained by early diagenetic dolomitization of aragonite along a spectrum of fluid- and sediment-buffered conditions. Dolostones from the outer platform formed under fluid-buffered conditions, whereas Dolostones on the inner platform and foreslope environment formed under sediment-buffered conditions. This spatial pattern of dolomitizing conditions is consistent with buoyant recirculation of glacial seawater within carbonate platforms driven by the deglacial sea-level rise and development of a meltwater surface ocean. Using a numerical diagenetic model to evaluate the geochemical differences between sediment- and fluid-buffered cap Dolostone units, we constrain the chemical and isotopic composition of both the dolomitizing fluid (glacial seawater [ δ 13 C ∼ 0–2‰]), the meltwater lens ( δ 13 C ∼−11‰), and the primary aragonite sediment ( δ 13 C ∼−6 to −3‰). These model end-members do not imply that primary geochemical variability did not exist but demonstrates that it is not necessary to change the chemistry of seawater to explain the global stratigraphic variability in the geochemistry of basal Ediacaran cap Dolostones. Our results provide a novel framework for understanding the geochemical variability of cap Dolostone units, including large excursions in carbon isotopes, and how this variability is the product of local diagenetic processes expressed globally in continental margin environments following the last Snowball Earth.
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The effects of diagenesis and dolomitization on Ca and Mg isotopes in marine platform carbonates: Implications for the geochemical cycles of Ca and Mg
Geochimica et Cosmochimica Acta, 2014Co-Authors: Matthew S. Fantle, John A. HigginsAbstract:Abstract The Ca, Mg, O, and C isotopic and trace elemental compositions of marine limestones and Dolostones from ODP Site 1196A, which range in depth (∼58 to 627 mbsf) and in depositional age (∼5 and 23 Ma), are presented. The objectives of the study are to explore the potential for non-traditional isotope systems to fingerprint diagenesis, to quantify the extent to which geochemical proxies are altered during diagenesis, and to investigate the importance of diagenesis within the global Ca and Mg geochemical cycles. The data suggest that Ca, which has a relatively high solid to fluid mass ratio, can be isotopically altered during diagenesis. In addition, the alteration of Ca correlates with the alteration of Mg in such a way that both can serve as useful tools for deciphering diagenesis in ancient rocks. Bulk carbonate δ 44 Ca values vary between 0.60 and 1.31‰ (SRM-915a scale); the average limestone δ 44 Ca is 0.97 ± 0.24‰ (1SD), identical within error to the average Dolostone (1.03 ± 0.15 1SD ‰). Magnesium isotopic compositions (δ 26 Mg, DSM-3 scale) range between −2.59‰ and −3.91‰, and limestones (−3.60 ± 0.25‰) and Dolostones (−2.68 ± 0.07‰) are isotopically distinct. Carbon isotopic compositions (δ 13 C, PDB scale) vary between 0.86‰ and 2.47‰, with average limestone (1.96 ± 0.31‰) marginally offset relative to average Dolostone (1.68 ± 0.57‰). The oxygen isotopic compositions (δ 18 O, PDB scale) of limestones (−1.22 ± 0.94‰) are substantially lower than the Dolostones measured (2.72 ± 1.07‰). The isotopic data from 1196A suggest distinct and coherent trends in isotopic and elemental compositions that are interpreted in terms of diagenetic trajectories. Numerical modeling supports the contention that such trends can be interpreted as diagenetic, and suggests that the appropriate distribution coefficient ( K Mg ) associated with limestone diagenesis is ∼1 to 5 × 10 −3 , distinctly lower than those values (>0.015) reported in laboratory studies. With respect to Mg isotopes, the modeling also suggest that diagenetic fractionation factors of ∼0.9955 (−4.5‰) and 0.9980 (−2‰) are appropriate for limestone diagenesis and dolomitization, respectively.
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Are basal Ediacaran (635 Ma) post-glacial ?cap Dolostones? diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:A layer of shallow-water Dolostone (?cap Dolostone?) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ? 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13C by 2 3? compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal ?13C curve over time with a net decline of 4.4?. In addition, a lateral gradient of 1.0?/100 km existed from the inner to outer bank. If the flooding was rapid (< 10 kyr), as suggested by ice-melting models, the ?13C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the ?13C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ? 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
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Are basal Ediacaran (635 Ma) post-glacial “cap Dolostones” diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:Abstract A layer of shallow-water Dolostone (“cap Dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼ 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13 C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal δ 13 C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank. If the flooding was rapid ( δ 13 C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the δ 13 C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO 2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼ 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
Daniel P. Schrag - One of the best experts on this subject based on the ideXlab platform.
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Are basal Ediacaran (635 Ma) post-glacial ?cap Dolostones? diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:A layer of shallow-water Dolostone (?cap Dolostone?) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ? 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13C by 2 3? compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal ?13C curve over time with a net decline of 4.4?. In addition, a lateral gradient of 1.0?/100 km existed from the inner to outer bank. If the flooding was rapid (< 10 kyr), as suggested by ice-melting models, the ?13C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the ?13C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ? 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
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Are basal Ediacaran (635 Ma) post-glacial “cap Dolostones” diachronous?
Earth and Planetary Science Letters, 2007Co-Authors: Paul F. Hoffman, John A. Higgins, Galen P. Halverson, Eugene W. Domack, Jonathan M. Husson, Daniel P. SchragAbstract:Abstract A layer of shallow-water Dolostone (“cap Dolostone”) with idiosyncratic sedimentary structures was deposited across continental margins world-wide in the aftermath of the terminal Cryogenian snowball Earth. The Dolostone has a global average thickness of 18.5 m and is interpreted stratigraphically in different ways in the current literature: as diachronous (top and bottom) and tracking glacioeustatic flooding, as semi-diachronous (bottom diachronous, top isochronous) and outlasting the flood, or as isochronous (top and bottom) and recording ocean-wide changes over time subsequent to deglaciation. Each interpretation carries a different implication for the timescale of cap Dolostones and their isotopic signatures, and therefore for their origin. In northern Namibia, we studied the Keilberg cap Dolostone (635 Ma) across the Otavi carbonate bank and down a contiguous submarine paleoslope to estimated depths of ∼ 0.5 km. We find giant wave ripples and other wave-generated structures in all areas, including the lower slope, pointing to a base-level change of large amplitude. No other formation in the carbonate succession contains wave-generated bedforms on the lower slope. Carbon isotope records from the bank are similar in shape and absolute value, irrespective of thickness. Slope records are also similar to one another, but different in shape and value from those on the bank. If the cap is isochronous, lower-slope waters were enriched in 13 C by 2–3‰ compared with the bank, which seems improbable. If diachronous, the lower slope, upper slope and bank records collectively describe a sigmoidal δ 13 C curve over time with a net decline of 4.4‰. In addition, a lateral gradient of 1.0‰/100 km existed from the inner to outer bank. If the flooding was rapid ( δ 13 C change may reflect strong surface warming, methane release, and kinetic isotope effects associated with rapid carbonate production. If the transgression was prolonged (> 100 kyr), as implied by actualistic interpretation of paleomagnetic reversals in this and other cap Dolostones, the δ 13 C change could record Rayleigh distillation associated with the drawdown of a large atmospheric CO 2 reservoir, built up during the preceding snowball glaciation. Either way, the sedimentology and isotopes support the diachronous interpretation, and are inconsistent with the semi-diachronous and isochronous models. The base-level rise of ∼ 0.5 km implies a glacioeustatic origin, meaning that cap Dolostone sedimentation was synchronous with land ice melting. This leaves the actualistic interpretation of reversal frequency and speed in cap Dolostones in conflict with ice-melting models.
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contrasting diagenetic evolution patterns of platform margin limestones and Dolostones in the lower triassic feixianguan formation sichuan basin china
Marine and Petroleum Geology, 2017Co-Authors: An Jiang Shen, Richard H Worden, Lei Jiang, Xun Yue HeAbstract:Abstract Deeply-buried carbonate-reservoirs from the Lower Triassic Feixianguan Formation in the Sichuan Basin of China host extensive natural gas resources. These reservoirs are predominantly found in oolitic shoals, with the reservoir quality of dolomitized zones being higher than that of undolomitized limestone counterparts. Here we present a combination of petrographic, isotopic, fluid inclusion, and quantitative porosity data in order to understand and predict the diagenetic processes that have impacted the reservoir quality of Dolostones and limestones. The porosity of limestones has been reduced to ∼7.5% due to calcite cementation, whereas the porosity in oolitic Dolostones is not cemented with calcite and typically has ∼23.5% porosity. Dolomitization and concurrent early-diagenetic gypsum growth played crucial roles on the development and preservation of high porosity in the oolitic Dolostone, first by stabilizing the rock fabric to inhibit loss of porosity during burial, and secondly through the generation of new porosity by dissolution of carbonate and anhydrite. A negative shift of δ 18 O and salinity values ( 13 C values (∼−20‰ VPDB) and δ 18 O (∼−10‰ VPDB) together with elevated calcite precipitation temperatures (>110 °C). It is likely that TSR was responsible for the formation of enlarged dissolution vugs that increased porosity by ∼2% in Dolostones due to: i) anhydrite dissolution, ii) production of significant amounts of water resulting in formation water undersaturated with respect to calcite and dolomite, iii) generation of H 2 S, and CO 2 , and the consequent reaction of H 2 S with the siderite (FeCO 3 ) component in calcite and dolomite. This study demonstrates the importance of diagenesis in the formation of deeply-buried, high-quality reservoirs in ooid-dominated grainstones influenced by the presence of evaporites. Our results should be useful for guiding future exploration and reservoir developments in similar paleogeographic and diagenetic settings.
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petrological and geochemical constraints on diagenesis and deep burial dissolution of the ordovician carbonate reservoirs in the tazhong area tarim basin nw china
Marine and Petroleum Geology, 2016Co-Authors: Lianqi Jia, Lei Jiang, Chunfang Cai, Ke Zhang, Wen ZhangAbstract:Abstract Deeply buried (4500–7000 m) Ordovician carbonate reservoirs in the Tazhong area, Tarim Basin, NW China show obvious heterogeneity with porosity from null in limestones and sweet Dolostones to 27.8% in sour Dolostones, from which economically important oils, sour gas and condensates are currently being produced. Petrographic features, C, O, Sr isotopes were determined, and fluid inclusions were analyzed on diagenetic calcite, dolomite and barite from Ordovician reservoirs to understand controls on the porosity distribution. Ordovician carbonate reservoirs in the Tazhong area are controlled mainly by initial sedimentary environments and eo-genetic and near-surface diagenetic processes. However, vugs and pores generated from eogenetic and telogenetic meteoric dissolution were observed to have partially been destroyed due to subsequent compaction, filling and cementation. In some locations or wells (especially ZG5-ZG7 Oilfield nearby ZG5 Fault), burial diagenesis (e.g. thermochemical sulfate reduction, TSR) probably played an important role in quality improvement towards high-quality reservoirs. C2 calcite and dolomite cements and barite have fluid inclusions homogenization temperatures (Ths) from 86 to 113 °C, from 96 to 128 °C and from 128 to 151 °C, respectively. We observed petrographically corroded edges of these high-temperature minerals with oil inclusions, indicating the dissolution must have occurred under deep-burial conditions. The occurrence of TSR within Ordovician carbonate reservoirs is supported by C3 calcite replacement of barite, and the association of sulfur species including pyrite, anhydrite or barite and elemental sulfur with hydrocarbon and 12C-rich (as low as −7.2‰ V-PDB) C3 calcite with elevated Ths (135–153 °C). The TSR may have induced burial dissolution of dolomite and thus probably improved porosity of the sour Dolostones reservoirs at least in some locations. In contrast, no significant burial dissolution occurred in limestone reservoirs and non-TSR Dolostone reservoirs. The deeply buried sour Dolostone reservoirs may therefore be potential exploration targets in Tarim Basin or elsewhere in the world.