| 1. |
- Komar, N., et al.
(författare)
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Understanding long-term carbon cycle trends : The late Paleocene through the early Eocene
- 2013
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Ingår i: Paleoceanography. - : American Geophysical Union (AGU). - 0883-8305 .- 1944-9186. ; 28:4, s. 650-662
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Tidskriftsartikel (refereegranskat)abstract
- The late Paleocene to the early Eocene (∼58–52 Ma) was marked by significant changes in global climate and carbon cycling. The evidence for these changes includes stable isotope records that reveal prominent decreases in δ18O and δ13C, suggesting a rise in Earth's surface temperature (∼4°C) and a drop in net carbon output from the ocean and atmosphere. Concurrently, deep-sea carbonate records at several sites indicate a deepening of the calcite compensation depth (CCD). Here we investigate possible causes (e.g., increased volcanic degassing or decreased net organic burial) for these observations, but from a new perspective. The basic model employed is a modified version of GEOCARB III. However, we have coupled this well-known geochemical model to LOSCAR (Long-term Ocean-atmosphere Sediment CArbon cycle Reservoir model), which enables simulation of seawater carbonate chemistry, the CCD, and ocean δ13C. We have also added a capacitor, in this case represented by gas hydrates, that can store and release13C-depleted carbon to and from the shallow geosphere over millions of years. We further consider accurate input data (e.g., δ13C of carbonate) on a currently accepted timescale that spans an interval much longer than the perturbation. Several different scenarios are investigated with the goal of consistency amongst inferred changes in temperature, the CCD, and surface ocean and deep ocean δ13C. The results strongly suggest that a decrease in net organic carbon burial drove carbon cycle changes during the late Paleocene and early Eocene, although an increase in volcanic activity might have contributed. Importantly, a drop in net organic carbon burial may represent increased oxidation of previously deposited organic carbon, such as stored in peat or gas hydrates. The model successfully recreates trends in Earth surface warming, as inferred from δ18O records, the CCD, and δ13C. At the moment, however, our coupled modeling effort cannot reproduce the magnitude of change in all these records collectively. Similar problems have arisen in simulations of short-term hyperthermal events during the early Paleogene (Paleocene-Eocene Thermal Maximum), suggesting one or more basic issues with data interpretation or geochemical modeling remain.
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| 2. |
- Agnini, Claudia, et al.
(författare)
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Stable isotope and calcareous nannofossil assemblage record of the late Paleocene and early Eocene (Cicogna section)
- 2016
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Ingår i: Climate of the Past. - : Copernicus GmbH. - 1814-9324 .- 1814-9332. ; 12:4, s. 883-909
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Tidskriftsartikel (refereegranskat)abstract
- We present records of stable carbon and oxygen isotopes, CaCO3 content, and changes in calcareous nannofossil assemblages across an 81 m thick section of upper Paleocene lower Eocene marine sedimentary rocks now exposed along the Cicogna Stream in northeast Italy. The studied stratigraphic section represents sediment accumulation in a bathyal hemipelagic setting from approximately 57.5 to 52.2 Ma, a multi-million-year time interval characterized by perturbations in the global carbon cycle and changes in calcareous nannofossil assemblages. The bulk carbonate delta C-13 profile for the Cicogna section, once placed on a common timescale, resembles that at several other locations across the world, and includes both a long-term drop in delta C-13 and multiple short-term carbon isotope excursions (CIEs). This precise correlation of widely separated delta C-13 records in marine sequences results from temporal changes in the carbon composition of the exogenic carbon cycle. However, diagenesis has likely modified the delta C-13 record at Cicogna, an interpretation supported by variations in bulk carbonate 8180, which do not conform to expectations for a primary signal. The record of CaCO3 content reflects a combination of carbonate dilution and dissolution, as also inferred at other sites. Our detailed documentation and statistical analysis of calcareous nannofossil assemblages show major differences before, during and after the Paleocene Eocene Thermal Maximum. Other CIEs in our lower Paleogene section do not exhibit such a distinctive change; instead, these events are sometimes characterized by variations restricted to a limited number of taxa and transient shifts in the relative abundance of primary assemblage components. Both long-lasting and short-lived modifications to calcareous nannofossil assemblages preferentially affected nannoliths or holococcoliths such as Discoaster,, Fasciculithus, Rhomboaster/Tribrachiatus, Sphenolithus and Zygrhablithus, which underwent distinct variations in abundance as well as permanent evolutionary changes in terms of appearances and disappearances. By contrast, placoliths such as Coccolithus and Tow eius, which represent the main component of the assemblages, were characterized by a gradual decline in abundance over time. Comparisons of detailed nannofossil assemblage records at the Cicogna section and at ODP Site 1262 support the idea that variations in the relative and absolute abundances, even some minor changes, were globally synchronous. An obvious link is through climate forcing and carbon cycling, although the linkages between variations in calcareous nannoplankton, changes in delta C-13 records and oceanography will need additional work.
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| 3. |
- Bhatnagar, Guarav, et al.
(författare)
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Analytical theory relating the depth of the sulfate‐methane transition to gas hydrate distribution and saturation
- 2011
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Ingår i: Geochemistry Geophysics Geosystems. - 1525-2027. ; 12:3, s. 1-21
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Tidskriftsartikel (refereegranskat)abstract
- [1] We develop a theory that relates gas hydrate saturation in marine sediments to the depth of the sulfate‐ methane transition (SMT) zone below the seafloor using steady state, analytical expressions. These expres- sions are valid for systems in which all methane transported into the gas hydrate stability zone (GHSZ) comes from deeper external sources (i.e., advective systems). This advective constraint causes anaerobic oxidation of methane to be the only sulfate sink, allowing us to link SMT depth to net methane flux. We also develop analytical expressions that define the gas hydrate saturation profile based on SMT depth and site‐specific parameters such as sedimentation rate, methane solubility, and porosity. We evaluate our analytical model at four drill sites along the Cascadia Margin where methane sources from depth dominate. With our model, we calculate average gas hydrate saturations across GHSZ and the top occurrence of gas hydrate at these sites as 0.4% and 120 mbsf (Site 889), 1.9% and 70 mbsf (Site U1325), 4.7% and 40 mbsf (Site U1326), and 0% (Site U1329), mbsf being meters below seafloor. These values compare favorably with average saturations and top occurrences computed from resistivity log and chloride data. The analyt- ical expressions thus provide a fast and convenient method to calculate gas hydrate saturation and first‐ order occurrence at a given geologic setting where vertically upward advection dominates the methane flux.
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| 4. |
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| 5. |
- Chatterjee, Sayantan, et al.
(författare)
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The impact of lithologic heterogeneity and focused fluid flow upon gas hydrate distribution in marine sediments
- 2014
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Ingår i: Journal of Geophysical Research - Solid Earth. - 2169-9313 .- 2169-9356. ; 119:9, s. 6705-6732
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Tidskriftsartikel (refereegranskat)abstract
- Gas hydrate and free gas accumulation in heterogeneous marine sediment is simulated using a two-dimensional (2-D) numerical model that accounts for mass transfer over geological timescales. The model extends a previously documented one-dimensional (1-D) model such that lateral variations in permeability (k) become important. Various simulations quantitatively demonstrate how focused fluid flow through high-permeability zones affects local hydrate accumulation and saturation. Simulations that approximate a vertical fracture network isolated in a lower permeability shale (k(fracture) >>k(shale)) show that focused fluid flow through the gas hydrate stability zone (GHSZ) produces higher saturations of gas hydrate (25-70%) and free gas (30-60%) within the fracture network compared to surrounding shale. Simulations with a dipping, high-permeability sand layer also result in elevated saturations of gas hydrate (60%) and free gas (40%) within the sand because of focused fluid flow through the GHSZ. Increased fluid flux, a deep methane source, or both together increase the effect of flow focusing upon hydrate and free gas distribution and enhance hydrate and free gas concentrations along the high-permeability zones. Permeability anisotropy, with a vertical to horizontal permeability ratio on the order of 10(-2), enhances transport of methane-charged fluid to high-permeability conduits. As a result, gas hydrate concentrations are enhanced within these high-permeability zones. The dip angle of these high-permeability structures affects hydrate distribution because the vertical component of fluid flux dominates focusing effects. Hydrate and free gas saturations can be characterized by a local Peclet number (localized, vertical, focused, and advective flux relative to diffusion) relative to the methane solubility gradient, somewhat analogous to such characterization in 1-D systems. Even in lithologically complex systems, local hydrate and free gas saturations might be characterized by basic parameters (local flux and diffusivity).
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| 6. |
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| 7. |
- Dickens, Gerald R., et al.
(författare)
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Core alignment and composite depth scale for the lower Paleogene through uppermost Cretaceous interval at Deep Sea Drilling Project Site 577
- 2013
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Ingår i: Newsletters on stratigraphy. - : Schweizerbart. - 0078-0421. ; 46:1, s. 47-68
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Tidskriftsartikel (refereegranskat)abstract
- Deep Sea Drilling Project Site 577 on Shatsky Rise (North Pacific Ocean) recovered a series of cores at three holes that contain calcareous nannofossil ooze of latest Cretaceous (late Maastrichtian) through early Eocene age. Several important records have been generated using samples from these cores, but the stratigraphy has remained outdated and confusing. Here we revise the stratigraphy at Site 577. This includes refining several age datums, realigning cores in the depth domain, and placing all stratigraphic markers on a current time scale. The work provides a template for appropriately bringing latest Cretaceous and Paleogene data sets at old drill sites into current paleoceanographic literature for this time interval. While the Paleocene Eocene Thermal Maximum (PETM) lies within core gaps at Holes 577* and 577A, the sedimentary record at the site holds other important events and remains crucially relevant to understanding changes in oceanographic conditions from the latest Cretaceous through early Paleogene.
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| 8. |
- Dickens, Gerald R.
(författare)
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Down the Rabbit Hole : toward appropriate discussion of methane release from gas hydrate systems during the Paleocene-Eocene thermal maximum and other past hyperthermal events
- 2011
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Ingår i: Climate of the Past Discussions. - : Copernicus GmbH. - 1814-9340 .- 1814-9359 .- 1814-9332. ; 7:3, s. 831-846
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Tidskriftsartikel (refereegranskat)abstract
- Enormous amounts of (13)C-depleted carbon rapidly entered the exogenic carbon cycle during the onset of the Paleocene-Eocene thermal maximum (PETM), as attested to by a prominent negative carbon isotope (delta(13)C) excursion and deep-sea carbonate dissolution. A widely cited explanation for this carbon input has been thermal dissociation of gas hydrate on continental slopes, followed by release of CH(4) from the seafloor and its subsequent oxidation to CO(2) in the ocean or atmosphere. Increasingly, papers have argued against this mechanism, but without fully considering existing ideas and available data. Moreover, other explanations have been presented as plausible alternatives, even though they conflict with geological observations, they raise major conceptual problems, or both. Methane release from gas hydrates remains a congruous explanation for the delta(13)C excursion across the PETM, although it requires an unconventional framework for global carbon and sulfur cycling, and it lacks proof. These issues are addressed here in the hope that they will prompt appropriate discussions regarding the extraordinary carbon injection at the start of the PETM and during other events in Earth's history.
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| 9. |
- Gu, Guansheng, et al.
(författare)
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Abundant Early Palaeogene marine gas hydrates despite warm deep-ocean temperatures
- 2011
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Ingår i: Nature Geoscience. - 1752-0894 .- 1752-0908. ; 4, s. 848-851
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Tidskriftsartikel (refereegranskat)abstract
- Abrupt periods of global warming between 57 and 50 million years ago—known as the Early Palaeogene hyperthermal events—were associated with the repeated injection of mas- sive amounts of carbon into the atmosphere1–4. The release of methane from the sea floor following the dissociation of gas hydrates is often invoked as a source5. However, seafloor temperatures before the events were at least 4–7 ◦ C higher than today1, which would have limited the area of sea floor suitable for hosting gas hydrates6,7. Palaeogene gas hydrate reservoirs may therefore not have been sufficient to provide a significant fraction of the carbon released. Here we use numer- ical simulations of gas hydrate accumulation8 at Palaeogene seafloor temperatures to show that near-present-day values of gas hydrates could have been hosted in the Palaeogene. Our simulations show that warmer temperatures during the Palaeogene would have enhanced the amount of organic carbon reaching the sea floor as well as the rate of methanogenesis. We find that under plausible temperature and pressure condi- tions, the abundance of gas hydrates would be similar or higher in the Palaeogene than at present. We conclude that methane hydrates could have been an important source of carbon during the Palaeogene hyperthermal events.
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| 10. |
- Ketzer, João Marcelo, et al.
(författare)
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Gas Hydrate Systems on the Brazilian Continental Margin
- 2022
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Ingår i: World Atlas of Submarine Gas Hydrates in Continental Margins. - Cham : Springer. - 9783030811853 - 9783030811860 ; , s. 343-352
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Bokkapitel (refereegranskat)abstract
- The existence of gas hydrate systems along Brazil’s vast continental margin has been known since the 1980s, based on observations of bottom simulating reflectors (BSRs) in two large shelf-slope depocenters: (1) the Amazon deep-sea fan in the Foz do Amazonas Basin and (2) Rio Grande Cone in Pelotas Basin. These depocenters are both undergoing gravitational collapse above deep detachment surfaces, resulting in upslope extensional and downslope compressional domains. The BSR is discontinuous across the Amazon deep-sea fan, mainly observed at water depths of 600–2800 m and at anticlines within an upper slope thrust-fold belt related to the compressional domain of the fan. Conversely, a fairly continuous BSR extends across Rio Grande Cone at water depths of 520–3500 m, within both extensional and compressional domains. Interestingly, the well-defined BSR that spans Rio Grande Cone rises to meet the seafloor at water depths of 515–520 m, forming an unusual ‘BSR outcrop’. This phenomenon has been observed previously in only a few locations worldwide. Gas hydrates have been recovered within piston cores taken from seafloor seeps in both depocenters, and analyses reveal that gas is dominated by methane of microbial origin.
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