| 1. |
- Kroger, Bjorn, et al.
(författare)
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Subaerial speleothems and deep karst in central Sweden linked to Hirnantian glaciations
- 2015
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Ingår i: Journal of the Geological Society. - : Geological Society of London. - 2041-479X .- 0016-7649. ; 172:3, s. 349-356
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Tidskriftsartikel (refereegranskat)abstract
- The limestones of the upper Katian Boda mud mounds (Ordovician) of the Siljan district in central Sweden are deeply fractured. The fissures were partly synsedimentary and are often lined with stromatolite-like crusts. These crusts thus far are the only known subaerial Ordovician speleothems. They reach depths of up to 30 m below the former mound top. Macroscopically the crusts form decimetre-sized, cone-shaped domal aggregates, stalactites and stalagmites. Microfabric and morphology identify them as microbially mediated speleothems in a dark environment. Combined Sr and C isotope values indicate a formation of the speleothems from meteoric waters without influence of a significant soil horizon. For the first time the age of the speleothems can be precisely constrained by delta C-13 whole-rock and brachiopod shell isotope data to the mid-Hirnantian. Repeated and/or prolonged subaerial exposure of the Boda mud mounds during the Hirnantian is evident from karst surfaces and early cements in the mound capping carbonates. The speleothems and the karst surfaces record an estimated sea-level fall in the range of 80-130 m within the time window of the Hirnantian Isotopic Carbon Excursion. This massive regression coincides with maximum ice sheet extent inferred from sections in West Gondwana.
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| 2. |
- Rasmussen, Christian, et al.
(författare)
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Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse.
- 2016
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 6
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Tidskriftsartikel (refereegranskat)abstract
- The Great Ordovician Biodiversification Event (GOBE) was the most rapid and sustained increase in marine Phanerozoic biodiversity. What generated this biotic response across Palaeozoic seascapes is a matter of debate; several intrinsic and extrinsic drivers have been suggested. One is Ordovician climate, which in recent years has undergone a paradigm shift from a text-book example of an extended greenhouse to an interval with transient cooling intervals - at least during the Late Ordovician. Here, we show the first unambiguous evidence for a sudden Mid Ordovician icehouse, comparable in magnitude to the Quaternary glaciations. We further demonstrate the initiation of this icehouse to coincide with the onset of the GOBE. This finding is based on both abiotic and biotic proxies obtained from the most comprehensive geochemical and palaeobiological dataset yet collected through this interval. We argue that the icehouse conditions increased latitudinal and bathymetrical temperature and oxygen gradients initiating an Early Palaeozoic Great Ocean Conveyor Belt. This fuelled the GOBE, as upwelling zones created new ecospace for the primary producers. A subsequent rise in δ(13)C ratios known as the Middle Darriwilian Isotopic Carbon Excursion (MDICE) may reflect a global response to increased bioproductivity encouraged by the onset of the GOBE.
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| 3. |
- Vickers, Madeleine L., et al.
(författare)
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Paleocene–Eocene age glendonites from the Mid-Norwegian Margin – indicators of cold snaps in the hothouse?
- 2024
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Ingår i: Climate of the Past. - : Copernicus Publications. - 1814-9324 .- 1814-9332. ; 20:1, s. 1-23
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Tidskriftsartikel (refereegranskat)abstract
- The International Ocean Discovery Program (IODP) Expedition 396 to the mid-Norwegian margin recovered > 1300 m of pristinely preserved, volcanic-ash-rich sediments deposited during the late Paleocene and early Eocene from close to the centre of the North Atlantic Igneous Province (NAIP). Remarkably, many of these cores contain glendonites, pseudomorphs after the purported cold-water mineral ikaite, from sediments dated to the late Paleocene and early Eocene. These time intervals span some of the hottest climates of the Cenozoic, including the Paleocene–Eocene Thermal Maximum (PETM). Global deep-ocean temperatures are not thought to have dropped below 10 ∘C at any point during this time, making the occurrence of supposedly cold-water (near-freezing temperature) glendonite pseudomorphs seemingly paradoxical. This study presents a detailed sedimentological, geochemical, and microscopic study of the IODP Exp. 396 glendonites and presents an updated model for the ikaite-to-calcite transformation for these glendonites. Specifically, we show that early diagenesis of basaltic ashes of the NAIP appear to have chemically promoted ikaite growth in the sediments in this region. Together with existing knowledge of late Paleocene and early Eocene glendonites from Svalbard to the north and early Eocene glendonites from Denmark to the south, these new glendonite finds possibly imply episodic, short-duration, and likely localized cooling in the Nordic Seas region, which may have been directly or indirectly linked to the emplacement of the NAIP.
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| 4. |
- Vickers, Madeleine L., et al.
(författare)
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The ikaite to calcite transformation : Implications for palaeoclimate studies
- 2022
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 334, s. 201-216
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Tidskriftsartikel (refereegranskat)abstract
- Marine sedimentary ikaite is the parent mineral to glendonite, stellate pseudomorphs found throughout the geological record which are most usually composed of calcite. Ikaite is known to be metastable at earth surface temperatures and pressures, readily breaking down to more stable carbonate polymorphs when exposed to warm (ambient) conditions. Yet the process of transformation of ikaite to calcite is not well understood, and there is an ongoing debate as to the palaeoclimatic significance of glendonites in the geological record. This study uses a combination of techniques to examine the breakdown of ikaite to calcite, outside of the ikaite growth medium, and to assess the palaeoclimatic and palaeoenvironmental significance of stable and clumped isotope compositions of ikaite-derived calcite. Powder X-ray diffraction shows that ikaite undergoes a quasi- solid-state transformation to calcite during heating of samples in air, yet when ikaite transforms under a high temperature differential, minor dissolution-recrystallisation may also occur with the ikaite structural waters. No significant isotopic equilibration to transformation temperature is observed in the resulting calcite. Therefore, in cases of transformation of ikaite in air, clumped and stable isotope thermometry can be used to reconstruct ikaite growth temperatures. In the case of ancient glendonites, where transformation of the ikaite occurred in contact with the interstitial waters of the host sediments over unknown timescales, it is uncertain whether the reconstructed clumped isotope temperatures reflect ikaite crystallisation or its transformation temperatures. Yet clumped and stable isotope thermometry may still be used conservatively to estimate an upper limit for bottom water temperatures. Furthermore, stable isotope along with element/Ca ratios shed light on the chemical environment of ikaite growth. Our data indicate that a range of (bio)geochemical processes may act to promote ikaite formation at different marine sedimentary sites, including bacterial sulphate reduction and anaerobic oxidation of methane. The colours of the ikaites, from light brown to dark brown, indicate a high organic matter content, favouring high rates of bacterial sulphate reduction as the main driver of ikaite precipitation. Highest Mg/Ca ratios are found in the most unstable ikaites, indicating that Mg acts to destabilise ikaite structure.
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