| 4551. |
- Peng, Shanchi, et al.
(författare)
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Trilobites
- 2020
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Ingår i: Geologic Timescale 2020. - 9780128243626 - 9780128243619 ; 1, s. 36-36
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Bokkapitel (refereegranskat)abstract
- Trilobites are marine arthropods that appeared in Cambrian Epoch 2 and became extinct at the end of the Permian. In some Paleozoic deposits, they number among the most abundant macrofossils. Trilobites are important for early Paleozoic biostratigraphy, especially Cambrian.
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| 4552. |
- Penn-Clarke, Cameron R., et al.
(författare)
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Early–Middle Devonian brachiopod provincialism and bioregionalization at high latitudes : A case study from southwestern Gondwana
- 2021
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Ingår i: Bulletin of the Geological Society of America. - 0016-7606. ; 133:3-4, s. 819-836
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Tidskriftsartikel (refereegranskat)abstract
- The paleobiogeography of Early–Middle Devonian (Pragian–Eifelian) brachiopods from West Gondwana was assessed to determine any potential controls (regional climatic differences or global eustasy) that may have driven bioregionalization. The Pragian–Eifelian interval of West Gondwana was examined because work by previous authors suggested that this was a period when regionally extensive areas of provincialism among marine invertebrates were present and most pronounced. Factors of particular interest in this study were the controls over brachiopod bioregionalization at high (60°–90°) southern latitudes, which the Malvinokaffric Realm is thought to have entirely occupied. A large presence-absence data matrix was compiled consisting of the occurrences of 206 genera from 17 localities across West Gondwana, and an array of multivariate methods (cluster analysis, nonmetric multidimensional scaling, and network analysis) was employed to assess regional bioregionalization trends. The results of our study suggest that regional climatic differences brought on by latitudinal effects were the determining driver for bioregionalism of brachiopods during the Pragian–Eifelian, and these trends were coincident with a global cooling period during the Early–Middle Devonian. Our study further suggests that of the three regional paleobiogeographic realms thought to be present in West Gondwana during the Early–Middle Devonian (Eastern Americas, Old World, and Malvinokaffric), only the Malvinokaffric Realm is valid as a single biogeographic area.Its area, however, is reduced; it is interpreted to have been a second-order biogeographic area and not a first-order area as suggested by previous authors. Given these factors, we suggest a new demonym for this area, the “Malvinoxhosan,” given the racially charged connotations of “Malvinokaffric.” We present a new biogeographic framework for West Gondwana that is free of preconceived biogeographic area and rank biases, with the understanding that a more globally expansive study should be undertaken to elucidate these areas and their rank within the correct hierarchy. Two first-order paleobiogeographic areas are recognized in West Gondwana and are named according to their latitudinal extent, namely, the high-latitude (60°S–90°S) and temperate-latitude (30°S–60°S) bioregions. The temperate-latitude bioregion consists of a single second-order paleobiogeographic area, the “Colombian–West African” bioregion. Two second-order bioregions are present in the high-latitude bioregion, namely, the Amazonian and Malvinoxhosan bioregions. Here, the Amazonian (~50°S–70°S), compositionally, was an intermediate region between the Colombian–West African (~30°S–50°S) and Malvinoxhosan (70°S–90°S) end members. Latitudinal effects may also have been responsible for dividing the Malvinoxhosan bioregion into two tentative third-order paleobiogeographic areas, namely, the Andeo– South African (~70°S–80°S) and the Paraná (~80°S–90°S) bioregions.
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| 4553. |
- Penn-Clarke, C. R., et al.
(författare)
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The Ordovician System of South Africa : a review
- 2023
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Ingår i: Geological Society Special Publication. - 0305-8719. ; 533, s. 175-197
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Bokkapitel (refereegranskat)abstract
- Outcrops of the Ordovician System in South Africa are extensive; they cover significant portions of the Northern, Western and Eastern Cape provinces as part of the Cape Fold Belt as well as the KwaZulu-Natal Province as supracrustal cover overlying the Natal sector of the Paleoproterozoic Namaqua-Natal metamorphic province. Within the Cape Fold Belt, Ordovician rocks of the Table Mountain Group (Piekenierskloof, Graafwater, Peninsula, Pakhuis and Cedarberg formations as well as the enigmatic Sardinia Bay Formation) outcrop extensively whilst pre-Cape rocks of the Kansa Group (Vaartwell, Uitvlug, Gezwinds Kraal and Schoongezigt formations) and Schoemanspoort Formation are present within the Kango Inlier encapsulated by the fold belt. The Natal Group (Durban and Mariannhill formations) is entirely located within KwaZulu-Natal. For the most part, these metasiliciclastic rocks are markedly unfossiliferous except for the world class fossil deposits of the Cedarberg Formation and important trace fossil sites in the Graafwater, Peninsula and Pakhuis formations. The lack of palaeontological material and other accurate geochronological proxies in these successions (as well as those of the Kansa and Natal groups and Schoemanspoort Formation) makes estimations of relative age constraints tenuous at best and difficult to correlate with global Ordovician chronostratigraphic frameworks. Regardless of the challenges faced in correlating these rocks within global frameworks, these rocks provide a unique low latitude glimpse into Earth surface processes and the feedback loops that ensued within the biological realm along the southern margin of Gondwana.
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| 4554. |
- Penn-Clarke, Cameron R., et al.
(författare)
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The rise and fall of the Malvinoxhosan (Malvinokaffric) bioregion in South Africa : Evidence for Early-Middle Devonian biocrises at the South Pole
- 2023
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Ingår i: Earth-Science Reviews. - 0012-8252. ; 246
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Forskningsöversikt (refereegranskat)abstract
- Global reconstructions, inclusive of environments and ecosystems, and biodiversity counts for the Devonian Period are often done so at the expense of high latitude regions given a historical lack of data presented from these areas. This has bearing on the recognition of biocrises (events marked by extinctions and faunal turnovers) at high latitudes as well as their controls and potential correlation with global, regional, and local tempos. The appearance and disappearance of high-latitude endemic Malvinoxhosan (synonymous with the “Malvinokaffric Realm” which it supersedes) marine invertebrate faunas from West Gondwana are often overlooked, in part owing to difficulties in correlating fossil-bearing strata with global frameworks given the absence and rarity of several key index taxa as well as detailed biostratigraphic appraisals in which to draw regional interbasinal correlations and comparisons. The Early to Middle Devonian Series of South Africa (upper Table Mountain, Bokkeveld and lower Witteberg groups) are a classic Malvinoxhosan-bearing section recording the rise of these endemic faunas, as well as their decline and replacement by cosmopolitan faunas. A detailed biostratigraphy of this interval was created following an assessment of fossil material curated at the Council for Geoscience and Iziko South African Museum, Cape Town as well as from literature. These data suggest that the Malvinoxhosan bioregion persisted as a cohesive unit during Rietvlei-Baviaanskloof to Waboomberg deposition (Pragian/Emsian-early Givetian) given that many representative taxa are found in these strata, however showing a trend of decreasing diversity with little origination through time. Above this interval, few representative taxa are known to continue into the upper Bokkeveld and Witteberg groups, disappearing entirely by the deposition of the Blinkberg Formation. The few fossils that are known in these strata and those succeeding it (e.g., the Swartruggens Formation) are entirely cosmopolitan in identity. Using novel multivariate statistical methods (non-metric multidimensional scaling and cluster analysis) in conjunction with network analysis (NA), the data were interrogated to indicate potential groupings of strata according to their fossil content as well as to track faunal changes through time. These analyses suggest the presence of at least seven to eight interval assemblage biozones housed within at least three larger faunal complexes (Eo-Malvinoxhosan, Malvinoxhosan and Post-Malvinoxhosan) based on their constituent faunal makeup. A closer inspection of these faunal complexes and interval assemblage biozones show a stepped decline in biodiversity with little to no origination and recovery through time that may be correlated with local base-level change at varying orders of magnitude. Declines in biodiversity show selectivity for taxa with epifaunal and semi-infaunal habits with respect to infaunal, deep infaunal and nektonic taxa. Environmental conditions associated with the collapse of the Malvinoxhosan bioregion are thought to have been catastrophic as few new (often short-ranging) immigrants are registered in Post-Malvinoxhosan strata. Further to this, those faunas that are prevalent in Post-Malvinoxhosan strata (e.g., Tropidoleptus) those with known high environmental tolerance and were already present in the region prior to the collapse of the Malvinoxhosan bioregion. Lastly, the observed biodiversity changes in South Africa with respect to local base-level show remarkable similarities with several time equivalent locales in South America suggesting that the decline and extinction of the Malvinoxhosan biota was regional and that the biostratigraphy presented herein has regional application. Here, it is thought regional tectonic controls are suggested to have brought on sea-level changes and entrained warmer waters into higher latitudes against the backdrop of overall rising temperatures from the late Givetian onwards. Whilst the decline of the Malvinoxhosan bioregion might associated with global Middle Devonian biocrises (e.g., Kačák; Taghanic) insufficient age constraints for these strata are available at present to make direct comparisons. Furthermore, an adequate driver for global sea-level change during the Devonian Period, needs to be established to tease out global and local signals in constructed local sea-level curves to establish if these changes (and their effects in changes in biodiversity) are truly global in extent.
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| 4555. |
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| 4556. |
- Perdikouri, Christina, et al.
(författare)
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Hydrothermal replacement of Aragonite by Calcite : interplay between replacement, fracturing and growth
- 2013
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Ingår i: European journal of mineralogy. - : Schweizerbart. - 0935-1221 .- 1617-4011. ; 25:2, s. 123-136
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Tidskriftsartikel (refereegranskat)abstract
- The hydrothermal transformation of single aragonite crystals into polycrystalline calcite has been studied under hydrothermal conditions. The transformation involves a fluid-mediated replacement reaction, associated with fracturing of the initial aragonite crystal and growth of calcite throughout various parts of the reacted aragonite. The observed overall preservation of the crystal morphology is typical of an interface-coupled dissolution-reprecipitation mechanism Analysis of the crystallographic orientation of the product calcite using electron backscatter diffraction (EBSD) showed little to no link between the two phases under the studied conditions, with calcite crystallites exhibiting dominantly different crystallographic orientations compared to those of the aragonite and of neighbouring calcite domains. The complexity of the observed textures is mainly a result of the combination of fracturing of the crystal and initiation of nucleation and growth at different points of the exposed aragonite surface. Experiments performed with solutions enriched in O-18 as a tracer for oxygen exchange and monitored by Raman spectroscopy, showed that carbonate ions in the starting solution are mixed with carbonate from the dissolving aragonite, resulting in an O-18 concentration in the product calcite which depended on the local fluid transport through the fractures. As replacement processes among the CaCO3 phases are relevant to a wide range of applications, understanding the mechanisms is essential for the interpretation of observations of natural and/or experimental samples. This study describes the interplay between the replacement process, the fracturing of the crystal and growth of the new phase, calcite, and provides new insights into the mechanism of the aragonite to calcite transition. The combination of the two methods, EBSD and Raman spectroscopy, demonstrates the importance of local fluid composition and transport pathways in determining the isotope and element exchange in mineral replacement reactions.
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| 4557. |
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| 4558. |
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| 4559. |
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| 4560. |
- Persson, katarina, et al.
(författare)
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Den dynamiska jorden
- 1999
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Ingår i: Ytor på djupet (Naturvetenskapliga forskningsrådets årsbok). - : Naturvetenskapliga forskningsrådet. ; , s. 39-51
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Bokkapitel (populärvet., debatt m.m.)
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