The Experts below are selected from a list of 16125 Experts worldwide ranked by ideXlab platform
Nadia Regina Do Nascimento - One of the best experts on this subject based on the ideXlab platform.
-
Dating kaolinite from the Neogene Içá Formation and overlying laterites, central Amazonia, Brazil: Constraints for a Stratigraphic Correlation
Palaeogeography Palaeoclimatology Palaeoecology, 2020Co-Authors: Thierry Allard, Luciana Pereira, Maximilien Mathian, E. Balan, Guilherme Taitson Bueno, Christophe Falguères, Nadia Regina Do NascimentoAbstract:The Neogene Içá Formation is a major sedimentary deposit in central Amazonia (Brazil) that is critical to the understanding and modeling of the evolution of the Amazon Basin. However, its chronostratigraphy is poorly defined, mostly because it is not fossiliferous and it does not contain volcanic ash levels. This study is the first attempt at the absolute dating of the sediments and overlying soils of the Içá Formation by electron paramagnetic resonance (EPR) of radiation-induced defects in kaolinite (RIDs). Four profiles of unconsolidated sediments overlaid by lateritic soils from the Barcelos region (Amazonia, Brazil) were studied. The concentration of RIDs was assessed from EPR spectra through a simplified procedure on a selected set of samples, excluding those showing too strong of interference from the signal of trace vanadyl complexes. It is shown that in such sedimentary material, both the microscopic distribution of thorium (together with uranium) and the origin of kaolinite, either detrital or authigenic, are critical for kaolinite dating. Considering an argued authigenic origin and two scenarios of Th distribution, it appears that most of the present chronologic data support the Stratigraphic Correlation with the Madre de Dios sedimentary Formation in Peru previously proposed in the literature for the Içá Formation (9.5–2.5 Ma). This study also shows a rejuvenation process of kaolinite both in the uppermost soil and at depth of the profiles. This rejuvenation occurred during paleoclimatic periods already revealed in other weathering covers from the Amazon Basin and possibly resulted from downward weathering in the topsoil and water table or lateral weathering at depth.
Thierry Allard - One of the best experts on this subject based on the ideXlab platform.
-
Dating kaolinite from the Neogene Içá Formation and overlying laterites, central Amazonia, Brazil: Constraints for a Stratigraphic Correlation
Palaeogeography Palaeoclimatology Palaeoecology, 2020Co-Authors: Thierry Allard, Luciana Pereira, Maximilien Mathian, E. Balan, Guilherme Taitson Bueno, Christophe Falguères, Nadia Regina Do NascimentoAbstract:The Neogene Içá Formation is a major sedimentary deposit in central Amazonia (Brazil) that is critical to the understanding and modeling of the evolution of the Amazon Basin. However, its chronostratigraphy is poorly defined, mostly because it is not fossiliferous and it does not contain volcanic ash levels. This study is the first attempt at the absolute dating of the sediments and overlying soils of the Içá Formation by electron paramagnetic resonance (EPR) of radiation-induced defects in kaolinite (RIDs). Four profiles of unconsolidated sediments overlaid by lateritic soils from the Barcelos region (Amazonia, Brazil) were studied. The concentration of RIDs was assessed from EPR spectra through a simplified procedure on a selected set of samples, excluding those showing too strong of interference from the signal of trace vanadyl complexes. It is shown that in such sedimentary material, both the microscopic distribution of thorium (together with uranium) and the origin of kaolinite, either detrital or authigenic, are critical for kaolinite dating. Considering an argued authigenic origin and two scenarios of Th distribution, it appears that most of the present chronologic data support the Stratigraphic Correlation with the Madre de Dios sedimentary Formation in Peru previously proposed in the literature for the Içá Formation (9.5–2.5 Ma). This study also shows a rejuvenation process of kaolinite both in the uppermost soil and at depth of the profiles. This rejuvenation occurred during paleoclimatic periods already revealed in other weathering covers from the Amazon Basin and possibly resulted from downward weathering in the topsoil and water table or lateral weathering at depth.
Yanhong Pan - One of the best experts on this subject based on the ideXlab platform.
-
revision of cypridea non marine ostracoda from the early cretaceous yixian formation of the beipiao yixian basin in western liaoning northeast china
Cretaceous Research, 2013Co-Authors: Yaqiong Wang, Jingeng Sha, Yanhong PanAbstract:Abstract The ostracod genus Cypridea Bosquet, which ranges in age from Kimmeridgian to Lower Eocene, frequently occurs abundantly in the Jianshangou and Jinggangshan beds of the Yixian Formation in western Liaoning, northeast China. However, the diversity of species has been considerably overestimated for many years, and this has led to a series of problems concerning taxonomic subdivision and Stratigraphic Correlation. Based on a large quantity of fossil material and more detailed descriptions of the morphological features, all of the published species of Cypridea from the Jianshangou and Jinggangshan beds are revised in this paper. As a result, 14 species erected previously are merged into three taxa: Cypridea liaoningensis , C. jingangshanensis and C. deplecta .
S C Finney - One of the best experts on this subject based on the ideXlab platform.
-
nd isotope records of late ordovician sea level change implications for glaciation frequency and global Stratigraphic Correlation
Palaeogeography Palaeoclimatology Palaeoecology, 2013Co-Authors: Chris Holmden, Charles E Mitchell, D F Laporte, William P Patterson, Michael J Melchin, S C FinneyAbstract:Abstract Detailed records of ice-sheet advances and retreats are reconstructed for the Hirnantian and Katian ages of the Late Ordovician using Nd isotopes (e Nd ) as a sea-level proxy in three study sections from the western margin of Laurentia: two shallow water platform sections located south and north of the paleo-equator, and one deep water section located in a continental slope-rise setting. When sea-level was high and paleo-shorelines had migrated eastward, the e Nd value of seawater in the vicinity of each of the study sections shifted toward the e Nd value of the eastern Panthalassa Ocean (~ − 4.0). By contrast, when sea-level was low and paleo-shorelines had migrated westward, the e Nd value of seawater shifted toward the e Nd value of the continental weathering flux from Laurentia (− 8.5 ± 0.2, 2σ mean ). These Stratigraphic patterns of changing e Nd values are interpreted to reflect the eustatic sea-level fluctuations that previous studies have documented in response to Gondwanan ice-sheet advances and retreats, thus linking the e Nd sea-level proxy to Late Ordovician global-scale climate changes. The e Nd profiles for the two platform sections yielded similar proxy sea-level curves with five cycles of oscillation recorded during the latest Katian and Hirnantian. Three additional cycles of late Katian sea-level change are recognized in the e Nd profile of the deep water continental slope-rise section. The combination of e Nd , δ 13 C and graptolite bioStratigraphic data facilitates a precise interregional Correlation of the Hirnantian Age and the paleoclimate changes that took place during this interval. The new Correlations support previous findings that the Hirnantian ice age comprised two major glacial periods separated by a minor interglacial during the early part of the Metabolograptus persculptus Biozone. The younger glacial (confined to mid M. persculptus Biozone time) led to more extensive sea surface cooling than did the earlier one, and resulted in extensive eustatic sea-level drawdown and C-cycle changes. It records the strata most often recognized as HICE (the Hirnantian Carbon Isotope Excursion) in sedimentary successions worldwide, such as Anticosti Island, Scotland, Estonia, Siberia, and South China. The results of this study support and strengthen the view that glaciation predated the Hirnantian Age in the Late Ordovician, and that the records of small positive δ 13 C excursions in Katian successions from Baltica and eastern North America are themselves proxy indicators of glaciation frequency and eustatic sea-level changes.
Shuhai Xiao - One of the best experts on this subject based on the ideXlab platform.
-
stable carbon isotopes of sedimentary kerogens and carbonaceous macrofossils from the ediacaran miaohe member in south china implications for Stratigraphic Correlation and sources of sedimentary organic carbon
Precambrian Research, 2017Co-Authors: Shuhai Xiao, Natalia Bykova, Alex Kovalick, Benjamin C GillAbstract:Abstract Ediacaran Stratigraphic Correlation depends on the integration of paleontological and δ13Ccarb data. However, these data often come from different lithologies (e.g., shales vs. carbonates), making it difficult to directly link bioStratigraphic and chemoStratigraphic markers. Ediacaran successions in South China consist of mixed carbonates and shales, offering an opportunity for integrated bioStratigraphic and chemoStratigraphic investigations. Black shales of the Ediacaran Miaohe Member in the Yangtze Gorges area contain macroalgae and Ediacara-type macrofossils, both preserved as carbonaceous compressions. The Miaohe Member has been traditionally regarded as an equivalent to Member IV of the uppermost Doushantuo Formation that records part of the Shuram negative δ13Ccarb excursion in South China. Thus, the Miaohe Member may have the potential to clarify the Stratigraphic relationship between Ediacara-type fossils and the Shuram excursion. Recently, however, the Miaohe Member has been partially correlated with the Shibantan Member of the Dengying Formation (the “A” Correlation) or partially with Member IV (the “Z” Correlation). These Correlations have different implications for the relationship between the Miaohe Member and the Shuram excursion. Here we test these two Correlations using δ13C data of sedimentary kerogen or total organic carbon (δ13CorgTOC), taking advantage of the distinct δ13CorgTOC values between the Shibantan Member and Member IV. Our data show that δ13CorgTOC of the Miaohe Member is more similar to that of Member IV than to Shibantan Member, providing tentative support for the “Z” Correlation. In addition, we also analyzed δ13Corg of individual carbonaceous compression fossils (δ13Corgtaxon), which have lower carbon isotope values than TOC. The data suggest that oxygenic photosynthesizers (e.g., macroalgae, cyanobacteria, and eukaryotic phytoplankton) were not the only contributors to sedimentary TOC. Methylotrophs, photoautotrophs, and chemoautotrophs living in anoxic waters or at the oxycline may have fixed recycled CO2 derived from organic carbon remineralization, thus contributing a significant amount of 13C-depleted organic carbon to sedimentary kerogen in the Miaohe Member.
-
integrated chemostratigraphy of the doushantuo formation at the northern xiaofenghe section yangtze gorges south china and its implication for ediacaran Stratigraphic Correlation and ocean redox models
Precambrian Research, 2012Co-Authors: Shuhai Xiao, Kathleen A Mcfadden, Sara Peek, Alan J Kaufman, Chuanming Zhou, Ganqing Jiang, Jie HuAbstract:Abstract The Ediacaran Doushantuo Formation in the Yangtze Gorges of South China plays an important role in our understanding of biological evolution, global Correlation, and ocean redox conditions, because of the availability of high-resolution paleontological and geochemical data and numerous radiometric dates. However, integrated study has been focused on the Jiulongwan section that was largely deposited below wave base in a restricted shelf lagoon ( Jiang et al., 2011 , Zhu et al., 2011 ). Studies of shallower water successions are lacking, and this presents a challenge to test Ediacaran Stratigraphic Correlation and ocean redox models. To fill this knowledge gap, we conducted a high-resolution integrated study of the Doushantuo Formation at the northern Xiaofenghe (NXF) section approximately 35 km to the northeast and paleogeographically updip of the Jiulongwan section. With the exception of the basal 20 m, NXF sediments were deposited above normal wave base. Integrated bioStratigraphic and chemoStratigraphic data indicate that the 140 m thick NXF section correlates with the lower Doushantuo Formation (Member I and much of Member II; i.e., the lower ca. 70 m of the formation) at Jiulongwan. Geochemical data from NXF and other Doushantuo sections indicate that euxinic conditions may have been limited to a shelf lagoon (represented by the Jiulongwan section) that was restricted between the proximal inner shelf and a distal shelf margin shoal complex, at least during early Doushantuo time following the deposition of the Doushantuo cap dolostone. Further integrated studies are necessary to test whether euxinic conditions existed in open marine shelves in South China and elsewhere during the Ediacaran Period.
