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Zhongli Ding - One of the best experts on this subject based on the ideXlab platform.
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drastic climatic shift at 2 8ma as recorded in eolian deposits of china and its implications for redefining the pliocene pleistocene boundary
Quaternary International, 2010Co-Authors: Shiling Yang, Zhongli DingAbstract:There has been a long-running debate over the position of the Pliocene-Pleistocene boundary, i.e. 1.8 Ma versus 2.6 Ma. This study presents litho- and pedo-stratigraphic characteristics, grain size, magnetic susceptibility, and dust sedimentation rates for ‘red clay’-loess sequences over northern China, in order to assess the drastic climatic Event(s) recorded in the quasi-continuous eolian deposits, and to readdress the position of the Pliocene-Pleistocene boundary. In complete Chinese loess sequences, 33 soil-loess couplets have been identified and labeled with Si-Li system. Traditionally, the base of Chinese loess has been designated at the bottom of loess unit L33. However, a typical loess layer within the uppermost ‘red clay’ is identified and formally designated here as L34 and the soil unit between L33 and L34 as S33, which lowers the basal age of Chinese loess from 2.6 Ma to w2.8 Ma. Although the Neogene ‘red clay’ and the overlying Quaternary loess are both of windblown origin, they have quite different pedogenic characteristics. The loess consists of alternating reddish soil and yellowish loess layers, while the ‘red clay’ is composed of soils with much redder colour and carbonate nodule layers. In general, the ‘‘red clay’’ has been subjected to stronger pedogenic processes than the overlying loess, and can be regarded as an extremely thick soil complex. The alternation of loess and soils reflects large-scale oscillations between glacial and interglacial conditions, whereas the well-developed ‘red clay’ deposition indicates a relatively stable climate condition. At w2.8 Ma, shortly before the Gauss/Matuyama boundary, the eolian sequences show a sedimentary transition from the well-weathered ‘red clay’ to the alternating loess and paleosol deposits, accompanied by coarser particle size, increased dust accumulation rates, and change of magnetic susceptibility signature. These indicate a large climatic shift from long-lasting warm-humid conditions to large-amplitude cold-dry and warm-humid fluctuations around 2.8 Ma, and a dramatic increase in aridity over the dust source region. In contrast, such a drastic Event is not identified around the top of the Olduvai Event or at any other time of the past 7.7 Ma. As the Gauss/Matuyama boundary offers a sound foundation for global time correlation and chronostratigraphic classification, these results still support the proposal of ICS and INQUA, that the conventional Pliocene-Pleistocene boundary should be lowered from 1.8 Ma to 2.6 Ma.
C.s. Grommé - One of the best experts on this subject based on the ideXlab platform.
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Geomagnetic polarity epochs: age and duration of the Olduvai normal polarity Event
Earth and Planetary Science Letters, 2002Co-Authors: C.s. GromméAbstract:Abstract New data show that the Olduvai normal geomagnetic polarity Event is represented in Olduvai Gorge, Tanzania, by rocks covering a time span of roughly from 0.1 to 0.2 my and is no older than 2.0 my. Hence the long normal polarity Event of this age that is seen in deep-sea sediment cores and in magnetic profiles over oceanic ridges should be called the Olduvai Event. The lava from which the GilsaEvent was defined may have been erupted during the Olduvai Event and, if so, the term Gilsashould now be abandoned. Many dated lavas that were originally assigned to the Olduvai Event represent one or two much shorter normal polarity Events that preceded the Olduvai Event; these are herein named the Reunion normal polarity Events. This revision brings the geomagnetic reversal time scale into conformity with the one implied by assumptions of uniform sedimentation rates on the ocean floor and uniform rates of sea-floor spreading.
Shiling Yang - One of the best experts on this subject based on the ideXlab platform.
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drastic climatic shift at 2 8ma as recorded in eolian deposits of china and its implications for redefining the pliocene pleistocene boundary
Quaternary International, 2010Co-Authors: Shiling Yang, Zhongli DingAbstract:There has been a long-running debate over the position of the Pliocene-Pleistocene boundary, i.e. 1.8 Ma versus 2.6 Ma. This study presents litho- and pedo-stratigraphic characteristics, grain size, magnetic susceptibility, and dust sedimentation rates for ‘red clay’-loess sequences over northern China, in order to assess the drastic climatic Event(s) recorded in the quasi-continuous eolian deposits, and to readdress the position of the Pliocene-Pleistocene boundary. In complete Chinese loess sequences, 33 soil-loess couplets have been identified and labeled with Si-Li system. Traditionally, the base of Chinese loess has been designated at the bottom of loess unit L33. However, a typical loess layer within the uppermost ‘red clay’ is identified and formally designated here as L34 and the soil unit between L33 and L34 as S33, which lowers the basal age of Chinese loess from 2.6 Ma to w2.8 Ma. Although the Neogene ‘red clay’ and the overlying Quaternary loess are both of windblown origin, they have quite different pedogenic characteristics. The loess consists of alternating reddish soil and yellowish loess layers, while the ‘red clay’ is composed of soils with much redder colour and carbonate nodule layers. In general, the ‘‘red clay’’ has been subjected to stronger pedogenic processes than the overlying loess, and can be regarded as an extremely thick soil complex. The alternation of loess and soils reflects large-scale oscillations between glacial and interglacial conditions, whereas the well-developed ‘red clay’ deposition indicates a relatively stable climate condition. At w2.8 Ma, shortly before the Gauss/Matuyama boundary, the eolian sequences show a sedimentary transition from the well-weathered ‘red clay’ to the alternating loess and paleosol deposits, accompanied by coarser particle size, increased dust accumulation rates, and change of magnetic susceptibility signature. These indicate a large climatic shift from long-lasting warm-humid conditions to large-amplitude cold-dry and warm-humid fluctuations around 2.8 Ma, and a dramatic increase in aridity over the dust source region. In contrast, such a drastic Event is not identified around the top of the Olduvai Event or at any other time of the past 7.7 Ma. As the Gauss/Matuyama boundary offers a sound foundation for global time correlation and chronostratigraphic classification, these results still support the proposal of ICS and INQUA, that the conventional Pliocene-Pleistocene boundary should be lowered from 1.8 Ma to 2.6 Ma.
Polat Wiers Gülsinem - One of the best experts on this subject based on the ideXlab platform.
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Palaeomagnetism och magnetisk anisotropi av Natronsjöns vulkano-sedimentära bergarter, norra Tanzania
Uppsala universitet Institutionen för geovetenskaper, 2019Co-Authors: Polat Wiers GülsinemAbstract:The East African Rift System diverges in the Lake Natron Basin of Northern Tanzania and is a major zone of continental extension and crustal thinning with resulting in active tectonics and volcanism. The discovery of Acheulean technology in Olduvai Gorge and Peninj as well as the presence of significant volcanic centers, has made in the region subject to studies in various disciplines. However, lack of precise radiometric age constraints due to the complex geology of the region is a major drawback. The basin is bordered on the western side by an escarpment that contains thick sequences of volcanic (nephelinites, basanites, hawaiites, alkali basalts), volcaniclastic and lacustrine strata that predates 1.2 Ma. This thesis is based on 41 rock samples that were collected from two geological sections, the Endukai Kete (EK) and Waterfall (WF) sections and aims to establish a preliminary geomagnetic polarity time scale (GPTS) for the Natron Escarpment, together with establishing possible flow directions of the volcanic lavas within these sections. Nephelinites of EK section have an inferred NW-SE direction of flow, based on study of anisotropy of magnetic susceptibility. They record a normal polarity that most likely correspond to the Cobb Mountain Event (CMT; 1.187-1.208 Ma), although there is an 80-ka discrepancy between the CMT Event and the dated lavas. The most probable source is the Mosonik that erupted nephelinitic lavas 1.28 Ma ago. The palagonitic tuff layer below the nephelinites displays reverse polarity and a NE-SW direction of flow. Due to the absence of approximately 200 m strata within the basanite series of the section, regional lithological correlation is used to constrain the GPTS pattern. Hajaro Beds of the Peninj Group to the north of the escarpment, postdates the Olduvai Event (1.71 to 1.86 Ma) and lacustrine strata of the escarpment for EK and WF sections are deposited over the same unconformity and share depositional similarities. Therefore, the lacustrine strata are correlative to Hajaro beds and the normal Event observed within the basanite series of both sections is attributed to the Réunion Event (2.116 – 2.137 Ma). The establishment of a preliminary magnetostratigraphic sequence presented in this thesis demonstrate that the rift escarpment in northern Tanzania is suitable for paleomagnetic dating. Future studies should be conducted to establish a more detailed and constrained magnetostratigraphic section, which will be of great use in this part of the African Rift where radiometric dating has been challenging
Tehquei Lee - One of the best experts on this subject based on the ideXlab platform.
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study of the polarity transition record of the upper Olduvai Event from wulochi sedimentary sequence of the coastal range eastern taiwan
Terrestrial Atmospheric and Oceanic Sciences, 1992Co-Authors: Tehquei LeeAbstract:This study presents the preliminary results of the upper Olduvai transition record analyzed from the Wulochi sedimentary sequence of the Coastal Range, eastern Taiwan. Variations of both the declination and paleolatitudes of VGP reveal an antisymmetric characteristic about the mid-point of the transition. However, variation of the VGP latitudes shows that the first half record is a three stage'stop and go' pattern similar to that of the same Event reported from Po river, Italy, by Tric et al. (1990) while the second half fluctuated more. The later part might also indicate that secular variation of the earth's magnetic field was still dominant during this transition. For the longitudes of the VGP path, they are found to be largely confined to a belt from meridian 200° to 250° centered around 230° which is about 40° west of the American continents. This result is dissimilar to many other paths which passed through them. However, the Coastal Range has undergone a clockwise rotation of about 30° during Plio-Pleistocene (Lee, 1989; Lee et al., 1990; 1991); the discrepancy might result from this clockwise rotation Event. After the correction of the rotation angle, it is found that the VGP path is much closer to the American continents. In addition, VGP path seems to show afar-field phenomenon and magnetite is identified to be the major magnetic mineral contained in our samples.
