Carina Hoorn | University of Amsterdam (original) (raw)
Books by Carina Hoorn
Papers by Carina Hoorn
Phytotaxa, 2015
Sabinaria magnifica is so far the only known species in the recently discovered tropical palm gen... more Sabinaria magnifica is so far the only known species in the recently discovered tropical palm genus Sabinaria (Arecaceae). Here we present a complete description of the pollen morphology of this palm species based on light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We also made SEM-based comparisons of Sabinaria with other genera within the tribe Cryosophileae. Pollen grains of Sabinaria magnifica resemble the other genera in the heteropolar, slightly asymmetric monads, and the monosulcate and tectate exine with perforate surface. Nevertheless, there are some clear differences with Thrinax, Chelyocarpus and Cryosophila in terms of aperture and exine. S. magnifica differs from its closest relative, Itaya amicorum, in the exine structure. This study shows that a combination of microscope techniques is essential for the identification of different genera within the Cryosophileae and may also be a necessary when working with other palynologically less distinct palm genera.
Species composition in the present-day Amazonian heartland has an imprint of past marine influenc... more Species composition in the present-day Amazonian heartland has an imprint of past marine influence. The exact nature, timing and extent of this marine influence, however, are largely unresolved. Here we use calcareous tests of foraminifera and marine palynomorphs from Miocene sediments in northwestern Amazonia to extend on current estimates for salinity ranges, paleoenvironments and paleogeography. Our samples mostly contain tests and/or organic linings of euryhaline (tolerant to a wide range of salinity) foraminifera of the genera Ammonia, Trochammina and Elphidium, with Ammonia being by far the dominant genus at all locations. Organic-walled dinoflagellate cysts (dinocysts), such as Spiniferites, Brigantedinium and Tuberculodinium vancampoae, are also common at a number of sites. This association of foraminifera and dinocyst taxa points at varying salinities, with aberrant forms of Ammonia indicating lower limits of 0-10 psu (practical salinity units) whereas dinocyst associations suggest more marine conditions. Such regional heterogeneity is common at the interface of shallow marine to freshwater environments, like estuaries. We conclude that during the early and middle Miocene marginal marine conditions reached at least 2000 km inland from the Caribbean portal. Global high sea level and fast subsidence in the sub-Andean zone are thought to be the controlling mechanisms of the marine incursions. Lowering of global sea level and a change in tectonic regime terminated the incursions in the course of the Plio-Pleistocene.
Climate change follows from the interaction between global atmospheric and oceanic processes with... more Climate change follows from the interaction between global atmospheric and oceanic processes with regional processes. In this chapter we review which factors determined climate evolution in Amazonia and the Brazilian Northeast and present a recompilation of Neogene palynological and paleobotanical records. These records suggest generally warm and humid conditions throughout this region during the Neogene. The uplift of the Andes, in the western half of South America, created a pattern of intense orographic precipitation in Western Amazonia that derived from humid air masses of Atlantic origin. During this time interval uplift of the Borborema Plateau, in the northern Brazilian Highlands, also formed an orographic barrier that allowed the development of a lush forest on the Atlantic side (Mata Atlantica), while rain shadow on the western flank gave place to the semi-arid Caatinga vegetation. The arid conditions in the Brazilian Northeast were further exacerbated by the evolution of the oceanic current system in the Atlantic, a system that was also responsible for the creation of the Namib Desert. The palynological and paleobotanical data illustrate the long history of Amazonia's tropical forest, but also the influence of the Andes formation on this region. The limited paleobotanical remains from the Brazilian Northeast tentatively point at longstanding humid conditions that, in the latter part of the Neogene transformed into the predominantly arid conditions that characterize the area at present.
Global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transi... more Global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transitional interval during the Eocene epoch characterized by incipient polar ice-sheet formation as well as shortlived warming events, of which the Middle Eocene Climatic Optimum (MECO) is most noticeable. Understanding this critical period requires high-resolution records that are being gathered in marine basins, but are still lacking in the terrestrial realm. Here, we provide a precisely-dated terrestrial record crossing the MECO time interval from the Xining Basin (NW China). We document a rapid aridification step and the onset of obliquity-dominated climate cyclicity indicated by lithofacies and pollen records dated at 40.0 Ma at the base of magnetochron C18n.2n. This shift is concomitant -within error -with the MECO peak warming in Ocean Drilling Program Site 1258 for which we reassessed the magnetostratigraphic age at 40.0 Ma (also at base of magnetochron C18n.2n). The rapidity of the shift observed in the Xining Basin and the region-wide aridification and monsoonal intensification reported around 40 Ma suggests Asian paleoenvironments were responding to global climate changes associated with the MECO. However, the Xining records show only the permanent shift but not the transient peak warming observed in marine MECO records. We thus relate this permanent aridification to occur during the post-MECO cooling. We propose the mechanisms linking global climate to Asian paleoenvironments may be eustatic fluctuations driving the stepwise retreat of the proto-Paratethys epicontinental sea or simply global cooling reducing moisture supply to the continental interior. In any case, Eocene global climate cooling from greenhouse to icehouse conditions seem to have played a primary role in shaping Asian paleoenvironments.
Hovikoski et al.(2005) acknowledge that their results contradict Campbell et al.(2000) who descri... more Hovikoski et al.(2005) acknowledge that their results contradict Campbell et al.(2000) who described the Madre de Dios Formation in SW Amazonia as the last major cycle of Cenozoic continental deposition. Yet they propose that their rhythmite statistics settle the debate (also with Hoorn, 1996) that the sediments are of marine origin. However, Hoorn et al. believe Hovikoski et al. have not provided sufficient proof for the existence of a long-lived marine connection between Amazonia and the sea during the Late Miocene.
Abstract Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collisi... more Abstract Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collision is held responsible for Asian aridification and monsoon intensification, but may also have gradually cooled global climate, leading to the 34 Ma Eocene-Oligocene transition. To unravel the interplay between Tibetan uplift and global climate, proxy records of Asian paleoenvironments constrained by accurate age models are needed for the Paleogene Period.
Abstract Northern South America harbours a highly diversified forest vegetation. However, it is n... more Abstract Northern South America harbours a highly diversified forest vegetation. However, it is not clear when this remarkable diversity was attained and how it was produced. Is the high diversity the product of a positive speciation–extinction balance that accumulated species over long time periods, or is it the product of high origination rates over short time periods, or both? Middle Cretaceous floras, although very poorly studied, are dominated by non-angiosperm taxa.
Abstract New biostratigraphic, isotopic, and well log data from exploration wells on the outer co... more Abstract New biostratigraphic, isotopic, and well log data from exploration wells on the outer continental shelf and uppermost Amazon deep-sea fan, Brazil, reveal that the Amazon River was initiated as a transcontinental river between 11.8 and 11.3 Ma ago (middle to late Miocene), and reached its present shape and size during the late Pliocene. Prior to the late Miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the Proterozoic basement in eastern Amazonia.
ON 17 NOVEMBER 2010, PRESIDENT OBAMA PRESENTED THE National Medals of Science and the National Me... more ON 17 NOVEMBER 2010, PRESIDENT OBAMA PRESENTED THE National Medals of Science and the National Medals of Technology and Innovation. These medals are the highest honor that the nation can bestow in science and technology, yet they are rarely mentioned by the popular media. Because Congress does not appropriate funds to implement the “outreach” of these medals, for many years the only national recognition was a private award ceremony with the President.
Journal of South …, Jan 1, 2012
The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.
Journal of Asian Earth …, Jan 1, 2011
Here we report new sedimentological and paleontological data from a 603.5 m thick Neogene sequenc... more Here we report new sedimentological and paleontological data from a 603.5 m thick Neogene sequence (Woma section) in the Gyirong Basin, a basin induced by east-west extension in the Himalayas of southern Tibet. We document the conglomeratic Danzengzhukang Formation, at the base of the section, and the overlying finer grained Woma Formation that includes a Hipparion fauna. Based on stratigraphic correlations and earlier thermochronology and magnetostratigraphic results, we bracket the depositional age of this section between 10.8 Ma and 1.7 Ma. Lithology, paleo-current directions and provenance analysis, together with palynological and paleontological data, have revealed three depositional environments for the deposition of the studied section. (1) Alluvial-fan to braided river environments with ESE transport directions (Danzengzhukang Formation, <10.8 to 7.2Ma)wereassociatedwithawarmandhumidconiferous−andbroad−leavedmixedforest.(2)Lacustrinedominatedconditions(LowerWomaFormation,7.2 Ma) were associated with a warm and humid coniferous-and broad-leaved mixed forest. (2) Lacustrine dominated conditions (Lower Woma Formation, 7.2Ma)wereassociatedwithawarmandhumidconiferous−andbroad−leavedmixedforest.(2)Lacustrinedominatedconditions(LowerWomaFormation,7.2 to 3.2 Ma) with WSW transport directions were associated with locally warm and humid environments in the low-lying areas while input from a new source area suggests the presence of a high-altitude, cold and arid deciduous coniferous-leaved forests. (3) A fan delta dominated environment (Upper Woma Formation, 3.2 to >1.7 Ma) with increased denudation and WSW paleo-currents was associated with a deciduous coniferous and broad-leaved mixed forest that suggests an increase in climate variability. Our data indicate that the Gyirong Basin was under overall warm and humid conditions throughout most of its history, in agreement with high-resolution oxygen and carbon isotope data collected from the same section (this issue). We interpret our warm climate in the Gyirong Basin to reflect the prevalence of the monsoonal influence and the distal pollen sources to result from orographic effects.
Palaeogeography, …, Jan 1, 2012
Climate models suggest that Asian paleoenvironments, monsoons and continental aridification were ... more Climate models suggest that Asian paleoenvironments, monsoons and continental aridification were primarily governed by tectonic uplift and sea retreat since the Eocene with potential contribution of global climate changes. However, the cause and timing of these paleoenvironmental changes remain poorly constrained. The recently well-dated continental mudflat to ephemeral saline lake sedimentary succession, situated in the Xining Basin at the northeastern margin of the Tibetan Plateau (NW China), provides a unique opportunity to develop additional proxy successions in this area that are placed accurately in time. Here, a palynological record from this succession is reported. High abundances of desert and steppe-desert taxa such as Ephedripites and Nitrariadites/Nitraripollis are found, which can be differentiated by the presence of broad leaved deciduous forest taxa in the lower part of the section (particularly up to 36.4 Ma; magnetochron C16r), and a sudden increase of Pinaceae (Pinuspollenites, Piceaepollenites and Abiespollenites) which is dated at 36.1 Ma (C16n.2n). Coexistence Approach (CoA) indicates that from 39.9 to 36.4 Ma (C17n.1n) regional climate was warm and wet, while from 36.4 to 33.5 Ma (C16n.2n-C13r) climate tends to be cooler and drier. The data indicate that paleoenvironmental and palynological changes on the NE part of the Tibetan Plateau resulted from a combination of long-term tectonic uplift forcing and long-and short-term climate changes. The increase of taxa such as Piceaepollenites and Abiespollenites indicates not only a cooling and drying trend prior to the Eocene/Oligocene (E/O) boundary, but also the existence of high altitude mountain habitats in the periphery of the Xining Basin. The sudden Pinaceae event correlates closely in time with a marked aridification step as viewed from the lithology of the Xining Basin that was linked to the sea retreat out of the Tarim Basin.
Palaeogeography, Palaeoclimatology, …, Jan 1, 2004
The coastal wetlands in Dhofar (Sultanate of Oman) provide a record of Late Holocene palaeoenviro... more The coastal wetlands in Dhofar (Sultanate of Oman) provide a record of Late Holocene palaeoenvironmental history. The age of the surface sediments at some selected sites in the estuaries, locally known as khawrs and qurms, ranges from 750 to 390 calendar years (cal y) BC to Present. Geomorphological data suggest that prior to 270 -420 cal y AD these estuaries were open to the sea and received a high input of fresh water from the mountains. At least from 270-420 cal y AD onwards, the physiography of the estuaries changed as a consequence of dwindling fresh water input from the mountains. As a result, the open estuaries changed into barrier dominated estuaries with periodical input of saline and fresh water. Palynological data indicate that the estuaries are mainly filled with (i) pollen and spores from the surrounding vegetation; (ii) a regional input of pollen and spores through surface and subsurface runoff from mountains, coastal plain and, wadis; (iii) pollen and spores from East Africa and the Indian subcontinent brought in by the monsoon. Between 750 -390 cal y BC and 270 -420 cal y AD the local input of khawr taxa prevailed whereas from 270 -420 cal y AD to Present coastal plain and wadi taxa prevail. Both geomorphological and palynological data suggest the ancient centers of trade such as Samhuram at Khawr Rawri (ca. 100 BC -100 AD) and Zafar at Khawr Al Balid (12th -15th centuries AD) were affected by important hydrological changes related to desertification of the region as a consequence of a weakening SW (summer) monsoon. D
SIDALC - Servicio de Informacion y Documentacion Agropecuaria de las Americas.
Palaios, Jan 1, 2006
The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sedime... more The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sediments of Middle to Late Miocene age. These sediments, here informally called Apaporis sand unit, are in nonconformable contact with the Precambrian basement, and were deposited in a low-sinuosity fluvial system with an anastomosing character that originated in the Guyana Shield. The predominantly sandy unit has organic-rich clay intervals that contain a palynological assemblage dominated by the mangrove Zonocostites ramonae (Rhizophora) and the palm Mauritiidites franciscoi (Mauritia). Abundant Zonocostites (25-85%), together with the occurrence of marine palynomorphs (dinoflagellates and foraminiferal inner-wall linings), indicate the presence of well-developed coastal mangrove forests and marine incursions. Occasional decrease of Zonocostites in favor of Mauritiidites suggests that the coastline fluctuated and the mangroves were replaced by palm vegetation. The Middle to Late Miocene age of these sediments is based on presence of the palynological marker species Grimsdalea magnaclavata and absence of the older biostratigraphic marker Crassoretitriletes vanraadshoovenii and the younger Asteraceae. This makes the unit equivalent in age to the upper Pebas/Solimões Formation. The marine ingression and coastal conditions in the heart of Amazonia possibly are related to a combination of global sea-level rise (Serravallian?) and subsidence in the periphery of the Guyana Shield. Modern analogues of the Miocene Apaporis fluvial/ coastal interface are present-day fluvial and coastal environments in Surinam and the Guyanas.
Scientific American, Jan 1, 2006
Phytotaxa, 2015
Sabinaria magnifica is so far the only known species in the recently discovered tropical palm gen... more Sabinaria magnifica is so far the only known species in the recently discovered tropical palm genus Sabinaria (Arecaceae). Here we present a complete description of the pollen morphology of this palm species based on light microscopy (LM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). We also made SEM-based comparisons of Sabinaria with other genera within the tribe Cryosophileae. Pollen grains of Sabinaria magnifica resemble the other genera in the heteropolar, slightly asymmetric monads, and the monosulcate and tectate exine with perforate surface. Nevertheless, there are some clear differences with Thrinax, Chelyocarpus and Cryosophila in terms of aperture and exine. S. magnifica differs from its closest relative, Itaya amicorum, in the exine structure. This study shows that a combination of microscope techniques is essential for the identification of different genera within the Cryosophileae and may also be a necessary when working with other palynologically less distinct palm genera.
Species composition in the present-day Amazonian heartland has an imprint of past marine influenc... more Species composition in the present-day Amazonian heartland has an imprint of past marine influence. The exact nature, timing and extent of this marine influence, however, are largely unresolved. Here we use calcareous tests of foraminifera and marine palynomorphs from Miocene sediments in northwestern Amazonia to extend on current estimates for salinity ranges, paleoenvironments and paleogeography. Our samples mostly contain tests and/or organic linings of euryhaline (tolerant to a wide range of salinity) foraminifera of the genera Ammonia, Trochammina and Elphidium, with Ammonia being by far the dominant genus at all locations. Organic-walled dinoflagellate cysts (dinocysts), such as Spiniferites, Brigantedinium and Tuberculodinium vancampoae, are also common at a number of sites. This association of foraminifera and dinocyst taxa points at varying salinities, with aberrant forms of Ammonia indicating lower limits of 0-10 psu (practical salinity units) whereas dinocyst associations suggest more marine conditions. Such regional heterogeneity is common at the interface of shallow marine to freshwater environments, like estuaries. We conclude that during the early and middle Miocene marginal marine conditions reached at least 2000 km inland from the Caribbean portal. Global high sea level and fast subsidence in the sub-Andean zone are thought to be the controlling mechanisms of the marine incursions. Lowering of global sea level and a change in tectonic regime terminated the incursions in the course of the Plio-Pleistocene.
Climate change follows from the interaction between global atmospheric and oceanic processes with... more Climate change follows from the interaction between global atmospheric and oceanic processes with regional processes. In this chapter we review which factors determined climate evolution in Amazonia and the Brazilian Northeast and present a recompilation of Neogene palynological and paleobotanical records. These records suggest generally warm and humid conditions throughout this region during the Neogene. The uplift of the Andes, in the western half of South America, created a pattern of intense orographic precipitation in Western Amazonia that derived from humid air masses of Atlantic origin. During this time interval uplift of the Borborema Plateau, in the northern Brazilian Highlands, also formed an orographic barrier that allowed the development of a lush forest on the Atlantic side (Mata Atlantica), while rain shadow on the western flank gave place to the semi-arid Caatinga vegetation. The arid conditions in the Brazilian Northeast were further exacerbated by the evolution of the oceanic current system in the Atlantic, a system that was also responsible for the creation of the Namib Desert. The palynological and paleobotanical data illustrate the long history of Amazonia's tropical forest, but also the influence of the Andes formation on this region. The limited paleobotanical remains from the Brazilian Northeast tentatively point at longstanding humid conditions that, in the latter part of the Neogene transformed into the predominantly arid conditions that characterize the area at present.
Global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transi... more Global climate cooling from greenhouse to icehouse conditions occurred across an enigmatic transitional interval during the Eocene epoch characterized by incipient polar ice-sheet formation as well as shortlived warming events, of which the Middle Eocene Climatic Optimum (MECO) is most noticeable. Understanding this critical period requires high-resolution records that are being gathered in marine basins, but are still lacking in the terrestrial realm. Here, we provide a precisely-dated terrestrial record crossing the MECO time interval from the Xining Basin (NW China). We document a rapid aridification step and the onset of obliquity-dominated climate cyclicity indicated by lithofacies and pollen records dated at 40.0 Ma at the base of magnetochron C18n.2n. This shift is concomitant -within error -with the MECO peak warming in Ocean Drilling Program Site 1258 for which we reassessed the magnetostratigraphic age at 40.0 Ma (also at base of magnetochron C18n.2n). The rapidity of the shift observed in the Xining Basin and the region-wide aridification and monsoonal intensification reported around 40 Ma suggests Asian paleoenvironments were responding to global climate changes associated with the MECO. However, the Xining records show only the permanent shift but not the transient peak warming observed in marine MECO records. We thus relate this permanent aridification to occur during the post-MECO cooling. We propose the mechanisms linking global climate to Asian paleoenvironments may be eustatic fluctuations driving the stepwise retreat of the proto-Paratethys epicontinental sea or simply global cooling reducing moisture supply to the continental interior. In any case, Eocene global climate cooling from greenhouse to icehouse conditions seem to have played a primary role in shaping Asian paleoenvironments.
Hovikoski et al.(2005) acknowledge that their results contradict Campbell et al.(2000) who descri... more Hovikoski et al.(2005) acknowledge that their results contradict Campbell et al.(2000) who described the Madre de Dios Formation in SW Amazonia as the last major cycle of Cenozoic continental deposition. Yet they propose that their rhythmite statistics settle the debate (also with Hoorn, 1996) that the sediments are of marine origin. However, Hoorn et al. believe Hovikoski et al. have not provided sufficient proof for the existence of a long-lived marine connection between Amazonia and the sea during the Late Miocene.
Abstract Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collisi... more Abstract Uplift of the Tibetan Plateau and the Himalayas since the onset of the Indo-Asia collision is held responsible for Asian aridification and monsoon intensification, but may also have gradually cooled global climate, leading to the 34 Ma Eocene-Oligocene transition. To unravel the interplay between Tibetan uplift and global climate, proxy records of Asian paleoenvironments constrained by accurate age models are needed for the Paleogene Period.
Abstract Northern South America harbours a highly diversified forest vegetation. However, it is n... more Abstract Northern South America harbours a highly diversified forest vegetation. However, it is not clear when this remarkable diversity was attained and how it was produced. Is the high diversity the product of a positive speciation–extinction balance that accumulated species over long time periods, or is it the product of high origination rates over short time periods, or both? Middle Cretaceous floras, although very poorly studied, are dominated by non-angiosperm taxa.
Abstract New biostratigraphic, isotopic, and well log data from exploration wells on the outer co... more Abstract New biostratigraphic, isotopic, and well log data from exploration wells on the outer continental shelf and uppermost Amazon deep-sea fan, Brazil, reveal that the Amazon River was initiated as a transcontinental river between 11.8 and 11.3 Ma ago (middle to late Miocene), and reached its present shape and size during the late Pliocene. Prior to the late Miocene the continental shelf was a carbonate platform that received moderate siliciclastic sediment supply from the Proterozoic basement in eastern Amazonia.
ON 17 NOVEMBER 2010, PRESIDENT OBAMA PRESENTED THE National Medals of Science and the National Me... more ON 17 NOVEMBER 2010, PRESIDENT OBAMA PRESENTED THE National Medals of Science and the National Medals of Technology and Innovation. These medals are the highest honor that the nation can bestow in science and technology, yet they are rarely mentioned by the popular media. Because Congress does not appropriate funds to implement the “outreach” of these medals, for many years the only national recognition was a private award ceremony with the President.
Journal of South …, Jan 1, 2012
The user has requested enhancement of the downloaded file. All in-text references underlined in b... more The user has requested enhancement of the downloaded file. All in-text references underlined in blue are added to the original document and are linked to publications on ResearchGate, letting you access and read them immediately.
Journal of Asian Earth …, Jan 1, 2011
Here we report new sedimentological and paleontological data from a 603.5 m thick Neogene sequenc... more Here we report new sedimentological and paleontological data from a 603.5 m thick Neogene sequence (Woma section) in the Gyirong Basin, a basin induced by east-west extension in the Himalayas of southern Tibet. We document the conglomeratic Danzengzhukang Formation, at the base of the section, and the overlying finer grained Woma Formation that includes a Hipparion fauna. Based on stratigraphic correlations and earlier thermochronology and magnetostratigraphic results, we bracket the depositional age of this section between 10.8 Ma and 1.7 Ma. Lithology, paleo-current directions and provenance analysis, together with palynological and paleontological data, have revealed three depositional environments for the deposition of the studied section. (1) Alluvial-fan to braided river environments with ESE transport directions (Danzengzhukang Formation, <10.8 to 7.2Ma)wereassociatedwithawarmandhumidconiferous−andbroad−leavedmixedforest.(2)Lacustrinedominatedconditions(LowerWomaFormation,7.2 Ma) were associated with a warm and humid coniferous-and broad-leaved mixed forest. (2) Lacustrine dominated conditions (Lower Woma Formation, 7.2Ma)wereassociatedwithawarmandhumidconiferous−andbroad−leavedmixedforest.(2)Lacustrinedominatedconditions(LowerWomaFormation,7.2 to 3.2 Ma) with WSW transport directions were associated with locally warm and humid environments in the low-lying areas while input from a new source area suggests the presence of a high-altitude, cold and arid deciduous coniferous-leaved forests. (3) A fan delta dominated environment (Upper Woma Formation, 3.2 to >1.7 Ma) with increased denudation and WSW paleo-currents was associated with a deciduous coniferous and broad-leaved mixed forest that suggests an increase in climate variability. Our data indicate that the Gyirong Basin was under overall warm and humid conditions throughout most of its history, in agreement with high-resolution oxygen and carbon isotope data collected from the same section (this issue). We interpret our warm climate in the Gyirong Basin to reflect the prevalence of the monsoonal influence and the distal pollen sources to result from orographic effects.
Palaeogeography, …, Jan 1, 2012
Climate models suggest that Asian paleoenvironments, monsoons and continental aridification were ... more Climate models suggest that Asian paleoenvironments, monsoons and continental aridification were primarily governed by tectonic uplift and sea retreat since the Eocene with potential contribution of global climate changes. However, the cause and timing of these paleoenvironmental changes remain poorly constrained. The recently well-dated continental mudflat to ephemeral saline lake sedimentary succession, situated in the Xining Basin at the northeastern margin of the Tibetan Plateau (NW China), provides a unique opportunity to develop additional proxy successions in this area that are placed accurately in time. Here, a palynological record from this succession is reported. High abundances of desert and steppe-desert taxa such as Ephedripites and Nitrariadites/Nitraripollis are found, which can be differentiated by the presence of broad leaved deciduous forest taxa in the lower part of the section (particularly up to 36.4 Ma; magnetochron C16r), and a sudden increase of Pinaceae (Pinuspollenites, Piceaepollenites and Abiespollenites) which is dated at 36.1 Ma (C16n.2n). Coexistence Approach (CoA) indicates that from 39.9 to 36.4 Ma (C17n.1n) regional climate was warm and wet, while from 36.4 to 33.5 Ma (C16n.2n-C13r) climate tends to be cooler and drier. The data indicate that paleoenvironmental and palynological changes on the NE part of the Tibetan Plateau resulted from a combination of long-term tectonic uplift forcing and long-and short-term climate changes. The increase of taxa such as Piceaepollenites and Abiespollenites indicates not only a cooling and drying trend prior to the Eocene/Oligocene (E/O) boundary, but also the existence of high altitude mountain habitats in the periphery of the Xining Basin. The sudden Pinaceae event correlates closely in time with a marked aridification step as viewed from the lithology of the Xining Basin that was linked to the sea retreat out of the Tarim Basin.
Palaeogeography, Palaeoclimatology, …, Jan 1, 2004
The coastal wetlands in Dhofar (Sultanate of Oman) provide a record of Late Holocene palaeoenviro... more The coastal wetlands in Dhofar (Sultanate of Oman) provide a record of Late Holocene palaeoenvironmental history. The age of the surface sediments at some selected sites in the estuaries, locally known as khawrs and qurms, ranges from 750 to 390 calendar years (cal y) BC to Present. Geomorphological data suggest that prior to 270 -420 cal y AD these estuaries were open to the sea and received a high input of fresh water from the mountains. At least from 270-420 cal y AD onwards, the physiography of the estuaries changed as a consequence of dwindling fresh water input from the mountains. As a result, the open estuaries changed into barrier dominated estuaries with periodical input of saline and fresh water. Palynological data indicate that the estuaries are mainly filled with (i) pollen and spores from the surrounding vegetation; (ii) a regional input of pollen and spores through surface and subsurface runoff from mountains, coastal plain and, wadis; (iii) pollen and spores from East Africa and the Indian subcontinent brought in by the monsoon. Between 750 -390 cal y BC and 270 -420 cal y AD the local input of khawr taxa prevailed whereas from 270 -420 cal y AD to Present coastal plain and wadi taxa prevail. Both geomorphological and palynological data suggest the ancient centers of trade such as Samhuram at Khawr Rawri (ca. 100 BC -100 AD) and Zafar at Khawr Al Balid (12th -15th centuries AD) were affected by important hydrological changes related to desertification of the region as a consequence of a weakening SW (summer) monsoon. D
SIDALC - Servicio de Informacion y Documentacion Agropecuaria de las Americas.
Palaios, Jan 1, 2006
The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sedime... more The lower Apaporis River area (Colombian Amazonia) is characterized by fluvial and coastal sediments of Middle to Late Miocene age. These sediments, here informally called Apaporis sand unit, are in nonconformable contact with the Precambrian basement, and were deposited in a low-sinuosity fluvial system with an anastomosing character that originated in the Guyana Shield. The predominantly sandy unit has organic-rich clay intervals that contain a palynological assemblage dominated by the mangrove Zonocostites ramonae (Rhizophora) and the palm Mauritiidites franciscoi (Mauritia). Abundant Zonocostites (25-85%), together with the occurrence of marine palynomorphs (dinoflagellates and foraminiferal inner-wall linings), indicate the presence of well-developed coastal mangrove forests and marine incursions. Occasional decrease of Zonocostites in favor of Mauritiidites suggests that the coastline fluctuated and the mangroves were replaced by palm vegetation. The Middle to Late Miocene age of these sediments is based on presence of the palynological marker species Grimsdalea magnaclavata and absence of the older biostratigraphic marker Crassoretitriletes vanraadshoovenii and the younger Asteraceae. This makes the unit equivalent in age to the upper Pebas/Solimões Formation. The marine ingression and coastal conditions in the heart of Amazonia possibly are related to a combination of global sea-level rise (Serravallian?) and subsidence in the periphery of the Guyana Shield. Modern analogues of the Miocene Apaporis fluvial/ coastal interface are present-day fluvial and coastal environments in Surinam and the Guyanas.
Scientific American, Jan 1, 2006
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The composition and distribution of organic matter in sediments of the Amazon Fan during the Plei... more The composition and distribution of organic matter in sediments of the Amazon Fan during the Pleistocene glacial/interglacial sequences has been studied using samples from Holes 940A, 944A, and 946A (Ocean Drilling Program Leg 155). The results show that glacial lowstand deposits are characterized by large concentrations of a palynological assemblage composed of Andean and tropical lowland taxa, minor amounts of reworked Paleozoic, Cretaceous, and Tertiary taxa, and large volumes of organic debris (e.g., wood, vessels, epidermis, and cuticles). In general, these intervals do not contain many taxa diagnostic for the lowland savannah areas. In all the studied sites, there is an increase of ferns, fern allies, reworked and corroded palynomorphs, and fungal spores at the time of the last glacial.