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Daniel D. Marshall - One of the best experts on this subject based on the ideXlab platform.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Precambrian Research, 2013Co-Authors: Francesca Furlanetto, Derek J Thorkelson, Daniel D. Marshall, Robert H. Rainbird, H. Daniel Gibson, William J. Davis, James L. Crowley, Jeffrey D. VervoortAbstract:a b s t r a c t The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with differ- ent scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obsta- cle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its pos- sible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplace- ment of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab vol- canics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimen- tary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present.
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The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia
Precambrian Research, 2013Co-Authors: Alexander B. Nielsen, Derek J Thorkelson, H. Daniel Gibson, Daniel D. MarshallAbstract:Abstract The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U–Pb titanite at 1599 Ma. Four U–Pb zircon ages on the Wernecke igneous clasts (1714–1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (
Derek J Thorkelson - One of the best experts on this subject based on the ideXlab platform.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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Paleoproterozoic closure of an Australia–Laurentia seaway revealed by megaclasts of an obducted volcanic arc in Yukon, Canada
Gondwana Research, 2016Co-Authors: Derek J Thorkelson, John R. LaughtonAbstract:Abstract The Slab volcanics are a late Paleoproterozoic volcanic succession in the Wernecke Mountains of Yukon Territory in northwestern Canada. Fragments of the succession are preserved as megaclasts in km-scale zones of hydrothermal breccia. The largest clast is 160 × 380 m and consists of 31 mafic lava flows and minor intercalated sandstone and tephra. Hydrothermal activity during the brecciation led to extensive metasomatic alteration, both sodic and potassic. Despite the alteration, an igneous geochemical signal is discernible. The rocks are mafic to intermediate and moderately alkalic. Their trace element profiles indicate derivation from a hydrated, enriched mantle source, consistent with an origin as a volcanic arc affected by a plume, rift or slab window. Major and trace element patterns demonstrate a cyclical magmatic evolution involving intervals of fractional crystallization punctuated by recharge with mafic magma. The Slab volcanics were previously thought to have been deposited on Laurentia but are herein regarded as part of the exotic terrane Bonnetia that was obducted onto the Laurentian continental margin in late Paleoproterozoic time. Bonnetia may have developed on or near the eastern margin of Australia as a continental or fringing arc. In that scenario, Bonnetia grew through arc magmatism as oceanic crust between Australia and Laurentia was consumed prior to terrane obduction and continental collision in the late Paleoproterozoic to early Mesoproterozoic. Sediment or tephra derived from the arc was transported westward into the interior of Australia, raising eNd values of late Paleoproterozoic sedimentary strata. Vigorous hydrothermal brecciation followed and led to fragmentation of the crust, including the obducted terrane Bonnetia. Giant blocks of the Slab volcanics and other units of Bonnetia foundered into breccia zones and moved downward for thousands of meters. Erosion removed the obducted terrane and the uppermost Laurentian crust, exposing the megaclasts.
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Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Precambrian Research, 2013Co-Authors: Francesca Furlanetto, Derek J Thorkelson, Daniel D. Marshall, Robert H. Rainbird, H. Daniel Gibson, William J. Davis, James L. Crowley, Jeffrey D. VervoortAbstract:a b s t r a c t The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with differ- ent scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obsta- cle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its pos- sible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplace- ment of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab vol- canics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimen- tary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present.
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The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia
Precambrian Research, 2013Co-Authors: Alexander B. Nielsen, Derek J Thorkelson, H. Daniel Gibson, Daniel D. MarshallAbstract:Abstract The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U–Pb titanite at 1599 Ma. Four U–Pb zircon ages on the Wernecke igneous clasts (1714–1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (
Safont Elisabet - One of the best experts on this subject based on the ideXlab platform.
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Flora and vegetation of the Guayana highlands : past dynamics, global warming and conservation guidelines; Flora i vegetació de les Terres de Guayana : dinàmica del passat, escalfament global i directrius de conservació
'Japanese Society of Applied Entomology & Zoology', 2017Co-Authors: Safont ElisabetAbstract:This thesis is aimed at the study of the flora and vegetation of the Guayana Highlands (GH), a set of -50 table mountains (called tepuis) located in the Guayana Shield of northern South America. Special emphasis is put on the potential upward migration of vascular plants as a response to global warming and the suggestion of conservation strategies. The tepui summits contain exceptional diversity and endemism of vascular plants, with more than 2400 known species, of which 30% would be endemics. In this study, the results of the Altitudinal Range Displacement analysis suggest that 30-50% of the GH endemic species could disappear due to total habitat loss by 2100 according to the most optimistic (B1) and pessimistic (A2) warming scenarios, respectively. An Environmental Impact Value has been calculated for each threatened species for a subsequent classification of these species into priority categories (10 for B1 and 13 for A2), which should be used in a chronological sequence to guide conservation and research actions. Subsequently, flora and vegetation approaches have been applied for the verification of the migratory phenomenon in the GH. Since this verification is based on the comparison of inventories conducted at different times, the first necessary step is to establish a baseline. The Roraima-tepui summit (2810 m elevation) has been selected for that purpose, as it is the best well-known tepui from the botanical point of view. The floristic survey of this tepuian summit has yielded 227 species, including 44 novelties since the publication of the Flora of the Venezuelan Guayana, one species new for science (Epidendrum sp. nov.), and 13 exotics, among which Poa annua and Polypogon elongatus (Poaceae) show the highest invasive potential. Five main plant communities have been identified: three herbaceous, one shrubland, and one forest formation, with their corresponding diagnostic species. An environmental characterization of these communities has been also performed. Finally, palaeoecological records may provide valuable information of the species' responses to past environmental changes, which is not available from short-term ecological observations. In this thesis, a palynological analysis of a sedimentary sequence from Uei-tepui (2150 m elevation) has been conducted to record the vegetation changes occurred in the last 2000 years in this tepui and to investigate the potential drivers involved. At -810 cal yr BP (AD 1140), a peak of fire incidence, coeval with a decrease in regional precipitation, reduced the woody elements of meadows (mainly Biophytum) and the Bonnetia cloud forests. Forest recovery to its previous abundance occurred slowly and elapsed several centuries to be completed after fire disturbance. In the mid-18th century, forests and meadows were replaced by Cyrilla racemiflora shrublands, coinciding with intense fires, synchronous with the arrival of Europeans to the Gran Sabana (GS) uplands. These results suggest that fire has been the main factor in the shaping of Uei¬tepui vegetation during the last two millennia. The documented local fire events were most likely caused by human activities and reached the summit from the GS, probably favoured by climatic conditions. The last local fire leading to forest clearing and the establishment of present-day vegetation most likely took place in the AD 1920s. The preservation of the GH flora in front of global warming should be addressed, at least partially, through ex situ conservation techniques, due to the nature of the threat (habitat loss).Proyectos BIOCON 08-188/09 y BIOCON 2004-90/05 (Fundación BBVA); CGL200907069-BOS (Ministerio de Ciencia e Innovación) CGL-200600974-BOS (Ministerio de Educación y CienciaPeer Reviewe
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Flora and vegetation of the Guayana highlands : past dynamics, global warming and conservation guidelines = Flora i vegetació de les Terres de Guayana : dinàmica del passat, escalfament global i directrius de conservació
Universitat de Barcelona, 2016Co-Authors: Safont ElisabetAbstract:This thesis is aimed at the study of the flora and vegetation of the Guayana Highlands (GH), a set of -50 table mountains (called tepuis) located in the Guayana Shield of northern South America. Special emphasis is put on the potential upward migration of vascular plants as a response to global warming and the suggestion of conservation strategies. The tepui summits contain exceptional diversity and endemism of vascular plants, with more than 2400 known species, of which 30% would be endemics. In this study, the results of the Altitudinal Range Displacement analysis suggest that 30-50% of the GH endemic species could disappear due to total habitat loss by 2100 according to the most optimistic (B1) and pessimistic (A2) warming scenarios, respectively. An Environmental Impact Value has been calculated for each threatened species for a subsequent classification of these species into priority categories (10 for B1 and 13 for A2), which should be used in a chronological sequence to guide conservation and research actions. Subsequently, flora and vegetation approaches have been applied for the verification of the migratory phenomenon in the GH. Since this verification is based on the comparison of inventories conducted at different times, the first necessary step is to establish a baseline. The Roraima-tepui summit (2810 m elevation) has been selected for that purpose, as it is the best well-known tepui from the botanical point of view. The floristic survey of this tepuian summit has yielded 227 species, including 44 novelties since the publication of the Flora of the Venezuelan Guayana, one species new for science (Epidendrum sp. nov.), and 13 exotics, among which Poa annua and Polypogon elongatus (Poaceae) show the highest invasive potential. Five main plant communities have been identified: three herbaceous, one shrubland, and one forest formation, with their corresponding diagnostic species. An environmental characterization of these communities has been also performed. Finally, palaeoecological records may provide valuable information of the species' responses to past environmental changes, which is not available from short-term ecological observations. In this thesis, a palynological analysis of a sedimentary sequence from Uei-tepui (2150 m elevation) has been conducted to record the vegetation changes occurred in the last 2000 years in this tepui and to investigate the potential drivers involved. At -810 cal yr BP (AD 1140), a peak of fire incidence, coeval with a decrease in regional precipitation, reduced the woody elements of meadows (mainly Biophytum) and the Bonnetia cloud forests. Forest recovery to its previous abundance occurred slowly and elapsed several centuries to be completed after fire disturbance. In the mid-18th century, forests and meadows were replaced by Cyrilla racemiflora shrublands, coinciding with intense fires, synchronous with the arrival of Europeans to the Gran Sabana (GS) uplands. These results suggest that fire has been the main factor in the shaping of Uei¬tepui vegetation during the last two millennia. The documented local fire events were most likely caused by human activities and reached the summit from the GS, probably favoured by climatic conditions. The last local fire leading to forest clearing and the establishment of present-day vegetation most likely took place in the AD 1920s. The preservation of the GH flora in front of global warming should be addressed, at least partially, through ex situ conservation techniques, due to the nature of the threat (habitat loss). However, the documentation in this study of other human impacts (trampling on vegetation, exotic species, changes in soil properties, etc.) and the emergence of fire as a potential disturbing agent of the GH biome highlight that in situ conservation through protected areas may also be very important. A management plan with the involvement of local indigenous communities should be implemented in the GH region (i.e., lowlands, uplands and highlands), in order to reduce the risks posed by global warming, human impacts, and the potential synergy among them.Aquesta tesi té com a objectiu l'estudi de la flora i la vegetació de les Terres Altes de Guayana (TAG), un conjunt de -50 muntanyes tabulars (anomenades tepuis) situades al nord d'Amèrica del Sud, amb especial èmfasi en la potencial migració altitudinal de les plantes vasculars com a resposta a l'escalfament global. Els cims dels tepuis contenen una diversitat i endemisme de plantes vasculars excepcional. Aquest estudi suggereix que el 30-50% de les espècies endèmiques de les TAG podrien desaparèixer per pèrdua total d'hàbitat cap a l'any 2100 segons els escenaris d'escalfament B1 (més optimista) i A2 (més pessimista), respectivament. S'ha calculat un Valor d'Impacte Ambiental per a cada espècie amenaçada per a la posterior classificació d'aquestes espècies en categories de prioritat (10 pel B1 i 13 per l'A2), les quals haurien de ser utilitzades en una seqüència cronològica per orientar les accions de conservació i recerca. Posteriorment, s'ha establert una línia base de diversitat florística, endemisme i vegetació al cim del Roraima-tepui per a la verificació del fenomen migratori a les TAG. L'inventari florístic d'aquest cim tepuià conté 227 espècies, incloent 44 nous registres, una espècie nova per a la ciència i 13 espècies exòtiques. S'han identificat cinc tipus de comunitats vegetals principals, amb les seves corresponents espècies diagnòstiques. També s'ha dut a terme una caracterització ambiental d'aquestes comunitats. Finalment, l'estudi paleoecològic de la seqüència sedimentària del Uei-tepui ha permès documentar la resposta de les espècies als canvis ambientals dels últims 2000 anys. Els resultats suggereixen que el foc ha estat el principal factor que ha modelat la vegetació del Uei durant aquest període. Els esdeveniments de focs locals molt probablement van ser causats per activitats humanes i van assolir el cim des de la Gran Sabana afavorits per les condicions climàtiques. La preservació de la flora de les TAG enfront de l'escalfament global s'hauria d'adreçar, com a mínim parcialment, mitjançant tècniques de conservació ex situ, a causa de la naturalesa de l'amenaça (pèrdua d'hàbitat). No obstant, la documentació en aquest estudi d'altres impactes humans (trepig de la vegetació, espècies exòtiques, canvis en les propietats del sòl, etc.) i l'aparició del foc com a potencial agent pertorbador del bioma tepuià posen de manifest que la conservació in situ mitjançant àrees protegides pot ser també molt important
Elisabet Safont Crespo - One of the best experts on this subject based on the ideXlab platform.
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Flora and vegetation of the Guayana highlands : past dynamics, global warming and conservation guidelines = Flora i vegetació de les Terres de Guayana : dinàmica del passat, escalfament global i directrius de conservació
2016Co-Authors: Elisabet Safont CrespoAbstract:This thesis is aimed at the study of the flora and vegetation of the Guayana Highlands (GH), a set of -50 table mountains (called tepuis) located in the Guayana Shield of northern South America. Special emphasis is put on the potential upward migration of vascular plants as a response to global warming and the suggestion of conservation strategies. The tepui summits contain exceptional diversity and endemism of vascular plants, with more than 2400 known species, of which 30% would be endemics. In this study, the results of the Altitudinal Range Displacement analysis suggest that 30-50% of the GH endemic species could disappear due to total habitat loss by 2100 according to the most optimistic (B1) and pessimistic (A2) warming scenarios, respectively. An Environmental Impact Value has been calculated for each threatened species for a subsequent classification of these species into priority categories (10 for B1 and 13 for A2), which should be used in a chronological sequence to guide conservation and research actions. Subsequently, flora and vegetation approaches have been applied for the verification of the migratory phenomenon in the GH. Since this verification is based on the comparison of inventories conducted at different times, the first necessary step is to establish a baseline. The Roraima-tepui summit (2810 m elevation) has been selected for that purpose, as it is the best well-known tepui from the botanical point of view. The floristic survey of this tepuian summit has yielded 227 species, including 44 novelties since the publication of the Flora of the Venezuelan Guayana, one species new for science (Epidendrum sp. nov.), and 13 exotics, among which Poa annua and Polypogon elongatus (Poaceae) show the highest invasive potential. Five main plant communities have been identified: three herbaceous, one shrubland, and one forest formation, with their corresponding diagnostic species. An environmental characterization of these communities has been also performed. Finally, palaeoecological records may provide valuable information of the species' responses to past environmental changes, which is not available from short-term ecological observations. In this thesis, a palynological analysis of a sedimentary sequence from Uei-tepui (2150 m elevation) has been conducted to record the vegetation changes occurred in the last 2000 years in this tepui and to investigate the potential drivers involved. At -810 cal yr BP (AD 1140), a peak of fire incidence, coeval with a decrease in regional precipitation, reduced the woody elements of meadows (mainly Biophytum) and the Bonnetia cloud forests. Forest recovery to its previous abundance occurred slowly and elapsed several centuries to be completed after fire disturbance. In the mid-18th century, forests and meadows were replaced by Cyrilla racemiflora shrublands, coinciding with intense fires, synchronous with the arrival of Europeans to the Gran Sabana (GS) uplands. These results suggest that fire has been the main factor in the shaping of Uei¬tepui vegetation during the last two millennia. The documented local fire events were most likely caused by human activities and reached the summit from the GS, probably favoured by climatic conditions. The last local fire leading to forest clearing and the establishment of present-day vegetation most likely took place in the AD 1920s. The preservation of the GH flora in front of global warming should be addressed, at least partially, through ex situ conservation techniques, due to the nature of the threat (habitat loss). However, the documentation in this study of other human impacts (trampling on vegetation, exotic species, changes in soil properties, etc.) and the emergence of fire as a potential disturbing agent of the GH biome highlight that in situ conservation through protected areas may also be very important. A management plan with the involvement of local indigenous communities should be implemented in the GH region (i.e., lowlands, uplands and highlands), in order to reduce the risks posed by global warming, human impacts, and the potential synergy among them.
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Flora and vegetation of the guayana highlands: past dynamics, global warming and conservation guidelines
2016Co-Authors: Elisabet Safont CrespoAbstract:Aquesta tesi te com a objectiu l'estudi de la flora i la vegetacio de les Terres Altes de Guayana (TAG), un conjunt de -50 muntanyes tabulars (anomenades tepuis) situades al nord d'America del Sud, amb especial emfasi en la potencial migracio altitudinal de les plantes vasculars com a resposta a l'escalfament global. Els cims dels tepuis contenen una diversitat i endemisme de plantes vasculars excepcional. Aquest estudi suggereix que el 30-50% de les especies endemiques de les TAG podrien desapareixer per perdua total d'habitat cap a l'any 2100 segons els escenaris d'escalfament B1 (mes optimista) i A2 (mes pessimista), respectivament. S'ha calculat un Valor d'Impacte Ambiental per a cada especie amenacada per a la posterior classificacio d'aquestes especies en categories de prioritat (10 pel B1 i 13 per l'A2), les quals haurien de ser utilitzades en una sequencia cronologica per orientar les accions de conservacio i recerca. Posteriorment, s'ha establert una linia base de diversitat floristica, endemisme i vegetacio al cim del Roraima-tepui per a la verificacio del fenomen migratori a les TAG. L'inventari floristic d'aquest cim tepuia conte 227 especies, incloent 44 nous registres, una especie nova per a la ciencia i 13 especies exotiques. S'han identificat cinc tipus de comunitats vegetals principals, amb les seves corresponents especies diagnostiques. Tambe s'ha dut a terme una caracteritzacio ambiental d'aquestes comunitats. Finalment, l'estudi paleoecologic de la sequencia sedimentaria del Uei-tepui ha permes documentar la resposta de les especies als canvis ambientals dels ultims 2000 anys. Els resultats suggereixen que el foc ha estat el principal factor que ha modelat la vegetacio del Uei durant aquest periode. Els esdeveniments de focs locals molt probablement van ser causats per activitats humanes i van assolir el cim des de la Gran Sabana afavorits per les condicions climatiques. La preservacio de la flora de les TAG enfront de l'escalfament global s'hauria d'adrecar, com a minim parcialment, mitjancant tecniques de conservacio ex situ, a causa de la naturalesa de l'amenaca (perdua d'habitat). No obstant, la documentacio en aquest estudi d'altres impactes humans (trepig de la vegetacio, especies exotiques, canvis en les propietats del sol, etc.) i l'aparicio del foc com a potencial agent pertorbador del bioma tepuia posen de manifest que la conservacio in situ mitjancant arees protegides pot ser tambe molt important. This thesis is aimed at the study of the flora and vegetation of the Guayana Highlands (GH), a set of -50 table mountains (called tepuis) located in the Guayana Shield of northern South America. Special emphasis is put on the potential upward migration of vascular plants as a response to global warming and the suggestion of conservation strategies. The tepui summits contain exceptional diversity and endemism of vascular plants, with more than 2400 known species, of which 30% would be endemics. In this study, the results of the Altitudinal Range Displacement analysis suggest that 30-50% of the GH endemic species could disappear due to total habitat loss by 2100 according to the most optimistic (B1) and pessimistic (A2) warming scenarios, respectively. An Environmental Impact Value has been calculated for each threatened species for a subsequent classification of these species into priority categories (10 for B1 and 13 for A2), which should be used in a chronological sequence to guide conservation and research actions. Subsequently, flora and vegetation approaches have been applied for the verification of the migratory phenomenon in the GH. Since this verification is based on the comparison of inventories conducted at different times, the first necessary step is to establish a baseline. The Roraima-tepui summit (2810 m elevation) has been selected for that purpose, as it is the best well-known tepui from the botanical point of view. The floristic survey of this tepuian summit has yielded 227 species, including 44 novelties since the publication of the Flora of the Venezuelan Guayana, one species new for science (Epidendrum sp. nov.), and 13 exotics, among which Poa annua and Polypogon elongatus (Poaceae) show the highest invasive potential. Five main plant communities have been identified: three herbaceous, one shrubland, and one forest formation, with their corresponding diagnostic species. An environmental characterization of these communities has been also performed. Finally, palaeoecological records may provide valuable information of the species' responses to past environmental changes, which is not available from short-term ecological observations. In this thesis, a palynological analysis of a sedimentary sequence from Uei-tepui (2150 m elevation) has been conducted to record the vegetation changes occurred in the last 2000 years in this tepui and to investigate the potential drivers involved. At -810 cal yr BP (AD 1140), a peak of fire incidence, coeval with a decrease in regional precipitation, reduced the woody elements of meadows (mainly Biophytum) and the Bonnetia cloud forests. Forest recovery to its previous abundance occurred slowly and elapsed several centuries to be completed after fire disturbance. In the mid-18th century, forests and meadows were replaced by Cyrilla racemiflora shrublands, coinciding with intense fires, synchronous with the arrival of Europeans to the Gran Sabana (GS) uplands. These results suggest that fire has been the main factor in the shaping of Ueitepui vegetation during the last two millennia. The documented local fire events were most likely caused by human activities and reached the summit from the GS, probably favoured by climatic conditions. The last local fire leading to forest clearing and the establishment of present-day vegetation most likely took place in the AD 1920s. The preservation of the GH flora in front of global warming should be addressed, at least partially, through ex situ conservation techniques, due to the nature of the threat (habitat loss). However, the documentation in this study of other human impacts (trampling on vegetation, exotic species, changes in soil properties, etc.) and the emergence of fire as a potential disturbing agent of the GH biome highlight that in situ conservation through protected areas may also be very important. A management plan with the involvement of local indigenous communities should be implemented in the GH region (i.e., lowlands, uplands and highlands), in order to reduce the risks posed by global warming, human impacts, and the potential synergy among them.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Gondwana Research, 2016Co-Authors: Francesca Furlanetto, Derek J Thorkelson, H.d. Gibson, Robert H. Rainbird, W. J. Davis, Daniel D. MarshallAbstract:Abstract The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 ± 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850–1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.
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Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Precambrian Research, 2013Co-Authors: Francesca Furlanetto, Derek J Thorkelson, Daniel D. Marshall, Robert H. Rainbird, H. Daniel Gibson, William J. Davis, James L. Crowley, Jeffrey D. VervoortAbstract:a b s t r a c t The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with differ- ent scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obsta- cle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wernecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its pos- sible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wernecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplace- ment of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wernecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab vol- canics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimen- tary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present.