Benjamin Eskesen, K. Fassmer, C. Münker, T. Ulrich, K. Szilas, Simon Wagner, J. Hoffmann, T. Nagel
We present petrological data and seven Lu-Hf garnet−amphibole−whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet−hornblende−whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.
{"title":"Neoarchean synkinematic metamorphic peak in the Isua supracrustal belt (West Greenland)","authors":"Benjamin Eskesen, K. Fassmer, C. Münker, T. Ulrich, K. Szilas, Simon Wagner, J. Hoffmann, T. Nagel","doi":"10.1130/g51564.1","DOIUrl":"https://doi.org/10.1130/g51564.1","url":null,"abstract":"We present petrological data and seven Lu-Hf garnet−amphibole−whole rock ages obtained from a single garnet-hornblende-mica schist sample from the Isua supracrustal belt (West Greenland). Garnets grew during prograde metamorphism toward regional amphibolite-facies peak conditions, and a mylonitic foliation formed during and after garnet growth. Garnet crystals show typical prograde zoning with no visible traces of a relict garnet generation. They do show various degrees of retrogression. While some crystals are perfectly euhedral with only minor chemical alteration along cracks, others are elongated in the foliation and either grew in this shape or were deformed. Six garnet splits were separated from crushed single crystals and one from a crushed bulk sample. Individual three-point garnet−hornblende−whole rock ages scatter between 2.603 ± 0.018 Ga and 2.432 ± 0.059 Ga for single garnets. The garnet split from the bulk sample defines an age of 2.463 ± 0.031 Ga, the data point farthest from the regression line for all data points (2.551 ± 0.074 Ga, mean square of weighted deviates = 25). We interpret these data to indicate partial retrogression of a Neoarchean garnet population not significantly older than the oldest obtained three-point age. Well-preserved garnet zoning, regional peak temperatures well below the closing temperature of the Lu-Hf system, and the small scatter of Lu-Hf ages preclude an interpretation of the observed metamorphism and deformation as being Eoarchean in age.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-16","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"47549075","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
B. Kelley, M. Yu, D. Lehrmann, D. Altıner, J. Payne
The tempo of biotic recovery following extinction reflects the time scales of evolutionary processes and the long-term consequences of degraded ecosystems, but recovery patterns are poorly resolved. In this study, we investigated the tempo of biotic recovery by evaluating metazoan-algal reef assembly following the end-Permian mass extinction. We combined satellite imagery analysis, field mapping, biostratigraphy, and quantitative petrography to assess recovery in the oldest-known and most stratigraphically extensive Lower to Middle Triassic platform-margin reef. The reef occurs in upper Spathian (upper Lower Triassic) to upper Anisian (lower Middle Triassic) strata of the Great Bank of Guizhou (GBG) isolated carbonate platform in south China. Previous work suggests that metazoan-algal reefs were absent for 8−10 m.y. following extinction but were biologically diverse from their Pelsonian (middle Anisian) initiation. This pattern implies that reefs can reassemble rapidly (<< 1 m.y.) when environmental conditions are favorable. In contrast, our analyses indicate that calcareous sponges, calcareous algae, and early scleractinian corals occurred progressively in the GBG reef and that biotic recovery metrics increased gradually. Unlike nonreef ecosystems, biodiverse metazoan-algal reefs were delayed until the late Pelsonian or early Illyrian, postdating broader marine ecosystem recovery and isotopic evidence for carbon-cycle stabilization by 2−4 m.y. Our findings suggest that reef and nonreef ecosystems differ in their recovery pattern and tempo. Reef recovery from severe environmental perturbation can require several million years, even after hospitable conditions return, highlighting the importance of modern reef conservation.
{"title":"Prolonged and gradual recovery of metazoan-algal reefs following the end-Permian mass extinction","authors":"B. Kelley, M. Yu, D. Lehrmann, D. Altıner, J. Payne","doi":"10.1130/g51058.1","DOIUrl":"https://doi.org/10.1130/g51058.1","url":null,"abstract":"The tempo of biotic recovery following extinction reflects the time scales of evolutionary processes and the long-term consequences of degraded ecosystems, but recovery patterns are poorly resolved. In this study, we investigated the tempo of biotic recovery by evaluating metazoan-algal reef assembly following the end-Permian mass extinction. We combined satellite imagery analysis, field mapping, biostratigraphy, and quantitative petrography to assess recovery in the oldest-known and most stratigraphically extensive Lower to Middle Triassic platform-margin reef. The reef occurs in upper Spathian (upper Lower Triassic) to upper Anisian (lower Middle Triassic) strata of the Great Bank of Guizhou (GBG) isolated carbonate platform in south China. Previous work suggests that metazoan-algal reefs were absent for 8−10 m.y. following extinction but were biologically diverse from their Pelsonian (middle Anisian) initiation. This pattern implies that reefs can reassemble rapidly (<< 1 m.y.) when environmental conditions are favorable. In contrast, our analyses indicate that calcareous sponges, calcareous algae, and early scleractinian corals occurred progressively in the GBG reef and that biotic recovery metrics increased gradually. Unlike nonreef ecosystems, biodiverse metazoan-algal reefs were delayed until the late Pelsonian or early Illyrian, postdating broader marine ecosystem recovery and isotopic evidence for carbon-cycle stabilization by 2−4 m.y. Our findings suggest that reef and nonreef ecosystems differ in their recovery pattern and tempo. Reef recovery from severe environmental perturbation can require several million years, even after hospitable conditions return, highlighting the importance of modern reef conservation.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"42389912","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
G. Volpe, G. Pozzi, M.E. Locchi, E. Tinti, M. Scuderi, C. Marone, C. Collettini
Although rheological heterogeneities are invoked to explain differences in fault-slip behavior, case studies where an interdisciplinary approach is adopted to capture their specific roles are still rare. In this work, we integrated geophysical, geological, and laboratory data to explain how rheological heterogeneities influence the earthquake activity at the roots of the seismogenic zone. During the 2016–2017 Central Italy sequence, following the major earthquakes, we observed a deepening of seismicity within the basement associated with a transient stress change. Part of this seismicity was organized in clusters of events, with similar sizes and waveforms. The structural study of exhumed basement rocks highlighted a heterogeneous fabric made of strong, quartz-rich lenses (up to 200 m) surrounded by a weak, interconnected phyllosilicate-rich matrix. Laboratory experiments simulating the main shock–induced increase in loading rate showed that the matrix lithology experienced an accelerating and self-decelerating aseismic creep, whereas the lens lithology showed dynamic instabilities. Our results suggest that the post–main shock loading rate increases favored accelerated creep within the matrix, which promoted, as a consequence, seismic instabilities within the lenses in the form of clustered seismicity. Our findings emphasize the strong connection between seismicity and the structural and frictional properties of the seismogenic zone.
{"title":"Rheological heterogeneities at the roots of the seismogenic zone","authors":"G. Volpe, G. Pozzi, M.E. Locchi, E. Tinti, M. Scuderi, C. Marone, C. Collettini","doi":"10.1130/g51432.1","DOIUrl":"https://doi.org/10.1130/g51432.1","url":null,"abstract":"Although rheological heterogeneities are invoked to explain differences in fault-slip behavior, case studies where an interdisciplinary approach is adopted to capture their specific roles are still rare. In this work, we integrated geophysical, geological, and laboratory data to explain how rheological heterogeneities influence the earthquake activity at the roots of the seismogenic zone. During the 2016–2017 Central Italy sequence, following the major earthquakes, we observed a deepening of seismicity within the basement associated with a transient stress change. Part of this seismicity was organized in clusters of events, with similar sizes and waveforms. The structural study of exhumed basement rocks highlighted a heterogeneous fabric made of strong, quartz-rich lenses (up to 200 m) surrounded by a weak, interconnected phyllosilicate-rich matrix. Laboratory experiments simulating the main shock–induced increase in loading rate showed that the matrix lithology experienced an accelerating and self-decelerating aseismic creep, whereas the lens lithology showed dynamic instabilities. Our results suggest that the post–main shock loading rate increases favored accelerated creep within the matrix, which promoted, as a consequence, seismic instabilities within the lenses in the form of clustered seismicity. Our findings emphasize the strong connection between seismicity and the structural and frictional properties of the seismogenic zone.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-14","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45909480","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Volcanoes exhibit a wide range of eruptive and geochemical behavior, which has significant implications for their associated risk. The suggested first-order drivers of intervolcanic diversity invoke a combination of crustal and mantle processes. To better constrain mantle-crustal-volcanic coupling, we used the well-studied Lesser Antilles island arc. Here, we show that melt flux from the mantle, identified by proxy in the form of boron isotopes in melt inclusions, correlates with the long-term volcanic productivity, the volcanic edifice height, and the geophysically defined along-arc crustal structure. These features are the consequence of a variable melt flux modulating the pressure-temperature-composition structure of the crust, which we inverted from xenolith mineral chemistry. Mafic to intermediate melts reside at relatively constant temperature (981 ± 52 °C; 2σ) in the middle crust (3.5−7.1 kbar), whereas chemically evolved (rhyolitic) melts are stored predominantly in the upper crust (<3.5 kbar) at maximum depths that vary geophysically along the arc (6−15 km). Our findings are applicable worldwide, where we see similar correlations among average magma geochemistry, eruptive magnitude, and rate of magma input.
{"title":"Eruptive dynamics reflect crustal structure and mantle productivity beneath volcanoes","authors":"O. Higgins, L. Caricchi","doi":"10.1130/g51355.1","DOIUrl":"https://doi.org/10.1130/g51355.1","url":null,"abstract":"Volcanoes exhibit a wide range of eruptive and geochemical behavior, which has significant implications for their associated risk. The suggested first-order drivers of intervolcanic diversity invoke a combination of crustal and mantle processes. To better constrain mantle-crustal-volcanic coupling, we used the well-studied Lesser Antilles island arc. Here, we show that melt flux from the mantle, identified by proxy in the form of boron isotopes in melt inclusions, correlates with the long-term volcanic productivity, the volcanic edifice height, and the geophysically defined along-arc crustal structure. These features are the consequence of a variable melt flux modulating the pressure-temperature-composition structure of the crust, which we inverted from xenolith mineral chemistry. Mafic to intermediate melts reside at relatively constant temperature (981 ± 52 °C; 2σ) in the middle crust (3.5−7.1 kbar), whereas chemically evolved (rhyolitic) melts are stored predominantly in the upper crust (<3.5 kbar) at maximum depths that vary geophysically along the arc (6−15 km). Our findings are applicable worldwide, where we see similar correlations among average magma geochemistry, eruptive magnitude, and rate of magma input.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-11","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"46053031","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Megabeds are exceptionally large submarine deposits interpreted to originate from single catastrophic events. Megabeds are significant components of deep-water basins and are critical for understanding geohazards. We discovered a succession of four megabeds within the upper 70 m of the western Marsili Basin, Tyrrhenian Sea, deposited within the past 50 k.y. The megabeds were imaged as distinctive acoustically transparent units with ponded geometries, 10−25 m thick, separated by parallel-bedded strata. Cores from Site 650 of Ocean Drilling Program Leg 107 revealed that three of the four megabeds are made of alternating volcaniclastic sand and mud, and one is a volcaniclastic debris flow. Abundant shallow-water benthic foraminifera within the megabeds suggest that they were not sourced locally from the active Marsili Seamount, but most likely originated from the Campanian volcanic province to the north. The time interval during which the megabeds were deposited includes the 39.8 ka Campanian ignimbrite supereruption of the Campi Flegrei caldera, Italy, which is among the largest known eruptions on Earth, and the 14.9 ka Neapolitan Yellow Tuff supereruption. Volume (minimum) estimates range from 1.3 to 13.3 km3. However, similar megabeds observed in the neighboring Vavilov Basin to the west suggest that the megabeds in both basins may be correlative, and thus volumes could be much larger. The newly discovered megabeds of the Marsili Basin reveal significant geohazard events for the circum−Tyrrhenian Sea coastlines with a recurrence interval on the order of ∼10−15 k.y.
{"title":"50,000 yr of recurrent volcaniclastic megabed deposition in the Marsili Basin, Tyrrhenian Sea","authors":"D. Sawyer, R. Urgeles, C. Lo Iacono","doi":"10.1130/g51198.1","DOIUrl":"https://doi.org/10.1130/g51198.1","url":null,"abstract":"Megabeds are exceptionally large submarine deposits interpreted to originate from single catastrophic events. Megabeds are significant components of deep-water basins and are critical for understanding geohazards. We discovered a succession of four megabeds within the upper 70 m of the western Marsili Basin, Tyrrhenian Sea, deposited within the past 50 k.y. The megabeds were imaged as distinctive acoustically transparent units with ponded geometries, 10−25 m thick, separated by parallel-bedded strata. Cores from Site 650 of Ocean Drilling Program Leg 107 revealed that three of the four megabeds are made of alternating volcaniclastic sand and mud, and one is a volcaniclastic debris flow. Abundant shallow-water benthic foraminifera within the megabeds suggest that they were not sourced locally from the active Marsili Seamount, but most likely originated from the Campanian volcanic province to the north. The time interval during which the megabeds were deposited includes the 39.8 ka Campanian ignimbrite supereruption of the Campi Flegrei caldera, Italy, which is among the largest known eruptions on Earth, and the 14.9 ka Neapolitan Yellow Tuff supereruption. Volume (minimum) estimates range from 1.3 to 13.3 km3. However, similar megabeds observed in the neighboring Vavilov Basin to the west suggest that the megabeds in both basins may be correlative, and thus volumes could be much larger. The newly discovered megabeds of the Marsili Basin reveal significant geohazard events for the circum−Tyrrhenian Sea coastlines with a recurrence interval on the order of ∼10−15 k.y.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"41743189","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
T. Clark, G. Roff, Tess Chapman, Hannah Markham-Summers, N. Hammerman, Faye Liu, Yue‐xing Feng, J. Pandolfi, Jian-xin Zhao
In the absence of detailed broad-scale studies, both spatially and temporally, the overall status (disturbed, recovering, or in decline) of many of the reefs that make up the Great Barrier Reef remains uncertain. Moreover, of the numerous and varied threats, their relative role in impacting individual reefs is generally unclear. Here, we adopt a novel approach to reliably reconstruct historical disturbance events at Rib and Davies Reefs, two mid-shelf reefs, using uranium-thorium (U-Th) dating of dead corals. Corrected 230Th ages obtained from dead Acropora spp. bracket time periods of lowest coral cover observed during independent ecological surveys in the late 1980s and 2000s at 1988.2 ± 1.3 and 2003.9 ± 6.2 CE, shortly after the arrival of crown-of-thorns starfish (CoTS) at Rib Reef in 1983−1985 and 2000−2002, respectively. At Davies Reef, 230Th ages dated to 1999.2 ± 1.2 CE when coral cover was halved as a result of Cyclone Tessi and an “unknown” disturbance in 2000−2001. Prior to modern surveys, there is remarkable overlap in 230Th ages between reefs, with repeated peaks in the age distribution having a return period of ∼10−15 years, akin to the periodicity of modern CoTS outbreaks. Our findings suggest that U-Th dating of dead corals can provide a robust foundation for understanding disturbance history and show promise in contributing to the effective monitoring of coral communities by providing a reliable benchmark with which to assess recovery.
{"title":"Reconstructing past disturbance in coral communities using U-Th dating of dead coral skeletons","authors":"T. Clark, G. Roff, Tess Chapman, Hannah Markham-Summers, N. Hammerman, Faye Liu, Yue‐xing Feng, J. Pandolfi, Jian-xin Zhao","doi":"10.1130/g51419.1","DOIUrl":"https://doi.org/10.1130/g51419.1","url":null,"abstract":"In the absence of detailed broad-scale studies, both spatially and temporally, the overall status (disturbed, recovering, or in decline) of many of the reefs that make up the Great Barrier Reef remains uncertain. Moreover, of the numerous and varied threats, their relative role in impacting individual reefs is generally unclear. Here, we adopt a novel approach to reliably reconstruct historical disturbance events at Rib and Davies Reefs, two mid-shelf reefs, using uranium-thorium (U-Th) dating of dead corals. Corrected 230Th ages obtained from dead Acropora spp. bracket time periods of lowest coral cover observed during independent ecological surveys in the late 1980s and 2000s at 1988.2 ± 1.3 and 2003.9 ± 6.2 CE, shortly after the arrival of crown-of-thorns starfish (CoTS) at Rib Reef in 1983−1985 and 2000−2002, respectively. At Davies Reef, 230Th ages dated to 1999.2 ± 1.2 CE when coral cover was halved as a result of Cyclone Tessi and an “unknown” disturbance in 2000−2001. Prior to modern surveys, there is remarkable overlap in 230Th ages between reefs, with repeated peaks in the age distribution having a return period of ∼10−15 years, akin to the periodicity of modern CoTS outbreaks. Our findings suggest that U-Th dating of dead corals can provide a robust foundation for understanding disturbance history and show promise in contributing to the effective monitoring of coral communities by providing a reliable benchmark with which to assess recovery.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49327018","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Impact melting and outgassing of impact melts were important processes shaping the crust and atmosphere of early Earth and Mars. The redox state of impact melts influences the gases they release, but the controls on impact melt redox are not well understood. Here, we present Fe micro−X-ray absorption near edge structure (μ-XANES) measurements of a suite of impact melt glasses from Lonar crater, India. As an impact into continental flood basalts, Lonar crater is an ideal analog for impacts into basaltic crust on early Earth and Mars. The Fe-μ-XANES technique permits robust characterization of the Fe3+/ΣFe ratio and therefore redox conditions in Lonar glasses. We found a range in Fe3+/ΣFe of 0.21−0.49 among Lonar glasses, including substantial intraclast variations. We conclude that a primary factor driving these variations was pre-impact oxidative weathering of target rocks. The coupling between crustal weathering regime and speciation of gases released from impact melts implies that impact melt outgassing could act as a feedback mechanism to reinforce prevailing surface redox conditions.
{"title":"Linking impact melt redox with crustal weathering regime","authors":"B. Black, M. Brounce","doi":"10.1130/g51405.1","DOIUrl":"https://doi.org/10.1130/g51405.1","url":null,"abstract":"Impact melting and outgassing of impact melts were important processes shaping the crust and atmosphere of early Earth and Mars. The redox state of impact melts influences the gases they release, but the controls on impact melt redox are not well understood. Here, we present Fe micro−X-ray absorption near edge structure (μ-XANES) measurements of a suite of impact melt glasses from Lonar crater, India. As an impact into continental flood basalts, Lonar crater is an ideal analog for impacts into basaltic crust on early Earth and Mars. The Fe-μ-XANES technique permits robust characterization of the Fe3+/ΣFe ratio and therefore redox conditions in Lonar glasses. We found a range in Fe3+/ΣFe of 0.21−0.49 among Lonar glasses, including substantial intraclast variations. We conclude that a primary factor driving these variations was pre-impact oxidative weathering of target rocks. The coupling between crustal weathering regime and speciation of gases released from impact melts implies that impact melt outgassing could act as a feedback mechanism to reinforce prevailing surface redox conditions.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49225639","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
The Devonian-Carboniferous boundary (DCB) Hangenberg extinction in Europe (ca. 359 Ma) and interval of black shale deposition has been considered a global anoxic event, the cause of which remains debated. However, the lack of a prominent global carbon isotope excursion and the duration of anoxia (3–4 m.y. at least) is inconsistent with other global anoxic events (e.g., Mesozoic ocean anoxic events) throughout the Phanerozoic. We examined geochemical and biostratigraphic data along with radiometric dating from purported Hangenberg event sections across the DCB in western Canada and demonstrate that the onset of anoxic pulses in Laurentian seas started before 363 Ma, diachronous with those in Baltica (Europe). Thus, DCB black shales do not record a single global anoxic event. We suggest instead that black shales near the DCB record multiple, but diachronous, Black Sea–like basins around the globe, promoted by the Late Devonian tectonic configuration. This drove a gradual biotic crisis in the form of regional punctuated benthic faunal extinction events.
{"title":"Multiple diachronous “Black Seas” mimic global ocean anoxia during the latest Devonian","authors":"M. Hedhli, S. Grasby, C. Henderson, B. Davis","doi":"10.1130/g51394.1","DOIUrl":"https://doi.org/10.1130/g51394.1","url":null,"abstract":"The Devonian-Carboniferous boundary (DCB) Hangenberg extinction in Europe (ca. 359 Ma) and interval of black shale deposition has been considered a global anoxic event, the cause of which remains debated. However, the lack of a prominent global carbon isotope excursion and the duration of anoxia (3–4 m.y. at least) is inconsistent with other global anoxic events (e.g., Mesozoic ocean anoxic events) throughout the Phanerozoic. We examined geochemical and biostratigraphic data along with radiometric dating from purported Hangenberg event sections across the DCB in western Canada and demonstrate that the onset of anoxic pulses in Laurentian seas started before 363 Ma, diachronous with those in Baltica (Europe). Thus, DCB black shales do not record a single global anoxic event. We suggest instead that black shales near the DCB record multiple, but diachronous, Black Sea–like basins around the globe, promoted by the Late Devonian tectonic configuration. This drove a gradual biotic crisis in the form of regional punctuated benthic faunal extinction events.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-07","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"45132269","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
J. Kendall, D. Schlaphorst, C. Rychert, N. Harmon, M. Agius, S. Tharimena
Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0−80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, ϕ, with SKS δt that are much longer (1.7−3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.
{"title":"Seismic anisotropy indicates organized melt beneath the Mid-Atlantic Ridge aids seafloor spreading","authors":"J. Kendall, D. Schlaphorst, C. Rychert, N. Harmon, M. Agius, S. Tharimena","doi":"10.1130/g51550.1","DOIUrl":"https://doi.org/10.1130/g51550.1","url":null,"abstract":"Lithospheric plates diverge at mid-ocean ridges and asthenospheric mantle material rises in response. The rising material decompresses, which can result in partial melting, potentially impacting the driving forces of the system. Yet the geometry and spatial distribution of the melt as it migrates to the ridge axis are debated. Organized melt fabrics can cause strong seismic anisotropy, which can be diagnostic of melt, although this is typically not found at ridges. We present anisotropic constraints from an array of 39 ocean-bottom seismometers deployed on 0−80 Ma lithosphere from March 2016 to March 2017 near the equatorial Mid-Atlantic Ridge (MAR). Local and SKS measurements show anisotropic fast directions away from the ridge axis, which are consistent with strain and associated fabric caused by plate motions with short delay times, δt (<1.1 s). Near the ridge axis, we find several ridge-parallel fast splitting directions, ϕ, with SKS δt that are much longer (1.7−3.8 s). This is best explained by ridge-parallel sub-vertical orientations of sheet-like melt pockets. This observation is much different than anisotropic patterns observed at other ridges, which typically reflect fabric related to plate motions. One possibility is that thicker sub-ridge lithosphere with steep sub-ridge topography beneath slower spreading centers focuses melt into vertical, ridge-parallel melt bands, which effectively weakens the plate. Associated buoyancy forces elevate the sub-ridge plate, providing greater potential energy and enhancing the driving forces of the plates.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"49395682","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
High-threat explosive silicic eruptions commonly contain banded pumice, reflecting magma mingling in the conduit prior to or during eruption. Heterogeneities in tuffs have been attributed to the draw-up of compositionally distinct magmas, in which low-viscosity magmas ascend more quickly than high-viscosity magmas. The Rattlesnake Tuff of the High Lava Plains in Oregon (northwestern United States) represents a zoned magma reservoir where at least five different rhyolite compositions are preserved in banded pumice samples in variable mingled combinations. Geochemical gradients recorded across band boundaries in pumice were modeled using a Monte Carlo least-square minimization procedure to find the complementary error function that best fit observed Si and Ba diffusion profiles by iteratively varying the concentration of each plateau (i.e., the concentration on either side of the band boundary), the center and spacing of the diffusion profile, diffusion length scale, and temperature. Modeling indicates maximum time scales between mingling and conduit ascent from minutes to hours. Viscosity calculations for each rhyolite composition confirm that highly viscous rhyolites have longer ascent times than low-viscosity magmas, strongly supporting a model of sequential tapping of a zoned chamber controlled by viscosity.
{"title":"Magma mingling and ascent in the minutes to hours before an explosive eruption as recorded by banded pumice","authors":"H. Shamloo, A. Grunder","doi":"10.1130/g51318.1","DOIUrl":"https://doi.org/10.1130/g51318.1","url":null,"abstract":"High-threat explosive silicic eruptions commonly contain banded pumice, reflecting magma mingling in the conduit prior to or during eruption. Heterogeneities in tuffs have been attributed to the draw-up of compositionally distinct magmas, in which low-viscosity magmas ascend more quickly than high-viscosity magmas. The Rattlesnake Tuff of the High Lava Plains in Oregon (northwestern United States) represents a zoned magma reservoir where at least five different rhyolite compositions are preserved in banded pumice samples in variable mingled combinations. Geochemical gradients recorded across band boundaries in pumice were modeled using a Monte Carlo least-square minimization procedure to find the complementary error function that best fit observed Si and Ba diffusion profiles by iteratively varying the concentration of each plateau (i.e., the concentration on either side of the band boundary), the center and spacing of the diffusion profile, diffusion length scale, and temperature. Modeling indicates maximum time scales between mingling and conduit ascent from minutes to hours. Viscosity calculations for each rhyolite composition confirm that highly viscous rhyolites have longer ascent times than low-viscosity magmas, strongly supporting a model of sequential tapping of a zoned chamber controlled by viscosity.","PeriodicalId":12642,"journal":{"name":"Geology","volume":" ","pages":""},"PeriodicalIF":5.8,"publicationDate":"2023-08-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"48261316","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":1,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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