George Stonadge, Andrew Miles, Daniel Smith, Simon Large, Thomas Knott
Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope−energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04−0.64 wt%) and show a progressive decrease in XCl/XOH (0.6−0.25) and an increase in XF/XCl (1.5−8) and XF/XOH (0.75−1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (<300 ppm), which show increasing XCl/XOH and XF/XOH together with constant XF/XCl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important ‘fertility’ filter for porphyry exploration.
{"title":"The volatile record of volcanic apatite and its implications for the formation of porphyry copper deposits","authors":"George Stonadge, Andrew Miles, Daniel Smith, Simon Large, Thomas Knott","doi":"10.1130/g51461.1","DOIUrl":"https://doi.org/10.1130/g51461.1","url":null,"abstract":"Volatile saturation influences the physicochemical behavior of magmas and is essential for the sequestration of metals in porphyry copper deposits. Tracking the evolution of volatile components (F, Cl, H2O, S) in arc systems is complicated by their mobility and tendency to rapidly re-equilibrate with late-stage melts. We demonstrate that accurate measurements of volatile concentrations in apatite offer a reliable method for identifying the occurrence of volatile saturation. Fluorine, Cl, S, and calculated OH concentrations in apatite obtained by scanning electron microscope−energy-dispersive X-ray spectroscopy and electron microprobe analysis were used to compare two end-member volcanic systems in the West Luzon Arc (Philippines): Pinatubo (a fluid-saturated analogue for porphyry copper deposits) and Taal (a barren and fluid-undersaturated comparator). Apatites from Pinatubo are S-rich (0.04−0.64 wt%) and show a progressive decrease in XCl/XOH (0.6−0.25) and an increase in XF/XCl (1.5−8) and XF/XOH (0.75−1.2) during crystallization. Modeling indicates that these changes result from efficient partitioning of Cl into a continuously saturated H2O-rich fluid, while high regions of S in apatite reflect episodic flushing by a separate S-rich flux. Little S is evident in apatites from Taal (&lt;300 ppm), which show increasing XCl/XOH and XF/XOH together with constant XF/XCl during crystallization. This cannot be explained using an H2O-saturated model, and instead reflects fluid-undersaturated crystallization and cooling in a reduced and/or S-depleted system. Measured volatiles in apatite therefore effectively discriminate volatile-saturated and undersaturated magmatic systems, providing an important ‘fertility’ filter for porphyry exploration.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135481260","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}
Kevin Mendes, Philippe Agard, Alexis Plunder, Clément Herviou
Continental subduction and collision are not merely follow-ups of oceanic subduction but mark the transition from lithospheric-scale deformation localized along the subduction interface to crustal-scale deformation distributed across the orogen. In order to unravel the processes typifying the dynamic changes from oceanic subduction to collision, we have characterized the pressure-temperature (P-T) and spatio-temporal evolution of rocks on each side of the tectonic contact (Briançonnais−Liguro-Piemont [Br-LP] contact) separating the subducted oceanic remnants from the subducted continental fragments along the Western Alps. Results indicate that the maximum temperature and pressure difference on each side of the contact is generally <30 °C and <0.3 GPa, evidencing that no significant metamorphic gap exists. The preservation of similar P-T conditions on both sides of the Br-LP contact is interpreted as resulting from offscraping of the Liguro-Piemont and later Briançonnais units at similar depths, as supported by the ∼10 m.y. gap between peak burial ages of both zones. The similar depth range reached by the various units reflects systematic variations of slicing and mechanical coupling along the plate interface suggesting that (1) similar slicing mechanisms and strain localization prevailed during both oceanic and continental subduction, and (2) the Br-LP contact represents a frozen-in subduction interface. The end of high-pressure and low-temperature metamorphism and continental subduction at ca. 33 Ma would mark the stalling of subduction interface dynamics and the onset of strain distribution across the plate interface and into the lower plate.
{"title":"Lithospheric-scale dynamics during continental subduction: Evidence from a frozen-in plate interface","authors":"Kevin Mendes, Philippe Agard, Alexis Plunder, Clément Herviou","doi":"10.1130/g51480.1","DOIUrl":"https://doi.org/10.1130/g51480.1","url":null,"abstract":"Continental subduction and collision are not merely follow-ups of oceanic subduction but mark the transition from lithospheric-scale deformation localized along the subduction interface to crustal-scale deformation distributed across the orogen. In order to unravel the processes typifying the dynamic changes from oceanic subduction to collision, we have characterized the pressure-temperature (P-T) and spatio-temporal evolution of rocks on each side of the tectonic contact (Briançonnais−Liguro-Piemont [Br-LP] contact) separating the subducted oceanic remnants from the subducted continental fragments along the Western Alps. Results indicate that the maximum temperature and pressure difference on each side of the contact is generally &lt;30 °C and &lt;0.3 GPa, evidencing that no significant metamorphic gap exists. The preservation of similar P-T conditions on both sides of the Br-LP contact is interpreted as resulting from offscraping of the Liguro-Piemont and later Briançonnais units at similar depths, as supported by the ∼10 m.y. gap between peak burial ages of both zones. The similar depth range reached by the various units reflects systematic variations of slicing and mechanical coupling along the plate interface suggesting that (1) similar slicing mechanisms and strain localization prevailed during both oceanic and continental subduction, and (2) the Br-LP contact represents a frozen-in subduction interface. The end of high-pressure and low-temperature metamorphism and continental subduction at ca. 33 Ma would mark the stalling of subduction interface dynamics and the onset of strain distribution across the plate interface and into the lower plate.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135644089","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}
Ken Yamaoka, Simon R. Wallis, Akira Miyake, Catherine Annen
A compilation of the thicknesses of contact metamorphic aureoles (CMAs) developed around intermediate to felsic plutons shows many CMAs are far broader than expected by commonly used thermal models for pluton emplacement. Shortfalls in the amount of heat potentially provided based on pluton size, compared to that needed to form the observed CMA, can be accounted for if some hot magma has been lost by volcanic eruption after passing through the pluton domain and replaced by new hot magma. A high ambient temperature may also contribute to broad CMA formation. However, the presence of coeval pairs of both narrow and broad CMAs in the same area requires contrasting types of pluton growth history. Our thermal modeling, constrained by the peak metamorphic temperature, shows the broad CMA of a well-developed pair of CMAs in the Hongusan area of Japan is due to a magmatic history, including magma tapping and replenishment. A global compilation of CMAs suggests more than 30% of plutons are associated with broad CMAs and fed contemporaneous volcanic eruption.
{"title":"Identifying plutons associated with long-lived volcanism by thermal modeling of contact metamorphic aureoles","authors":"Ken Yamaoka, Simon R. Wallis, Akira Miyake, Catherine Annen","doi":"10.1130/g51563.1","DOIUrl":"https://doi.org/10.1130/g51563.1","url":null,"abstract":"A compilation of the thicknesses of contact metamorphic aureoles (CMAs) developed around intermediate to felsic plutons shows many CMAs are far broader than expected by commonly used thermal models for pluton emplacement. Shortfalls in the amount of heat potentially provided based on pluton size, compared to that needed to form the observed CMA, can be accounted for if some hot magma has been lost by volcanic eruption after passing through the pluton domain and replaced by new hot magma. A high ambient temperature may also contribute to broad CMA formation. However, the presence of coeval pairs of both narrow and broad CMAs in the same area requires contrasting types of pluton growth history. Our thermal modeling, constrained by the peak metamorphic temperature, shows the broad CMA of a well-developed pair of CMAs in the Hongusan area of Japan is due to a magmatic history, including magma tapping and replenishment. A global compilation of CMAs suggests more than 30% of plutons are associated with broad CMAs and fed contemporaneous volcanic eruption.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-04","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135644213","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}
Miaofa Li, Binggui Cai, Slobodan B. Marković, Luo Wang, Qingzhen Hao, Andy Baker, Milivoj B. Gavrilov, Liyuan Jiang, Fang Wang, Xuefeng Wang, Lisheng Wang, Zhibang Ma, Jule Xiao, Zhengtang Guo
The North Atlantic jet stream (NAJ) has a profound impact on the climate of the North Atlantic−European sector, especially in winter. Observations show that the winter NAJ (NAJw) has strengthened over the past ∼140 yr. However, it remains unclear whether this long-term trend has deviated from the natural variability. Here, we present a 2500-yr-long reconstruction of NAJw strength using high-quality stalagmite δ18O records from southeastern Europe. Our results show that the NAJw weakened during both the Roman Warm Period (B.C. 300−A.D. 200) and the Medieval Warm Period (A.D. 900−1250) but that it has strengthened under anthropogenic warming (since A.D. 1850). This indicates that its current trend has already deviated from the natural variability. The best explanation for this present anomalous trend of NAJw strength is that it was triggered by the appearance of the North Atlantic warming hole under anthropogenic forcing. This anomalous trend suggests that continued global warming may further strengthen the NAJw in the future.
{"title":"Strength of the winter North Atlantic jet stream has deviated from its natural trend under anthropogenic warming","authors":"Miaofa Li, Binggui Cai, Slobodan B. Marković, Luo Wang, Qingzhen Hao, Andy Baker, Milivoj B. Gavrilov, Liyuan Jiang, Fang Wang, Xuefeng Wang, Lisheng Wang, Zhibang Ma, Jule Xiao, Zhengtang Guo","doi":"10.1130/g51329.1","DOIUrl":"https://doi.org/10.1130/g51329.1","url":null,"abstract":"The North Atlantic jet stream (NAJ) has a profound impact on the climate of the North Atlantic−European sector, especially in winter. Observations show that the winter NAJ (NAJw) has strengthened over the past ∼140 yr. However, it remains unclear whether this long-term trend has deviated from the natural variability. Here, we present a 2500-yr-long reconstruction of NAJw strength using high-quality stalagmite δ18O records from southeastern Europe. Our results show that the NAJw weakened during both the Roman Warm Period (B.C. 300−A.D. 200) and the Medieval Warm Period (A.D. 900−1250) but that it has strengthened under anthropogenic warming (since A.D. 1850). This indicates that its current trend has already deviated from the natural variability. The best explanation for this present anomalous trend of NAJw strength is that it was triggered by the appearance of the North Atlantic warming hole under anthropogenic forcing. This anomalous trend suggests that continued global warming may further strengthen the NAJw in the future.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135689533","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}
Naoma McCall, Sean P.S. Gulick, Kaidi Karro, Argo Jõeleht, Jakob Wilk, Gisela Pösges
The Ries impact structure (southern Germany) formed ca. 15 Ma and is 22−26 km in diameter, making it one of the youngest and best-preserved mid-size terrestrial impact craters, yet the subsurface has not been studied with modern geophysics. We present the first high-resolution seismic profiles of the Ries impact structure; the profiles show discontinuous intra-basement reflectors and a central crater floor without a significant central topographic high. The inner crystalline ring sits adjacent to, not on top of, the crater terrace zone. These morphologies indicate that during the crater modification stage, the rebounding central uplift at Ries rose and then collapsed without the continued outward motion required to form a fully developed peak ring. The Ries impact structure may be best considered a transitional complex crater form between a central-peak crater and a peak-ring crater as documented on the Moon and other rocky planets. A series of high-amplitude, discontinuous, topographically influenced reflectors overlying the basement implies that the suevite within the crater basin was emplaced via lateral transport.
{"title":"Understanding the Ries impact structure subsurface from high-resolution seismic data","authors":"Naoma McCall, Sean P.S. Gulick, Kaidi Karro, Argo Jõeleht, Jakob Wilk, Gisela Pösges","doi":"10.1130/g51503.1","DOIUrl":"https://doi.org/10.1130/g51503.1","url":null,"abstract":"The Ries impact structure (southern Germany) formed ca. 15 Ma and is 22−26 km in diameter, making it one of the youngest and best-preserved mid-size terrestrial impact craters, yet the subsurface has not been studied with modern geophysics. We present the first high-resolution seismic profiles of the Ries impact structure; the profiles show discontinuous intra-basement reflectors and a central crater floor without a significant central topographic high. The inner crystalline ring sits adjacent to, not on top of, the crater terrace zone. These morphologies indicate that during the crater modification stage, the rebounding central uplift at Ries rose and then collapsed without the continued outward motion required to form a fully developed peak ring. The Ries impact structure may be best considered a transitional complex crater form between a central-peak crater and a peak-ring crater as documented on the Moon and other rocky planets. A series of high-amplitude, discontinuous, topographically influenced reflectors overlying the basement implies that the suevite within the crater basin was emplaced via lateral transport.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135385146","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}
Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium "nutrient capacitor" model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.
{"title":"Ammonium “nutrient capacitor” model for δ15N signatures associated with marine anoxic events","authors":"Benjamin T. Uveges, Ann Pearson","doi":"10.1130/g51527.1","DOIUrl":"https://doi.org/10.1130/g51527.1","url":null,"abstract":"Geochemical records of ancient periods of warm climate can be useful to help understand the looming effects of modern anthropogenic warming, including changes to biogeochemical nutrient cycles. Stable nitrogen isotope compositions of marine sediments archive the balance of processes in the global nitrogen cycle. However, the unusual isotopic signals of Mesozoic oceanic anoxic events (OAEs) remain enigmatic, thus hindering our understanding of nitrogen cycle processes and dynamics under conditions of ocean deoxygenation. Here, we present an ammonium \"nutrient capacitor\" model of the water-column nitrogen cycle to explain the anomalously negative isotopic compositions seen in Mesozoic OAE sediments. Our model applies isotopic inferences derived from high-resolution records of Lake Kivu sediments to show how periodic chemocline overturning of redox-stratified water columns during Mesozoic OAEs may have delivered ammonium to the photic zone in excess of primary producer requirements. Smoothed, stochastic sampling of the changing fluxes within the nitrogen cycle across these events can simulate OAE nitrogen isotope records.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135385445","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}
Mantle metasomatism under ultrahigh-pressure (UHP) conditions is widely recognized in orogenic peridotites and pyroxenites from UHP terranes. However, the processes by which the deeply subducted continental crust reacts with the mantle remain obscure. To investigate the metasomatic regime under UHP conditions, we conducted layered reaction experiments between gneiss and peridotite at 5 GPa and 800−1100 °C, with free water added in some of the experiments. At temperatures below the gneiss solidus, the major metasomatic agent is aqueous fluid, with the main reaction products being orthopyroxene + phlogopite + K-richterite. At temperatures above the gneiss solidus, hydrous melt becomes the major metasomatic agent, and the major reaction products vary from orthopyroxene + phlogopite to orthopyroxene + garnet with increasing degree of melting. The transformation from phlogopite to garnet occurred between 1000 °C and 1100 °C for runs without water added and between 800 °C and 900 °C for runs with water added. Pyroxenites in UHP terranes are mainly characterized by the metasomatic growth of orthopyroxene and garnet at 4−6 GPa and 750−1000 °C. Only experiments with water added reproduced the metasomatic assemblage at similar pressure-temperature conditions, indicating that mantle metasomatism in UHP terranes is probably mainly induced by water-fluxed melting of the continental crust under UHP conditions.
{"title":"Mantle metasomatism induced by water-fluxed melting of subducted continental crust at ultrahigh pressures","authors":"Mingdi Gao, Stephen F. Foley, Haijin Xu, Yu Wang","doi":"10.1130/g51547.1","DOIUrl":"https://doi.org/10.1130/g51547.1","url":null,"abstract":"Mantle metasomatism under ultrahigh-pressure (UHP) conditions is widely recognized in orogenic peridotites and pyroxenites from UHP terranes. However, the processes by which the deeply subducted continental crust reacts with the mantle remain obscure. To investigate the metasomatic regime under UHP conditions, we conducted layered reaction experiments between gneiss and peridotite at 5 GPa and 800−1100 °C, with free water added in some of the experiments. At temperatures below the gneiss solidus, the major metasomatic agent is aqueous fluid, with the main reaction products being orthopyroxene + phlogopite + K-richterite. At temperatures above the gneiss solidus, hydrous melt becomes the major metasomatic agent, and the major reaction products vary from orthopyroxene + phlogopite to orthopyroxene + garnet with increasing degree of melting. The transformation from phlogopite to garnet occurred between 1000 °C and 1100 °C for runs without water added and between 800 °C and 900 °C for runs with water added. Pyroxenites in UHP terranes are mainly characterized by the metasomatic growth of orthopyroxene and garnet at 4−6 GPa and 750−1000 °C. Only experiments with water added reproduced the metasomatic assemblage at similar pressure-temperature conditions, indicating that mantle metasomatism in UHP terranes is probably mainly induced by water-fluxed melting of the continental crust under UHP conditions.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135535230","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}
Rebecca G. Englert, Age J. Vellinga, Matthieu J.B. Cartigny, Michael A. Clare, Joris T. Eggenhuisen, Stephen M. Hubbard
Submarine channels parallel river channels in their ability to transport sediment. However in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (>1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average >5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope.
{"title":"Controls on upstream-migrating bed forms in sandy submarine channels","authors":"Rebecca G. Englert, Age J. Vellinga, Matthieu J.B. Cartigny, Michael A. Clare, Joris T. Eggenhuisen, Stephen M. Hubbard","doi":"10.1130/g51385.1","DOIUrl":"https://doi.org/10.1130/g51385.1","url":null,"abstract":"Submarine channels parallel river channels in their ability to transport sediment. However in contrast to rivers, sediment transport and bed-form development in submarine channels are less well understood. Many steep (&gt;1°), sandy submarine channels are dominated by upstream-migrating bed forms. The flow conditions required to form these upstream-migrating bed forms remain debated because the interactions between turbidity currents and active bed forms are difficult to measure directly. Consequently, we used a depth-resolved numerical model to test the role of flow parameters that are hypothesized to control the formation of upstream-migrating bed forms in submarine channels. While our modeling results confirmed the importance of previously identified flow parameters (e.g., densiometric Froude number), we found that basal sediment concentration in turbidity currents is the strongest predictor of upstream-migrating bed-form formation. Our model shows how locally steep gradients enable high sediment concentrations (average &gt;5 vol%) in the basal parts of flows, which allow the development of cyclic step instabilities and their associated bed forms. This new insight explains the previously puzzling observation that upstream-migrating bed forms are abundant in proximal, steep, sandy reaches of submarine channels, while their occurrence becomes more intermittent downslope.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135535536","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}
Warren W. Wood, Ward E. Sanford, John A. Cherry, Warren T. Wood
Groundwater solute mass flux discharged from the continents to the oceans is between 56% and 63% of particulate sediment transport mass flux. Herein we utilized newly developed continental geospatial groundwater concentration estimates that were multiplied by groundwater volumetric recharge flux to provide a continental-scale discharge mass flux to the oceans of 7.3 Pg DS/yr (petagrams dissolved solutes per year). This mass flux was evaluated from six continental ecosystems: direct ocean discharge (0.28 Pg DS/yr), endorheic basins (0.59 Pg DS/yr), cold-wet exorheic basins (0.55 Pg DS/yr), cold-dry exorheic basins (1.1 Pg DS/yr), warm-dry exorheic basins (0.82 Pg DS/yr), and warm-wet exorheic basins (4.0 Pg DS/yr), thus providing insight into the role of rainfall and temperature on continental weathering and denudation. A new, robust molar silicate/carbonate ratio of 0.42 was calculated for weathering of continental rocks, which is important in the Urey model of climate change. We estimate that rock weathering accounts for ∼50% of the total solute mass flux discharged from the continents, the remainder being from externally derived marine aerosols and organic-derived bicarbonate.
{"title":"\"Wrecking the rocks\": Continental weathering by groundwater","authors":"Warren W. Wood, Ward E. Sanford, John A. Cherry, Warren T. Wood","doi":"10.1130/g51571.1","DOIUrl":"https://doi.org/10.1130/g51571.1","url":null,"abstract":"Groundwater solute mass flux discharged from the continents to the oceans is between 56% and 63% of particulate sediment transport mass flux. Herein we utilized newly developed continental geospatial groundwater concentration estimates that were multiplied by groundwater volumetric recharge flux to provide a continental-scale discharge mass flux to the oceans of 7.3 Pg DS/yr (petagrams dissolved solutes per year). This mass flux was evaluated from six continental ecosystems: direct ocean discharge (0.28 Pg DS/yr), endorheic basins (0.59 Pg DS/yr), cold-wet exorheic basins (0.55 Pg DS/yr), cold-dry exorheic basins (1.1 Pg DS/yr), warm-dry exorheic basins (0.82 Pg DS/yr), and warm-wet exorheic basins (4.0 Pg DS/yr), thus providing insight into the role of rainfall and temperature on continental weathering and denudation. A new, robust molar silicate/carbonate ratio of 0.42 was calculated for weathering of continental rocks, which is important in the Urey model of climate change. We estimate that rock weathering accounts for ∼50% of the total solute mass flux discharged from the continents, the remainder being from externally derived marine aerosols and organic-derived bicarbonate.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-27","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135536463","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}
Youqiang Yu, Frederik Tilmann, Stephen S. Gao, Kelly H. Liu, Jiaji Xi
The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction.
{"title":"Insights into initial continental rifting of marginal seas from seismic evidence for slab relics in the mid-mantle of the Woodlark rift, southwestern Pacific","authors":"Youqiang Yu, Frederik Tilmann, Stephen S. Gao, Kelly H. Liu, Jiaji Xi","doi":"10.1130/g51528.1","DOIUrl":"https://doi.org/10.1130/g51528.1","url":null,"abstract":"The initiation and evolution of marginal seas, especially those developing under a convergent setting, is one of the more enigmatic aspects of plate tectonics. Here, we report the presence of slab relics in the mid-mantle of the Woodlark rift in the southwestern Pacific based on a new map of the topography of the mantle discontinuities from a receiver function analysis and evidence from body-wave tomography. The widespread mantle transition-zone thickening rules out active mantle upwelling, and the revealed slab relics in both the upper and middle mantle may hydrate the upper mantle, which can be expected to further weaken the overlying lithosphere. Such a process can then promote initial continental rifting when this lithosphere is exposed to tensional stress like slab-pull stretching originating from the nearby active subduction.","PeriodicalId":12642,"journal":{"name":"Geology","volume":null,"pages":null},"PeriodicalIF":0.0,"publicationDate":"2023-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"135718694","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}