2020 Vol. 42, No. 7
Display Method:
2020, 42(7): 1-15.
doi: 10.3969/j.issn.0253-4193.2020.07.001
Abstract:
River runoff plays a dominant role in the freshwater flux to the Arctic Ocean, and the corresponding changes in river runoff may have important effects on many processes in the Arctic Ocean. In this study, the sensitivity of ocean temperature, salinity, sea ice and circulation to river runoff was investigated by using a global high-resolution coupled ocean-sea ice model. By comparing the simulated results from a sensitivity experiment without river runoff with the results from a control simulation with climatology river runoff, we found that the river runoff can significantly modulate the temperature, salinity, sea ice, and ocean currents in the Arctic Ocean. In the sensitivity experiment without river runoff, the sea surface temperature on the shelf near the estuaries decreases, and the salinity increases. At 500 m depth, the temperature decreases, and the salinity increases in the sensitivity experiment. The sea ice concentration and thickness also increase near the estuaries in the sensitivity experiment. The shutdown of the river runoff also has an impact on the circulation, which leads to weaker and fresher East Greenland Current and the Transpolar Drift Stream. The shutdown of runoff forcing can also slow down the Arctic Circumpolar Boundary Current along the Eurasian Shelf and speed up the Bering Strait inflow by lowering the sea surface height in the shelf seas. In the sensitive experiment, a colder Atlantic layer, a smaller salinity gradient and a thinner upper halocline can be identified by comparing with the temperature and salinity profiles from the control experiment.
River runoff plays a dominant role in the freshwater flux to the Arctic Ocean, and the corresponding changes in river runoff may have important effects on many processes in the Arctic Ocean. In this study, the sensitivity of ocean temperature, salinity, sea ice and circulation to river runoff was investigated by using a global high-resolution coupled ocean-sea ice model. By comparing the simulated results from a sensitivity experiment without river runoff with the results from a control simulation with climatology river runoff, we found that the river runoff can significantly modulate the temperature, salinity, sea ice, and ocean currents in the Arctic Ocean. In the sensitivity experiment without river runoff, the sea surface temperature on the shelf near the estuaries decreases, and the salinity increases. At 500 m depth, the temperature decreases, and the salinity increases in the sensitivity experiment. The sea ice concentration and thickness also increase near the estuaries in the sensitivity experiment. The shutdown of the river runoff also has an impact on the circulation, which leads to weaker and fresher East Greenland Current and the Transpolar Drift Stream. The shutdown of runoff forcing can also slow down the Arctic Circumpolar Boundary Current along the Eurasian Shelf and speed up the Bering Strait inflow by lowering the sea surface height in the shelf seas. In the sensitive experiment, a colder Atlantic layer, a smaller salinity gradient and a thinner upper halocline can be identified by comparing with the temperature and salinity profiles from the control experiment.
2020, 42(7): 16-31.
doi: 10.3969/j.issn.0253-4193.2020.07.002
Abstract:
The northeastern South China Sea is oligotrophic. In summer, the northeastern South China Sea is low in phytoplankton chlorophyll a (Chl a) concentration, and the typhoon “wind pump” induced upper ocean turbulence can transport nutrients upwards, and therefore increase the Chl a concentration in the surface and subsurface layers. Previous studies have generally focused on the effects of tropical cyclonic wind stress and ocean mesoscale eddies on the Chl a concentration in the upper ocean. This study investigates the possible effect of the Kuroshio to the Chl a concentration in the euphotic layer after typhoon using CTD data, in situ Chl a concentration, Argo profiles and remote sensing data. The results show that the Typhoon Linfa in 2015 caused a looping path of the Kuroshio intrusion into the South China Sea (Area A) through the northwestern Luzon Strait, which weakened the pre-existing cyclonic eddy after the typhoon. This typhoon “wind pump” enhanced Kuroshio inhibited the uplift of nutrients to the surface layer (0−40 m) through the typhoon Ekman Pumping, and accumulated the nutrient in the subsurface layer (60−90 m), thereby inhibiting the increase of surface Chl a concentration but promoting the growth of subsurface Chl a concentration. The increase of Chl a concentration over the western Luzon in the South China Sea (Area B) was not only the vertical redistribution of the phytoplankton, but especially the growth of the phytoplankton. The typhoon-induced looping path of Kuroshio over the northwestern Luzon promoted the formation of the cyclonic eddy over the western Luzon. This continuously enhancing cyclonic eddy provided sufficient nutrients for the significant increase of the chlorophyll in the surface layer (0−40 m).
The northeastern South China Sea is oligotrophic. In summer, the northeastern South China Sea is low in phytoplankton chlorophyll a (Chl a) concentration, and the typhoon “wind pump” induced upper ocean turbulence can transport nutrients upwards, and therefore increase the Chl a concentration in the surface and subsurface layers. Previous studies have generally focused on the effects of tropical cyclonic wind stress and ocean mesoscale eddies on the Chl a concentration in the upper ocean. This study investigates the possible effect of the Kuroshio to the Chl a concentration in the euphotic layer after typhoon using CTD data, in situ Chl a concentration, Argo profiles and remote sensing data. The results show that the Typhoon Linfa in 2015 caused a looping path of the Kuroshio intrusion into the South China Sea (Area A) through the northwestern Luzon Strait, which weakened the pre-existing cyclonic eddy after the typhoon. This typhoon “wind pump” enhanced Kuroshio inhibited the uplift of nutrients to the surface layer (0−40 m) through the typhoon Ekman Pumping, and accumulated the nutrient in the subsurface layer (60−90 m), thereby inhibiting the increase of surface Chl a concentration but promoting the growth of subsurface Chl a concentration. The increase of Chl a concentration over the western Luzon in the South China Sea (Area B) was not only the vertical redistribution of the phytoplankton, but especially the growth of the phytoplankton. The typhoon-induced looping path of Kuroshio over the northwestern Luzon promoted the formation of the cyclonic eddy over the western Luzon. This continuously enhancing cyclonic eddy provided sufficient nutrients for the significant increase of the chlorophyll in the surface layer (0−40 m).
2020, 42(7): 32-40.
doi: 10.3969/j.issn.0253-4193.2020.07.003
Abstract:
Based on tide gauge observation data, sea level change impact investigation information and runoff observation data, the changing characteristics and causes of salt tide intrusions at Changjiang River Estuary during 2009−2018 were emphatically analyzed. Results show that: (1) Seasonal variations of salt tide intrusions at Changjiang River Estuary are obvious, which usually begin from September to October, and end in April or May of the following year. The most frequent occurrence is in March, reaching 12 times. The occurrence and duration of salt tide intrusions marked downward trend as a whole during 2009−2018. The intrusion occurrence was the most in 2009, reaching 13 times, all appeared during October to April of the following year. Annual variation of salt tide intrusion duration was obvious, which was the longest in 2011, reaching 55 days. The occurrence and duration of salt tide intrusions all decreased obviously during 2015−2018, and there was no salt tide intrusion in 2018. (2) From January to April, the seasonal sea level is low, and runoff is small in the Changjiang River Estuary. As influenced by the East-Asian Monsoon, the surge-runoff impact index is high, valued 1.5, 1.9 and 1.6 for January, February and March respectively during 2009−2018. Salt tide intrusion is mainly influenced by surge during this period. For the time of May to July, the runoff increases significantly and becomes dominant, and the sea-level-runoff impact indexes are less than zero. In August, the runoff of the Changjiang River begins to decrease. Seasonal sea level is high, but there is continuous negative surge during this time. Correspondingly, the sea level-runoff impact index is 0.1, and the surge-runoff impact index is −1.6. And salt tide intrusion hardly happens. In September, the seasonal sea level is high, and surge is obvious. The sea level-runoff as well as surge-runoff impact indexes is 1.2 and 1.0 respectively, and salt tide intrusion is very likely to happen. For the time of October and November, the sea-level-runoff impact indexes are 1.5 and 0.8 respectively, and the Changjiang River Estuary is prone to salt tide intrusions. (3) Two-thirds of the 48 salt tide intrusions in the past 10 years occurred in the period of astronomical high tide. The basic sea level is high in some years in the Changjiang River Estuary. In the cases of continuous surge and astronomical high tide, the impacts of the saltwater intrusion will be aggravated.
Based on tide gauge observation data, sea level change impact investigation information and runoff observation data, the changing characteristics and causes of salt tide intrusions at Changjiang River Estuary during 2009−2018 were emphatically analyzed. Results show that: (1) Seasonal variations of salt tide intrusions at Changjiang River Estuary are obvious, which usually begin from September to October, and end in April or May of the following year. The most frequent occurrence is in March, reaching 12 times. The occurrence and duration of salt tide intrusions marked downward trend as a whole during 2009−2018. The intrusion occurrence was the most in 2009, reaching 13 times, all appeared during October to April of the following year. Annual variation of salt tide intrusion duration was obvious, which was the longest in 2011, reaching 55 days. The occurrence and duration of salt tide intrusions all decreased obviously during 2015−2018, and there was no salt tide intrusion in 2018. (2) From January to April, the seasonal sea level is low, and runoff is small in the Changjiang River Estuary. As influenced by the East-Asian Monsoon, the surge-runoff impact index is high, valued 1.5, 1.9 and 1.6 for January, February and March respectively during 2009−2018. Salt tide intrusion is mainly influenced by surge during this period. For the time of May to July, the runoff increases significantly and becomes dominant, and the sea-level-runoff impact indexes are less than zero. In August, the runoff of the Changjiang River begins to decrease. Seasonal sea level is high, but there is continuous negative surge during this time. Correspondingly, the sea level-runoff impact index is 0.1, and the surge-runoff impact index is −1.6. And salt tide intrusion hardly happens. In September, the seasonal sea level is high, and surge is obvious. The sea level-runoff as well as surge-runoff impact indexes is 1.2 and 1.0 respectively, and salt tide intrusion is very likely to happen. For the time of October and November, the sea-level-runoff impact indexes are 1.5 and 0.8 respectively, and the Changjiang River Estuary is prone to salt tide intrusions. (3) Two-thirds of the 48 salt tide intrusions in the past 10 years occurred in the period of astronomical high tide. The basic sea level is high in some years in the Changjiang River Estuary. In the cases of continuous surge and astronomical high tide, the impacts of the saltwater intrusion will be aggravated.
2020, 42(7): 41-50.
doi: 10.3969/j.issn.0253-4193.2020.07.004
Abstract:
The ocean response to typhoon is usually characterized by a cooling on the sea surface. In August 2012, however, a warming (instead of cooling) event occurs in the Yellow Sea outside Mokpo, South Korea, as the typhoon Bolaven (2012) passes. This study gives a report of this abnormal sea surface warming, and provides an exploration of the underlying dynamical processes. It is found that the event coincides in location with the Yellow Sea Surface Cold Patch (SCP) outside Mokpo; Typhoon Bolaven (2012) suppresses the surface cooling due to SCP and hence leads to the abnormal warming. Of the two basic mechanisms that account for the formation of SCPs, i.e., tidal mixing and upwelling, the latter shows a drastic response to the typhoon passage−the upwelling is turned into a downwelling. The downwelling arises from the onshore Ekman transport generated by the northward wind stress on the left hand side of the Mokpo SCP.
The ocean response to typhoon is usually characterized by a cooling on the sea surface. In August 2012, however, a warming (instead of cooling) event occurs in the Yellow Sea outside Mokpo, South Korea, as the typhoon Bolaven (2012) passes. This study gives a report of this abnormal sea surface warming, and provides an exploration of the underlying dynamical processes. It is found that the event coincides in location with the Yellow Sea Surface Cold Patch (SCP) outside Mokpo; Typhoon Bolaven (2012) suppresses the surface cooling due to SCP and hence leads to the abnormal warming. Of the two basic mechanisms that account for the formation of SCPs, i.e., tidal mixing and upwelling, the latter shows a drastic response to the typhoon passage−the upwelling is turned into a downwelling. The downwelling arises from the onshore Ekman transport generated by the northward wind stress on the left hand side of the Mokpo SCP.
2020, 42(7): 51-63.
doi: 10.3969/j.issn.0253-4193.2020.07.005
Abstract:
In this work, we compare the actual prediction skills and potential predictability of the Indian Ocean Dipole (IOD) index among different models, using the North American Multi-Model Ensemble dataset. If defined by a 0.5 anomaly correlation skill, we find that skillful predictions of the IOD strongly vary among models, ranging from 2- to 4-month leads. The diversity in the prediction skills of Sea Surface Temperature Anomalies (SSTA) over the east pole of the IOD is more obvious than that over the west pole, suggesting the model errors and the initial errors have more significant effects on SSTA over the east pole region. Also, there is a significant linear relationship between the actual prediction skills and potential predictability of the IOD. Finally, we find the actual prediction skills diversity is due to the El Niño/Southern Oscillation (ENSO) influences onto the IOD in different models. The models with enhanced ENSO/IOD link tend to have high actual prediction skills.
In this work, we compare the actual prediction skills and potential predictability of the Indian Ocean Dipole (IOD) index among different models, using the North American Multi-Model Ensemble dataset. If defined by a 0.5 anomaly correlation skill, we find that skillful predictions of the IOD strongly vary among models, ranging from 2- to 4-month leads. The diversity in the prediction skills of Sea Surface Temperature Anomalies (SSTA) over the east pole of the IOD is more obvious than that over the west pole, suggesting the model errors and the initial errors have more significant effects on SSTA over the east pole region. Also, there is a significant linear relationship between the actual prediction skills and potential predictability of the IOD. Finally, we find the actual prediction skills diversity is due to the El Niño/Southern Oscillation (ENSO) influences onto the IOD in different models. The models with enhanced ENSO/IOD link tend to have high actual prediction skills.
2020, 42(7): 64-77.
doi: 10.3969/j.issn.0253-4193.2020.07.006
Abstract:
Typhoon is one of the most serious natural disasters in southeast coastal region of China. In this paper, the traditional Monte-Carlo simulation and the advanced empirical track model are respectively used to predict the extreme wind speed (10-min mean at 10 m height) of typhoons in the southeast coastal region of China, and the prediction results of two methods are compared. An area extending 200 km inland from the coastline is divided into 0.25°×0.25° grid cells and each grid is taken as the research point. Firstly, the Monte-Carlo method is used to generate virtual typhoons of 1 000-year for each research point. Then, we use the empirical track model to construct virtual typhoons of 1 000-year in the Northwest Pacific Ocean, from which typhoon events affecting each research site are extracted by using the simulation circle method. Next, the Yan Meng wind field model is used to calculate the wind speed of the extracted typhoons, from which samples of maximum wind speed can be derived. Finally, the extreme wind speed of different return periods for each research point is predicted by the extreme value distribution. Through comparison, we find that in some inland areas, the predicted wind speeds by empirical track model are slightly higher than those by Monte-Carlo method, and in most coastal areas the opposite is true. This is mainly caused by the difference of the central pressure of the virtual typhoons constructed by two methods and the uncertainty of the model itself. The research results of this paper can provide useful reference for the disaster prevention and mitigation system.
Typhoon is one of the most serious natural disasters in southeast coastal region of China. In this paper, the traditional Monte-Carlo simulation and the advanced empirical track model are respectively used to predict the extreme wind speed (10-min mean at 10 m height) of typhoons in the southeast coastal region of China, and the prediction results of two methods are compared. An area extending 200 km inland from the coastline is divided into 0.25°×0.25° grid cells and each grid is taken as the research point. Firstly, the Monte-Carlo method is used to generate virtual typhoons of 1 000-year for each research point. Then, we use the empirical track model to construct virtual typhoons of 1 000-year in the Northwest Pacific Ocean, from which typhoon events affecting each research site are extracted by using the simulation circle method. Next, the Yan Meng wind field model is used to calculate the wind speed of the extracted typhoons, from which samples of maximum wind speed can be derived. Finally, the extreme wind speed of different return periods for each research point is predicted by the extreme value distribution. Through comparison, we find that in some inland areas, the predicted wind speeds by empirical track model are slightly higher than those by Monte-Carlo method, and in most coastal areas the opposite is true. This is mainly caused by the difference of the central pressure of the virtual typhoons constructed by two methods and the uncertainty of the model itself. The research results of this paper can provide useful reference for the disaster prevention and mitigation system.
2020, 42(7): 78-92.
doi: 10.3969/j.issn.0253-4193.2020.07.007
Abstract:
The Mn (manganese)-rich brown layers (brown layers) distribute widely in the Arctic Ocean seafloor which controlled by sea ice, surface currents, material supply and so on. In this study, we investigate the provenance and composition feature of brown layers based on changes of contents on rare earth and trace elements, coarse fraction, inorganic carbon and colour reflectance of core ARC07-E25 retrieved from the Mendeleev Ridge, western Arctic Ocean. Total rare earth elements (∑REE) changes between 122.37×10−6 to 231.94×10−6, the north American Shale normalized records of the sediments generally show enrichment in medium rare earth element (MREE) and light rare earth element (LREE) which dominated by lanthanum (La), cerium (Ce) and neodymium (Nd), respectively. With the increase of coarse fraction, such as ice rafted detritus (IRD), the decrease trend of ∑REE indicates REE mainly enrich in fine grain fraction in the Mendeleev Ridge sediments. The differences of REE contents between brown and gray layers, which are classified in the core based on various characteristics of ∑REE, demonstrate distinct Ce anomalies in the brown layer and grey layer associated with different redox conditions: it will be oxidized from Ce3+ to Ce4+ in oxic brown layers and be reduced from Ce4+ to Ce3+ in gray layers. This behavior favors the large variation of ∑LREE content and affects the ∑REE content in the sediments. Consequently, the ∑REE contents are increasing in the brown layers representing oxidation condition and decreasing in the gray layer representing reduction condition. The results of R-type factor analysis show that the REE in sediments have a good correlation with the detritus elements (Nb, U and Th), and mainly derived from inshore erosion of the East Siberian continental shelf, New Siberian Islands and input of the Lena River.
The Mn (manganese)-rich brown layers (brown layers) distribute widely in the Arctic Ocean seafloor which controlled by sea ice, surface currents, material supply and so on. In this study, we investigate the provenance and composition feature of brown layers based on changes of contents on rare earth and trace elements, coarse fraction, inorganic carbon and colour reflectance of core ARC07-E25 retrieved from the Mendeleev Ridge, western Arctic Ocean. Total rare earth elements (∑REE) changes between 122.37×10−6 to 231.94×10−6, the north American Shale normalized records of the sediments generally show enrichment in medium rare earth element (MREE) and light rare earth element (LREE) which dominated by lanthanum (La), cerium (Ce) and neodymium (Nd), respectively. With the increase of coarse fraction, such as ice rafted detritus (IRD), the decrease trend of ∑REE indicates REE mainly enrich in fine grain fraction in the Mendeleev Ridge sediments. The differences of REE contents between brown and gray layers, which are classified in the core based on various characteristics of ∑REE, demonstrate distinct Ce anomalies in the brown layer and grey layer associated with different redox conditions: it will be oxidized from Ce3+ to Ce4+ in oxic brown layers and be reduced from Ce4+ to Ce3+ in gray layers. This behavior favors the large variation of ∑LREE content and affects the ∑REE content in the sediments. Consequently, the ∑REE contents are increasing in the brown layers representing oxidation condition and decreasing in the gray layer representing reduction condition. The results of R-type factor analysis show that the REE in sediments have a good correlation with the detritus elements (Nb, U and Th), and mainly derived from inshore erosion of the East Siberian continental shelf, New Siberian Islands and input of the Lena River.
Mineral chemistry and geological significance of plagioclases hosted by basalts from the Cocos Ridge
2020, 42(7): 93-107.
doi: 10.3969/j.issn.0253-4193.2020.07.008
Abstract:
During the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344, tholeiitic basalt basement samples of the Cocos Ridge offshore central America were drilled in holes U1381A and U1381C at Site U1381. Studying on these samples will provide some important clues for understanding their petrogenesis. In this study we carried out in-situ major and trace elements analysis for plagioclase phenocrysts and microlites hosted by these basalt samples. The results show that plagioclases are bytownite, labradorite, with a small number of andesine. Some of the plagioclase phenocrysts show positive compositional zoning, while others only have weak compositional changes from the core to the rim of the phenocrysts. The trace element compositions of plagioclase phenocrysts and microlites are quite different. The plagioclase phenocrysts are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and have obvious positive Eu anomalies. The contents of most incompatible elements of microlites are higher than those of phenocrysts. We used the classical igneous plagioclase thermometer to calculate the crystallization temperature of plagioclase phenocrysts and microlites as follows, i.e., 1 050−1 253℃ for plagioclase phenocrysts, and 866−1 033℃ for plagioclase microlites. Finally, we suggest that the core of plagioclase phenocrysts is crystallized from earlier relatively primitive magma, while the rim of the phenocrysts and microlites are crystallization products of continuous evolved magma compositions. The corrosion structure and positive compositional zoning of plagioclase phenocrysts may be mainly caused by some processes such as upwelling decompression and magma recharging. We suggest that the Cocos Ridge basaltic magma was originated from a magma chamber with continuous injection and convection of primitive magma.
During the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344, tholeiitic basalt basement samples of the Cocos Ridge offshore central America were drilled in holes U1381A and U1381C at Site U1381. Studying on these samples will provide some important clues for understanding their petrogenesis. In this study we carried out in-situ major and trace elements analysis for plagioclase phenocrysts and microlites hosted by these basalt samples. The results show that plagioclases are bytownite, labradorite, with a small number of andesine. Some of the plagioclase phenocrysts show positive compositional zoning, while others only have weak compositional changes from the core to the rim of the phenocrysts. The trace element compositions of plagioclase phenocrysts and microlites are quite different. The plagioclase phenocrysts are enriched in light rare earth elements and large ion lithophile elements, depleted in high field strength elements, and have obvious positive Eu anomalies. The contents of most incompatible elements of microlites are higher than those of phenocrysts. We used the classical igneous plagioclase thermometer to calculate the crystallization temperature of plagioclase phenocrysts and microlites as follows, i.e., 1 050−1 253℃ for plagioclase phenocrysts, and 866−1 033℃ for plagioclase microlites. Finally, we suggest that the core of plagioclase phenocrysts is crystallized from earlier relatively primitive magma, while the rim of the phenocrysts and microlites are crystallization products of continuous evolved magma compositions. The corrosion structure and positive compositional zoning of plagioclase phenocrysts may be mainly caused by some processes such as upwelling decompression and magma recharging. We suggest that the Cocos Ridge basaltic magma was originated from a magma chamber with continuous injection and convection of primitive magma.
2020, 42(7): 108-118.
doi: 10.3969/j.issn.0253-4193.2020.07.009
Abstract:
The sediment provenance is important information to study on the transport process of shelf sediments, paleo-environmental changes and the formation of heavy mineral placers on the shelf. However, most shelf sediments are heavily reworked and repeatedly sorted, which cause the difficulty in indentifying their sources. This study firstly reported the work on heavy mineral provenance on the northeastern shelf of the South China Sea by using mineral chemistry, which has been proved to be a useful method to decrease the effect of hydrodynamic sorting. The provenance of shelf sediments was inferred by comparing the mineral assemblage and mineral chemical compositions of tourmaline and garnet between six marine sediment samples on the outer shelf and six offshore and coastal samples. In the study area, tourmaline and amphibole are dominant and monazite is present in some samples. Mineral chemical composition of tourmaline suggested the Zhujiang River and Hanjiang River are two dominant sediment suppliers. Sediments from the two big rivers contain abundant tourmaline from igneous rocks with the content of about 60% which are associated with widely-spreaded granitic rocks in the South China. Some small rivers distribute within the Lianhuashan and Changle-Nanao fracture zones are characterized by the high content of metamorphic tourmaline. These sediments can be traced on the northeastern shelf of the South China Sea by using chemical composition of tourmaline, suggesting small rivers are also important sediment suppliers on this studied shelf. Garnet sourced from sharn rocks is found on the eastern most shelf. The study highlights the importance of single-mineral chemistry in judging the provenance of shelf sediments which suffer from multiple sorting and have complex sources.
The sediment provenance is important information to study on the transport process of shelf sediments, paleo-environmental changes and the formation of heavy mineral placers on the shelf. However, most shelf sediments are heavily reworked and repeatedly sorted, which cause the difficulty in indentifying their sources. This study firstly reported the work on heavy mineral provenance on the northeastern shelf of the South China Sea by using mineral chemistry, which has been proved to be a useful method to decrease the effect of hydrodynamic sorting. The provenance of shelf sediments was inferred by comparing the mineral assemblage and mineral chemical compositions of tourmaline and garnet between six marine sediment samples on the outer shelf and six offshore and coastal samples. In the study area, tourmaline and amphibole are dominant and monazite is present in some samples. Mineral chemical composition of tourmaline suggested the Zhujiang River and Hanjiang River are two dominant sediment suppliers. Sediments from the two big rivers contain abundant tourmaline from igneous rocks with the content of about 60% which are associated with widely-spreaded granitic rocks in the South China. Some small rivers distribute within the Lianhuashan and Changle-Nanao fracture zones are characterized by the high content of metamorphic tourmaline. These sediments can be traced on the northeastern shelf of the South China Sea by using chemical composition of tourmaline, suggesting small rivers are also important sediment suppliers on this studied shelf. Garnet sourced from sharn rocks is found on the eastern most shelf. The study highlights the importance of single-mineral chemistry in judging the provenance of shelf sediments which suffer from multiple sorting and have complex sources.
2020, 42(7): 119-129.
doi: 10.3969/j.issn.0253-4193.2020.07.010
Abstract:
Future global changes may increase the intensity and frequency of extreme events, coupled with the rapid rise in sea level, bringing the issue of updated protection standards to the coastal areas. Due to the short length of observational records, it is necessary to find other alternative records to extend typhoon records other than meteorological observations. The sedimentary records and historical documents provide alternative approaches to extend typhoon records. This study attempts to reconstruct typhoon events on a 100-year scale on the inner shelf of the East China Sea through coupling of meteorological observations and sedimentary records. This reconstruction is compared with the meteorological disaster records to explore the preservation potential, path and intensity of typhoon events in the shelf mud area. In May 2018, a 2 m Core J1 was collected in the offshore area of Zhejiang Province. Typhoon events were comprehensively identified by multiple indicators (grain size distribution, D90, Zr/Fe, Sr/Al and Ca). The radioisotope 210Pb dating method was used to determine the age model of Core J1. The results show that the sedimentation rate of Core J1 is about 1.1 cm/a, and the time span is about 1836 to 2018. 25 typhoon events are identified in Core J1 and 10 of which match the meteorological observation period (1950−2018). The average wind speed of the 10 typhoons is close to the one of all typhoons affecting Core J1, and most of them belong to the strong typhoons of type III and IV landing on Zhejiang Province and Fujian Province. Through multivariate comprehensive analysis, it is found that the preservation potential of typhoon events in sedimentary records is close to 30% for Core J1. In addition, the typhoon events recorded in historical documents may be biased towards stronger landing typhoons, thus the number of typhoons recorded in literatures is significantly less than that actually affected. The knowledge obtained here helps to not only make better use of sedimentary records to expand the time span of typhoon records, but also guide future sampling to obtain more complete typhoon sedimentary records.
Future global changes may increase the intensity and frequency of extreme events, coupled with the rapid rise in sea level, bringing the issue of updated protection standards to the coastal areas. Due to the short length of observational records, it is necessary to find other alternative records to extend typhoon records other than meteorological observations. The sedimentary records and historical documents provide alternative approaches to extend typhoon records. This study attempts to reconstruct typhoon events on a 100-year scale on the inner shelf of the East China Sea through coupling of meteorological observations and sedimentary records. This reconstruction is compared with the meteorological disaster records to explore the preservation potential, path and intensity of typhoon events in the shelf mud area. In May 2018, a 2 m Core J1 was collected in the offshore area of Zhejiang Province. Typhoon events were comprehensively identified by multiple indicators (grain size distribution, D90, Zr/Fe, Sr/Al and Ca). The radioisotope 210Pb dating method was used to determine the age model of Core J1. The results show that the sedimentation rate of Core J1 is about 1.1 cm/a, and the time span is about 1836 to 2018. 25 typhoon events are identified in Core J1 and 10 of which match the meteorological observation period (1950−2018). The average wind speed of the 10 typhoons is close to the one of all typhoons affecting Core J1, and most of them belong to the strong typhoons of type III and IV landing on Zhejiang Province and Fujian Province. Through multivariate comprehensive analysis, it is found that the preservation potential of typhoon events in sedimentary records is close to 30% for Core J1. In addition, the typhoon events recorded in historical documents may be biased towards stronger landing typhoons, thus the number of typhoons recorded in literatures is significantly less than that actually affected. The knowledge obtained here helps to not only make better use of sedimentary records to expand the time span of typhoon records, but also guide future sampling to obtain more complete typhoon sedimentary records.
2020, 42(7): 139-146.
doi: 10.3969/j.issn.0253-4193.2020.07.012
Abstract:
The Fairway Basin is located on the eastern side of the Lord Howe Ridge in the southwestern Pacific Ocean and on the west side of the New Caledonia. It is the frontier of global oil and gas exploration. However, the current research on the tectonic evolution of the basin is still weak, which limits the further exploration and development on the oil and gas resources in the basin. A large amount of geophysical data from the Tasman Sea resource bank in the New Zealand are collected in this paper. The tectonic evolution simulation was carried out by using the 2D Move balance profile technique, and combining regional dynamics, the tectonic evolution of the northern and southern basins is divided into seven phases: (1) intracontinental rift stage from early Cretaceous to late Cretaceous; (2) late Cretaceous fault transition period; (3) the early depression stage of the Eocene; (4) a tectonic inversion stage in the late Eocene; (5) the thermal sedimentation stage from Eocene to Oligocene; (6) secondary tectonic inversion from Oligocene to Miocene; (7) ocean subsidence stage. Since no obvious secondary tectonic inversion stage was found in the central basin, the structural evolution of the central basin is divided into five stages: (1) intracontinental rift stage from early Cretaceous to late Cretaceous; (2) late Cretaceous fault transition period; (3) the early depression stage of the Eocene; (4) the tectonic inversion stage in the late Eocene; (5) the stage of marine subsidence from the Eocene to the present. At this stage, the basin as a whole descended and gradually formed its present appearance. The northern part of the Fairway Basin is greatly affected by regional tectonic activities, and the Cretaceous strata have more fault structures. More diapir structure occurred in the late Cretaceous strata in the central part of the basin. Since the formation of the southern part of the basin, it has been less affected by tectonic activities, the developmental strata are intact, and the pre-Cenozoic strata are thicker. The tectonic activity is strong in the northern part of the Fairway Basin, with a weaker central part and a smaller southern part. The sedimentary strata are thickened and thinned from north to south.
The Fairway Basin is located on the eastern side of the Lord Howe Ridge in the southwestern Pacific Ocean and on the west side of the New Caledonia. It is the frontier of global oil and gas exploration. However, the current research on the tectonic evolution of the basin is still weak, which limits the further exploration and development on the oil and gas resources in the basin. A large amount of geophysical data from the Tasman Sea resource bank in the New Zealand are collected in this paper. The tectonic evolution simulation was carried out by using the 2D Move balance profile technique, and combining regional dynamics, the tectonic evolution of the northern and southern basins is divided into seven phases: (1) intracontinental rift stage from early Cretaceous to late Cretaceous; (2) late Cretaceous fault transition period; (3) the early depression stage of the Eocene; (4) a tectonic inversion stage in the late Eocene; (5) the thermal sedimentation stage from Eocene to Oligocene; (6) secondary tectonic inversion from Oligocene to Miocene; (7) ocean subsidence stage. Since no obvious secondary tectonic inversion stage was found in the central basin, the structural evolution of the central basin is divided into five stages: (1) intracontinental rift stage from early Cretaceous to late Cretaceous; (2) late Cretaceous fault transition period; (3) the early depression stage of the Eocene; (4) the tectonic inversion stage in the late Eocene; (5) the stage of marine subsidence from the Eocene to the present. At this stage, the basin as a whole descended and gradually formed its present appearance. The northern part of the Fairway Basin is greatly affected by regional tectonic activities, and the Cretaceous strata have more fault structures. More diapir structure occurred in the late Cretaceous strata in the central part of the basin. Since the formation of the southern part of the basin, it has been less affected by tectonic activities, the developmental strata are intact, and the pre-Cenozoic strata are thicker. The tectonic activity is strong in the northern part of the Fairway Basin, with a weaker central part and a smaller southern part. The sedimentary strata are thickened and thinned from north to south.
2020, 42(7): 147-154.
doi: 10.3969/j.issn.0253-4193.2020.07.013
Abstract:
The Yintan Beach in the Beihai, Guangxi, is a typical macro-tidal beach characterized by wide beach surface and pure white sand. In recent years, Yintan Beach has been eroded, which result in the retreat of coastline and the disappearance each shoulder. To study the dynamic of beach erosion, the elevation, slope and retreat distance of beach surface were comprehensively analyzed. The change of beach surface elevation caused by Typhoon 1621 and Typhoon 1713, as well as the water level process and offshore wave characteristics during the two typhoons were studied. The relationship between typhoon track and beach erosion was discussed. The results show that since 1985, the position line of average sea level has retreated by 0−80 m. Beach erosion has resulted in a decrease in the slope of the upper beach surface and a more convex beach profile. The main reason for beach erosion is the frequency increase of typhoon landing in recent years. Typhoons that originate in the western Pacific Ocean and enter the Beibu Gulf through the northern part of the Hainan Island or Leizhou Peninsula can form strong storms and cause higher coastal water increase along the coast of the Guangxi, which is most likely to lead to strong beach erosion events.
The Yintan Beach in the Beihai, Guangxi, is a typical macro-tidal beach characterized by wide beach surface and pure white sand. In recent years, Yintan Beach has been eroded, which result in the retreat of coastline and the disappearance each shoulder. To study the dynamic of beach erosion, the elevation, slope and retreat distance of beach surface were comprehensively analyzed. The change of beach surface elevation caused by Typhoon 1621 and Typhoon 1713, as well as the water level process and offshore wave characteristics during the two typhoons were studied. The relationship between typhoon track and beach erosion was discussed. The results show that since 1985, the position line of average sea level has retreated by 0−80 m. Beach erosion has resulted in a decrease in the slope of the upper beach surface and a more convex beach profile. The main reason for beach erosion is the frequency increase of typhoon landing in recent years. Typhoons that originate in the western Pacific Ocean and enter the Beibu Gulf through the northern part of the Hainan Island or Leizhou Peninsula can form strong storms and cause higher coastal water increase along the coast of the Guangxi, which is most likely to lead to strong beach erosion events.
2020, 42(7): 130-138.
doi: 10.3969/j.issn.0253-4193.2020.07.011
Abstract:
To overcome the problem of the low contrast and blurred details of underwater images, we propose a method to enhance the details of underwater images based on unsharp mask guided filtering in this paper. Firstly, the original image is guided filtering to obtain a detail layer image, and the speckle noise in detail layer image is removed by the median filtering based on noise detection. Then the unsharp mask based on mean filtering is operated on the original image to obtain a sharpened image, which is used as guidance image to perform guided filtering on the original image, and the base layer image is obtained. It is magnified to boost the details so that the details of the output image are obtained. Finally, the filtered detail layer is magnified to boost the details, and the enhanced underwater image is the combination of the boosted detail layer and the base layer. The image processing results are compared and analyzed through three objective evaluation indexes: information entropy, local contrast and average gradient. Subjective and objective test results show that the proposed algorithm can effectively improve image contrast and detail information, which is beneficial to improve the accuracy of underwater image data interpretation.
To overcome the problem of the low contrast and blurred details of underwater images, we propose a method to enhance the details of underwater images based on unsharp mask guided filtering in this paper. Firstly, the original image is guided filtering to obtain a detail layer image, and the speckle noise in detail layer image is removed by the median filtering based on noise detection. Then the unsharp mask based on mean filtering is operated on the original image to obtain a sharpened image, which is used as guidance image to perform guided filtering on the original image, and the base layer image is obtained. It is magnified to boost the details so that the details of the output image are obtained. Finally, the filtered detail layer is magnified to boost the details, and the enhanced underwater image is the combination of the boosted detail layer and the base layer. The image processing results are compared and analyzed through three objective evaluation indexes: information entropy, local contrast and average gradient. Subjective and objective test results show that the proposed algorithm can effectively improve image contrast and detail information, which is beneficial to improve the accuracy of underwater image data interpretation.
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