Huguangyan Maar Lake is a typical maar lake in the southeast of China. It is well preserved and not disturbed by anthropogenic activities. In this study, microbial community structures in sediment and water samples fr...Huguangyan Maar Lake is a typical maar lake in the southeast of China. It is well preserved and not disturbed by anthropogenic activities. In this study, microbial community structures in sediment and water samples from Huguangyan Maar Lake were investigated using a high-throughput sequencing method. We found significant differences between the microbial community compositions of the water and the sediment. The sediment samples contained more diverse Bacteria and Archaea than did the water samples. Actinobacteria, Betaproteobacteria, Cyanobacteria, and Deltaproteobacteria predominated in the water samples while Deltaproteobacteria, Anaerolineae, Nitrospira, and Dehalococcoidia were the major bacterial groups in the sediment. As for Archaea, Woesearchaeota (DHVEG-6), unclassified Archaea, and Deep Sea Euryarchaeotic Group were detected at higher abundances in the water, whereas the Miscellaneous Crenarchaeotic Group, Thermoplasmata, and Methanomicrobia were significantly more abundant in the sediment. Interactions between Bacteria and Archaea were common in both the water column and the sediment. The concentrations of major nutrients (NO^3-, PO4^3-, SiO3^2- and NH4^+) shaped the microbial population structures in the water. At the higher phylogenetic levels including phylum and class, many of the dominant groups were those that were also abundant in other lakes;however, novel microbial populations (unclassified) were often seen at the lower phylogenetic levels. Our study lays a foundation for examining microbial biogeochemical cycling in sequestered lakes or reservoirs.展开更多
The dissolved organic carbon(DOC)in the surface water of Huguangyan Maar Lake was continuously monitored based on once per week sampling frequency of 4 locations from June 2018 to May 2019.The DOC concentrations and i...The dissolved organic carbon(DOC)in the surface water of Huguangyan Maar Lake was continuously monitored based on once per week sampling frequency of 4 locations from June 2018 to May 2019.The DOC concentrations and its seasonal variation were discussed by correlating with the rainfall,water temperature,and p H of lake water.The results showed that the weekly DOC varied from 8.15 to 32.65 mg/L with an annual average concentration of 17.96 mg/L.There was a significant difference in the average DOC concentrations between the rainy and dry seasons as the monthly average DOC concentration was 21.72 mg/L for the wet season compared to the dry season concentration of 14.21 mg/L.The rainfall shows a significant effect on the DOC concentration of lake water,as DOC concentration was much high during the wet season.There were no significant spatial variations in the average monthly concentration among the four sampling locations except occasional variation during the wet season.The reason for the wet season DOC differences among four sampling locations is likely due to the uneven runoff and underground water inputs and the relatively slow circulation of lake water.Finally,the seasonal fluctuation of DOC concentration in this closed lake water suggests that dissolved soil organic matter inputs through the rainfall related to surface runoff and subsurface infiltration from the surrounding watershed land is important to the primary production and organic matter deposition in the lake sediments.展开更多
The Huguangyan Maar Lake（HML）（21°9＇N,110°17＇E）,situated on the Leizhou Peninsula in the southernmost of China's Mainland,is the deepest recent crater lake among the identified volcanic structures i...The Huguangyan Maar Lake（HML）（21°9＇N,110°17＇E）,situated on the Leizhou Peninsula in the southernmost of China's Mainland,is the deepest recent crater lake among the identified volcanic structures in the Leiqiong Volcanic Field.The bi-lobate lake.展开更多
To mark the occasion of the 175th anniversary of the Rheinische Naturforschende Gesellschaft in 2009 and of the centennial of the Mainz Natural History Museum in 2010, we present a short account of our present knowled...To mark the occasion of the 175th anniversary of the Rheinische Naturforschende Gesellschaft in 2009 and of the centennial of the Mainz Natural History Museum in 2010, we present a short account of our present knowledge of the Eckfeld Maar after 20 years of continuous research. This paper does not attempt to include all of the detailed results on the geology of the Eckfeld site or its biota. To date, nearly 250 papers and books have been published since the start of our project. An up-to-date list of these publications can be found at www.eckfeldermaar.de.展开更多
The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was tho...The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.展开更多
The Bambili maars are twin contemporaneous maars embedded in trachytic rocks. The two maars are separated by a low lying inter-Maar dome whose inner walls dip towards the Southern Maar. The Bambili Northern Maar (BNM)...The Bambili maars are twin contemporaneous maars embedded in trachytic rocks. The two maars are separated by a low lying inter-Maar dome whose inner walls dip towards the Southern Maar. The Bambili Northern Maar (BNM) contains water and its floor is found at an altitude lower than the Bambili Southern Maar (BSM) (50 m) which is swampy and drains into the BNM. Both maars have very steep vertical walls. Three different units are identified in the area. Unit 1 is 1 m thick sequence, characterize by the presence of matrix supported fine-grained fallouts (ash and lapilli size) with stratified diffused bedding planes. This unit is further subdivided into three different layers: L1, L2 and L3 from bottom to top with increasing sizes of particles respectively. This layer shows reverser grading and its grain sizes rarely exceed 2 cm and this unit shows no depositional structures. Unit 2 is an unstratified unit predominantly made up of tephra deposits. Within these deposits are found blocks of sizes ranging between 2 and 10 cm of different petrographic types. They include scoria, basalts, trachyte, and granite cognate. The last unit, Unit 3 is distinctly stratified characterized by well sorted, heterolithologic, graded bedding layers which are matrix supported. Base on the composition and grain sizes, this unit was subdivided into 7 horizons: h1 - h7. Within this unit, two sequences of deposits are distinguished: U1 and U2. Both sequences are characterized by alternation of lapilli and tuff beds. The clast comprises of juvenile materials, mudstone which are highly vesicular and altered rock fragments. Other products identified within the area are ferolithic volcanic bombs (5 - 20 cm), xenocrystals (2 - 4 cm) within finer deposits. They are elongated and show visible twinned planes. Explosion breccias are equally identified occurring as bolders. Along the inner walls of the maars are also found large basaltic and trachytic blocks which are remnants of pre-maar unit within maar deposits. The model proposed indicates the maars were emplaced simultaneously from adjacent vents followed by a succession of lava flows and pyroclastic ejections.展开更多
Located in Xilin Gol League of Inner Mongolia Autonomous Region,the Xilinhot-Abaga volcanic cluster is in the centre of the Daxing’anling-Datong volcanic eruption belt.More than 300 Quaternary basaltic volcanoes of d...Located in Xilin Gol League of Inner Mongolia Autonomous Region,the Xilinhot-Abaga volcanic cluster is in the centre of the Daxing’anling-Datong volcanic eruption belt.More than 300 Quaternary basaltic volcanoes of different types are developed in the volcanic eruption belt.Maars are found for the first time in the study area,with those in Haotewula in the southeast of Abagaqi,and Chelewula and Esigewula in the northwest of Abagaqi to be the most representative.They all are large in scale,with cone diameters generally 3~4km,with the largest up to 6.5km.The maars show intact structure,a similar two-ringed mountain landscape and obviously phased eruption which contained a vigorous phreatomagmatic explosion at the earlier stage,and weak magmatic explosion at the later stage,and basaltic lava effusion in the last stage.The eruption sequence reflects the interaction between magma and water,the change of intrusive speed and outflow rate of magma.The Middle Pleistocene deposits of sand and gravel in the valley are covered by basal surge deposits.The TL age of sandstone-shale 'xenolith'which was found in the spatter deposits close to the crater is( 0.112 ± 0.0096) Ma,indicating that the eruption time of the Maars was the early Late Pleistocene.展开更多
This study explores the magmatic and eruptive processes of the Manengouba maars,located along the Cameroon Volcanic Line(CVL).Petrographic analyses of the samples reveal a diverse mineralogical composition with clinop...This study explores the magmatic and eruptive processes of the Manengouba maars,located along the Cameroon Volcanic Line(CVL).Petrographic analyses of the samples reveal a diverse mineralogical composition with clinopyroxenes(20%-35%),plagioclases(19%-35%),olivines(5%-15%),and sanidines(up to 15%).Positive Europium(Eu)anomalies observed suggest crystallization under reducing conditions.X-ray diffraction analyses show a significant presence of smectite(25.99%to 76.05%),kaolinite,and other clay minerals,indicating extensive hydrothermal alteration.Geochemical diagrams reveal clear trends of magmatic fractionation,with an increase in Zr(up to 200 ppm)with increasing SiO_(2)(45%to 60%)and a decrease in MgO(from 10%to less than 5%)accompanied by an increase in Nb(from 10 to 50 ppm).These trends suggest magmatic differentiation prior to the eruption.Explosive interactions between ascending magma and groundwater led to phreatomagmatic explosions,forming complex diatremic structures.The presence of mantle xenoliths and host rock fragments indicates significant assimilation of the host rocks during magma ascent,modifying its chemical composition.The results allow proposing a diatreme growth model,highlighting the importance of phreatomagmatic interactions and magmatic fractionation processes.Compared to other volcanic regions,the geology of Manengouba shows significant similarities with other volcanoes along the CVL and with maars and diatremes studied elsewhere in the world.In conclusion,this study enriches our understanding of volcanic and magmatic processes in the Manengouba region,providing crucial insights into eruptive dynamics and associated geochemical and mineralogical conditions.展开更多
There exist five primary periods of 2 930, 1 140, 490, 250 and 220 a in the Holocene climatic variations in the Huguangyan Maar Lake, according to the energy-spectrum and filter analyses of high-resolution time sequen...There exist five primary periods of 2 930, 1 140, 490, 250 and 220 a in the Holocene climatic variations in the Huguangyan Maar Lake, according to the energy-spectrum and filter analyses of high-resolution time sequences (10-15 a) of the sediment dry density. The peak values of the three temperature-decreasing periods with the 2 930 a cycle occur at about 7 300, 4 250 and 1 200 Cal. aBP. There are 7-8 temperature-decreasing periods with the 1 140 a cycle, and the climate fluctuation range is largest in the early Holocene, and reduces gradually in the middle and late Holocene. The millennial-scale climatic change in the Holocene may adjust the global water cycle and the thermohaline circulation intensity through the harmonic tones of the earth’s precession cycle, which in turn influences the global climate change.展开更多
The concentrations of biogenic silica, total organic carbon, total nitrogen and total hydrogen inferred from the sediments of tropical Maar Lake Huguangyan, southern China, provide a climate record of the last deglaci...The concentrations of biogenic silica, total organic carbon, total nitrogen and total hydrogen inferred from the sediments of tropical Maar Lake Huguangyan, southern China, provide a climate record of the last deglaciation with century resolution. The records fully demonstrate the existence of the two-step shape of the last deglaciation in tropic East Asia, and they point out noticeable differences between the low and high latitudes in the Northern Hemisphere. Thus, the Boiling first warming at the last deglaciation in the low latitude may have preceded that of the high latitude, whereas the cooling of the Younger Dryas occurred synchronously in the two regions. These results likely suggest that the links between the low and high latitude climates in the Northern Hemisphere during this period are complexity.展开更多
Varved lacustrine sediments have been found from high latitude to low latitude, even in tropical area. Paleoenvironmental and paleoclimatic research has increasingly relied on the use of varved sediments to provide hi...Varved lacustrine sediments have been found from high latitude to low latitude, even in tropical area. Paleoenvironmental and paleoclimatic research has increasingly relied on the use of varved sediments to provide high-resolution geochronologies and climatic reconstructions. However, a special depositional environment is necessary for varve formation and preservation. Generally, varves can be found in the sediments of maar lakes because of their special geological and hydrological environment. The preliminary research showed that a few carbonate laminations are developed in the Huguangyan maar lake, south China. They are poorly developed and not annual laminations, and may be related to periodically drought. Diatomaceous laminations are found in the maar lakes in northeast China. Diatomaceous laminations with cyclic nature corresponding with seasonal diatom blooming are the credible natural clock.展开更多
As the second common type of volcanic vent on Earth, maar-diatreme volcanoes and their post-eruptive lacustrine sediments are a main focus of volcanology, palaeolimnology, palaeoclimatology and palaeontology. A number...As the second common type of volcanic vent on Earth, maar-diatreme volcanoes and their post-eruptive lacustrine sediments are a main focus of volcanology, palaeolimnology, palaeoclimatology and palaeontology. A number of maar-type volcanoes have been found in Leizhou Peninsula, South China, but little is known about their eruption processes and detailed stratigraphy of the post-eruptive sediments. We present a combined geophysical and geological analysis to study the eruptive history and post-eruptive sediment stratigraphy of a large maar, the elliptical(1.8 × 3.0 km^(2)) Jiudouyang(JDY) maar. The lacustrine stratigraphy revealed by drilling cores shows that the JDY maar lake has three major stages of evolution:(i) deep-lake sedimentary environment characterized by high autochthonous diatom productivity;(ii) shallow lake to swamp with very low water levels, characterized by a high total organic carbon(TOC) and abundant wood fragments;and,(iii) intermittent shallow lake and alluvial deposits composed of clay minerals and sand. The electrical resistivity tomography(ERT) values and lithological features are highly consistent, which clearly reveal the presence of ca. 50 m thick lacustrine sediments, directly underlain by a ca. 70 m thick basaltic lava rather than diatreme breccia in the crater. This infill sequence implies an alternation of eruption style from phreatomagmatic to Strombolian and/or lava flow, due to high magma flux and ascent rate of the Hainan Plume during the middle Pleistocene. The ERT data also reveal the initial phreatomagmatic crater floor at ca. 120 m depth. The initial crater had a large diameter/depth ratio(ca. 17), with an elongated shape(major axis to minor axis = 0.6), implying possible lateral vent migration during the eruption. A significant erosion under tropical weathering condition during the last few hundred thousand years, accounted for the large size of the maar crater. The study provides insights into the eruptive history and post-eruptive evolution of a large maar, as well as the spatial distribution of the lacustrine sediments.展开更多
A high-resolution pollen record of the past 13000 a from Huguangyan Maar Lake reveals the vegetation and environment changes in southern China during the Holocene. It shows that (i) pollen percentage of trees and shru...A high-resolution pollen record of the past 13000 a from Huguangyan Maar Lake reveals the vegetation and environment changes in southern China during the Holocene. It shows that (i) pollen percentage of trees and shrubs reached 56% during the early Holocene (11600―7800 cal a BP), of which the pollen percentage of tropical trees reached a maximum at 9500―8000 cal a BP, reflecting a hot and wet envi- ronment; (ii) during the mid-Holocene (7800―4200 cal a BP), the pollen percentage of montane conif- erous trees and herbs increased, while the percentage of tropical-subtropical trees decreased, indi- cating lower temperature and humidity; (iii) in the late Holocene spanning from 4200 to 350 cal a BP, the pollen percentage of herbs and montane conifer increased significantly, indicating a marked decrease of temperature and humidity. Our pollen data reveal that the time period 9500―8000 cal a BP in south- ern China represents a climatic optimum for the Holocene characterized by hot and wet conditions. This is consistent with the Holocene optimum found in lower latitude regions globally. We speculate that strong insolation might cause the northward migration of the ITCZ and subtropical summer mon- soon front, which resulted in an early Holocene optimum in the Huguangyan area. The dry tendency and climate fluctuations of the middle and late Holocene could be associated with a decrease in solar insolation and frequent ENSO event.展开更多
Varve counts with AMS 14 C,137 Cs and 210 Pb dating of sediments(0-900 cm) from Erlongwan Maar Lake,NE China were used to establish a high-resolution chronology series for the late Quaternary.Dry density,total organic...Varve counts with AMS 14 C,137 Cs and 210 Pb dating of sediments(0-900 cm) from Erlongwan Maar Lake,NE China were used to establish a high-resolution chronology series for the late Quaternary.Dry density,total organic carbon(TOC) content,total nitrogen(TN) content,TOC/TN ratios and stable organic carbon isotope(13 C org) ratios were continuously analyzed on this sediment profile.On the basis of lithological characters,sporo-pollen assemblages and geochemical analyses,we identified 6 climate stages within the last 14 ka BP.The time before the Holocene(14-11.4 ka BP) represents a higher-order oscillation climatic transitional period(I).The entire Holocene climate development(from 11.4 ka BP to present) exhibited an increasing temperature trend,although there were cold and warm alternations(II-VI).The periods included were:II(11.4-9.05 ka BP) warm-wet stage,III(9.05-7.4 ka BP) cold and warm fluctuation stage,IV(7.4-4.2 ka BP) smoothly warming climate stage,V(4.2-1.67 ka BP) climate optimum stage,and VI(from 1.67 ka BP to present) cool and drier stage.Each climate stage began with a warming event and ended with an abrupt cooling event.This climate change cycle had unequal time spaces that were progressively shorter over time.Several abrupt climate shifts occurred at about 9.4-9.05,8.5-8.2,7.8-7.4,4.6-4.2,3.7-3.25,2-1.67 and 0.3-0.03 ka BP.Thus,it can be seen that the climate has been warming since 1920 AD,which indicates a new climate stage.展开更多
The results of sedimentary characteristics, grain-size, clay mineral, C/N and pollen analysis reveal the evolution history of the Shuangchi Maar Lake of Hainan Island as well as the short-term climate variations in tr...The results of sedimentary characteristics, grain-size, clay mineral, C/N and pollen analysis reveal the evolution history of the Shuangchi Maar Lake of Hainan Island as well as the short-term climate variations in tropical areas. It is testified that the Maar sediment system was sensitive in recording the rapid climate changes in the Holocene. These multidisciplinary results demonstrate that the thermal maximum in the Holocene climate lasted from 7200 to 2700 aBP, with several abrupt ephemeral vibrations occurring between 7250-6120 aBP and at around 4460, 3850 and 2700 aBP. In the late Holocene, particularly at ~2700 aBP, a distinct environmental change indicating an important climate transition in the tropical eastern Asia was recorded, probably responding to the enhancement of ENSO phenomena in the Pacific Ocean. In addition, the high-resolution algae record clearly indicates periodical climate fluctuations at 500 a cyclicity during the last 2500 years.展开更多
To understand the origin and its climatic implications of the Huguang Maar sediments, we conducted laser grain size analysis and rock mangtic measurement for the recent deposits from the Huguang Maar and for the volca...To understand the origin and its climatic implications of the Huguang Maar sediments, we conducted laser grain size analysis and rock mangtic measurement for the recent deposits from the Huguang Maar and for the volcanic rocks of the caldera for comparison. The grain size distributions are dominated by clayey silts for both the bulk sediments and magnetic separates. Acquisition of isothermal remanent magnetization (IRM) and thermal demagnetization of IRM suggest (titano) magnetite as the dominating magnetic mineral in the sediments; high coercivity minerals are not detected. Therefore, the ratio of the susceptibility of anhysteretic remanent magnetization (kARM) to the weak field magnetic susceptibility (k), kARM/k can be used to estimate the grain size of magnetite. The sediments and volcanic rocks yield values of 13.7 and 11.8 for the kARM/k, respectively, corresponding to grain size values of ～0.03 μm and ～0.05 μm. The magnetic granulometry of lake sediments is distinctive from one of the Chinese loess, in which published data suggest a value of 6.0 for kARM/k, and grain size of ～0.1 μm, but is akin to the nearby volcanic rocks. Although a Loess Plateau source of the lake sediments cannot be fully ruled out, the catchment is most probably the main source. We explain the slightly finer magnetite grain size in sediments than in the volcanic rocks as being disintegrated of particles during erosion and transportation. We conclude that there is no evidence for aeolian input of the Loess Plateau dusts into the Huguang Maar, and the relationship between magnetic parameters of the lake sediments and winter monsoon intensity is probably more complicated than previously thought.展开更多
基金Supported by the National Natural Science Foundation of China(Nos.41576123,41706129)the Guangdong Natural Science Foundation(Nos.2015A030313326,2016A030312004)+2 种基金the International Science and Technology Cooperation Project(No.GASI-IPOVI-04)the Project of Enhancing School with Innovation of Guangdong Ocean University(No.GDOU2016050243)the Program for Scientific Research Start-Up Funds of Guangdong Ocean University(No.E15030)
文摘Huguangyan Maar Lake is a typical maar lake in the southeast of China. It is well preserved and not disturbed by anthropogenic activities. In this study, microbial community structures in sediment and water samples from Huguangyan Maar Lake were investigated using a high-throughput sequencing method. We found significant differences between the microbial community compositions of the water and the sediment. The sediment samples contained more diverse Bacteria and Archaea than did the water samples. Actinobacteria, Betaproteobacteria, Cyanobacteria, and Deltaproteobacteria predominated in the water samples while Deltaproteobacteria, Anaerolineae, Nitrospira, and Dehalococcoidia were the major bacterial groups in the sediment. As for Archaea, Woesearchaeota (DHVEG-6), unclassified Archaea, and Deep Sea Euryarchaeotic Group were detected at higher abundances in the water, whereas the Miscellaneous Crenarchaeotic Group, Thermoplasmata, and Methanomicrobia were significantly more abundant in the sediment. Interactions between Bacteria and Archaea were common in both the water column and the sediment. The concentrations of major nutrients (NO^3-, PO4^3-, SiO3^2- and NH4^+) shaped the microbial population structures in the water. At the higher phylogenetic levels including phylum and class, many of the dominant groups were those that were also abundant in other lakes;however, novel microbial populations (unclassified) were often seen at the lower phylogenetic levels. Our study lays a foundation for examining microbial biogeochemical cycling in sequestered lakes or reservoirs.
基金the Special Talent Support Program of Guangdong Ocean University(No.R17001)the Project of Marine geochemistry and climate change of Guangdong Ocean University(No.002026002004)。
文摘The dissolved organic carbon(DOC)in the surface water of Huguangyan Maar Lake was continuously monitored based on once per week sampling frequency of 4 locations from June 2018 to May 2019.The DOC concentrations and its seasonal variation were discussed by correlating with the rainfall,water temperature,and p H of lake water.The results showed that the weekly DOC varied from 8.15 to 32.65 mg/L with an annual average concentration of 17.96 mg/L.There was a significant difference in the average DOC concentrations between the rainy and dry seasons as the monthly average DOC concentration was 21.72 mg/L for the wet season compared to the dry season concentration of 14.21 mg/L.The rainfall shows a significant effect on the DOC concentration of lake water,as DOC concentration was much high during the wet season.There were no significant spatial variations in the average monthly concentration among the four sampling locations except occasional variation during the wet season.The reason for the wet season DOC differences among four sampling locations is likely due to the uneven runoff and underground water inputs and the relatively slow circulation of lake water.Finally,the seasonal fluctuation of DOC concentration in this closed lake water suggests that dissolved soil organic matter inputs through the rainfall related to surface runoff and subsurface infiltration from the surrounding watershed land is important to the primary production and organic matter deposition in the lake sediments.
基金financially supported by the China Geological Survey(grant no.1212011120045)the NSFC(grant no.41274074)
文摘The Huguangyan Maar Lake（HML）（21°9＇N,110°17＇E）,situated on the Leizhou Peninsula in the southernmost of China's Mainland,is the deepest recent crater lake among the identified volcanic structures in the Leiqiong Volcanic Field.The bi-lobate lake.
基金supported by the Government of Rhineland-PalatinateThe German Science Foundation and the Stiftung Rheinland-Pfalz fur Innovation have financially supported several projects and for this we are grateful.
文摘To mark the occasion of the 175th anniversary of the Rheinische Naturforschende Gesellschaft in 2009 and of the centennial of the Mainz Natural History Museum in 2010, we present a short account of our present knowledge of the Eckfeld Maar after 20 years of continuous research. This paper does not attempt to include all of the detailed results on the geology of the Eckfeld site or its biota. To date, nearly 250 papers and books have been published since the start of our project. An up-to-date list of these publications can be found at www.eckfeldermaar.de.
文摘The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 m-thick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.
文摘The Bambili maars are twin contemporaneous maars embedded in trachytic rocks. The two maars are separated by a low lying inter-Maar dome whose inner walls dip towards the Southern Maar. The Bambili Northern Maar (BNM) contains water and its floor is found at an altitude lower than the Bambili Southern Maar (BSM) (50 m) which is swampy and drains into the BNM. Both maars have very steep vertical walls. Three different units are identified in the area. Unit 1 is 1 m thick sequence, characterize by the presence of matrix supported fine-grained fallouts (ash and lapilli size) with stratified diffused bedding planes. This unit is further subdivided into three different layers: L1, L2 and L3 from bottom to top with increasing sizes of particles respectively. This layer shows reverser grading and its grain sizes rarely exceed 2 cm and this unit shows no depositional structures. Unit 2 is an unstratified unit predominantly made up of tephra deposits. Within these deposits are found blocks of sizes ranging between 2 and 10 cm of different petrographic types. They include scoria, basalts, trachyte, and granite cognate. The last unit, Unit 3 is distinctly stratified characterized by well sorted, heterolithologic, graded bedding layers which are matrix supported. Base on the composition and grain sizes, this unit was subdivided into 7 horizons: h1 - h7. Within this unit, two sequences of deposits are distinguished: U1 and U2. Both sequences are characterized by alternation of lapilli and tuff beds. The clast comprises of juvenile materials, mudstone which are highly vesicular and altered rock fragments. Other products identified within the area are ferolithic volcanic bombs (5 - 20 cm), xenocrystals (2 - 4 cm) within finer deposits. They are elongated and show visible twinned planes. Explosion breccias are equally identified occurring as bolders. Along the inner walls of the maars are also found large basaltic and trachytic blocks which are remnants of pre-maar unit within maar deposits. The model proposed indicates the maars were emplaced simultaneously from adjacent vents followed by a succession of lava flows and pyroclastic ejections.
基金sponsored by the Special Fund of Public Research,China Earthquake Administration(200782713)Special Fund of the Geological Heritage Protection,Department of Land and Resources,Inner Mongolia Autonomous Region,China(20101030)
文摘Located in Xilin Gol League of Inner Mongolia Autonomous Region,the Xilinhot-Abaga volcanic cluster is in the centre of the Daxing’anling-Datong volcanic eruption belt.More than 300 Quaternary basaltic volcanoes of different types are developed in the volcanic eruption belt.Maars are found for the first time in the study area,with those in Haotewula in the southeast of Abagaqi,and Chelewula and Esigewula in the northwest of Abagaqi to be the most representative.They all are large in scale,with cone diameters generally 3~4km,with the largest up to 6.5km.The maars show intact structure,a similar two-ringed mountain landscape and obviously phased eruption which contained a vigorous phreatomagmatic explosion at the earlier stage,and weak magmatic explosion at the later stage,and basaltic lava effusion in the last stage.The eruption sequence reflects the interaction between magma and water,the change of intrusive speed and outflow rate of magma.The Middle Pleistocene deposits of sand and gravel in the valley are covered by basal surge deposits.The TL age of sandstone-shale 'xenolith'which was found in the spatter deposits close to the crater is( 0.112 ± 0.0096) Ma,indicating that the eruption time of the Maars was the early Late Pleistocene.
文摘This study explores the magmatic and eruptive processes of the Manengouba maars,located along the Cameroon Volcanic Line(CVL).Petrographic analyses of the samples reveal a diverse mineralogical composition with clinopyroxenes(20%-35%),plagioclases(19%-35%),olivines(5%-15%),and sanidines(up to 15%).Positive Europium(Eu)anomalies observed suggest crystallization under reducing conditions.X-ray diffraction analyses show a significant presence of smectite(25.99%to 76.05%),kaolinite,and other clay minerals,indicating extensive hydrothermal alteration.Geochemical diagrams reveal clear trends of magmatic fractionation,with an increase in Zr(up to 200 ppm)with increasing SiO_(2)(45%to 60%)and a decrease in MgO(from 10%to less than 5%)accompanied by an increase in Nb(from 10 to 50 ppm).These trends suggest magmatic differentiation prior to the eruption.Explosive interactions between ascending magma and groundwater led to phreatomagmatic explosions,forming complex diatremic structures.The presence of mantle xenoliths and host rock fragments indicates significant assimilation of the host rocks during magma ascent,modifying its chemical composition.The results allow proposing a diatreme growth model,highlighting the importance of phreatomagmatic interactions and magmatic fractionation processes.Compared to other volcanic regions,the geology of Manengouba shows significant similarities with other volcanoes along the CVL and with maars and diatremes studied elsewhere in the world.In conclusion,this study enriches our understanding of volcanic and magmatic processes in the Manengouba region,providing crucial insights into eruptive dynamics and associated geochemical and mineralogical conditions.
文摘There exist five primary periods of 2 930, 1 140, 490, 250 and 220 a in the Holocene climatic variations in the Huguangyan Maar Lake, according to the energy-spectrum and filter analyses of high-resolution time sequences (10-15 a) of the sediment dry density. The peak values of the three temperature-decreasing periods with the 2 930 a cycle occur at about 7 300, 4 250 and 1 200 Cal. aBP. There are 7-8 temperature-decreasing periods with the 1 140 a cycle, and the climate fluctuation range is largest in the early Holocene, and reduces gradually in the middle and late Holocene. The millennial-scale climatic change in the Holocene may adjust the global water cycle and the thermohaline circulation intensity through the harmonic tones of the earth’s precession cycle, which in turn influences the global climate change.
文摘The concentrations of biogenic silica, total organic carbon, total nitrogen and total hydrogen inferred from the sediments of tropical Maar Lake Huguangyan, southern China, provide a climate record of the last deglaciation with century resolution. The records fully demonstrate the existence of the two-step shape of the last deglaciation in tropic East Asia, and they point out noticeable differences between the low and high latitudes in the Northern Hemisphere. Thus, the Boiling first warming at the last deglaciation in the low latitude may have preceded that of the high latitude, whereas the cooling of the Younger Dryas occurred synchronously in the two regions. These results likely suggest that the links between the low and high latitude climates in the Northern Hemisphere during this period are complexity.
文摘Varved lacustrine sediments have been found from high latitude to low latitude, even in tropical area. Paleoenvironmental and paleoclimatic research has increasingly relied on the use of varved sediments to provide high-resolution geochronologies and climatic reconstructions. However, a special depositional environment is necessary for varve formation and preservation. Generally, varves can be found in the sediments of maar lakes because of their special geological and hydrological environment. The preliminary research showed that a few carbonate laminations are developed in the Huguangyan maar lake, south China. They are poorly developed and not annual laminations, and may be related to periodically drought. Diatomaceous laminations are found in the maar lakes in northeast China. Diatomaceous laminations with cyclic nature corresponding with seasonal diatom blooming are the credible natural clock.
基金This work was supported by the National Key R&D Program of China(Grant No.2016 YFA0600500)the National Natural Science Foundation of China(Grant Nos.41472143 and 41661144003)the Guangdong Province Introduction of Innovative R&D Team of Geological Processes and Natural Disasters around the South China Sea(Grant No.2016ZT06N331)。
文摘As the second common type of volcanic vent on Earth, maar-diatreme volcanoes and their post-eruptive lacustrine sediments are a main focus of volcanology, palaeolimnology, palaeoclimatology and palaeontology. A number of maar-type volcanoes have been found in Leizhou Peninsula, South China, but little is known about their eruption processes and detailed stratigraphy of the post-eruptive sediments. We present a combined geophysical and geological analysis to study the eruptive history and post-eruptive sediment stratigraphy of a large maar, the elliptical(1.8 × 3.0 km^(2)) Jiudouyang(JDY) maar. The lacustrine stratigraphy revealed by drilling cores shows that the JDY maar lake has three major stages of evolution:(i) deep-lake sedimentary environment characterized by high autochthonous diatom productivity;(ii) shallow lake to swamp with very low water levels, characterized by a high total organic carbon(TOC) and abundant wood fragments;and,(iii) intermittent shallow lake and alluvial deposits composed of clay minerals and sand. The electrical resistivity tomography(ERT) values and lithological features are highly consistent, which clearly reveal the presence of ca. 50 m thick lacustrine sediments, directly underlain by a ca. 70 m thick basaltic lava rather than diatreme breccia in the crater. This infill sequence implies an alternation of eruption style from phreatomagmatic to Strombolian and/or lava flow, due to high magma flux and ascent rate of the Hainan Plume during the middle Pleistocene. The ERT data also reveal the initial phreatomagmatic crater floor at ca. 120 m depth. The initial crater had a large diameter/depth ratio(ca. 17), with an elongated shape(major axis to minor axis = 0.6), implying possible lateral vent migration during the eruption. A significant erosion under tropical weathering condition during the last few hundred thousand years, accounted for the large size of the maar crater. The study provides insights into the eruptive history and post-eruptive evolution of a large maar, as well as the spatial distribution of the lacustrine sediments.
基金Supported by the National Natural Science Foundation of China for Distinguished Youth Scholar (Grant No. 40325002)the Key Research Project of the Knowledge Innovation Program of CAS (Grant Nos. KZCX3-SW-145 and KZCX2-YW-117)+1 种基金the National Basic Research Program of China (Grant No. 2005CB422002-2)the National Natural Science Foundation of China (Grant No. 40331011)
文摘A high-resolution pollen record of the past 13000 a from Huguangyan Maar Lake reveals the vegetation and environment changes in southern China during the Holocene. It shows that (i) pollen percentage of trees and shrubs reached 56% during the early Holocene (11600―7800 cal a BP), of which the pollen percentage of tropical trees reached a maximum at 9500―8000 cal a BP, reflecting a hot and wet envi- ronment; (ii) during the mid-Holocene (7800―4200 cal a BP), the pollen percentage of montane conif- erous trees and herbs increased, while the percentage of tropical-subtropical trees decreased, indi- cating lower temperature and humidity; (iii) in the late Holocene spanning from 4200 to 350 cal a BP, the pollen percentage of herbs and montane conifer increased significantly, indicating a marked decrease of temperature and humidity. Our pollen data reveal that the time period 9500―8000 cal a BP in south- ern China represents a climatic optimum for the Holocene characterized by hot and wet conditions. This is consistent with the Holocene optimum found in lower latitude regions globally. We speculate that strong insolation might cause the northward migration of the ITCZ and subtropical summer mon- soon front, which resulted in an early Holocene optimum in the Huguangyan area. The dry tendency and climate fluctuations of the middle and late Holocene could be associated with a decrease in solar insolation and frequent ENSO event.
基金supported by the National Natural Science Foundation of China (40802038)the President Fund of GUCAS
文摘Varve counts with AMS 14 C,137 Cs and 210 Pb dating of sediments(0-900 cm) from Erlongwan Maar Lake,NE China were used to establish a high-resolution chronology series for the late Quaternary.Dry density,total organic carbon(TOC) content,total nitrogen(TN) content,TOC/TN ratios and stable organic carbon isotope(13 C org) ratios were continuously analyzed on this sediment profile.On the basis of lithological characters,sporo-pollen assemblages and geochemical analyses,we identified 6 climate stages within the last 14 ka BP.The time before the Holocene(14-11.4 ka BP) represents a higher-order oscillation climatic transitional period(I).The entire Holocene climate development(from 11.4 ka BP to present) exhibited an increasing temperature trend,although there were cold and warm alternations(II-VI).The periods included were:II(11.4-9.05 ka BP) warm-wet stage,III(9.05-7.4 ka BP) cold and warm fluctuation stage,IV(7.4-4.2 ka BP) smoothly warming climate stage,V(4.2-1.67 ka BP) climate optimum stage,and VI(from 1.67 ka BP to present) cool and drier stage.Each climate stage began with a warming event and ended with an abrupt cooling event.This climate change cycle had unequal time spaces that were progressively shorter over time.Several abrupt climate shifts occurred at about 9.4-9.05,8.5-8.2,7.8-7.4,4.6-4.2,3.7-3.25,2-1.67 and 0.3-0.03 ka BP.Thus,it can be seen that the climate has been warming since 1920 AD,which indicates a new climate stage.
基金the National Natural Science Foundation of China (Grant No. 40071084)
文摘The results of sedimentary characteristics, grain-size, clay mineral, C/N and pollen analysis reveal the evolution history of the Shuangchi Maar Lake of Hainan Island as well as the short-term climate variations in tropical areas. It is testified that the Maar sediment system was sensitive in recording the rapid climate changes in the Holocene. These multidisciplinary results demonstrate that the thermal maximum in the Holocene climate lasted from 7200 to 2700 aBP, with several abrupt ephemeral vibrations occurring between 7250-6120 aBP and at around 4460, 3850 and 2700 aBP. In the late Holocene, particularly at ~2700 aBP, a distinct environmental change indicating an important climate transition in the tropical eastern Asia was recorded, probably responding to the enhancement of ENSO phenomena in the Pacific Ocean. In addition, the high-resolution algae record clearly indicates periodical climate fluctuations at 500 a cyclicity during the last 2500 years.
基金supported by the Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Acad-emy of Sciences (Grant No. MSGL08-20)
文摘To understand the origin and its climatic implications of the Huguang Maar sediments, we conducted laser grain size analysis and rock mangtic measurement for the recent deposits from the Huguang Maar and for the volcanic rocks of the caldera for comparison. The grain size distributions are dominated by clayey silts for both the bulk sediments and magnetic separates. Acquisition of isothermal remanent magnetization (IRM) and thermal demagnetization of IRM suggest (titano) magnetite as the dominating magnetic mineral in the sediments; high coercivity minerals are not detected. Therefore, the ratio of the susceptibility of anhysteretic remanent magnetization (kARM) to the weak field magnetic susceptibility (k), kARM/k can be used to estimate the grain size of magnetite. The sediments and volcanic rocks yield values of 13.7 and 11.8 for the kARM/k, respectively, corresponding to grain size values of ～0.03 μm and ～0.05 μm. The magnetic granulometry of lake sediments is distinctive from one of the Chinese loess, in which published data suggest a value of 6.0 for kARM/k, and grain size of ～0.1 μm, but is akin to the nearby volcanic rocks. Although a Loess Plateau source of the lake sediments cannot be fully ruled out, the catchment is most probably the main source. We explain the slightly finer magnetite grain size in sediments than in the volcanic rocks as being disintegrated of particles during erosion and transportation. We conclude that there is no evidence for aeolian input of the Loess Plateau dusts into the Huguang Maar, and the relationship between magnetic parameters of the lake sediments and winter monsoon intensity is probably more complicated than previously thought.
