A multi‐proxy record of changing environments from ca. 30 000 to 9000 cal. a BP: Onepoto maar palaeolake,Auckland, New Zealand

We present a high‐resolution record of lacustrine sedimentation spanning ca. 30 000 to 9000 cal. a BP from Onepoto maar, northern North Island, New Zealand. The multi‐proxy record of environmental change is constrained by tephrochronology and accelerator mass spectrometric ¹⁴C ages and provides evidence for episodes of rapid environmental change during the Last Glacial Coldest Period (LGCP) and Last Glacial–Interglacial Transition (LGIT) from northern New Zealand. The multi‐proxy palaeoenvironmental record from Onepoto indicates that the LGCP was cold, dry and windy in the Auckland region, with vegetation dominated by herb and grass in a beech forest mosaic between ca. 28 500 and 18 000 cal. a BP. The LGCP was accompanied by more frequent fires and influx of clastic sediment indicating increased erosion during the LGCP, with a mid‐LGCP interstadial identified between ca. 25 000 and 23 000 cal. a BP. Rapid climate amelioration at ca. 18 000 cal. a BP was accompanied by increased terrestrial biomass exemplified by the expansion of lowland podocarp forest, especially Dacrydium cupressinum. Increasing biomass production is reversed briefly by LGIT perturbations which are apparent in many of the proxies that span ca. 14 000–10 500 cal. a BP, suggesting generally increased wetness and higher in situ aquatic plant productivity with reduced terrestrial organic matter and terrigenous detrital influx. Furthermore, conditions at that time were probably warmer and frosts rare based on the increasing importance of Ascarina. The subsequent early Holocene is characterised by podocarp conifer forest and moist mild conditions. Postglacial sea‐level rise breached the crater rim and deposited 36 m of estuarine mud after ca. 9000 cal. a BP. Copyright © 2011 John Wiley & Sons, Ltd.     

Volcanology,history and myths of the Lake Albano maar (Colli Albani volcano,Italy)

The polygenetic Albano maar is the most recent centre of the Colli Albani volcano, located just few kilometres to the south-east of Roma. Presently the maar hosts a 167.5 m deep crater lake, the deepest in Europe. The maar is to be considered quiescent, as phreatic activity is documented throughout the Holocene. This paper illustrates the close relationships between the activity of the maar and the history of settlement in the Roman region as recorded in the geology, archaeology, history and legends of the area. Severe fluctuations of the groundwater table and catastrophic overflows of the Lake Albano from the maar rim had occurred prior to and after the early prehistoric settlements dated in the maar area at the Eneolithic times (ca. III millennium B.C.). Repeated lahars occurred along the northwestern slope of the maar filling in the paleodrainage network and forming a vast plain. Paleohydraulic analyses on fluvial and lahar deposits originated from the Holocene phreatic activity of the Albano maar indicate sediment–water flows in excess of hundreds of cubic metres per second. Absolute age determinations of the paleosoil underlying one of the most recent deposits of the lahar succession at 5800 ± 100 yr B.P. (¹⁴C CAL) are in perfect agreement with the age of the overlying Eneolithic age settlements. The last catastrophic overflow is described in the Roman literature as a consequence of the anger of Poseidon against the Romans in 398 B.C. for their war against the Etruscans. In 394 B.C. the Romans decided to prevent the repetition of such events by the excavation through the maar crater wall of a 1.5 km long drain tunnel, which is still operational, keeping the lake 70 m below the lowest point of the maar rim. This tunnel drain may be regarded as the first prevention device for volcanic hazard in history and shows an unprecedented development of the engineering technology under the pressure of hazard perception. The surprising and still largely unknown results of this study are very important to redefine the hazard of the Roman region.     

东北四海龙湾玛珥湖沉积物纹层计年与137Cs、210Pb测年

对东北四海龙湾玛珥湖SHLF6孔纹层沉积物的¹³⁷Cs放射性测量表明:¹³⁷Cs比活度的最大值出现在55cm处,对应于1963年世界原子弹试爆高峰期。纹层计年表明0～6cm共有35个纹层层偶。从75cm到65cm,¹³⁷Cs比活度从256±009dpm/g急剧增加到1868±017dpm/g,可能65cm对应于1954年。通过测量²²⁶Ra子核²¹⁴Pb和²¹⁴Bi（能量为295keV,352keV和609keV）放射的光子数获得²²⁶Ra比活度数据,然后求得过剩²¹⁰Pb_{比活度（²¹⁰Pbuns）。²¹⁰Pbuns比活度随深度增加而呈指数衰减,其异常波动可能与人类活动以及沉积速率变化有关,例如55cm处²¹⁰Pb比活度较高,与¹³⁷Cs的峰值对应,这可能与1963年前后人工核实验的高峰有关,因为核试验不仅产生¹³⁷Cs,而且可以产生208Pb和²¹⁰Pb;45cm处²¹⁰Pb比活度较低,而²²⁶Ra较高,可能与人类活动加剧,导致沉积速率增加有关。根据²¹⁰PbunsCRS模式,SHLF6孔0～19cm的平均沉积速率为20mg/cm²·a,或约为011cm/a。²¹⁰Pb测年数据与¹³⁷Cs时标及纹层计年均有很好的一致性。四海龙湾玛珥湖发育的纹层为年纹层,可以建立高分辨率时间序列。}     

40Ar/39Ar laser probe evidence concerning the age and associated hazards of the Lake Nyos Maar,Cameroon

The waters of Lake Nyos are impounded by a fragile natural dam composed of pyroclastic rocks ejected during the formation of the lake crater (maar). Lateral erosion of this dam has reduced its width from over 500 m to only 45 m. Published whole-rock K-Ar ages of about 100 ka on juvenile basalt from the dam suggests that erosion has been slow and that the dam poses no imminent threat. New apparent ⁴⁰Ar/³⁹Ar ages of 1.4 to 232 Ma on xenocrystic K-feldspar contained in the basalt show that the xenocrysts, whose source is the 528-Ma crystalline basement, are carriers of inherited radiogenic ⁴⁰Ar and would cause the whole-rock K-Ar ages to be too old. The best estimate for the age of the maar is provided by a ¹⁴C age of 400 ± 100 yr BP on charcoal from the base of the dam. This young age indicates that the dam is eroding at a relatively rapid rate; its failure, perhaps within a few decades, would result in a major flood and imperil thousands of people living downstream in Cameroon and eastern Nigeria.     

Ar/Ar and C geochronology of the Albano maar deposits: Implications for defining the age and eruptive style of the most recent explosive activity at Colli Albani Volcanic District,Central Italy

New ⁴⁰Ar/³⁹Ar and ¹⁴C ages have been found for the Albano multiple maar pyroclastic units and underlying paleosols to document the most recent explosive activity in the Colli Albani Volcanic District (CAVD) near Rome, Italy, consisting of seven eruptions (Albano 1 = oldest). Both dating methodologies have been applied on several proximal units and on four mid-distal fall/surge deposits, the latter correlated, according to two current different views, to either the Albano or the Campi di Annibale hydromagmatic center. The ⁴⁰Ar/³⁹Ar ages on leucite phenocrysts from the mid-distal units yielded ages of ca. 72 ka, 73 ka, 41 ka and 36 ka BP, which are indistinguishable from the previously determined ⁴⁰Ar/³⁹Ar ages of the proximal Albano units 1, 2, 5 and 7, thus confirming their stratigraphic correspondence.     

雷州半岛玛珥湖区玄武岩的年代、地球化学特征及其意义

我国南部雷州半岛是亚洲季风影响的核心区,区域内田洋、青桐洋和九斗洋这3个干玛珥湖沉积序列可为揭示低纬地区亚洲季风的轨道尺度变化提供关键证据。查明这些玛珥湖的形成年代、基底玄武岩的地球化学组成及其风化产物特征,为进一步获得沉积序列的年代控制、揭示沉积物替代指标的古气候意义提供了基础。本研究对3个干玛珥湖基底的玄武岩进行了K-Ar测年、地球化学成分和玛珥湖周围风化剖面粘土矿物的测试分析,得出了以下结论：1）田洋玛珥湖湖盆的形成年代为0.73~0.87 Ma,首次报道的青桐洋和九斗洋湖盆的形成时代分别为0.82 Ma和0.81~1.08 Ma,它们均属于早更新世晚期到中更新世早期的石峁岭期火山射气喷发形成的。2）田洋、青桐洋和九斗洋玛珥湖基底玄武岩均属于石英拉斑玄武岩。同一时期喷发的石英拉斑玄武岩广泛分布在玛珥湖的周围,这些玄武岩风化的产物以高岭石、三水铝石以及赤铁矿为主,在降水较为充沛的时期,物理或化学风化的产物随地表径流汇入湖盆,是玛珥湖沉积的主要来源。本研究为正在进行的低纬地区更新世轨道尺度季风降水重建提供了基础。

     

火山射气岩浆喷发作用研究进展

射气岩浆喷发是一种特殊类型的火山活动,水在这类火山活动中起到至关重要的作用,且其喷发产物——低平火山口和基浪堆积物在我国乃至全世界都有广泛的分布。国际上对射气岩浆喷发的研究始于1921年,迄今为止已有80余年的历史。国内外许多学者运用火山地质学、岩石学、沉积学、物理火山学及数值模拟等多学科研究手段,对射气岩浆喷发作用及其产物进行详细的野外观测描述,并探讨其成因机制。本文在前人研究基础上,以我国南方北部湾周边第四纪火山区大量存在的射气岩浆喷发成因的低平火山口和基浪堆积物为研究对象,深入讨论了基浪堆积物的地质特征、射气岩浆喷发形成的基本条件、喷发过程的动力学机制以及基浪流的搬运过程等几方面重要问题,并对已有的研究成果进行了概括和总结,提出有待解决的难点,揭示了这类火山活动特有的属性。     

Phreatomagmatic volcanic hazards where rift-systems meet the sea, a study from Ambae Island, Vanuatu

Ambae Island is a mafic stratovolcano located in the northern Vanuatu volcanic arc and has a NE–SW rift-controlled elongated shape. Several hundred scoria cones and fissure-fed lava fields occur along its long axis. After many decades of quiescence, Ambae Island erupted on the 28th of November 2005, disrupting the lives of its 10,000 inhabitants. Its activity remained focused at the central (crater-lake filled) vent and this is where hazard-assessments were focused. These assessments initially neglected that maars, tephra cones and rings occur at each tip of the island where the eruptive activity occurred < 500 and < 300 yr B.P. The products of this explosive phreatomagmatic activity are located where the rift axis meets the sea. At the NE edge of the island five tephra rings occur, each comparable in size to those on the summit of Ambae. Along the NE coastline, a near-continuous cliff section exposes an up to 25 m thick succession of near-vent phreatomagmatic tephra units derived from closely spaced vents. This can be subdivided into two major lithofacies associations. The first association represents when the locus of explosions was below sea level and comprises matrix-supported, massive to weakly stratified beds of coarse ash and lapilli. These are dominant in the lowermost part of the sequence and commonly contain coral fragments, indicating that the loci of explosion were located within a reef or coral sediment near the syn-eruptive shoreline. The second type indicate more stable vent conditions and rapidly repeating explosions of high intensity, producing fine-grained tephra with undulatory bedding and cross-lamination as well as megaripple bedforms. These surge and fall beds are more common in the uppermost part of the succession and form a few-m-thick pile. An older tephra succession of similar character occurs below, and buried trees in growth position, as well as those flattened within base surge beds. This implies that the centre of this eruption was very near the coastline. The processes implied by these deposits are amongst the most violent forms of volcanism on this island. In addition, the lowland and coastal areas affected by these events are the most heavily populated. This circumstance is mirrored on many similar volcanic islands, including the nearby SW Pacific examples of Taveuni (Fiji), Upolu and Savai'i (Samoa), and Ambrym (Vanuatu). These locations are paradoxically often considered safe areas during summit/central-vent eruptions, simply because they are farthest from the central sources of ash-fall and lahar hazard. The observations presented here necessitate a revision of this view.     

The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: From a Surtseyan to an energetic maar eruption

The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km², and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.