Holocene evolution of mud depocentres on a high-energy, low-accumulation shelf (NW Iberia)

The high-energy, low-accumulation NW Iberian shelf features three confined Holocene mud depocentres. Here, we show that the evolution of such depocentres follows successive steps. The flooding of inner shelf zones and river catchment areas by the late deglacial sea-level rise provided the precondition for shelf mud deposition. Following this, the Holocene deceleration of the sea-level rise caused a rapid refill of the accommodation space within river valleys. Subsequently, the export of major amounts of fines was initiated. The initial onset and loci of shelf mud deposition were related to deposition-favouring conditions in mid-shelf position or to the presence of morphological highs, which act as sediment traps by providing protection against stronger hydrodynamic energy. The detailed reconstruction of the Holocene depocentre evolution shows for the first time that the expansion of such shelf mud deposits cannot only occur by linear growth off the associated sediment source. Rather, they might develop around centres that are fully disconnected from the source of original sediment supply, and expand later into specific directions. Based on these differences and on the connection of the individual mud depocentres to the material source we propose a conceptual subdivision of the group “mid-shelf mud depocentres”.     

Dynamic landscape changes in Glen Roy and vicinity,west Highland Scotland,during the Younger Dryas and early Holocene: a synthesis

This paper introduces a special issue devoted to the sequence of events in and around Glen Roy during the Loch Lomond or Younger Dryas Stadial, the short but important cold period dated to between ∼12,900 and 11,700 years ago, during which glaciers last expanded to occupy the Scottish Highlands, and during the subsequent transition to warmer conditions at the start of the Holocene. The Glen Roy area is internationally famous for the ‘Parallel Roads’, pre-eminent examples of ice-dammed lake shorelines which were formed during the stadial. What makes these shorelines unique, however, is their role as distinctive time markers, allowing the order of formation of landforms and sediments to be construed with unprecedented detail. Varved lake sediments preserved within Glen Roy and nearby Loch Laggan provide a precise timescale – the Lochaber Master Varve Chronology (LMVC) – for establishing the rates and timing of some of the events. This introductory paper first sets the geological context for those new to this topic, with a digest of key advances in understanding made between the nineteenth century and the publication of the LMVC in 2010. It then summarises the evidence and ideas that have emerged from new research investigations reported in this special issue for the first time, and which shine new light on the subject. Two final sections synthesise the new data and consider future prospects for further refinement of the precise sequence and timing of events. A major conclusion to emerge from this new body of work is that the ice-dammed lakes, and the glaciers that impounded them, persisted in the area until around 11,700 to perhaps 11,600 years ago. This conflicts with recently promoted suggestions that the last glaciers in Scotland were already in a state of considerable decline by ∼12,500 years ago.     

云南腾冲全新世火山岩地球化学特征及其成因

通过对腾冲火山群的黑空山、打莺山和马鞍山火山岩主量元素、微量元素、Sr-Nd-Pb同位素地球化学的研究表明,腾冲全新世火山岩为高钾钙碱性系列,包括粗面玄武岩、玄武粗安岩、粗面安山岩和英安岩。该套火山岩富集大离子亲石元素和轻稀土元素,亏损Nb-Ta-Ti不相容元素,具有高的⁸⁷Sr/⁸⁶Sr比值(0.705862～0.710614),低的¹⁴³Nd/¹⁴⁴Nd比值(0.511941～0.512526)和较高的放射性成因Pb同位素组成(²⁰⁸Pb/²⁰⁴Pb=38.962～39.155;²⁰⁷Pb/²⁰⁴Pb=15.620～15.659;²⁰⁶Pb/²⁰⁴Pb=17.872～18.269)。主量、微量和同位素地球化学研究表明,腾冲全新世三座火山的岩浆源区为富集地幔,地幔富集与古洋壳俯冲有关。它们是由相同的地幔源区经历了不同的岩浆演化阶段的产物,打莺山岩浆演化程度最高,黑空山次之,马鞍山岩浆演化程度最弱。通过腾冲全新世火山岩与藏南碰撞后钾质火山岩的地球化学成分对比,Sr-Nd-Pb同位素地球化学成分存在着明显的不同,暗示两者的源区存在差异。     

Aspartic acid racemization dating of Holocene brachiopods and bivalves from the southern Brazilian shelf, South Atlantic

The extent of racemization of aspartic acid (Asp) has been used to estimate the ages of 9 shells of the epifaunal calcitic brachiopod Bouchardia rosea and 9 shells of the infaunal aragonitic bivalve Semele casali. Both taxa were collected concurrently from the same sites at depths of 10 m and 30 m off the coast of Brazil. Asp D/L values show an excellent correlation with radiocarbon age at both sites and for both taxa (r²_{Site 9 B. rosea} = 0.97, r²_{Site 1 B. rosea} = 0.997, r²_{Site 9 S. casali} = 0.9998, r²_{Site 1 S. casali} = 0.93). The Asp ratios plotted against reservoir-corrected AMS radiocarbon ages over the time span of multiple millennia can thus be used to develop reliable and precise geochronologies not only for aragonitic mollusks (widely used for dating previously), but also for calcitic brachiopods. At each collection site, Bouchardia specimens display consistently higher D/L values than specimens of Semele. Thermal differences between sites are also notable and in agreement with theoretical expectations, as extents of racemization for both taxa are greater at the warmer, shallower site than at the cooler, deeper one. In late Holocene marine settings, concurrent time series of aragonitic and calcitic shells can be assembled using Asp racemization dating, and parallel multi-centennial to multi-millennial records can be developed simultaneously for multiple biomineral systems.     

云南鲁春铜铅锌矿床鲁春火山岩锆石U-Pb年龄、地球化学及其地质意义

位于三江地区的云南鲁春铜铅锌矿床和鲁春火山岩有密切的关系.鲁春火山岩锆石U-Pb年龄为270.0±4.6Ma.稀土和微量元素分布图显示该火山岩相对富集轻稀土元素、强烈亏损Na、Ta、和Ti,表明该火山岩具有岛弧火山岩的地球化学特点,反映了该火山岩形成于早二叠世晚期金沙江洋盆消减阶段的岛弧构造环境.鲁春铜铅锌矿床矿石具有条带状、层状构造等沉积作用的特点;矿石的不活泼元素比值和矿区火山岩相近,表明鲁春铜铅锌矿床经历了海西期火山喷流沉积成矿作用.     

Aeolian activity in northern Amazonia: optical dating of Late Pleistocene and Holocene palaeodunes

Palaeodunes were examined on the eastern margin of the Rio Branco–Rupununi savanna, northeast Amazonia. Optical dating suggests that the onset of aeolian activity was between 17 000 and 15 000 yr ago, just after the Last Glacial Maximum. Both the palaeodune axes and modern dominant wind directions have northeast to east‐northeast directions, implying no significant shift in atmospheric circulation patterns over northeast Amazonia during the Late Pleistocene to Holocene. Major regional climate change events, such as the Younger Dryas, do not appear to have had any effect on the rates of aeolian deposition at the study site. Aeolian activity appears to have continued to the present day, showing a remarkably constant deposition rate of around 0.13 m kyr^?1 initially, increasing smoothly to the present. Until more palaeodunes in northern Amazonia are dated, it is impossible to determine if this record of gradual aeolian deposition is a reliable regional palaeoclimate indicator, rather than being the result of local bioclimatic and geomorphological effects. Copyright © 2004 John Wiley & Sons, Ltd.     

The influence of pretreatment chemistry on the radiocarbon dating of Campanian Ignimbrite-aged charcoal from Kostenki 14 (Russia)

The presence of an independently dated marker in an archaeological site offers rare opportunities for assessing the reliability of radiocarbon dates, especially when these are close to the age limit of the technique. Two different pretreatment protocols (routine ABA and more rigorous ABOx-SC) were employed in the chemical preparation of the same charcoal sample from a layer closely associated to the Campanian Ignimbrite tephra at the Russian Palaeolithic site of Kostenki 14 (Markina Gora). The ABA-treated fraction gave an age of ∼33 ¹⁴C ka BP, comparable to a previous determination from the same layer, whereas the ABOx-SC produced an older age of ∼35 ¹⁴C ka BP. This is the first radiocarbon determination of an archaeological sample to provide an age consistent with the “calendar” age for the CI tephra marker.     

渤海湾老狼坨子海岸带^14C,^137Cs、^210Pb测年与现代沉积速率的加速趋势

通过对渤海湾西岸L剖面中同一关键层的碳酸钙和原生腹足类壳债权的^14C测年对比研究,确定了晚全新世的2800cal BP的层位及沉积速率0.045cm/a;8个样柱的^137Cs和^210Pbex强度、蓄积量揭示了距今约120年来堤后盐沼的平均沉积速率约为0.35cm/a,而面向开放海湾的潮坪上部则达到约2-3cm/a。研究区近1个世纪以来沉积速率的加速趋势,是渤海湾西岸泥质海岸带的特征之一。     

西准噶尔塔尔巴哈台-谢米斯台地区火山热液型铜矿床(点)地质及含矿火山岩年代学、地球化学特征

西准噶尔塔尔巴哈台-谢米斯台地区研究和找矿勘探工作十分薄弱,近年来随着谢米斯台铜矿的发现,本项目组陆续发现了喀因德、乌兰浩特、阿依德、巴汗等铜矿点,指示该区具有与火山热液活动有关的铜成矿作用有的潜力。本文对这些矿床(点)开展了地质特征、年代学和地球化学研究显示,区内发育的阿尔木强、谢米斯台铜矿床以及喀因德、乌兰浩特、阿依德、巴汗等铜矿点与火山岩地层密切相关,矿化主要表现为黄铁矿化、孔雀石化,发育绿帘石化、绿泥石化、硅化、碳酸岩化等蚀变,具有火山热液型铜矿特点。锆石LA-ICP-MS U-Pb测年获得喀因德铜矿点火山岩年龄为455.1±5.4Ma,乌兰浩特铜矿点火山岩年龄为428.6±4.6Ma,阿依德铜矿点火山岩年龄为428.8±7.2Ma,谢米斯台铜矿床火山岩年龄为424.3±4.3Ma,阿尔木强铜矿床火山岩年龄为426.7Ma,巴汗铜矿点火山岩年龄为411.7±4.7Ma,可分为晚奥陶世、中志留世、早泥盆世三个阶段,以中志留世为主。地球化学特征显示晚奥陶世、中志留世、早泥盆世三个阶段的火山岩均形成于岛弧环境;且具有类似的岩浆源区和演化过程;岩浆在上升侵位过程中没有明显的受到外来物质混染。综合地质、年代学和地球化学特征显示,塔尔巴哈台-谢米斯台地区与火山热液有关的铜矿床(点)主要受控于构造背景、地层组合、岩石类型、蚀变、控矿构造等因素,其中中志留世中基性火山岩、与火山机构相关的深部可能存在的次火山岩或浅成侵入岩分布区具有较好的找矿潜力。     

Holocene and Pleistocene fringing reef growth and the role of accommodation space and exposure to waves and currents (Bora Bora,Society Islands,French Polynesia)

Holocene fringing reef development around Bora Bora is controlled by variations in accommodation space (as a function of sea‐level and antecedent topography) and exposure to waves and currents. Subsidence ranged from 0 to 0·11 m kyr^?1, and did not create significant accommodation space. A windward fringing reef started to grow 8·7 kyr bp , retrograded towards the coast over a Pleistocene fringing reef until ca 6·0 kyr bp , and then prograded towards the lagoon after sea‐level had reached its present level. The retrograding portion of the reef is dominated by corals, calcareous algae and microbialite frameworks; the prograding portion is largely detrital. The reef is up to 13·5 m thick and accreted vertically with an average rate of 3·12 m kyr^?1. Lateral growth amounts to 13·3 m kyr^?1. Reef corals are dominated by an inner Pocillopora assemblage and an outer Acropora assemblage. Both assemblages comprise thick crusts of coralline algae. Palaeobathymetry suggests deposition in 0 to 10 m depth. An underlying Pleistocene fringing reef formed during the sea‐level highstand of Marine Isotope Stage 5e, and is also characterized by the occurrence of corals, coralline algal crusts and microbialites. A previously investigated, leeward fringing reef started to form contemporaneously (8·78 kyr bp ), but is thicker (up to 20 m) and solely prograded throughout the Holocene. A shallow Pocillopora assemblage and a deeper water Montipora assemblage were identified, but detrital facies dominate. At the Holocene reef base, only basalt was recovered. The Holocene windward–leeward differences are a consequence of less accommodation space on the eastern island side that eventually led to a more complex reef architecture. As a result of higher rates of exposure and flushing, the reef framework on the windward island side is more abundant and experienced stronger cementation. In the Pleistocene, the environmental conditions on the leeward island side were presumably unfavourable for fringing reef growth.     

Late Pleistocene and Holocene glaciation of the Fish Lake valley,northeastern Alaska Range,Alaska

We reconstructed a chronology of glaciation spanning from the Late Pleistocene through the late Holocene for Fish Lake valley in the north‐eastern Alaska Range using ¹⁰Be surface exposure dating and lichenometry. After it attained its maximum late Wisconsin extent, the Fish Lake valley glacier began to retreat ca. 16.5 ka, and then experienced a readvance or standstill at 11.6 ± 0.3 ka. Evidence of the earliest Holocene glacial activity in the valley is a moraine immediately in front of Little Ice Age (LIA) moraines and is dated to 3.3–3.0 ka. A subsequent advance culminated at ca. AD 610–900 and several LIA moraine crests date to AD 1290, 1640, 1860 and 1910. Our results indicate that ¹⁰Be dating from high‐elevation sites can be used to help constrain late Holocene glacial histories in Alaska, even when other dating techniques are unavailable. Close agreement between ¹⁰Be and lichenometric ages reveal that ¹⁰Be ages on late Holocene moraines may be as accurate as other dating methods. Copyright © 2009 John Wiley & Sons, Ltd.     

新藏公路奇台达坂晚中新世火山岩的发现及~(40)Ar-~(39)Ar定年

在新藏公路奇台达坂东约10krn、海拔5600m的晚三叠世花岗岩之上发现厚约10m的玄武岩和粗面英安岩.地球化学数据显示,该火山岩高碱,富集大离子亲石元素和轻稀土元素,亏损重稀土元素,Eu中等负异常,高Sr,低Nd,属钾玄岩系列,可能源于壳幔混合层.测定全岩~(40)Ar-~(39)Ar坪年龄为8.27Ma±0.32Ma(900～1400℃,~(39)Ar累积释放量为66%),与受火山岩烘烤后花岗岩的磷灰石裂变径迹年龄(7.9Ma±1.0Ma)在误差范围内一致,表明该火山岩喷发于约8Ma的晚中新世,与相邻的大红柳滩火山岩的时代(7.97Ma±0.14Ma)相近.新藏公路奇台达坂晚中新世火山岩的发现丰富了青藏高原西北缘晚新生代岩浆活动的资料,表明在晚中新世-上新世康西瓦-泉水沟一带火山活动非常频繁,并显示火山活动与大型断裂带运动的关系非常密切.     

内蒙古架子山钼(银)矿床辉钼矿Re-Os同位素年龄及其地质意义

本文通过对内蒙古架子山斑岩型钼(银)矿床7件辉钼矿样品的Re-Os同位素测定与分析,获得了辉钼矿的模式年龄介于143.1±2.1～147.2±2.4 Ma之间,加权平均年龄为145.8±1.4 Ma(MSWD=1.5),对应的等时线年龄为147.4±1.4 Ma (MSWD=1.5),表明架子山钼(银)矿床形成于晚侏罗世,属中国东部燕山期大规模钼成矿期成矿。架子山钼(银)矿床Re含量在37.62×10^-6～401.34×10^-6之间,平均值为121.28×10^-6,表明架子山钼(银)矿床的成矿物质具有壳幔混源特征。     

K-Ar dating of episodic Mesozoic fluid migrations along the fault system of Gernsbach between the Moldanubian and Saxothuringian (Northern Black Forest,Germany)

Along the SW-NE trending fault zone of Gernsbach (northern Black Forest), over a distance of about 45 km between Baden-Baden and Pforzheim, basal series of the sedimentary cover are extensively altered by hydrothermal solutions. The fluids were conveyed in the fault zone and because of favourable permeabilities they spread out along the unconformity between the basement and sedimentary cover. K-Ar dates on authigenic white micas (illites) and hydrothermally altered detrital micas point to the occurrence of two major episodes of fluid migration. The extensive alteration along the entire fault system occurred about 150 Ma ago during the Jurassic. In relation to these hydrothermal activities, the epigenetic strata-bound uranium deposit of Müllenbach originated in Upper Carboniferous sediments near the fault and the fluorite-quartz vein-type mineralization of Käfersteige was formed along the fault about 100 Ma ago during the Cretaceous. The results indicate various geological processes of the fault system of Gernsbach, which is located between the Saxothuringian and Moldanubian zone. The mobilization of the fluids and the formation of the ore deposits were probably caused by and connected with tectonic activities in mid-Europe.     

滇西腾冲地块东南缘早白垩世火山岩锆石U-Pb定年及地质意义

滇西腾冲地块东侧高黎贡变质岩带,发育一套浅变质的沉积岩地层,其时代不明。本次1:5万区域地质调查在潞西市轩岗乡团坡地层剖面中,确定了该套地层含有酸性火山岩夹层,获得火山岩的锆石U-Pb年龄为121.4±1.4Ma。这一结果表明该套地层的时代为早白垩世;同时也揭示研究区存在早白垩世火山活动,其成因可能与怒江洋盆的闭合有关。     

Late Quaternary river development in the southwest Chad Basin: OSL dating of sediment from the Komadugu palaeofloodplain (northeast Nigeria)

The Komadugu river system is the major Nigerian tributary to Lake Chad. Its large alluvial complex represents an important archive of the late Quaternary environmental history of the Chad Basin. Modern floodplains are incised into an older fluvial terrace that extends over 5000 km². Evidence from satellite images suggests that the ancient river system was dammed up by the Bama Beach Ridge during high stands of mega Lake Chad. This caused the formation of extensive wetlands with a chaotic network of relic channels. The first set of luminescence ages presented here fits well into the previously established environmental history of the Chad Basin. Both the early and mid‐Holocene pluvial periods as well as a final relatively humid period during the late Holocene are reflected by point bar to overbank deposits or channel fills. The onset of the Holocene was characterised by erosion of older dune fields as indicated by grain‐size distributions of the alluvium. Satellite images and sedimentological observations show that the floodplain was dominated by meandering channels shifted by frequent avulsion, reflecting a high variability in precipitation and discharge patterns at the beginning and end of Holocene humid periods. Copyright © 2007 John Wiley & Sons, Ltd.     

西藏长青温池波弄贡变质带片麻岩锆石 ELA-ICP-MS U-Pb年龄及地质意义

西藏林芝至通麦一带出露大面积变质岩,变质岩中含长柱状锆石。长柱状锆石晶体形态特征、矿物包裹体、CL图像及Th/U比值在0.1～0.4之间,表明锆石主要来自火山岩,受后期变质流体改造。长柱状锆石ELA-ICP-MSU-Th-Pb年龄为(40.7±0.3)Ma,MSWD=2.35。变质流体改造边缘部分年龄和核部年龄基本一致。长柱状锆石特征及年龄表明长青温池至通麦一带片麻岩原岩为新生代含火山物质的沉积岩而不是元古宙变质基底。     

南秦岭勉略带铧厂沟火山岩锆石U-Pb年代学及地球化学研究

陕西省西南部铧厂沟火山岩以英安岩为主(～ 90vol％),夹玄武岩构造透镜体(～ 10vol％).玄武岩SiO2含量为43.6％ ～ 54.7％,具有低K、Ti,高Na、Mg的特征;稀土总量为24×10-6 ～29×10-6,中稀土轻微富集,Eu、Sr轻微正异常;具有正Rb、Ba异常及负Nb、Zr异常,LaN/YbN值为1.81 ～2.87,Th/Yb值为0.19 ～0.23,Th/Nb值为0.11 ～0.20,Nb/La值为0.26～0.70,Hf/Th值为0.50 ～ 0.67,显示亚碱性弧玄武岩的特征.英安岩SiO2含量为59.5％～ 72.3％,稀土总量较低(116×10-6～187 × 10-6),为右倾式配分模式,Eu负异常,富集大离子亲石元素(如Rb、Ba、Th、K等),亏损高场强元素(如Nb、P、Ti、Ta等),显示弧火山岩地球化学特征.获得玄武岩的锆石SHRIMP U-Pb年龄为801.7±4.7Ma (MSWD=1.18;n=12),英安岩的锆石LA-ICP-MS U-Pb年龄为802.1±5.3Ma (MSWD=1.02;n=19),二者在误差范.内一致.因此,铧厂沟火山岩是一套火山弧环境的亚碱性玄武质-英安质火山岩组合,表明新元古代曾有大洋板块向南俯冲到扬子古板块北缘之下;这套火山岩裹挟于泥盆系沉积地层中,与泥盆系地层一起,共同组成了一套由晚古生代-三叠纪勉略洋闭合所致的构造混杂岩带.