国内外前寒武纪条带状铁建造研究现状

条带状铁建造(BIFs)主要发育于早前寒武纪时期(3.8~1.8Ga),记录了早期地球演化的重要信息且蕴含丰富的铁矿石资源。本文梳理总结了国内外BIF相关领域的研究认识及存在问题:1统计对比显示,BIF沉积事件与地幔柱、地壳增生等重大地质事件相关;2稀土元素及Nd同位素示踪表明,Fe来源于海水与海底高温热液的混合溶液,其中高温热液与海水比例为1:1000;3 BIFs缺乏负Ce异常且富集重Fe同位素,暗示沉积时古海洋整体处于缺氧环境以避免Fe~(2+)发生氧化;4一些重要科学问题尚未解决,例如Si的主要来源、沉淀机制及条带成因等;5华北克拉通BIFs多形成于约2.54Ga,BIF类型、形成时代与富矿成因等问题有待深入研究。本文认为,加强国内外典型BIFs的对比研究并适当应用现代先进测试技术,有利于探索BIF沉积的精细过程及古老克拉通的早期演化。     

Reconstructing historic Glacial Lake Outburst Floods through numerical modelling and geomorphological assessment: Extreme events in the Himalaya

Recession of high‐mountain glaciers in response to climatic change frequently results in the development of moraine‐dammed glacial lakes. Moraine dam failure is often accompanied by the release of large volumes of water and sediment, termed a Glacial Lake Outburst Flood (GLOF). Chukhung Glacier is a small (~3 km²) receding valley glacier in Mt. Everest (Sagarmatha) National Park, Nepal. Unlike many Himalayan glaciers, which possess a thick mantle of supraglacial debris, its surface is relatively clean. The glacier terminus has receded 1.3 km from its maximum Holocene position, and in doing so provided the space for an ice‐contact moraine‐dammed lake to develop. The lake had a maximum volume of 5.5 × 10⁵ m³ and drained as a result of breaching of the terminal moraine. An estimated 1.3 × 10⁵ m³ of material was removed from the terminal moraine during breach development. Numerical dam‐breach modelling, implemented within a Generalised Likelihood Uncertainty Estimation (GLUE) framework, was used to investigate a range of moraine‐dam failure scenarios. Reconstructed outflow peak discharges, including failure via overtopping and piping mechanisms, are in the range 146–2200 m³ s^‐1. Results from two‐dimensional hydrodynamic GLOF modelling indicate that maximum local flow depths may have exceeded 9 m, with maximum flow velocities exceeding 20 m s^‐1 within 700 m of the breach. The floodwaters mobilised a significant amount of material, sourced mostly from the expanding breach, forming a 300 m long and 100 m wide debris fan originating at the breach exit. moraine‐dam. These results also suggest that inundation of the entire floodplain may have been achieved within ten minutes of initial breach development, suggesting that debris fan development was rapid. We discuss the key glaciological and geomorphological factors that have determined the evolution of a hazardous moraine‐dammed lake complex and the subsequent generation of a GLOF and its geomorphological impact. © 2014 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.     

Geochemical processes controlling the spatial distribution of selenium in soil and water,west central South Dakota,USA

Selenium (Se) is essential in the human diet, but has a low threshold for toxic concentration. It is recommended that nutrients such as Se should be consumed through foods as part of a normal diet. Se concentrations in crops and meat depend on the amount of labile Se in the soil where crops and forage are grown. Therefore, managing agriculture for optimal Se in grain crops and forage requires an understanding of the distribution and mobility of Se. Elevated concentrations of Se occur in waters, soils, and forage 120 km west of Pierre in west central South Dakota, USA. The research site lies in an elevated, dissected plain where soils developed on gently dipping Pierre Shale. Soils were sampled along catena transects and waters collected from soil, ponds, and shallow borings in areas of known elevated forage and crop Se. Soil extracts from saturated-paste extraction and acid (aqua regia and hydrofluoric acid) extraction were analyzed. Selenate was the dominant Se species in both acid and saturated-paste extracts; selenite and organic Se were below detection (<0.2 ppb) in the same soil extracts. On average, 98% of soil Se was not water-soluble. The distribution of total Se shows much less spatial variation than water-soluble Se in the areas sampled. Se shows correlation with organic carbon in soils and waters, suggesting its association with organic carbon. Ca shows some correlation to Se in acid extracts, but not in saturated-paste extracts or in waters. Total Se shows no significant correlation to Na, Mg, and total S in the soils. Se in saturated-paste extracts and water samples shows good correlations with Na, Mg, and SO₄, suggesting that evaporitic Na–Mg–sulfate minerals may temporarily concentrate water-soluble Se in shallow soils. The dissolution and precipitation of these Na–Mg–sulfate salts together with pH and oxidation–reduction conditions apparently control water-soluble Se distribution and mobilization in shallow subsurface environments.     

Recognition of the Cambrian Timan–Severnaya Zemlya orogen and timing of geological evolution of the North Kara sedimentary basin based on detrital zircon dating

New data on the ages of detrital zircons from folded basement rocks and cover sediments of the Severnaya Zemlya archipelago and Izvestiy TSIK islands have been obtained. The basement age is defined as Cambrian (pre-Ordovician). The Ordovician and Silurian sandstones were mainly formed by erosion of the basement rocks. The Devonian sandstones were formed by debris sourced from the Caledonian orogen. The Carboniferous–Early Permian molasse was formed simultaneously with the erosion of the Carboniferous granitoids and weathering of the Ordovician volcanic arc rocks and the Cambrian basement. The North Kara basin was formed in the Ordovician as a back-arc basin. It experienced its main compression deformations at the boundary of the Devonian and Carboniferous and in the Carboniferous.     

Using atmospheric fallout to date organic horizon layers and quantify metal dynamics during decomposition

High concentrations of metals in organic matter can inhibit decomposition and limit nutrient availability in ecosystems, but the long-term fate of metals bound to forest litter is poorly understood. Controlled experiments indicate that during the first few years of litter decay, Al, Fe, Pb, and other metals that form stable complexes with organic matter are naturally enriched by several hundred percent as carbon is oxidized. The transformation of fresh litter to humus takes decades, however, such that current datasets describing the accumulation and release of metals in decomposing organic matter are timescale limited. Here we use atmospheric ²¹⁰Pb to quantify the fate of metals in canopy-derived litter during burial and decay in coniferous forests in New England and Norway where decomposition rates are slow and physical soil mixing is minimal. We measure ²¹⁰Pb inventories in the O horizon and mineral soil and calculate a 60-630 year timescale for the production of mobile organo-metallic colloids from the decomposition of fresh forest detritus. This production rate is slowest at our highest elevation (∼1000 m) and highest latitude sites (>63°N) where decomposition rates are expected to be low.We calculate soil layer ages by assuming a constant supply of atmospheric ²¹⁰Pb and find that they are consistent with the distribution of geochemical tracers from weapons fallout, air pollution, and a direct ²⁰⁷Pb application at one site. By quantifying a gradient of organic matter ages with depth in the O horizon, we describe the accumulation and loss of metals in the soil profile as organic matter transforms from fresh litter to humus. While decomposition experiments predict that Al and Fe concentrations increase during the initial few years of decay, we show here that these metals continue to accumulate in humus for decades, and that enrichment occurs at a rate higher than can be explained by quantitative retention during decomposition alone. Acid extractable Al and Fe concentrations are higher in the humus layer of the O horizon than in the mineral soil immediately beneath this layer: it is therefore unlikely that physical soil mixing introduces significant Al and Fe to humus. This continuous enrichment of Al and Fe over time may best be explained by the recent suggestion that metals are mined from deeper horizons and brought into the O horizon via mycorrhizal plants. In sharp contrast to Al and Fe, we find that Mn concentrations in decomposing litter layers decrease exponentially with age, presumably because of leaching or rapid uptake, which may explain the low levels of acid extractable Mn in the mineral soil. This study quantifies how metals are enriched and lost in decomposing organic matter over a longer timescale than previous studies have been able to characterize. We also put new limits on the rate at which metals in litter become mobile organo-metallic complexes that can migrate to deeper soil horizons or surface waters.     

雄安新区雾迷山组岩溶热储成储机制及发育模式

雄安新区雾迷山组地热资源丰富,研究其岩溶热储特征及其形成机理对于雄安新区清洁地热资源的开发利用具有重要的理论意义和实用价值。前人已经对该区雾迷山组油气储集层的分布、古岩溶、成岩作用、储集空间等特征做过一定的研究,但对于该区雾迷山组地热资源的成储机制、特别是3期岩溶等成岩特征尚缺乏系统性的研究。充分利用野外露头、岩心、薄片和测录井等地质及地球物理资料,并结合区域地质背景的分析,对雄安新区雾迷山组的岩石学特征、成岩作用、岩溶热储的成储机制进行了深入研究,并建立了雄安新区雾迷山组岩溶热储的发育模式。结果表明：(1)雄安新区雾迷山组主要岩性为白云岩,主要的储集空间为次生孔隙和构造缝—构造溶蚀缝,热储孔渗变化较大;(2)雄安新区最主要的建设性成岩作用是3种溶蚀作用,包括同生—准同生溶蚀、表生溶蚀和埋藏溶蚀,主要破坏性成岩作用是压实作用和胶结作用;(3)根据构造演化、岩心及测井资料,将表生溶蚀作用划分为3期,分别是芹峪期、印支期和燕山—喜山期。芹峪期雾迷山组局部出露遭受淋滤,形成的溶孔经后期改造后不易识别;印支期大部分区域抬升并遭受淋滤,形成高孔渗储集层,后期遭受了进一步改造;燕山—喜山期是雄安新...     

Pollen records of vegetation and inferred climate change in southwestern Madhya Pradesh during the last ca. 3800 years

Pollen analysis of 1.5m deep sediment core from Sapna Lake, Betul District (M.P.) has demonstrated that between 3,800 and 2,700 yr BP, open Acacia-dominant scrub forests interspersed with stretches of herbaceous vegetation comprising grasses, sedges, Asteraceae, Cheno/Am, etc., occurred in the region under a regime of dry climate. A few trees of Madhuca indica, Holoptelea, Shorea robusta, Lagerstroemia parviflora, etc. were also sparingly distributed therein. The vicinity of the lake was under cereal-based agricultural practice as portrayed by the presence of Cerealia and other culture pollen taxa viz., Cheno/Am, Caryophyllaceae, Artemisia and Cannabis sativa. Around 2,700 to 1,260 yr BP the Acacia-dominant scrub forests were succeeded by the open mixed deciduous forests as evidenced from the improvement in frequencies of Madhuca indica, Sapotaceae, Holoptelea, Shorea robusta as well as sporadic invasion of Terminalia, Mitragyna, Flacourtia, Grewia, Lannea coromandelica, etc. This enhancement in the forest floristic reflects the initiation of a warm and humid climate possibly in response to increasing monsoon precipitation. Owing to the prevailing favourable climatic condition there was acceleration in the agricultural practice as indicated by the rising trend of Cerealia and other culture pollen taxa. Since 1,260 yr BP onwards the depletion in the prominent forest constituents such as Madhuca indica, Sapotaceae, Shorea robusta, Holoptelea and other associated trees implies that the forests turned more sparse and less diversified due to the inception of a warm and less humid climate, attributable to reduced monsoon precipitation. However, the agricultural practice continued with almost same intensity as before, because the Cerealia and other culture pollen taxa remain unchanged.