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Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas) 总被引：1，自引：0，他引：1
CHRISTINA GLUNK CHRISTOPHE DUPRAZ OLIVIER BRAISSANT KIMBERLEY L. GALLAGHER ERIC P. VERRECCHIA PIETER T. VISSCHER 《Sedimentology》2011,58(3):720-736
Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments. 相似文献
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Microbial primary dolomite from a Norian carbonate platform: northern Calabria, southern Italy 总被引：1，自引：1，他引：1
ADELAIDE MASTANDREA EDOARDO PERRI FRANCO RUSSO ALESSANDRA SPADAFORA MAURICE TUCKER† 《Sedimentology》2006,53(3):465-480
The origin of fine‐grained dolomite in peritidal rocks has been the subject of much debate recently and evidence is presented here for a microbial origin of this dolomite type in the Norian Dolomia Principale of northern Calabria (southern Italy). Microbial carbonates there consist of stromatolites, thrombolites, and aphanitic dolomites. High‐relief thrombolites and stromatolites characterize sub‐tidal facies, and low‐relief and planar stromatolites, with local oncoids, typify the inter‐supratidal facies. Skeletal remains are very rare in the latter, whereas a relatively rich biota of skeletal cyanophycea, red algae and foraminifera is present in the sub‐tidal facies. Some 75% of the succession consists of fabric‐preserving dolomite, especially within the microbial facies, whereas the rest is composed of coarse dolomite with little fabric preservation. Three end‐members of dolomite replacement fabric are distinguished: type 1 and type 2, fabric retentive, with crystal size <5 and 5–60 μm, respectively; and type 3, fabric destructive, with larger crystals, from 60 to several hundred microns. In addition, there are dolomite cements, precipitated in the central parts of primary cavities during later diagenesis. Microbialite textures in stromatolites are generally composed of thin, dark micritic laminae of type 1 dolomite, alternating with thicker lighter‐coloured laminae of the coarser type 2 dolomite. Thrombolites are composed of dark, micritic clotted fabrics with peloids, composed of type 1 dolomite, surrounded by coarser type 2 dolomite. Marine fibrous cement crusts are also present, now composed of type 2 dolomite. Scanning electron microscope observations of the organic‐rich micritic laminae and clots of the inter‐supratidal microbialites reveal the presence of spherical structures which are interpreted as mineralized bacterial remains. These probably derived from the fossilization of micron‐sized coccoid bacteria and spheroidal–ovoidal nanometre‐scale dwarf‐type bacterial forms. Furthermore, there are traces of degraded organic matter, probably also of bacterial origin. The microbial dolomites were precipitated in a hypersaline environment, most likely through evaporative dolomitization, as suggested by the excess Ca in the dolomites, the small crystal size, and the positive δ18O values. The occurrence of fossilized bacteria and organic matter in the fabric‐preserving dolomite of the microbialites could indicate an involvement of bacteria and organic matter degradation in the precipitation of syn‐sedimentary dolomite. 相似文献
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Lilypad stromatolites, up to 3 m long and 1·5 m wide, were found to be actively growing in the shallow marginal waters of Frying Pan Lake and its outflow channel. These stromatolites, composed of Phormidium (> 90%), Fischerella, and a variety of other microbes, develop through a series of distinct growth stages. Dark green microbial mats cover the floor of the outflow channel and give rise to columns of various sizes and shapes in the shallower marginal waters. Once the columns reach the water level, the mats spread laterally to form a lilypad stromatolite. The lilypads are characterized by a raised, dark green rim, 4–5 mm high, that encircles a flat interior covered with a distinctive orange-red mat. The microbes forming the columns and lilypad plate are being actively silicified. The stromatolites are formed of: (i) flat-lying Phormidium filaments (P-laminae), (ii) upright filaments of Phormidium that are commonly associated with Fischerella (U-laminae), and (iii) mucus, diatoms and pyrite framboids (M-laminae). P-laminae dominate most of the columns, with tripartite cycles of P-, U-, to M-laminae being found mostly in the upper parts of the stromatolites. The transition from the P- to U-laminae is marked by a change in the growth pattern of the Phormidium and branching of Fischerella, which was probably triggered by a change in environmental conditions. In the Frying Pan Lake outflow channel, this change may be related to fluctuations in water level and flow rates that are caused by periods of heavy rain, seasonal changes, long-term variations in rainfall, and/or the unique 40-day hydrological cycle that exists between Frying Pan Lake and Inferno Crater, which is a nearby hydrothermal crater lake. 相似文献
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With more than 10 km of total length, Holocene microbialites in Laguna Bacalar, Mexico, belong to the largest freshwater microbialite occurrences. Microbialites include domes, ledges and oncolites. Domal forms can grow to diameters and heights of 3 m. Microbialites are composed of low magnesium calcite which is, to a large extent, precipitated due to the metabolic activity of the cyanobacteria Homeothrix and Leptolyngbya, and associated diatoms. Photosynthesis removes carbon dioxide and triggers carbonate precipitation. Also, an elevated carbonate concentration in lagoon waters, derived from dissolution of Cenozoic limestone in a karst system, supports carbonate precipitation. Trapping and binding of detrital grains is also observed, but is not as common as precipitation. Bacalar microbialites are largely thrombolitic, however, stromatolitic sections occur as well. The bulk of Bacalar microbialites probably formed in the Late Holocene (ca 1 kyr BP until present). According to 14C dating, microbialites accreted 9 to 8 cal kyr BP; however, these ages may be too old as a result of a strong hard water effect. This effect is seen in 14C ages of living bivalve and gastropod mollusc shells from Bacalar Lagoon, which are 8 to 7 cal kyr BP. The modern associated fauna of microbialites is characterized by low diversity and high abundance of the bivalve mollusc Dreissena sp. and the gastropod Pomacea sp. The abundant grazing gastropods presumably hamper modern microbialite formation. A comparison of Bacalar microbialites with other modern microbialite occurrences worldwide shows only a few patterns: sizes, shapes, microbial taxa, mineralogy, type of accretion and settings including water properties of microbialite occurrences exhibit high variability. A trend may be seen in the grazing metazoa, which are rare to absent in the marine and brackish examples, but apparently present in all the freshwater occurrences of microbialites. Also, freshwater examples are usually characterized by elevated concentrations of carbonate and/or calcium ions in the surrounding waters. 相似文献
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以前的微生物岩识别和分类只根据裸眼可见的中观结构,而对显微镜下的微观结构未予使用。微生物的显微结构有什么基本特征,是否可以用于微生物岩的识别和分类是本文关注的焦点。作者从微生物岩形成机制和实际材料的观察来探讨这个问题。在诱导钙化实验中观察到,蓝藻鞘丝藻Lyngbya的胶鞘表面有碳酸盐矿物形成: 先是在胶鞘表面形成点状碳酸盐颗粒,后来碳酸盐几乎包裹整个丝体,形成一个矿物壳。微生物岩形成的模式推演表明,微生物岩一般都有微生物留下的孔(模孔)和包围模孔的矿物壳2种基本单元。对3个地点的现代微生物碳酸盐岩和1个地点的古代(寒武纪)微生物岩的观察,发现在显微尺度上都具有模孔,此外还可能有矿物壳或胶结物。模孔的形状、大小、排列方式是微生物碳酸盐岩显微结构研究的核心内容,在今后的研究中可以用于微生物岩的识别和分类,以及作为分析形成机制和形成环境的基础。 相似文献
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鄂尔多斯盆地奥陶系马家沟组广泛发育岩溶角砾岩,结构特征复杂。近期研究发现,这些角砾岩的组构和分布特征难以能单纯地用经典的风化壳岩溶模式解释,各类角砾岩的成因机理值得进一步探究。文中基于大量岩心及薄片观察,对苏里格气田东区奥陶系马家沟组上组合不同层位角砾岩进行归纳和判识,明确其空间展布和序列结构特征,分析形成机理。研究发现: (1)根据成因与发育环境,苏格里气田东区奥陶系马家沟组上组合角砾岩可划分为5类,分别是近原地解离角砾岩(B1)、近地表堆积角砾岩(B2)、洞穴堆积角砾岩(B3)、洞顶(壁)碎裂角砾岩(B4)和坍塌角砾岩(B5),它们具有纵向多层叠置、横向可对比的“准层状”特征; (2)不同类型角砾岩的纵向配置和演化与周期性的向上变浅沉积序列和暴露溶蚀有关,共归纳为3种类型,即B1主发育型、B2-B3-B4组合发育型、B5主发育型; (3)准层状角砾岩形成于早成岩期,受控于古地貌起伏背景下的高频海平面变化,由暴露溶蚀透镜体横向迁移连片和纵向多旋回叠置而成。研究结果不仅有助于了解鄂尔多斯盆地马家沟组多样化的岩溶角砾岩特征与成因,也为白云岩早成岩期岩溶研究提供了新的素材。 相似文献
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贵州罗甸二叠纪末生物大灭绝事件后沉积的微生物岩时代和沉积学特征 总被引：9，自引：1，他引：8
二叠纪末期发生的显生宙以来最大的生物绝灭事件,使海洋生态系统和陆地生态系统均受到重创之后,微生物岩广泛分布于全球正常浅海地区。研究认为,扬子地台在二叠纪末期存在一次海平面降低的事件,造成研究区二叠纪地层与早三叠世微生物岩之间存在沉积间断或剥蚀,并使三叠纪牙形石混入二叠纪末期的沉积物中。二叠-三叠系界线位于微生物岩层的底界;微生物岩形成于早三叠世最早期,相当于Hindeodus parvus带,是早三叠世最早期开始的海侵事件为其提供生长所需的可容纳空间。在Isarcicella staeschei带-I. isarcica带早期再次发生相对海平面降低事件,之后海平面开始快速上升。研究区早三叠世早期的微生物岩以凝块构造发育为特征,具有斑状、层状、枝状和网状凝块构造4种典型中型构造。结合前人的工作,认为微生物群落通过生物沉积和物理沉积作用形成球状体,球状体汇聚形成不同的中型凝块构造。研究扬子地台早三叠世凝块石的确切时代和结构、构造类型特征,为准确恢复生物大灭绝事件前后的环境变迁以及生物演化事件与环境变化的相互作用关系提供重要的证据。 相似文献
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微生物碳酸盐岩是近年来地学界研究的一个热点,其时空分布非常广泛,并可形成重要的油气储集层。在鄂尔多斯盆地北部苏里格气田的中奥陶统马家沟组马五1+2段和马五4段,笔者分别发现了非钙化微生物成因的凝块石微生物岩。依据显微结构,将凝块石划分为蠕虫状凝块石、网状凝块石和房室孔洞状凝块石3种类型。通过与基里巴斯现代超盐湖微生物碳酸盐形成环境的对比,推测在华北地台云坪广布、坪中有湖的大背景下,凝块石是在相对海平面上升、盐度大约在60‰~149‰之间的条件下形成的,而含石膏结核碳酸盐岩是在相对海平面下降、盐度达到149‰以上的条件下形成的。进一步研究发现,蠕虫状凝块石泥粉晶白云岩的白云石晶间孔和溶蚀孔非常发育,网状凝块石和房室孔洞状凝块石的溶蚀残余孔较发育,推测凝块石微生物岩具有较好的物性,是未来油气勘探的重要目标岩性之一。 相似文献