Crystallization pathways in the Great Artesian Basin (Australia) spring mound carbonates: Implications for life signatures on Earth and beyond

Recent studies of continental carbonates revealed that carbonates with similar fabrics can be formed either by biotic, biologically-induced, biologically-influenced or purely abiotic processes, or a combination of all. The aim of this research is to advance knowledge on the formation of carbonates precipitated (or diagenetically altered) in extreme, continental environments by studying biotic versus abiotic mechanisms of crystallization, and to contribute to the astrobiology debate around terrestrial analogues of Martian extreme environments. Both fossil (upper Pleistocene to Holocene) and active carbonate spring mounds from the Great Artesian Basin (South Australia) have been investigated. These carbonates consist of low-Mg to high-Mg calcite tufa. Four facies have been described: (i) carbonate mudstone/wackestone; (ii) phytohermal framestone/boundstone; (iii) micrite boundstone; and (iv) coarsely crystalline boundstone. The presence of filaments encrusted by micrite, rich in organic compounds, including ultraviolet-protectants, in phytohermal framestone/boundstone and micrite boundstone is clear evidence of the existence of microbial mats at the time of deposition. In contrast, peloidal micrite, despite commonly being considered a microbial precipitate, is not directly associated with filaments in the Great Artesian Basin mounds. It has probably formed from nanocrystal aggregation on colloid particulate. Thus, where biofilms have been documented, it is likely that bacteria catalyzed the development of fabrics. It is less certain that microbes induced calcium carbonate precipitation elsewhere. Trace elements, including rare earth element distribution from laminated facies, highlight strongly evaporative settings (for example, high Li contents). Carbon dioxide degassing and evaporation are two of the main drivers for an increase in fluid alkalinity, resulting in precipitation of carbonates. Hence, although the growth of certain fabrics is fostered by the presence of microbial mats, the formation of carbonate crystals might be independent from it and mainly driven by extrinsic factors. More generally, biological processes may be responsible for fabric and facies development in micritic boundstone whilst micrite nucleation and growth are driven by abiotic factors. Non-classical crystallization pathways (aggregation and fusion of nanoparticles from nucleation clusters) may be more common than previously thought in spring carbonate and this should be carefully considered to avoid misinterpretation of certain fabrics as by-products of life. It is proposed here that the term ‘organic-compound catalyzed mineralization’ should be used for crystal growth in the presence of organic compounds when dealing with astrobiological problems. This term would account for the possibility of multiple crystallization pathways (including non-classical crystallization) that occurred directly from an aqueous solution without the direct influence of microbial mats.     

External controls on the distribution,fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon,Cayo Coco (Cuba)

Active, carbonate‐mineralizing microbial mats flourish in a tropical, highly evaporative, marine‐fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea‐level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light–dark shifts in conjunction with microelectrode O₂ and HS^? profiles. High rates of O₂ production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate‐driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water‐level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The macrofabrics are influenced by the hydrodynamic regime: wind‐driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in‐depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings.     

华北地台西缘早-中寒武世过渡期核形石：微生物群落对浅海缺氧环境的响应*

地质记录表明在早-中寒武世过渡期发生了一次重要的生物灭绝事件,但对其成因的认识仍存在分歧。作者试图通过对华北地台该时段广泛发育的核形石微组构和矿化过程研究,揭示微生物群落对灾变事件和海洋环境变化的响应。研究发现核形石内富含细菌化石;包壳的微组构和有机矿化特征表明其形成于细菌硫酸盐还原(BSR)作用活跃的高碱度海水条件,密集的莓状黄铁矿微粒和异养细菌残余指示为缺氧环境;而沉积相分析显示核形石发育于浅海陆棚背景。研究认为,同期的核形石及其他可对比微生物岩不仅在华北广泛分布,在其他大陆和板块上也有良好记录,表明在这个生物灾变期浅海环境有广泛的微生物群爆发,并可能与大火成岩省喷发、全球气温升高以及深部缺氧海水向陆棚侵进引起的浅海广泛缺氧相关。早-中寒武世过渡期广泛发育的微生物岩可能记录了底栖动物大量灭绝后,微生物群在缺氧水体中的快速繁盛与生态扩张过程。     

Petrology of Lower Cretaceous carbonate mud mounds (Albian, N. Spain): insights into organomineralic deposits of the geological record

The process of organomineralization is increasingly well understood with respect to modern carbonate sediments accumulating adjacent to tropical reef atolls and reef caves. Mineralization related to non-living organic substrates results in autochthonous micrite production (‘automicrites’). ‘Automicrites’ are the main constructive element of Lower Cretaceous (Albian) carbonate mud mounds in northern Spain. These slope mud mounds occur within transgressive and early highstand system tracts encompassing several macrobenthic ecological zones. They are clearly separated from the biocalcifying carbonate factory (Urgonian carbonate platforms), in both space and time. Within these build-ups, most ‘automicrites’ were initially indurated and accreted to form a medium-relief growth framework. ‘Automicrites’ have a uniform, presumably high-Mg-calcite precursor mineralogy. They show an inorganic stable-isotope signature (?¹³C around +3·3‰) within the range of early marine cements, and skeletal compounds lacking major vital effects. Epifluorescence microscopy shows that they have facies-specific fluorescence, which is similar to skeletal compounds of Acanthochaetetes, but clearly different from allomicritic sediment and cements, which are mostly non-fluorescent. The EDTA-soluble intracrystalline organic fraction (SIOF) of Albian automicrites shows an amino acid spectrum that is similar to shallow subsurface samples from their modern counterparts. Gel electrophoresis of the SIOF demonstrates an exclusively acidic character, and a mean molecular size range between 20 and 30 kDa. Experiments in vitro (inhibition tests) indicate that the SIOF has a significant Ca²⁺-binding capacity. Fluorescence and chemical characteristics of SIOF point to a main substance class, such as humic and fulvic acids, compounds that form from pristine organic matter during early diagenesis. Biomarker analyses provide evidence for the crucial role of biodegradation by heterotrophic microorganisms, but no biomarker for cyanobacteria has been found. Primary sources of organic material should have been manifold, including major contributions by metazoans such as sponges. It is concluded that many carbonate mud mounds are essentially organomineralic in origin and that the resulting fabric of polygenetic muds (‘polymuds’) may represent ancestral metazoan reef ecosystems, which possibly originated during the Neoproterozoic.     

从生物矿化作用衍生出的有机矿化作用：地球生物学框架下重要的研究主题

早期"生物矿化作用"的概念,被定义为生物形成矿物的作用,并进一步分为生物控制和生物诱导两大类型。这个宽泛的概念,被修订为生物以生命活型(living form)影响矿物物质的沉淀作用;相应地,"生物矿物"是在严格的生物控制下、从局部环境中选择性地吸收元素并融合成具有生物功能构造的矿物。"有机矿化作用",则被定义为"与那些无生命活力的有机物质相关联的矿物形成作用"。与生物矿化作用相对应,有机矿化作用的产物被定义为"有机矿物",用来指那些通过有机聚合物、生物的和(或)非生物的有机化合物所导致的矿物沉淀作用,但是,有机矿物并非活着的细胞所直接形成。有机矿物与生物矿物的重要区别是,有机矿物没有被融合成受到生物严格控制的功能性构造。生物学家和化学家将生物矿化作用作为关注"生命体系中复杂的化学过程"的研究主题,超越了地质学范畴并使生物矿化作用的研究成为多学科关注的迷人领域,也大大促进了有机矿化作用的研究;考虑到有机矿物是沉积岩的重要组成,而且与生物的出现同步,还是潜在性的地外生命的遗迹,因此,从生物矿化作用衍生出的有机矿化作用的研究,自然就成为与生物矿化作用存在紧密关联的、地球生物学框架下又一个重要的研究主题     

Processes of carbonate precipitation in modern microbial mats

Microbial mats are ecosystems that arguably greatly affected the conditions of the biosphere on Earth through geological time. These laminated organosedimentary systems, which date back to > 3.4 Ga bp, are characterized by high metabolic rates, and coupled to this, rapid cycling of major elements on very small (mm-µm) scales. The activity of the mat communities has changed Earth's redox conditions (i.e. oxidation state) through oxygen and hydrogen production. Interpretation of fossil microbial mats and their potential role in alteration of the Earth's geochemical environment is challenging because these mats are generally not well preserved.Preservation of microbial mats in the fossil record can be enhanced through carbonate precipitation, resulting in the formation of lithified mats, or microbialites. Several types of microbially-mediated mineralization can be distinguished, including biologically-induced and biologically influenced mineralization. Biologically-induced mineralization results from the interaction between biological activity and the environment. Biologically-influenced mineralization is defined as passive mineralization of organic matter (biogenic or abiogenic in origin), whose properties influence crystal morphology and composition. We propose to use the term organomineralization sensu lato as an umbrella term encompassing biologically influenced and biologically induced mineralization. Key components of organomineralization sensu lato are the “alkalinity” engine (microbial metabolism and environmental conditions impacting the calcium carbonate saturation index) and an organic matrix comprised of extracellular polymeric substances (EPS), which may provide a template for carbonate nucleation. Here we review the specific role of microbes and the EPS matrix in various mineralization processes and discuss examples of modern aquatic (freshwater, marine and hypersaline) and terrestrial microbialites.     

Questioning the microbial origin of automicrite in Ordovician calathid–demosponge carbonate mounds

Calathid–demosponge carbonate mounds are a feature of Early to Middle Ordovician shallow‐marine carbonate depositional environments of tropical to subtropical palaeolatitudes. These mounds contain an important amount of autochthonous non‐skeletal microcrystalline calcium‐carbonate (automicrite) conventionally considered microbial in origin. Here, the automicrite of calathid–demosponge carbonate mounds (Tarim Basin, north‐west China) is broken down into five distinct fabrics: an in situ peloidal–spiculiferous fabric (AM‐1), an in situ peloidal fabric (AM‐2), an aphanitic–microtubular fabric (AM‐3), a minipeloidal fabric (AM‐4) and a laminoid–cerebroid fabric (AM‐5). Type AM‐1 occurs with AM‐2 being succeeded by an assemblage of AM‐3 and AM‐4. Types AM‐4 and AM‐5 are separated by an erosional disconformity. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably via the permineralization of non‐living organic substrates adsorbing dissolved metal–humate complexes. Using a spreadsheet with six parameters and 17 characters, AM‐1 to AM‐4 turn out to be non‐microbial in origin. Instead, these automicrites represent relics of calcified metazoan tissues, such as siliceous sponges, non‐spiculate sponges or the basal attachment structures of stalked invertebrates. Fabric AM‐5 is a microbial carbonate but is post‐mound in origin forming a drape within a reefal framework established by AM‐4. The five automicritic fabrics, individually or as an assemblage, are a common element of Ordovician calathid–demosponge carbonate mounds in general. The reassessment of the origins of these automicritic fabrics holds consequences for understanding of the Great Ordovician Biodiversification Event in terms of community structure, reef ecology and reef evolution. Episodically, these fabrics are also present in other carbonate build‐ups stretching from the Neoproterozoic over the entire Phanerozoic Eon. The massive calcification of metazoan soft tissue (AM‐1 to AM‐4) characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of p CO₂, Ca concentration and Mg/Ca ratio at the scale of individual sedimentary basins.     

Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

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.     

中元古代微指状叠层石: 超微组构和有机矿化过程

微指状叠层石(microdigitate stromatolites,以下简称MDS)是新太古代—中元古代一种重要的沉积构造。由于其内部以纤维状组构为重要特征,而缺乏直接的微生物化石证据,被认为代表特定海洋化学条件下形成的一种海底碳酸盐沉淀,属无机成因。本文对华北中元古代雾迷山组硅化MDS的显微组构研究发现,其柱体由亚毫米级浅色微亮晶纹层(平均厚约65μm)和暗色微晶纹层(平均厚约680μm)交互叠加而成。前者含较少细菌残余,重结晶显著;后者富含细菌残余、微晶多面体及微球粒,并进一步分为具密集连续次级微纹层的(平均厚约380μm)和具稀疏断续微纹层的(平均厚约300μm)两种暗纹层。这三种纹层在纵向上的规律性交互可能反映了季节性变化。毛发状垂向纤维贯穿于整个柱体,但在亮纹层内稀疏。这种纤维可能由垂向生长的丝状菌束(宽<10μm)矿化而成,有些由微球粒(粒径为10～30μm)定向富集构成。微球粒富含细菌残余、胞外聚合物(EPS)以及与之密切共生的纳米颗粒(粒径<45nm)。纳米颗粒可粘结形成亚μm级多面体,构成碳酸盐晶体生长的基点。包围微球粒的微亮晶环边和纤维组构间的微亮晶条带内少细菌残余,可能属微生物影响的矿化成因,而纤维体和微球粒则是微生物诱发矿化的结果。故中元古代MDS属微生物成因,它所展现的有机矿化过程可能也适用于更古老的叠层石。此外,MDS内有机矿物从纳米颗粒到微球粒的有序聚合可能代表了有机矿化的普遍过程,并可用作判定微生物成因碳酸盐岩的重要标识。