Stromatolites are widely regarded as layered, early lithified, authigenic microbial structures – often domical or columnar in form – that developed at the sediment water interface in freshwater, ...marine and evaporitic environments
Microbial carbonates were major components of early Paleozoic reefs until coral-stromatoporoid-bryozoan reefs appeared in the mid-Ordovician. Microbial reefs were augmented by archaeocyath sponges ...for ~15 Myr in the early Cambrian, by lithistid sponges for the remaining ~25 Myr of the Cambrian, and then by lithistid, calathiid and pulchrilaminid sponges for the first ~25 Myr of the Ordovician. The factors responsible for mid–late Cambrian microbial-lithistid sponge reef dominance remain unclear. Although oxygen increase appears to have significantly contributed to the early Cambrian ‘Explosion’ of marine animal life, it was followed by a prolonged period dominated by ‘greenhouse’ conditions, as sea-level rose and CO2 increased. The mid–late Cambrian was unusually warm, and these elevated temperatures can be expected to have lowered oxygen solubility, and to have promoted widespread thermal stratification resulting in marine dysoxia and hypoxia. Greenhouse conditions would also have stimulated carbonate platform development, locally further limiting shallow-water circulation. Low marine oxygenation has been linked to episodic extinctions of phytoplankton, trilobites and other metazoans during the mid–late Cambrian. We propose that this tendency to dysoxia-hypoxia in shallow marine environments also limited many metazoan reef-builders. In contrast, during the mid–late Cambrian, the ability of lithistid sponges to withstand low oxygen levels allowed them to create a benthic association with microbial carbonates that dominated global reefs. These conditions ameliorated during the Ordovician, as temperature decline promoted ocean ventilation. The prolonged time gap occupied by low diversity reefs between the ‘Cambrian Explosion’ and the ‘Great Ordovician Biodiversification Event’ reflects elevated temperatures and reduced marine oxygenation that limited metazoan diversification in shallow marine environments.
•2800 Ma Steep Rock carbonate platform is an early example of a marine oxygen oasis.•REE analyses show that Steep Rock limestones precipitated from oxygenated water.•We infer that oxygenation ...promoted Ca-carbonate over Fe-carbonate precipitation.•Oxygenation could be linked to the locally common stromatolites in the limestone.•The Archean inception of marine carbonate platforms reflects oxygenation.
The early Earth was essentially anoxic. A number of indicators suggest the presence of oxygenic photosynthesis ∼2700–3000 million years (Ma) ago, but direct evidence for molecular oxygen (O2) in seawater has remained elusive. Here we report rare earth element (REE) analyses of ∼2800 million year old shallow-marine limestones and deep-water iron-rich sediments at Steep Rock Lake, Canada. These show that the seawater from which extensive shallow-water limestones precipitated was oxygenated, whereas the adjacent deeper waters where iron-rich sediments formed were not. We propose that oxygen promoted limestone precipitation by oxidative removal of dissolved ferrous iron species, Fe(II), to insoluble Fe(III) oxyhydroxide, and estimate that at least 10.25μM oxygen concentration in seawater was required to accomplish this at Steep Rock. This agrees with the hypothesis that an ample supply of dissolved Fe(II) in Archean oceans would have hindered limestone formation. There is no direct evidence for the oxygen source at Steep Rock, but organic carbon isotope values and diverse stromatolites in the limestones suggest the presence of cyanobacteria. Our findings support the view that during the Archean significant oxygen levels first developed in protected nutrient-rich shallow marine habitats. They indicate that these environments were spatially restricted, transient, and promoted limestone precipitation. If Archean marine limestones in general reflect localized oxygenic removal of dissolved iron at the margins of otherwise anoxic iron-rich seas, then early oxygen oases are less elusive than has been assumed.
Defined here as ‘essentially in place calcareous deposits created by sessile organisms’, Organic Reefs are diverse and complex structures with a long geological history. Their classification has been ...the subject of fierce debate, often characterized by reliance on subjective features such as wave-resistance and qualitative attempts to discriminate between ‘first’ and ‘second class’ reefs. In contrast, emphasis is here placed on the objective characteristic of the type of sedimentary support, which largely determines the sedimentary composition of the deposit.
Constructional and depositional processes result in three principal sedimentary components:
matrix (M), essentially in place
skeletons (S) and
cavity/
cement (C), whose proportions can be represented on MSC triangular plots. Separately or together, these components also provide the structural support for the reef. On these compositional and structural bases, three main categories of Organic Reef are recognized:
1.
Matrix-supported reefs (Agglutinated Microbial Reefs, Cluster Reefs, Segment Reefs),
2.
Skeleton-supported reefs (Frame Reefs),
3.
Cement-supported reefs (Cement Reefs).
Agglutinated Microbial Reefs: possess laminated, clotted, or aphanitic fabrics created by microbial trapping of particulate sediment; in place skeletons and large primary cavities are rare; early cementation may provide added support; topographic relief is limited by the need for currents to provide sediment to accreting surfaces.
Cluster Reefs: skeletal reefs in which essentially in place skeletons are adjacent, but not in contact, resulting in matrix support; characterized by relatively high matrix/skeleton ratios and low volumes of extra-skeletal early cement. Sediment trapping is an important corollary of skeletal growth and Cluster Reef organisms are tolerant of loose sediment. Absence of framework limits the topographic relief that Cluster Reefs can attain relative to spatial extent, and may permit bedding to develop within the reef.
Close Cluster Reefs have skeletons up to 1 unit-distance apart.
Spaced Cluster Reefs have skeletons more than 1, and up to 2 unit-distances apart; with increasing separation of skeletons they grade to level-bottom communities.
Segment Reefs: matrix-supported reefs in which skeletons are adjacent, and may be in contact, but are mostly disarticulated and mainly parauthochtonous. Matrix abundance is high, and early cement relatively low. Moderate relief can develop in response to intense on-reef sediment production.
Frame Reefs: skeletal reefs in which essentially in place skeletons (including calcified microbes) are in contact; characterized by relatively high skeleton/matrix ratio. Skeletal support enables them to raise themselves above the substrate independently of cementation and particulate sedimentation. Simultaneously, by creating partly open shelter cavities, skeletal support may facilitate early cementation. Both relief and early lithification promote marginal talus formation. Skeletal shape and orientation distinguish:
conical/stick-like,
dendritic,
domical, and
laminar frames. Each of these may be open or filled.
Open Frame Reefs: cavities remain open during the early stages of reef growth and are occupied by cryptic encrusters, early cements and internal sediment; exposed skeleton encourages endoliths.
Filled
Frame Reefs: inter-skeletal spaces penecontemporaneously occluded by surficial sediment during reef-growth.
Cement Reefs: reefs created by cementation of essentially in place organisms. Cement provides strength and volume, mimicking skeletal growth, and can form on non-skeletal as well as skeletonized organisms.
Non-skeletal Cement Reefs: created by synsedimentary cementation of essentially in place non-skeletal organisms. This converts a soft deposit with relatively poor preservation potential into a rigid lithified mass: e.g.,
Tufa Cement Reefs (phytoherms) in rivers and lakes and possibly
Travertine Cement Reefs associated with hot springs. If the organisms are skeletal, synsedimentary cementation imparts extra strength and stability to what otherwise would be a Cluster or Frame Reef, and results in
Skeleton–Cement Reefs. Cement Reefs exhibit complex relationships between cement, matrix and skeletons.
Agglutinated Microbial, Cluster and Segment reefs tend to be structurally simple, have low primary relief, and may show bedding. Frame (including microbial Microframe) and Cement Reefs tend to be unbedded, structurally complex, and can have high relief.
Carbonate Mud Mounds: carbonate mud-dominated deposits with topographic relief and few or no stromatolites, thrombolites or in place skeletons.
Low Relief Carbonate Mud Mounds are typically thin.
High Relief Carbonate Mud Mounds are thick, and internal bedding, slumping, stromatactis cavity systems, and steep marginal slopes may be common. Whereas Organic Reefs are biogenic, calcareous, and are created by essentially in place organisms, Carbonate Mud Mounds can be organic and/or inorganic in origin and it can be difficult to distinguish their origins.
Summary
Deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the Archaean. Geologically best known in seas and lakes, microbial carbonates ...are also important at the present day in fluviatile, spring, cave and soil environments. The principal organisms involved are bacteria, particularly cyanobacteria, small algae and fungi, that participate in the growth of microbial biofilms and mats. Grain‐trapping is locally important, but the key process is precipitation, producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation. Various metabolic processes, such as photosynthetic uptake of CO2 and/or HCO3– by cyanobacteria, and ammonification, denitrification and sulphate reduction by other bacteria, can increase alkalinity and stimulate carbonate precipitation. Extracellular polymeric substances, widely produced by microbes for attachment and protection, are important in providing nucleation sites and facilitating sediment trapping.
Microbial carbonate microfabrics are heterogeneous. They commonly incorporate trapped particles and in situ algae and invertebrates, and crystals form around bacterial cells, but the main component is dense, clotted or peloidal micrite resulting from calcification of bacterial cells, sheaths and biofilm, and from phytoplankton‐stimulated whiting nucleation. Interpretation of these texturally convergent and often inscrutable fabrics is a challenge. Conspicuous accumulations are large domes and columns with laminated (stromatolite), clotted (thrombolite) and other macrofabrics, which may be either agglutinated or mainly composed of calcified or spar‐encrusted microbes. Stromatolite lamination appears to be primary, but clotted thrombolite fabrics can be primary or secondary. Microbial precipitation also contributes to hot‐spring travertine, cold‐spring mound, calcrete, cave crust and coated grain deposits, as well as influencing carbonate cementation, recrystallization and replacement. Microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water, and thus relies uniquely on a combination of biotic and abiotic factors. This overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas, and in the past by temporal variation in abundance of marine microbial carbonates. Patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry.
Stromatolites appeared at ∼3450 Ma and were generally diverse and abundant from 2800 to 1000 Ma. Inception of a Proterozoic decline variously identified at 2000, 1000 and 675 Ma, has been attributed to eukaryote competition and/or reduced lithification. Thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the Palaeozoic, and reappeared in the Late Devonian. Microbial carbonates retained importance through much of the Mesozoic, became scarcer in marine environments in the Cenozoic, but locally re‐emerged as large agglutinated domes, possibly reflecting increased algal involvement, and thick micritic reef crusts in the late Neogene. Famous modern examples at Shark Bay and Lee Stocking Island are composite coarse agglutinated domes and columns with complex bacterial–algal mats occurring in environments that are both stressed and current‐swept: products of mat evolution, ecological refugia, sites of enhanced early lithification or all three?
The term keratolite is proposed for keratosan sponge carbonate dominated by vermiform fabric that preserves the outlines of the original spongin skeleton. Thinly (<~2 cm) interlayered ...keratosan–microbial carbonate consortia in peritidal sediments near the Cambrian–Ordovician boundary in Newfoundland, Canada, are macroscopically indistinguishable from stromatolites. These carbonate domes and columns consist of approximately equal proportions of keratolite and stromatolite. The keratolite is characterized by pervasive microscopic vermiform fabric, which reflects the original spongin framework. The stromatolite is characterized by fine-grained carbonate with cross-cutting laminae, which primarily formed by sediment trapping. The intimate association of keratolite and stromatolite in these deposits indicates that the sponges and microbes involved shared similar environmental tolerances and requirements. Synchronicity of sponge colonization, followed by stromatolite regrowth, across adjacent columns suggests coordinated responses by both sponges and microbes to local ecophysiological stimuli. Due to their macroscopic similarity, keratolite and fine-grained stromatolite may commonly have been confused with one-another throughout the Phanerozoic, and possibly longer.
•The name keratolite (new term) is proposed for keratosan sponge carbonate.•Keratolite-stromatolite consortia and pure stromatolite are indistinguishable in the field.•Microscopically, keratolite is distinguished by pervasive “vermiform” fabric.•Keratose sponges and microbes shared similar environmental tolerances and requirements.•Keratolite may have been widely mistaken for stromatolite throughout the Phanerozoic.
Animal evolution transformed microbial mat development. Canonically inferred negative effects include grazing, disturbance and competition for space. In contrast, ancient examples of cooperation ...between microbial mats and invertebrates have rarely been reported. Late Cambrian (~485 million years) Cryptozoön is widely regarded as the first stromatolite to have received a taxonomic name and has been compared with present‐day examples at Shark Bay, Australia. Here, we show that Cryptozoön is an interlayered consortium of keratose (‘horny’) sponge and microbial carbonate in roughly equal proportions. Cryptozoön's well‐defined layering reflects repeated alternation of sponge and microbial mat. Its distinctive lateral growth is due to the ability of keratosans to colonize steep and overhanging surfaces. Contrary to the perception of Phanerozoic stromatolites as anachronistic survivors in a eukaryotic world, Cryptozoön suggests mutualistic behaviour in which sponges and microbial mats cooperated to gain support, stability and relief, while sharing substrates, bacteria and metabolites. Keratosan‐microbial consortia may have been mistaken for stromatolites throughout the record of the past 500 million years, and possibly longer.
Microbial carbonates are long-ranging, essentially bacterial, aquatic sediments. Their calcification is dependent on ambient water chemistry and their growth is influenced by competition with other ...organisms, such as metazoans. In this paper, these relationships are examined by comparing the geological record of microbial carbonates with metazoan history and secular variations in CaCO
3 saturation state of seawater. Marine abundance data show that microbial carbonates episodically declined during the Phanerozoic Eon (past 545 Myr) from a peak 500 Myr ago. This abundance trend is generally inverse to that of marine metazoan taxonomic diversity, supporting the view that metazoan competition has progressively limited the formation of microbial carbonates. Lack of empirical values concerning variables such as seawater ionic composition, atmospheric partial pressure of CO
2, and pH currently restricts calculation of CaCO
3 saturation state for the Phanerozoic as a whole to the use of modeled values. These data, together with palaeotemperature data from oxygen isotope analyses, allow calculation of seawater CaCO
3 saturation trends. Microbial carbonate abundance shows broad positive correspondence with calculated seawater saturation state for CaCO
3 minerals during the interval 150–545 Myr ago, consistent with the likelihood that seawater chemistry has influenced the calcification and therefore accretion and preservation of microbial carbonates. These comparisons suggest that both metazoan influence and seawater saturation state have combined to determine the broad pattern of marine microbial carbonate abundance throughout much of the Phanerozoic. In contrast, for the major part of the Precambrian it would seem reasonable to expect that seawater saturation state, together with microbial evolution, was the principal factor determining microbial carbonate development. Interrelationships such as these, with feedbacks influencing organisms, sediments, and the environment, are central to geobiology.
Microbialites are volumetrically abundant components in Last Glacial Maximum and deglacial reefs in the Australian Great Barrier Reef sampled by IODP Expedition 325 in 34 holes from 17 sites ...(M0030–M0058), along four transects on the shelf edge. Detailed radiometric datings show that four distinct reef phases developed between 28and 10 ka, displaying offlapping and then backstepping patterns. The reef boundstone facies include coralgal, coralgal-microbialite and microbialite boundstone. The microbialite consists of combinations of micrite/microspar, bioclasts, siliciclastic grains (up to 14.5%), fenestrae and encrusting epibionts. The micrite/microspar is high-magnesian calcite commonly irregularly clotted, fenestral and peloidal. Mesoscale microbialite fabrics include laminated, structureless, digitate, intraskeletal and boring-filling, and coatings on debris. Intraskeletal and boring-filling is the first fabric to develop in skeletal voids and borings. It is usually followed by structureless and laminated microbialite, locally overlain by digitate fabric. Microbialite-coated debris can occur at any stage in this succession, including in bioclastic accumulations where the scarce in situ framework builders are mainly encrusting corals. Lipid biomarkers of intermediate to high specificity for sulfate-reducing bacteria, together with δ13C values of these lipids, indicate that microbialite formation was favored by sulfate-reducing bacteria in anoxic microenvironments, probably under high nutrient levels. The microbialite in fore-reef deposits accumulated in the photic zone in water depths of a few to several tens of meters, within small spaces generated by large bioclasts and encrusting corals in the topmost centimeters of the sediment. These crusts that formed on the illuminated surface constitute a previously unrecognized style of microbialite formation in Quaternary reefs. As with the cryptic crusts described from other reef locations, its greatest development occurred during the Last Glacial Maximum and early deglaciation. Microbial carbonate formation during this interval may reflect elevated seawater carbonate saturation corresponding with relatively low levels of atmospheric CO2.
•Microbialite is a main component in LGM and deglacial reefs in NE Australian shelf.•Microbialite was mediated by bacterial sulfate reduction in anoxic micro-environments.•Microbialite-coated debris is common in fore-reef bioclastic accumulations.•Fore-reef microbialite formed in the photic zone in shallow water depths.•LGM and deglacial microbialites may reflect elevated seawater carbonate saturation.
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