Robert Riding

Structure and composition of organic reefs and carbonate mud mounds: concepts and categories

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:

The stable isotope record of environmental and climatic signals in modern terrestrial microbial carbonates from Europe

Stable carbon and oxygen isotope data from over 80 samples of Recent freshwater microbial carbonates from western Europe, confirm that these deposits record environmental and climatic information. Our sample area tested whether recent microbial carbonates record environmental signals over large regions with differing δ18O compositions for rainfall (δw), particularly in the Alps where δw is influenced by lower condensation temperatures caused by the orographic effect. Microbial crusts in Alpine areas are clearly distinguished, i.e., have isotopically lower δ18O values by up to 4‰, from those forming in lowland areas on the east side of the mountains. Bavarian lakes and rivers which receive runoff from the Alps also have δ18O compositions that reflect the Alpine meteoric water input. Microbial crusts in the higher Alpine sites have δ13C values around −4‰, which are between 2 and 6‰ higher than values from lowland sites. This difference probably reflects a smaller soil-zone carbon component in the mountain sites where soils are thin, poorly vegetated, or absent. Oxygen and carbon isotope values do not vary significantly between different types of microbial precipitate (e.g., undifferentiated crusts, Rivularia colonies, moss tufa, etc.) at a site. The oxygen isotope compositions of freshwater lacustrine mussel shell aragonite and associated (sometimes shell encrusting) microbial carbonates differ by <1‰, and both are probably close to equilibrium isotopic values. However, δ13C values in mussel shell aragonite are consistently lower, typically by 5‰, than the associated microbial carbonate value. This suggests that the δ13C of the microbial carbonate is affected by the microenvironmental photosynthetic processes of the microbes. These microenvironmental effects are only evident at lake shore sites where water flushing rates are low. These data suggest that selected ancient microbial carbonates may contain clear records of palaeoclimate (particularly relative changes in temperature) and environmental change. Pools behind fluvial barrage tufas are probably the best targets for reasonably continuous, long, dateable records with isotopic conditions least affected by microenvironmental processes.

Cyanobacterial tufa calcification in two freshwater streams: ambient environment, chemical thresholds and biological processes

Abstract Calcareous freshwater streams near Bad Urach, southwest Germany, were studied to determine the environmental limits to cyanobacterial calcification. Daily variations in temperature, pH, calcium concentration, and alkalinity were measured at seasonal intervals from September 1993 to January 1995 in two small woodland streams with lengths of 1.0 and 1.5 km. The principal cause of supersaturation in these fast-flowing streams is inorganic carbon dioxide outgassing from resurging groundwater, locally enhanced by turbulence at waterfalls and cascades. Photosynthetic uptake of carbon dioxide and temperature effects are negligible. Nonetheless, organic substrates, particularly cyanobacteria-dominated microbial mats and biofilm, significantly localize precipitation by providing suitable nucleation sites. Precipitation rates on artificial substrates, up to 2.2 mm/yr on limestone, correlate with high Saturation Index values. Copper substrates inhibited microbial colonization and received negligible encrustation. Tufa formation and external calcium carbonate encrustation of cyanobacteria are conspicuous where the annual WATEQ4F Saturation Index exceeds 0.8, and are slightly below 0.7. Calcium carbonate impregnation of cyanobacterial sheaths has not been observed. We infer that in these fast-flowing streams cyanobacteria utilize CO2 in photosynthesis whereas elsewhere, in sluggish freshwater, cyanobacteria utilize HCO3−, leading to sheath impregnation by calcium carbonate even where Saturation Index is only 0.2–0.3. Thus, photosynthetic influence on cyanobacterial calcification appears to be negligible in fast-flowing CO2-rich streams and cyanobacterial calcification takes the form of external encrustation. In slow-flowing CO2-poor streams and lakes cyanobacteria preferentially utilize bicarbonate and sheath impregnation can result. Modern tropical marine carbonate shelves have saturation indices commonly in the range 0.62–0.82 and cyanobacterial calcification is negligible. Extrapolation suggests that cyanobacterial calcification could occur in present-day seas if Saturation Index exceeded 0.8. This level, corresponding with heavy calcification in the freshwater streams studied, may have been widespread in marine environments in the geological past at times when calcified cyanobacteria and environmentally influenced non-skeletal calcium carbonate precipitates were abundant.

Diversity of coralline red algae: Origination and extinction patterns from the Early Cretaceous to the Pleistocene

Abstract Data from a comprehensive literature survey for the first time provide stage-level resolution of Early Cretaceous through Pleistocene species diversity for nongeniculate coralline algae. Distributions of a total of 655 species in 23 genera were compiled from 222 publications. These represent three family-subfamily groupings each with distinctive present-day distributions: (1) Sporolithaceae, low latitude, mainly deep water; (2) Melobesioid corallinaceans, high latitude, shallow water, to low latitude, deep water; (3) Lithophylloid/mastophoroid corallinaceans, mid- to low latitude, shallow water. Raw data show overall Early Cretaceous–early Miocene increase to 245 species in the Aquitanian, followed by collapse to only 43 species in the late Pliocene. Rarefaction analysis confirms the pattern of increase but suggests that scarcity of publications exaggerates Neogene decline, which was actually relatively slight. Throughout the history of coralline species, species richness broadly correlates with published global paleotemperatures based on benthic foraminifer δ18O values. The warm-water Sporolithaceae were most species-abundant during the Cretaceous, but they declined and were rapidly overtaken by the Corallinaceae as Cenozoic temperatures declined. Trends within the Corallinaceae during the Cenozoic appear to reflect environmental change and disturbance. Cool- and deep-water melobesioids rapidly expanded during the latest Cretaceous and Paleocene. Warmer-water lithophylloid/mastophoroid species increased slowly during the same period but more quickly in the early Oligocene, possibly reflecting habitat partitioning as climatic belts differentiated and scleractinian reef development expanded near the Eocene/Oligocene boundary. Melobesioids abruptly declined in the late Pliocene–Pleistocene, while lithophylloid/mastophoroids increased again. Possibly, onset of glaciation in the Northern Hemisphere (∼2.4 Ma) sustained or accentuated latitudinal differentiation and global climatic deterioration, disrupting high-latitude melobesioid habitats. Simultaneously, this could have caused moderate environmental disturbance in mid- to low-latitude ecosystems, promoting diversification of lithophylloids/mastophoroids through the “fission effect.” Extinction events that eliminated >20% of coralline species were most severe (58–67% of species) during the Late Cretaceous and late Miocene–Pliocene. Each extinction was followed by substantial episodes of origination, particularly in the Danian and Pleistocene.

Mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria

Filamentous and shrub-like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca. 1200 Ma Society Cliffs Formation, Baffin and Bylot Islands, Arctic Canada, are 400 m.y. older than previously reported examples. In vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms (CCMs) and is a direct ecophysiological link to atmospheric CO 2 concentration. CCMs are induced in present-day cyanobacteria under experimental conditions when pCO 2 is below ∼0.36% (∼10 times present atmospheric level, PAL). Society Cliffs calcified cyanobacteria consequently imply pCO 2 levels of 2 of 7–10 PAL in the late Mesoproterozoic. Combined, petro-graphic, experimental, and modeling results therefore suggest that Mesoproterozoic pCO 2 concentrations were not substantially different from Phanerozoic values and were significantly less than previous estimates of up to 200 PAL. Assuming 10% lower solar luminosity in the late Mesoproterozoic, pCO 2 levels of 10 PAL or less require the presence of additional greenhouse gases for maintenance of an ice-free Earth. At 10 PAL pCO 2 , methane concentrations of 100–200 ppm would have been sufficient to sustain warm Earth surface conditions. The low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of Earth9s Proterozoic biosphere.

Internal structure of segment reefs: Halimeda algal mounds in the Mediterranean Miocene

Halimeda reefs in the upper Miocene strata (∼6 Ma) of the Sorbas basin, southeastern Spain, shed light on the internal structure of more extensive but less accessible Holocene counterparts, and challenge conventional reef concepts. Coarse discoid segments, released by Halimeda during life or immediately after death, dominate the lenslike mounds. Their chaotic, loose appearance disguises the reefal nature of the mounds. Segments, accumulating at or very close to sites of growth, were quickly stabilized by microbial and cement crusts that bound them into distinctive rigid gravel fabrics. This early lithification generated relief but inhibited off-mound export of sediment, although large blocks detached locally and moved downslope. Encrustation of parautochthonous Halimeda gravel created a unique reef type: segment reefs.

Structure and diversity of oldest sponge-microbe reefs: Lower Cambrian, Aldan River, Siberia

The oldest sponge reef is a small Early Cambrian bioherm at the base of the Tommotian Stage (∼535–540 Ma) in southeast Siberia. The mainly archaeocyath construction may be a response to turbid conditions. Cambrocyathellus bowls fused to create a rigid cavernous frame colonized by cryptic Archaeolynthus and calcified microbes ( Renalcis ). In addition to these constructors and binders, other reef guilds are present: bafflers (other archaeocyaths, spiculate sponges, and hyoliths) and dwellers (hyoliths, mollusks, and many others). This is the oldest known reef possessing an open skeletal frame structure built by animals and a mixed animal-autotroph composition. It provides a blueprint for younger Phanerozoic reefs.

Bahamian giant stromatolites: microbial composition of surface mats

Subtidal columnar stromatolites up to 2.5 m high near Lee Stocking Island in the Exuma Cays, Bahamas, have surface mats approximately equally composed of algae and cyanobacteria. The stromatolites are composed of fine-medium ooid and peloid sand. This sediment is supplied to the growing stromatolite surfaces by strong tidal currents which lift grains into suspension and sweep migrating dunes over the columns. The algae include an unidentified filamentous chlorophyte, and numerous diatom species mostly belonging to Mastogloia, Nitzschia and Navicula. The dominant cyanobacteria are two oscillatoriacean species, both probably belonging to Schizothrix. Trapping of sediment is mainly effected by the unidentified chlorophyte which is veneered by epiphytic diatoms. Grains are bound into a mucilaginous mat composed of diatoms and cyanobacteria. Cyanobacteria alone would not be able to trap and bind coarse sediment so effectively in this environment. In being coarse-grained and having a significant eualgal component to their mats, these stromatolites are similar to subtidal columnar stromatolites at Shark Bay, Western Australia. The Lee Stocking stromatolites are physically stressed by high velocity tidal currents and mobile sediment. The Shark Bay stromatolites are stressed by hypersalinity. In both cases stress deters grazers, encrusters and bioeroders. These coarse-grained eualgal stromatolites contrast with micritic and predominantly prokaryotic stromatolites of most Recent marine environments, and are not analogues for most pre- Phanerozoic stromatolites. They appear to be a response to changing stromatolitic mat components in the Cenozoic.

Dasycladalean Algal Biodiversity Compared with Global Variations in Temperature and Sea Level over the Past 350 Myr

Abstract Dasycladalean green algae show marked fluctuation in genus and species biodiversity from the Carboniferous to the Pliocene. Diversity lows (<10 species) alternate with peaks (>70 species) over periods of ∼20–50 Myr. Relatively few taxa are recorded for the earliest Carboniferous, Early Triassic, Early to Mid-Jurassic, Late Cretaceous, and Pliocene. Diversity maxima occur in the Permian, Early Cretaceous, and Paleocene. With the exception of the Late Cretaceous, biodiversity broadly tracked temperature from the Carboniferous to the Pliocene. Diversity minima generally correspond with low sea level, and diversity maxima with periods of intermediate sea level. Dasycladaleans were most diverse when their main habitats—warm shallow seas—were most extensive. This observation does not preclude the influence of additional important factors on dasycladalean evolutionary history, but it suggests a strong link between long-term patterns of dasycladalean diversity and global fluctuations in temperature and sea level.

The lower Wenlock reef sequence of Gotland: Facies and lithostratigraphy

Abstract The sequence which was previously described by J. E. Hede as Lower Visby Marl, Upper Visby Marl, Hogklint Group, and Tofta Limestone is a shallowing marine sequence dominated by the development of numerous large stromatoporoid-tabulate-algal-cyanobacterial patch reefs and disconformably overlain by restricted, locally high energy, oncolitic and stromatoporoidal limestones associated with erosion surfaces. The total sequence at any one point is generally approximately 45 m thick, and is well-exposed in sea-cliffs along much of the north-west coat of Gotland. It represents the lowest part of the exposed Gotland successon and is of latest Llandovery to earliest Wenlock age. The sum of the maximum thicknesses of the individual formations is approximately 56 m. The conformable underlying sequence is known from boreholes. The following revised lithostratigraphic scheme is proposed: (1) Visby Formation: 1 Ygne Member (11 m), 2 Rovar Lilja Member (6.6 m). These members are approximately equivalent to the...