Abstract The degradation of exopolymeric substances (EPS) by heterotrophic bacteria, concomitant release of calcium ions and precipitation of carbonates were studied in a temperate mountain lake, Lac d'Ilay, Jura France. Phytoplankton blooms in this lake produced large amounts of exopolymeric substances (EPS; 1.8–3.0 mg L −1 ), probably inhibiting CaCO 3 precipitation by binding Ca 2+ as shown by the saturation index of calcite and aragonite remaining well below 1. EPS settled to the sediments, where additional polymeric substances were produced by the benthic community. The total amount of EPS decreased downcore from ca 50 μg/g dry sediment near the surface to ca 1.5 μg/g dry sediment at the bottom (120 cm depth). A decrease in acidity, protein and sugar content, and calcium‐binding capacity of EPS with depth coincided with active calcite precipitation. Aerobic and anaerobic EPS‐degrading heterotrophic enrichments were obtained from the top, middle and bottom of the core. Doubling times of aerobic cultures from the top were six times shorter than those of cultures obtained from the bottom of the core, but anaerobic growth rates were similar across all enrichments. Aerobic turnover rates of organic compounds decreased by a factor of 4–5 from top to bottom; anaerobic rates were similar at all depths, except for the turnover of polymers, which was negligible at the surface compared to rates at the middle and bottom. All enrichments released calcium when grown on EPS. Growth on calcium‐saturated EPS in anaerobic cultures obtained from the bottom of the core was the slowest, but still released 26% of the Ca in 20 days. This release during EPS degradation explained an increase in free calcium ions with depth reported in a previous study and may account for a large fraction of the carbonate mud. This suggests that sediments should be considered as an important source of biogenic carbonates.

Abstract Although admired and examined since antiquity, carbonate sediment and rock research really began with Charles Darwin who, during a discovery phase , studied, documented and interpreted their nature in the mid‐19th century. The modern discipline, however, really began after World War II and evolved in two distinct phases. Breakthroughs during an advancement phase came about via the numerous studies by many researchers globally on both modern and ancient carbonates that were eventually merged into carbonate sedimentology as a whole in the 1970s. Formulation of the factory concept and integration of recurring facies into facies models solidified the discipline and allowed integration to move ahead. Stromatolites were discovered growing in modern environments and it was realised that the spectrum of diverse fossil structures called reefs also existed in the modern world. Internal reef structure was largely interpreted as ecological. At the same time, development of radiogenic and stable isotopes allowed Pleistocene carbonate stratigraphy and processes to be accurately dated, and precipitates related specific formation waters. Cementation, long postulated as mostly meteoric, was also realised to be a common synsedimentary phenomenon. Breakthroughs during the more recent refinement phase were likewise synthesised into books, but incrementally as the science progressed. Research on the genesis of critical components such as ooids, stromatolites and carbonate muds was at last convincing. The whole new realm of cool‐water carbonates was revealed with resulting different interpretations of rock record facies and the critical role of aragonite in diagenesis. Reconciling the relationship between sea water nutrient levels, factory type and taphonomy was a major advancement to interpreting the stratigraphic record. Sequence stratigraphy, coupled with shallow drilling and isotopes, led to progressively more dynamic interpretations, especially of reefs. Use of stable isotopes became more sophisticated, and development of clumped isotopes held out possibilities of even more precise interpretations. Finally, although prediction of a future phase is futile, much more needs to be done on mixed carbonate‐siliciclastic systems, and the implications of mixed carbonate ironstone and chert in Precambrian accumulations, especially using new geochemical techniques. Perhaps, philosophically, it is time to use attributes of the exquisite carbonate rock record to tell us about the future of our rapidly changing world.

Abstract The fate of sediment‐laden density flows is strongly controlled by the density contrast between the flow and the basin fluid, which fundamentally influences the character of the sedimentary deposit. However, differentiating the deposits of hyperpycnal and hypopycnal flows, in addition to slope‐failure‐related turbidity currents, remains a source of debate. In part, this can be attributed to the difficulty in determining the density of the fluid in an ancient depositional basin. In this study of a nearly modern succession of fine‐grained (silt‐clay) glaciogenic deposits, basin fluid conditions are well‐constrained and the sedimentological make‐up of these sediments was determined using X‐ray computed tomography and microscopy. Deposits of bottom‐hugging hyperpycnal flows consist of a distinctive alternating pattern of silt‐rich and clay‐rich laminae attributed to the rhythmic alternation of shear thinning and shear thickening processes in the mm‐ to sub‐mm‐thick, non‐Newtonian very‐near‐bed region. A similar pattern is observed in fine‐grained turbidites, suggesting similarity in depositional mechanism. This similarity suggests that differentiating the deposits of fine‐grained hyperpycnal flows from turbidity currents based solely on physical stratal attributes is challenging. Alternatively, hypopycnal flows form buoyant plumes from which sediment settles, spawning bottom‐hugging secondary turbidity currents. In these flows, insufficient sediment concentration and shear stress prevent shear thinning and shear thickening processes from operating, and instead, entropic Brownian fluid motion results in an unstratified deposit with a disorganised fabric. Additionally, these strata contain irregularly spaced, well‐sorted silt lenses that range from a single silt grain to a few silt grains thick and record outer flow disturbances that reworked the previously deposited silt and clay. Whereas differentiating fine‐grained hypopycnal flow deposits from hyperpycnal flows or turbidity currents may be straightforward, differentiating them from pure suspension fallout would appear to rely on the recognition of the thin, well‐sorted silt lenses indicating advection rather than pure particle settling.

Abstract Worldwide, reefs are under significant pressure, and hence, understanding the consequences of natural and anthropogenically driven sediment influx to reef systems is crucial to planning future protection strategies. Most reef systems are associated with clear water settings, but reefs also evolved in turbid water environments stressed by high rates of sediment influx. Mixed carbonate–clastic environments have been considered unfavourable to reef‐building organisms. Currently, we lack generally applicable models for (i) reef growth under the stress of siliciclastic sediment influx and (ii) tools that diagnose ancient reefs that developed in sediment‐stressed environments. Case studies of sediment‐stressed reefs from the Devonian to the recent reviewed here demonstrate that reef organisms show the ability to survive, and even thrive, under clastic sediment influx. These case studies were selected based on (i) the presence of a mixed carbonate‐clastic matrix and (ii) the existence of a coral framework. For each example, the system was characterised in terms of sediment input, organism growth forms and the overall reef architecture. The host sediment from Cenozoic reefs is typically better described than that within Palaeozoic and Mesozoic communities. This may be due to the closer affinity between Cenozoic communities and recent species compared to more ancient systems. The same reasoning accounts for the paucity of data describing the internal structure of many fossil reefs, a feature also related to outcrop quality. Moreover, the juxtaposition of siliciclastic interbeds and ancient reefal bodies should not be taken as conclusive evidence that clastic influx was contemporaneous with the active growth stages of the framework organisms. Based on the data reviewed here, no relationship was identified between the nature of the reef builders, the character of the siliciclastic component and the reef structure. We suggest that this lack of understanding of mixed carbonate‐clastic reef systems significantly compromises potential forecasts of future reef development.

Abstract Grand Lake is a large 250 m deep fjord lake located in Labrador, Canada. Previous studies on short and shallow sediment cores identified seasonal hydrological signals and connections with North Atlantic modes of climate variability. This study presents a new 20 m composite sequence from the deepest basin of Grand Lake, providing high‐resolution insights into sedimentary processes over the last ca. 3300 years. As a potential key environmental archive for north‐eastern Canada, a region where high‐resolution palaeoclimate records are scarce, Grand Lake offers a unique opportunity to examine long‐term sedimentary and climatic interactions. Previous research did not examine temporal changes in sedimentary processes or the specific mechanisms driving mass sediment deposition, limiting the distinction and interpretation of climate controls on longer time scales. Here, sedimentological and geochemical characteristics are used to reconstruct sedimentation dynamics and erosional processes. Several rapidly deposited layers are characterised over changing depositional environments during the Late Holocene, from a phase when the lake was connected to the sea to a more stable state conducive to varve formation. A combination of end‐member modelling analysis, lithofacies descriptions and high‐resolution μ‐XRF proxies revealed density currents as the dominant sedimentation process. Their origins ranged from proximal sources (gully systems) to distal sources (tributary rivers), with contributions varying over time, reflecting the transition from a marine‐influenced system to a post‐glacial fjord lake. The results provide a framework for future palaeoclimate studies in the region by contributing to a better understanding of sedimentary dynamics in a deep glacial lake, with implications for regional palaeoclimate reconstructions. Additionally, this study highlights the broader applicability of statistical unmixing for interpreting grain‐size variations in both lacustrine and marine environments.

Abstract Phanerozoic patch reefs are extensively studied because they are abundant and are potential reservoirs for valuable fluids. In contrast, geologic studies of modern reefs have largely focused on platform margins, leaving patch reefs comparatively neglected. What conditions favour patch‐reef development? What is their potential as reservoirs or as coral refugia under deteriorating environmental conditions? Where other environmental conditions are favourable, the availability of lithified Holocene sediment or Pleistocene rock as substrates for coral colonization appears to control patch‐reef distribution in The Bahamas. A census of patch reefs from satellite imagery of the eastern Great Bahama Bank (GBB) tallies 23 reefs/km2, for a total of 70,000 reefs over 3,000 km2. Higher resolution images of favorable areas tally up to 80 reefs/km2. Patch reefs accumulate sediment typically 3 m, and exceptionally 10 m, above the surrounding sea floor and amalgamate into complexes up to 2150 m long, illustrating their potential to form reservoirs. Patch‐reef corals have adapted to conditions of elevated suspended sediment, salinity and temperature, suggesting they could provide refuge for corals as platform‐margin reefs are decimated by climate change. Data on the distribution, abundance, morphology and health of patch reefs on the GBB may stimulate further research on modern patch reefs.

Abstract Saline‐alkaline lakes are common in tectonically active, semi‐arid regions, resulting from the interplay between tectonic, hydrothermal, surface processes and climate. This study investigates their contribution to the evolution of the saline‐alkaline succession in the intramontane Ibar Basin (Southern Serbia). The succession is investigated using detailed sedimentological analysis, in situ U–Pb geochronology of carbonates acquired by laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS), major and trace element geochemistry and fluid inclusion analysis. Dominantly clastic sedimentation is represented by alluvial fan, flood plain, delta and marginal and profundal lake facies associations. Microbialite and littoral to sublittoral oil shales formed in the areas protected from clastic input. The stratigraphic succession shows a transition from alluvia to a hydrologically open and closed lacustrine environment. U–Pb geochronology of microbialite of a close lake phase (~17 Ma) suggests basin evolution during the Early to Middle Miocene age. The hydrologically closed lake phase is marked by borate‐rich facies, which comprise mainly colemanite overgrown by secondary ulexite within profundal lake facies associations. The textural features suggest that colemanite initially precipitated at or below the sediment/water interface. The subsequent growth, as observed from the primary fluid inclusions in colemanite and calcite, indicates precipitation from the evolving bittern brine under evaporitic, redox conditions, which during diagenesis reached temperatures of 200–220°C. High Ca2+/Na+ ratios in the brine favour colemanite precipitation. Gradual Ca2+ depletion and clay breakdown (dewatering) lead to ulexite and borax precipitates. The results imply that B, Ca and Na were mobilised and transported into the basin by hydrothermal fluids that previously leached bedrock and/or by streams carrying products of the volcanic rocks' weathering. However, brine saturation and syndepositional precipitation were initiated by an arid climate through evaporation, while further growth was driven by reflux of fluids during diagenesis. This study highlights the importance of the tectonics of the collisional orogens and associated processes, arid climate and basin hydrological regime on the deposition of B, Ca‐rich deposits in saline‐alkaline lakes.

Abstract Significant progress has been made in understanding lake basin evolution through climatic and tectonic changes using sedimentology, sequence stratigraphy, geochemistry, hydrology and watershed characteristics to interpret three main lake basin types: overfilled, balanced‐fill and underfilled, including fluvio‐lacustrine systems. Groundwater constraints, however, have been mostly overlooked, especially as they affect the formation and preservation of climate‐sensitive indicators (‘proxies’). The incorporation of subsurface water flow factors into lake basin analysis has led to the realisation that more diverse and complex facies associations can result from its influence, particularly in balanced‐fill and underfilled lake basins. Better understanding of palaeohydrogeology of lacustrine systems results from integrated analysis, including sediment and water delivery pathways, components of potential accommodation and conditions in the lake water column studied in modern lakes and ancient lake strata representing deep‐time lacustrine basins. This special issue intends to act as a pivotal point for future studies on limnogeology. Our goal is to improve our understanding of water delivery pathways (precipitation, overland, groundwater) into fluvio‐lacustrine basins, their influences on such systems and their record of climate and tectonics and to update the existing models for them. The integration of the full range of variables holds great promise to help us better constrain controls and factors affecting the evolution of inland water systems through time and space, to Mars and beyond.