Recent interest in Mg-rich silicate formation stems from their role as valuable paleoclimatic indicators in fluvio-lacustrine environments and their insights into metal geochemical cycling. Traditionally, Mg-silicate genesis in lacustrine contexts is linked to alkaline or saline conditions in closed, evaporitic basins. However, the discovery of interparticle amorphous kerolite-like Mg-silicates in the sediments of Lake Clifton, a currently hypersaline coastal lagoon in Western Australia with circumneutral pH and moderate alkalinity, challenges existing models. In this study, petrographic, hydrochemical, and microbial genomic data from different Lake Clifton sub-environments (episodically submerged and subaerial settings) and substrates (pustular microbial mats and non-lithifying microbial sediments) were integrated with geochemical modelling to quantify the mechanisms underlying the formation of Mg-silicates and aragonite peloids as lake shoreline sediments. Geochemical modelling suggests that neither evaporation-driven alkalinity fluctuations nor mixing of lake water with groundwater can solely explain the kerolite-like/carbonate association observed in lakebed sediments. Kerolite-like phases nucleate in association with twisted microbial extracellular polymeric substances (EPS) and organic-rich bacterial remains; this, combined with the identification of diatom- and cyanobacteria-powered photosynthesis, putative anoxygenic photosynthesis, and sulphate-reducing metabolisms, suggests an intimate link between biologically induced processes and the co-precipitation of aragonite peloids and interparticle kerolite-like phases in the lake. Moreover, the contribution of dead diatom frustule dissolution towards kerolite-like authigenesis was geochemically simulated, revealing that the precipitation of observable amounts of kerolite-like at pH values measured in Lake Clifton waters would prevent the formation of aragonite, questioning the feasibility of a scenario dominated by large inputs of dissolved biogenic silica. Discovery of kerolite-like Mg-silicates in microbial-bearing sediments of a hypersaline coastal lagoon prompts a holistic re-evaluation of the environmental and microbiological factors influencing Mg-silicate-carbonate co-precipitation in lacustrine-peri-marine settings. Studying modern Mg-silicate-bearing lacustrine sediments offer the opportunity to better understand the early diagenetic biotic- abiotic processes that may have had limited petrographic preservation potential in ancient saline lake deposits.

During the late Pliocene-early Pleistocene Tunisian calcretes were formed in a soil environment. Fabrics at the macro and micro scale show that these deposits are similar to rhizogenic modern analogue systems forming in Spain. We infer that a similar climate prevailed in Tunisia in the past, specifically winter-season rainfall and a dry summer with annual rainfall close to 430 mm/year in the center of Tunisia. Here, we provide further constraints on the climate under which the Tunisian calcrete formed by looking at vegetation structure, water advection and amount of seasonal rainfall.To investigate (1) the palaeo-vegetation type(s) if they are C3 or C4 plants and (2) the source(s) of meteoric water of Tunisian calcretes during that era, stable isotopes of C and O were analysed in 25 samples taken from 5 horizontal laminar calcretes cores, from 3 sites: North (N36º.43.713; E10º.06.681’), Center (N35º.07.077’; E10º.14.545’), South (N33º.28.898’; E10º.23.602’). Results are expressed relative to the VPDB reference.In the North, the δ18O samples show values varying from – 4.78 ‰ to -6.91 ‰. Likewise, central site cores are characterised by δ18O values ranging from -5.32 ‰ to -6.97 ‰. In contrast, the δ18O values from the South are more depleted (-8.82 ‰ to -7.20 ‰). Concerning the carbon isotope results, both central and southern sites show similarly enriched δ13C values with an average of -6 ‰, while the North site shows more depleted values (-11.3 ‰ to -9.6 ‰). The δ18O values are similar to those determined in the last deglaciation/early Holocene speleothem carbonates from caves in the Tunisian Atlas Mountains, indicating a North Atlantic source. The north-to-south difference in both isotope systems reveals a decoupling between precipitation δ18O and vegetation effects. The southern site shows more depleted water isotopes (a consequence of internal water recycling effects) and more enriched δ13C, consistent with C4 vegetation and/or lower soil respiration. The central site shows low water recycling, but southern-like dryland vegetation. The northern site shows low water recycling and C3 vegetation and/ or higher soil respiration. Consequently, although there is enhanced humidity in all three sites, the sites do not record the same amount of rainfall and the same response of the landscape to form calcretes.

The North African calcretes formed during the Plio-Pleistocene are a prominent and stratigraphically significant palaeoclimatic marker. We studied the calcrete along a north-south transect in Tunisia that crosses the climatic boundaries between the latitudes 33 degrees and 37 degrees N to elucidate the palaeoenvironmental and climatic significance of the calcrete for the Plio-Pleistocene. Macroscopic, petrographic, Scanning Electron Microscopy (SEM), and Cathodoluminescence (CL) observations obtained in the field and from thin sections of the non-powdery horizons of all sites allowed to identify 4 facies: Pisolithic (limestones with clast(s) coated by laminae), laminar (limestones with a succession of laminae), massive brecciated (brecciated limestones), and groundwater facies (limestones with no biogenic features). Notably, soil-forming processes were evident at all sites except for those situated further south, which exhibited characteristics indicative of a groundwater environment. The comparison with modern analogues suggests for our study area the climate was similar to modern central Spain (Hot-summer Mediterranean climate Csa) in central Tunisia which is, in turn, similar to the modern climate in North Tunisia. The climate was characterized by strong seasonal contrast: higher winter rainfall than today followed by long dry hot summers modified by a North-South gradient. This gradient was the result of the interplay between westerly winter winds and the orographic effect of the Atlas Mountains confirming the North Atlantic origin of precipitations. We conclude that during the late Pliocene to early Pleistocene, the climate in Northern Africa was more humid than today, with enhanced winter rainfall at all study sites probably at the same time of the establishment of Saharan humid periods. These humid phases may have been long enough, spanning more than 10.000 years, to reduce recharged groundwater salinity and, subsequently, form thick calcareous soil.

Highly alkaline (pH 9-12) waters can arise from a range of globally significant and environmentally impactful industrial processes such as lime, steel and cement production, alumina refining and energy generation (e.g. combustion ashes). Such residue storage sites are often characterized by extreme geochemical conditions that can be hazardous to aquatic life but are quickly becoming a critical focus for resource recovery and carbon capture initiatives. The very high rates of mineral precipitation at these sites can give rise to the formation of transient minerals that are not currently well understood. As such our estimates of carbon budgets and understanding of trace metal dynamics at highly alkaline sites is currently limited. This study provides a significant advancement in the basis for characterising hyperalkaline carbonate systems through identification and chemical analysis of transient minerals forming in sites receiving high pH (>11) steel slag leachate in northern England. Whilst most of the secondary deposits at the study sites appear to be dominated by calcite, this study provides the first account of ikaite (CaCO3.6H(2)O) crystallization within steel-slag leachate, using novel field (Fourier Transform Infra-Red) supported by rapid laboratory (X-Ray Diffraction) validation. This study suggests that ikaite is a secondary mineral with a primary phase being amorphous calcium carbonate (ACC). Trace element analysis of ikaite forming in these steel-slag leachate affected waters is demonstrates its strong affinity to incorporate relatively large inventories of potentially harmful metals (e.g. lead and cadmium). Importantly, ikaite is only stable at low temperatures (-4 to 8 degrees C) and thus is of significant concern given its potential to release hazardous pulses of contamination during warming events in the spring. The findings provide an improved understanding of carbonate precipitation processes at highly alkaline sites which in turn should influence future research endeavours around mineral carbonation, trace metal dynamics and environmental remediation at these sites globally.

Travertine-depositing hot springs can generate various carbonate minerals with or without the participation of microorganisms. They thus serve as good natural laboratories to study abiotic and biotic factors controlling the precipitation of different carbonate minerals and CaCO3 polymorphism. Through tens of years investigations on travertines, considerable advances about carbonate mineral precipitation have been made. Here, we summarized general hydrological features of travertine systems, and driving forces, microenvironments, and mechanisms of the precipitation of different carbonate minerals in travertine-depositing hot springs by integrating present advances and conducting hydrochemical simulations. Travertine-depositing hot springs can be divided into near-neutral pH hot springs and hyperalkaline hot springs and contain four types of microenvironments: hypogean solid-water interface, epigean solid-water interface, air-water interface, and subaerial exposure surface. Both abiotic (passive CO2 degassing, atmospheric CO2 uptake, evaporation, and fluid mixing) and biotic ( metabolism, organism-related crystal nucleation, and trapping and binding) processes may drive carbonate formation. The specific driving forces, however, depend on bulk water hydrochemistry and microenvironments. Calcite and aragonite are the most common minerals in travertines and the calcite-aragonite polymorphism might be under decisive influences from bulk water chemistry, especially [Mg2+]/[Ca2+], in some conditions. However, calcium carbonate precipitation in hot springs is controlled by solute transport in microenvironments, and any local change induced by hydrodynamics, microorganisms, and extracellular polymeric substances may significantly modify CaCO3 precipitation and polymorphism. Such integrated control from bulk water composition and microenvironments also affects the formation of other carbonate minerals, but their exact roles remain unclear. Overall, despite the fruitful recent findings, further investigations, especially those focusing on microenvironments, are still imperative to better understand carbonate precipitation in hot springs. These microenvironment-scale studies might also provide insights on carbonate precipitation in other environments. (C) 2022 Elsevier B.V. All rights reserved.

Continental carbonates constitute an interesting topic of study since they are important archives recording climate and paleoenvironmental changes. In Tunisia, calcretes are formed during the Pliocene-early Pleistocene (Villafranchian). They mainly occur in the center and on the coastal plain of Djeffara (Southern East) while their presence is more sporadic in the North of the country. Continental carbonates may form in the soil, groundwater, and palustrine and lacustrine environments. Four criteria are used to differentiate these different environments: host rock, components and micromorphological texture, subaerial exposure (seasonality), and flora and fauna. The purpose of this study is to investigate the palaeoenvironmental significance of Tunisian calcretes for the Plio-Pleistocene. For this aim, well-developed carbonate beds were studied along a north-south transect that crosses the climatic boundaries between the latitudes 33 ° and 37 ° N: North (N36º.43.713 E10º.06.681’) Center (N35º.07.077 'E10º. 14.545 ') and South (N33º.28.898' E10º.20.597 '). Based on the macroscopic, petrographic, and cathodoluminescence observations in association with scanning electron microscopy results, we classified thin sections extracted from the massive horizons in all sites into 8 facies types: The Northern site is characterized by 1) pisolithic calcretes and 2) laminar calcretes showing several beta microfabrics such as rhizolith, peloids, intraclasts, coated grains, bioclast debris, cracks, alveolar septal, spherulite, and organic matter. Moreover, 3) pseudo microkarst carbonates were detected in Northern Tunisia by the presence of vertical layers, peloids, intraclasts, circum crack grain, pedotubules, and rhizoconcretions, and bioclasts. At the central site, 4) laminar calcretes are characterized by planar layers. Both 5) massive brecciated calcretes and 6) mottled nodular brecciated calcretes are distinguished by the abundance of cracks. All these facies types’ shows similar pedogenic components such as peloids, coated grains, gastropod shell, alveolar septal, ooids, and bioclasts. The southern site is characterized on the one hand by 7) laminar calcretes composed of peloids, nodules, coated grains, cracks, and on the other hand by 8) groundwater nodules showing a massive aspect with alpha microfabrics. In all sites, the nodular horizons are pedogenic. Thus, our Plio-early Pleistocene calcretes formed in three main depositional environments: pedogenic, groundwater, and palustrine. The groundwater calcretes are formed under phreatic conditions while the Tunisian pedogenic calcretes and palustrine carbonates exhibit subaerial exposure characteristics (cracks). Except for southern groundwater calcretes, all other types of carbonates show biogenic traces. The palustrine carbonates developed above lacustrine mud. Whereas, pedogenic and groundwater calcretes, develop on different types of host rocks (siliceous and clayey). The difference between fabrics indicates that during the Pliocene-early Pleistocene calcrete form in the north in palustrine settings while it forms in groundwater context in the south. Furthermore, the transition between the different environments is controlled mainly by variation in the water table suggesting a variable but generally more humid climate during the Plio-Pleistocene in North Africa.

Abstract The influence of extracellular polymeric substances on carbonate mineral growth in natural settings remains one of the most poorly understood contributors to the growth of non‐marine carbonate sediments. The influences of these materials are complicated by their association with living cells creating local microenvironments via metabolism and enzyme production, and by our uncertainty about the extracellular polymeric substances materials themselves. Different mixtures of extracellular polymeric substance molecules may behave in different ways, and differences in the local physical environment may alter how the mixtures influence mineral formation, and even result in different patterns of polymerization. Here, the influence of extracellular polymeric substances on calcite precipitation rate and Mg/Cacalcite in the absence of cells is investigated using extracts of extracellular polymeric substances from temperate fluvial tufa biofilm. The influence is complex, with the concentration of extracellular polymeric substances in solution altering deposition rate and trace element incorporation. Moreover, the results show interaction of the presence/absence of extracellular polymeric substances and both temperature and salinity. However, despite extracting extracellular polymeric substances from the same parent sample, a uniform influence was not found in these experiments, implying that the mixture is sufficiently variable within a sample for microenvironments within the biofilm to either promote or inhibit mineralization. As sedimentologists, we can no longer take the view that extracellular polymeric substances are a bystander material, or that they have a single set of coherent and predictable or intuitive influences. Rather, the emphasis must be on investigating the specific mixtures present in nature, and their complex and dynamic interaction with both mineral surfaces and hydrochemical conditions.

Understanding the flow of carbon through hyperalkaline lakes is a key means of understanding their biogeochemistry, sedimentology, and their paleoenvironmental and paleoclimatic records. Furthermore, understanding how mineral precipitation is regulated in these lakes can provide insights into how their sequestration of carbon can be managed. We report geophysical surveys of Mono Lake, California, USA, which show unanticipated geomorphological control on the recent/contemporary formation of lacustrine carbonate formations (“tufa”). Acquired shallow-penetration seismic data show a fault zone below the lake floor, but despite the regional evidence for geothermal waters rising up these fractures, we find no evidence for tufa precipitation at the surface exposure of this structure, either in the seismic data or in the swath bathymetry. However, we do find sub-lacustrine tufa columns in these data elsewhere, which is the first time these have been reported directly. We find and report on a strong link between column location and meteoric Ca supply to the lake, with the latter sourced either through surface runoff or groundwater. For example, a region close to a creek inlet has more frequent and larger tufa bodies, which grow at a wider depth range than another region far from an inlet but close to the fault. This demonstrates the importance of meteoric water ingress in regulating carbonate mineral formation in these basins, and raises the possibility that management of water within the catchment could be a means to enhance carbon capture in natural and artificial hyperalkaline lakes.

Abstract This study aims to compare the fabrics of anthropogenic carbonates downstream of lime and steel disposal sites with models of carbonate precipitation from natural systems to elucidate potential drivers, precipitation mechanisms, morphological similarities, predictability in 3D facies distributions and depositional models of these systems. For this purpose, a combination of sedimentological, mineralogical and geochemical approaches is conducted including the analysis of microfacies (petrographic microscope and electron microscopy), X‐ray diffraction and hydrochemistry. The data show that both anthropogenic and natural systems exhibit clear proximal, middle and distal zones of carbonate precipitation. The main macroscopic differences are the dominance of swamp environments in the proximal zone and the prevalence of tufa‐like barrage‐and‐pool sequences in the middle and distal zones. Microfabrics in anthropogenic sites are comparable to travertines but lack the sub‐surface facies and at extreme pH exhibit sparry crusts without clear equivalents in travertines. This sedimentology of anthropogenic carbonates shows that calcite mineral formation is complex and not homogeneous or purely driven by thermodynamic processes. Considering the sedimentology of these human‐induced systems will help us understand precipitation mechanisms which will aid both efforts to utilise anthropogenic alkaline wastes for carbon capture and manage their environmental impacts.

Introduction of the special issue on non-marine carbonates. (A) Sub-lacustrine spring pinnacles from alkaline Mono Lake (California, USA); scale is 2 m. (B) Cascade tufa from Sarikavak (TK); scale is 175 cm. (C) Travertine deposits from Pamukkale (TK); scale is 170 cm. (D) Speleothems and flowstones from Kaklik Cave (TK); hammer for scale. (E) Temporarily exposed microbial bioherms at Bridger Bay shoreline, Antelope Island, Great Salt Lake (Utah, USA); sedimentologists for scale. (F) Upper Jurassic rhizolite laminar calcrete (base at hammer edge) from Rochaforte (Portugal; courtesy of V.P. Wright; Wright & Azeredo, 2006). (G) The Brook Bottom leachate deposit (Harpur Hill, Derbyshire, UK). Anthropocene carbonate deposit forming downstream of a legacy deposit of impure lime, left by historical industrial activity at the site (courtesy of Laura Bastianini); scale is about 10 m.

Ancient and recent terrestrial carbonate-precipitating systems are characterised by a heterogeneous array of deposits volumetrically dominated by calcite. In these environments, calcite precipitates display an extraordinary morphological diversity, from single crystal rhombohedral prisms, to blocky crystalline encrustations, or spherulitic to dendritic aggregates. Despite many decades of thorough descriptive and interpretative work on these fabrics, relating calcite micro-morphology with sedimentary hydrogeochemical conditions remains a challenge. Environmental interpretations have been hampered by the fact that calcite morphogenesis results from the complex interaction between different physico-chemical parameters which often act simultaneously (e.g., carbonate mineral supersaturation, Mg/Ca ratio of the parental fluid, organic and inorganic additives). To try to experimentally address the sedimentological causes of calcite morphogenesis, an experimental approach yielding a first attempt at a calcite growth-form phase diagram is presented here. The initial aim was to account for the carbonate products experimentally nucleated in alkaline, saline lake settings. These are the result of at least two competing calcite precipitation ‘driving forces’ that affect morphogenesis: the calcite supersaturation level of the parental fluid, and the concentration of microbial-derived organic molecules (alginic acid). A key finding of this study is that common naturally-occurring calcite products such as calcite floating rafts, rhombohedral prismatic forms, di-pyramid calcite crystals, spherulitic calcite grains, or vertically stacked spheroidal calcite aggregates, can be related to specific hydrogeochemical contexts, and their physical transitions pinpointed in a phase diagram. By exploring binary or ternary responses to forcing in morphological phase-space, links between calcite growth forms and (palaeo)environmental conditions can be determined. This provides a truly process-oriented means of navigating questions around carbonate precipitate morphogenesis for the future.

The ice-rafted-debris (IRD) record of the open Northwest Pacific points towards the existence of substantial glacial ice on the Northeast Siberian coast during the late Quaternary. However, the scale and timing of glaciation and de-glaciation remains controversial due to the dearth of both onshore and offshore records. Existing IRD data suggests at least one event of dynamic and abrupt change during mid-late Marine Isotope Stage (MIS 3) which mimics the massive collapse of the Laurentide ice sheet during Heinrich Events. It is uncertain whether other events of this magnitude occurred during the late Quaternary. Here we present a ∼160,000 yr IRD series, planktic foraminiferal counts and an age model, derived from a benthic δ18O curve, radiocarbon dates and tephrochronology, from core ODP 1207A (37.79°N, 162.75°E), revealing the presence of low but episodic flux of IRD. We conclude that glacial Northwest Pacific icebergs spread further south than previously thought, with icebergs emanating from Northeast Siberia being transported to the transition region between the subpolar and subtropical waters, south of the subarctic front during at least the Quaternary's last two glacial periods. The episodic nature of the 1207A IRD record during the last glacial, combined with coupled climate-iceberg modelling, suggests occasional times of much enhanced ice flux from the Kamchatka-Koryak coast, with other potential sources on the Sea of Okhotsk coast. These findings support the hypothesis of a variable but extensive ice mass during the last glacial over Northeast Siberia, particularly early in the last glacial period, behaving independently of North American and Eurasian ice masses. In strong contrast, IRD was absent during much of the penultimate glacial Marine Isotope Stage (MIS) 6 suggesting the possibility of very different Northeast Siberian ice coverage between the last two glacial periods.

The tufa deposits in the Kurkur–Dungul area, southern Egypt, date from marine isotope stage (MIS) 11 to MIS 1. Springs across the region were active during glacial periods (with sea-level below –50 m), reflecting changed atmospheric circulation over the Indian Ocean, as well as peak interglacial periods. During times of low sea-level, reduced Indonesian throughflow promoted formation of an Indian Ocean Warm Pool, and anomalous rainfall on its western margin. We suggest that Egypt lies at the intersection of westerly (‘maghrebian’) and easterly (‘mashriqian’) rainfall provinces, which show different timing with relation to orbital forcing and different source water regions. Tufa-growth periods are therefore not mechanistically linked to ‘humid periods’ or ‘sapropel events’ identified elsewhere. Stable isotope and T (Δ 47 ) data are also inconsistent with these spring systems being part of a larger system spanning northern Africa, and lack a clear interaction between northern hemisphere heating and mid-latitude rainfall. We also follow previous researchers in concluding that formation of springline deposit formation was probably delayed compared with rainfall, owing to aquifer flow distances. This delay is unlikely to be sufficient to explain why rainfall is out of phase with movements of the monsoon belts, but may complicate interpretation of these records. Supplementary material: A lithofacies description and supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.5246661

Abstract Lacustrine non‐skeletal carbonates exhibit a diversity of petrographies due to interactions between physico‐chemical and biologically influenced mechanisms. Despite the suggestion that evaporative concentration was involved in the formation of spherulite and shrubby‐bearing carbonate successions in the Pre‐Salt Cretaceous alkaline lakes of the South Atlantic, no consensus exists about the water chemistries promoting these exotic mineral textures. In this work, an experimental approach was developed to evaluate how changes in salinity (NaCl) and biopolymer concentrations (alginic acid) impact calcite growth dynamics from saline and alkaline synthetic solutions. Hydrochemical and petrographical data from selected modern saline/alkaline environments were compared with experimental datasets to further estimate how the underlying (bio)chemical conditions and lake locations probably converge to allow the formation of calcite spherulite grains in evaporitic settings. Spherulitic calcite from Recent saline lakes and experiments arise from waters with moderate to high [Calcium]/[Alkalinity] ratios ([Ca]/[Alk]) rather than in calcium‐depleted and alkaline‐rich environments which tend to produce single‐crystal calcites during abiotic water mixing or lake evaporation. This observation is consistent with the assembly of polycrystalline textures being a kinetically controlled feature, forced by remarkably high rates of nucleation. Also, the data analysed do not support a causative relationship between evaporite‐driven salinity fluctuations and the preferential formation of spherulites, shrubs or their intermediate textures. Ubiquitous in saline lakes, organic substances can lower the kinetic thresholds for spherulitic calcite aggregation while microbial photosynthesis can also raise pH, altogether enhancing calcite supersaturation and promoting spherulite formation in waters with moderate‐high [Ca]/[Alk] ratios and high salinities. Localised observations of abiotic spherulites in Recent soda lakes can occur in restricted mixing zones where [Ca]/[Alk] ratios are enhanced. This work highlights the roles of concentration regimes associated with biopolymers and microbial metabolism against the background salinity fluctuations in determining the morphological and textural transitions in lacustrine carbonate minerals.

Over the last decennia, alkaline leachates from the weathering of legacy steel slag disposal sites have affected the surrounding soils and drainage streams. The hyperalkaline and hypersaline conditions around these sites are comparable to extreme paleo environments such as alkaline lakes in rift volcanic settings. Investigating the carbonate deposits forming in these man-made systems provides a unique opportunity to link the ongoing physical and microbial processes to their resultant carbonate morphologies. Here we present data from 3 sites across Dene Burn, a slag drainage stream in Consett, County Durham, UK. After 100 years, iron and steel production ceased in 1980, leaving over 20 million tons of slag in the form of several large mounds. Analysis showed Dene Burn to be typical of slag drainage waters with an elevated pH (>9) and saturated with different secondary phase minerals- particularly calcite. However, the physical distribution of carbonates is more comparable with estimated local kinetic precipitation rate than it is to thermodynamic saturation, indicating that the fundamental control on carbonate formation arises from crystal surface processes. A microbial community comprising predominantly Proteobacteria (Alpha-, Gamma-, Beta- and Deltaproteobacteria), Cyanobacteria, Bacillariophyta (diatoms) and Bacteroidetes (Flavobacterium) was identified at the 3 sites. The microbial communities and an abundance of extracellular polymeric substances (EPS) were shown in close association with the mineral phases detected at the sites. The presence and composition of these biofilms appears to control local carbonate mineralisation rates and carbonate morphologies. Drainage streams from steel slag provide a unique opportunity to study carbonate mineral formation under extreme environmental conditions. Furthermore, maximising carbonate formation at such sites could be utilised as a remediation and carbonate sequestration technique.

With an estimated annual production of two billion tonnes globally, alkaline industrial wastes can be considered both major global waste streams, and materials that offer significant options for potential resource recovery. Alkaline wastes are usually derived from high temperature production (e.g. steel and alumina) or disposal (e.g. incineration) processes and are increasingly abundant given rising global demand for steel and alumina and the drive for waste incineration in some jurisdictions. Although relatively long-standing afteruses have been adopted for these materials providing opportunities for value recovery (e.g. steel slag use as an aggregate), they are not sufficient to consume all residues generated or completely limit potential environmental impacts. These impacts can include the generation of fugitive dusts, challenges associated with revegetation, and effects on the water environment. These wastes can produce highly alkaline leachates enriched with trace metals (e.g. As, Cr, Mo, V) and persist over decades after site closure. Vanadium, one of the most hazardous ecotoxins in leachates, is also a valuable commodity for renewable energy technologies, unifying the often divergent needs of resource recovery and remediation. Case studies are included to illustrate routes to resource recovery from wastes from two major industrial sectors: steel production and alumina production.

The closure of Late Miocene Mediterranean-Atlantic gateways and the restriction of Mediterranean Outflow Water (MOW) led to the Messinian Salinity Crisis (MSC), the dynamics of which is not well understood. However, restriction of the Mediterranean-Atlantic exchange and the Mediterranean Outflow Water (MOW) is one of the prerequisites to generate hypersaline conditions for evaporitic deposition. During the Late Miocene, MOW circulation was active through a Mediterranean-Atlantic exchange of the Betic, Riffian, and possibly Gibraltar gateways. This connection is thought to have ceased or reduced with the onset of the MSC, before re-establishing through the Gibraltar gateway since the Pliocene to the present. In this study, we define the sedimentary evolution of the Neogene Basins of the Gulf of Cádiz to investigate MOW evolution during the latest Miocene. Seismic interpretation shows an Upper Messinian sedimentary unit of transparent seismic facies. It could also be found in the lower Guadalquivir and Gharb basins, and towards the West Portuguese margin. Biostratigraphic dating indicate an onset of deposition predating the MSC. Distribution of this transparent unit implicates the dominant deposition of hemipelagic/pelagic deposits during a period of quiescence in the Atlantic margins, subsequent to MOW disconnection. This suggests that weakening or cut-off of the intermediate bottom currents of the Mediterranean-Atlantic exchange through the Betic-Gibraltar-Riffian paleo-gateways precedes the onset of MSC evaporites. This work is crucial for the understanding of sedimentary, paleoceanographic and climatic implications of the Latest Miocene Mediterranean isolation in the Atlantic margins.

To investigate past changes in the Mediterranean Overflow Water (MOW) to the Atlantic, we analyzed the strength of the MOW and benthic δ 13 C along the last 250 kyr at Integrated Ocean Drilling Program (IODP) Site U1389 in the Gulf of Cadiz, near the Strait of Gibraltar. Both the strength of the MOW and the benthic δ 13 C were mainly driven by precession‐controlled fluctuations in the Mediterranean hydrologic budget. Reduced/enhanced Nile discharge and lower/higher Mediterranean annual rainfall at precession maxima/minima resulted in higher/lower MOW strengths at Gibraltar and stronger/weaker Mediterranean overturning circulation. At millennial scale, the higher heat and freshwater loss to the atmosphere during Greenland stadials increased buoyancy loss in the eastern Mediterranean. This enhanced the density gradient with Atlantic water, resulting in a higher MOW velocity in the Gulf of Cadiz. Unlike non‐Heinrich stadials, a lower‐amplitude increase in velocity was seen during Heinrich stadials (HSs), and a significant drop in velocity was recorded in the middle phase. This weak MOW was especially recognized in Termination I and II during HS1 and HS11. These lower velocities at the depth of Site U1389 were triggered by MOW deepening due to the lower densities of Atlantic intermediate water caused by freshwater released from the Laurentide and Eurasian ice sheets. The intrusion of salt and heat at deeper depths in the Atlantic during HSs and its shoaling at the end could have contributed to drive the changes in the Atlantic Meridional Overturning Circulation during Terminations.