New and interesting

Plate tectonic control of strontium concentration in Phanerozoic and Neoproterozoic seawater: Evidence from fluid inclusions in marine halite

Chemical analyses of 1371 fluid inclusions in 131 halite samples with marine 87Sr/86Sr values were used to reconstruct the strontium concentrations [Sr]SW of Phanerozoic and Neoproterozoic seawater. [Sr]SW varied seven-fold and oscillated twice between high- and low-Sr concentrations over the past 550 million years (Myr), in rhythm with Ca-rich and SO4-poor paleoseawater intervals and calcite-aragonite seas. Variations in the [Sr]/[Ca]SW ratio from fluid inclusions were not significant over the past ∼270 Myr, and are within ±3 µmol/mmol of the modern [Sr]/[Ca]SW ratio of ∼8.5 µmol/mmol. These results agree with the [Sr]/[Ca]SW ratios obtained from fossil corals, benthic foraminifera, brachiopods, belemnites, and rudists. [Sr]/[Ca]SW in the early and middle Paleozoic was ≈ 2 times the modern [Sr]/[Ca]SW ratio. A major shift of the [Sr]/[Ca]SW ratio in the late Permian coincided with the initial rifting of the Pangean supercontinent. Seawater 87Sr/86Sr ratios plotted against 1/[Sr]SW show two distinct linear correlations: negative correlation from 515 to 252 Ma and positive correlation from 150 to 0 Ma, suggesting different controls on the global Sr cycle between these intervals. The negative correlation coincides with the long-term assembly of Pangea in the Paleozoic (∼500–250 Ma). The positive correlation from 150 to 0 Ma parallels the break-up of Pangea and the decrease of mid-ocean ridge (MOR) hydrothermal fluid flux and subduction zone length in the Mesozoic and Cenozoic

Figure 1 Secular variation in Phanerozoic and Neoproterozoic seawater [Stlsw and [Srl/(Ca)sw ratios. Marine 87S/86Sr ratio (blue curve) are divided into three phases by McArthur (2010). [Sr]sw and [Sr|/|Ca|sw ratio (circles) were calculated from [Sr]*. [Ca]* published |Ca|sw (Weldeghebriel et al., 2022), and the partition coefficient (Kds) of gypsum (Kushnir, 1980). The indigo curves connect mean [Sr )sw and the [Sr|/| Ca lsw ratios calculated using mean Kdsr = 0.43. Gray bands around indigo curves show uncertainties in accuracy of calculated [Sr]sw and the [Sr]/|Ca]sw that is associated with Kdsr range between 0.2 and 0.8 and [Ca]sw range (Table 2) (Weldeghebriel et al., 2022). The red curve shows [Sr)sw calculated using the Doerner-Hoskins relation and ks of 0.7. [Sr]/| Ca]sw data from fossil brachiopods, belemnites, and rudists (crosses) (Steuber and Veizer, 2002), corals (triangles) (Gothmann et al., 2015), benthic foraminifera (diamonds) (Lear et al., 2003), mid-ocean ridge flank calcite veins (blue squares) (Coggon et al., 2010) and (cyan squares) (Rausch et al., 2013). [Sr]* and [Ca]* are [Sr]F and [CalF from fluid inclusions divided by degree of evaporation to correct tor their increase in concentration during evaporation. Dashed lines are best fitt curves to the ISrI* and |Cal* over the last 550 Ma. Uncertainties. shown by two SE bars (95% confidence interval), were calculated from the standard deviation of (n) fluid inclusion analyses in halite using STD/sqrt(n) (Table 2). Supercontinent cycle (dispersal, assembly) from Frizon De Lamotte et al. (2015) and Kroner et al. (2021). Blue and pink shading show SO4-rich and Ca-rich seawater, respectively (Weldeghebriel et al., 2022). Horizontal bars at the top and bottom of the figure are greenhouse-icehouse climates (McKenzie et al., 2016), MgSO4-KCI potash evaporites (Hardie, 1996), and aragonite calcite seas (Sandberg. 1983).

Broken Beds, but better science; using multiple hypotheses to interpret geological data

In their study, the authors propose a multiple hypotheses approach to improve interpretations of limited remotely sensed datasets, such as sparsely exposed outcrops, subsurface datasets, or planetary objects using semi-quantitative scoring and ranking of observable features. This method is demonstrated using an outcrop example from the Broken Beds of the Upper Jurassic?Lower Cretaceous Purbeck Limestone Group exposed along Britain's Jurassic Coast. Four published hypotheses regarding their origin are refined, represented in matrix form, scored and ranked based on carefully selected outcrop features. Semi-quantitative scoring utilises knowledge of likely processes governing the occurrence of a range of features, some of which might be ignored or down-played to favour a single hypothesis. Furthermore, by integrating expertise from different sub-disciplines (e.g. basin analysis, sedimentology, diagenesis), we also consider the combined evidence of multiple features. This new method results in an interpretation that favours a multi-process origin for the Broken Beds due to evaporite dissolution, overpressure release and tectonic folding, with identified uncertainty, all useful to guide further data collection.

Figure 1.  Field images of the Broken Beds. (a) The Cypris Freestone and the Broken Beds as seen in the field at Fossil Forest location (50°36'58.37"N; °14'23.09"W). Modified from Gallois et al. (2018). (b) The Broken Beds breccia with chaotically arranged boulder size blocks from the Cvpris Freestone unit as seen in the field on a close-up view at Lulworth Cove (50°37'1.27"N: 2°14'55.18"W). Modified from Gallois et al. (2018). (c) Laterally abrupt faces change from in situ Cypris Freestone beds on the left to the Broken Beds on the right as seen at Fossil Forest location (50°36'58.37"N: 014'23.09"W). (d) Folded structures in the Cvoris Freestone orientated northwards (towards the Purbeck Fault) with bedded evaporite at the base as seen at Mupe Bav (50°36'58.37"N: 2013'42.20”W).

Ca and SO4 in Phanerozoic and terminal Proterozoic seawater from fluid inclusions in halite: The significance of Ca-SO4 crossover points

Chemical analyses of 2,618 (1,640 new and 978 published) fluid inclusions in marine halite were used to define paleoseawater [Ca] and [SO4] over the past 550 million years (Myr). Three types of fluid inclusion brine chemistries were recognized based on measured [Ca] and [SO4]: (1) SO4-rich with [SO4] ≫ [Ca]; (2) Ca-rich with [Ca] ≫ [SO4]; and (3) Ca-SO4 crossover points with [Ca] ≈ [SO4]. The SO4-rich and Ca-rich fluid inclusion chemistries oscillated twice in the terminal Proterozoic and Phanerozoic. Transitions between SO4-rich and Ca-rich seas, here called “Ca−SO4 crossover points” occurred four times: terminal Proterozoic–Early Cambrian (544–515 Ma), Late Pennsylvanian (309–305 Ma), Triassic–Jurassic boundary (∼200 Ma), and Eocene–Oligocene (36–34 Ma). New fluid inclusion analyses using laser ablation-inductively coupled plasma-mass spectrometry better defined the [Ca] and [SO4] in seawater at the Late Pennsylvanian and Eocene–Oligocene crossover points and the timing of the Triassic–Jurassic crossover point. Crossover points coincide with shifts in seawater Mg/Ca ratios, the mineralogies of marine non-skeletal carbonates and shell building organisms (aragonite vs. calcite) and potash evaporites (MgSO4 vs. KCl types). Phanerozoic and terminal Proterozoic trends in seawater [Ca] and [SO4] also coincide with supercontinent breakup, dispersal, and assembly cycles, greenhouse–icehouse climates, and modeled atmospheric pCO2. Paleoseawater [Ca] and [SO4] were calculated from the fluid inclusion data using the assumption that the [Ca] × [SO4] ranged from 150 to 450 mmolal2, which is 0.5–1.5 times the [Ca] = 11 × [SO4] = 29 product in modern seawater (319 mmolal2). Two additional end-member scenarios, independent of the [Ca] × [SO4] = 150–450 mmolal2 assumption, were tested using constraints from fluid inclusion [Ca] and [SO4]: (1) constant [SO4] = 29 mmolal as in modern seawater, and variable [Ca], and (2) constant [Ca] = 11 mmolal as in modern seawater and variable [SO4]. Mg/Ca ratios calculated from the three scenarios were compared to independent data on the Mg/Ca ratios from skeletal carbonates (echinoderms and corals) and mid-ocean ridge flank calcite veins. Constant [Ca] of 11 mmolal is unlikely because this relatively low concentration generated unreasonably low seawater [SO4] during most of the past 550 Myr and high Mg/Ca ratios compared to independent data. Constant [SO4] of 29 mmolal produced unreasonably high seawater [Ca] and lower Mg/Ca ratios than those derived from fluid inclusions, echinoderms, corals, and calcite veins. Variable [Ca] and [SO4] showed the best agreement with the Mg/Ca ratios derived from fluid inclusions, echinoderms, corals, and calcite veins.

Figure 1. Secular variation in Phanerozoic and terminal Proterozoic seawater [Ca and SO4], showing raw data (uncorrected for varying concentration factor) from 2618 brine inclusions in halite. Circles and diamonds are [SO4] and [Ca], respectively; white circles and orange diamonds are new data whereas blue circles and red diamonds are published SO4 and Ca, respectively; the indigo curve connects the median value of each dataset. The large range of SO4 and Ca from one particular age is due to halite precipitation at varying concentration factors-this is corrected. Orange curve is cumulative proportion of young zircon grains from globally distributed clastic sedimentary rocks used to track the spatiotemporal distribution of continental volcanic arcs. Blue curve is modeled Phanerozoic and Late Proterozoic atmospheric CO2 concentrations. Supercontinent cycle (dispersal, assembly) from Frizon de Lamotte et al. (2015) and Kroner et al. (2021). Blue and pink shading show SO4-rich and Ca-rich seawater, respectively. Horizontal bars at the top and bottom of the figure are greenhouse-icehouse climates, MgSO4-KCI potash evaporites , and aragonite-calcite seas. 

High-amplitude water-level fluctuations at the end of the Mediterranean Messinian Salinity Crisis: Implications for gypsum formation, connectivity and global climate

The formation and dissolution of salt giants impacts ocean chemistry on thousand-million year timescales. Gypsum precipitation and weathering changes the oceanic calcium concentration with implications for the carbon cycle and global temperatures. However, the connectivity of salt giants with the global ocean is necessarily restricted, making the timing of Ca2+ extraction and return more uncertain. Here we reconstruct the final phase of gypsum precipitation of the Late Miocene Mediterranean Salt Giant using micropaleontology, sedimentology and 87Sr/86Sr analyses on the most complete record preserved at Eraclea Minoa on Sicily and explore its implications for global climate. Precessional gypsum-marl couplets (Upper Gypsum) characterize the last 200 kyrs (Stage 3) of the Messinian Salinity Crisis (MSC; 5.97–5.332 Ma) in both intermediate (500–1000 m) and deep (>1000 m) Mediterranean basins. The interbedded selenitic gypsum layers contain well-preserved calcareous nannofossil assemblages dominated by Reticulofenestra minuta, a marine species which tolerates stressful conditions. Marine water is also required to explain the gypsum 87Sr/86Sr data, which describe a small range of ratios (0.708704–0.708813) lower than coeval ocean water. Mass-balance calculations indicate that during gypsum precipitation, the Atlantic made up ≤20% of a Mediterranean (“Lago-Mare”) water mass dominated by low salinity discharge from large river systems and Eastern Paratethys. This suggests episodic extraction of calcium and sulfate ions from the ocean throughout MSC Stage 3. The marls commonly contain shallow (30–100 m) brackish-water ostracods of Paratethyan (Black Sea) origin. Marls with Paratethyan ostracods are also found in both marginal (<500 m) and deep Mediterranean settings. This indicates that marl-deposition was not synchronous across the basin, but that it occurred in intermediate and deep basins during base-level lowstands at insolation minima and on the shallow Mediterranean margins during insolation maxima-driven highstands. These high-amplitude base-level fluctuations exposed the evaporites to weathering, but ponded the products in the Mediterranean basin until reconnection occurred at the beginning of the Pliocene.

Figure 1. Lithofacies a of a complete UG cycle as a function of precession cycle and Mediterranean base-level according to Manzi et al. (2009) and this study. A comparison with the PLG interpretation is also shown (re-drawn after Roveri et al., 2008). The question marks in the figure to the left refer to the uncertainty over the paleodepth during deposition of sub-units A, B and C. Note that our new interpretation of the UG challenges the long-lasting idea that PLG and UG reflect similar depositional mechanisms and paleoenvironmental conditions (Manzi et al., 2009; Roveri et al., 2014a). This is consistent in a number of ways: (1) UG and PLG sequences comprise different facies (UG contains more abundant balatino gypsum and is missing the banded and branching selenite lithofacies of the PLG). (2) Euxinic shales interbedded with the PLG contain possibly in-situ marine fossils (e.g. Corbí and Soria, 2016), whereas the marls interbedded with the UG contain brackish and shallow water ostracods and brackish water foraminifera such as A. tepida (Grossi et al., 2015). This is likely to reflect greater proportion of marine water in the Mediterranean during Stage 1. (3) 87 Sr/86 Sr isotopes are consistent with the above interpretation, being, on average, close to, or within error of coeval ocean values on stage gypsums (average of 0.708941; Lugli et al., 2010; Reghizzi et al., 2018) and substantially higher than those measured 3 on Stage 3 gypsum samples (average of 0.708752). (4) The Stage 3 gypsum 87Sr/86Sr data are either constant within the layer or show a subtle decline from base to top. This is in a contrast with a larger base to top increase in 87Sr/86Sr ratio in Stage 1 gypsum layers from the Vena del Gesso Basin (Northern Apennines; Reghizzi et al., 2018). (5) The PLG is largely restricted to marginal basins (e.g. Lugli et al., 2010; García-Veigas et al., 2018), whereas the UG is only found in intermediate-deep basins. b: Summary outline of the main sedimentological (i), paleontological (ii) and geochemical (iii) data for each sub-units, their interpretation in terms of salinity (iv) and relationship with the base-level curve (V) and their paleoenvironmental and paleohydrological interpretation proposed in this work (vi).

Global metallogeny in relation to secular evolution of the Earth and supercontinent cycles

Mineral systems with their core of ore deposits require a rare conjunction of geodynamic settings, crustal or lithospheric fertility, crustal architecture and suitable host rocks, and preservation potential. They are thus an integral component of Earth’s thermal and tectonic evolution which also control the supercontinent cycles with progressive assembly and breakup of Ur, Kenorland, Columbia, Rodinia, Gondwana, and Pangea. Despite the ongoing debate, some form of plate tectonics has operated on Earth since the Eoarchean. However, the hotter Archean mantle generated a long-term double-layered convection system which was disrupted by episodic mantle overturns, with the largest in the early Neoarchean potentially enriching the mantle in metals that form the Earth’s core (Figure 1). Cratons with thick subcontinental mantle lithosphere (SCLM) or tectosphere keels commenced to form in the Mesoarchean as small continents amalgamated. The conjunction of pre-4.0 Ga crust, giant Ti, Cr, Fe, Ni and PGE-enriched layered mafic intrusions and major diamond fields provide strong evidence that the Kaapvaal and Zimbabwe Cratons and Wyoming Craton formed part of Ur with its early potentially core-metal-fertilized SCLM. Orogenic gold deposits and VMS Cu-Zn-Pb deposits with their high preservation potential were deposited in subduction-related convergent margins that activated the assembly of all supercontinents with giant provinces related to assembly of Kenorland, Columbia and Gondwana-Pangea. Erosion-susceptible porphyry Cu-Au and epithermal Au-Ag deposits were most abundant at the time of Gondwana and Pangea and in Cenozoic convergent margins and collisional orogens, although there are rare examples associated with assembly of all supercontinents. Magmatic intrusion-related Ni-Cu-PGE, and magmatic-hydrothermal IOCG Cu-Au and Kiruna-type Fe-P deposits formed near craton margins. However, although giant Ni-Cu-PGE deposits formed during the breakup of all supercontinents, giant IOCG deposits were largely restricted to extensional episodes related to Kenorland and Columbia and Kiruna-type deposits to those involved in Columbia. The evolution of the Earth’s atmosphere-hydrosphere-biosphere was an additional influence on that of the supercontinent cycle in terms of the evolution of metallogenic provinces (Figure 2). The Great Oxidation Event (GOE) at ca. 2.4–2.0 Ga witnessed the end of the great era of deposition of BIFs that became the hosts to high-grade Fe and Mn deposits which formed under more oxidizing conditions, with Oligocene sediment-hosted Mn deposits and late-Cenozoic Mn nodules becoming the dominant Mn resources and potential resource, respectively. The GOE was also responsible for the evolution of U deposits from the Mesoarchean paleoplacer uraninite deposits of the Witwatersrand, through Mesoproterozoic unconformity-related deposits to Phanerozoic sandstone roll deposits. The Cambrian ‘explosion of life’, following a second GOE event, magnified the importance of organisms, particularly those secreting Ca and Mg, carbonate in the formation or ore deposits in sedimentary basins. Late Paleoproterozoic-Mesoproterozoic shale-hosted SEDEX Zn-Pb-Cu deposits were progressively replaced by Phanerozoic carbonate-hosted MVT Pb-Zn deposits and Neoproterozoic-Cambrian Zambian-type Cu-Co deposits hosted in calcareous sedimentary sequences. Carlin-type Au-Ag deposits hosted by calcareous and carbonaceous sequences appeared in the Cretaceous to Paleogene epochs to rival the more ubiquitous orogenic gold deposits in terms of global importance. It is evident that the evolution of the great metallogenic belts of the Earth was intrinsically linked to the thermal and tectonic evolution of the Earth and particularly to plate tectonics and the supercontinent cycles. The nature of contained mineral deposits of elements with multiple valency states and those requiring particularly reactive host rocks was strongly influenced by the evolution of the atmosphere-hydrosphere-biosphere system.

Figure 1. Evolution of plate motions and ore deposit drivers in time. (a) and (b) Schematic plate tectonic sketch showing Archean double-layered mantle convection and modern whole mantle convection. (c) Double-layered mantle convection in the Archean in between major overturn events.

Figure 2. Evolution of the atmosphere and hydrosphere. (a) Schematic compilation of the major phases during the secular evolution of Earth, environment, and life. (b) Major events in environmental and life evolution during Earth history. The age ranges of some of the supercontinents are also shown. GOE - Great Oxidation Event. NOE- Neoproterozoic Oxidation Event.

Cryptic sulfur cycling during the formation of giant gypsum deposits (Messinian Primary Lower Gypsum)

Salt giants are large-scale, basin-wide deposits formed sporadically in the geological past, from the early Paleozoic to the late Cenozoic. Their role as sinks for seawater dissolved ions is well known, however the biogeochemical conditions that accompany salt giant formation and their effects on carbon cycling remain poorly constrained. Here we show that massive gypsum deposits of the Mediterranean salt giant – the youngest salt giant on Earth – formed in a particularly dynamic biogeochemical environment controlled by orbitally-driven climate oscillations at the precessional scale. Using multiple sulfur isotopes combined with a steady-state sulfur cycle model, we show that, prior to gypsum precipitation, more than 80% of its constituting sulfate was first microbially reduced into sulfide, possibly stored as elemental sulfur, and then almost completely microbially reoxidized back to sulfate. This “cryptic” sulfur cycling contemporaneous to gypsum precipitation implies both negligible net sulfate consumption and sulfide production, despite a significant benthic flux of organic carbon remineralized through microbial sulfate reduction. This is the first known evidence of cryptic sulfur cycling in the geological past.

Figure 1. Schematic representation of the biogeochemical conditions that accompanied The deposition of the Messinian Primary Lower Gypsum (PLG) deposition. (A) Deposition of organic rich marls during humid phases (precession minima). (B) Deposition of gypsum in the margins and marls in the depocenter during arid phases (precession maxima). The relative size of the arrows represents the approximate magnitude of fluxes. (C) Details of a gypsum twin.

Episodic venting of extreme subsalt overpressure through a thick evaporitic seal

Episodic venting from deep overpressured layers has rarely been documented in sedimentary basins, so the pressure evolution leading to these cyclical phenomena is poorly understood at present. Using three-dimensional (3D) seismic data from the West Nile Deep Sea Fan, the authors interpret >480 fluid escape pipes within a region of prolific leakage across a > 2 km thick sealing sequence of salt and claystones. Within this region, they map five linear trails of pipes with pockmarks or small mud volcanoes at their outlets and interpret them to have formed by episodic venting of overpressured fluids from beneath the salt, over a ∼2–3 Myr period, coeval with basinward displacement of the pipes by flow of the salt. Importantly, the pipe trails root to the crests of stratigraphic traps at the base of the salt seal. Direct measurements from nearby exploration wells are used to construct pressure-depth and pressure-time plots over the lifetime of the pipe trails and demonstrate that overpressure near to or in excess of the lithostatic pressure must be repeatedly achieved in the stratigraphic traps to breach the salt seal every 50–150 kyrs. This pressure evolution is best described by a sawtooth pattern with overpressure relieved by venting, followed by re-seal of the trap, and pressure recharge. The pressure recharge for this episodic venting can most easily be explained here by biogenic gas generation and aquifer pressure transfer in the traps, with some minor contribution from sustained disequilibrium compaction during the Pliocene to Recent. The conditions necessary for this type of episodic venting should occur more widely, particularly in basins with thick evaporite deposits. The recognition of episodic venting has important implications for prospect de-risking, drilling safety and CO2 and hydrogen storage.

Figure 1. Seismic stratigraphy, base salt structure and mud volcanoes, pipes and pipe trails in the Western Nile region of the Mediterranean. A: A seismic cross-section through the 3D seismic of the studied area highlighting the key marker horizons and seismic stratigraphy. OM Oligo-Miocene, IOM Intra-Oligo-Miocene, LEU Lower Evaporitic Unit, UEU Upper Evaporitic Unit. B: IOM to Base Salt isopach map showing regional and anomalous thickness distribution of OM2 across the study area and erosional remnants (ER) and erosional depressions (ED). The location of all the mud volcanoes, pipes and 5 pipe trails (PT) are show. The erosional remnants correlate with increases in thickness of OM2. The erosional depressions and also circular to elliptical mud depletion zones that underly the mud volcanoes correlate with decreases in thickness of OM2.

Trace metals in saline waters and brines from China: Implications for tectonic and climatic controls on basin-related mineralization

Research into mineralization using a deposit-centric approach has identified three principal factors (evolution of the hydrosphere–atmosphere, secular decrease in global heat production, and long-term global tectonic trends) that have influenced the global pattern of metallogenesis. However, these factors do not explain the peak occurrences of basin-related mineralization during particular periods of Earth’s evolution. Here we shift the research perspective from an ore-deposit-centric approach to one focusing on the formation processes of ore-forming fluids by using mass spectrometry to analyze the concentrations of metallic elements (U, Cr, Fe, Mn, Co, Ni, Cu, Mo, Cd, Sn, Pb, Zn, and V) of naturally occurring saline water bodies (brines) in currently developing basins in China. Our aim is to determine the presence of metal-bearing fluids and establish the formation processes of ore-forming fluids that can subsequently generate mineral deposits. The concentrations of trace metals in evaporated seawater and surficial saline waters (brines) in continental basins can reach up to mg/L level, with the increase in salinity resulting from the significant evaporation that occurs in semi-arid to arid environments. The high salinities increase the density of metal-bearing brines, which in turn causes these brines to circulate and mix with groundwater, resulting in enriched metal element concentrations in the groundwater. Groundwater containing dissolved evaporites also shows relatively high concentrations of metal elements (up to mg/L level) that compare with fresh groundwater. These metal-bearing brines can be regarded as the initial ore-forming fluids of basin-related mineral deposits, such as unconformity-type U deposits, red-bed Cu deposits, and some stratiform Pb–Zn deposits. The metal-bearing brines are the result of substantial evaporation under arid to semi-arid climates. Therefore, we conclude that climate is an important control on the formation of basin-related mineralization and that the tectonoclimatic setting plays an essential role in the formation of metal-bearing brines in closed inland basins. The major climatic events, such as the snowball and greenhouse event can affect the basin-related mineralization globally. The super mantle plume event could affect the global basin-related mineralization by changing the global climate significantly.

Figure 1. Formation processes of metal-bearing saline lakes in closed basins under the influence of semi-arid climatic conditions

Are fluid inclusions in gypsum reliable paleoenvironmental indicators? An assessment of the evidence from the Messinian evaporites

The paleosalinity of water from which the gypsum precipitated during the Messinian salinity crisis is a controversial issue. Recent microthermometry studies on primary fluid inclusions in gypsum provided very low salinity values not compatible with precipitation from seawater, and suggested strong mixing between seawater and nonmarine waters enriched in calcium sulfate. We applied a new microthermometric protocol on gypsum crystals from nine Mediterranean sections that were experimentally stretched to measure a larger population of fluid inclusions. The results show salinities ranging from 9 to 238 wt‰ NaCl equivalent, largely falling within the evaporation path of normal seawater. The data from previous studies were obtained mostly from those fluid inclusions capable of nucleating a stable bubble after a weak stretching, which probably correspond to those having a lower salinity acquired through post-depositional crack-and-seal processes. Our data suggest instead that the primary gypsum precipitated from a marine brine, later modified by post-trapping processes during tectonics and exhumation.

Figure 1. (A) Growth mode of a swallowtail gypsum crystal. Yellow lines represent example isochrons within the growing crystal. (B) Crystal growth produces growth bands both in reentrant angle and in the clear portion at the side of the crystal, generating growth bands parallel to the twin plane 100. (C) Classical pyramidal-shaped fluid inclusion (FI) in reentrant angle (type A). (D) FIs showing very different salinities along same lateral growth band (type B); black arrows point to FIs falling within the expected field from seawater, whereas white arrows indicate Fls at lower salinities. Fls at 71%0, 65%0, 27%, and 86%0 show clear signs of leakage. (E) Perspective view of swallowtail gypsum crystal; larger Fls intercept more 010 cleavage planes. (F) Fl trains marking multiple lateral growth bands. (G) Examples of clearly modified Fls (leakage, necking-down, and presence of vapor bubble before any stress cycles).

Depositional and diagenetic model of the Aptian potash-bearing Loémé evaporites in onshore Congo

The evaporites of the Aptian Loémé Formation occur along the stable structural domain of onshore Congo as horizontally well-stratified depositional halite interbedded with Mg-poor potash minerals including carnallite, bischofite, tachyhydrite and sylvite. Eleven correlative depositional recharge-to-evaporation cycles are composed of suites of organic-prone shale followed by primary halite and carnallite beds, sometimes punctuated by a bischofite-tachyhydrite end-member. A sedimentological and petrographical analysis performed on cores and thin sections allowed an interpretation of nine facies associations, named as FA1 to FA9, relying upon the occurrence of bands of fluid inclusions, grain-size, dissolution features, detrital sediments, minute inclusions and sedimentary structures. A depositional model reconciles the spatial distribution of primary textures, hydrology, brine saturation and palaeo-depth during a standard regressive evaporation cycle. Shallow burial eo-diagenesis led to compaction-driven dissolution and recrystallization into fine-grained halite and carnallite laminites but poorly affected the overall preservation of primary deposits. Measurements and profile analysis of bromine trace element concentrations reveal a palaeo-salinity increase of the parent brine for cycle II to base IX caused by a relative brine level fall and a gradual confinement of the salt basin, likely associated with an excess of saline elements inflow from hydrothermal vents. Similar depositional textures and bromine concentration variations are observed in the lower cycles of the Aptian evaporites from the counter-part Sergipe Basin in the northern part of the Brazilian margin, testifying of a progressive brine salinity rise due to a palaeogeographic confinement along the northern segment of the South Atlantic Ocean. In Congo, the sylvite displays horizontal “pseudo-stratifications” that unconformably overlay the crests of anticlines and propagate along flanks as a sharp-based caprock. The flushing and transformation of depositional carnallite – halite into secondary sylvite – halite is interpreted as having been caused by down-stepping incremental dissolutions sourced by younger pervasive connate to phreatic waters. In Sergipe, the sylvite is interpreted as secondary but further results from syndepositional dissolution of carnallite and tachyhydrite due to a regional North Atlantic Ocean ingression.

Figure 1. Depositional model displaying spatial and lateral distribution of halite and carnallite-prone facies associations FA2 to FA8, with respect to each parent brine saturation stages and salinity increase.

Figure 2. Photographs of FA7 facies and micro-facies. A) Microcrystalline red carnallite cumulates and interbedded halite laminites (K1 well, 647.47 m, cycle IV). B) and C) Granoblastic equigranular fine-grained red carnallite texture, respectively under natural and polarized light (K55 well, 675.67 m, Top cycle IV). D) and E) Details of tinted fine-grained carnallite cumulates of B) (natural light). F) Example of white translucid to red medium-grained carnallite interlayered with red carnallitic laminites (natural light) (K55 well, 675.67 m, Top cycle IV). G) Detail of white to red medium-grained carnallite texture illustrated in F) (natural light).

Lacustrine carbonate towers of Lake Abhe, Djibouti: Interplay of hydrologic and microbial processes

Lake Abhe, located at the triple junction between the Red Sea, Gulf of Aden, and Main Ethiopian rift trends, is a hypersaline and alkaline closed lake that is known for its exposures of massive carbonate chimneys on the flat-lying sediments of the lake's eastern margin. This study describes the morphological, textural, and petrographic characteristics of these chimneys at the basin- to microscopic-scale in order to constrain the carbonate depositional system in the context of basin hydroclimate history. Chimneys occur in three main fields and are assessed according to 1) large-scale (>5 m) morphological variations, 2) meso-scale (cm to <5 m) textures, and 3) micro-scale (<1 cm) fabrics. The dominant chimney fabric is a porous crystalline framework of trigonal prismatic and dendritic calcite crystals that locally contain spherulites, sickle-cell calcite fabrics, and entombed microbial cocci. In addition to the crystalline chimneys, other carbonate sediments include stromatolitic crusts composed of microdigitate laminated columns dominated by crystalline fan microfabrics, as well as carbonate-rich mudstones and relatively rare, localized, carbonate-rich diatomites. Chimneys are interpreted to form primarily as products of mixing between hydrothermal sublacustrine springs and lake waters during lake highstand intervals, while stromatolitic crusts are interpreted to form during lowstand lake levels. Our interpretation of mixing processes is supported by the δ18O composition of chimney calcite, which is representative of lake water and area hot spring endmembers. The observation of stromatolitic crusts with more positive values of δ13C and δ18O than crystalline chimneys indicates that crusts formed during periods of high evaporation and low lake level. Crusts also contain Mg-silicate minerals, which are not present in crystalline chimneys, and further support the interpretation of lowstand depositional conditions. Although the age of chimney formation is not well constrained, evidence from seismic reflection data suggests a pattern of chimney formation during lake level rise and highstand times, followed by lake level fall, subaerial exposure and weathering that occurred at several times throughout the Late Pleistocene and Holocene.

Figure 1. Large-scale chimney forms. A -Massive (base of exposure) and pinnacle (top of exposure) chimney shapes, illustrating how massive forms may arise from amalgamation c individual pinnacles. B -Isolated pinnacles next to a massive chimney; note that the massive form is relatively flat-topped in comparison to pinnacle forms. C -Bulbous chimney forms, characterized by rounded shapes. D -Barrel chimney form, characterized by more columnar shape. E -Frondose chimney forms, characterized by branching tubular structures forming tree-like structures.

Figure 2. Meso-scale chimney textures. A -Branching chimney texture defined by cm-scale dendritic branches; scale bar on the right (black box = 1 cm). B -Swallow's-nest texture defined by hollow, cup-like "nests". C -Honeycomb texture, characterized by hollow chambers separated by thin horizontal and/or vertical layers. Note that honeycomb textures are flatter and more box-like than swallow's-nest textures. D -Interchimney carbonate-rich mudstones, showing mm-to cm-scale horizontal bedding.

Geochemical proxies for water-soil interactions in the hyperarid Atacama Desert, Chile

The Atacama Desert is the oldest and driest non-polar desert on Earth. Millions of years of hyperaridity enabled salt accumulations through atmospheric deposition. These salts can serve as proxies to decipher the interaction between water and soil as well as to understand the habitability with changing environmental settings. Therefore, we investigated four soil profiles regarding their mineralogy, salt abundance, and sulfate stable isotopic composition. The profiles were located along an elevation transect in the hyperarid region southeast of Antofagasta, Chile. The two lower sites situated on the distal parts of inactive alluvial fan deposits were subject to occasional fog occurrences. The upper steeper-sloped sites experienced no fog but are subject to minimal erosion. In all soil profiles, sulfates are the dominant salts showing a downward transition from gypsum to anhydrite that is accompanied by an increase of highly soluble salts and a decrease of sulfate δ34S and δ18O values. These trends are consistent with downward directed water infiltration during rare rain events causing salt dissolution followed by precipitation within the deeper soil column. This conclusion is also supported by our Rayleigh fractionation model. We attribute the presence of anhydrite at > 40 cm depth to the cooccurrence of nitrate and chloride salts, which decreases water activity during sulfate precipitation and therefore drives anhydrite formation. Along the elevation transect, the total salt inventories of each profile show a trend for nitrates and chlorides concentration decreasing with elevation. This observation together with the sulfate stable isotopes indicates a fog-independent source and suggests remobilization of soluble salts through enhanced washout from hillslopes to alluvial fans. These findings are essential for assessing the long-term regional habitability of hyperarid environments and have also relevance for Mars.

Figure 1. Location of study area. a) overview map shows arid and hyperarid regions in South America (after Houston and Hartley 2003), black rectangle indicates study region; b) topographic map of the study region located southeast of Antofagasta, in the southern part of the hyperarid core of the Atacama Desert (topography based on ASTER DEM (USGS)), the black rectangle shows the study area; c) topographic map of the study site in the Yungay valley: three sites (HP, HM, HD) are located on the southern slopes of the mountain Cerro Herradura and one (TD) on the northern slopes of the mountain Cerro de las Tetas, eye symbol with indicated angle of view marks the position from which the photograph in d) was taken; d) panoramic photograph taken with ai unmanned aerial vehicle at ~ 1400 m a.s.l. showing the studied valley, covered by fog on 9 August 2019 8:22 AM, reaching the 1200 m a.s.l. threshold. All elevation values refer to sea level (a.s.l.).

Figure 2. Sketch of salt input and distribution within the Yungay valley. SO? and NO are brought into the system homogeneously by dry fall-out, eolian dust, or rain. Cl is more limited to lower elevations as the source is mainly the ocean, distributed by fog and eolian dust or saline groundwater, also redistributed by eolian dust. Once in the soil, salts can be redistributed again. Rare rain events lead to downward in-soil salt transport. However, there are indications on rare fluvial run-off and downslope transport, due to steeper hill slopes at HP and HM. All three transport mechanisms lead to incomplete leaching of the salts, causing a fractionation of salts by solubility, within the soil column as well as along the investigated transect.

Leaky salt: Pipe trails record the history of cross-evaporite fluid escape in the northern Levant Basin, Eastern Mediterranean

Despite salt being regarded as an extremely efficient, low-permeability hydraulic seal, an increasing number of cross-evaporite fluid escape features have been documented in salt-bearing sedimentary basins. Because of this, it is clear that our understanding of how thick salt deposits impact fluid flow in sedimentary basins is incomplete. The paper examines the causes and evolution of cross-evaporite fluid escape in the northern Levant Basin, Eastern Mediterranean. High-quality 3D seismic data offshore Lebanon image hundreds of supra-salt fluid escape pipes distributed widely along the margin. The pipes consistently originate at the crest of prominent sub-salt anticlines, where overlying salt is relatively thin. The fact the pipes crosscut the salt suggests that hydrofracturing occurred, permitting focused fluid flow. Sequential pipes from unique emission points are organized along trails that are several kilometres long, and which are progressively deformed due to basinward gravity gliding of salt and its overburden. Correlation of pipes in 12 trails suggests margin-wide fluid escape started in the Late Pliocene/Early Pleistocene, coincident with a major phase of uplift of the Levant margin. The consequent transfer of overpressure from the central basin area, in addition to gas exsolution from hydrocarbons already trapped in sub-salt anticlines, triggered seal failure and cross-evaporite fluid flow. Other causes of fluid escape in the Eastern Mediterranean, such as subsurface pressure changes driven by sea-level variations and salt deposition associated with the Messinian Salinity Crisis, played only a minor role in triggering cross-evaporite fluid flow in the northern Levant Basin. Further phases of fluid escape are unique to each anticline and cannot be easily correlated across the margin. Therefore, despite a common initial cause, long-term fluid escape proceeded according to structure-specific characteristics, such as local dynamics of fluid migration and anticline geometry. Hence, mechanisms triggering cross-evaporite fluid flow in salt basins vary in time and space.

Figure 1. Seismic expression of different fluid escape pipe geometries. (a) Upward tapering conical pipe showing a clear pockmark terminus marked by an amplitude anomaly; (b) cylindrical pipe roughly maintaining the same diameter; (c) downward tapering conical pipes with large pockmark at the seafloor and in the shallow subsurface; (d) mud volcano feeder conduit showing lateral migration of fluid into the hosting sediments

Figure 2. Sketch of pipe trails evolution. An increased uplift of the Levant margin at ca. 1.8 Ma triggered enhanced fluid migration towards the sub-salt anticlines from the deeper areas of the basin (b). As a consequence, supra-lithostatic overpressure formed inside the anticlines leading to hydrofracture and the formation of the first pipes. During the subsequent period and until present, local processes govern overpressure build-up and the formation of cross-evaporite fluid escape.

The northern Gulf of Mexico offshore super basin: Reservoirs, source rocks, seals, traps, and successes

The northern Gulf of Mexico federal offshore area easily qualifies as a super basin based upon estimated petroleum endowment of more than 100 BOE and cumulative production of 60 BOE. Like other super basins, it has multiple petroleum systems and stacked reservoirs. Examination of four key elements of these petroleum systems (reservoirs, source rocks, seals, and traps) yields important insights to the geologic processes that result in such an exceptional habitat for conventional hydrocarbons.The bulk of hydrocarbon resources in federal offshore waters is in Cenozoic sandstone reservoirs such as the Paleogene Wilcox reservoir of deep-water subsalt areas. 

Overall, Cenozoic sandstone reservoirs in both suprasalt and subsalt fields yield the highest flow rates and cumulative production volumes. Notable is the recent addition of the deep-water Jurassic Norphlet sandstone play, the newest and second largest by ultimately technically recoverable resources. Overall, Gulf of Mexico reservoirs are diverse, formed in paleoenvironments ranging from aeolian to deep water.Powering this super basin are three primary marine source rocks centered in the Oxfordian, Tithonian, and Cenomanian–Turonian Stages. These source rock intervals commonly act as top seals, but other Neogene and Mesozoic shales and even carbonate mudstones are also important trap-sealing elements, as proven by analytical work and downhole pressure measurements. 

High rates of Cenozoic deposition on a mobile salt substrate also generated a myriad of salt tectonic structures, ranging from simple diapiric closures and extensional fault traps to complex subsalt configurations such as salt-cored compressional anticlines, salt-cutoff traps, and bucket weld traps. 

Geologically, salt is important because it can radically alter how petroleum basins evolve. Compared to other sedimentary rocks, it migrates easily through the Earth, creating space for oil and gas to collect. It helps moderate heat and keeps hydrocarbon sources viable longer and deeper. And it is a tightly packed mineral that seals oil and gas in large columns, setting up giant fields.

Exploration success in the past 20 yr is a direct result of improved seismic imaging around and below salt, as well as advances in drilling, completing, and producing wells and fields. 

A ccording to the paper, the bulk of the northern offshore basin's potential remains in giant, deepwater oil fields beneath the salt blanket. Although reaching them is expensive and enormously challenging, Snedden believes they represent the best future for fossil fuel energy. That's because the offshore -- where many of the giant fields are located -- offers industry a way of supplying the world's energy with fewer wells.

Figure 1. Location map of the United States (northern) Gulf of Mexico Super Basin as defined here. The dashed line separates Mesozoic (MZ) fields and discoveries on the north from Cenozoic (CZ) fields and discoveries located basinward. PW = Paluxy-Washita supersequence; SH = Sligo-Hosston supersequence.

Figure 2. Regional map of the northern Gulf of Mexico shelf and continental slope showing the four major tectonically defined exploration provinces. From Weimer et al. (2016b) and reproduced with permission from the Gulf Coast Association of Geological Societies. Colors indicate age of stratigraphic fill in the Basins province: dark blue = Miocene; medium blue = Pliocene; light blue = Pleistocene. AC = Alaminos Canyon; AT = Atwater Valley; DC = Desoto Canyon; EB = East Breaks; EW = Ewing Bank; FP = Florida Plain; GB = Garden Banks; GC = Green Canyon; HE = Henderson; KC = Keathley Canyon; LL = Lloyd; LS = Lund South; LU = Lund; MC = Mississippi Canyon; SE AT = Sigsbee Escarpment Amery Terrace; WR = Walker Ridge.

Biomarker similarities between the saline lacustrine Eocene Green River and the Paleoproterozoic Barney Creek Formations

The Paleoproterozoic Barney Creek Formation, which is currently interpreted as a restricted, deep marine paleoenvironment, plays a disproportionate role in our understanding of Proterozoic ocean chemistry and the rise of complex life. The Barney Creek Formation hosts several unusual biomarker features, specifically its methylhopane and carotenoid signatures. Herein, we demonstrate that the saline lacustrine Eocene Green River Formation shares a similar distribution of methylhopanes and carotenoids, which is characteristic of saline lacustrine organic matter more generally. These distinct methylhopane and carotenoid patterns are not observed together in marine organic matter of any geologic age. These results imply a saline lacustrine depositional environment for the Barney Creek Formation, which agrees with earlier but now abandoned depositional models of this formation. As a result, models of Proterozoic ocean chemistry and emergence of complex life that rely on a marine Barney Creek Formation should be reexamined. Alternatively, if Paleoproterozoic marine biomarker signatures resemble those of younger saline lacustrine systems, then this must be recognized to accurately interpret geologic biomarker and paleoenvironmental records.

Figure 1.  Saturated C40 carotanes. ß-carotane and y-carotane are labeled in the total ion chromatograms (TICs) of the saturated fractions of samples from different lake phases in the Uinta Basin PR-15-7c core (left). The ß-Car value is the percentage of the ß-carotane peak height relative to the average peak height in the TIC. The BCF saturated fraction TIC ) and a TIC for a saturated fraction from an interval of immature, marine Cretaceous lower Eagle Ford Group are shown for comparison at upper right and lower right, respectively.

Gypsum Deltas at the Holocene Dead Sea linked to Grand Solar Minima

Unique gypsum structures: large capes (termed ?gypsum deltas?) and small pitted gypsum mounds are exposed along the western shores of the currently retreating Dead Sea, the hypersaline terminal lake in the Dead Sea Basin. The gypsum deltas were formed during time-intervals of low lake stands (?420±10 m below mean sea level), when sulfate-rich Ca chloride brines discharged from the coastal aquifer via saline springs, mixed with the Dead Sea brine and precipitated the gypsum (outsalting process). The ages of formation of the gypsum structures coincide with times of North Atlantic cooling events and grand solar minima suggesting a direct impact of the latter on the Dead Sea hydrology and high sensitivity of the regional hydrology (controlling lake level) to global solar-related events. The temporal occurrence and numbers of the gypsum structures appear to follow the Hallstat Cycle that approaches minima at ?3000-2000 years before present. 

The "gypsum delta" of Cape Qedem: (a) An oblique aerial photo of Cape Qedem depicting the sites of the three stratigraphic sections (the green lines) conducted on the cape, cutting through the gypsum delta. The location of Qedem drillhole is marked by the red dot. The photo was taken on January 2020, when lake level was at 434.4 m bmsl. (b) Schematic cross section along Gypsum-2 extending from the major fault line in the west to the Dead Sea in the east. The section depicts the: 1. Cretaceous marine carbonate rocks of the Judea Group, the lacustrine sediments of the last Glacial Lisan Fm.; 2. The unconformity between Lisan Fm. and the overlaying gypsum-rich layers of Cape Qedem; 3. Discordant gypsum mounds ; 4. The saline springs that discharge the Ein Qedem brine. The brine rises from a depth of ~1 km depth along the fault plain (thick solid purple line) filling the coastal aquifer (partly transparent purple area) and discharges at the shore; and 5. The interface between the Dead Sea-Ein Qedem brines. (c) Cross-section along the same line as in b depicting Cape Qedem during the time of the gypsum precipitation (e.g. ~3000-1000 y BP).

Ingredients for microbial life preserved in 3.5 billion-year-old fluid inclusions

It is widely hypothesised that primeval life utilised small organic molecules as sources of carbon and energy. However, the presence of such primordial ingredients in early Earth habitats has not yet been demonstrated. Here we report the existence of indigenous organic molecules and gases in primary fluid inclusions in c. 3.5-billion-year-old barites (Dresser Formation, Pilbara Craton, Western Australia). The compounds identified (e.g., H2S, COS, CS2, CH4, acetic acid, organic (poly-)sulfanes, thiols) may have formed important substrates for purported ancestral sulfur and methanogenic metabolisms. They also include stable building blocks of methyl thioacetate (methanethiol, acetic acid) – a putative key agent in primordial energy metabolism and thus the emergence of life. Delivered by hydrothermal fluids, some of these compounds may have fuelled microbial communities associated with the barite deposits. Our findings demonstrate that early Archaean hydrothermal fluids contained essential primordial ingredients that provided fertile substrates for earliest life on our planet.

Black baryte associated with originally sulfidic stromatolites in the 3.45 Ga  Dresser Fm. near Marble Bar, West Australia

Inserts a, b represent enlargements of respective areas in the chromatogram marked by dashed lines. Triangles denote oxygen-bearing compounds, circles denote aromatic hydrocarbons and stars denote sulfur-bearing compounds. n-Hexane (Hex) was used as a retention time standard (RT std.). COS carbonyl sulfide, Ea ethanal, MT methanethiol, Bu but-1-ene, Pa prop-2-enal, Pa’ propanal, ET ethanethiol, MSM (methylsulfanyl)methane, Po propan-2-one, Ba but-2-enal, Ox oxolane, TP thiophene, B benzene, Ac acetic acid, TL thiolane. Note the presence of methanethiol and acetic acid, the stable building blocks of activated acetic acid

Dams and reservoirs in karst? Keep away or accept the challenges

The distribution and flow of groundwater in karstified rocks can be extremely complex and not readily predictable, a far from friendly environment for constructing dams and reservoirs. There have been many expensive failures such as unacceptable leakage rates at and around dams, and/or reservoirs that could not be filled to the design levels. This is never the fault of site geology but always of human mistakes due to inadequate investigation programmes and/or erroneous interpretation of the karst processes at work. Remedial works are expensive, time-consuming and frequently do not justify the money invested. As a result, those undertaking engineering works in karst terrains may approach with two fears—of the exceptional risk and/or of a failure. The key question, so often, is whether to build the dam in karstified rocks or keep away from such a risky environment. However, construction of water storage reservoirs is essential in many karst regions for socio-economic development. The challenge must be accepted. Based on much field experience, the best practices for selection of adequate dam and reservoir sites are defined and illustrated with specific examples from many different climatic, topographic, lithologic and hydrogeologic settings in Europe and Asia. This work emphasises that the amount of certainty or uncertainty in the crucial parameters—geological structure, groundwater regime, intensity and depth of karstification—should be recognized.

Flow chart of the engineering karstology approach In dam and reservoir construction

Towards a low-carbon society: A review of lithium resource availability, challenges and innovations in mining, extraction and recycling, and future perspectives

The demand for lithium has skyrocketed in recent years primarily due to three international treaties—Kyoto Protocol, Paris Agreement and UN Sustainable Development Goals—all of which are pushing for the integration of more renewable energy and clean storage technologies in the transportation and electric power sectors to curb CO2 emissions and limit the adverse effects of CO2-promoted climate change. Over 60% of lithium produced in 2019 were utilised for the manufacture of lithium-ion batteries (LIBs), the compact and high-density energy storage devices crucial for low-carbon emission electric-based vehicles (EVs) and secondary storage media for renewable energy sources like solar and wind. In 2019, the global market value of lithium reached around US$213 B and is forecasted to grow by around 20–25% until 2025. In this review, the current state of global lithium resources, global lithium material flow, and forecasts of future lithium supply–demand dynamics are discussed. Persistent challenges in mining, processing and industrial-scale recycling operations are also examined and recent innovations to address these issues are introduced. Finally, unconventional lithium sources like submarine/deep-sea ferromanganese (Fe-Mn) nodules and crusts, industrial wastes (e.g., desalination brines, geothermal brines and coal fly ashes), mining wastes and effluents, and extra-terrestrial materials are explored.

(a) Global lithium consumption from 2010 to 2019; (b) Global production (supply) and consumption (demand) from 2010 to 2019 (excluding US production), their growth forecasts until 2025 pre-COVID-19 and updated 2020 supply-demand estimates factoring in the effects of COVID-19; and (c) Global Li demand of the three main Li consumers battery, ceramics & glass, and grease-and projections of their growth until 2025.

Global lithium resources, resource type and production in 2019 (excluding US production)

Rift and salt-related multi-phase dolomitization: example from the northwestern Pyrenees

The Meillon (Callovo-Oxfordian) and Mano (Tithonian) Formations are dolomitized carbonate reservoirs that actively produce oil and gas (Aquitaine Basin, France). In this study, the dolomitization conditions of their counterparts exhumed in the northwestern Pyrenees are detailed using a combination of field observations, petrography, fluid inclusion microthermometry, elemental and isotopic geochemistry, and carbonate U–Pb geochronology. Dolomitization occurred in several stages spanning from the Neocomian (pre-rift) to the Albian (syn-rift, associated with mantle exhumation and active salt tectonics). Both formations were first massively dolomitized in near-surface to shallow burial conditions during the Berriasian-Valanginian, likely triggered by the influx of marine-derived waters. Between the Barremian and the Albian, the Early Cretaceous rifting caused the upward influx of hot fluids associated with the partial to complete recrystallization of the initial dolomites. During the Albian, subsequent dolomites precipitated in both formations as high-temperature (T > 160 °C) vein- and pore-filling cement. Distinct fluid inclusion chlorinities and rare earth element patterns between the Meillon and Mano Formations point to fluid compartmentalization during this stage. Whereas dolomite cements indicate the involvement of evaporite-derived brines in the Meillon Formation, precipitation was likely related to clay-derived water in the Mano Formation. Lastly, a final episode of dolomite cementation occurred only in the vicinity of faults and volcanic intrusions during the Albian when the highest temperatures were recorded in both formations (T > 250 °C). These saddle dolomites precipitated from hydrothermal water with a mixture of mantle-, crustal-, and evaporite-derived waters channeled by faults and active diapirs. Subsequent quartz and calcite cement precipitation reveals a temperature decrease in a post-rift to inversion setting (post-Cenomanian) and indicates fluid compartmentalization between both formations. This study highlights the major control exerted by rifting, combined with the presence of diapiric salt, on dolomitization, making carbonate platforms of modern salt-rich passive margins potential targets for exploration.

General model of the evolution of fluid chemistry, fluid circulation, and diagenesis phases set in the schematic reconstruction of the Mail Arrouy chaînon and its structural and geochemical evolution. The rightmost sketches detail the inferred fluid circulations in the near-sampled area.

Coupling of paleo-environment and biogeochemistry of deep-time alkaline lakes: A lipid biomarker perspective

Studies of alkaline lakes have critical biological–environmental–economic properties, but deep-time alkaline lakes are challenging to investigate. Lipid biomarkers can provide valuable insights into such lakes and their biogeochemical significance. The paper reviews and compares typical examples of ancient alkaline lakes across the world. Lipid biomarker evidence, including C30-steranes, Pr/Ph, Pr/n-C17-Ph/n-C18, (β − +γ-carotane)/n-Cmax, and gammacerane/C30αβH values, suggests these alkaline lakes were reducing, hypersaline, and stratified. The n-alkanes, steranes/hopanes, C28-St/C27–29-St%, and C28/C29-St values indicate that the preserved biomass of the alkaline lakes were dominated by algae and bacteria, with less input from higher plants. The algae were mainly halotolerant green algae, rather than cyanobacteria. The different alkaline lakes have some subtle differences in their sedimentary environments. The paleoenvironmental setting and biomass of the alkaline lakes co-vary systematically. The ratio of algae/bacteria is positively correlated with increasingly reducing and saline conditions, because the increase in salinity improves the competitiveness of halotolerant green algae. The changes in these extreme alkaline environments are too small to cause obvious variations in the proportion of green algae/total algae, and the abundance of cyanobacteria, photoautotrophs, and/or type I methanotrophic proteobacteria. Lipid biomarker data show that the primary controlling factor on the biomass of saline and alkaline lakes is their geologic age and, to a lesser extent, their salinity. The abundance of organic matter in these sediments varies greatly, and the types of organic matter are generally good for hydrocarbon generation. The formation of oil and gas is controlled by factors such as abundance of organic matter, thermal maturity, size of lake basin, and thickness of the organic-rich sediments.

Alkaline saline lake across time and their stratified palaeohydrologies, as indicated by their preserved lipid biomarkers