Meteoric Lens Research Articles

Hydroacoustic Observations Reveal Drivers of Mixing and Salinization of a Karst Subterranean Estuary During Intense Precipitation

AbstractKarst subterranean estuaries within globally ubiquitous carbonate aquifers are coastal groundwater ecosystems that provide an essential water resource for human populations. To understand the drivers of salinization within a coastal aquifer in the Yucatan Peninsula (Mexico), we employed hydroacoustics in flooded caves to observe how oceanic and atmospheric events facilitate mixing between the meteoric lens (fresh‐brackish groundwater) and the saline groundwater on tidal and episodic timescales. Precipitation during Tropical Storm Carlotta increased the flow and salinity of the meteoric lens without evidence for vertical mixing across the halocline. We postulate that vertical migration of haloclines in the conduit relative to those within the rock matrix during precipitation creates lateral density gradients that drive mixing, and ultimately creates a brackish layer within the meteoric lens. These results provide a mechanistic explanation for vertical and lateral exchange in a coastal carbonate aquifer, which has implications for groundwater response to future climatic change.

Microbial biogeography of the eastern Yucatán carbonate aquifer.

The extensive Yucatán carbonate aquifer, located primarily in southeastern Mexico, is pockmarked by numerous sinkholes (cenotes) that lead to a complex web of underwater caves. The aquifer hosts a diverse yet understudied microbiome throughout its highly stratified water column, which is marked by a meteoric lens floating on intruding seawater owing to the coastal proximity and high permeability of the Yucatán carbonate platform. Here, we present a biogeographic survey of bacterial and archaeal communities from the eastern Yucatán aquifer. We apply a novel network analysis software that models ecological niche space from microbial taxonomic abundance data. Our analysis reveals that the aquifer community is composed of several distinct niches that follow broader regional and hydrological patterns. This work lays the groundwork for future investigations to characterize the biogeochemical potential of the entire aquifer with other systems biology approaches.

Frasnian reef mounds in the Durbuy–Bomal area (eastern border of the Dinant Synclinorium, Belgium)

This paper focuses on the Frasnian reefal development in the eastern border of the Dinant Synclinorium. Classical sections from the Durbuy–Bomal area were reevaluated for lithostratigraphy, microfacies, magnetic susceptibility (MS) and diagenesis. As regards the middle Frasnian succession, the studied area is located in a transitional zone between the Pont de la Folle/Philippeville Formations and the Moulin Liénaux/Grand Breux Formations. This succession is topped by Petit-Mont Member reef mounds. Massive mound microfacies are characteristic of the Petit-Mont and Lion Members, with fossil associations respectively dominated (from deepest to shallowest) by sponges; sponges, crinoids and corals; corals, crinoids, stromatoporoids and cyanobacteria; and microbes. Flank and off-mound microfacies consist of microbioclastic, bioclastic, crinoidal or lithoclastic bedded limestones. MS values are related to the depositional environment and regularly decrease from the off-mound to the reef mound microfacies. The reef mound diagenetic sequence is similar to that identified in other Petit-Mont buildups: cementation in the marine phreatic zone preliminary to drowning, then the development of a meteoric lens at the time of a marine regression, with dysoxic facies in the distal zones of the aquifer and, finally burial cementation and dolomitization during the Variscan tectonism.

An integrative re-evaluation of Typhlatya shrimp within the karst aquifer of the Yucat\xe1n Peninsula, Mexico

The Yucatán Peninsula, Mexico is a carbonate platform well-known for extensive karst networks of densely stratified aquifer ecosystems. This aquifer supports diverse anchialine fauna, including species of the globally distributed anchialine shrimp genus Typhlatya (Atyidae). Four species (T. campecheae, T. pearsei, T. dzilamensis and T. mitchelli) are endemic to the Peninsula, of which three are federally listed in Mexico. This first integrative evaluation (i.e., molecular, morphological, broad geographic and type locality sampling, and environmental data) of Yucatán Typhlatya reveals considerable species identity conflict in prior phylogenetic assessments, broad species ranges, syntopy within cave systems and five genetic lineages (of which two are new to science). Despite sampling from the type locality of endangered T. campecheae, specimens (and molecular data) were indistinguishable from vulnerable T. pearsei. Ancestral/divergence reconstructions support convergent evolution of a low-salinity ancestor for a post-Paleogene arc Yucatán + Cuba Typhlatya clade within the anchialine Atyidae clade. A secondary adaptation for the coastal-restricted euryhaline (2–37 psu), Typhlatya dzilamensis (unknown conservation status) was identified, while remaining species lineages were low-salinity (< 5 psu) adapted and found within the meteoric lens of inland and coastal caves. This study demonstrates the need for integrative/interdisciplinary approaches when conducting biodiversity assessments in complex and poorly studied aquifers.

Methane oxidation dynamics in a karst subterranean estuary

Chemical gradients between fresh, brackish and saline waters shape biogeochemical reactions and organic matter transformation within subterranean estuaries. In the Yucatán Peninsula’s karst subterranean estuary (KSE), methane and dissolved organic matter generated during the anaerobic decomposition of tropical forest vegetation are transported into flooded cave networks where microbial consumption greatly reduces their concentrations in the groundwater. To test the hypothesis that chemoclines associated with salinity gradients of the KSE are sites of methane oxidation, we obtained methane concentration and δ13C profiles of unprecedented vertical resolution from within a fully-submerged cave system located 6.6 km inland from the coastline using the ‘OctoPiPi’ (OPP) water sampler. Along a 12–24 cm thick low-salinity-halocline at ∼4.5 m water depth, salinity increased from fresh to brackish (0.2–1.8 psu), methane concentrations decreased, and δ13C values increased, as expected for microbial methane oxidation. The underlying brackish water had elevated oxygen concentrations compared to the always anoxic freshwater, suggesting that aerobic methane oxidation is the dominant process facilitating methane consumption. By contrast, as salinity increased from 1.8 to 36 psu through a 24–36 cm thick high-salinity-halocline between the meteoric lens and the saline groundwater at ∼20 m water depth, methane concentrations and δ13C values were constant. Conservative mixing and kinetic isotope models incorporating the methane data confirm a hotspot for microbial methane oxidation at the low-salinity-halocline. At least 98% of methane originating in the anoxic freshwaters was removed before its transport via channelized flow towards the coastline. These findings provide novel insight into the spatial constraints of methane dynamics within a karst subterranean estuary.

The characteristics and controlling factors of facies-controlled coastal eogenetic karst: insights from the Fourth Member of Neoproterozoic Dengying Formation, Central Sichuan Basin, China

Karst porosity system is widely developed in the Fourth Member of Neoproterozoic Dengying Formation in Central Sichuan Basin of China. The result of the previous exploration in this area based on the theory of telogenetic karst, which mainly focused on the fracture/fault belt especially in the karst slope, leaves much to be desired. Though a systematically geological research in this study, it is found that the dissolution holes are mainly concentrated on the mud mound deposits and granular rocks. Both of them have a greater primitive porosity and permeability than the micrite that originally formed in low-energy environments, which is facies controlled. On the basis of the above and the fact that a tectonic movement has caused a massive subaerial exposure of the formation that only experienced a shallow burial, it is proposed that the paleokarst porosity system is the product of eogenetic karst. Two-stage eogenetic karsts have been interpreted: the syngenetic exposure caused by the short-term relative sea-level fluctuation and the exposure caused by the uplift of the second phase of Tongwan tectonic movement. The meteoric alteration in syngenetic period is commonly marked by the development of intergranular pores but on a small scale. It is believed to be important for providing a good pore-permeability foundation for the later eogenetic karst. The later exposure occurs immediately at the end of the deposition of the Fourth Member of Dengying Formation that only experienced a shallow burial. Since the effect of compaction and cementation in this shallow burial before uplift is relatively weak, only the micrite has been densified and retaining the majority of original pores in the carbonate mounds and grain shoals as paths for karst flow. Moreover, combined with the geological background and seismic profile, it can be ascertained that the karst occurs in a coastal environment. Twelve meteoric lenses have been interpreted after recovering the karst palaeogeomorphology. The scale of the meteoric lens and the reservoir performance have shown a decreasing tendency from the coastal to the inland area, which also confirms this view. The results of this research have put emphasis on the lithologic differentiation in eogenetic karst and the geographical location of karst development thus might provide a reference for further reservoir prediction in this area and other similar places.

Organic matter sources and lateral sedimentation in a Bahamian karst basin (sinkhole) over the late Holocene: Influence of local vegetation and climate

Karst basins (e.g., blueholes, sinkholes) accumulate well-preserved sedimentary successions that provide transformative paleoclimatic and paleoenvironmental information. However, the sedimentary processes within these basins are not yet fully understood. Here we present stable carbon isotopic values (δ13Corg) and C:N ratios of bulk organic matter in well-dated sediment cores from Blackwood Sinkhole (Abaco, The Bahamas) to investigate the changing flux of organic matter into the sinkhole during the late Holocene. The provenance of preserved organic matter changed through the late Holocene between three primary sources, as determined by three-endmember mixing modeling: wetland organic matter from the adjacent epikarst surface, authigenic primary productivity in the oligohaline meteoric lens, and terrestrial organic matter from the surrounding landscape. Expansion of wetlands on the adjacent epikarst surface played a critical role by increasing the flux of wetland organic matter to the sinkhole, especially during the last 1000 years. Hurricanes and regional rainfall may have mediated organic matter delivery to the benthos, either through hampering wetland development (prior to 1000 cal yr BP) or by changing dissolved nutrient concentrations available in the basin for primary producers. These results demonstrate that organic matter provenance in karst basins is not constant through time, and is significantly dependent upon both landscape vegetation on the epikarst surface and changing hydrographic conditions that impacts nutrient availability to primary producers.

Hurricanes Ingrid and Manuel (2013) and their impact on the salinity of the Meteoric Water Mass, Quintana Roo, Mexico

We report on measurements of the salinity and temperature in the Yax Chen cave system on the Yuacatan peninsula. This paper is submitted together with Kovacs et al. (2017). Kovacs et al. focuses on the salinity levels of the meteoric lens, while this paper uses the observed results to elucidate the hydrodynamics. The cave passages have water depths on the order of 10m, with flow on the order of ten centimeters a second, and as such is a hydrodynamic, as opposed to a porous, system. The measurements reveal that episodes of significant mixing between the fresh meteoric lens and the underlying salty water are driven by meteorological events (e.g., Hurricane Rina in 2011, and the twin Hurricanes Ingrid and Manuel in 2013). We find evidence that after the hurricanes in 2013, the water column remains unstable for several months. Through wavelet analysis, we find that the marine Water Mass (WM) exhibits much less low period activity compared to the meteoric WM. We hypothesize that the open cenotes are locations of high mixing intensity, with turbulent fronts propagating away from the sites of direct mixing into the cave network. We perform laboratory experiments and numerical simulations to explore this phenomenon, and find that mixing preferentially occurs on the flanks of regions of strong, stable density stratification (i.e., on the periphery of pycnoclines), and leads to entrainment of fluid into the turbulent region. Using high resolution direct numerical simulation, we explore the detailed manner in which turbulent entrainment can drive flow toward the mixing region, and lead to mixing of passive tracers. Finally, we discuss the implications of these results for the mixing of passive tracers, such as suspended chemicals.

Origin and hydrology of a large, intact Early Cambrian paleocave system and its role in overlying fluidisation structures, Arctic Canada

Paleokarst is most commonly expressed as subtle stratigraphic surfaces rather than large void systems penetrating deeply into the paleo-subsurface. In contrast, a regional Precambrian-Cambrian unconformity on Victoria Island (NWT, Canada), is associated with exceptional exposure of large, intact Cambrian paleocaverns (100m diameter; tens of m high). The paleocaves are distributed along a paleo-horizontal plane, and an associated gryke network is present in the 30–60m of Neoproterozoic dolostone between cave rooves and the base of overlying Cambrian sandstone; both are filled by Cambrian sandstone. The formation and preservation of such karst features require aggressive dissolution along a stable paleo-water-table shortly before transgression and deposition of shallow-marine sand over the dolostone. During the transgression, the karst network acted as a conduit for flowing groundwater that was discharged through overlying, unconsolidated Cambrian shallow-marine sand, producing water-escape structures (sand volcanoes and their conduits). The conduits are preserved as cylindrical remnants of the sand volcanoes' feeder pipes. Sediment fluidisation was probably caused by variations in the hydraulic-head gradient in a meteoric lens near the Cambrian coastline under a tropical climate with abundant, probably seasonally variable rainfall that caused pulses in subsurface fluid flow. Spatial distribution of the paleocaves and sand volcanoes suggests their formation on the southeast side of a recently faulted horst of Proterozoic carbonate bedrock that formed a nearshore island during early Cambrian sea-level rise. Fluidisation structures such as those reported here have generally been difficult to interpret owing to a lack of data on the fluid hydraulics of the underlying aquifer. This is the first report linking the hydraulics of a well-characterised paleokarst to development of fluid-escape structures. Such structures are widely known from sandstones overlying the sub-Cambrian unconformity around the circumference of Laurentia.

Geochemical signatures of pervasive meteoric diagenesis of Early Miocene syn-rift carbonate platform, Red Sea, NW Saudi Arabia

Different diagenetic environments have been recognized in the Early Miocene carbonate platform of Musayr Formation in the Red Sea rift area. Early marine diagenesis includes micritisation that occurs as thin envelope around skeletal and non-skeletal grains in low-energy mud dominated facies and isopachous fibrous calcite in high-energy grain-dominated facies. Pervasive meteoric water diagenesis resulted in cementation of the carbonates by coarse-crystalline blocky-drusy calcite and meniscus cements. Depletion of oxygen (avg. –9.08‰), carbon (avg. –1.6‰) isotopes and trace elements concentrations (avg. values of Fe: 1387 ppm; Mn: 1444 ppm; Sr: 419 ppm; Na: 1194 ppm) in conjunction with negative correlation between Mn2+ and oxygen isotope data suggest variable degrees of fluid-rock interactions and pervasive meteoric diagenesis. The formation of meteoric diagenesis in the Musayr Formation can be explained by two subsequent mechanisms: (1) the presence of meteoric lenses during the time of deposition might have been associated with active freshwater input from the hinterland (NE) due to fall in the relative sea level; (2) later uplift episode during Plio-Pleistocene may have also contributed to the pervasive meteoric diagenetic alterations of the carbonates of the Musayr Formation. The first mechanism is supported by the cement stratigraphy where the blocky-drusy cements postdate the meniscus cement. The latter mechanism seems to have more pronounced effect on the alteration of Musayr carbonate sequence by observing the occurence of late cements such as blocky calcite in most of the samples. The impact of meteoric diagenesis on the studied samples suggest that dissolution is less severe than cementation, hence the visible porosity is very low. Understanding the timing of meteoric diagenesis provides useful information about the reservoir quality distribution in syn-rift carbonate sequences.

Karstification of reef-bank facies carbonate and its control on pore distribution: A case study of Upper Ordovician Lianglitage Formation in eastern Tazhong area, Tarim Basin, NW China

Abstract The karstification type and characteristics and its control on pore distribution in reef-bank carbonate rock in the Upper Ordovician Lianglitage Formation (O 3 l) in eastern Tazhong area were studied using cores, thin sections and well logging data. The area experienced two types of karstification: syndepositional karstification and early diagenetic karstification near surface. During the depositional stage of reef-bank complex, meteoric diagenetic lens with lots of selective dissolution pores were formed by the syndepositional karstification, they were developed in the middle and upper parts of reef-bank sedimentary cycles, and a single porous/cavity layer has a thickness of 10–30 m, increasing the absolute porosity by 3%–4%. After depositional stage, sediments were uplifted and exposed without experiencing burial diagenesis, the depositional topography was influenced by early diagenetic karstification near surface and turned into karst paleotopography and formed four karstification zones within 100 m below the top of the Upper Lianglitage Formation. The surface karstification zone and radial flow karsitifiction zone have favorable porous connectivity, increasing the absolute porosity by 4%-8%. These two types of karsitifiction are continuous in diagenesis stage and the formed porosity is successive in distribution. The favorable space-time coupling is the key reason for effective pores being largely developed in the O 3 l 3 –O 3 l 1 members.

Sea-level rise and coastal circulation controlled Holocene groundwater development in Bermuda and caused a meteoric lens to collapse 1600years ago

The recent report from Working Group II of the Intergovernmental Panel on Climate Change (IPCC) called for an increased understanding of how groundwater interacts with the ocean-atmospheric system. This remains a prerequisite to better understand how groundwater will respond to climate change. However, achieving this goal is hampered by several problems, including: (1) our minimal understanding of how current groundwater conditions evolved and responded to previous climatic perturbations, and (2) the scarcity of techniques available for evaluating prehistoric groundwater.

ECOLOGY OF FORAMINIFERA AND HABITAT VARIABILITY IN AN UNDERWATER CAVE: DISTINGUISHING ANCHIALINE VERSUS SUBMARINE CAVE ENVIRONMENTS

Seventy-five surface (<4 cm) sediment samples were collected throughout Green Bay Cave System, Bermuda to investigate foraminiferal ecology and habitat variability in underwater coastal caves. This cave is ideal for studying different cave environments because it consists of an anchia-line cave environment connected to a submarine cave environment. Each sediment sample was analyzed for foraminifera, δ13Corg, C:N, organic matter content, CaCO3, and granulometry. Measurements of pH, salinity, dissolved oxygen, and temperature in the coastal aquifer distinguished the meteoric lens and saline groundwater. Q-mode cluster analysis on the foraminifera produced a dendrogram that segregates the anchialine and submarine cave environments, and subdivides each environment into distinct habitats consistent with local hydrogeology and sedimentology. The anchialine cave environment near the sinkhole is characterized by two groups of foraminifera: 1) the Meteoric Lens Assemblage living in the brackish meteoric lens within 60 cm of sea level, and 2) the Anchialine Cave Assemblage living in the saline groundwater. Helenina anderseni , Discorinopsis aguayoi , and other marsh foraminifera can persist in the brackish meteoric lens, which transitions into a more diverse assemblage dominated by Bolivina striatula and Rosalina globularis below the halocline. The boundary between the anchialine (terrestrially dominated) and submarine cave (marine-dominated) environments is demarcated by gross foraminiferal and sedimentary changes (δ13Corg from −24‰ to −18‰, C:N from 11.2 to 8.3) that correspond to the maximum point where terrestrial influences routinely impact the cave benthos. Three assemblages of foraminifera inhabit the submarine cave environment: 1) the Entrance Assemblage in the first ~60 m of the submarine cave, dominated by Quinqueloculina ; 2) the Circulated Submarine Cave Assemblage dominated by Spirillina vivipara and Triloculina oblonga , and 3) the Isolated Submarine Cave Assemblage dominated by Spirophthalmidium emaciatum . Planktic tintinnids suggest that tidally forced saline groundwater circulation is transporting more nutrients and particulate organic matter to the Circulated Submarine Cave Assemblage than the Isolated Submarine Cave Assemblage. These results indicate that coastal caves are partitioned into specific environments that can be further subdivided into habitats by groundwater masses, sediment fluxes (terrestrial versus marine), and groundwater circulation. This implies that that cave foraminifera can be useful paleohydrogeologic, paleoclimatic, and Quaternary sea-level proxies.

Linkages between Holocene paleoclimate and paleohydrogeology preserved in a Yucatan underwater cave

Three sediment cores spanning the last 4200 years from Aktun Ha Cave on the Yucatan Peninsula (Mexico) demonstrate that underwater caves can document changes to regional hydrogeology and climate. Benthic microfossils (testate amoebae, foraminifera), organic matter geochemistry (δ 13C, δ 15N, C/N), and particle size distributions were analyzed. However, microfossil paleoecology proved the most useful for indicating three salinity phases in the Aktun Ha sediment cores. Phase 1 (>4300 yr BP) contains predominantly foraminifera ( Physalidia simplex, 78%) that indicate the meteoric lens flooding the cave was initially brackish (salinity >3.5 g L −1). Phase 2 (2800–4300 Cal yr BP) has both freshwater testate amoebae ( Centropyxis spp. 40%) and P. simplex (42%), which indicates a slight freshening of the meteoric lens to 1.5–2 g L −1. Phase 3 (<2800 yr BP) is demarcated by an increase in testate amoebae abundance (90%) and diversity, including the colonization of Lagenodifflugia vas and Difflugia oblonga, with a reduction in P. simplex (10%). This last faunal shift represents the initiation of modern freshwater conditions in the cave (1.5 g L −1). This final freshening is synchronous with a significant reduction in the C/N ratio (e.g., Core 2: ∼27 to 19), which suggests an expansion of primary productivity in the adjacent cenote. The δ 15N values ranged from 1.5 to 3.5‰ with observed cycles likely from intervals of increased terrestrial OM input into the cave during high rainfall events (e.g., hurricanes). The observed paleoenvironmental shifts in the cave correlate well with regional precipitation patterns, aquifer recharge, and storm activity caused by southward migration of the Intertropical Convergence Zone. Therefore, regional climate change impacted eastern Yucatan groundwater during the mid to late Holocene. However, decelerating Holocene sea-level rise and aquifer occlusion are likely contributing factors. These results demonstrate that underwater cave sediments and microfossils can be useful proxies for aquifer evolution and climate change.

Karst and Early Fracture Networks in Carbonates, Turks and Caicos Islands, British West Indies

Abstract Historically, studies of Quaternary carbonates have not adequately addressed the influence of early fracture networks on diagenesis. Because of this lack of detail, understanding and predicting fracture-related diagenetic heterogeneities and preferential fluid flow pathways in ancient carbonate successions is particularly challenging. The Pleistocene oolitic grainstones of the Caicos platform provide an excellent opportunity to evaluate the relative importance of fractures on early diagenetic alteration styles, and are a suitable analog for subsurface carbonate reservoirs. Detailed analyses of fractures (e.g., orientation, aperture, spacing, fill material) from Caicos outcrops combined with high-resolution, three-dimensional ground-penetrating radar (3D GPR), assisted in exploring the causality and distribution of fractures and relationship to karst. Four models were evaluated to explain the observed distribution of dolines: (1) gravitational, fractured-margin controlled, (2) tectonic-fracture controlled, (3) antecedent-topography controlled, and (4) a hybrid model. Based on observations of numerous fractures (n = 306) on the western Caicos platform, early fractures are abundant and dominantly margin-parallel. These fracture networks are well established in limestones prior to mineralogical stabilization, thereby indicating that diagenetic heterogeneities evolve very early in carbonate diagenesis. The spatial distribution of dolines on Providenciales is likely the result of a complex interplay between the antecedent topography, margin-parallel fracture systems, and meteoric fluids. Resultant diagenetic alteration is far more complicated than simple, unconfined, meteoric lenses associated with topographic highs. Any attempts to model early diagenesis in carbonates should not dismiss the role of fractures as diagenetic facilitators and diagenetic anisotropy templates.

Diagenesis of the Middle Eocene Upper Dammam Subformation, Qatar: Petrographic and isotopic evidence

The subsurface Middle Eocene Upper Dammam Subformation in Qatar displays a complex diagenetic history related to sea level variations. The studied rocks are made up of dolomitic limestones and pervasive dolomites. Two main types of dolomite are identified based on an integrated petrographic/isotopic/mineralogic study: dolomites type (I) and (II), which affect the lower (the Umm Bab member) and the upper (the Abaruq member) parts of the Upper Dammam Subformation respectively. Dolomite type (I) is pervasive and composed of relatively coarse crystals that exhibit polymodal size distribution and different shapes of crystal boundaries (non-planar, planar-s and planar-e). Meanwhile, dolomite type (II) is selective, fine-textured and made up of unimodal equant euhedral rhombs. The marine d18O (+0.6 to +2.8%o) values of dolomite type (I) are consistent with formation during an early transgressive phase where the rising sea level possibly caused marine pore-water to push mixing and meteoric waters ahead of it. The active circulation in the marine pore-water zone led to pervasive dolomitization. The depleted d13C (-5.9 to -8.9%o PDB) values of dolomite type (I) indicate that dolomitization possibly occurred within the sulphate reduction zone beneath the sediment/water interface when an oxygen-minimum zone impinged the bottom surface of the carbonate platform. The presence of mimetic and non-mimetic dolomite textures is related to kinetic factors including the rate of dissolution of the CaCO3 precursor, reactant surface area and precipitation kinetics. Dolomite type (II) was formed during a subsequent highstand period during shelf progradation. The d18O (+0.4 to +2.6%o PDB) and d13C (-0.9 to -4.6%o PDB) values, the fine crystalline fabric and the planar intercrystalline boundaries argue for its early diagenetic origin within the subtidal zone above the oxygen-minimum zone. During further shelf progradation the studied rocks have been emplaced within a mixing marine-meteoric zone. During this period the dolomitization process was interrupted by a silicification phase. The precipitation of quartz and the resultant silica fabrics are controlled by variations in the PCO2 and the pH of the porewater. At the later stages of the highstand system tract, when sea level started to fall, a meteoric lens possibly developed at the shelf margin. The section was subjected to meteoric diagenesis. A late meteoric origin is suggested for calcite mosaics, that fill some secondary pores, based on their extremely depleted d18O values (-5.7 to -11.8%o), variable d13C values (-2.6 to -7.8%o PDB), low-Mg composition and their equant form.

Meteoric versus burial control on porosity evolution of the Smackover Formation

Whole-rock 13C, 18O, and Sr compositions of the uppermost ooid grainstones of the Smackover Formation at Black Creek field in Mississippi reveal characteristics of a meteoric system in this area. The trend in 13C signatures indicates that meteoric water acquired its low carbon values at a soil zone and was buffered completely with the host rock before entering the meteoric lens. Identical 18O data suggest that allochems stabilized at the same temperatures in waters of similar 18O compositions during early meteoric diagenesis; that is, calcitization continued to completion before the onset of burial. Sr concentrations reveal differential transformation mechanisms. Low Sr values in the phreatic zones are attributed to slow stabilization by exchange with the bulk solution. High Sr compositions in the vadose zone resulted from rapid, incremental transformation.Meteoric cementation was sparse; therefore, grainstones entered the burial realm with all of their porosity intact. Modification of porosity was a burial process and was directly related to the intensity of chemical compaction, which itself was controlled by three factors: (1) the degree of early cementation, (2) grain type, and (3) grain size. Early-cemented intervals experienced low to moderate compaction regardless of grain size or grain type. In the absence of early cementation, grain type and grain size were the dominant factors of porosity control. Coarse-grained intervals that were composed of micritic grains experienced less compaction than fine-grained, oolitic intervals.Average volumetric percentages of cements as determined by point counting of 63 samples are as follows: (1) marine and meteoric cements = 1.5%, (2) prebitumen calcite = 6.5%, (3) saddle dolomite = 1%, (4) postbitumen calcite = 4.3%, (5) pyrobitumen = 1%. Total cement (sum of 1–5), total burial cement (sum of 2–5), and total burial carbonate cement volumes (sum of 2–4) average 14, 13, and 12%, respectively. Out of an original porosity of 40%, 13 porosity units were lost by cementation, and 27 porosity units were reduced by compaction (mechanical and chemical).Present-day intergranular porosity of these grainstones is zero, whether they belonged to the vadose zone or the phreatic interval. Burial cementation and compaction destroyed all intergranular porosity, regardless of early diagenetic overprint or textural variables. The reservoir became compartmentalized during burial. Some intervals were active producers of calcium carbonate; others were passive recipients.

Diagenesis at exposure surfaces in a transgressive systems tract in a third-order sequence (Lower Carboniferous, Belgium)

Viséan limestones in Belgium show evidence for marine early diagenesis followed by meteoric and then deeper burialdiagenesis. The fourth-order sequence boundaries have no influence on the diagenetic pattern of the limestones. The third-order, Type 1 sequence boundary exerts a distinct influence on the diagenetic pattern of the carbonates due to the development of a meteoric lens that persisted even after sedimentation resumed.

Questioning carbonate diagenetic paradigms: evidence from the Neogene of the Bahamas

Carbonate diagenetic models have been heavily influenced by numerous studies of exposed Quaternary limestones. As a result, meteoric diagenesis is often assumed to be the principle means of altering aragonite-rich sediments into calcitic limestones. However, these models are limited by the scarcity of examples of aragonite-rich sediments buried in seawater that have never been influenced by meteoric fluids. The Bahamas transect cores recovered originally aragonite-rich sediments deposited in deep water beyond the easy reach of meteoric waters and provide an opportunity to test current diagenetic paradigms. The Bahamas transect consists of seven cores drilled in the prograding western margin of Great Bahama Bank. The two proximal cores (Clino and Unda) were drilled on the platform top and recovered shallow-water platform to reef facies overlying deeper margin and proximal slope facies. The five distal cores were drilled by ODP Leg 166 in up to 660 m of water and recovered carbonate slope facies. All studied sections are Neogene to Pleistocene in age. Diagenetic environments were identified based on petrographic and scanning electron microscopy (SEM) observations, XRD mineralogy, carbon and oxygen stable isotopic data, and trace elements. The upper 100–150 m of the two proximal cores were altered in meteoric to mixing-zone diagenetic environments but all other intervals were altered exclusively in marine pore fluids during seafloor, marine-burial, and deep-burial diagenesis. Several of the findings of this study question current carbonate diagenetic paradigms. These include: (1) large-scale sea level lowstands may not have chemically active meteoric lenses as we found no meteoric alteration at the −120 m elevation of the latest Pleistocene lowstand. Rather, phreatic meteoric diagenesis appears restricted to within ≈10 m of the land surface. (2) Mixing-zone diagenesis includes aragonite dissolution and minor LMC cementation but does not show the cavernous porosity or dolomitization predicted by mixing-zone diagenetic models. Current models are based on coastal mixing zones, which do not appear to be applicable to these more inland, and perhaps more typical, locations. (3) Marine-burial diagenesis produces a mature limestone with fabrics formerly considered diagnostic for meteoric diagenesis such as moldic porosity, aragonite neomorphism, blocky calcite spar and calcite microspar. However, oxygen stable isotopic data (average δ 18O=+1‰) indicate alteration in marine pore fluids only. The character of marine-burial diagenesis is partially controlled by the nature of the sediment being altered. We have identified two end-member styles, an open-system style characterized by dissolution of aragonite without significant cementation and a more closed-system style with aragonite dissolution accompanied by calcite cementation. The sediments examined were deposited well above the aragonite compensation depth, so seawater entering the sediment is saturated with respect to aragonite. The under-saturation needed to drive diagenesis is likely the result of bacterial oxidation of organic matter using sulfate. (4) Microspar forms in these sediments as a cement based on petrographic and SEM examination of partly to completely altered samples. This contradicts the common assumption that microspar forms by aggrading neomorphism of micrite. (5) Strontium content of sediments altered in marine pore fluids can show an extreme range of values, formerly thought to indicate different environments. The opportunity to finally examine the diagenesis of aragonite-rich sediments buried in seawater challenges current diagenetic paradigms and emphasizes the importance of integrated studies.

Limitations on lowstand meteoric diagenesis in the Pliocene-Pleistocene of Florida and Great Bahama Bank: Implications for eustatic sea-level models

Data from three Bahamian and South Floridan cores with deeper water facies show that, contrary to current models, there is no phreatic meteoric diagenesis that can be attributed to large-scale sea-level lowstands. Models of meteoric diagenesis in carbonate sediments commonly assume major phreatic alteration during glacio-eustatic sea-level lowstands. However, the diagenesis previously attributed to large-scale (>80 m) lowstands is found in shallow-water facies that have been repetitively exposed. In order to document diagenesis from large-scale lowstands, an interval is needed that was deposited in deeper water and not immediately exposed to meteoric fluids. Three cores with such deeper-water facies indicate that the majority of phreatic meteoric diagenesis in Florida and the Bahamas happened during glacio-eustatic highstands, not during lowstands. The data also suggest that diagenetically active meteoric lenses in Florida and the Bahamas are restricted to within 60 m, and perhaps less, of the land surface. The most likely reasons deeper lenses are chemically inactive are (1) the greater percolation distance allows the water to reach saturation prior to entering the lens, and (2) the large distance exceeds the reach of soil-derived organic matter, known to drive diagenesis in meteoric lenses. Models that currently assume a constant rate for alteration in meteoric fluids need to accommodate this variation with thickness of the vadose zone. This study also questions the use of paleophreatic lenses as records of eustatic sea level, as the large-scale falls may not leave any record.