Precipitation Of low-Mg Calcite Research Articles

Glacial-interglacial lake hydrochemistry in step with the Pleistocene Indian summer monsoon at the southeastern Tibetan Plateau

The lake state and chemistry are sensitive to climate change at short timescales, but it is unclear how lake hydrochemistry responds to monsoonal climate, especially over glacial-interglacial cycles, owing to lack of well-dated and continuous sedimentary records. The types and geochemical compositions of authigenic carbonates from lacustrine sediments can be used to reconstruct past hydrochemistry. Yet, little is known about the significance and influencing factors of the Mg/Ca and Sr/Ca ratios of authigenic carbonates over glacial-interglacial cycles. Here, trace-element data of a 666 m long sediment sequence from the Heqing basin, a paleo-lake situated at the southeastern margin of the Tibetan Plateau, provide detailed glacial-interglacial hydrochemical conditions and their association with Indian summer monsoon (ISM) evolution during the entire Pleistocene. In interglacials, variation in both Mg/Ca and Sr/Ca ratios can serve as proxies for salinity as a result of precipitation of low-Mg calcite, constrained by enhanced ISM intensity and catchment weathering product. Instead, in glacials, high Mg/Ca and Sr/Ca ratios reflect high-Mg calcite and aragonite precipitation under low lake level conditions. There is a nonlinear response of the Sr/Ca ratio of lacustrine sediment to salinity variation when high-Mg calcite and aragonite precipitate in a hydrologically closed and evaporative lake. Based upon the sedimentary Sr/Ca and Mg/Ca ratios in the Heqing paleo-lake, the lake hydrochemistry during the Pleistocene experienced obvious glacial-interglacial ISM cycles at the life span of the lake itself: (1) in the old interval (2640–1840 ka), the salinity was low with cycles at the early stage of lake development owing to strong ISM; (2) in the middle interval (1840–920 ka), the lake was at a balance state and its salinity was more constrained by weathering solute, rather than ISM; and (3) in the young interval (920–130 ka), the salinity fluctuated with large amplitude over glacial-interglacial cycles and subsequent increase paced the weakened ISM. This study highlights the sensitivity of the types of lacustrine authigenic carbonates and lake chemistry to ISM changes over glacial-interglacial cycles.

THE APPLICATION OF SEM-EDS FOR THE SURFACE TEXTURE OF FORAMINIFERA: IDENTIFICATION OF EARLY DIAGENESIS OF REWORKED L. DIMIDIATUS WITHIN MODERN COASTAL DEPOSITS

Sediment recycling has been known to occur within Quaternary coastal barriers of the greater Coorong Coastal Plain, southern Australia. A high degree of reworking of skeletal carbonates from Late Pleistocene deposits (Robe Range) is evident based on the novel application of amino acid dating of the single-foraminifera species Lamellodiscorbis dimidiatus. More importantly, some apparently transparent, well-preserved tests indicate anomalously high extents of amino acid racemization, implying that reworked fossils could not be easily distinguished based on taphonomic signatures such as corrosion. Here, we examine the surface microtexture of this species, constrained with well-preserved specimens, on a modern beach of Canunda, southern Australia, using scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS). The aim is to identify surface features of foraminifera in more detail, capturing signs of early diagenesis associated with weakly consolidated Late Pleistocene coastal barriers. The results reveal that some well-preserved tests show localized blocky calcite cementation, most notably within intraseptal spaces or impact sites. The EDS spectra of cement indicate lower Mg content than unaltered foraminifera surface. This suggests low-Mg calcite precipitation due to meteoric diagenesis experienced by the onshore Late Pleistocene coastal barrier. It implies that these foraminifera shells are reworked fossils originated from older successions and were subsequently redeposited within the present-day beach. SEM-EDS used in this study demonstrates its capability in examining small-scale carbonate diagenesis products beyond the traditional binocular microscope. Thus, this tool is recommended to aid amino acid dating in detecting reworked fossils.

Mg isotopic systematics during early diagenetic aragonite-calcite transition: Insights from the Key Largo Limestone

The seawater magnesium (Mg) isotopic composition (δ26MgSW) can be applied to quantify marine Mg cycle in Earth's history. However, reconstruction of δ26MgSW by using marine carbonate has been challenged by the large range of variation in Mg isotopic composition of carbonate (δ26Mgcarb). Such variation, on the one hand, reflects, e.g. the mineralogy, temperature, and precipitation rate dependent isotopic fractionation during Ca‑carbonate precipitation. On the other hand, the pristine seawater record could be modified in carbonate diagenesis, during which less stable carbonate minerals would spontaneously convert to more stable mineral phases. In the modern ocean with high Mg/Ca (molar) ratio (~5), aragonite precipitation is favored. However, aragonite is thermodynamically less stable than low-Mg calcite at ambient surface environment, resulting in the spontaneous transition from aragonite to low-Mg calcite in the earliest stage of diagenesis (i.e. the aragonite-calcite transition). As a common process in the modern ocean, Mg isotopic fractionation in aragonite-calcite transition may provide valuable constraints on the variation of δ26Mgcarb. In this study, we measured Mg isotopic compositions of Pleistocene Key Largo Limestone (Florida), which has been experiencing the aragonite-calcite transition during early diagenesis. The Key Largo Limestone samples consist of a mixture of calcite and aragonite. δ26Mg of aragonite (δ26Mgarag) shows higher and more stable values, while calcite has lower yet more variable Mg isotopes (δ26Mgcal). We suggest a model conceptualizing a water-film attached to the mineral surface, in which the aragonite-calcite transition initiates with aragonite dissolution and the Mg exchange with external fluid can be variable, followed by Mg isotopes fractionate during low-Mg calcite precipitation. In this model, δ26Mgcal is controlled by (1) the chemical composition of water-film, which is dependent on the mineralogical composition of aragonite (i.e. Mg content and δ26Mgarag), the composition of external fluid (i.e. fresh water, seawater, or mixed fresh water and seawater), and the mixing ratio between dissolved aragonite and external fluid, and (2) the isotopic fractionation during calcite precipitation. Because the isotopic fractionation in calcite is sensitive to the rate of calcite precipitation that is dependent on the calcite saturation state of solution. Therefore, δ26Mgcal could be highly heterogeneous depending on the aragonite dissolution and calcite precipitation processes, further complicating the interpretation of carbonate data. Thus, the process-controlled Mg isotopic fractionation should be taken into consideration when reconstructing δ26MgSW by using carbonate sediments/rocks.

Microterracettes in Sabkha Oum Dba (Western Sahara, Morocco): Physical and Biological Interactions in the Formation of a Surface Micromorphology.

Small-scale terracing (microterracettes) is a surface geomorphic feature that recurs under a range of environmental settings, such as those existing in high to low temperature geothermal springs and evaporitic environments, through the single or combined action of physicochemical agents and microbiological processes. Such morphology can also be observed in a confined sector of the Sabkha Oum Dba, which is an inland sabkha of the Western Sahara (Morocco), where field and laboratory investigations revealed that they primarily depend on the accumulation of naviculoid diatoms. Through their biofilm production ability, these benthic diatoms are able to stabilize surface morphologies and make organic alveolar frameworks where the precipitation of low Mg calcite occurs in areas subjected to active oxygenic photosynthesis. Because microterracettes arise in a specific set of environmental conditions, they have environmental significance and, thanks to a high fossilization potential due to mineral precipitation, they can be an effective source of biomorphological and chemical evidence for life. The relationship with aqueous environments, considered to be widespread on Mars especially during a period of intense hydrologic activity as in the late Noachian and Hesperian periods, make the understanding of surficial processes useful (such as the formation of microterracettes) whose formation is frequent in terrestrial analogues for martian environments, such as ephemeral saline continental lakes (sabkhas) and related to the products of bacterial and eukaryotic life, as in the case of biofilms, in search for similar life forms beyond Earth.

Fractionation of Mg isotopes by clay formation and calcite precipitation in groundwater with long residence times in a sandstone aquifer, Ordos Basin, China

Compared with the numerous studies on river and soil waters, studies on Mg isotopes of groundwater are limited. In this study, a sandstone aquifer in the Ordos Basin, China with contrast contents of Mg in shallow and deep groundwater is selected to examine the behavior of Mg isotopes during groundwater circulation. The δ26Mg values of shallow groundwater are within the range of widely reported results of groundwater, while those of deep groundwater are found to be as light as −3.30‰ to −2.13‰. Assuming that shallow groundwater is an endmember, 87Sr/86Sr ratios show that calcite dissolution has contribution to low δ26Mg of deep groundwater, but mixing alone cannot explain the coupled low δ26Mg and low Mg contents. The removal of Mg in deep groundwater is found to be mainly caused by incorporating into neoformed clay minerals, which further lowers δ26Mg. For the deep groundwater samples denoted as G1 and G3, the relationship between δ26Mg and 1/Mg has been quantitatively explained by the superposition of calcite dissolution and clay formation with a fractionation factor (αclay–water) of 1.0003. For samples denoted as G2, in addition to calcite dissolution and clay formation, high proportion of Mg in the residual solution are further removed via precipitation of low-Mg calcite, which leads to increased δ26Mg. There are increasingly stronger degrees of clay formation in G3, G1, and G2 due to the increasingly longer travel distances and travel times of groundwater from recharge to discharge areas. This study enhances understanding on the factors controlling Mg isotopes of groundwater, as well as the geochemical processes of subsurface water-rock interactions in sandstone aquifers.

Hydrothermal carbonate chimneys from a continental rift (Afar Rift): Mineralogy, geochemistry, and mode of formation

Carbonate chimney-like deposits up to 60m high are scattered or arranged in rows at the shores of a desiccating hypersaline and alkaline lake from a continental rift setting (Lake Abhé, Afar Rift, Djibouti). The chimneys formed sub-aqueously in the lake water body at a higher water level than observed today. Alternating calcite and low-Mg calcite+silica concentric layers compose the chimney structures. Mineralogical and geochemical investigations of the chimneys, lake water, and hot spring (hydrothermal) fluids suggest that the chimneys are a result of rapid carbonate precipitation during the mixing of hydrothermal fluids with lake water. In contrast to the hot spring fluid, lake water is enriched in HREE and possesses a pronounced positive Ce anomaly, features that are preserved in the carbonate chimney layers. Mixing calculations based on Sr-isotope and concentration data indicate a hydrothermal fluid contribution of ~45% in the chimney interior, which decreases to ~4% in the external chimney layer. Sr in the hydrothermal fluids is predominantly leached from the underlying volcanic rocks, whereas the lake's Sr budget is dominated by riverine input. Considering the fluid mixing ratios calculated by Sr-data, the measured C and O isotope compositions indicate that chimney carbonates precipitated at temperatures between 14°C (internal part) and 22°C (external part) with δ13C-carbonate mainly controlled by isotope equilibrium exchange of lake water with atmospheric CO2. The low-Mg calcite layers, including the outermost layer, have enhanced signals of lake water inheritance based on elevated concentrations of immobile elements, ΣREE, and Sr and Ca isotope compositions. Ca-isotope data reveal that internal chimney layers formed by non-equilibrium calcite precipitation with a predominantly hydrothermal Ca source. The external low-Mg calcite layer received Ca contributions from both hydrothermal fluid and lake water, with the latter being the dominant Ca source. Highly positive δ44/40Ca of lake water likely reflects non-equilibrium Ca-carbonate precipitation during lake water evaporation with resulting 44Ca enrichment of residual lake water. The strong degree of 44Ca enrichment may point towards multiple lake drying and Ca-reservoir depletion events. Coupled C–O–Ca-isotope data of the sampled carbonate chimney suggest late-stage (low-temperature) hydrothermal carbonate chimney formation during strongly evaporative lake conditions at the time of low-Mg calcite precipitation. U–Th age dating suggests that the chimneys formed no earlier than 0.82kyrBP (0.28±0.54).

Giant calcite concretions in aeolian dune sandstones; sedimentological and architectural controls on diagenetic heterogeneity, mid-Cretaceous Iberian Desert System, Spain

Aeolian dune sandstones of the Iberian erg system (Cretaceous, Spain) host giant calcite concretions that constitute heterogeneities of diagenetic origin within a potential aeolian reservoir. The giant calcite concretions developed in large-scale aeolian dune foresets, at the transition between aeolian dune toeset and damp interdune elements, and in medium-scale superimposed aeolian dune sets. The chemical composition of the giant concretions is very homogeneous. They formed during early burial by low Mg-calcite precipitation from meteoric pore waters. Carbonate components with yellow/orange luminescence form the nuclei of the poikilotopic calcite cement. These cements postdate earlier diagenetic features, characterized by early mechanical compaction, Fe-oxide cements and clay rims around windblown quartz grains resulting from the redistribution of aeolian dust over the grain surfaces. The intergranular volume (IGV) in friable aeolian sandstone ranges from 7.3 to 15.3%, whereas in cemented aeolian sandstone it is 18.6 to 25.3%. The giant-calcite concretions developed during early diagenesis under the influence of meteoric waters associated with the groundwater flow of the desert basin, although local (e.g. activity of fluid flow through extensional faults) and/or other regional controls (e.g. variations of the phreatic level associated with a variable water influx to the erg system and varying sea level) could have favoured the local development of giant-calcite concretions. The spatial distribution pattern of carbonate grains and the main bounding surfaces determined the spatial distribution of the concretions. In particular, the geometry of the giant calcite concretions is closely associated with main bounding aeolian surfaces. Thus, interdune, superimposition and reactivation surfaces exerted a control on the concretion geometries ranging from flat and tabular ones (e.g. bounded by interdunes) to wedge-shaped concretions at the dune foresets (e.g. bounded by superimposition and reactivation surfaces) determining the spatial distribution of the heterogeneities of diagenetic origin in the aeolian reservoir.

Discriminating the Role of Photosynthetic and Heterotrophic Microbes Triggering Low-Mg Calcite Precipitation in Freshwater Biofilms (Lake Geneva, Switzerland)

Field and laboratory experiments in freshwater Lake Geneva (Switzerland) have recently demonstrated a substantial role of biofilms triggering the formation of ooids. However, the precise function and specificity of the microbes involved in this process remained elusive. In this study we compared biofilms lining ooid depressions in the natural environment with those grown under laboratory conditions in a BG11 media. Confocal microscopy, SEM and TEM as well as PCR-DGGE results have been used to accomplish this comparison at both microscopical and geochemical levels, respectively. Carbonate precipitates in the natural environment show that cyanobacteria dominate the microbial community in association with green algae and diatoms all embedded within the EPS. Analogously, low-Mg calcite crystals were formed in the laboratory experiments under light conditions. These crystals are found within the EPS and always associated with photosynthetic microorganisms. The latter in concert with the lack of evidence for the presence of sulfate-reducers, allow us to postulate that photosynthetic activity through increasing pH in EPS is the main factor triggering low-Mg calcite precipitation in Lake Geneva.

Magnesium-isotope fractionation during low-Mg calcite precipitation in a limestone cave – Field study and experiments

The chemical and isotopic composition of speleothem calcite and particularly that of stalagmites and flowstones is increasingly exploited as an archive of past environmental change in continental settings. Despite intensive research, including modelling and novel approaches, speleothem data remain difficult to interpret. A possible way foreword is to apply a multi-proxy approach including non-conventional isotope systems. For the first time, we here present a complete analytical dataset of magnesium isotopes (δ 26Mg) from a monitored cave in NW Germany (Bunker Cave). The data set includes δ 26Mg values of loess-derived soil above the cave (−1.0 ± 0.5‰), soil water (−1.2 ± 0.5‰), the carbonate hostrock (−3.8 ± 0.5‰), dripwater in the cave (−1.8 ± 0.2‰), speleothem low-Mg calcite (stalactites, stalagmites; −4.3 ± 0.6‰), cave loam (−0.6 ± 0.1‰) and runoff water (−1.8 ± 0.1‰) in the cave, respectively. Magnesium-isotope fractionation processes during weathering and interaction between soil cover, hostrock and solute-bearing soil water are non-trivial and depend on a number of variables including solution residence times, dissolution rates, adsorption effects and potential neo-formation of solids in the regolith and the carbonate aquifer. Apparent Mg-isotope fractionation between dripwater and speleothem low-Mg calcite is about 1000ln α Mg-cc-Mg(aq) = −2.4‰. A similar Mg-isotope fractionation (1000ln α Mg-cc-Mg(aq) ≈ −2.1‰) is obtained by abiogenic precipitation experiments carried out at aqueous Mg/Ca ratios and temperatures close to cave conditions. Accordingly, 26Mg discrimination during low-Mg calcite formation in caves is highly related to inorganic fractionation effects, which may comprise dehydration of Mg 2+ prior to incorporation into calcite, surface entrapment of light isotopes and reaction kinetics. Relevance of kinetics is supported by a significant negative correlation of Mg-isotope fractionation with the precipitation rate for inorganic precipitation experiments.

Seawater chemistry, coccolithophore population growth, and the origin of Cretaceous chalk

The magnesium/calcium ratio (Mg/Ca) and calcium (Ca) con- centration of seawater have oscillated throughout geologic time; our experiments indicate that these variables have strongly influ- enced biomineralization and chalk production by coccolithophores. The high Mg/Ca ratio of modern seawater favors precipitation of high-Mg calcite and/or aragonite. In contrast, the low Mg/Ca ratio of imputed Cretaceous seawater favored precipitation of low-Mg calcite. We have discovered that some coccolithophore species to- day secrete skeletal elements of high-Mg calcite, rather than low- Mg calcite, as conventionally believed. These species incorporated less Mg when the ambient Mg/Ca ratio was lowered, secreting low- Mg calcite in imputed Cretaceous seawater. Calcification stimu- lates coccolithophore population growth by contributing CO2 to photosynthesis. Three extant coccolithophore species multiplied much faster as the composition of ambient seawater was shifted toward that estimated for Cretaceous seas. Two of these species secreted high-Mg calcite in ambient seawater having Mg/Ca . 1, and incorporation of Mg in a calcite crystal inhibits growth. Cal- cification of the third species, which secreted low-Mg calcite at all ambient Mg/Ca ratios, is hindered by the high Mg/Ca ratio and low absolute concentration of Ca of modern seawater. We conclude that the ionic composition of Cretaceous seawater enabled cocco- lithophores to produce massive chalk deposits, and conversely, that the ionic composition of modern seawater inhibits population growth for most extant coccolithophore species, which occupy nu- trient-poor waters and fail to respond to fertilization by nitrate, phosphate, or iron.

Paleoenvironmental significance of Messinian post-evaporitic lacustrine carbonates in the northern Apennines, Italy

The post-evaporitic Messinian in the northern sector of the Adriatic foredeep is characterized by the occurrence of abundant terrigenous deposits which have been subdivided into two allostratigraphic units (p-ev 1, p-ev 2) separated by an erosional unconformity of regional importance. The p-ev 1 consists of basal complex of resedimented evaporites overlain by monotonous succession of mudstone/siltstone alternations (thin bedded turbidites) with an overall coarsening and shallowing upwards trend. It is possibly related to the rapid basin infill after the end of the salinity crisis. The upper unit (p-ev 2) shows, in the most marginal areas, a clear cyclical stacking pattern made up of three fining-up sequences with fan-delta conglomerates at the base of each cycle. Here, a peculiar feature of the p-ev 2 deposits is the occurrence of three distinct laminated limestone horizons (termed colombacci) interbedded with fine grained, varved mudstones. These limestones are abiotic, despite the occurrence of abundant Ostracod assemblages (Lago–Mare fauna) in the interbedding pelites, and are interpreted as inorganically precipitated, deposited in a relatively deep-water environment, possibly anoxic and related to permanent stratification of water mass events during periods of lake level maxima. The combination of petrographical and geochemical observations on carbonate layers reveals that they correspond to events characterized by significant variations of environmental conditions. However, δ 18O (−5.30/+0.2) and δ 13C (−9.28/+1.68) signatures suggest that the precipitation of low-Mg calcite often occurred in a non-marine environment and that meteoric water input had a considerable influence on late Messinian sedimentation. Moreover, the limestones have a uniform 87Sr/ 86Sr composition (0.70864–0.70875) in the range of other Lago–Mare sediments from elsewhere in the Mediterranean, suggesting that some of the major Mediterranean basins were likely interconnected during the Lago–Mare phase. A strongly reduced rate of hydrological exchange with the ocean is also possible, but the possibility of marine influence during this stage is not completely excluded.

Quaternary dune carbonates from the Mediterranean Coast of Egypt: Petrography and diagenesis

The Quaternary carbonates of the Mediterranean coast of Egypt between Alexandria and Salum appear as parallel limestone ridges rising up to 100 m above sea level. These ridges are dominated by dunal carbonates which differ not only in their primary composition but also by distinct grades of meteoric water diagenesis. Oolitic facies dominates the younger aeolianites of the first and second ridges. Bioclastic facies with abundant coralline algae, benthonic foraminifers, molluscs, echinoderms and intraclasts represents the major rock type in the older aeolianites. Features of meteoric water diagenesis include precipitation of increasing amounts of avoid-filling low Mg-calcite spar, dissolution of aragonite and stabilization of aragonite and high Mg-calcite to low Mg-calcite. Aeolianites below paleosol horizons contain abundant calcrete cements, micritized fossils and detrital grains which are commonly corroded and replaced by calcite. Three stages of progressive meteoric diagenesis are recognised in the Egyptian Quaternary aeolianites. In stage 1 minor precipitation of low Mg-calcite occurs at the grain boundaries. Stage 2 is marked by partial dissolution of aragonite, partial loss of high Mg-calcite and precipitation of low Mg-calcite in some pore spaces. In stage 3, most of the remaining pores are occluded by cementation. At the end of stage 3, Mg is removed from high Mg-calcite but some aragonite still persists. The early vadose cements are represented by miniscus, bridge and pendant cements. The phreatic cements were precipitated as bladed spar in the isopachous rims and equant spar in the intergranular and mouldic porosity. The late vadose cements are represented by micritic cements that were related to calcrete formation. Elemental behaviour during meteoric water diagenesis indicates that it leads to a gradual decrease in bulk Sr along with Na in progressively altered aeolianites. Mn distribution is controlled by the carbonate mineralogy (aragonite versus calcite) as well as the proximity of the aeolianites to the overlying paleosol horizons.

Diagenetic processes in Holocene carbonate sediments: Florida Bay mudbanks and islands

ABSTRACTThe sedimentology, mineralogy and pore fluid chemistry of seven cores from the Holocene sediments of Florida Bay were studied to determine the physical processes and diagenetic reactions affecting the sediments. The cores were taken in a transect from a shallow mudbank onto a small adjacent island, Jimmy Key. Steady state models of pore fluid chemistry are used to estimate the rates of various reactions.In the mudbank sediments, little carbonate mineral diagenesis is taking place. No change in sediment mineralogy is detectable and pore water profiles of Ca2+, Mg2+ and Sr2+ show only minor variation. Chloride concentrations indicate substantial biological mixing of seawater from the bay into the sediments in one of the cores. Pore water analyses of sulphate and alkalinity show only a low degree of sulphate depletion and a decreasing extent of sulphate reduction downcore. Models of sulphate reduction in the mudbank show that there is substantial chemical exchange between the sediment pore fluids and water from the bay probably as a result of bio‐irrigation. The sulphate and alkalinity data also suggest that the underlying Pleistocene rocks contain water of near normal seawater composition.Stratigraphic analysis and δ13C analyses of the organic carbon in the sediments of the island cores show that the sediments were primarily deposited in a subtidal mudbank setting; only the upper 20–30 cm is supratidal in origin. Nevertheless, island formation had a significant effect on pore fluid chemistry and the types of diagenetic reactions throughout the sediment column. Chloride in the sediment pore fluids is more than twice the normal seawater concentrations over most of the depth of the cores. The constant, elevated chloride concentrations indicate that hypersaline fluids which formed in ponds on the island are advected downward through the sediments. Models of the chloride profiles yield an estimate of 2·5 cm yr−1 as a minimum advective velocity. Changes in pore water chemistry with depth are interpreted as indicating the following sequence of reactions: (1) minor high‐Mg calcite dissolution and low‐Mg calcite precipitation, from 0 to 35 cm; (2) Ca‐ or Mg‐sulphate dissolution and low‐Mg calcite precipitation, from 5 to 35 cm; (3) dolomite or magnesite precipitation together with sulphate reduction, from 35 to 55 cm; and (4) little reaction below 55 cm. In addition, one or more as yet unidentified reactions must be taking place from 5 to 55 cm depth as an imbalance in possible sources and sinks of alkalinity is observed. The imbalance could be explained if chloride is not completely conservative. Despite the pore fluid chemical evidence for diagenetic reactions involving carbonate minerals, no changes in sediment mineralogy were detected in X‐ray diffraction analyses, probably because of the comparatively young age of the island.

The Concentration of Uranium in Freshwater Vadose and Phreatic Cements in a Holocene Ooid Cay: A Method of Identifying Ancient Water Tables

ABSTRACT The concentration of uranium has been measured using the fission track technique in vadose and freshwater phreatic cements from South Joulters Cay, in The Bahamas. These analyses reveal dramatic differences in the U concentration of vadose (< 0.28 ± 0.11 ppm) and freshwater phreatic (1.80 ± .75 ppm) cements. This distinctive change in concentration is proposed to result from 1) variations in the CO2 of the waters of the phreatic and vadose environments, and 2) selective exclusion of U during the precipitation of calcite. The pCO2 is important because it controls the pH and hence the concentration of CO32- ions in the pore fluids as well as the amount of dissolution of the U rich aragonitic precursors. For example, an increas in pCO2 will lower the pH and decrease the concentration of CO32- present in the porewaters. As the U, present as the uranyl ion (UO22+), is mainly complexed with CO32-, a decrease in the concentration of CO32- will not only alter the speciation of U but also increase the UO2(CO3)22-/ CO32- ratio of the fluid and hence the concentration of U in the precipitated carbonate cements. The second mechanism for explaining the increase of U in phreatic cements relies on the low distribution coefficient for the incorporation of U into low-Mg calcite (LMC). The precipitation of LMC therefore leads to an increase in the concentration of U in the residual solution. As fluids do not remain in the vadose zone, but rather accumulate in the freshwater phreatic portion of the lens, phreatic waters become relatively enriched in U compared to vadose fluids. The clear difference in the U concentrations of cements in these two zones may enable the U concentration of cements to be used as an indicator of paleo-water tables in ancient rocks even where other geochemical and petrographic evidences are equivocal.

Calcrete and near-surface geology of Kuwait City and suburbs, Kuwait, Arabian Gulf

Near-surface accumulation of pedogenic carbonate (calcrete) is a common and characteristic feature of the landscape of Kuwait City and its suburbs. The principal host material for these calcretes is a thick sequence of unconsolidated quartzose sand. A typical fully developed soil profile can be divided into five zones: in descending order these are (1) a top calcareous aeolian soil; (2) a zone of friable sand with powdery or tightly cemented calcrete lumps (nodules); (3) a massive calcrete zone; (4) a mottled calcrete zone; and (5) unaltered parent material. Episodic formation of calcrete is indicated by the occurrence of multiple profiles in which zones are repeated. Calcrete ooids and pseudo-ooids of accretionary origin are characteristic of the upper calcrete zone. The carbonate fraction in the upper calcrete zone consists dominantly of low-Mg calcite with minor amounts of dolomite. Replacement was a major process in formation of the massive calcrete and the dominant cementing material is largely low-Mg calcite and/or microcrystalline dolomite. Aeolian dust is considered to be the major source of ions for calcrete formation in Kuwait City and suburbs. An earlier-introduced model* is applied to explain the mechanisms of formation and precipitation of low-Mg calcite and dolomite. The calcrete thickness in the study area is explained by episodic deposition of fluviatile sand and calcrete formation during the Oligocene through Lower Miocene period.

Rapid burial diagenesis of deep-water carbonates: Exuma Sound, Bahamas

Well-lithified, deep-water, periplatform limestones of late Miocene age occur at subsurface depths from 113 m to total depth of 283 m at Ocean Drilling Program (ODP) Site 632 in Exuma Sound, Bahamas. This occurrence, marked by a dramatic increase in compressional wave velocities up to 5.0 km/s, represents the most rapid transformation of ooze to limestone yet discovered in either periplatform or open-ocean settings. The burial diagenetic pathway is similar to those for meteoric and deep-burial diagenetic environments: aragonite dissolves, magnesian calcites exsolve or dissolve, low-Mg calcite precipitates as cement, and secondary moldic/vuggy porosity develops. Stable isotope ratios suggest precipitation of low-Mg calcite from cold, marine-derived pore waters. Accelerated diagenesis is a response to the influx of large quantities (up to 80%) of bank-derived, metastable carbonates and rapid burial via high accumulation rates (up to 210 m/m.y.). Our findings (1) document carbonate mineral stabilization and lithification in a seawater-mediated, shallow-burial diagenetic environment; (2) highlight the very high diagenetic potential of periplatform carbonates; and (3) call into question the use of seismic velocities as support for the Bahama megabank hypothesis.

East African Rift stromatolites

Summary The East African Rift System contains many lacustrine basins in which biologically precipitated stromatolite carbonates are found. The development of these carbonates results from low-Mg calcite precipitation by cyanophytes and bacteria; the binding and trapping of sediments are not involved. A general outline for the distribution of stromatolites has been established by analysing data from different sites in East Africa. In the fluviatile environment, travertines are associated with a biotype of flowing waters with high calcium content. During exceptional rainy seasons, inundation of floodplains promotes seasonal growth of oncolites. Chimneys and pool-rim dams associated with hydrothermal springs may be sub-lacustrine, or may occur along lake shores. In the lacustrine environment, stromatolites form shorelines that indicate former high stands. The study of Quaternary stromatolites and their modern equivalents reveals their exceptional potential for reconstructing sedimentary palaeoenvironments in rifts. Stromatolites represent a particular continental carbonate facies, somewhat analogous to marine reefs. Furthermore they constitute an important source of other geological data: material for isotopic dating; determining altitudes of former lake levels; and providing evidence for hydrological and physico-chemical conditions in palaeolakes. All these may be combined to assist palaeoclimatic reconstruction.

Geochemical differences between tropical (Ordovician) and subpolar (Permian) carbonates, Tasmania, Australia

The Ordovician Gordon Limestone in Tasmania is a low-paleolatitude (10°N) peritidal carbonate with a tropical chlorozoan assemblage (calcareous green algae and corals), diverse nonskeletal grains, extensive early diagenetic dolomites, and some evaporites. It is characterized by small Mn, moderate Na, and large Sr concentrations as in Holocene tropical aragonitic carbonates. The range of Sr/Na (2.6 to 4.6) in the Gordon Limestone is related to aragonitic original mineralogy, dolomitization, and evaporite formation and to channel, prograding tidal flat, and stratigraphic reef environments. The Upper Permian Berriedale Limestone is a sequence of inter-bedded limestones and calcareous shales deposited at a paleolatitude near 80° S during the Gondwanan ice age. The shallow-marine limestone beds formed during the lowering of sea level in colder periods. As sea level rose in warmer periods, a shower of dropstones preceded shale deposition. The Berriedale Limestone contains a cold-water foramol fauna (bryozoans, brachiopods, and pelecypods), intraclasts, and abundant orthochems. Low Mg, equal amounts of Sr and Na (Sr/Na = 1.0), and large Mn concentrations in this limestone indicate original low-Mg calcite precipitation from sea water at a temperature