Carbonate precipitation in southern high-latitude environments remains poorly constrained in terms of its controlling mechanisms, particularly in continental settings where climatic extremes, volcanic substrates, and inherited glacial processes converge. Here we investigate carbonate formation in an alkaline lake hosted within Middle Pleistocene maars of the Pali Aike Volcanic Field, southern Patagonia. This setting provides a natural laboratory to investigate interactions between silicate weathering, carbon cycling, and CaCO₃ supersaturation. Mineralogical and geochemical data, including 87Sr/86Sr, δ13C, and δ18O, from carbonates, basalts, soils, sediments, and lake waters were used to constrain the processes controlling carbonate precipitation. The δ13C composition of dissolved inorganic carbon (DIC) indicates that silicate weathering involving soil-derived and atmospheric CO₂ is the primary source of carbon to the lake system. Carbonate speciation reveals a strong dominance of HCO₃− and CO₃2− over dissolved CO₂ in the lake carbon pool. Strontium isotopic compositions identify mafic rock weathering as the principal source of dissolved Ca. Combined evidence from Sr partitioning, weathering indices, aqueous geochemistry, and stable isotopes indicates carbonate precipitation under sustained supersaturation, modulated by kinetic constraints rather than simple equilibrium conditions. These results define an integrated geochemical framework in which silicate weathering intensity, carbon cycle dynamics, and precipitation kinetics jointly regulate carbonate formation, with implications for interpreting lacustrine carbonate archives in high-latitude continental systems.

Complex interactions are known to govern calcium carbonate precipitation in alkaline lake water conditions. Whilst comprising a small fraction of Earth's water volume, they exhibit unique sedimentological and geochemical characteristics that may be crucial for understanding palaeoclimate and planetary surface evolution. Here, I investigate the geochemical dynamics of CaCO3 supersaturation in alkaline lakes and the possible roles of Mg/Ca ratios and total dissolved phosphate (Sigma PO4) in modulating CaCO3 precipitation kinetics. Through a comprehensive analysis of global alkaline lake water data and thermodynamic modelling, it is suggested that 80-93 % of these lakes are supersaturated with respect to calcite, despite generally low calcium concentrations (defined in terms of saturation state or saturation index, expressed as Omega calcite >= 1 and SIcalcite >= 0, respectively). To account for the presence of Sigma PO4 in generally high ionic strength and alkaline waters, this assessment is underpinned by original contributions, comprising the incorporation of amorphous and metastable Ca-carbonate and Ca-phosphate phases in a geochemical thermodynamic database and an implementation of a Pitzer ion interaction activity coefficient model that explicitly considers the species H3PO4, H2PO4- , HPO42- and PO43- . These implementations enable a more robust characterisation of phosphate speciation and distribution at high ionic strengths and over the range of relevant pH values (mostly bracketed between 8 and 11 in these systems) in continental waters. This fundamental approach provides further insight into our current understanding of individual phosphate species' involvement in the inhibition of CaCO3 precipitation in natural alkaline waters, with H2PO4- and PO43- being the predominant species in the lower and higher pH values, respectively. However, individual phosphate species concentrations and distribution are also strongly dependent on the concentration of Ca2+ ion, as well as dependent on the solid phosphate phase setting the solubility threshold (likely either octacalcium phosphate, OCP, or amorphous calcium phosphate, ACP), as shown by simple model calculations. In line with observations from recent studies, total phosphate concentrations tend to increase with increasing alkalinity (ALKT) and dissolved inorganic carbon (DIC). Consequently, it may promote further CaCO3 supersaturation through its known inhibitory effect. High CaCO3 supersaturation lowers the necessary critical energy of nucleation, promoting the nucleation and growth of metastable polymorphs of CaCO3, such as monohydrocalcite (MHC) and amorphous calcium carbonate (ACC) and/or calcium-Mg carbonate (ACMC) phases. High supersaturation implies most actual calcite forms far from equilibrium in these lakes. Based on actual lake water chemistry data and theoretical considerations, likely MHC is the predominant precursor of calcite in alkaline lake waters, whilst ACC/ACMC phases are limited to very high calcite supersaturation conditions which may not be very commonly achieved. Calcite minerals forming via these pathways may not only deviate compositionally and in overall geochemical properties from those conditions expected at equilibrium, but are also likely to develop growth textures incorporating more defects and consisting of polycrystalline phases with non-coherent optical properties. The latter may be the case of familiar examples of unusual calcite crystal morphologies such as those now widely documented and studied in the Cretaceous continental carbonates of the South Atlantic sedimentary basins.

Alkaline carbonate-rich systems may, under some circumstances, result in significant accumulation of aqueous phosphorus. Additionally, the geochemistry of modern alkaline lakes, combined with current understanding of CaCO3 precipitation kinetics, indicates that the establishment of high alkalinity, high dissolved inorganic carbon (DIC), and CaCO3 supersaturation require compounds that inhibit or interfere with CaCO3 precipitation. Furthermore, the interplay between these factors may potentially affect fabric development in non-skeletal carbonates. In order to understand how these interlinked processes may have regulated PO4 concentrations and the processes and products of non-skeletal CaCO3 precipitation in the distant past, we conducted a detailed geological and geochemical study in two cores drilled in different portions of the Santos Basin, offshore southeastern Brazil, consisting of lacustrine carbonate rocks from the Lower Cretaceous Barra Velha Formation. We find that phosphorus in the sediments comes from two main products, phosphatic bioclasts and authigenic phosphate (of both primary and diagenetic origins). We also find that phosphorus occurs in concentrations of up to several thousand ppm in some intervals and is preferentially associated with some calcite textures potentially reflecting the primary sedimentary depositional conditions (e.g., upward-radiating aggregates of fibrous calcite, or "shrubs" and spherulitic aggregates of fibrous calcite, or "spherulites"). Microanalytical data from selected samples and high-Mg CaCO3 textures show median P values can be particularly high in crusts, shrubs and spherulitic calcite minerals interpreted here to have formed as primary to early diagenetic deposits, and roughly vary between 1000 and 2000 ppm (10.5-21.1 mmol/kg). The average P/(Ca + Mg) molar ratio values based on the same data for these three calcite textures are 1.45 +/- 0.98, 1.13 +/- 0.97 and 1.09 +/- 0.90 mmol/mol, respectively, being as high as 4.80 mmol/mol in specific spots in "shrubby" textures. Although multiple source-to-sink mechanisms were likely involved in the P budget in lake waters and in the sediments, such as recycling and remobilisation of skeletal- and organic-bound phosphate, high alkalinity may have enhanced its accumulation in the lake's waters. In light of current understanding of CaCO3 precipitation kinetics, we hypothesise that phosphate may have played a central role in maintaining high calcium carbonate saturation (14-45 < Omega(calcite) <= 100-1 80). These conditions would have favoured growth of calcite crystals on pre-existing substrates over nucleation of finely particulate carbonate sediment. By combining these observations with theoretical geochemical considerations, we propose that phosphate inhibition of calcium carbonate precipitation was enhanced by high alkalinity, further sustaining CaCO(3 )supersaturation and favouring distinctive carbonate growth textures. This unique and relatively well-preserved example of non-skeletal carbonate production implies that similar regulation of CaCO3 precipitation kinetics and PO4 concentrations may have operated across other intervals of time in Earth's history, both in non-marine and marine environments that pre-date the emergence of skeletal calcification. (C) 2021 The Author(s). Published by Elsevier Ltd.

This study presents new stable carbon and oxygen isotope data from Lower Cretaceous lacustrine carbonate rock samples recovered from a well drilled in the Santos Basin, offshore southeast Brazil. These samples represent a record of a continental environment just prior to the opening of the South Atlantic Ocean and the ultimate break-up of Gondwanaland. The geochemical data, along with carbonate mineralogy, indicate repeated cycles of lake level variation that could be attributed to climatic oscillations. Despite the absence of correlations between δ13C and δ18O values, facies analysis and the isotopic and mineralogical data suggest that lake hydrology was essentially closed for most of the depositional interval studied here. The existence of persisting trends of nearly constant δ13C values with a spread in δ18O values though, suggests long water residence times in the palaeolake, equilibrium between atmosphere and lake water CO2, as well as significant evaporation of water. The overall geological model that emerges unveils a more comprehensive picture of the depositional conditions that favoured the continuity of a significant carbonate factory in the middle of the Gondwanan continent, corroborating previous studies that suggested the lasting existence of a large and somewhat shallow endorheic lake in the area during the Early Cretaceous. As a result of this recorded trend strongly suggesting equilibrium between lake waters DIC (dissolved inorganic carbon) reservoir and atmospheric CO2, the data are most consistent with lacustrine deposition rather than precipitation of travertine, contrasting with some suggestions for the genesis of the carbonates of the Barra Velha Formation. Finally, this apparent equilibrium with the atmosphere likely left a preserved record in the continental carbonates of the final stages that preceded a major global environmental disturbance associated with an increase in atmospheric CO2, known for this time as the Oceanic Anoxic Event (OAE) 1a. If this is correct, it also helps to put further time constraints on this studied interval, which should not be younger than Barremian age, and to provide a regional continental perspective on a global event.

Large oil accumulation discoveries in the last decade in the pre-salt rock succession off the Brazilian coast aroused renewed interest in carbonate reservoirs, prompting several studies aiming at unravelling the conditions of formation of these rocks. Despite that, there are still many gaps in the knowledge concerning environmental conditions of the deposition of the pre-salt carbonates of Santos Basin, located at the southeastern Brazilian margin. In this study, we present and discuss a new suite of geochemical and biostratigraphic data of the lacustrine rift to post-rift succession (broadly correlated with Itapema and Barra Velha Formations) of the Santos Basin, whose deposition occurred in the Aptian. The integration of the petrographic, biostratigraphic and geochemical data serves the ultimate purpose of the study, which is to apply the new radiogenic strontium data as a proxy for the reconstruction of the evolution of the lake hydrology associated with the deposition of the carbonates of the Itapema and Barra Velha Formations. These data are integrated into numerical models, whose results bring new ideas on the general hydrologic conditions of the lake system and on its evolution through time. The mass balance calculations indicate that two end-members alone can account as potential sources of Sr to the palaeolake: a mafic and a felsic component, respectively. The latter likely contributed with significantly higher fluxes, roughly one to two, and up to four, orders of magnitude higher than the former end-member for most of the depositional history in order to generate the 87Sr/86Sr trends observed in the carbonate rocks. Finally, the results obtained from the integration of the observations with the numerical models point to decreasing fluxes of water with time as contributed from both sources, suggesting that the formation of the deposits became very likely constrained to progressively more evaporitic and alkaline lake waters. These results also open up a new window into the environmental conditions of the interior Gondwana continent immediately prior to the definitive opening of the South Atlantic Ocean.

ABSTRACT: Empty elliptical vesicles are observed in outcrops of Barremian very fine clayey sandstone to siltstone lacustrine slurry deposits of the Pitanga Member (Maracangalha Formation), exposed in the Maré Island, Southern Recôncavo Basin, Brazil. These sedimentary features have been traditionally interpreted as water escape structures triggered by the diapirism of the underlying shales of the Candeias Formation. This work proposes that vesicles were generated during massive gas hydrate dissociation as a result of tectonic activity in a paleolake system. Tectonic uplift would have triggered both the reduction of the confining pressure as well as an increase in lake bottom temperature, resulting in the instability of gas hydrate and causing intense release of both methane - or carbon dioxide (CO2) - and water. On one hand, this proposal has a strong impact on paleoenvironmental interpretations, giving support to the current hypothesis that rocks related to the Pitanga Member would have been deposited under water columns deep enough for gas hydrate formation and subsequent dissociation. On the other hand, it provides new insights on the genesis of fluid escape structures in sedimentary rocks, both lacustrine and marine, providing a paleobathymetric indicator.

The scope of this work was to evaluate the levels of polycyclic aromatic hydrocarbons (PAHs) and their possible sources in two shallow sediment cores from an estuarine area located in the south region of Brazil, the Guaratuba Bay. The estuary is encompassed by an Environmental Protection Area (EPA) of 1306km2, which is still considered to be pristine, despite recent urban growth. To assess levels of 14 selected PAHs in the sediments, among those listed as “priority pollutants” by the US Environmental Protection Agency, the cores were divided in several sections, followed by extraction, clean-up, fractionation and subsequent analysis using HPLC coupled to a fluorescence detector. Among the PAHs detected, phenanthrene and fluoranthene showed the highest concentrations. PAHs with more than four rings were usually present in low concentrations or were undetectable. To assist in the identification of possible sources, the ratios An/(An+Phe) and Flt/(Flt+Py) (anthracene to anthracene plus phenanthrene and fluoranthene to fluoranthene plus pyrene) and a PCA model were also used. Total concentrations of PAHs ranged between 1.5 and 3130ng/g (mean 495) dry weight for the core collected in the inner part of the estuary, and between 78.5 and 3270ng/g (mean 899) dry weight for the other core, collected in the outer part. Compared to values found in other studies in coastal zones along South American eastern margin, this area can be considered to range from low polluted to moderately polluted. From the data, it was also possible to conclude that there is predominance of petroleum sources, and essentially close to the more urbanised areas. Combustion sources have only minor contribution and are episodic, when compared to the latter.

The contamination of soil and groundwater as a result of the mobility of elements used in farming is of major concern. This paper will present and analyze results of a study aiming to evaluate the mobility of Zn, Cu and Cd contained in NPK fertilizers and simple super phosphate, used on a toposequence formed by a latossol and a podzolic soil. The A and B horizons were analyzed in four different locations of the toposequence. The studied area consists of latossols and podzolic soil profiles (A and B horizons) located on the Marambaia Farm, Petropolis, Rio de Janeiro, Brazil. A tropical climate prevails in this mountainous area, with heavy rains being mostly concentrated within the months of November and March. Undisturbed samples of the soils were collected in the field and submitted, in laboratory column experiments, to infiltration by solutions of Zn, Cu and Cd, under concentrations representing the amount of these elements in fertilizers sprayed on the site. This procedure allowed the evaluation of transport parameters that describe the migration of chemical species through porous media under controlled conditions in the laboratory. The leachate collected from each experiment was analyzed by flame atomic absorption spectrometry to determine their metal concentration. The results showed that metals were retained in the superficial horizons of the soils. Due to superficial erosive processes that occur in the area, it can also be concluded that the metals could be leached from the soil and dispersed by nearby rivers.