The investigation of mechanisms involved in barite formation in the mesopelagic zone has served to demonstrate the importance of extracellular polymeric substances (EPS) in promoting microenvironments in which Ba can precipitate. Barite formation in the ocean was not fully understood until experimental work and observations from microenvironments of intense organic matter mineralization in the ocean water column demonstrated the role of bacteria and EPS in concentrating Ba. The organomineralization processes leading to barite formation are expected to be similar to those involved in the formation of other biominerals in which bacterial cells and EPS provide charged surfaces that bind metals inducing mineralization. Thus, EPS production plays a major role in promoting locally high concentrations of Ba leading to barite precipitation. Regarding the crystallization pathway, scanning and high-resolution transmission electron microscopy analyses have shown the occurrence of P-rich amorphous precursor phases, which supports that phosphate groups in EPS and bacterial cells are the main sites for binding Ba. These P-rich amorphous particles evolve into poorly crystallized barite and eventually into micrometer-sized barite crystals. The ubiquitous presence of bacteria and EPS in aquatic systems, and in the mesopelagic zone at depths of intense organic matter mineralization, and their inherent ability to biomineralize, make them extremely important agents in driving the Ba biogeochemical cycle. Thus, further investigating microbial processes in the open ocean is essential to better understand metal cycling. The strong link between organo-mineralization and microbial processes further supports the microbial role in biogeochemical cycles. Other than Ba, EPS may also play an important role in concentrating other metals in seawater, which still needs further investigation.

The early lithification of carbonate mud during the subaerial exposure stage under semiarid conditions has been proposed to facilitate dolomite formation. However, how the biogeochemical processes during subaerial diagenesis promote dolomite formation remains unclear. Here, we employ a multiproxy approach to investigate the process of dolomite formation by analysing modern dolomite crusts forming in lagoon Brejo do Espinho. Petrological analysis reveals that the crusts consist of coexisting high-Mg calcite and dolomite. Low Fe and Mn concentrations indicate the formation of dolomite under oxic conditions, whereas a higher Sr concentration in well-lithified crust suggests primary bacterial-induced dolomite precipitation. The Mg isotopic composition of the crusts exhibits a lighter value than that of modern sabkha dolomite, suggesting different dolomitization processes and Mg sources. The more negative δ 13 C values of the crusts than those of the unlithified carbonate mud in lagoon Brejo do Espinho indicate the incorporation of 13 C-depleted organic carbon. The biogeochemical processes related to decaying organic matter during subaerial diagenesis generate partially oxidized organic matter that promotes Mg 2+ dehydration and enhances the dissolution of primary high-Mg calcite, ultimately triggering the transition of high-Mg calcite to dolomite and/or the direct precipitation of dolomite. The ancient ‘dolomite factory’ operated through the cyclic deposition of carbonate sediments and penecontemporaneous subaerial diagenesis. Thematic collection: This article is part of the Towards unravelling the ‘Dolomite Problem’: new approaches and novel perspectives collection available at: https://www.lyellcollection.org/topic/collections/towards-unravelling-the-dolomite-problem

Brejo do Espinho coastal lake (LBE) is one of the few places in the world where dolomite [CaMg(CO3)2] is precipitating in the modern environment under microbially induced processes and low oxygen conditions. We use pore morphometry of the foraminifera Ammonia cf. A. veneta to evaluate paleo-O2 dynamics during the dolomitic depositional phase that took place at LBE in the late Holocene. Foraminiferal community structure was also investigated, and results were compared to bulk isotopic composition of carbonates, total organic carbon (TOC), and X-ray Diffraction of sediments (XRD). The correlation matrix (Spearman method) showed that Ammonia test pores morphometric parameters displayed significant correlations with overall biotic and geochemical data, with pore area presenting a relatively higher association. Ammonia test pores were primarily controlled by the degradation of organic matter (Pore area-TOC, r = - 0.84), and foraminifera density appeared to be influenced by oxygen changes, with a higher abundance in the highest porosity intervals (Pore area-N, r = 0.82), indicating a direct effect of oxygen penetration on species dominance. These data also reveal a tolerant behavior of the low-O2 bioindicator species Quinqueloculina laevigata and A. veneta. Understanding microbemineral interactions is critical for interpreting paleo records, and our data provide strong support for coupling assemblage and pores analysis as paleo-O2 bioindicators for paleo-redox coastal settings.

Over the few past decades, the concept of microbial sulfur cycling catalyzing the precipitation of CaMg (CO3)2 at low temperatures (<40 °C) has been studied intensely. In this respect, two hypersaline lagoons, Lagoa Vermelha and Brejo do Espinho, in Brazil, have been the subject of numerous studies investigating sedimentary Ca/Mg carbonate formation. Here, we present the sulfur and oxygen isotopic compositions of dissolved sulfate from surface water, as well as sulfate and sulfide from pore-water (δ34SSO4, δ18OSO4, and δ34SH2S), the sulfur isotopic composition of sedimentary pyrite (δ34SCRS), and sulfur and oxygen isotopic compositions of carbonate-associated sulfate (CAS, δ34SCAS and δ18OCAS). The pore-water profiles at Lagoa Vermelha indicate ongoing bacterial sulfate reduction by increasing δ34SSO4, δ18OSO4 and δ34SCRS values downcore. At Brejo do Espinho, the pore-water profiles displayed no depth-dependent isotope trends; the Ca/Mg ratio was, on average, lower, and the δ18OSO4 values in both surface and pore-water were strongly enriched in 18O. There was an overall mismatch between δ34SSO4 and the significantly higher δ34SCAS values. A negative correlation was observed between the Ca/Mg ratio and higher δ34SCAS values. The results show that the size difference between the two lagoons induces differences in the intensity of evaporation, which leads to the increased secretion of extrapolymeric substances (EPSs) by microbes in the smaller Brejo do Espinho. EPS provides the microenvironment where Ca/Mg carbonate can nucleate and preserve increased δ34SCAS values. Apart from EPS, increased sulfur oxidation is proposed to be a second factor causing relative enrichment of Ca/Mg carbonates at Brejo do Espinho. Our results emphasize the role of evaporative processes on Ca/Mg carbonate formation, and indicate that the respective δ34SCAS values reflect microenvironments rather than preserving an open marine δ34SSO4 signature.

ABSTRACTThe Lagoa Salgada is located in the Paraíba do Sul river delta plain on the coast of Rio de Janeiro state, Brazil, and is one of the few lagoons in the world that have well-developed recent stromatolites. Lagoa Salgada is a hypersaline lagoon formed in a very complex environmental system subjected to terrestrial and oceanic influences under different sea level regimes and climate variations. In addition, sediment and stromatolites are characterized by unusually positive inorganic δ13C VPDB values. For this reason, it has been the target of several geological and paleoenvironmental studies, which, in their great majority, require a geochronological technique in order to determine the changes in the environment over time. When radiocarbon (14C) dating is used, it is necessary to consider some details as the source of 14C in the environment and perform 14C ages calibration accordingly. In the present paper, a bibliographic survey was carried out in order to review the data treatment and improve the environmental evolution discussion based on accurate calibration. Using the Marine20 curve and an undetermined ΔR, we generated growth and depositional models to establish an overview of the formation of this lagoon.

Lipid-biomarkers have been used to reconstruct environmental changes in lacustrine systems on a range of time scales. Lake sediments are excellent archives to apply these tools due to their rapid and amplified response to environmental pressures. For the past thirty years, the hypersaline lagoons of the Rio de Janeiro coastal plain have been studied as natural laboratories for the observation of the biogeochemical processes involved in modern dolomite precipitation. Here, we apply a multiproxy approach to characterize two depositional stages during the Holocene that may have triggered primary dolomite formation in these lagoonal environments. A first stage, with two sub-stages (1A − 6.1 to 4.2 kyr. BP; 1B − 4.2 to ∼3.6 kyr. BP) was deposited during the sea-level rise, with sediments containing an abundance of long-chain n-alkanes with 2H-depleted (δ2Hn-alk) signatures indicating riverine inputs of terrestrial organic carbon during prevailing wet conditions. A second stage (<∼3.6 kyr. BP), comprising lacustrine facies, was characterized by high amounts of authigenic carbonate precipitates (calcite, Mg-calcite, Ca-dolomite, and dolomite). The carbonates are the result of physico-chemical changes in the water after the isolation of the lagoons from both the Atlantic Ocean and the neighboring Lagoa de Araruama due to a fall in sea level and aridification associated with intensification of the coastal upwelling after 2.2 kyr. BP. The n-alkanes deposited during this phase contain variable proportions of long and short-chain homologues indicating a mixed source of organic matter (terrestrial higher plants and microorganisms), as well as changes in vegetation associated with the driest conditions, inferred from the 2H-enriched n-alkane homologous. These results clearly demonstrate a climatic influence on dolomite formation in coastal hypersaline environments linked to sea-level change and coastal upwelling phenomena. With these observations, we hypothesize that the existence of similar palaeoceanographic and environmental conditions in the geologic past may have triggered the formation of extensive microbial dolomite deposits. This study provides new elements to interpret the formation of massive dolomite deposits in the geological record, for example, along the Late Triassic Tethys margin.

In this study, we present a correlation between δ18OC values of carbonate in tooth enamel samples from the modern Brazilian population and the available δ18ODW data for the meteoric water from the Global Network of Isotopes in Precipitation (GNIP). Tooth enamel from 119 Brazilian individuals from five different regions of the country were analyzed. The δ18OC isoscape obtained is in good agreement with the isoscape based on regional meteoric and drinking water. The regression matrix obtained for the δ18O values of the carbonate tooth enamel and meteoric water was used to build an isoscape using the regression-kriging approach. Our data show that Brazil can be divided in two main regions with respect to the δ18O values of the carbonate tooth enamel: (1) the most easterly part of the northeast region, which is characterized by a warm and dry climate and (2) the remainder of the country, stretching from the Amazon rain forest to the more southernly regions. The data herein reported can be used for forensic purposes related to human identification.

In recent years resulting investigations in living microbialites have provided significant data that have been critical to disentangle the role of the various biotic and abiotic processes contributing to their development. Despite these efforts separating the impact and magnitude of these processes remain a difficult task. At present the Maquinchao Basin in northeastern Patagonia, Argentina, contains both fossil and living microbialites. Thus, the region provides a unique opportunity to investigate the impact of intrinsic and extrinsic parameters in carbonate precipitation. Early investigations (Austral summer 2011) in living microbialites concluded that organomineralization was related to both photosynthetic activity in the more surficial layer (green), and sulfate-reduction in the lower part (beige). Field investigations in the same area four years later showed that the pounds previously containing abundant active mats had dried out, and in general revealed the absence of globular structured clusters of minerals in the microbial mats. Here we present microscale investigations using optical microscopy and SEM along with the 16SrRNA gene sequence diversity, and the physico-chemical parameters of the hosting waters. They were carried out in successive seasonal samplings in November 2015, April-May 2016, August 2016, February 2017, and March 2018. All microbialite samples show regular occurrences of sulfate reducing bacteria (SRB) along with filaments of unknown origin. Carbonates are observed associated with erect filaments in shallow and active running water locations whereas the mineral phase is located below organic matter film in comparatively deeper and calmer water areas. Additionally, seasonal changes in the physico-chemical properties of the hosting waters indicate that extrinsic parameters, especially evaporation, might play a more substantial role in the precipitation of these carbonates than previously proposed. The environmental differences between 2011 and 2015 in meteorological conditions, regional volcanic activity and associated deposits in the basin are analyzed. We concluded that they are likely responsible of the decrease of the mineralization processes, and particularly those associated with photosynthetic activity. These results call for caution when interpreting the degree of biological impact on the formation of microbialites in the geological record. Local extrinsic factors might have a changeable impact over time switching mineral precipitation from biotic to abiotic and vice-versa, which can be undistinguishable in fossilized microbialites.

Summary The coastal region of the Rio de Janeiro state (Brazil) is characterized by a semi-arid microclimate associated with the upwelling coastal system of nearby Cabo Frio, which affects the hydrological and biogeochemical cycles in the region. Lakes and lagoons are natural laboratories to study biogeochemical signals that occur over geological times because they generally have higher sedimentation rates than the oceans. Lagoa Vermelha (LV) and Brejo do Espinho (LBE) beside amplify signals can register fluctuation in the local climate which is related to ocean circulation and biomineralization. These lagoons represent one of the few places in the world where modern precipitation of dolomite occurs. This study uses a multiproxy approach to characterize the deposition of carbonate sediments at these evaporitic environments. Sedimentary cores from LBE and LV, 1.6 and 6.1 cal kyr BP respectively, demonstrated mixed organic matter source reaching the lagoons with large input of terrestrial components. The dolomite-rich layers deposited ~2.1 cal kyr BP displayed enriched |18O and depleted |13C suggesting intense microbial activity and dryness. Using an approach combining organic and inorganic geochemical proxies has led to the recognition of dryness as an important regional climatic characteristic on the carbonate sedimentation in these hypersaline coastal lagoons.

Geneses of microbialites and, more precisely, lithification of microbial mats have been studied in different settings to improve the recognition of biogenicity in the fossil record. Living microbial mats and fossil microbialites associated with older paleoshorelines have been studied in the continental Maquinchao Basin in southernmost South America. Here, we investigate carbonate crusts from a former pond where active mineralizing microbial mats have been previously studied. Petrographic observations revealed the presence of abundant erect and nonerect microfilaments and molds with diameters varying from 6 to 8 micrometers. Additionally, smaller pores and organic matter (OM) remains have been identified in areas containing less filaments and being dominated by carbonate. A Mg, Al and Si-rich phase has also been identified in the carbonate matrix associated with the dominant micritic calcite. Moreover, mineralized sheaths contain mixed carbonate (calcite) with Mg, Al and Si, where the latter elements are associated with authigenic clays. The presence of mineralized sheaths further attests to biologically induced processes during the uptake of CO2 by photosynthetic microorganisms. Additionally, the high density of the micritic phase supports the subsequent mineralization by nonphotosynthetic microorganisms and/or physicochemical processes, such as evaporation. Since the micritic filament microstructure of these recent crusts is very similar to that observed in fossil microbialites, they can be used to bridge the gap between living mats and fossil buildups.

The Regiao dos Lagos situated along the coast east of Rio de Janeiro, Brazil, is dominated by a semiarid microclimate attributed to the occurrence of oceanographic upwelling at nearby Cabo Frio. The upwelling is strongly associated with the dominance of NE winds during austral spring/summer and the directional change of the shoreline orientation. Some coastal hypersaline lagoons from this region have been studied intensively over the last 25 yr because they represent relatively rare sites of modern primary dolomite precipitation. Comparison of environmental signals in three lagoons indicates that during the last similar to 3.0 kyr, changes in oceanographic parameters may have influenced biogeochemical processes associated with the production of carbonate-bearing sediments. The timing of a decrease in sea level may have influenced a period of more intense upwelling, which coincides with precipitation of stoichiometric dolomite at similar to 2.3 kyr BP in the lagoons located along the coast directly west of Cabo Frio. The dolomite found in the lower sections of cores from Lagoa Vermelha and Brejo do Espinho contains more positive delta O-18 values, indicating greater evaporation with a period of increased semiarid conditions corresponding to greater terrestrial input. The lower delta C-13 values indicate re-equilibration with the input of new carbonate ions derived from the decomposition of organic matter during dolomite formation. In contrast, the sediment core from Lagoa Salgada, located northeast of Cabo Frio, contains no dolomite, and very positive delta C-13 values recorded in carbonate sediments are attributed to microbially mediated methanogenesis, whereas delta O-18 values remain relatively constant throughout the core at around zero.

In a seminal paper regarding the mechanisms of carbonate stromatolite formation, Ginsburg (1991, Controversies in Modern Geology , pp. 25–36) emphasized the need to question the relative role of microbes versus environment in their formation. The Maquinchao Basin is a continental lacustrine system in southern Argentina. It provides an ideal site to study carbonate buildups, the role of microbes and environmental stressors in their development and their implications in palaeoenvironmental reconstructions. Presently, the basin encompasses two lakes (Carri Laufquen Grande and Carri Laufquen Chica) joined by the ephemeral Maquinchao River. Fossil microbialites are found south and southwest of the largest lake. Preferential areas of development for fossil microbialites have been mapped using a high‐resolution differential Global Positioning System. Outcrops are located between 820 and 830 m elevation, higher than actual lake levels and the Maquinchao River where living microbialites have been observed. Field data along with microscopical observations and X‐ray diffraction analyses have revealed a heterogeneity in both distribution and macro‐morphotypes since carbonate buildups display different morphologies such as crust, columns, open flower‐like, rounded and ellipsoids. Conversely, on the meso and micro‐scale they show more homogeneous morphologies including laminations and shrubs. These microbial buildups are associated with basaltic substrates of variable size from pebbles to boulder. The homogeneity in meso and micro‐structures argue in favour of stable intrinsic parameters (i.e. microbial communities) whereas the variable macro‐morphotypes indicate changing extrinsic constraints such as steepness, energy and turbidity. The occurrence of distinctive morphotypes in buildups separated by outcrop and topography suggest that the Maquinchao microbialites are indicative of a former larger lake. Thus, the Maquinchao microbial buildups are a valuable proxy for water‐level evolution and therefore palaeoenvironmental reconstructions. They can be further used to interpret the apparently random distribution of morphological types and extension of microbialites in the geological past.

Dolomite is a very common carbonate mineral in ancient sediments, but is rarely found in modern environments. Because of the difficulties in precipitating dolomite in the laboratory at low temperatures, the controls on its formation are still debated after more than two centuries of research. Two important parameters to constrain the environment of dolomitization are the temperature of formation and the oxygen isotope composition of the fluid from which it precipitated. Carbonate clumped isotopes (expressed with the parameter Delta(47)) are increasingly becoming the method of choice to obtain this information. However, whereas many clumped isotope studies treated dolomites the same way as calcite, some recent studies observed a different phosphoric acid fractionation for Delta(47) during acid digestion of dolomite compared to calcite. This causes additional uncertainties in the Delta(47) temperature estimates for dolomites analyzed in different laboratories using different acid digestion temperatures. To tackle this problem we present here a (proto-)dolomite-specific Delta(47)-temperature calibration from 25 to 1100 degrees C for an acid reaction temperature of 70 degrees C and anchored to widely available calcite standards. For the temperature range 25 to 220 degrees C we obtain a linear Delta(47)-T relationship based on 289 individual measurements with R-2 of 0.864: Delta(47) (CDES 70 degrees C) = 0.0428 +/- 0.0020 x (10(6)/T-2) + 0.1481 +/- 0.0160 (T in Kelvin) When including two isotopically scrambled dolomites at 1100 degrees C, the best fit is obtained with a third order polynomial temperature relationship (R-2 = 0.924): Delta(47) (CDES 70 degrees C) (parts per thousand) = -0.0002 x (10(6)/T-2)(3) +0.0041 x (10(6)/T-2)(2) + 0.0115 x (10(6)/T-2) + 0.2218. Applying a calcite Delta(47)-T relationship produced under identical laboratory conditions results in 3 to 16 degrees C colder calculated formation temperatures for dolomites (with formation temperature from 0 to 100 degrees C) than using the (proto-)dolomite specific calibration presented here. For the synthetic samples formed between 70 and 220 degrees C we also determined the temperature dependence of the oxygen isotope fractionation relative to the water. Based on the similarity between our results and two other recent studies (Vasconcelos et al., 2005 and Horita, 2014) we propose that a combination of the three datasets represents the most robust calibration for (proto-)dolomite formed in a wide temperature range from 25 to 350 degrees C. 10(3)alpha(CaMg-carbonates-Water) = 2.9923 +/- 0.0557 (10(6)/T-2) - 2.3592 +/- 0.4116 Because of the uncertainties in the phosphoric acid oxygen and clumped isotope fractionation for (proto-)dolomite, we promote the use of three samples that are available in large amounts as possible inter-laboratory reference material for oxygen and clumped isotope measurements. A sample of the middle Triassic San Salvatore dolomite from southern Switzerland, the NIST SRM 88b dolomite standard already reported in other Delta(47) studies and a lacustrine Pliocene dolomite from La Roda (Spain). This study demonstrates the necessity to apply (proto-)dolomite specific Delta(47)-T relationships for accurate temperature estimates of dolomite formation, ideally done at identical acid digestion temperatures to avoid additional uncertainties introduced by acid digestion temperature corrections. In addition, the simultaneous analyses of dolomite reference material will enable a much better comparison of published dolomite clumped and oxygen isotope data amongst different laboratories.

Holocene palaeoceanography and climatic reconstructions were evaluated based on proxy analyses of fish otoliths from shellmounds located along the coast of Rio de Janeiro State, Brazil. Modern coastal seasonal upwelling is associated with the ascension of a deep and cold-water mass as a consequence of persistent NE winds that affect the climate in southeastern Brazil. However, this seasonal influence and the effect of the Holocene palaeoenvironment on shallow water are poorly known. In this work, coastal palaeotemperatures were estimated based on the geochemical analyses of otolith. The results of the delta O-18 and delta C-13 analyses from Holocene otoliths, obtained from coastal shellmounds separated by a distance of almost 200 km, show two distinctive otolith-derived palaeotemperature signals in a coastal region under upwelling influence. Additionally, this analysis suggests that the fish lived in marine waters, showing similar microchemistry values composition when compared with modern marine otoliths.

In his 1974 paper titled “Introduction to Comparative Sedimentology of Carbonates”, Robert N. Ginsburg stated that combining research on modern and ancient carbonates has led to significant discoveries. For example, he noted that “the discovery of synsedimentary dolomite or protodolomite on recent tidal flats” confirmed “the often-mentioned connection between well-stratified dolomites and shallow, nearshore environments”. He predicted that this discovery would result in research leading to new concepts and approaches. Indeed, in the subsequent 50 plus years since the discovery of modern dolomite forming beneath the coastal sabkhas of Qatar (Wells, 1962) and Abu Dhabi, U.A.E. (Illing et al., 1965), numerous research efforts to evaluate the biogeochemical processes in hypersaline coastal environments have led to significant advances towards resolving the long-standing “Dolomite Problem”. For instance, recognition of microbial activity as a major factor overcoming the kinetic inhibition of dolomite precipitation in coastal and deep-sea environments is currently driving dolomite research in entirely new directions.