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New Constraints on Precambrian Ocean Composition
Abstract
The Precambrian record of carbonate and evaporite sedimentation is equivocal. In contrast to most previous interpretations, it is possible that Archean, Paleoproterozoic, and to a lesser extent, Meso to Neoproterozoic seawater favored surplus abiotic carbonate precipitation, as aragonite and (hi-Mg?) calcite, in comparison to younger times. Furthermore, gypsum/anhydrite may have been only rarely precipitated prior to halite precipitation during evaporation prior to about 1.8 Ga. Two effects may have contributed to these relationships. First, sulfate concentration of seawater may have been critically low prior to about 1.9 Ga so the product mCa++ x mSO4-- would not have produced gypsum before halite, as in the Mesoproterozoic to modern ocean. Second, the bicarbonate to calcium ratio was sufficiently high so that during progressive evaporation of seawater, calcium would have been exhausted before the gypsum field was reached. The pH of the Archean and Paleoproterozoic ocean need not have been significantly different from the modern value of 8.1, even at CO2 partial pressures of a tenth of an atmosphere. Higher CO2 partial pressures require somewhat lower pH values.
... Botryoidal and spherulitic aragonite are spectacular fabrics found in Precambrian to Holocene marine (Ginsburg and James, 1976;Taviani and Rabbi, 1984;Aissaoui, 1985;Grotzinger and Kasting, 1993;Jaramillo-Vogel et al., 2019) and non-marine (Pentecost, 2005;Fouke, 2011;Capezzuoli et al., 2014;Della Porta, 2015 and references therein) carbonates. Hence, these carbonate growth-forms have been described from a wide range of depositional environments including: marine peritidal settings Grotzinger, 2000, 2004); reefs at carbonate platform margins and slopes (Ginsburg and James, 1976;Taviani and Rabbi, 1984;Aissaoui, 1985;Della Porta et al., 2003;van der Kooij et al., 2010;Jaramillo-Vogel et al., 2019); lacustrine environments (Della Porta, 2015;Pace et al., 2016), methane cold seeps (Peckmann et al., 1999;Feng et al., 2008;Himmler et al., 2010) and hydrothermal travertines (Folk, 1994;Pentecost, 2005;Fouke, 2011;Della Porta, 2015;Jones, 2017a, b). ...
... As both, botryoidal and spherulitic aragonite consist of aragonite fibers with a radial arrangement, they are often considered closely related fabrics (Ginsburg and James, 1976;Davies, 1977;Aissaoui, 1985). In marine settings, individual botryoids or similar fibrous crystal fan fabrics can range in size from a few μm (Pleistocene; Taviani and Rabbi, 1984) to more than 1 m (Archean; Grotzinger and Kasting, 1993). Mostly reported from platform margin reefs, botryoidal and spherulitic aragonite generally nucleate on the walls of voids and grow in the direction of free pore space (Ginsburg and James, 1976;Davies, 1977;Taviani and Rabbi, 1984;Aissaoui, 1985). ...
... We document and discuss petrological features of botryoidal and spherulitic aragonites from three Holocene depositional environments with microbial mats and different water physicochemical properties in order to enlighten the complex origins and formation processes of these fabrics associated with microbial mats. Results shown here are relevant as the recognition and interpretation of botryoidal and spherulitic aragonite in fossil records are important in both paleoceanography and general carbonate research, providing important information regarding water chemistry and depositional and diagenetic environments (Mazzullo, 1980;Aissaoui, 1985;Grotzinger and Kasting, 1993;Grotzinger, 2000, Sumner andGrotzinger, 2004). We acknowledge that similar fabrics have also been reported from non-carbonate lithologies, (e.g. ...
Similar carbonate fabrics may result from different pathways of precipitation and diagenetic replacement. Distinguishing the underlying mechanisms leading to a given carbonate fabric is relevant, both in terms of an environmental and diagenetic interpretation. Prominent among carbonate fabrics are aragonite botryoids and spherulites, typically interpreted as direct seawater precipitates and used as proxies for fluid properties and depositional environments. This study investigated μm to mm-scale Holocene botryoidal and spherulitic aragonite from marine and non-marine carbonate settings associated with microbial mats, and reports two distinct formation mechanisms: 1) early diagenetic replacement, and 2) primary precipitation via nanocrystal aggregation. In the intertidal microbial mats of Khawr Qantur (Abu Dhabi), botryoidal and spherulitic aragonite are replacement products of heavily micritized bioclasts. To form the botryoidal and spherulitic aragonite, skeletal rods and needles, resulting from disintegration of micritized bioclasts, recrystallize into nanocrystals during early marine diagenesis. These nanocrystals then grow into fibrous crystals, forming botryoidal and spherulitic aragonite. In the lacustrine microbial bioherms of the hypersaline Great Salt Lake (United States) and in the hydrothermal travertines of Bagni San Filippo (Italy), botryoidal and spherulitic aragonite evolve from nanocrystals via precipitation. The nanocrystals are closely associated with extracellular polymeric substances in microbial biofilms and aggregate to form fibrous crystals of botryoidal and spherulitic aragonite. The studied fabrics form a portion of the bulk sediment and show differences in terms of their formation processes and petrological features compared to the often larger (few mm to over 1 m) botryoidal and spherulitic aragonite described from open-marine reefal cavities. Features shown here may represent modern analogues for ancient examples of carbonate depositional environments associated with microbialites. The implication of this research is that botryoidal and spherulitic aragonite associated with microbial mats are relevant in paleoenvironmental interpretations, but must be combined with a detailed evaluation of their formation process. Care must be taken as the term “botryoidal and spherulitic aragonite” may in fact include, from the viewpoint of their nucleation and formation mechanism, similar fabrics originated from different pathways. At present, it seems unclear to which degree the μm to mm-scale botryoids and spherulites described here are comparable to their cm-to dm-size counterparts precipitated as cements in the open pore space of reefal environments. However, it is clear that the investigation of ancient botryoidal and spherulitic aragonite must consider the possibility of an early diagenetic replacement origin of these precipitates.
... The temperature was not significantly different from today [25], because the lower luminosity of the sun was compensated for by the higher partial CO 2 pressure [26]. The pH of the ocean was close to 7 [25,27,28], one pH unit lower than today. The concentration of Fe 2+ may have been in the range of 0.02 mM [29] to 0.2 mM [30]. ...
... The concentration of Fe 2+ may have been in the range of 0.02 mM [29] to 0.2 mM [30]. The partial pressure of CO 2 was probably between 0.1 and 0.2 bar [25,27]. Under the assumption that the ionic strength of the ocean was similar to that of today, ca. ...
We address the chemical/biological history of H2O2 back at the times of the Archean eon (2.5–3.9 billion years ago (Gya)). During the Archean eon the pO2 was million-fold lower than the present pO2, starting to increase gradually from 2.3 until 0.6 Gya, when it reached ca. 0.2 bar. The observation that some anaerobic organisms can defend themselves against O2 has led to the view that early organisms could do the same before oxygenic photosynthesis had developed at about 3 Gya. This would require the anaerobic generation of H2O2, and here we examine the various mechanisms which were suggested in the literature for this. Given the concentration of Fe²⁺ at 20–200 μM in the Archean ocean, the estimated half-life of H2O2 is ca. 0.7 s. The oceanic H2O2 concentration was practically zero. We conclude that early organisms were not exposed to H2O2 before the arrival of oxygenic photosynthesis.
... Calcium carbonate seafloor fans are common in early Precambrian (Archaean and Palaeoproterozoic) marine sequences (Kah and Knoll, 1996;Sumner and Grotzinger, 1996) but became environmentally restricted through the intertidal settings of the Mesoproterozoic (Bartley et al., 2000) and occasionally found in the carbonate successions that cap low-latitude glacial deposits of the Neoproterozoic (Kennedy, 1996;Hoffman et al., 1998;James et al., 2001;Hoffman and Schrag, 2002). The stratigraphic distribution of carbonate and carbonate fans precipitation suggests that certain environmental conditions were instrumental in their development and preservation and thereby imply that the nature of carbonate sedimentation and ocean carbonate chemistry must have been fundamentally different from modern conditions (Kempe and Degens 1985;Grotzinger 1989;Kazmierczak 1990, 1994;Grotzinger and Kasting 1993;Bartley et al., 2000). Studies during the past two decades on the numerous Archaean and pre-Neoproterozoic radial-fibrous and laminated textures have revealed the diversity and complexity of the sedimentary precipitates into sharper focus (Seong-Joo and Golubic, 1999, 2000; Bartley et al., 2000). ...
... The stratigraphic distribution of carbonate fans, their overall rarity in post-Proterozoic sections, and their occurrence in otherwise enigmatic carbonates (i.e. Snowball Earth and Lower Triassic carbonates) suggests that rare environmental factors are required for their formation Kazmierczak 1990 1994;Grotzinger 1989;Grotzinger and Kasting 1993). Exceptionally, sea floorprecipitated carbonate fans of cm-sized preserved and recorded in the Neoproterozoic, Rainstorm Member of the Ediacaran Johnnie Formation, Death Valley region, eastern California (Pruss et al., 2008). ...
Present study records the Fan-Fabric Structures from the late Palaeoproterozoic Kajrahat Limestone of the Vindhyan Supergroup, India exposed in Katni district, M. P. Centimeter (cm) size carbonate fans (1 to 10 cm in length) radiating in upward direction are part of a stromatolite dominated Kajrahat Limestone in the area. The Kajrahat FFS represent their widespread occurrence in the Proterozoic successions of India. Our study establishes that these fans were originally precipitated and not the result of a late diagenesis or any other post sedimentation process. These fan-fabric structures were deposited in intertidal to sub-tidal environments. Globally, fan-fabrics structures are considered a common feature of the Archaean to early Mesoproterozoic carbonate platforms.
... First, ejecta weathering and/or more intense volcanic activity during the early Earth favor the accumulation of bicarbonate and the consumption of calcium, building a "soda ocean" based on thermodynamic, kinetic, and mass balance considerations (Kempe and Degens, 1985;Kadoya et al., 2020). Second, the extensive distribution of primary sedimentary minerals of dolomite, magnesite, and nahcolite in Precambrian and the lack of massive gypsum/anhydrite deposition before 1.8 Ga imply both the high alkalinity and low calcium concentration in the early oceans (Tucker, 1982;Grotzinger and Kasting, 1993;Lowe and Worrell, 1999;Frank and Fielding, 2003;Sugitani et al., 2003). Third, "soda oceans" with high alkalinity and low Ca concentration could chemically facilitate the biogenesis because low calcium prevents the binding of Ca 2+ onto phosphate and high alkalinity promotes the synthesis of polypeptides Kazmierczak, 1994, 2011). ...
... sedimentary rocks indicate that seawater chemistry has significantly changed through Earth's history (Hardie, 1996;Lyons et al., 2014). It has been proposed that the alkalinity of Precambrian oceans was higher than that of modern seawater, which promotes biogenesis (Kempe and Degens, 1985;Grotzinger and Kasting, 1993). However, the appraisal of the seawater alkalinity of the early Earth has received limited attention so far due to the lack of reliable approaches. ...
The chemical evolution of the ocean is one major component of the puzzle of how climate and life have co-evolved over the Earth's history. A “soda ocean” with high alkalinity, high pH, and low calcium concentration in Precambrian has been proposed to explain the emergence and evolution of early life. However, this hypothesis has not been widely accepted due to the lack of reliable tracers for the chemical composition of Precambrian seawater. Evaporite is formed during seawater/brine evaporation and thus has been widely used to reconstruct the ancient seawater/brine chemical composition. Here, evaporation experiments were conducted using Qinghai Lake (QHL) water, a modern soda lake, to provide an analogy of the Precambrian “soda ocean” evaporation and investigate the mineralogical and Mg isotopic signatures of alkaline brine-derived evaporites. Our results show that the evaporation path of QHL water overall covers the stages of hydrous Mg carbonates (hydromagnesite), halite, and bloedite [Na2Mg(SO4)2·4H2O] precipitation. The precipitation of hydrous Mg carbonates and bloedite is distinct from the modern seawater evaporation and is accompanied by the removal of up to 85% Mg from brine. The brine gradually becomes enriched in heavy Mg isotopes during evaporation due to the preferential incorporation of light Mg isotopes into precipitates. The fractionation of Mg isotopes is dominantly controlled by the bond structure during hydrous Mg carbonates and bloedite precipitation, and the latter is also slightly influenced by the kinetics in the highly concentrated brine. The Mg isotope fractionation during bloedite dissolution is limited due to the rapid congruent dissolution. The significant Mg isotopic fractionation observed within the hydrous Mg carbonates precipitation during the earliest QHL water evaporation indicates the potential Rayleigh distillation of Mg by alkalinity in the “soda ocean”, which is absent in the Phanerozoic oceans. Therefore, the large Mg isotope fractionation during alkaline brine evaporation can be applied to test the existence of Precambrian “soda oceans”.
... The dominance of calcium carbonates results from the fact that modern seawater is oversaturated with calcium carbonate due to weathering of Ca-dominant silicate rocks (Walker et al., 1981) and preexisting carbonate rocks. Ca appears to have dominated for even the earliest part of Earth history that preserves a sedimentary rock record (Grotzinger & James, 2000;Grotzinger & Kasting, 1993). Since marine sediments dominate Earth's stratigraphic record, deposits of calcium carbonate overwhelm the tiny proportion of primary magnesium carbonate precipitation on Earth. ...
... Sedimentary magnesite deposits also occur in the Precambrian rock record (Alderman & von der Borch, 1961;Melezhik et al., 2001). Here, Ca-bearing sulfate minerals are likely absent due to both the depleted marine sulfate reservoir (compared to present day), and the seawater chemistry that favored Ca-carbonate precipitation instead of Ca-bearing sulfate (Frank & Fielding, 2003;Grotzinger, 1989;Grotzinger & Kasting, 1993). ...
Magnesium carbonates have been identified within the landing site of the Perseverance rover mission. This study reviews terrestrial analog environments and textural, mineral assemblage, isotopic, and elemental analyses that have been applied to establish formation conditions of magnesium carbonates. Magnesium carbonates form in five distinct settings: ultramafic rock‐hosted veins, the matrix of carbonated peridotite, nodules in soil, alkaline lake, and playa deposits, and as diagenetic replacements within lime—and dolostones. Dominant textures include fine‐grained or microcrystalline veins, nodules, and crusts. Microbial influences on formation are recorded in thrombolites, stromatolites, crinkly, and pustular laminites, spheroids, and filamentous microstructures. Mineral assemblages, fluid inclusions, and carbon, oxygen, magnesium, and clumped isotopes of carbon and oxygen have been used to determine the sources of carbon, magnesium, and fluid for magnesium carbonates as well as their temperatures of formation. Isotopic signatures in ultramafic rock‐hosted magnesium carbonates reveal that they form by either low‐temperature meteoric water infiltration and alteration, hydrothermal alteration, or metamorphic processes. Isotopic compositions of lacustrine magnesium carbonate record precipitation from lake water, evaporation processes, and ambient formation temperatures. Assessment of these features with similar analytical techniques applied to returned Martian samples can establish whether carbonates on ancient Mars were formed at high or low temperature conditions in the surface or subsurface through abiotic or biotic processes. The timing of carbonate formation processes could be constrained by ¹⁴⁷Sm‐¹⁴³Nd isochron, U‐Pb concordia, ²⁰⁷Pb‐²⁰⁶Pb isochron radiometric dating as well as ³He, ²¹Ne, ²²Ne, or ³⁶Ar surface exposure dating of returned Martian magnesium carbonate samples.
... By the late Neoproterozoic, macroscopic seafloor precipitates were increasingly scarce, supplanted by micritic carbonate textures, except where prominently associated with Neoproterozoic post-glacial cap carbonates (Hoffman et al. 1998). These observations led to the hypothesis that a secular decrease in inorganic carbon concentration and, hence, marine carbonate saturation (Grotzinger, 1989;Grotzinger & Kasting, 1993) occurred through the Precambrian. This decrease in carbonate saturation has been plausibly linked to an observed increase in stromatolite morphological diversity (Grotzinger & Knoll, 1999), followed by a pronounced decrease in complexity and diversity (Grotzinger, 1990), as well as to the distribution and abundance of molar-tooth carbonate through time (Pollock et al. 2006). ...
... micritic vs microbial) then resulted in different degrees of lithification, wherein breccia clasts were more likely to be ductile, or incompletely lithified, when associated with microbial or stromatolitic carbonate (Fig. 2b) and more likely to be brittle, or fully lithified, when associated with mechanically laminated or unlaminated carbonate from the seafloor (Fig. 4). Spatial distributions of specific time-sensitive featuresnotably acicular precipitates, herringbone carbonate precipitates and molar-tooth structurefurther indicate that carbonate saturation state, although likely elevated compared to that of the modern ocean (Grotzinger & Kasting, 1993;Bartley & Kah, 2004;Kah & Riding, 2007), may have lain at a threshold that required additional modification to overcome energy barriers to nucleation and precipitation of carbonate. Although these elements do not occur in high volumes in any of the studied basins, each provides important constraints on the timing of lithification. ...
Time-distinctive features within carbonate rocks, specifically the presence, abundance and distribution of stromatolites, molar-tooth fabric and specific morphologies of marine cement, have been identified as potential indicators of global-scale changes in the chemistry of marine environments. Recently, Cantine et al. (2019) introduced a database approach seeking to quantify spatial and temporal patterns in these carbonate features through the Precambrian. Despite the coarse temporal scale, results support earlier inferences of temporal change in carbonate sedimentation. Here, we use original field notes to dissect late Mesoproterozoic (˜1.3 to 1.0 Ga) carbonate strata at a high resolution, analyse time-distinctive carbonate fabrics within a database context and compare sedimentological patterns within this narrow time range to observations of the Proterozoic as a whole. Late Mesoproterozoic strata contain a variety of features (e.g. stromatolites, seafloor precipitates, herringbone carbonate, molar-tooth carbonate), often in close spatial and temporal proximity, that are commonly considered to be temporally restricted during the Precambrian. The spatial distribution of such features within Mesoproterozoic basins demonstrates the importance of recognizing even rare occurrences of time-distinctive facies and permits inference of environmental drivers that may have interacted to affect carbonate precipitation. Such spatial variability reflects a subtle division of Mesoproterozoic carbonate platform environments driven by globally high sea level, elevated carbonate saturation and a low-oxygen water column. The heterogeneous, mosaic nature of environments appears to be a hallmark of Mesoproterozoic carbonate sedimentation and emphasizes the importance of these basins in understanding longer-term trends in carbonate deposition.
... Regarding the ocean in the early Earth, previous research indicates that seawater had weak acidity (pH 4-7) because the carbon dioxide concentration in the atmosphere was high compared to today [6][7][8][9][10], or it was strongly acidic (pH 1-2) because a large amount of halogen substance in the high-temperature vapor atmosphere eventually dissolved into seawater [11]. Previously, pH levels, and concentrations of CO 2 and other elements were considered [6], but these estimations did not provide information about the quantitative chemical compositions of seawater. ...
... In contrast, the pH value of the primordial ocean was estimated to be in the range of 4.9-6.7. Such values are consistent with geologically and theoretically estimated values for the early Archean era (>5.7; [8]). Therefore, even if plate tectonics started after ocean formation, a mildly acidic ocean could have been maintained by the Archean era probably because the neutralization effect of the later water-rock reactions in the oceanic crust could be canceled out by the CO 2 -rich atmosphere. ...
The Hadean was an enigmatic period in the Earth’s history when ocean formation and the emergence of life may have occurred. However, minimal geological evidence is left from this period. To understand the primordial ocean’s composition, we focused on the ocean’s formation processes from CO2- and HCl-bearing water vapor in the high-temperature atmosphere. When the temperature of the lower atmosphere fell below the critical point, high-temperature rain reached the ground surface. Then, hydrothermal reactions between the subcritical fluid and primordial crust started. Eventually, a liquid ocean emerged on the completely altered crust as the temperature decreased to approximately 25 °C. Here, we conducted two experiments and modeling to simulate the reactions of hypothetical primordial crustal rock (basalt or komatiite). The results indicate that the primordial ocean was mildly acidic and rich in CO2, Mg, and Ca relative to Na, irrespective of the rock type, which is different from the modern equivalents. Therefore, unlike the present seawater, the primordial seawater could have been carbonic, bitter, and harsh rather than salty.
... Temporal restriction and abundance of MTS within the Meso-to Neoproterozoic carbonates has been attributed to combined eAects of (i) long term changes in the depositional condition, (ii) unique Proterozoic ocean water chemistry, and (iii) biological factors. In general, the Precambrian carbonates shows a gradual decrease from the Bbrous seaCoor-encrusting cement, aragonite fan, tufas during Archean to increased production of micrite and stromatolite with sediment binding texture in shallow water in Mesoproterozoic (Grotzinger 1989(Grotzinger , 1990Grotzinger and Kasting 1993;Grotzinger 1996, 2000;Grotzinger and James 2000;Bose et al. 2012). In addition, this gradual change in depositional condition accompanied with long-term decrease in the gradient of the CaCO 3 saturation during mid-proterozoic. ...
... Similarly, the demise of MTS in geological time scale is more difBcult to explain (Frank and Lyons 1998;Shields 2002; and references therein) and attributed to major glaciations, increased tectonic activity, decrease in microbial colonies and appearance of skeletal carbonates, increased bioturbation, increased plate motion leading to rapid subsidence and destruction of shallow water carbonate platforms and near attainment of Phenerozoic carbonate saturation (Grotzinger and Kasting 1993). However, a few isolated occurrences of MTS, e.g., in post-Marinoan Keilberg cap carbonate in Namibia and ca. ...
Molar-tooth structures (MTS) are enigmatic, micro-crystalline calcite filled fissures, confined in Proterozoic carbonates. Here we present petrography, carbon isotope, total organic carbon (TOC) and morphological attributes in context of interpreted palaeoenvironment to understand its development in the Mesoproterozoic carbonates of Lesser Himalaya. Lack of any detrital infill, uniform crystal size and gradational contact with host limestone indicate rapid calcite precipitation in fluid filled cracks. Reworking of MT as intraclast, folding and offset of MT ribbons supports for early formation before significant lithification. Moderate TOC (0.1 to 0.9) is possibly due to organic matter preservation under sub-oxic to slightly anoxic/ dysoxic condition. Storm generated bed forms indicates deposition in between fair weather- and storm wave base. Average 1.4‰ depletion of δ13C in MT relative to host limestone, presence of relict microbial laminae along the margin of the MT cracks and storm generated bed forms at outcrop scale indicates that the cracks might have formed by the combined effect of degassing of CO2 generated during the microbial oxidation of organic matter and wave loading by storm. Precipitation of microcrystalline calcite within the cracks may have been triggered by alkalinity generated by the mixing of the outflowing CO2 with sea water.
... While sulfates (SO 4 2− ) and their organic derivatives represent core (in)organic nutrients utilized in contemporary biochemistry 1 , the prevalence of only low concentrations of sulfates during Earth's early Archean period suggests a limited bioavailability of vital sulfates 2,3 . Sulfur in the +IV oxidation state presents an alternate source of sulfur for the first terrestrial microorganisms 4 . ...
For the last century, the source of sulfur in Earth’s very first organisms has remained a fundamental, unsolved enigma. While sulfates and their organic derivatives with sulfur in the S(+VI) oxidation state represent core nutrients in contemporary biochemistry, the limited bioavailability of sulfates during Earth’s early Archean period proposed that more soluble S(+IV) compounds served as the initial source of sulfur for the first terrestrial microorganisms. Here, we reveal via laboratory simulation experiments that the three simplest alkylsulfonic acids—water soluble organic S(+IV) compounds—can be efficiently produced in interstellar, sulfur-doped ices through interaction with galactic cosmic rays. This discovery opens a previously elusive path into the synthesis of vital astrobiological significance and untangles fundamental mechanisms of a facile preparation of sulfur-containing, biorelevant organics in extraterrestrial ices; these molecules can be eventually incorporated into comets and asteroids before their delivery and detection on Earth such as in the Murchison, Tagish Lake, and Allende meteorites along with the carbonaceous asteroid Ryugu.
... The precipitation of calcium carbonate and sulfate (Walker, 1983) and the high ratio of calcium to carbonate alkalinity (Blättlet et al., 2017) suggest an acidic early ocean, whereas rare-earth element anomalies found in sedimentary rocks argue for a neutral to weakly alkaline Archean Ocean (Friend et al., 2008). The scarcity of gypsum pseudomorphs indicates that the early ocean pH may be within the range of 5.7 to 8.6 (Grotzinger and Kasting, 1993). The evolution of atmospheric partial pressure of CO2 (pCO2) can also constrain ocean pH, but the exact evolutionary path of pCO2, especially in the early Earth, remains highly debated (e.g., Sheldon, 2006;Driese et al., 2011;Kah and Riding, 2007;Kanzaki and Murakami, 2015). ...
The ocean pH is a fundamental property that regulates various aspects of Earth system evolution. However, the early ocean pH remains controversial, with estimates ranging from strongly acidic to alkaline. Here we show that, by coupling global carbon cycle with ocean charge balance, and by using Earth interior processes to specify the history of volatile distribution and ocean chemistry, a rapid increase in ocean pH is likely during the Hadean to early Archean, with the pH evolving from 5 to neutral by ~ 4.0 Ga. This rapid pH evolution is attributed primarily to the elevated rates of both seafloor and continental weathering during the Hadean, which in turn result from high surface temperatures, efficient CO 2 supply, rapid formation and destruction of both continental and oceanic crusts, and elevated levels of divalent cations in the crust. Earth likely transformed from a hostile state to a habitable one by the end of Hadean, which has important implications for planetary habitability and the origin of life.
... changes to degassing rates through time) and internal model parameters (e.g. the extent of ocean anoxia). More generally, the modelling of carbonate chemistry has been used to explain the relative scarcity of Archean and Paleoproterozoic gypsum deposits by way of exhaustion of [Ca 2+ ] sw due to extensive evaporitic calcium carbonate formation when oceans were bicarbonate-rich (Grotzinger and Kasting 1993;Warren 2021), although substantial gypsum deposits occasionally formed (i.e. during the Lomagundi Event; Schröder et al. 2008;Blättler et al. 2018). ...
Understanding the long-term variance of seawater sulfate concentrations ([SO 4 ²⁻ ] sw ) is critical to understanding the dynamic relationship between the sulfur, carbon, calcium, and oxygen cycles, and their influence on Earth's habitability. Here, we explore how [SO 4 ²⁻ ] sw has changed throughout the Phanerozoic, and its impact on other elemental cycles. We do this by utilising the biogeochemical box model GEOCARBSULFOR. The model suggests that [SO 4 ²⁻ ] sw rose throughout the Paleozoic, declined during the Mesozoic, and then rose once more in the Cenozoic, generally matching geochemical proxies. Atmospheric oxygen mirrors [SO 4 ²⁻ ] sw changes during the Paleozoic and Mesozoic, but intriguingly, decouples during the Cenozoic. We further explored controls on [SO 4 ²⁻ ] sw by modifying the modelled gypsum fluxes via the incorporation of evaporite data from the geological record. We find that forcing gypsum burial with the observed evaporite deposition data causes the model to better match proxy records at some times, but worsens predictions at others. Finally, we investigate model reliance on a prescribed record of marine calcium concentrations, finding that it is a dominant control on modelled Phanerozoic [SO 4 ²⁻ ] sw , and that removing this control seriously degrades model predictions. We conclude that no model can yet simulate a reasonable evolution of both the calcium and sulfur cycles.
Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/topic/collections/sulfur-in-the-earth-system
Supplementary material: https://doi.org/10.6084/m9.figshare.c.7164928
... However, calcified cyanobacteria were barely found in the laminations of loopites, which possibly demonstrates that p (CO 2 ) is much larger than 10 PAL (present atmosphere level) (Riding 2011). In addition, the precipitation of fibrous aragonite was identified in the laminations of laminites and loopites (Fig. 7F), which may indicate that the carbonate saturation was much higher than that in modern oceans and that the bacterial sulfate reduction (BSR) played an important role in consuming organic matter and sulfate to produce bicarbonate to ensure precipitation, which means that loopites were deposited in an anoxic and carbonate-supersaturated environment with a relatively high microbial biomass (Grotzinger and Kasting 1993;Grotzinger and Knoll 1999;Brocks et al. 2005;Tang et al., 2013c). Thus, under possible similar environmental conditions, the advent of loopites is controlled by the activities of the microbial community, which can be influenced by illumination and microtopography, and their morphologies and sizes are controlled by hydrodynamics. ...
Within the lower Wumishan Formation at the eastern edge of the Tai-hang Mountains in North China, a ~ 10 m stratigraphic interval contains alternately "bright and dark" laminites with enigmatic loop structures (2.5–27.5 cm in length and 0.6–12 cm in height), preserved in cross-sectional and named "loopites" in this study. The loopites are composed of cores and annulate laminations. Based on the different morphologies, they can be divided into three different types: type I, II and III. Although the loopites are similar to the loop beddings, the formation mechanisms are different. The former is possibly microbially induced sedimentary structures (MISS), while the loop beddings preserve evidence of soft-sediment deformation structures (SSDS) such as boudinage or chain structures, joints and small-scale tensional faults. All three types of loopites have cores. The type I core is made up of relicts of previous microbial mat and the microhighlands, while the type II and III loopites have cores defined by debris and rock fragments. The cores are completely wrapped by microbial mats of later generation. Thus, we can conclude that the formation of loopites is due to the growth, wrapping and deposition of microbial mats, while loop beddings are generated by external triggering mechanism such as earthquake. Furthermore, the discovery and possible formation of loopites may provide a new type of MISS and indicate a stable, anoxic and carbonate-supersaturated environment favorable for microbial mats to form annulate structures, which are controlled by illumination, microtopography and hydrodynamics.
... In the intervening Paleo and Meso-proterozoic interval, stromatolites with hybrid fabrics are most common, and this is argued to reflect growth by a combination of chemical precipitation and sediment trapping and binding. This transition has been suggested to represent a long-term decline in Precambrian seawater carbonate supersaturation (Grotzinger and Kasting 1993), with a consequent S switch in importance from chemical precipitation to microbial trapping and binding as the dominant mechanisms of stromatolite growth. This connection between seawater carbonate saturation and microbialite abundance has been further tested in the Phanerozoic with compilations of abundance data for calcified cyanobacteria and microbial carbonates (Riding and Liang 2005 and references therein) showing a positive correlation with marine carbonate saturation. ...
... 2− ] sw after the LJE is supported by the absence of sulfate in evaporites at~1.9 Ga (Grotzinger and Kasting, 1993;Pope and Grotzinger, 2003) and the occurrence of a fundamental change in the sedimentary sulfur isotopic composition (Och and Shields-Zhou, 2012;Scott et al., 2014). Although pseudomorphs replacing sulfate crystals have been reported from younger Paleoproterozoic and Mesoproterozoic successions (e.g., 1.7-1.6 ...
... The significance of the oxygen isotopic composition in Phanerozoic and Precambrian precipitates (such as chert and carbonate rocks) is still unresolved, as suggested by Veizer et al. (1999) and Jacobsen and Kaufman (1999). As per the works reviewed by Grotzinger and Kasting (1993), there were temporal changes in the chemistry of seawater and sediment during the Precambrian. ...
The term ‘chert’ ideally refers to fine-grained siliceous (micro/cryptocrystalline) mineral and is also often used for rock with such siliceous mineral aggregate of chemical, biochemical, and organic origin. Petrologically, inorganic non-sedimentary origin or even volcanic derivatives formed by devitrification of metastable felsic volcanic glass can also be included within chert. A new classification scheme for Precambrian cherts is proposed, especially for field workers. Despite several worldwide studies on chert, simple comprehensive classification of chert is not available to date. There are notable differences amongst Archaean, Palaeoproterozoic and Meso-Neoproterozoic cherts. This paper reviews all the Precambrian cherts to divide them into three categories from global context. Archaean and Palaeoproterozoic cherts mostly imply precipitation from silica gel material supplied vide submarine volcanism. This paper also focuses on diagenetic chert concretion, nodules, and geodes in detail. Finally, the Mesoproterozoic Nagari Formation in Cuddapah Basin, India is shown as a case to explain the diagenetic conditions, which could favour chert development by silica supersaturation in the pores. Diagenetic sub-environments are categorized systematically as eogenetic, mesogenetic, and telogenetic types with evidences of each based on photomicrography and outcrop studies. A comprehensive analysis is attempted to understand the development of concretions, nodules and geodes due to diagenesis with respect to the Eastern Ghats Orogeny, which has played a significant role in the prominent development of diagenetic features during mesodiagenetic and telodiagenetic processes.
... Albarede et al., 2020). It likely had a higher CO 2 content (Grotzinger and Kasting, 1993) and very low sulfate concentrations (Canfield and Raiswell, 1999) which would have had significant impact on the chemistry of hydrothermal fluids and their precipitates, regardless of tectonic setting. ...
... The ability to remain soluble under different conditions could represent an important selection factor during the formation of the early protein alphabet, as prebiotically relevant environments spanned from alkaline hydrothermal vents to acidic lakes. 24,25 We measured the solubility profiles of these peptide libraries in the pH range 3−11 and also at low vs high ionic strength, spectrophotometrically by absorption of peptide bonds at 215 nm and fluorometrically in case of library 19F due to its very poor solubility ( Figure 2). In general, ionic strength did not significantly modulate solubility, with the exception of the 11Y library (see below). ...
Whereas modern proteins rely on a quasi-universal repertoire of 20 canonical amino acids (AAs), numerous lines of evidence suggest that ancient proteins relied on a limited alphabet of 10 "early" AAs and that the 10 "late" AAs were products of biosynthetic pathways. However, many nonproteinogenic AAs were also prebiotically available, which begs two fundamental questions: Why do we have the current modern amino acid alphabet and would proteins be able to fold into globular structures as well if different amino acids comprised the genetic code? Here, we experimentally evaluate the solubility and secondary structure propensities of several prebiotically relevant amino acids in the context of synthetic combinatorial 25-mer peptide libraries. The most prebiotically abundant linear aliphatic and basic residues were incorporated along with or in place of other early amino acids to explore these alternative sequence spaces. The results show that foldability was likely a critical factor in the selection of the canonical alphabet. Unbranched aliphatic amino acids were purged from the proteinogenic alphabet despite their high prebiotic abundance because they generate polypeptides that are oversolubilized and have low packing efficiency. Surprisingly, we find that the inclusion of a short-chain basic amino acid also decreases polypeptides' secondary structure potential, for which we suggest a biophysical model. Our results support the view that, despite lacking basic residues, the early canonical alphabet was remarkably adaptive at supporting protein folding and explain why basic residues were only incorporated at a later stage of protein evolution.
... The SET, however, extends from present-day seawater, and Archean seawater had a distinctly different halogen signature ( Fig. 1A; Burgess et al., 2020). An Archean SET would have had its origin in the iodine-rich Archean seawater, and its trajectory would not have led toward the Pampalo halogen signatures because Archean evaporites were dominated by halite and sylvite (Grotzinger and Kasting, 1993), which would have resulted in roughly constant Br/I ratios in residual fluids, as is the case today (Fig. 1C). There is also no geological record of evaporites in the Ilomantsi greenstone belt that would support the presence of bittern brines, even if a hypothetical Archean SET did have a different trajectory than its present-day counterpart. ...
Halogens (Cl, Br, I) are exceptional provenance tracers in crustal fluids because their ratios are not strongly altered during most fluid-rock interaction processes. The halogen systematics of metamorphic fluids are of particular interest because such fluids are key drivers of crustal-scale element fluxes and ore formation in orogenic belts, but they remain poorly studied due to analytical challenges. We present novel triple-halogen laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) fluid-inclusion data from metamorphic systems ranging in age from Archean to Phanerozoic. Our results show that the halogen signatures in Phanerozoic metamorphic fluids are controlled by variable degrees of organic-matter interaction in their source rocks, leading to increased I/Cl and decreased Br/I ratios relative to seawater. By contrast, Archean metamorphic fluids from organic matter–rich source rocks have low I/Cl and very high Br/I ratios, distinctly different from any known fluid source signature. We propose that these signatures nevertheless are consistent with organic-matter interaction because dominantly prokaryotic Archean lifeforms did not yet produce iodine-bearing metabolites. This prevented biosequestration and accumulation of iodine-rich organic matter in sediments and imposed halogen signatures onto Archean metamorphic fluids entirely unlike those in younger fluids.
... Both natural observation of basaltic rocks associated with subduction zones and experiments involving the incorporation of volatiles into basaltic melts under high pressure and temperature indicate that such melts can contain up to ∼4 wt% H 2 O (Lesne et al., 2011a;Plank et al., 2013), up to 1500 ppm CO 2 (Lesne et al., 2011b), up to 3 to 4 wt% Cl (primarily as NaCl with lesser amounts of KCl and CaCl 2 ) (Webster et al., 1999(Webster et al., , 2015Lesne et al., 2011a;Thomas & Wood, 2022), and up to 2000 to 3000 ppm S (Mathez, 1976;Lesne et al., 2011a). The S would occur in an oxidised form as SO 3 since most of the S would likely have originated from subducted CaSO 4 -bearing oceanic sediments (Grotzinger & Kasting, 1993). In addition to a high solubility in basaltic melts (Mathez, 1976;O'Neill & Mavrogenes, 2002;Jugo et al., 2005;Metrich et al., 2009;Metrich & Mandeville, 2010;Morizet et al., 2010;de Moor et al., 2013;Plank et al., 2013), CaSO 4 has been shown to have a high solubility in saline to hypersaline fluids at high P-T as opposed to CO 2 (Newton & Manning, 2005). ...
Systematic changes in whole-rock chemistry, mineralogy, mineral textures, and mineral chemistry, are seen along a ca. 95 km traverse of late Archean granitoid orthogneisses in the Shevaroy Block, Eastern Dharwar Craton, southern India. The traverse passes from amphibolite-grade gneisses in the north to granulite-grade rocks (charnockite) in the south. Changes include whole-rock depletion of Rb, Cs, Th, and U in the granulite grade rocks as relative to the amphibolite grade gneisses, and oxidation trends regionally from highly oxidized granulite-facies rocks near the magnetite-hematite buffer to relatively reduced amphibolite-facies rocks below the fayalite-magnetite-quartz. Rare earth elements show limited mobility and are hosted a variety of minerals whose presence is dependent on the metamorphic grade ranging from titanite and allanite in the amphibolite-facies rocks to monazite in the vicinity of the orthopyroxene-in isograd to apatite in the granulite-facies charnockite.
Cathodoluminesence (CL) and back scattered electron (BSE) sub-grain imaging and Sensitive High-Resolution Ion MicroProbe (SHRIMP) analysis of zircon from 29 samples of dioritic, tonalitic, and granitic orthogneiss from the traverse reveals magmatic zircon cores that record the emplacement of the granitoid protoliths mostly about 2580 to 2550 Ma, along with a few older mid to late Archean tonalites. Protolith zircon was modified during metamorphism by overgrowth and/or replacement. Relative to igneous cores, U-enriched metamorphic zircon, dominant in the amphibolite-grade gneisses, formed at ca. 2530 Ma, predating retrograde titanite growth at ca. 2500 Ma. Uranium-depleted mantles grew on zircon between 2530 and 2500 Ma in granulite-grade samples south of the orthopyroxene-in isograd. In some of these samples, the U-depleted metamorphic zircon is preceded by mantles of U-undepleted zircon, indicating a progression of metamorphic zircon growth with increasing depleted compositions between 2530 and 2500 Ma .
With increasing metamorphic grade (from amphibolite to granulite) and oxidation state, allanite and monazite disappear from the assemblage and zircon became depleted in U and Th. Whole-rock U-Th compositions became decoupled from relict magmatic zircon compositions, reflecting the development of U-depleted magmatic zircon and indicating that whole-rock chemical differences along the traverse were produced during metamorphism, rather than just reflecting differences in dioritic vs. granitic protoliths. Although in situ anatexis and melt extraction may have played a role, whole-rock and zircon depletion of trace elements can be explained by the action of externally-derived, oxidizing, low-H2O activity hypersaline fluids migrating up through the mid to lower crust. Fluids and element migration during metamorphism may be the end result of subduction related processes that cumulated in the collision and concatenation of island arcs and continental blocks. These tectonic processes assembled the Dharwar Craton at the end of the Archean.
... Previously it has been proposed that Precambrian seawater Fe 2+ concentrations under steady-state conditions were buffered by the solubility of siderite, corresponding to values of 0.01-0.12 mM (Grotzinger and Kasting, 1993). This assumption has, however, been recently challenged based on the solubility of ferrous carbonates in ferrous silicate-carbonate systems, suggesting that greenalite, not siderite, was the predominant control on dissolved Fe 2+ concentrations (Jiang and Tosca, 2019). ...
Iron in the early anoxic oceans of Archean age (4000-2500 million years ago) is believed to have been oxidized to form banded iron formations (BIF). Previously, it has been proposed that iron was oxidized either by free oxygen, H2O2, microbial oxidation, or photo-oxidation. However, these mechanisms are difficult to reconcile with evidence for the oceans at that time having been largely devoid of dissolved oxygen and oxidants, together with the rarity of microbial remains in BIF and restrictively slow rates of photo-oxidation. Experiments reported here show that ferrous iron readily oxidizes in analogs of Archean anoxic seawater following the precipitation of ferrous hydroxide. Once precipitated, ferrous hydroxide undergoes decomposition to elemental iron that reacts with water at room temperature to form ferric iron and release hydrogen gas. The ferric iron may then be incorporated into green rust, a mixed ferrous-ferric phase that ages into iron minerals commonly found in BIF. Our finding suggests that anoxic iron oxidation may have contributed to the formation of oxide-facies BIF, especially Algoma-type BIF that likely formed in semi-restricted basins where ferrous hydroxide saturation was more easily achieved. Additionally, ferrous hydroxide decomposition would have contributed to early Earth's oxidation, as a result of hydrogen escape to space, thus providing new insights into environmental and biological conditions on early Earth.
... The Archean ocean composition is poorly constrained which is mainly dominated by volcano-exhalative activity and rock-water interactions (Raudsepp, 2012). Atmospheric CO 2 was very high, pH was >6.5, the temperatures were >57°C and the salinity was comparable with the modern seawater (Holland and Kasting 1992;Grotzinger and Kasting, 1993). Metal redox states were prevalent in the Archean oceans due to anoxic ocean water and atmospheric conditions. ...
The Banded iron formations (BIFs) and manganese formations of Chitradurga, Shimoga and Sandur greenstone belts of Dharwar craton, associated with the stromatolitic carbonates, carbonaceous phyllites and shales along with gold mineralization, are best geological entities to evaluate the Archean biogeochemical processes and transformation of a habitable Earth. The geochemical anomalies along with C, O and S isotopic signatures of stromatolitic carbonates, carbonaceous phyllites and sulphidic BIFs reflect on biogenic signatures, fluctuating Archean ocean temperatures from 25–75°C and anoxic to euxinic redox conditions. The U-Pb detrital zircon ages of these stromatolitic carbonates indicate 3.5 to 2.6 Ga whereas the carbonaceous shales indicate 3.2–2.2 Ga reflecting the transportation of organic matter to the ocean basin during the growth of stromatolitic carbonates. The gold content of carbonaceous phyllites and sulphidic BIFs indicates hydrothermal source. The O2 produced due to stromatolitic activity has deposited Mn and Fe of the Archean oceans as BIFs and Mn formations. The biogenic matter of the stromatolites along with other siliciclastic material, gold and sulphides derived from the volcanic activity mixed and formed as carbonaceous shales in the ocean basin under euxinic conditions. The comprehensive geological, geochemical including isotopic studies on these rock types collectively indicate the interaction of lithosphere-hydrosphere-atmosphere-biosphere in the Archean oceans which paved the way for the advanced forms of life.
... Compared with Phanerozoic strata, sulfate minerals are relatively rare in Precambrian sedimentary archives (Warren 2021). This uneven distribution of sulfate minerals in the geological record is typically explained as a consequence of much lower sulfate concentrations in Precambrian seawater (Grotzinger and Kasting 1993;Kah et al. 2004;Canfield and Farquhar 2009;Bristow and Grotzinger 2013;Algeo et al. 2015;Blättler et al. 2020). Wherever they occur in Precambrian marine strata, sulfate minerals are often used as direct mineralogical evidence for a transient increase in sulfate levels indicative of an increasingly oxygenated environment (Kah et al. 2001;Melezhik et al. 2005;Schröder et al. 2008;Reuschel et al. 2012;Turner and Bekker 2016;Blättler et al. 2018;Prince et al. 2019). ...
Compared with the Phanerozoic strata, sulfate minerals are notably rare in the Precambrian record largely due to lower concentrations of sulfate in dominantly anoxic oceans. Here, we present a compilation of sulfate minerals, including diagenetic barite (BaSO4), pseudomorphs of gypsum (CaSO4·2H2O) and anhydrite (CaSO4), and celestine (SrSO4) that are stratigraphically associated with the Ediacaran Shuram excursion (SE) — arguably the largest negative carbon isotope excursion in Earth history. In this study, we investigated 15 SE-equivalent sections including: the upper Doushantuo Formation in South China, the Shuram Formation in Oman, the Wonoka Formation in Australia, the Krol Group in India, the Nama Group in southern Namibia, the Rainstrom Member in western USA, the upper Clemente Formation in Mexico, and the Alyanchskaya Formation in Siberia. All of the studied successions reveal the presence of sulfate minerals and/or concentration enrichment in carbonate-associated sulfate ([CAS]). The sulfur isotope (δ34S) trends of both sulfate and sulfide during the SE show progressively decreasing values, suggesting an increasing sulfur reservoir coincident with independent U isotope evidence for oxygenation of the oceans. To test the hypothesis of an increased oceanic sulfate level, we conducted a petrographic search for evidence of sulfate-bearing minerals, including barite, evaluated stratigraphic trends of barium concentration ([Ba]) in five SE-equivalent sections, with a primary focus on South China. Where [Ba] data are available, it reveals considerable enrichments relative to pre- and post-SE intervals. Based on our compiled observations, we propose that elevated seawater sulfate concentrations during the SE — triggered by enhanced oxidative weathering of terrestial pyrite or evaporite dissolution — faciliated the formation of sulfate minerals as primary precipitates and secondary authigenic cements. At the same time, a larger influx of dissolved Ba to the Ediacaran basins further faciliated barite deposition. Enhanced seawater sulfate concentrations would also stimulate microbial sulfate reduction and anaerobic oxidation of organic matter (including methane) in the sulfate-methane transition zone (SMTZ). Our study highlights the dynamic interplay of biogeochemical C, S, and Ba cycles in increasingly oxygenated Ediacaran surface environments.
... Compared with Phanerozoic strata, sulfate minerals are relatively rare in Precambrian sedimentary archives (Warren 2021). This uneven distribution of sulfate minerals in the geological record is typically explained as a consequence of much lower sulfate concentrations in Precambrian seawater (Grotzinger and Kasting 1993;Kah et al. 2004;Canfield and Farquhar 2009;Bristow and Grotzinger 2013;Algeo et al. 2015;Blättler et al. 2020). Wherever they occur in Precambrian marine strata, sulfate minerals are often used as direct mineralogical evidence for a transient increase in sulfate levels indicative of an increasingly oxygenated environment (Kah et al. 2001;Melezhik et al. 2005;Schröder et al. 2008;Reuschel et al. 2012;Turner and Bekker 2016;Blättler et al. 2018;Prince et al. 2019). ...
Compared with Phanerozoic strata, sulfate minerals are relatively rare in the Precambrian record likely due to the lower concentrations of sulfate in dominantly anoxic oceans. Here, we present a compilation of sulfate minerals that are stratigraphically associated with the Ediacaran Shuram excursion (SE) — the largest negative δ ¹³ C excursion in Earth history. We evaluated 15 SE sections, all of which reveal the presence of sulfate minerals and/or concentration enrichment in carbonate-associated sulfate, suggesting a rise in sulfate reservoir. Notably, where data are available, the SE also reveals considerable enrichments in [Ba] relative to pre- and post-SE intervals. We propose that elevated seawater sulfate concentrations during the SE may have faciliated authigenesis of sulfate minerals. At the same time, the rise of Ba concentrations in shelf environments further facilitated barite deposition. A larger sulfate reservoir would stimulate microbial sulfate reduction and anaerobic oxidation of organic matter (including methane), contributing to the genesis of the SE. The existence of sulfate minerals throughout the SE suggests that oxidant pools were not depleted at that time, which challenges previous modelling results. Our study highlights the dynamic interplay of biogeochemical C, S, and Ba cycles in response to the Shuram oxygenation event.
Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system
Supplementary material: https://doi.org/10.6084/m9.figshare.c.5602560
... The search for at least two independent proxies to constrain the carbonate system has proven challenging beyond the Cenozoic. Of the six variables, only atmospheric pCO 2 is moderately well-constrained for the Phanerozoic (e.g., Foster et al., 2017;Royer, 2006), but it has larger uncertainties in the Precambrian (Blättler et al., 2017;Grotzinger & Kasting, 1993). Surface ocean Ω with respect to carbonate minerals can be calculated by tracking the carbonate compensation depth (Pälike et al., 2012;Tyrrell & Zeebe, 2004) but subduction of oceanic lithosphere limits these records beyond the Jurassic. ...
Ocean chemistry and carbonate sedimentation link Earth's climate, carbon cycle, and marine pH. The carbonate system in seawater is complex and there are large uncertainties in key parameters in deep time. Here, we link sedimentary textures formed in arid coastal environments and preserved in the rock record to past seawater carbonate chemistry. Prior to the mid‐Mesozoic, tepee structures and pisoids – features associated with peritidal environments – co‐vary with available shelf area during cycles of supercontinent formation and rifting. In contrast, tepees and pisoids are consistently scarce after the mid‐Mesozoic, which coincides with a radiation in pelagic calcifiers as well as the breakup of Pangea. Numerical models suggest that the global and temporal abundances of tepee structures and pisoids are correlated with secular shifts in seawater chemistry, and that trends likely reflect the underlying influence of tectonics and biotic innovation on marine alkalinity and the saturation states of carbonate minerals. As independent sedimentary proxies, tepees and pisoids serve as benchmarks for global carbon cycle models and provide a new proxy record of seawater chemistry that can help discern links among tectonics, biotic innovation, and seawater chemistry.
... Despite the difficulties in accurately estimating the composition of Paleoarchean seawater interacting with the Dresser Caldera, the zoned mineral assemblages developed in the Mount Ada Basalt (Units III, IV, and V of Terabayashi et al., 2003) can be utilized as a reliable reference for Paleoarchean seawater-dominated hydrothermal alteration (Terabayashi et al., 2003). The alteration assemblages in the Mount Ada Basalt are not related to hydrothermal silica veins, and are largely carbonate-bearing reflecting high CO 2 concentrations in the Paleoarchean atmosphere and oceans (Grotzinger and Kasting, 1993;Rouchon and Orberger, 2008;Terabayashi et al., 2003). Furthermore, the Mount Ada alteration assemblages form stratigraphically-related patterns that are remarkably similar to those from modern mid-ocean ridges. ...
Hydrothermal fluids played a key role in establishing the environmental conditions in which ancient stromatolites grew within the North Pole Chert of the ~3.48 Ga Dresser Formation (Pilbara Craton, Western Australia). However, there has been uncertainty as to the physicochemical conditions of the hydrothermal system in relation to (i) the distribution of hydrothermal alteration, (ii) the relative contribution of seawater and/or magmatic volatiles to the hydrothermal fluids, and (iii) the origin of some of the major elements mobilized in the hydrothermal fluids.
This study examines the hydrothermal alteration of the underlying North Star Basalt in order to better understand the nature of the circulating fluids and to determine the processes responsible for the transport and accumulation of metals and metalloids to the near surface environment. Detailed geological mapping reveals a complex distribution of alteration mineral assemblages that is controlled at all stratigraphic depths by the distance to the major fluid pathways that are now represented by hydrothermal silica veins. With increasing distance from the vein margins, alteration assemblages change from argillic (kaolinite–quartz) to phyllic (illite–quartz), and then to propylitic (chlorite–albite–epidote) and actinolitic (actinolite–albite–chlorite–epidote) at more distal positions. Illite Ar–Ar dating of argillic–altered basalt proximal to major hydrothermal veins immediately below the North Pole Chert confirms a syn–depositional hydrothermal origin of alteration, and demonstrates that the mineralogical and chemical features developed through the circulation of hydrothermal fluids were largely preserved after subsequent thermal overprints at 3.25, 3.06, and 2.29 Ga.
The spatial distribution of the alteration mineral assemblages indicates that the fluids circulating in the hydrothermal system were highly acidic (pH < 3) for at least some time during the evolution of the Dresser Caldera. Such highly acidic fluid conditions were likely promoted by the input of magmatic volatile phases, such as HCl, SO2, H2S, and F. Bulk geochemical analyses of altered basalts reveal that large amount of metals, including Fe, Mg, Ni, and Zn, were leached from the North Star Basalt during hydrothermal alteration and delivered to the surface. Furthermore, our data indicate that K and Ba were introduced into the hydrothermal system from external reservoir(s). Although the contribution of K–rich seawater cannot be completely discounted, we argue that the bulk of K and Ba was sourced from an underlying magma chamber undergoing fractional crystallization of a melt with TTG–like composition.
... We do not currently include precipitation of gypsum (CaSO 4 ) in our model description (Fig. 4b). Gypsum is an evaporite mineral that precipitates during regional and episodic events of supersaturation and was likely a less important sulfur sink on a globally integrated basis during Precambrian time or during any other period in which ocean [SO 2− 4 ] was relatively low (Grotzinger and Kasting, 1993;Crowe et al., 2014;Fakhraee et al., 2019). Indeed, there is still some debate as to the time-integrated impact of sulfate evaporites on the steady-state global sulfur cycle even during more recent periods of Earth's history (Halevy et al., 2012;Canfield, 2013). ...
The coupled biogeochemical cycles of iron and sulfur are central to the long-term biogeochemical evolution of Earth's oceans. For instance, before the development of a persistently oxygenated deep ocean, the ocean interior likely alternated between states buffered by reduced sulfur (“euxinic”) and buffered by reduced iron (“ferruginous”), with important implications for the cycles and hence bioavailability of dissolved iron (and phosphate). Even after atmospheric oxygen concentrations rose to modern-like
values, the ocean episodically continued to develop regions of euxinic or ferruginous conditions, such as those associated with past key intervals of organic carbon deposition (e.g. during the Cretaceous) and extinction events (e.g. at the Permian–Triassic boundary). A better understanding of the cycling of iron and sulfur in an anoxic ocean, how geochemical patterns in the ocean relate to the available spatially heterogeneous
geological observations, and quantification of the feedback strengths between nutrient cycling, biological productivity, and ocean redox requires a spatially resolved representation of ocean circulation together with
an extended set of (bio)geochemical reactions.
Here, we extend the “muffin” release of the intermediate-complexity Earth
system model cGENIE to now include an anoxic iron and sulfur cycle (expanding the existing oxic iron and sulfur cycles), enabling the model to simulate ferruginous and euxinic redox states as well as the precipitation of reduced iron and sulfur minerals (pyrite, siderite, greenalite) and
attendant iron and sulfur isotope signatures, which we describe in full.
Because tests against present-day (oxic) ocean iron cycling exercises only a small part of the new code, we use an idealized ocean configuration to explore model sensitivity across a selection of key parameters. We also present the spatial patterns of concentrations and δ56Fe and
δ34S isotope signatures of both dissolved and solid-phase Fe and S species in an anoxic ocean as an example application. Our sensitivity analyses show that the first-order results of the model are relatively robust against the choice of kinetic parameter values within the Fe–S system and that simulated concentrations and reaction rates are comparable to those observed in process analogues for ancient oceans (i.e. anoxic lakes). Future model developments will address sedimentary recycling and benthic iron fluxes back to the water column, together with the coupling of nutrient (in particular phosphate) cycling to the iron cycle.
... In cGENIE, the eventual sink for sulphur is the precipitation of pyrite (reaction 19) (ignoring sulphide reacting with organic matter -see Hülse et al. (2019)). We do not currently include precipitation of gypsum (CaSO 4 ) in our model description (Fig. 3) -a sink likely less important on a globally integrated basis during Precambrian time, or during any other period in which 15 ocean [SO 2− 4 ] was relatively low (Grotzinger and Kasting, 1993;Crowe et al., 2014;Fakhraee et al., 2019). Planned future developments to cGENIE will incorporate an explicit gypsum cycle. ...
The coupled biogeochemical cycles of iron and sulphur are central to the long-term biogeochemical evolution of Earth's oceans. For instance, before the development of a persistently oxygenated deep ocean, the ocean interior likely alternated between states buffered by reduced sulphur ('euxinic') vs. buffered by reduced iron ('ferruginous'), with important implications for the cycles and hence bioavailability of dissolved iron (and phosphate). Even after atmospheric oxygen concentrations rose to modern-like values, the ocean continued, episodically, to develop regions of euxinic or ferruginous conditions, 5 such as associated with past key intervals of organic carbon deposition (e.g. during the Cretaceous) as well as extinction events (e.g. at the Permian/Triassic boundary). A better understanding of the cycling of iron and sulphur in an anoxic ocean, how geochemical patterns in the ocean relate to the available spatially heterogeneous geological observations, and quantification of the feedback strengths between nutrient cycling, biological productivity, and ocean redox, requires a spatially-resolved representation of ocean circulation together with an extended set of (bio)geochemical reactions. 10 Here, we extend the 'muffin' release of the intermediate-complexity Earth system model cGENIE, to now include an anoxic iron and sulphur cycle, enabling the model to simulate ferruginous and euxinic redox states as well as the precipitation of reduced iron and sulphur minerals (pyrite, siderite, greenalite) and attendant iron and sulphur isotope signatures, which we describe in full. While we cannot make direct model comparison with present-day (oxic) ocean observations, we use an idealized ocean configuration to explore model sensitivity across a selection of key parameters. We also present the spatial patterns of 15 concentrations and δ 56 F e isotope signatures of both dissolved and solid-phase Fe species in an anoxic ocean as an example application. Our sensitivity analyses show how the first-order results of the model are relatively robust against the choice default kinetic parameters within the Fe-S system, and that simulated concentrations and reaction rates are comparable to those observed in process analogues for ancient oceans (i.e., anoxic lakes). Future model developments will address sedimentary recycling and benthic iron fluxes back to the water column, together with the coupling of nutrient (in particular phosphate) 20 cycling to the iron cycle.
... The physical basis of marine phosphorite formation is poorly constrained due to the number of possible reactions, both inorganic and biologically mediated, that may help to establish calcium phosphate and carbonate supersaturated porewaters (Kolodny, 1969;Sheldon, 1981;Cook and Shergold, 1986;Kholodov, 2014;Oxmann and Schwendenmann, 2014). Calcium and fluoride ions in both modern and Cambrian seawaters are/were readily available for the precipitation of fluorapatite and carbonatefluorapatite (Kazmierczak and Kempe, 1984;Tucker, 1992;Grotzinger and Kasting, 1993;Lowenstein et al., 2001;Brennan et al., 2004); however, biological utilization of phosphorus typically results in low phosphate availability (Filippelli, 2008), limiting the potential for calcium phosphate mineralization. Therefore, phosphorite formation requires a mechanism to generate elevated levels of phosphate ions. ...
This paper addresses the taphonomic processes responsible for fossil preservation in calcium phosphate, or phosphatization. Aside from silicification and rarer examples of carbonaceous compression, phosphatization is the only taphonomic mode claimed to preserve putative subcellular structures. Because this fossilization window can record such valuable information, a comprehensive understanding of its patterns of occurrence and the geochemical processes involved in the replication of soft tissues are critical endeavors. Fossil phosphatization was most abundant during the latest Neoproterozoic through the early Paleozoic, coinciding with the decline of non-pelletal phosphorite deposits. Its temporal abundance during this timeframe makes it a particularly valuable window for the study of early animal evolution. Several occurrences of phosphatization from the Ediacaran through the Permian Period, including Doushantuo-type preservation of embryo-like fossils and acritarchs, phosphatized gut tracts within Burgess Shale-type carbonaceous compressions, Orsten-type preservation of meiofaunas, and other cases from the later Paleozoic are reviewed. In addition, a comprehensive description of the geochemical controls of calcium phosphate precipitation from seawater is provided, with a focus on the rates of phosphate nucleation and growth, favorable nucleation substrates, and properties of substrate tissue and pore-fluid chemistry. It is hoped that the paleontological and geochemical summaries provided here offer a practical and valuable guide to the Neoproterozoic–Paleozoic phosphatization window.
... The stone records indicate that water was present, and surface temperatures were at approximately 85 to 110°C (Kasting & Ackerman, 1986). Oxygen was absent (Canfield, 2005;Lyons & Gill, 2010), and due to the high CO2 partial pressure, the Hadean ocean was likely mildly acidic (Grotzinger & Kasting, 1993). Life on Earth is carbon-based, and consequently, the formation of life requires the formation of carbon-carbon bonds from inorganic precursor molecules under the aforementioned conditions. ...
... Mesoproterozoic through Phanerozoic time (Grotzinger & Kasting, 1993; Pope & 80 Grotzinger, 2003 Each proxy material has different, irregularly spaced temporal distributions (Figure 1a). To 143 estimate Phanerozoic δ 34 S trends, each proxy record was interpolated at 50 kyr resolution 144 (Figure 1b). ...
The δ34S of seawater sulfate reflects processes operating at the nexus of sulfur, carbon, and oxygen cycles. However, knowledge of past seawater sulfate δ34S values must be derived from proxy materials that are impacted differently by depositional and post-depositional processes. We produced new timeseries estimates for the δ34S value of seawater sulfate by combining 6710 published data from three sedimentary archives—marine barite, evaporites, and carbonate-associated sulfate—with updated age constraints on the deposits. Robust features in multiple records capture temporal trends in the δ34S value of seawater and its interplay with other Phanerozoic geochemical and stratigraphic trends. However, high-frequency discordances indicate that each record is differentially prone to depositional biases and diagenetic overprints. The amount of noise, quantified from the variograms of each record, increases with age for all δ34S proxies, indicating that post-depositional processes obscure detailed knowledge of seawater sulfate’s δ34S value deeper in time.
Understanding the marine environment of early Earth is crucial for understanding the evolution of climate and early life. However, the master variable of Archean and Proterozoic seawater, the pH, is poorly constrained, and published ideas about the pH range encompass ~7 pH units from mildly acidic to hyperalkaline. To better infer ancient seawater pH, we examine the possibility of a seawater pH proxy using rare earth elements (REEs) in marine carbonates. The principle is based on increasing concentrations of heavy rare earth elements in solution relative to the light REEs with decreasing pH due to REE complexation and scavenging. We calibrated such an REE pH proxy using pH variability in modern seawater and tested the proxy with ~100 REE measurements from 13 separate carbonate formations. We compared our pH estimates derived from the REE proxy to published pH estimates of Cenozoic and Neoproterozoic seawater that use the established pH proxy of boron isotopes (δ ¹¹ B). REE-pH estimates agree with the Cenozoic and the Ediacaran δ ¹¹ B-pH proxy based on the type of carbonate and boron isotopic composition at corresponding times. The uncertainty in our REE-pH proxy can probably be explained by model assumptions, noise from freshwater influence, siliciclastic input, and diagenesis. This proof-of-concept study demonstrates that the REE-pH method provides pH estimates comparable to boron isotope pH estimates within uncertainties, which potentially could constrain changes in Precambrian seawater pH to better understand the coevolution of life and early Earth’s environment.
Carbonate I/(Ca + Mg) has been used as a proxy to track shallow-seawater oxygen levels through Earth's history. However, due to diagenetic alteration and homogenization of iodine in carbonates formed in a redox-stratified water column or in porewater, bulk-rock I/(Ca + Mg) values—and thus the oxygen levels in Precambrian shallow seawater—could have been significantly underestimated. Here, we report a mineralogy-based sequential dissolution method using dilute nitric acid (0.03% v/v) to obtain I/(Ca + Mg) values of water-column precipitated calcite during the ∼1.57 Ga oxygenation event in North China. The results show that at the peak of the oxygenation event, the I/(Ca + Mg) ratios of primary calcites are up to ∼11 μmol/mol, which are significantly higher than the bulk-rock I/(Ca + Mg) values (up to ∼4 μmol/mol). The new data imply that local shallow seawater O2 concentrations at ∼1.57 Ga were higher than previously estimated and sufficient to support the respiratory needs of eukaryotes including animals. The delay of complex eukaryote and ecosystem evolution during the mid-Proterozoic (1.8–0.8 Ga) was not due to the lack of local oxic niches for eukaryotes but a consequence of temporal and spatial redox instability in shallow-marine environments.
The chapter reflects on events occurring in the several-billion-year time interval that linked apatite geochemistry with the formation of molecules critical to the emergence of life. Among its topics, the chapter addresses prebiotic chemistry and minerals as bioactive surfaces, nucleoside and nucleotide synthesis from simple molecules, mineral and the RNA World hypothesis, and skeletonization as a crucial evolutionary step. In this context, minerals provided a surface for adsorption and concentration of organic compounds and inorganic ions; they afforded a surface for chemical reactions; they served as a source of chemical reagents; and, for some minerals, they possessed a chirality which dictated structural features of the adsorbed reagent. From this perspective, it is plausible to consider the possibility that apatite as well as clays, silica, calcium carbonate and many other minerals enhanced reactions leading to the synthesis of more complex molecules, especially those associated with membrane function, replication and information storage. In the succeeding evolution of living organisms, the fossil record indicates that, within the Cambrian period, the tissue deposition of mineral was so widespread that it was evident in not only eukaryotes, but also prokaryotes, including bacteria and archaea. During this time, there is also abundant evidence of the appearance of both bone- and cartilage-like tissues as well as primitive dental tissues. Moreover, the record suggests that the mineralization step was so critical that it was expressed a number of different times through the millennia.
Within the lower Wumishan Formation at the eastern edge of the Tai-hang Mountains in North China, a ~ 10 m stratigraphic interval contains of alternately “bright and dark” laminites, with enigmatic, cross-sectional preserved loop structures (2.5 ~ 27.5 cm in length and 0.6 ~ 12 cm in height), named as “loopites” by this study. Based on the different morphologies and formations, the loopites, composed of the cores and annulate laminations, can be divided into three different types which are type Ⅰ, Ⅱ and Ⅲ. Despite the loopites are similar to the loop beddings of soft-sediment deformation structures, we suggest that they are previously undescribed microbial mat structures (MMS). The formation of type Ⅰ is interpreted as the initial microbial mat, grows on the micro-highland of carbonate deposits, are wrapped by the subsequent microbial mats. The core is the initial microbial mat, but the micro-highland, which are formed by increased carbonate sedimentation rate, are not wrapped together. In contrast, the formation of type Ⅱ and Ⅲ is interpreted that the micro-highlands, which also can be treated as the cores and may be formed by the thrombolites, rock debris and the fragments of microbial mats lie on the flat microbial mats, are wrapped by the top-covered and underlaid microbial mats. In consequence, differing from the earthquake-induced loop beddings, the formation of loopites is due to the growth, wrapping and deposition of the microbial mats. Furthermore, the discovery and the possible formation of the loopites may provide a new type of MMS and indicate a stable, anoxic and carbonate supersaturated environment with a relatively weak hydrodynamics for microbial mats to form the annulate structures which is controlled by illumination, microtopography and hydrodynamics.
The nitrogen isotopic composition of organic matter is controlled by metabolic activity and redox speciation and has therefore largely been used to uncover the early evolution of life and ocean oxygenation. Specifically, positive δ15 N values found in well-preserved sedimentary rocks are often interpreted as reflecting the stability of a nitrate pool sustained by water column partial oxygenation. This study adds much-needed data to the sparse Paleoarchean record, providing carbon and nitrogen concentrations and isotopic compositions for more than fifty samples from the 3.4 Ga Buck Reef Chert sedimentary deposit (BRC, Barberton Greenstone Belt). In the overall anoxic and ferruginous conditions of the BRC depositional environment, these samples yield positive δ15 N values up to +6.1‰. We argue that without a stable pool of nitrates, these values are best explained by non-quantitative oxidation of ammonium via the Feammox pathway, a metabolic co-cycling between iron and nitrogen through the oxidation of ammonium in the presence of iron oxides. Our data contribute to the understanding of how the nitrogen cycle operated under reducing, anoxic, and ferruginous conditions, which are relevant to most of the Archean. Most importantly, they invite to carefully consider the meaning of positive δ15 N signatures in Archean sediments.
Authigenic carbonate (AC) forms in siliciclastic marine sediments in part as a result of the degradation of organic matter and methane. Degradation pathways vary considerably and each impact the chemical evolution of sediments and porewater differently. The relative importance of these reactions in contributing to carbonate authigenesis are poorly understood, especially from a global perspective. Modern porewater geochemical data and authigenic carbonate carbon isotope compositions (δ¹³Cac) of globally distributed marine sediment sites allow direct assessment of the relative importance of diagenetic pathways. Common correlations between bulk δ¹³Cac and depth reveal that AC tends to form progressively by multiple reaction pathways, rather than within a particular sediment horizon. A general lack of shallow AC within sediments 1) containing porewater sulfate and 2) exhibiting decreasing dissolved inorganic carbon (DIC) δ¹³C with depth suggest that organotrophic sulfate reduction does not promote significant authigenesis. Instead, the anaerobic oxidation of methane (AOM) may (in part) account for most AC expressing ¹³C-depleted isotope compositions, including those with δ¹³C values between −25 (the approximate marine organic matter value) and 0‰ VPDB. Widespread increases in δ¹³Cac with depth in conjunction with values that exceed seawater DIC compositions indicate precipitation in sediments exhibiting methanogenesis, likely aided by contemporaneous marine silicate weathering. Deeper AC formation may occur in sediments experiencing thermal decarboxylation. When data from all sites are considered collectively, sediments experiencing AOM and methanogenesis emerge as the most significant in yielding AC. The relative importance of authigenesis pathways controls potential impacts of AC deposition on marine carbon budgets and provides insight into the geochemical signatures exhibited by ancient carbonate concretions.
Significance
Halogens play a critical role in biochemistry and are useful to understand how planets formed and evolved. As we found that the traditional way of constraining the halogen budget within Earth is unreliable, we developed a method that better utilizes relevant geochemical data and estimated halogen abundances in various silicate reservoirs of Earth. Our halogen budget indicates that the majority of halogens are more concentrated in the mantle than in the surface and suggests that halogens have likely experienced early degassing and subsequent net regassing. This study also provides an important key to deciphering the geological history of ocean chemistry.
This chapter covers a lot of territory including the atmosphere and hydrosphere and their interactions with each other through time. It discusses Earth's primitive atmosphere and oceans as well as the postlunar formation atmosphere, including atmospheric composition and the faint young Sun paradox. It includes a discussion of the carbon and sulfur cycles and carbon and sulfur isotopic anomalies and a discussion of paleoclimates both in the Phanerozoic and in the Precambrian. It summarizes the geologic history of oceans including secular changes in seawater composition and variations in temperature and volume of seawater through time.
The large-scale dynamics of ocean oxygenation have changed dramatically throughout Earth's history, in step with major changes in the abundance of O 2 in the atmosphere and changes to marine nutrient availability. A comprehensive mechanistic understanding of this history requires insights from oceanography, marine geology, geochemistry, geomicrobiology, evolutionary ecology, and Earth system modeling. Here, we attempt to synthesize the major features of evolving ocean oxygenation on Earth through more than 3 billion years of planetary history. We review the fundamental first-order controls on ocean oxygen distribution and summarize the current understanding of the history of ocean oxygenation on Earth from empirical and theoretical perspectives—integrating geochemical reconstructions of oceanic and atmospheric chemistry, genomic constraints on evolving microbial metabolism, and mechanistic biogeochemical models. These changes are used to illustrate primary regimes of large-scale ocean oxygenation and to highlight feedbacks that can act to stabilize and destabilize the ocean–atmosphere system in anoxic, low-oxygen, and high-oxygen states.
Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Placozoa remain an ancient multicellular system with a dynamic body structure where calcium ions carry out a primary role in maintaining the integrity of the entire animal. Zinc ions can compete with calcium ions adsorption. We studied the effect of zinc ions and l-cysteine molecules on the interaction of Trichoplax sp. H2 cells. The regularity of formless motion was diminished in the presence of 20–25 μM of Zn2+ ions leading to the formation of branching animal forms. Locomotor ciliated cells moved chaotically and independently of each other leaving the Trichoplax body and opening a network of fiber cells. Application of 100 μM cysteine resulted in dissociation of the plate into separate cells. The combined chemical treatment shifted the effect in a random sample of animals toward disintegration, i.e. initially leading to disorder of collective cell movement and then to total body fragmentation. Two dissociation patterns of Trichoplax plate as “expanding ring” and “bicycle wheel” were revealed. Analysis of the interaction of Ca2+ and Zn2+ ions with cadherin showed that more than half (54%) of the amino acid residues with which Ca2+ and Zn2+ ions bind are common. The contact interaction of cells covered by the cadherin molecules is important for the coordinated movements of Trichoplax organism, while zinc ions are capable to break junctions between the cells. The involvement of other players, for example, l-cysteine in the regulation of Ca2+-dependent adhesion may be critical leading to the typical dissociation of Trichoplax body like in a calcium-free environment. A hypothesis about the essential role of calcium ions in the emergence of Metazoa ancestor is proposed.
During the Aptian (Cretaceous), in what is now the South Atlantic, the largest chemogenic (abiotic) carbonate factory so far identified in the Phanerozoic geological record developed as a vast hyper‐alkaline lake system. This covered at least 330 000 km², producing carbonates, locally over 500 m thick, in what are now the offshore Santos and Campos Basins (Brazil), and Kwanza Basin (Angola). Current evidence supports the view that almost all of this carbonate was chemogenic in origin, precipitated from hyper‐alkaline, shallow lake waters, probably by evaporation. This unit, best documented from offshore Brazil and known as the Barra Velha Formation (Santos Basin) and the Macabu Formation (Campos Basin), consists of just two basic carbonate components, millimetre to centimetre sized crystal shrubs and spherulites. These are commonly in situ but can also be reworked into a range of detrital facies. Demonstrable microbialites are generally rare. These carbonates are associated with Mg silicates (as clays) which had a profound influence not only on the textural development of the in situ carbonates, but also on their diagenesis. The dissolution of the clays produced much of the porosity in these limestones, which are the hosts for multi‐billion barrel oil fields. The source of the carbonate was most likely from metasomatic alteration of mafic rocks such as continental flood basalts related to Atlantic opening, with some contribution from much older continental basement. Clear evidence that serpentinization of possible exhumed mantle is lacking but mantle CO2 is likely to have been a critical factor in determining the composition of the fluids from which the carbonates formed and the high alkalinities of the lake waters.
Constraints on the biochemical composition of life derive from the relative abundance of elements in the Universe and at the surface of the Earth. Earth formed about 4.5 billion years ago (bya), and its atmosphere and oceans were derived from gaseous elements that were delivered as part of solid materials during the accretion process. Life arose 3.5–3.8 bya by the condensation of materials to primitive organic forms. Although evidence for photosynthetic forms extends to 3.5 bya, Earth's atmosphere remained anoxic until about 2.5 bya. The release of oxygen to Earth's surface allowed a rapid proliferation of the oxidation and reduction metabolisms that control Earth's biogeochemistry today. Mars and Venus remain the best replicates for how Earth might look in the absence of life.
The degree of saturation of CaC03 in the Pacific and Atlantic oceans has been calculated
from measurements of the partial pressure of CO2 gas and of the total content of dissolved inorganicCO2. Lyman's apparent dissociation constants for carbonic and boric acid at 1 bar, MacIntyre's apparent solubility product of CaC03 in seawater at 1 bar, and the Disteches' determinations of the pressure dependence of these parameters were used for the calculation.The results indicate that the crossover from supersaturation to undersaturation for CaCO3 occurs at a water depth between 500 and 3000 meters for calcite and at about 300 meters for aragonite in the Pacifiic, and between 4000 and 5000 meters for calcite and between 1000 and 2500 meters for aragonite in the North Atlantic Ocean. This difference between the Pacific and Atlantic oceans may be attributed to a difference in the amount of CO2 dissolved in these waters. Such a difference may, in turn, be caused by a difference in the residence times in these waters. The marked decrease of CaCO3 content in the ocean bottom sediments observed at water depths greater than 3500 meters in the Pacific and 4500 meters in the Atlantic appears to reflect a transition of the sea water from saturation to undersaturation with respect to calcite.
It is suggested that the partial pressure of carbon dioxide in the atmosphere is buffered, over geological time scales, by a negative feedback mechanism, in which the rate of weathering of silicate minerals (followed by deposition of carbonate minerals) depends on surface temperature, which in turn depends on the carbon dioxide partial pressure through the greenhouse effect. Although the quantitative details of this mechanism are speculative, it appears able to partially stabilize the earth's surface temperature against the steady increase of solar luminosity, believed to have occurred since the origin of the solar system.
Tentative geochemical cycles for the pre-biologic Earth are developed by comparing the relative fluxes of oxygen, dissolved iron, and sulfide to the atmosphere and ocean. The flux of iron is found to exceed both the oxygen and the sulfide fluxes. Because of the insolubility of iron oxides and sulfides the implication is that dissolved iron was fairly abundant and that oxygen and sulfide were rare in the atmosphere and ocean. Sulfate, produced by the oxidation of volcanogenic sulfur gases, was the most abundant sulfur species in the ocean, but its concentration was low by modern standards because of the absence of the river-borne flux of dissolved sulfate produced by oxidative weathering of the continents. These findings are consistent with the geologic record of the isotopic composition of sedimentary sulfates and sulfides. Except in restricted environments, the sulfur metabolism of the earliest organisms probably involved oxidized sulfur species not sulfide.
The effect of variations in cloud cover, optical properties, and fractional distribution with altitude on the mean surface temperature of a model of the early earth has been investigated. In all cases examined, cloud-climate feedbacks result in temperatures greater than those in models with no cloud feedbacks. If the model of hydrospheric feedback effects is correct, then cloud feedbacks are as important to the climate as changes in solar luminosity and atmospheric composition during the earth's atmospheric evolution. In particular, the early earth need not become completely ice-covered if strong negative cloud feedbacks occur. However, until a proper understanding of cloud feedbacks is available, conclusions regarding conditions in the early atmosphere must remain in doubt.
This paper reviews the principal results from paleoclimate studies and includes background material slanted toward climate modelers. The inferred temperature history of the last 4.6 billion years indicates major changes in the components of the earth's climate system. A secular change in global insolation receipt is due to a 20–30% increase in solar luminosity since the formation of the earth. A CO 2 ‐H 2 O greenhouse effect may have offset the lower luminosity during early earth history. Inferred fluctuations of global temperature have occurred over a broad range of time scales. On time scales of 10 ⁶ –10 ⁸ years, paleogeographic factors (e.g., continental drift and sea level changes) have contributed significantly to temperature changes associated with transitions between nonglacial and glacial states. Preliminary modeling efforts indicate that additional factors (e.g., CO 2 , changes in atmospheric circulation) must also be considered in order to explain the origin of nonglacial climates. The origin of polar ice caps may result from ocean circulation changes that were caused by plate tectonic processes. Fluctuations of ice volume on a time scale of 10³–10 ⁵ years correlate with insolation variations caused by orbital perturbations. Feedback interactions within the land‐sea‐air‐ice system (e.g., ocean circulation changes and bedrock dynamics) have been responsible for a significant modulation of the orbital signal. Ice ages may be due to orbitally induced temperature changes superimposed on a global cooling of terrestrial origin.
This isolated intracratonic basin contains an upper Proterozoic to mid-Paleozoic stratigraphic succession of shallow-marine sediments that, in places, exceeds 14 km in thickness. The Gillen Member of the Bitter Springs Formation includes evaporites which are among the world's earliest (-0,8 to 0.7 Ga). Deposition of the evaporites was cyclic and followed the patterns identified in other major evaporite basins. The apparent sea-level high may relate to basin dynamics, whereas the cyclicity of the evaporites may be due to eustatic sea-level controls acting on the barrier to allow intermittent inflow of sea water. -from Author
Describes 140 to 160 shallowing-upward cycles (1 to 15 m thick) that contain distinctive marker beds and can be correlated for > 200 km parallel to strike and > 120 km perpendicular to strike. Cycles are grouped according to cycle base lithology, which reflects paleogeographic position on the platform.-from Author
Large concentrations of atmospheric CO2 in the atmosphere of the early earth have been proposed as a possible explanation of the apparent absence of frozen earth in spite of a faint early sun. However, the most thorough treatments of this question, by Owen et al. (1979) and Kasting et al. (1984), apparently disagree as to the warming effects of large amounts of CO2. We recalculate the evolution of surface temperature over the last 4.25 billion year time period, using the same scenario for CO2 partial pressures and solar constant as employed by the previous authors. We find good agreement with Kasting et al. (1984) and also explain why the results of Owen et al. are at variance with our findings and those of Kasting et al. Using the concept of direct radiative forcing, we present analytic relations between the solar luminosity and CO2 mixing ratio required to maintain the troposphere close to its present thermal structure. As a by-product, we present new broadband parameterizations for the 961 and 1064 cm−1 CO2 bands that can be used in climate models. We also consider the direct radiative forcing of large amounts of CH4, or changes in model clouds, and discuss how these might reduce the CO2 mixing ratio required to balance the faint early sun.
The stromatolites of the Belingwe Greenstone Belt (approximately 2700 Ma old) are perhaps the best-developed Archaean stromatolites yet found. Exposures occur on two stratigraphic levels, both part of the “Bulawayan” in Rhodesian stratigraphic terminology (Wilson et al., 1978). The extensive outcrops show a wide variety of stromatolites, including forms similar to Baicalia, Conophyton, Irregularia and Stratifera. Many stromatolites occur in cyclic units, possibly reflecting periodic changes in lagoonal conditions. Associated sedimentary rocks were deposited in a very shallow-water environment and some display well-developed desiccation features. Currently held concepts concerning the evolution of stromatolites and their usefulness in biostratigraphy do not appear to be supported by the evidence from Belingwe.
In analogy to modern soda lakes commonly associated with volcanic regions, it is postulated that the ancient sea had a high alkalinity, a high pH and low Ca and Mg concentrations. The change towards NaCl dominance as observed in the present-day ocean can best be explained by a series of mineral equilibria, crustal differentiation and life processes. In principle hydrothermal leaching of chlorine from the oceanic crust caused this ion to accumulate in the sea while at the same time dissolved carbonates became gradually removed by organisms and carbonates. The switch from a soda to a halite ocean was accomplished ca. 1 Ga ago. Crititcal biochemical events marking the chemical evolution of the Precambrian sea and their implications for the PCO2 in air will be discussed.
Presents and discusses criteria to distinguish marine from non-marine evaporites.-from Author
Fluid inclusion homogenisation temperatures from the magnesites of the Celia Dolomite and the Coomalie Dolomite of the Rum Jungle area of the Pine Creek Geosyncline, N.T., indicate that temperature is a major control of the two distinctive morphological forms. The rhombohedral form recrystallised in association with fluids that homogenised mainly at temperatures less than 150°C, whereas the bladed (tabular) form developed by recrystallisation at a higher temperature. Previously the two forms have been interpreted as pseudomorphs after halite and gypsum, respectively.
Sulphate reduction generally causes isotopic fractionation of sulphur1. Modern sedimentary sulphide is largely produced by biogenic reduction of sulphate and is typically enriched in 32S (ref. 2). This is balanced by excess 34S in the oceanic sulphate reservoir and evaporites3. High-temperature, inorganic reduction of sulphate may also cause fractionation4,5. Since the work of Ault and Kulp6, there has been interest in finding the beginnings of sulphate reduction in the sedimentary record. This is important for several reasons. First, sulphate-respiring bacteria are a milestone of evolution7,8. Second, it established the exogenic sulphur cycle in an essentially modern form. This, with the interconnected oxygen and carbon cycles, regulates the composition of atmosphere and oceans9–11. Third, widespread evidence of sulphate reduction in rocks of a given age and younger indicates that sulphate was established as a major constituent of seawater. In addition to identifying a stage in the evolution of an oxygenated environment10, this has important metallogenic implications. Schidlowski8 has recently concluded that dissimilatory reduction commenced at 2,800–3,100 Myr in an Archaean ocean that had relatively high concentrations of sulphate. I review here the published data and present additional sulphur isotope analyses obtained from the early Precambrian of South Africa. These results indicate that sulphate was a minor component of Archaean and early Proterozoic ocean water, probably <0.001 mol l−1. The concentration had increased by ~2,350 Myr to levels allowing significant biogenic and inorganic fractionation and the partitioning of 32S/34S in the exogene cycle.
CLOUDS dominate the albedo of the Earth and hence have a vital role in the global radiation balance. They are one of the most important physical properties of the atmosphere but the most difficult to parameterise. The forecasting of short-term climatological excursions (weather) requires numerical integration of complex dynamical models. However, over longer time periods it may be possible to include cloud cover without resorting to explicit atmospheric dynamics. Here we suggest that over evolutionary time periods (108-109 yr) the Earth's percentage cloud cover has remained approximately constant. This is in general agreement with present ideas about the stability of the Earth's evolution1-3. Over medium-term climatological periods (104-107 yr) we have found that the position of large cloud masses may be directly related to the changing surface configuration, caused, for example, by continental drift. Global cloud cover fluctuates about a mean, which is near the present-day value and reinforces albedo changes caused by surface configuration; this could be highly significant for theories of climatic change.
CURRENT models for the evolution of the Sun require an increase in solar luminosity by 25% since the formation of the Solar System1. Such an increase in the solar constant should have profound effects on the terrestrial climate, but there is no evidence from the fossil record of a corresponding change in the Earth's global mean temperature2. This apparent conflict cannot be explained by the apparent inability of solar models to account for the low observed neutrino flux3. Even models that are forced to fit the neutrino data require a similar increase in the solar luminosity. As Newman and Rood1 state: ``a faint young Sun is one of the most unavoidable consequences of stellar structure considerations''. We discuss here whether CO2-H2O in a weakly reducing atmosphere could have caused this change in the early Earth's temperature by the so-called greenhouse effect.
Early Proterozoic marine carbonates of the Rocknest Formation (1.93-1.89 Ga) have very depleted delta18O values (about 80/00) relative to younger, Late Proterozoic marine carbonates that formed in similar depositional environments. Two isotopic trends are superimposed on the data for open-marine components. The first involves stabilization of tidal-flat sediments during early, possibly reflux-type dolomitization by evaporative pore fluids enriched in delta18O The second trend toward isotopically light delta18O values was established during dolomitization of open-marine facies in contact either with meteoric waters (mixing zone) or under conditions of higher temperatures during burial. This resulted in precipitation of blocky, pore-occluding cements. The isotopically most enriched ooids are the best preserved normal marine components and may suggest that the delta18O of seawater was about -9.750/00 ±1.00/00 (SMOW) at 1.9 Ga. This composition would require a major change in the balance of high-temperature oxygen isotopic exchange between seawater and basalt and low-temperature weathering in order to explain the 80/00 positive shift in inferred seawater delta18O between 1.9 and 1.0 Ga. Alternatively, the depleted delta18O values represent an approximately 30-35 °C higher temperature of surface waters at 1.9 Ga. The heaviest carbonate delta13C values are +1.75permil;, more enriched than previously reported for the Early Proterozoic on the basis of bulk-rock data.
The assumptions of standard solar evolution theory are mentioned briefly, and the principle conclusions drawn from them are described. The result is a rationalization of the present luminosity and radius of the Sun. Because there is some uncertainty about the interior composition of the Sun, a range of models is apparently acceptable.
To decide which model is the most accurate, other more sensitive comparisons with observations must be made. Recent measurements of frequencies of dynamical oscillations are particularly valuable in this respect. The most accurate observations are of the five-minute oscillations, which suggest that the solar composition is not atypical of other stars of the same age as the Sun.
The theory predicts that the solar luminosity has risen steadily from about 70% of its current value during the last 4.7 x 109yr. Superposed on this there might have been variations on shorter timescales. Variations lasting about 107yr and occurring at intervals of 108yr have been suggested as being the cause of terrestrial ice ages. Moreover, there may be other variations, associated with instabilities arising from the coupling between the convection zone and the radiative interior, that occur on a timescale of 105yr and which also have climatic consequences. These issues are quite uncertain.
We do know that the Sun varies magnetically with a period of about 22 yr, and that this oscillation is modulated irregularly on a timescale of centuries. This appears to be a phenomenon associated with the convection zone and its immediate neighbourhood, though control from a more deeply-seated mechanism is not out of the question. There is a small luminosity variation associated with this cycle, and the way by which this might come about is discussed in terms of certain theories of the solar dynamo.
Finally, there must be small surface flux variations associated with the dynamical oscillations mentioned above. Though the total luminosity variations are extremely small, the flux in any specific direction, and in particular that of the earth, may be measurable.
The early Precambrian Hamersley Group of Western Australia contains both major and minor occurrences of well-preserved carbonate sedimentary rock. The Carawine Dolomite and the middle or Paraburdoo Member of the Wittenoom Formation are the two most extensive occurrences. The Carawine Dolomite is restricted to the eastern part of the Hamersley Basin and contains abundant stromatolites, oncolites, and wave ripples, as well as local occurrences of evaporite crystal pseudomorphs and oolitic to pisolitic textures. These carbonate strata were deposited in a shallow-water paleoenvironment herein referred to as the Carawine Platform. In contrast, the Wittenoom Formation is restricted to the centraland western parts of the Hamersley Basin, and such shallow-water features are entirely absent from its carbonate strata. These strata consist largely of thinly laminated lutite, but also include some thin carbonate turbidites. The carbonate sedimentary rocks of the Wittenoom Formation, as well as some in the Carawine Dolomite which display similar characteristics, are therefore interpreted as sediments that were deposited off-platform in deeper-water paleoenvironments. Paleocurrent, thickness, and grain size trends of the carbonate turbidites indicate they were deposited by paleoflows moving south and west. Even though the Carawine Dolomite lies to the northeast of the Wittenoom Formation, stratigraphic correlations using a newly recognized marker bed of probable impact origin suggest that the carbonate strata of the Carawine Dolomite are younger than the Paraburdoo Member of the Wittenoom Formation. If true, the Carawine Platform could not have been a source of sediment for most of the carbonate sedimentary rocks in the Wittenoom Formation, but it may havebeen the sediment source for similar carbonate turbidites in younger formations such as the Mt. McRae Shale and the Dales Gorge Member of the Brockman Iron Formation. This, in turn, suggests that carbonate sediments were accumulating in the shallower parts of the Hamersley Basin while some of the BIFs were being deposited simultaneously in the deeper parts.
Thermodynamics and kinetics are both important in the operation of the ocean-atmosphere system. The carbonate chemistry of the oceans can be treated in terms of perturbations of a well-defined equilibrium state. The equilibrium state for silicates in sea water is less well defined, and thermodynamic arguments are much less persuasive. Kinetics are of overwhelming importance in determining—among other parameters—the nitrate concentration of sea water and the pressure of oxygen and nitrogen in the atmosphere. It is likely that a similar mix of attainment and non-attainment of equilibrium prevailed during the entire history of the oceans, and that the sedimentary record must be interpreted accordingly.
Photochemical calculations indicate that in the prebiotic atmosphere of the Earth ammonia would have been irreversibly converted to N2 in less than 40 years if the ammonia surface mixing ratio were ≤ 10−4. However, if a continuous outgassing of ammonia were maintained, radiative equilibrium calculations indicate that a surface mixing ratio of ammonia of 10−5 or greater would provide a sufficient greenhouse effect to keep the surface temperature above freezing. With a 10−4 mixing ratio of ammonia, 60 to 70% of the present day solar luminosity would be adequate to maintain surface temperatures above freezing. A lower limit to the time constant for accumulation of an amount of nitrogen equivalent to the present day value is 10 my if the outgassing were such as to provide a continuous surface mixing ratio of ammonia ≥ 10−5.
The photochemistries of methane and HCN are discussed in the context of the primitive terrestrial atmosphere, using a detailed numerical model. In the absence of abundant O 2 , absorption of solar EUV (λ < 1023Å) by N 2 provides a large thermospheric source of atomic nitrogen. Methane is oxidized cleanly and efficiently, provided CO 2 is more abundant than CH 4 . Otherwise, a large fraction of the methane present is polymerized, forming alkanes in the troposphere and polyacetylenes and nitriles in the upper atmosphere. The combination of low O 2 , high N 2 , and moderately high levels of CO 2 would have made the ancient terrestrial atmosphere a favorable environment for the production of HCN from CH 4 . Once formed, HCN is rather long‐lived; it is removed from the atmosphere either by direct photodissociation at Ly α (∼100 years) or by rainfall (∼10 years). Chemical loss would have been unimportant. Owing to its stability, transport of HCN from the top to the bottom of the atmosphere can be efficient; nevertheless, our results are sensitive to the assumed eddy diffusion profile. For small amounts of methane a small constant fraction of order 0.1% to 1% of the carbon in the methane is returned to the surface as hydrocyanic acid rain. For larger methane sources exceeding a critical value of order 10 ¹¹ molecules cm ⁻² s ⁻¹ (corresponding to ƒ(CH 4 ) of order 10 ⁻⁴ –10 ⁻³ ), rainout of HCN increases abruptly to more than 10% of the carbon supplied as methane, limited by the primary production of N. Under favorable conditions, hydrolysis of HCN could have supported atmospheric NH 3 mixing ratios approaching 1 ppm.
The potential impact of high carbon dioxide partial pressure on ocean chemistry is examined in order to investigate what constraints are imposed by the known record of chemical sedimentation through time. The evidence consists of the persistence of calcium carbonate and sulfate precipitation throughout almost the entire sedimentary rock record. A uniformitarian point of view that assumes no very great change in the conditions for the deposition of these chemical sediments. The methods of Holland (1972) are used to set limits on the composition of the water from which precipitation occurred. No inconsistencies between the sedimentary rock record and presumed higher partial pressure of carbon dioxide early in earth history, provided that high partial pressure was accompanied by a generally lower pH for seawater, higher concentrations of calcium and biocarbonate ions, and lower concentrations of carbonate and sulfate ions.
The Tumbiana Formation, about 2700 million years old, was largely deposited in ephemeral saline lakes, as judged by the unusual
evaporite paragenesis of carbonate and halite with no sulfate. Stromatolites of diverse morphology occur in the lacustrine
sediments, some with palimpsest fabrics after erect filaments. These stromatolites were probably accreted by phototropic microbes
that, from their habitat in shallow isolated basins with negligible sulfate concentrations, almost certainly metabolized by
ozygenic photosynthesis.
The Upper Proterozoic (approx 800-700 Ma) Akademikerbreen Group, Spitsbergen, comprises 2000 m of carbonates, with only minor intercalations of quartz arenite and shale and about 45 percent limestone. Stromatolites are conspicuous in outcrop but constitute only 25 percent of the total section. Micrites and coarser intraclastic carbonates derived mainly from micritic precursors comprise 60 percent of the group, while oolites make up the remaining 15 percent. Distinctive sedimentary features are discussed. Carbonate sedimentology reinforces data from other sources which indicate the last 200 to 300 Ma of the Proterozoic Eon was a distinctive interval of Earth history. -from Authors
Simple (one-dimensional) climate models suggest that carbon dioxide concentrations during the Archean must have been at least 100-1000 times the present level to keep the Earth's surface temperature above freezing in the face of decreased solar luminosity. Such models provide only lower bounds on CO2, so it is possible that CO2 levels were substantially higher than this and that the Archean climate was much warmer than today. Periods of extensive glaciation during the early and late Proterozoic, on the other hand, indicate that the climate at these times was relatively cool. To be consistent with climate models CO2 partial pressures must have declined from approximately 0.03 to 0.3 bar around 2.5 Ga ago to between 10(-3) and 10(-2) bar at 0.8 Ga ago. This steep decrease in carbon dioxide concentrations may be inconsistent with paleosol data, which implies that pCO2 did not change appreciably during that time. Oxygen was essentially absent from the Earth's atmosphere and oceans prior to the emergence of a photosynthetic source, probably during the late Archean. During the early Proterozoic the atmosphere and surface ocean were apparently oxidizing, while the deep ocean remained reducing. An upper limit of 6 x 10(-3) bar for pO2 at this time can be derived by balancing the burial rate of organic carbon with the rate of oxidation of ferrous iron in the deep ocean. The establishment of oxidizing conditions in the deep ocean, marked by the disappearance of banded iron formations approximately 1.7 Ga ago, permitted atmospheric oxygen to climb to its present level. O2 concentrations may have remained substantially lower than today, however, until well into the Phanerozoic.
This paper uses arguments of geochemical mass balance to arrive at an estimate of the partial pressure of carbon dioxide in the terrestrial atmosphere very early in earth history. It appears that this partial pressure could have been as large as 10 bars. This large estimate depends on two key considerations. First, volatiles were driven out of the interior of the earth during the course of earth accretion or very shortly thereafter. This early degassing was a consequence of rapid accretion,which gave the young earth a hot and rapidly convecting interior. Second, the early earth lacked extensive, stable continental platforms on which carbon could be stored in the form of carbonate minerals for geologically significant periods of time. In the absence of continental platforms on the early earth, the earth's carbon must have been either in the atmosphere or ocean or in the form of shortlived sedimentary deposits on ephemeral sea floor.
Solar evolution implies, for contemporary albedos and atmospheric composition, global mean temperatures below the freezing
point of seawater less than 2.3 aeons ago, contrary to geologic and paleontological evidence. Ammonia mixing ratios of the
order of a few parts per million in the middle Precambrian atmosphere resolve this and other problems. Possible temperature
evolutionary tracks for Earth and Mars are described. A runaway greenhouse efect will occur on Earth about 4.5 aeons from
now, when clement conditions will prevail on Mars.
The roughly 25 percent increase in luminosity over the life of the sun shared by many different solar models is shown to be a very general result, independent of the uncertainties suggested by the solar neutrino experiment. Superficially, this leads to a conflict with the climatic history of the earth, and if basic concepts of stellar evolution are not fundamentally in error, compensating effects must have occurred, as first pointed out by Sagan and Mullen. One possible interpretation supported by recent detailed models of the earth's atmosphere is that the greenhouse effect was substantially more important than at present even as recently as 1 billion to 2 billion years ago.
The rate at which ammonia would have been destroyed in the earth's atmosphere under assumed NH3 mixing ratio conditions of 10 to the -8th to 0.0001 is calculated by a one-dimensional photochemical model, and the destruction rates are compared with possible biotic and abiotic ammonia sources. It is found that, while the mixing ratio of 10 to the -8th needed for the evolution of life could have been maintained by abiotic sources, the value of 0.00001 needed for the production of significant greenhouse warming could not have been sustained abiotically. The increase of atmospheric ammonia due to biological activities during the Archean is also considered lower than the level required for the generation of measurable thermal effects.
The photochemical behavior of methane in the early terrestrial atmosphere is investigated with a detailed model in order to determine how much CH4 might have been present and what types of higher hydroocarbons could have been formed. It is found that any primordial methane accumulated during the course of earth accretion would have been dissipated by photochemical reactions in the atmosphere in a geologically short period of time after the segregation of the core. Abiotic sources of methane are not likely to have been large enough to sustain CH4 mixing ratios as high as 10 to the -6th, the threshold for a possible methane greenhouse, with a CO-rich atmosphere being a possible exception. After the origin of life an increasing biogenic source of methane may have driven CH4 mixing ratios well above 10 to the 6th. The rise of atmospheric oxygen in the early Proterozoic may have led to a more rapid photochemical destruction of methane, lowering the mixing ratio to its present value.