L. Paul Knauth

High Archean climatic temperature inferred from oxygen isotope geochemistry of cherts in the 3.5 Ga Swaziland Supergroup, South Africa

New and compiled oxygen isotope data combined with the results of geological and sedimentological studies demonstrate that enclaves of synsedimentary to very early diagenetic cherts are widely preserved in the 3.5-3.2 Ga Swaziland Supergroup, Barberton greenstone belt, South Africa. The low δ 1 8 O values of these cherts indicate extremely high ocean temperatures of 55-85 °C. Previously, the large depletion in 1 8 O shown by all Barberton cherts relative to their Phanerozoic counterparts has been attributed to low 1 8 O in Archean oceans, chert formation during late diagenesis, wholesale loss of 1 8 O during alteration, and/ or regional silicification of sediments around hot springs. These alternative explanations are not compatible with the new results. Cherts in the Onverwacht Group display an isotopic stratigraphy that is inversely repeated in conglomerates in the overlying Fig Tree and Moodies Groups, demonstrating that the chert δ 8 0 O values were fixed prior to Archean uplift and erosion, which started at 3.26 Ga. The maximum δ 1 8 O value in Barberton cherts (+22‰) is lower than the minimum values (+23‰) in Phanerozoic bedded cherts, precluding late diagenesis as the explanation of the overall low δ 1 8 O values. Regional metamorphic, hydrothermal, or long-term resetting of original δ 1 8 O values is also precluded by preservation of δ 1 8 O values across different metamorphic grades and by systematic δ 1 8 O differences among interbedded chert types, stratigraphic units, and conglomerate clasts. The 7‰ δ 1 8 O variation of these Archean cherts is similar to that of Phanerozoic deep-sea cherts-formed when opal converted to microquartz during burial-but the actual Archean values are ∼10‰ lower. Marine opal was apparently converted to microquartz during burial to depths of <1 km. Cherts with δ 1 8 O < 15‰ reflect conversion during deepest burial or in local areas of enhanced geothermal gradient and/ or hydrothermal activity. Cherts with higher δ 1 8 O values formed during early diagenesis and indicate an extremely hot Archean ocean and surface environment.

The late Precambrian greening of the Earth

Many aspects of the carbon cycle can be assessed from temporal changes in the 13C/12C ratio of oceanic bicarbonate. 13C/12C can temporarily rise when large amounts of 13C-depleted photosynthetic organic matter are buried at enhanced rates, and can decrease if phytomass is rapidly oxidized or if low 13C is rapidly released from methane clathrates. Assuming that variations of the marine 13C/12C ratio are directly recorded in carbonate rocks, thousands of carbon isotope analyses of late Precambrian examples have been published to correlate these otherwise undatable strata and to document perturbations to the carbon cycle just before the great expansion of metazoan life. Low 13C/12C in some Neoproterozoic carbonates is considered evidence of carbon cycle perturbations unique to the Precambrian. These include complete oxidation of all organic matter in the ocean and complete productivity collapse such that low-13C/12C hydrothermal CO2 becomes the main input of carbon. Here we compile all published oxygen and carbon isotope data for Neoproterozoic marine carbonates, and consider them in terms of processes known to alter the isotopic composition during transformation of the initial precipitate into limestone/dolostone. We show that the combined oxygen and carbon isotope systematics are identical to those of well-understood Phanerozoic examples that lithified in coastal pore fluids, receiving a large groundwater influx of photosynthetic carbon from terrestrial phytomass. Rather than being perturbations to the carbon cycle, widely reported decreases in 13C/12C in Neoproterozoic carbonates are more easily interpreted in the same way as is done for Phanerozoic examples. This influx of terrestrial carbon is not apparent in carbonates older than ∼850 Myr, so we infer an explosion of photosynthesizing communities on late Precambrian land surfaces. As a result, biotically enhanced weathering generated carbon-bearing soils on a large scale and their detrital sedimentation sequestered carbon. This facilitated a rise in O2 necessary for the expansion of multicellular life.

Isotope geochemistry of fluid inclusions in Permian halite with implications for the isotopic history of ocean water and the origin of saline formation waters

Abstract δD and δ18O values have been determined for fluid inclusions in 45 samples of Permian halite. The inclusions are enriched in 18O relative to the meteoric water line but are depleted in D relative to ocean water. Inclusions with the more positive δ-values coincide with the isotopic composition expected for evaporating sea water which follows a hooked trajectory on a δD-δ18O diagram. Inclusions with more negative δ-values may represent more highly evaporated sea water but probably reflect synsedimentary or diagenetic mixing of meteoric water with evaporite brines. The isotope systematics in these inclusions are sufficiently similar to those of a modern evaporite pan to indicate that Permian sea water was isotopically similar to modern sea water. Connate evaporite brines can have negative δ-values because of the probable hooked isotope trajectory of evaporating sea water and/or synsedimentary mixing of evaporite brines with meteoric waters. Subsurface formation waters composed of mixtures of remnant primary evaporite brines and later meteoric waters may be more common than previous isotopic evidence has suggested.

Life on Land in the Precambrian

Microfossils have been discovered in cavity-fill and replacement silica that occurs between chert-breccia clasts in 1200-million-year-old paleokarst at the top of the Mescal Limestone, central Arizona, and in ∼800-million-year-old paleokarst at the top of the Beck Spring Dolomite, southeastern California. Microbial communities on Precambrian (>550 million years ago) land may have been extensive enough to affect weathering, erosion, sedimentation, and geochemical processes.

Oxygen isotope geochemistry of cherts from the Onverwacht Group (3.4 billion years), Transvaal, South Africa, with implications for secular variations in the isotopic composition of cherts

δ18O values for 87 chert samples from the 3.4-b.y.-old Onverwacht Group, South Africa, range from +9.4 to +22.1‰. δ-values for cherts representing early silicified carbonates and evaporites, and possible primary precipitates range from +16 to +22‰ and are distinctly richer in18O than silicified volcaniclastic debris and cherts of problematical origin. The lower δ-values for the latter two chert types are caused by isotopic impurities such as sericite and feldspar, and/or late silicification at elevated temperature during burial. Cherts with δ-values below +16‰ are thus not likely to yield geochemical data relevant to earth surface conditions. Fine-grained chert is less than 0.7‰ depleted in18O relative to coexisting coarse drusy quartz. Because coarse quartz preserves its isotopic composition with time, the maximum amount of post-depositional lowering of the δ-values of cherts by long-term isotopic exchange with meteoric groundwaters does not exceed 0.7‰ in 3.4 b.y. In response to metamorphism the δ-values of Onverwacht cherts appear to remain unchanged or to have increased by as much as 4‰. Neither metamorphism nor long-term isotopic exchange with groundwaters can explain why Onverwacht cherts are depleted in18O relative to their Phanerozoic counterparts. Meteoric waters with a δ18O range of at least 3‰ appear to have been involved in Onverwacht chert diagenesis. δ-values for possible primary cherts or cherts representing silicified carbonates and evaporites are compatible with the depositional and diagenetic environments deduced from field and petrographic evidence. Onverwacht cherts appear to have formed with δ-values at least 8‰ lower than Phanerozoic cherts. The new Onverwacht data combined with all published oxygen isotope data for cherts suggest a secular trend similar to that initially suggested by Perry (1967) in which younger cherts are progressively enriched in18O. However, Precambrian cherts appear to be richer in18O than Perrys original samples and can be reasonably interpreted in terms of declining climatic temperatures from ∼70°C at 3.4 b.y. to present-day values, as initially suggested by Knauth and Epstein (1976). This surface temperature history is compatible with existing geological, geochemical, and paleontological evidence.

A model for the origin of chert in limestone

It is proposed that many nodular cherts in limestone have formed in the ground water of mixed meteoric-marine coastal systems where dissolution of biogenic opal and mixing of marine and fresh waters can produce waters highly supersaturated with respect to quartz and undersaturated with respect to calcite and aragonite. Aspects of cherts readily explained by this model include the observed isotopic ratios in cherts, typical field relationships, the relative resistance of dolomite to silicification, the source-of-silica problem, the preservation of siliceous fossils in cherts, and aspects of chert morphology and mineralogy.

Impact origin of sediments at the Opportunity landing site on Mars

Mars Exploration Rover Opportunity discovered sediments with layered structures thought to be unique to aqueous deposition and with minerals attributed to evaporation of an acidic salty sea. Remarkable iron-rich spherules were ascribed to later groundwater alteration, and the inferred abundance of water reinforced optimism that Mars was once habitable. The layered structures, however, are not unique to water deposition, and the scenario encounters difficulties in accounting for highly soluble salts admixed with less soluble salts, the lack of clay minerals from acid–rock reactions, high sphericity and near-uniform sizes of the spherules and the absence of a basin boundary. Here we present a simple alternative explanation involving deposition from a ground-hugging turbulent flow of rock fragments, salts, sulphides, brines and ice produced by meteorite impact. Subsequent weathering by intergranular water films can account for all of the features observed without invoking shallow seas, lakes or near-surface aquifers. Layered sequences observed elsewhere on heavily cratered Mars and attributed to wind, water or volcanism may well have formed similarly. If so, the search for past life on Mars should be reassessed accordingly.

Stable chlorine isotopes in the Palo Duro Basin, Texas: Evidence for preservation of Permian evaporite brines

Experimental evaporation of seawater yields brines with δ37Cl from 0.0‰ (initial) to −0.9‰. In the Palo Duro Basin, brines with δ37Cl values overlapping the −0.1 to 0.4‰ range of halite evaporite can be generated by a set of processes including dissolution of halite in meteoric water. Such brines occur above and below an evaporite aquitard. Brines with δ37Cl values of −0.4 to −1.0‰ in the evaporite aquitard and in a deep brine aquifer cannot be generated by dissolution of halite. Considered with Br and Cl content, δD and noble gas content, the δ37Cl data indicate that such brines originated as evapoconcentrated seawater. High Br evaporite brine formed directly from seawater on the eastern side of the basin, whereas low Br evaporite brines on the western side formed after an influx of meteoric water at the time of evaporite formation. There has not been detectable vertical flow of meteoric water across the evaporite aquitard. Strata beneath the evaporite contain compartments that have been isolated geochemically since the Permian.

Stable isotope geochemistry of cherts and carbonates from the 2.0 Ga Gunflint Iron Formation : implications for the depositional setting, and the effects of diagenesis and metamorphism

Abstract δ 18 O values for all but three of 97 samples of chert along a ∼220 km transect through the 2.0 Ga Gunflint Iron Formation of the Animikie Basin, North America, range from 21.3 to 24.7%o. There is no correlation between δ 18 O and the type of chert (i.e., deeper-water lutitic chert, shallower water arenitic chert, and peritidal algal chert). The fact that the Gunflint Iron Formation has been exposed to thermal conditions only slightly above burial diagenesis suggests that the δ values of the chert have not been significantly affected by metamorphism. Field and petrographic observations, and the small δ 18 O range of all the chert types suggests that silicification, lithification, and any conversion to quartz chert from a hydrous silica precursor occurred close to the sediment-water interface. The uniform δ 18 O values of all the different chert types suggest that all cherts precipitated over a relatively common temperature interval (∼20°C) from a common parent water having a relatively uniform δ 18 O. The isotopic data suggest silica saturation and precipitation was not the result of the mixing of seawater and meteoric water, or the result of evaporative processes. Siderite in the deep-water, banded facies of the Gunflint displays a δ 18 O range of −6.0 to −2.6%o, whereas siderite in the more shoreward, organic-rich, shale facies is significantly enriched in 13 C ( δ 13 C=−2.5 to +0.5%o). δ 18 C of siderite does not appear to have been altered as a result of neomorphic recrystallization, and authigenic microsparitic siderite along stylolites displays the same δ 13 C range as primary and neomorphosed siderite. However, neomorphic and authigenic siderite are depleted in 18 O by up to 7%o relative to primary, unaltered microspheroidal siderite. The lower δ 13 C of banded facies siderite (relative to the shale facies siderite) does not seem to be the result of thermal decarboxylation or anaerobic oxidation (abiotic or bacterial) of organic matter, but rather is better explained by primary precipitation in an ocean system that was layered with respect to carbon isotopes. The more basinward banded facies siderite precipitated from a 13 C depleted, Fe 2+ -rich, hydrothermal water mass that was moved by advective upwelling from submarine regions of magmatic/volcanic-hydrothermal activity onto the continental shelf. The siderite of the shale facies precipitated from a more near-shore, shallow seawater mass that was comparatively enriched in 13 C ( δ 13 C≈0±2% o ). Ankerite in the Gunflint Iron Formation occurs as a replacement of primary siderite in the banded facies, and as a replacement of greenalite and chert in the arenite facies. Many ankerite samples have δ 13 C values that are similar to those of the shale facies siderite which suggests that shallow seawater was a major component of the diagenetic fluid. Later neomorphism of the replacement ankerite at higher burial temperatures and probably in the presence of 18 O depleted waters resulted in a wide range of δ 18 O values (13.8–22.5%o). The unique mineralogy of the upper most Gunflint Iron Formation (calcite, serpentine and magnetite) compared to the majority of the Formation, the very low δ values, and overlying diabase sills suggest that the non-ferroan nature of the carbonate in the upper Gunflint is the result of contact metamorphism.

Preserved stable isotopic signature of subaerial diagenesis in the 1.2-b.y. Mescal Limestone, central Arizona: implications for the timing and development of a terrestrial plant cover.

A Precambrian exposure surface in the 1.2-b.y. Mescal Limestone has been isotopically examined for indications of a carbon isotopic signature that might indicate the presence of a subaerial vegetative cover in the middle Proterozoic. δ 18 O values of the Mescal carbonates show two distinct data sets: (1) dolomites from an unaltered zone which were unaffected by sub-aerial diagenesis having δ 18 O values ranging from +19.9 to +25.6‰ Standard Mean Ocean Water (SMOW) with cherts averaging +30‰, and (2) dolomites from a dissolution zone subaerially exposed in the Precambrian with δ 18 O values ranging from +13.9 to +22.4‰ and cherts averaging +25‰. δ 13 C of dolomite ranges from +3.7‰ Pee Dee Belemnite (PDB) in the unaltered zone to 0‰ in the dissolution zone. The dissolution zone consists of a karst breccia of recemented dolomite and chert fragments with numerous clastic solution dikes. Isotopic and field data indicate that the δ 18 O of the unaltered dolomite was fixed during early meteoric-water diagenesis, including dissolution-silicification of evaporites and dolomitization. During a later subaerial exposure event, a large flux of meteoric water flushed through the dissolution zone and produced the same isotopic patterns that Allan and Matthews documented for younger examples as indicative of a vegetatively covered land surface. Alternative explanations for producing the observed δ 13 C variations in the absence of vegetation do not seem feasible. We therefore suggest that the subaerial environment 1.2 b.y. ago was a biologically active zone.