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The origin and evolution of Earth’s biosphere were shaped by the physical and chemical histories of the oceans. Marine chemical sediments and altered oceanic crust preserve a geochemical record of these histories. Marine chemical sediments, for example, exhibit an increase in their 18O/16O ratio through time. The implications of this signal are ambiguous but are typically cast in terms of two endmember (but not mutually exclusive) scenarios. The oceans may have been much warmer in the deep past if they had an oxygen isotope composition similar to that of today. Alternatively, the nature of fluid–rock interactions (including the weathering processes associated with continental emergence) may have been different in the past, leading to an evolving oceanic oxygen isotope composition. Here we examine approximately 3.24-billion-year-old hydrothermally altered oceanic crust from the Panorama district in the Pilbara Craton of Western Australia as an alternative oxygen isotope archive to marine chemical sediments. We find that, at that time, seawater at Panorama had an oxygen isotope composition enriched in 18O relative to the modern ocean with a δ18O of 3.3 ± 0.1‰ VSMOW. We suggest that seawater δ18O may have decreased through time, in contrast to the large increases seen in marine chemical sediments. To explain this possibility, we construct an oxygen isotope exchange model of the geologic water cycle, which suggests that the initiation of continental weathering in the late Archaean, between 3 and 2.5 billion years ago, would have drawn down an 18O-enriched early Archaean ocean to δ18O values similar to those of modern seawater. We conclude that Earth’s water cycle may have gone through two separate phases of steady-state behaviour, before and after the emergence of the continents.

The global biosphere is commonly assumed to have been less productive before the rise of complex eukaryotic ecosystems than it is today1. However, direct evidence for this assertion is lacking. Here we present triple oxygen isotope measurements (∆17O) from sedimentary sulfates from the Sibley basin (Ontario, Canada) dated to about 1.4 billion years ago, which provide evidence for a less productive biosphere in the middle of the Proterozoic eon. We report what are, to our knowledge, the most-negative ∆17O values (down to −0.88‰) observed in sulfates, except for those from the terminal Cryogenian period2. This observation demonstrates that the mid-Proterozoic atmosphere was distinct from what persisted over approximately the past 0.5 billion years, directly reflecting a unique interplay among the atmospheric partial pressures of CO2and O2and the photosynthetic O2flux at this time3. Oxygenic gross primary productivity is stoichiometrically related to the photosynthetic O2flux to the atmosphere. Under current estimates of mid-Proterozoic atmospheric partial pressure of CO2(2–30 times that of pre-anthropogenic levels), our modelling indicates that gross primary productivity was between about 6% and 41% of pre-anthropogenic levels if atmospheric O2was between 0.1–1% or 1–10% of pre-anthropogenic levels, respectively. When compared to estimates of Archaean4–6and Phanerozoic primary production7, these model solutions show that an increasingly more productive biosphere accompanied the broad secular pattern of increasing atmospheric O2over geologic time8. Triple oxygen isotope measurements of 1.4-billion-year-old sedimentary sulfates reveal a unique mid-Proterozoic atmosphere and demonstrate that gross primary productivity in the mid-Proterozoic was between 6% and 41% of pre-anthropogenic levels.

Sulfur (S) isotope fractionation by sulfate-reducing microorganisms is a direct manifestation of their respiratory metabolism. This fractionation is apparent in the substrate (sulfate) and waste (sulfide) produced. The sulfate-reducing metabolism responds to variability in the local environment, with the response determined by the underlying genotype, resulting in the expression of an “isotope phenotype”. Sulfur isotope phenotypes have been used as a diagnostic tool for the metabolic activity of sulfate-reducing microorganisms in the environment. Our experiments with Desulfovibrio vulgarisHildenborough (DvH) grown in batch culture suggest that the S isotope phenotype of sulfate respiring microbes may lag environmental changes on time scales that are longer than generational. When inocula from different phases of growth are assayed under the same environmental conditions, we observed that DvH exhibited different net apparent fractionations of up to −9‰. The magnitude of fractionation was weakly correlated with physiological parameters but was strongly correlated to the age of the initial inoculum. The S isotope fractionation observed between sulfate and sulfide showed a positive correlation with respiration rate, contradicting the well-described negative dependence of fractionation on respiration rate. Quantitative modeling of S isotope fractionation shows that either a large increase (≈50×) in the abundance of sulfate adenylyl transferase (Sat) or a smaller increase in sulfate transport proteins (≈2×) is sufficient to account for the change in fractionation associated with past physiology. Temporal transcriptomic studies with DvH imply that expression of sulfate permeases doubles over the transition from early exponential to early stationary phase, lending support to the transport hypothesis proposed here. As it is apparently maintained for multiple generations (≈1–6) of subsequent growth in the assay environment, we suggest that this fractionation effect acts as a sort of isotopic “memory” of a previous physiological and environmental state. Whatever its root cause, this physiological hysteresis effect can explain variations in fractionations observed in many environments. It may also enable new insights into life at energetic limits, especially if its historical footprint extends deeper than generational.

Dissimilatory sulfate reduction (DSR) has been a key process influencing the global carbon cycle, atmospheric composition and climate for much of Earth’s history, yet the energy metabolism of sulfate-reducing microbes remains poorly understood. Many organisms, particularly sulfate reducers, live in low-energy environments and metabolize at very low rates, requiring specific physiological adaptations. We identify one such potential adaptation—the electron carriers selected for survival under energy-limited conditions. Employing a quantitative biochemical-isotopic model, we find that the large S isotope fractionations (>55‰) observed in a wide range of natural environments and culture experiments at low respiration rates are only possible when the standard-state Gibbs free energy (ΔG′°) of all steps during DSR is more positive than −10 kJ mol−1. This implies that at low respiration rates, only electron carriers with modestly negative reduction potentials are involved, such as menaquinone, rubredoxin, rubrerythrin or some flavodoxins. Furthermore, the constraints from S isotope fractionation imply that ferredoxins with a strongly negative reduction potential cannot be the direct electron donor to S intermediates at low respiration rates. Although most sulfate reducers have the genetic potential to express a variety of electron carriers, our results suggest that a key physiological adaptation of sulfate reducers to low-energy environments is to use electron carriers with modestly negative reduction potentials.

Mass-independent fractionation of sulfur isotopes (MIF-S) in Archaean sediments results from photochemical processing of atmospheric sulfur species in an oxygen-depleted atmosphere. Geological preservation of MIF-S provides evidence for microbial sulfate reduction (MSR) in low-sulfate Paleoarchaean (3.8–3.2 billion years ago (Ga)) and Neoarchaean (2.8–2.5 Ga) oceans, but the significance of MSR in Mesoarchaean (3.2–2.8 Ga) oceans is less clear. Here we present multiple sulfur and iron isotope data of early diagenetic pyrites from 2.97-Gyr-old stromatolitic dolomites deposited in a tidal flat environment of the Nsuze Group, Pongola Supergroup, South Africa. We identified consistently negative Δ33S values in pyrite, which indicates photochemical reactions under anoxic atmospheric conditions, but large mass-dependent sulfur isotope fractionations of ~30‰ in δ34S, identifying active MSR. Negative pyrite δ56Fe values (−1.31 to −0.88‰) record Fe oxidation in oxygen-bearing shallow oceans coupled with biogenic Fe reduction during diagenesis, consistent with the onset of local Fe cycling in oxygen oases ~3.0 Ga. We therefore suggest the presence of oxygenated near-shore shallow-marine environments with ≥5 μM sulfate at this time, in spite of the clear presence of an overall reduced Mesoarchaean atmosphere. Oxidized sulfur, formed in photochemical reactions in an anoxic atmosphere, fuelled microbial sulfate reduction in Mesoarchaean oxygenated near-shore seas, according to sulfur and iron isotopes in pyrite.

Dissimilatory sulfate reduction (DSR) has been a key process influencing the global carbon cycle, atmospheric composition and climate for much of Earth’s history, yet the energy metabolism of sulfate-reducing microbes remains poorly understood. Many organisms, particularly sulfate reducers, live in low-energy environments and metabolize at very low rates, requiring specific physiological adaptations. We identify one such potential adaptation—the electron carriers selected for survival under energy-limited conditions. Employing a quantitative biochemical-isotopic model, we find that the large S isotope fractionations (>55‰) observed in a wide range of natural environments and culture experiments at low respiration rates are only possible when the standard-state Gibbs free energy (ΔG′°) of all steps during DSR is more positive than −10 kJ mol−1. This implies that at low respiration rates, only electron carriers with modestly negative reduction potentials are involved, such as menaquinone, rubredoxin, rubrerythrin or some flavodoxins. Furthermore, the constraints from S isotope fractionation imply that ferredoxins with a strongly negative reduction potential cannot be the direct electron donor to S intermediates at low respiration rates. Although most sulfate reducers have the genetic potential to express a variety of electron carriers, our results suggest that a key physiological adaptation of sulfate reducers to low-energy environments is to use electron carriers with modestly negative reduction potentials.

The stable isotope ratios of meteoric waters change predictably over orographic barriers. We present a new approach to determine stable isotope ratios in ancient meteoric waters from spatial patterns of hydrothermal alteration in continental volcanic fields. In the San Juan Volcanic Field, Colorado, USA we reconstruct water δ18O values feeding hydrothermal systems of −7 to −10‰ from 35 to 20 Ma, followed by a drop to −17 to −18‰ between 20 and 12 Ma. This drop is consistent with greater magmatic water input to older hydrothermal systems, ∼1 km of rock exhumation, ∼2–3 km of surface uplift, or a combination of all three. Our approach returns water isotope compositions integrated over spatial (kms to 10s kms) and temporal (104− 107years) scales of continental hydrothermal systems. Such length‐ and time‐scales approach those of continental tectonics, potentially alleviating issues with diagenetic space and time limitations associated with other paleo‐meteoric water proxies. Past surface elevations are commonly reconstructed from the stable isotope ratios of oxygen and hydrogen in paleo‐precipitation. These ratios change systematically as air lifts up and over mountains and other topographic barriers. Direct samples of ancient precipitation do not exist so we rely on proxies instead. The most common proxies used are individual minerals, such as calcium carbonate, or fossils, such as leaves. We present a new approach, where we instead use the isotopic “footprint” of large hydrothermal cells associated with ancient volcanoes as a record of precipitation isotope ratios. By processing these large patterns with an inverse model to “undo” the alteration, we find that there was a large change in oxygen isotope values in precipitation between 20 and 12 million years ago in the San Juan Volcanic Field, Colorado, USA. This is consistent with surface uplift of 2–3 km over this timeframe. Unlike other isotopic proxies for paleo‐precipitation our technique captures an isotopic signal integrated over the space‐ and timescales of hydrothermal alteration, which approach those of the tectonic processes that make and break mountains. Continental hydrothermal systems record meteoric water oxygen isotope compositions over broad space‐ and time‐scalesIsotopic alteration in hydrothermal systems in the San Juan Volcanic Field, Colorado, USA provides a test of this approachInverse modeling suggests a decrease in meteoric oxygen isotope ratios between 23 and 15 Ma, consistent with 2–3 km of surface uplift Continental hydrothermal systems record meteoric water oxygen isotope compositions over broad space‐ and time‐scales Isotopic alteration in hydrothermal systems in the San Juan Volcanic Field, Colorado, USA provides a test of this approach Inverse modeling suggests a decrease in meteoric oxygen isotope ratios between 23 and 15 Ma, consistent with 2–3 km of surface uplift

We present a degassing study of Long Valley Caldera that explores the structural controls upon emissions of magmatic carbon dioxide gas. A total of 223 soil gas samples were collected and analyzed for stable carbon isotopes using a field‐portable cavity ring‐down spectrometer. This novel technique is flexible, accurate, and provides sampling feedback on a daily basis. Sampling sites included major and minor volcanic centers, regional throughgoing faults, caldera‐related structures, zones of elevated seismicity, and zones of past and present hydrothermal activity. The classification of soil gases based on their δ13C and CO2values reveals a mixing relationship among three end‐members: atmospheric, biogenic, and magmatic. Signatures dominated by biogenic contributions (~4 vol %, −24‰) are found on the caldera floor, the interior of the resurgent dome, and areas associated with the Hilton Creek and Hartley Springs fault systems. With the introduction of the magmatic component (~100 vol %, −4.5‰), samples acquire mixing and hydrothermal signatures and are spatially associated with the central caldera and Mammoth Mountain. In particular, they are concentrated along the southern margin of the resurgent dome where the interplay between resurgence‐related reverse faulting and a bend in the regional fault system has created a highly permeable fracture network, suitable for the formation of shallow hydrothermal systems. This contrasts with the south moat, where despite elevated seismicity, a thick sedimentary cover has formed an impermeable cap, inhibiting the ascent of fluids and gases to the surface. Regional and resurgence faults control magmatic CO2emissions from Long ValleyCO2in soil gases reflects a mixture of magmatic, biogenic, and atmospheric CCRDS enables field‐based, same day measurements of CO2and δ13C

Organic haze and sulfur gases are ubiquitous in planetary atmospheres and were likely present in Earth's Archean atmosphere. Currently, there are few experiments investigating how H2S influences organic haze chemistry on Archean Earth. Here, we present results from laboratory haze‐analog experiments probing the role of H2S in the composition and total mass of aerosol produced from precursor mixtures of Archean‐like gas fluxes (e.g., pCO2∼3–50xPAL). We show that trace H2S enhances organic aerosol production at all carbon dioxide mixing ratios studied, and we observe both organic and inorganic sulfur aerosol products. Our finding challenges predictions that H2SO4and S8were the primary sulfur reservoirs in Earth's Archean atmosphere, and these results suggest that inorganic sulfur and organic haze chemistry are tightly coupled during the formation of organic hazes in the atmospheres of the Archean Earth and likely Archean‐like exoplanets. The Archean Eon (4.0–2.5 billion years ago) atmosphere was much different from the modern one, with very little oxygen and higher amounts of carbon dioxide (CO2) and methane (CH4). Light from the sun would have jump‐started chemical reactions leading to a mixture of organic molecules and particles (“organic haze”). Gases in smaller amounts (trace gases) can greatly influence “haze” chemistry. One common trace gas is hydrogen sulfide (H2S). We conducted laboratory experiments that attempted to reproduce this “haze” chemistry with gas mixtures of CO2, CH4, and H2S in a nitrogen background. When H2S was added to the gas mixture, organosulfur molecules were formed, and the mass of organic particles greatly increased. These results challenge two current assumptions about Archean organic haze and sulfur chemistry. The first is that the production of organic particles in a CO2/CH4haze should decrease when CO2is increased; we show that organic particle mass is essentially independent of CO2when trace H2S is present. The second is that sulfur particles would be composed of inorganic sulfur; we show that organosulfur accounts for a significant portion of the sulfur. Our results may affect the current understanding of the history and evolution of Earth's atmosphere. In haze‐analog experiments with carbon dioxide (CO2)/methane (CH4)/N2precursors, addition of trace H2S increases aerosol production by at least a factor of ∼3.8In CO2/CH4/N2gas mixtures, high CO2inhibits organic aerosol formation. With trace H2S, organic aerosol formation is independent of CO2Inorganic and organic sulfate aerosol forms when trace H2S is present, suggesting that organic sulfur is an important sulfur reservoir In haze‐analog experiments with carbon dioxide (CO2)/methane (CH4)/N2precursors, addition of trace H2S increases aerosol production by at least a factor of ∼3.8 In CO2/CH4/N2gas mixtures, high CO2inhibits organic aerosol formation. With trace H2S, organic aerosol formation is independent of CO2 Inorganic and organic sulfate aerosol forms when trace H2S is present, suggesting that organic sulfur is an important sulfur reservoir