Seafloor Weathering Research Articles

Three-stage formation of cap carbonates after Marinoan snowball glaciation consistent with depositional timescales and geochemistry

At least two global “Snowball Earth” glaciations occurred during the Neoproterozoic Era (1000-538.8 million years ago). Post-glacial surface environments during this time are recorded in cap carbonates: layers of limestone or dolostone that directly overlie glacial deposits. Postulated environmental conditions that created the cap carbonates lack consensus largely because single hypotheses fail to explain the cap carbonates’ global mass, depositional timescales, and geochemistry of parent waters. Here, we present a global geologic carbon cycle model before, during, and after the second glaciation (i.e. the Marinoan) that explains cap carbonate characteristics. We find a three-stage process for cap carbonate formation: (1) low-temperature seafloor weathering during glaciation generates deep-sea alkalinity; (2) vigorous post-glacial continental weathering supplies alkalinity to a carbonate-saturated freshwater layer, rapidly precipitating cap carbonates; (3) mixing of post-glacial meltwater with deep-sea alkalinity prolongs cap carbonate deposition. We suggest how future geochemical data and modeling refinements could further assess our hypothesis.

Mineralogy and sulfur isotopic compositions of sulfides from Yunzang (25.3°S) hydrothermal field, South Mid-Atlantic Ridge: Implications for formation mechanism and maturation of sulfide chimneys

A newly discovered hydrothermal field, named Yunzang and predominantly hosted by mafic rocks, was recently identified at 25.3°S on the South Mid-Atlantic Ridge (SMAR). This is the first report on the sulfide mineralogy and sulfur isotopic composition of sulfide chimney and massive sulfide ores collected from Yunzang field, SMAR. Based on mineralogical morphology, texture, crystallinity, assemblage and zonation, five specific stages of mineralization can be divided for the sulfide chimney at Yunzang. These stages range from low- to high-temperature hydrothermal conditions, along with an additional stage of seafloor weathering. The observed variations in morphology and crystallinity from low- and medium-temperature stages to high- and high- to medium-temperature stages suggest an evolution in the ore-forming conditions. Initially, the environment was characterized by instability and low-temperature conditions, with significant seawater influx. Subsequently, there was a shift towards a more stable regime dominated by high-temperature ore-forming fluid condition. The positive δ34S values of all the sulfides from Yunzang hydrothermal field suggest 79–92 % and 69–79 % of sulfur were generated from the leaching of basalt for sulfide chimney and massive sulfide, respectively, the rest sulfur were from reduced seawater sulfate. The δ34S values of massive sulfide (3.5–6.8 ‰, average = 5.1 ± 0.9 ‰) are mostly higher than that of sulfide chimney (1.1–4.5 ‰, average = 2.8 ± 0.8 ‰) are mainly caused by sub-surface reactions between preexisting sulfate and re-circulating fluid. An incremental increase of δ34S values from the external edge to the interior orifice indicating the Yunzang chimney maturation is relatively lower and the duration of hydrothermal activity was short. These results enrich our comprehension of mineralization in the seafloor hydrothermal system.

Iron oxidation and porosity generation in serpentinized abyssal peridotite

Serpentinization, the water-driven alteration of olivine-rich rocks, is a fundamental process contributing to planetary habitability and the maintenance of rock-hosted life on Earth. Serpentinization generates molecular hydrogen via the reduction of water and the coupled oxidation of Fe, which partitions into serpentine, brucite and magnetite. Magnetite formation controls the magnetic properties of serpentinites, and researchers have highlighted a link between increasing magnetic susceptibility and decreasing density of peridotites with increasing degree of serpentinization. Several analytical and theoretical studies have suggested that these increases in magnetization and decreases in density are accompanied by increasing porosity. Here, we investigate the potential correlation between porosity generation and serpentinization by analyzing the geochemical composition, Fe redox state, and porosity of 28 abyssal serpentinites recovered via drilling from the Atlantic Ocean subseafloor. Principal component analysis applied to the resulting dataset revealed no direct links between these properties, which we suggest results from the introduction of variability due to low-temperature seafloor weathering processes. Seafloor weathering can modify the porosity of serpentinites by dissolving brucite, and it affects the Fe redox state of serpentinites by inducing the replacement of magnetite by Fe(III) minerals like maghemite, hematite, and goethite. Moreover, we also suggest that ophiolitic serpentinites would not show direct correlations between porosity and Fe redox state, due to geochemical transformations induced by metamorphism and porosity reworking linked to deformation induced by tectonic stresses. Ultimately, our results demonstrate that weathering processes can significantly affect the chemical and physical properties of serpentinites and thus must be considered in geochemical and geophysical evaluations of these rocks.

Stable Chromium Isotope Fractionation During the Alteration of Abyssal Peridotite: Implications for Marine Chromium Isotope Mass Balance

AbstractThe stable chromium isotope system has been widely used as a redox proxy to reconstruct the oxygenation history of ocean atmosphere systems. However, the Cr isotope mass balance in modern oceans (i.e., inputs and outputs) remains poorly constrained. To investigate the influence of seawater‐peridotite reaction on the global marine Cr isotope mass balance, we report high‐precision Cr isotope data (δ53Cr) on a series of fresh and altered abyssal peridotites from the Gakkel Ridge and the Southwest Indian Ridge (SWIR). The least altered peridotites give a δ53Cr value of −0.08 ± 0.06‰ (2SD, n = 4) for the oceanic mantle which is consistent with the established δ53Cr of the Bulk Silicate Earth. Compared to fresh peridotites, a subset of altered peridotites exhibit a loss of isotopically light Cr with relatively positive δ53Cr values (up to 0.04‰). These altered peridotites are characterized by significant Cr loss and likely have been subject to serpentinization. By contrast, seafloor weathering has limited influence on the Cr concentrations and isotopic compositions of the altered peridotites. Monte Carlo (MC) simulations of marine alteration suggest a net Cr flux into seawater from altered abyssal peridotites of ∼3.5 × 108 mol/yr, which is on the same order of magnitude as the riverine input flux of 108–109 mol/yr. Furthermore, the MC results suggest that the peridotite‐sourced net Cr flux has a negative δ53Cr signature (−0.33 ± 0.21‰, 2SD). Thus, seawater‐peridotite interactions must be considered when evaluating the modern oceanic Cr isotope mass balance.

Metallogenetic process of Xunmei hydrothermal field (26°S), South Mid-Atlantic Ridge: Constraints from in-situ sulfur isotope and trace elements of sulfides

Seafloor hydrothermal sulfides at slow-spreading mid-ocean ridges (MORs) are of economic and scientific significance. The formation of newly discovered submarine hydrothermal sulfides on the South Mid-Atlantic Ridge (SMAR), particularly with respect to the fluid evolution, remains poorly understood. The Xunmei hydrothermal field is a typical volcanic dome-type hydrothermal field at 26°S on the SMAR. Based on the mineralogical zonation and assemblages, six distinct ore-forming stages were identified from low- to high-temperature stages with a seafloor weathering stage. The crystallinity of pyrite/marcasite and grain size of chalcopyrite increase gradually from the outer rim to the inner conduit of a sulfide chimney. The positive δ34SV-CDT values of pyrite/marcasite (2.1‰–8.2‰), chalcopyrite (2.5‰–5.6‰), and sphalerite (2.9‰–7.0‰) suggest 62%–91% of S was derived from the leaching of basement basalts, 9%–38% from reduction of seawater sulfate. In situ trace element data for sulfides show enrichments in Mn, Ag, Tl, and Pb in outer zones A and B, while enrichments in Se, In, and Sn in inner zones C and D, indicating an increase in precipitation temperature during chimney growth. Low pyrite Co contents, low Tl/Pb, low Sb/Pb, low Bi/Pb, high As, Ag, Cu, and Pb concentrations, suggesting that the pyrites precipitated from fluids that may have undergone supercritical phase separation during their ascent. High-temperature, Cl-depleted, vapor-rich hydrothermal fluids discharged into ambient cold seawater over multiple stages and probably evolved from low (<240 °C) to medium (∼ 263 °C) and high (∼ 317 °C) temperatures, reached a highest temperature of ∼335 °C, and then evolved to medium to low temperatures (∼ 270 °C to <240 °C) during the wanning of hydrothermal venting. The sulfur fugacity (fS2) likely evolved from a relatively low to high condition within the scope of intermediate sulfidation, and then decreased to low condition. The redox states (fO2) probably were strongly affected by seawater influx and evolved from relatively oxidized to original reduced conditions. The salinity decreased during chimney growth. Sulfide precipitation in the Xunmei hydrothermal field probably was the result of a combination of fluid-seawater mixing and phase separation. We propose a chimney growth and hydrothermal evolution models for the Xunmei hydrothermal field.

Hydrothermal alteration of basalts in the ultramafic-associated Tianxiu Vent Field, Carlsberg Ridge

The Tianxiu vent field (TVF) is the first active ultramafic-associated hydrothermal field to be discovered at the termination of a detachment fault on the slow-spreading Carlsberg Ridge. Most hydrothermal chimneys in the TVF are developed on the basaltic apron. This paper presents mineralogical, petrological, and geochemical data for altered basalts in and surrounding the TVF. The basalt samples can be classified into seafloor-weathered and hydrothermally altered (i.e., slightly saponite-altered, highly saponite-altered, and silicified) types. At the periphery of the TVF, the basalts have experienced seafloor weathering, which has resulted in slight mineralogical and geochemical changes. In the TVF, most basalts have interacted with diffuse low-temperature hydrothermal fluids, leading to variable degrees of saponite alteration. As the distance from the vents decreases, basaltic glass, olivine, and pyroxene are progressively replaced by saponite; Zn, Cu, Co, and U are enriched; and Ca and Mn are depleted. Magnesium, Fe, and Li have been lost due to olivine replacement by saponite, and have been gained due to pyroxene replacement by saponite. In the fluid flow conduits, the basalts have interacted with focused medium- to high-temperature fluids, which has led to the replacement of saponite by opal; depletion in Mg, Fe, Li, and rare earth elements; and enrichment in Zn, Cu, Co, and U. We propose a new hydrothermal alteration model for basalts in an ultramafic-associated vent field. Based on a comparison with mafic-hosted vent fields along mid-ocean ridges, we propose that: (1) the permeable host rocks promote lateral fluid flow, widespread mixing with cold seawater, and pervasive low-temperature hydrothermal alteration; and (2) H2S-poor reducing fluids reduce the solubility of U and inhibit the precipitation of Fe, leading to high Cu/Fe and Zn/Fe ratios, and high U concentrations. The hydrothermal alteration model for the TVF is applicable to similar vent fields and can guide exploration for seafloor massive sulfide deposits along slow-spreading ridges.

The effects of submarine alteration and phosphatization on igneous rocks: Implications for Sr-, Nd-, Pb-isotope studies

A suite of fresh and variably phosphatized submarine volcanic rock samples from NW Pacific seamounts from the Mid Cretaceous were subject of a detailed trace element and isotopic leaching study to examine the effect of phosphate alteration on the use of Sr-, Nd-, and Pb- radiogenic isotope systems. The study particularly focuses on the behavior of the respective parent/daughter element pairs during phosphatization and leaching in order to evaluate the impact on radiogenic ingrowth correction (“age correction”) for old seafloor samples and their use in geochemical studies. For this purpose, variably intense leaching procedures (no leaching; 2 M HCl at 70 °C for 1 h; 6 M HCl at 130 °C for 3 h) were applied to diverse sample materials (rock powder, rock chips and fresh volcanic glass chips).Phosphatization causes significant modification of isotope ratios and elemental proportions of most trace elements including the rare earth elements. As opposed to common seafloor weathering, Rb/Sr decreases in the lavas with increasing phosphatization, while U/Pb enrichment appears slightly amplified. Mild acid leaching of rock chips, however, generally yields Sr, Nd and Pb isotope ratios largely in accordance with values from fresh glass fractions of the same samples and comparable with the results for similarly leached rock powders. Applying original (non-leached) elemental concentration ratios for the radiogenic age correction of mildly leached sample chips produces mixed results for 87Sr/86Srin but generally agrees with expected 143Nd/144Ndin, 206Pb/204Pbin, 207Pb/204Pbin, and 208Pb/204Pbin values.All residua of strongly leached powders yield significantly more radiogenic 206Pb/204Pbm, 143Nd/144Ndm and higher U/Pb than mildly leached rock chips or powders or even non-leached powder of the respective samples, opposite to what is expected when removing seawater-derived secondary phases by stronger acid leaching. We suggest that this enrichment, after removal of most of the magmatic Pb and Nd, reflects minor refractory components that formed in association with the phosphatization and developed high 206Pb/204Pb ratios by radiogenic ingrowth since the eruption of these lavas in the Mid Cretaceous due to high U/Pb.Utilizing modeled parent/daughter ratios based on magma evolution systematics of associated fresh glass yields no satisfying initial isotopic compositions for the strongly leached samples despite supposed removal of secondary isotopic signals upon leaching. Applying the modeled parent/daughter ratios to age correct mildly leached sample chips and powder otherwise slightly improves the accordance with their respective fresh glass fractions for 208Pb/204Pbin but yields similar results as if using the actual parent/daughter ratios for 143Nd/144Ndin, 86Sr/87Srin, 206Pb/204Pbin, and 207Pb/204Pbin.

Weathering-driven porosity generation in altered oceanic peridotites

Ultramafic rocks exposed at slow and ultra-slow spreading mid-ocean ridges represent a significant and extremely reactive portion of the oceanic lithosphere. Thus, mechanistic understanding of the processes by which seawater infiltrates into and reacts with these rocks is essential for constraining their contribution to the chemistry of the oceans and the coupled carbonate-silicate cycle. Recent observations indicate that nanoscale processes contribute to seawater-driven alteration of ultramafic rocks, but conventional petrographic and tomographic observations of the associated physical features are challenging to link to these nanoscale features. Moreover, multiple generations and varying conditions of fluid infiltration often obscure the relative roles of higher-temperature serpentinization, where reactions are mostly isochemical, and lower-temperature weathering reactions, where observations suggest the release of massive amounts of magnesium. Here we bridge these scales and investigate the specific role of weathering processes in dissolution-driven porosity generation by integrating focused ion beam scanning electron microscopy nanotomography and micro-computed X-ray tomography imaging of the pore structures preserved in drill cores of serpentinized oceanic peridotites. Relict olivine crystals in all imaged samples contain abundant etch pits, and those in the higher-resolution FIB-SEM imagery show the presence of channel-like dissolution structures. The pore channels preferentially affect olivine along grain boundaries and show anisotropic distribution likely controlled by crystallographic features. The pores formed via olivine dissolution are interpreted to result from dissolution of serpentinized peridotite at conditions where serpentine and carbonate precipitation are kinetically inhibited, i.e., at weathering conditions. Importantly, the calculated connectivity of the imaged pore structures increases as the scale of investigation increases, suggesting that weathering-driven olivine dissolution facilitates further seawater infiltration and olivine dissolution, a positive feedback that can sustain continued magnesium extraction until the rocks are ultimately cut off from seawater circulation via sedimentation. Thus, while much attention has been directed towards constraining geochemical fluxes from the higher-temperature alteration of ultramafic rocks, our results support literature studies suggesting that mineral dissolution, and hence elemental fluxes, are significant at the lower temperatures of seafloor weathering. Our data thus provide mechanistic evidence of the physical process contributing to the observed elemental loss from weathered oceanic peridotites.

Rates of seafloor and continental weathering govern Phanerozoic marine phosphate levels

Phosphate is an essential macronutrient for all organisms with a key role in setting levels of marine primary productivity. Despite its importance for marine biogeochemical cycles and its role in shaping the evolution of marine organisms, the factors controlling phosphate bioavailability on geologic timescales remain poorly understood. Here we develop a statistical model of the coupled cycles of phosphate, carbon, oxygen and calcium to constrain the weathering-derived fluxes and seawater concentrations of phosphate through Phanerozoic time (541 million years ago to the present). Our model includes input parameters and time-dependent forcings derived from geologic and geochemical data. We find that the climate sensitivity of chemical weathering of the oceanic crust by low-temperature fluids exerts a first-order control on phosphate availability. Specifically, continental weathering is a source of the limiting nutrient phosphate, but seafloor weathering is considered to be a minor phosphate sink. Consequently, times in Earth history during which seafloor weathering constituted a large fraction of the total (seafloor + continental) weathering were also times during which phosphate influxes to and concentrations in the ocean were relatively low. Lower seawater phosphate levels during those times probably resulted in lower primary productivity and oceanic and atmospheric oxygen concentrations.

Iron isotopic variations in basalts from oceanic crust due to low-temperature seawater alteration

Iron (Fe) isotopic compositions of seafloor basalts can provide important insights into high-temperature processes in the upper mantle. However, low-temperature seawater alteration (i.e., seafloor weathering) may significantly affect the primary Fe isotopic compositions. In this paper, we report whole-rock Fe isotopes, major and trace elements, and SrNd isotopes, along with in situ elemental maps for slightly altered Site U1434 basalts collected on IODP Expedition 349 from the South China Sea. We use these data to investigate the influence of low-temperature seawater alteration on the Fe isotopic compositions of seafloor basalts. The basalts have δ57Fe values of +0.12‰ to +0.27‰. No correlation between δ57Fe values versus Sr or Nd isotopes that are source-sensitive and no correlation between δ57Fe values and La/Yb ratios that are melting-sensitive indicate the δ57Fe values have not been affected by mantle source heterogeneity and partial melting. The negative correlations of MgO versus CaO and Al2O3 contents suggest limited clinopyroxene and plagioclase fractionation during magmatic evolution. Possible influence of olivine fractionation on the Fe isotopic variations can further be ruled out by a positive correlation between MgO and SiO2 contents and a weak negative correlation between δ57Fe values and SiO2 contents because olivine fractionation can form basalts with increasing SiO2 and δ57Fe with decreasing MgO. The strong correlations between δ57Fe values and MgO, Al2O3, and TiO2 contents indicate the Fe isotopic variations were caused by low-temperature seawater alteration. This is also evident from in situ elemental mapping of representative Site U1434 basalt sample, which shows that low-temperature alteration was characterized by decreases in MgO and SiO2 contents, and increases in Al2O3 and TiO2 contents. We propose a two-stage alteration model to explain the Fe isotopic variations of the Site U1434 basalts. The alteration appeared to occur under oxidizing conditions: Fe2+-rich external fluid likely derived from the progressive alteration inside the oceanic crust was oxidized at the surface of the oceanic crust, which caused the alteration, Fe isotopic fractionation, and increase in δ57Fe values of the seafloor basalts. This study highlights how low-temperature seawater alteration under oxidizing conditions can modify the Fe isotopic compositions of slightly altered oceanic crust, with critical implications for understanding the Fe cycle in the oceans.

Abundant phosphorus expected for possible life in Enceladus’s ocean

Saturn's moon Enceladus has a potentially habitable subsurface water ocean that contains canonical building blocks of life (organic and inorganic carbon, ammonia, possibly hydrogen sulfide) and chemical energy (disequilibria for methanogenesis). However, its habitability could be strongly affected by the unknown availability of phosphorus (P). Here, we perform thermodynamic and kinetic modeling that simulates P geochemistry based on recent insights into the geochemistry of the ocean-seafloor system on Enceladus. We find that aqueous P should predominantly exist as orthophosphate (e.g., HPO42-), and total dissolved inorganic P could reach 10-7 to 10-2 mol/kg H2O, generally increasing with lower pH and higher dissolved CO2, but also depending upon dissolved ammonia and silica. Levels are much higher than <10-10 mol/kg H2O from previous estimates and close to or higher than ∼10-6 mol/kg H2O in modern Earth seawater. The high P concentration is primarily ascribed to a high (bi)carbonate concentration, which decreases the concentrations of multivalent cations via carbonate mineral formation, allowing phosphate to accumulate. Kinetic modeling of phosphate mineral dissolution suggests that geologically rapid release of P from seafloor weathering of a chondritic rocky core could supply millimoles of total dissolved P per kilogram of H2O within 105 y, much less than the likely age of Enceladus's ocean (108 to 109 y). These results provide further evidence of habitable ocean conditions and show that any oceanic life would not be inhibited by low P availability.

Controls on the evolution of Cenozoic seawater chemistry

A simple process-based model of the Li, Mg, Ca, Sr and alkalinity cycles in the ocean has been developed with the aim of better understanding the information contained in the evolution of Cenozoic seawater chemistry. In the model, changes in seawater chemistry are forced by changing climatic (temperature) and tectonic (ocean crust accretion rate and continental weatherability) conditions. These drive time-varying element and isotope fluxes from continental and seafloor weathering, mid-ocean ridge hydrothermal systems and sedimentation. Inverse modelling reveals that a range of model parameter values can simultaneously reproduce the key features of the Cenozoic history of seawater chemistry for all species considered. In addition to Cenozoic cooling, a decrease in the rate of ocean crust accretion and/or an increase in continental weatherability are required to fit the data. However, paleoseawater compositions do not allow the relative roles of these tectonic forcings to be determined. While other processes (e.g., changes in surface lithology) may have played a role in generating the observed variations in seawater chemical and isotopic composition, they are not required to explain the general tends in the data. The increase in Mg/Ca over the Cenozoic in acceptable models is largely driven by a decrease in the Ca/alkalinity of the net continental, diagenetic and hydrothermal fluxes, and not by an increase in the Ca/alkalinity of the carbonate mineral sink. Thus, the increase in seawater Mg/Ca over the Cenozoic is readily explained without any decrease in dolomite formation. Notably, all acceptable models have a stronger temperaturedependence of seafloor than continental weathering. This means that climatically driven changes in chemical fluxes in the model come dominantly from seafloor, not continental, weathering.

Habitable zones with an earth climate history model

A simulation of Earth's climate system through geological time was produced with a view to estimating the sun's continuously habitable zone (CHZ). The model consists of four boxes (air, land, ocean, and rock) and parameterizes the carbonate-silicate cycle as a five-step process, comprising volcanism, dissolution of airborne carbon dioxide in seawater, continental weathering, neritic deposition of carbonates on the sea floor, and seafloor weathering. Weathering, surface albedo, and planetary albedo are affected by the growth of continents over time, and by changes in cloud cover with planetary rotation rate. Radiative transfer is handled with a semigray method based on conditions on preindustrial Earth and present-day Mars. The model includes a process for recovering from Snowball Earth events, involving changing surface albedo due to volcanic soot deposition. The simulation produces a runaway greenhouse effect inside 0.982 AU and “limit cycling” between temperate and freezing temperatures outside 1.18 AU. Because continental weathering is less reactive to temperature than previously thought, the results indicate that Earth is more vulnerable to catastrophic climate runaways at the outer CHZ boundary than foreseen earlier. The narrow spacing ratio (R ​= ​1.20) implies that habitable planets will be less frequent than previously expected.

The role of planetary interior in the long-term evolution of atmospheric CO2on Earth-like exoplanets

Context.The long-term carbonate silicate cycle plays an important role in the evolution of Earth’s climate and, therefore, may also be an important mechanism in the evolution of the climates of Earth-like exoplanets. However, given the large diversity in the possible interiors for Earth-like exoplanets, the ensuing evolution of the atmospheric CO2pressure may be widely different.Aims.We assess the role of the thermal evolution of the planetary interior on the long-term carbon cycle of Earth-like exoplanets. In particular, we investigate the effects of radiogenic mantle heating, core size, and planetary mass on the atmospheric partial CO2pressure, and the ability of a long-term carbon cycle driven by plate tectonics to control the atmospheric CO2pressure.Methods.We developed a box-model which connects carbon cycling to parametrized mantle convection. Processes considered in the carbon cycle are temperature-dependent continental weathering, seafloor weathering, subduction, and degassing through ridge and arc volcanism. The carbon cycle was coupled to the thermal evolution via the plate speed, which was parametrized in terms of the global Rayleigh number.Results.We find decreasing atmospheric CO2pressure with time, up to an order of magnitude over the entire main sequence lifetime of a solar-type star. High abundances of radioactive isotopes allow for more efficient mantle degassing, resulting in higher CO2pressures. Within the spread of abundances found in solar-type stars, atmospheric CO2pressures at 4.5 Gyr were found to vary from 14 Pa to 134 Pa. We find a decreasing Rayleigh number and plate speed toward planets with larger core mass fractionsfc, which leads to reduced degassing and lower atmospheric CO2pressure. In particular forfc≳ 0.8, a rapid decrease of these quantities is found. Variations in planet mass have more moderate effects. However, more massive planets may favor the development of more CO2rich atmospheres due to hotter interiors.Conclusions.The dependence of plate tectonics on mantle cooling has a significant effect on the long-term evolution of the atmospheric CO2pressure. Carbon cycling mediated by plate tectonics is efficient in regulating planetary climates for a wide range of mantle radioactive isotope abundances, planet masses and core sizes. More efficient carbon cycling on planets with a high mantle abundance of thorium or uranium highlights the importance of mapping the abundances of these elements in host stars of potentially habitable exoplanets. Inefficient carbon recycling on planets with a large core mass fraction (≳0.8) emphasizes the importance of precise mass-radius measurements of Earth-sized exoplanets.

Waterworlds May Have Better Climate Buffering Capacities than Their Continental Counterparts

Abstract The long-term habitability of a planet is often assumed to be controlled by its ability to cycle carbon between the solid planetary interior and atmosphere. This process allows the planet to respond to external forcings (i.e., changes in insolation, changes in volcanic outgassing rates, etc.) and regulate its surface temperature through negative feedbacks on atmospheric CO2 involved in silicate weathering. Continental weathering and seafloor weathering rates have different, non-linear dependencies on pCO2 and will respond differently to changes in external forcings. Because waterworlds (planets with only seafloor weathering) have a weaker pCO2 dependence than continental worlds (such as modern Earth), we find that waterworlds are better at resisting changes in surface temperature resulting from perturbations in insolation than their continental counterparts, and may be more habitable in this respect.

Features of seafloor hydrothermal alteration in metabasalts of mid-ocean ridge origin from the Chrystalls Beach Complex

ABSTRACT The Taieri Mouth locale of the Chrystalls Beach Complex (CBC) in the South Island of New Zealand includes well preserved to strongly deformed pillow lavas and flattened veins of epidote, quartz and chlorite intercalated with basalt flows and volcanoclastic breccias. The tectonic affinity for this rare igneous portion of the predominantly sedimentary CBC has not been well established in the context of its regional metamorphic geology. New field, petrographic, geochemical and isotopic evidence suggest a mid-ocean ridge origin for the Taieri metabasalts. Further, paleo-vertical networks of epidote-quartz-chlorite veins and cross-cutting faults provide a record of seafloor fracturing and fluid-flow. Altered pillows and epidote separates have δ18O isotope values ranging from 9.3 to 13.1‰. This indicates slightly enriched δ18O fractionation resulting from seafloor weathering and low-temperature (<250°C) exchange between seawater and hydrothermal fluids in basaltic fractures. Age-corrected 87Sr/86Sr ratios between 0.704135 and 0.70624 show low temperature fluid-rock interactions where the altered pillows and veins did not succumb to major mineralogic changes or isotopic re-equilibration after formation. In contrast, compressed s-fold epidote and coarse quartz veins near metasediments are suggestive of the elevated temperatures and pressures during accretion. We differentiate between episodic seafloor venting and accretional wedge-related alteration recorded within these metabasalts.

The Effect of Seafloor Weathering on Planetary Habitability

Abstract Conventionally, a habitable planet is one that can support liquid water on its surface. Habitability depends on temperature, which is set by insolation and the greenhouse effect, due mainly to CO2 and water vapor. The CO2 level is increased by volcanic outgassing and decreased by continental and seafloor weathering. Here, I examine the climate evolution of Earth-like planets using a globally averaged climate model that includes both weathering types. Climate is sensitive to the relative contributions of continental and seafloor weathering, even when the total weathering rate is fixed. Climate also depends strongly on the dependence of seafloor weathering on CO2 partial pressure. Both these factors are uncertain. Earth-like planets have two equilibrium climate states: (i) an ice-free state where outgassing is balanced by both weathering types, and (ii) an ice-covered state where outgassing is balanced by seafloor weathering alone. The second of these has not been explored in detail before. For some planets, neither state exists, and the climate cycles between ice-covered and ice-free states. For some other planets, both equilibria exist, and the climate depends on the initial conditions. Insolation increases over time due to stellar evolution, so a planet usually encounters the ice-covered equilibrium first. Such a planet will remain ice covered, even if the ice-free state appears subsequently, unless the climate receives a large perturbation. The ice-covered equilibrium state covers a large fraction of phase space for Earth-like planets. Many planets conventionally assigned to a star’s habitable zone may be rendered uninhabitable as a result.

Cycling phosphorus on the Archean Earth: Part II. Phosphorus limitation on primary production in Archean ecosystems

Several lines of evidence point to low rates of net primary production (NPP) in Archean oceans. However, whether Archean NPP was limited by electron donors or nutrients, particularly phosphorus (P), and how these factors might have changed over a billion years of recorded Archean history, remains contentious. One major challenge is to understand quantitatively the biogeochemical cycling of P on the early Earth. In Part I of this series (Hao et al., 2020), we estimated the weathering flux of P to the oceans as a function of temporally increasing continental emergence and elevation through Archean time. In Part II, we conduct thermodynamic and kinetic simulations to understand key processes of P cycling within the Archean ocean, including seafloor weathering, recycling of organic P, the solubility and precipitation of secondary phosphate minerals, and the burial diagenesis of P precipitates. Our calculations suggest low solubilities of apatite minerals in Archean seawater, primarily due to nearly neutral pH and high levels of Ca. This low solubility, in turn, implies a negligible contribution of apatite dissolution to P bioavailability in Archean seawater.We also simulate the solubility limits of common secondary P-bearing minerals, showing that vivianite would have been the least soluble P mineral in ferruginous Archean seawater (0.1–0.3 µM), even at moderate supersaturation states (Ω = 100 or 1000). If vivianite precipitation was kinetically favorable by microbial activities and mineral adsorption, the sinking flux of P as vivianite in Archean seawater could have reached the modern sinking flux, implying that vivianite precipitation was a potentially major sink for P in Archean oceans. During burial diagenesis, however, vivianite in porewater would have become less stable than Ca-phosphates of lower solubility. At elevated temperatures (>100 °C) associated with burial diagenesis and low-grade metamorphism, vivianite is predicted to react irreversibly with calcite to form apatite.Optimistic assumptions about the recycling efficiency of P on the Archean Earth lead us to estimate that by the end of the eon the total flux of P (continental weathering + recycling) could have supported NPP at levels up to 7% of the modern. The total flux of P would have been much lower on the early and middle Archean Earth, whereas fluxes of electron donors could have been higher, suggesting very low productivity and P-limitation of marine ecosystems during much of the eon. Comparing our estimates of NPP as limited by P supply with the estimate by Ward et al. (2019), in which NPP was limited by electron donors and metabolic efficiency, there could have been a transition between P-limited productivity in the early to middle Archean to electron donor-limitation closer to the eon’s end (assuming no oxygenic photosynthesis). Once oxygenic photosynthesis reached ecological significance, probably near the end of the Archean, our estimated flux of P would allow rapid oxidation of atmosphere.

Probable Cold and Alkaline Surface Environment of the Hadean Earth Caused by Impact Ejecta Weathering

AbstractConstraining the surface environment of the early Earth is essential for understanding the origin and evolution of life. The release of cations from silicate weathering depends on climatic temperature and , and such cations sequester into carbonate minerals in or on the seafloor, providing a stabilizing feedback on climate. Previous studies have suggested that this carbonate‐silicate cycle can keep the early Earth's surface temperature moderate by increasing to compensate for the faint young Sun. However, the Hadean Earth experienced a high meteorite impactor flux, which produced ejecta that is easily weathered by carbonic acid. In this study, we estimated the histories of surface temperature and ocean pH during the Hadean and early Archean using a new model that includes the weathering of impact ejecta, empirically justified seafloor weathering, and ocean carbonate chemistry. We find that relatively low and surface temperatures are probable during the Hadean, for example, at 4.3 Ga, (in bar) is and temperature is K. Such a low would result in a circumneutral to basic pH of seawater, for example, at 4.3 Ga. A probably cold and alkaline marine environment is associated with a high impact flux. Hence, if there was an interval of an enhanced impact flux, that is, Late Heavy Bombardment, similar conditions may have existed in the early Archean. Therefore, if the origin of life occurred in the Hadean, life likely emerged in a cold global environment and probably spread into an alkaline ocean.

Osmium isotope compositions and highly siderophile element abundances in abyssal peridotites from the Southwest Indian Ridge: Implications for evolution of the oceanic upper mantle

Abyssal peridotites are extensively exposed on the seafloor of amagmatic segments at the ultraslow spreading Southwest Indian Ridge (SWIR). This study presents abundances of highly siderophile elements (HSEs; Os, Ir, Ru, Pt, Pd and Re) and Re–Os isotope compositions as well as major and trace element concentrations of nine spinel-facies peridotites from the Dragon Bone amagmatic segment (~53°E) and the easternmost part (~63.5°E and 69.5°E) of the SWIR. These peridotites have experienced intense serpentinization and seafloor weathering in addition to variable degrees of melt depletion (bulk rock Al2O3 = 0.74–2.73 wt%). Low-temperature alterations, however, have a negligible effect on Os isotope compositions and concentrations of platinum-group elements. The 63.5°E peridotites displayed sub-chondritic Os/Ir and chondritic Pt/Ir ratios and slightly variable Re/Ir and Pd/Ir ratios. The observed positive covariance between 187Re/188Os ratios and γOs, Re and Pd abundances can be explained most likely by interaction of mantle residues with radiogenic basaltic melts, which is consistent with the petrographic observations indicative of melt infiltration. The 53°E peridotites, on the other hand, displayed larger variations in Os isotopes and relative HSE abundances, indicating a more complex evolutionary history. High Re abundances and 187Os/188Os ratios in some 53°E samples attest to Re and radiogenic Os addition, probably through reaction with S-undersaturated hydrous melts. Specifically, sample 21V-S9-D5‐2 exhibited a high 187Os/188Os ratio (0.148) and enrichment of Cu and Pt abundances, implying interaction with an oxidizing agent. Although the Re–Os isotopes of SWIR peridotites were superimposed by variable metasomatic processes, they preserve the ancient Os isotope signature (~1Ga) and record the melting episode at ~0.7 Ga during the closure of the Mozambique Ocean. We infer that ancient depleted and hence buoyant mantle exists beneath the SWIR, which probably supports the weakly magmatic rifted Marion Rise.