Around 720-640 million years ago, much of the Earth's surface was covered in ice during a glaciation that lasted millions of years. Explosive underwater volcanoes were a major feature of this 'Snowball Earth', according to new research led by the University of Southampton.
Many aspects of this extreme glaciation remain uncertain, but it is widely thought that the breakup of the supercontinent Rodinia resulted in increased river discharge into the ocean. This changed ocean chemistry and reduced atmospheric CO2 levels, which increased global ice coverage and propelled Earth into severe icehouse conditions.
Because the land surface was then largely covered in ice, continental weathering effectively ceased. This locked the planet into a 'Snowball Earth' state until carbon dioxide released from ongoing volcanic activity warmed the atmosphere sufficiently to rapidly melt the ice cover. This model does not, however, explain one of the most puzzling features of this rapid deglaciation; namely the global formation of hundreds of metres thick deposits known as 'cap carbonates', in warm waters after Snowball Earth events.
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Lead author of the study Dr Tom Gernon, Lecturer in Earth Science at the University of Southampton, said: "When volcanic material is deposited in the oceans it undergoes very rapid and profound chemical alteration that impacts the biogeochemistry of the oceans. We find that many geological and geochemical phenomena associated with Snowball Earth are consistent with extensive submarine volcanism along shallow mid-ocean ridges."
During the breakup of Rodinia, tens of thousands of kilometres of mid-ocean ridge were formed over tens of millions of years. The lava erupted explosively in shallow waters producing large volumes of a glassy pyroclastic rock called hyaloclastite. As these deposits piled up on the sea floor, rapid chemical changes released massive amounts of calcium, magnesium and phosphorus into the ocean.
Dr Gernon explained: "We calculated that, over the course of a Snowball glaciation, this chemical build-up is sufficient to explain the thick cap carbonates formed at the end of the Snowball event.
Weathering Profiles in Phosphorus-Rich Rocks at Gusev Crater, Mars, Suggest Dissolution of Phosphate Minerals into Potentially Habitable Near-Neutral Waters
Authors:
Adcock et al
Abstract:
Abundant evidence indicates that significant surface and near-surface liquid water has existed on Mars in the past. Evaluating the potential for habitable environments on Mars requires an understanding of the chemical and physical conditions that prevailed in such aqueous environments. Among the geological features that may hold evidence of past environmental conditions on Mars are weathering profiles, such as those in the phosphorus-rich Wishstone-class rocks in Gusev Crater. The weathering profiles in these rocks indicate that a Ca-phosphate mineral has been lost during past aqueous interactions. The high phosphorus content of these rocks and potential release of phosphorus during aqueous interactions also make them of astrobiological interest, as phosphorus is among the elements required for all known life. In this work, we used Mars mission data, laboratory-derived kinetic and thermodynamic data, and data from terrestrial analogues, including phosphorus-rich basalts from Idaho, to model a conceptualized Wishstone-class rock using the reactive transport code CrunchFlow. Modeling results most consistent with the weathering profiles in Wishstone-class rocks suggest a combination of chemical and physical erosion and past aqueous interactions with near-neutral waters. The modeling results also indicate that multiple Ca-phosphate minerals are likely in Wishstone-class rocks, consistent with observations of martian meteorites. These findings suggest that Gusev Crater experienced a near-neutral phosphate-bearing aqueous environment that may have been conducive to life on Mars in the past.
PALEOENVIRONMENTAL IMPLICATIONS OF TWO PHOSPHOGENIC EVENTS IN NEOPROTEROZOIC SEDIMENTARY SUCCESSIONS OF THE TANDILIA SYSTEM, ARGENTINA
Authors:
Gómez-Peral et al
Abstract:
Two phosphogenic events of potential economic importance (preserving P2O5 abundances up to 25 and 35%, respectively) are recognized in the Neoproterozoic Tandilia System. The older of the two lies atop a quartz-arkosic facies association of the Villa Mónica Formation. The age of this unit remains controversial, but has previously been considered as Tonian to Cryogenian based on stromatolite assemblages and carbon isotope trends; strontium isotope abundances of less than 0.7071 currently support an earliest Cryogenian age for the formation. The younger phosphate level lies at the base of the Cerro Negro Formation above a widespread karstic surface in a succession containing Cloudina, so is most-likely to be Ediacaran in age. In both cases, the phosphatic concretionary levels are related to relative sea-level fall and exposure that may be related to glacial eustacy. In order to reconstruct the paleoenvironment associated with the formation of these concretionary phosphates, we combined field observations with thin-section petrography, XRD, SEM and geochemical analyses. Total rare-earth element (REE) contents range from 311 to 1010 ppm in concretions from the Villa Mónica Formation and from 290 to 1471 ppm those from the Cerro Negro Formation. Villa Mónica concretions reveal no anomalies in Ce abundance, but clearly positive Eu anomalies (ranging from 1 to 1.4). These results suggest reduced conditions in the marine depositional environment. In contrast, Eu anomalies are not recorded in Cerro Negro concretions, while negatve Ce anomalies, ranging from −0.14 to −0.18, are noted. These results are consistent with oxic seawater conditions in mixed platform facies. This study suggests that depositional conditions where phosphate was concentrated in the Neoproterozoic Tandilla System were markedly different.
Two billion years ago the Earth system was recovering from perhaps the single-most profound modification of its surface environments: the oxygenation of the atmosphere and oceans. This led to a series of major changes in global biogeochemical cycles, as a team around Aivo Lepland of the Norwegian Geological Survey NGU reports in the latest online edition of "Nature Geoscience".
This also resulted in the distribution of one of life's key elements, phosphorous. Studies on the unique organic-rich Zaonega rock formation preserved in Carelia, NW Russia, with an age of around two billion years has revealed an astonishing result: "The formation of Earth's earliest phosphorites was influenced strongly, if not controlled completely, by the activity of sulfur bacteria", says co-author Richard Wirth of the GFZ German Research Centre for Geosciences, who analyzed the rock samples with an electron microscope. "This activity occurred in an oil field setting that had been influenced by active volcanism and associated venting and seeping." In the modern world, sulfur bacteria inhabit upwelling vent and seep areas known as "Black Smokers" and mediate phosphorite formation. The authors therefore conclude that the formation of the earliest worldwide phosphorites 2 billion years ago can be linked to the establishment of sulfur bacteria habitats, triggered by the oxygenation of the Earth.
