Francis A. Macdonald

Divergent models persist for the emergence of animals. Lipid biomarkers suggest animals originated before 635 million years ago (Ma), but their body fossil record extends to only 574 Ma. Molecular clocks might differentiate between these records, but we demonstrate how their accuracy and precision hinge on maximum age calibrations. We evaluate the Ediacaran Weng’an Biota (China) as a maximum calibration utilizing microfossils from the Kheseen Biota (Mongolia). These two biotas have similar preservation and microfossil assemblages, containing no definitive animals. The Kheseen Biota, deposited at a time (∼550–526 Ma) when animals were undoubtedly present, questions the validity of the Weng’an calibration. We discuss the uncertainty in maximum calibration strategies and propose multiple pre-Ediacaran deposits with favorable taphonomy, preserving both non-animal body fossils and biomarkers, as currently the most reliable calibrations. Our analyses support a late Tonian to mid-Cryogenian (∼800–700 Ma) origin for Metazoa.

Glaciation cools the crust by setting its surface temperature to 0°C or well below. During the Cryogenian Snowball Earth glaciations, these conditions were in place long enough to cool the entire crustal column. Although crustal thermal history can be deciphered by thermochronology, these data are typically interpreted to record depth changes due to burial or erosion. Thermal models of the crust under Cryogenian boundary conditions predict that thermochronometers were substantially altered in the upper quarter to half of the crust. This Snowball Earth refrigeration, with little to no coeval erosion, may have imparted a cooling signal mimicking up to 3‒5 km of glacial erosion. Rock cooling previously attributed to massive Snowball erosion and linked to the Great Unconformity may instead reflect a great refrigeration of Earth’s crust, with implications for thermochronology across other climate events.

GRIND (Geological Research through Integrated Neoproterozoic Drilling), sponsored by the International Continental Scientific Drilling Program (ICDP), aims to create a core archive of the Neoproterozoic Era via three phases: Tonian (GRIND-TON), Cryogenian (GRIND-CRY), and Ediacaran–Cambrian Transition (GRIND-ECT). The latter is the first phase undertaken and focuses on the geological and biogeochemical context of Late Ediacaran environmental perturbations and biological turnovers that culminated in the Cambrian Explosion. Mixed siliciclastic–carbonate successions of Namibia, Brazil, and China were chosen as GRIND-ECT targets. Here we report on the completed Namibian and Brazilian components. The Namibian stratigraphy drilled consists of the Kuibis–Schwarzrand sub-groups of the late Ediacaran–early Cambrian Nama Group on the western margin of the Kalahari craton in southern Namibia. These strata contain a rich record of Ediacaran fauna, including Cloudina and related small calcifiers, and simple to complex trace fossils that inhabited environments spanning shorelines to deeper-marine ramps. In addition, cores were drilled through the Late Ediacaran Corumbá Group in Brazil, which records a marine carbonate platform developed along the southeastern margin of the Amazon craton. These units are marked by a transition from a rimmed shelf to a distally steepened ramp and host the first appearance of Cloudina in Brazil and a myriad of acritarchs and trace fossils. Collectively, six Namibian cores totalling 2379 m and two Brazilian cores totalling 306 m were drilled. All cores were split and fully characterized and are housed at the Bundesanstalt für Geowissenschaften und Rohstoffe, Berlin–Spandau, Germany, and either at the Ministry of Mines in Windhoek, Namibia, or at the Universidade de São Paulo, Brazil. Ongoing efforts are integrating stable isotope and redox-sensitive element geochemistry and U–Pb and Re–Os geochronology throughout these cores, with detailed facies analyses to reconstruct the conditions under which metazoa expanded; diversified; and, in some cases, went extinct. GRIND-ECT cores offer exceptional opportunities to create and test hypotheses about the causes and consequences of Earth system transformation during the evolution of animals.

Twice in the Cryogenian Period (720–635 Ma), during the Sturtian and Marinoan glaciations, ice sheets extended to equatorial latitudes for millions of years. These climate extremes have been interpreted to record the Snowball climate state, in which all of Earth’s oceans were covered with ice. During a Snowball Earth, the hydrological cycle would have been curtailed and silicate weathering greatly reduced. In this scenario, deep ocean chemistry should have evolved toward mantle values through hydrothermal exchange at mid-ocean ridges. Specifically, seawater strontium isotopes (87Sr/86Sr) are predicted to exhibit unradiogenic mantle-like values. However, cap carbonates that overlie the Cryogenian glacial deposits have yielded radiogenic 87Sr/86Sr values similar to those of seawater prior to glaciation, inconsistent with the central geochemical prediction of the Snowball Earth hypothesis. Here we report the discovery of 87Sr/86Sr values of 0.7034 in marine carbonate and authigenic barite that rest directly above Sturtian glacial deposits in Dhofar, Oman. These values record either a local unradiogenic source or Snowball Earth deep-water values that have not been previously identified. If it is a global signal, these new data and geochemical modeling support an extreme Snowball Earth scenario with near-complete ice cover and define one of the largest geochemical perturbations to seawater in Earth history.

Twice during the Neoproterozoic Era, Earth experienced runaway ice-albedo catastrophes that resulted in multimillion year, low-latitude glaciations: the Sturtian and Marinoan snowball Earths. In the snowball climate state, CO ₂ consumption through silicate weathering collapses, and atmospheric CO ₂ accumulates via volcanic outgassing until a sufficiently strong greenhouse causes deglaciation. The duration and extent of ice cover are critical for planetary habitability, both on exoplanets and on Earth where animals emerged between the two glaciations. Radioisotopic ages have defined the duration of the Sturtian glaciation to 56 Myr, but the duration of the Marinoan glaciation (4 to 15 Myr) currently has 11 Myr of uncertainty. Here, we show that the Marinoan glaciation in Namibia lasted ca. 4 Myr with less than 10 m of vertical ice grounding line motion through glacial advance-retreat cycles. The stability of a low-latitude ice grounding line is consistent with the strong hysteresis of a hard snowball state. The disparity in durations demonstrates different routes to deglaciation, through slower CO ₂ accumulation for the longer Sturtian and radiative perturbation for the Marinoan. The short duration of the Marinoan glaciation may have been essential for the survival and evolution of animals and illustrates an additional path toward habitability on exoplanets.

The triple oxygen isotope composition of sulphate minerals has been used to constrain the evolution of Earth’s surface environment (e.g., pO₂, pCO₂ and gross primary productivity) throughout the Proterozoic Eon. This approach presumes the incorporation of atmospheric O₂ atoms into riverine sulphate via the oxidative weathering of pyrite. However, this is not borne out in recent geological or modern sulphate records, where an atmospheric signal is imperceptible and where terrestrial pyrite weathering occurs predominantly in bedrock fractures that are physically more removed from atmospheric O₂. To better define the transition from a Proterozoic to a modern-like weathering regime, here we present new measurements from twelve marine evaporite basins spanning the Phanerozoic. These data display a step-like transition in the triple oxygen isotope composition of evaporite sulphate during the mid-Paleozoic (420 to 387.7 million years ago). We propose that the evolution of early root systems deepened the locus of pyrite oxidation and reduced the incorporation of O₂ into sulphate. Further, the early Devonian proliferation of land plants increased terrestrial organic carbon burial, releasing free oxygen that fueled increased redox recycling of soil-bound iron and resulted in the final rise in pO₂ to modern-like levels.

The Snowball Earth hypothesis predicts global ice cover; however, previous descriptions of Cryogenian (720-635 Ma) glacial deposits are limited to continental margins and shallow marine basins. The Tavakaiv (Tava) sandstone injectites and ridges in Colorado, USA, preserve a rare terrestrial record of Cryogenian low-latitude glaciation. Injectites, ridges, and chemically weathered crystalline rock display features characteristic of fluidization and pervasive deformation in a subglacial environment due to glacial loading, fluid overpressure, and repeated sand injection during meltwater events. In situ hematite U-Pb geochronology on hematite-quartz veins, which crosscut and are cut by Tava dikes, constrain sand injection at ~690-660 Ma. We attribute early Tava sand injection episodes to basal melting associated with rifting and geothermal heating, and later injections to meltwater generation during ~661 Ma Sturtian deglaciation. A modern analog is provided by the Ross Embayment of Antarctica, where rift-related faults border sediment-filled basins, overpressurized fluids circulate in confined aquifers below ice, and extensive preglacial topography is preserved. Field evidence and geochronology in Colorado further highlight that deep chemical weathering of Proterozoic bedrock and denudation associated with the Great Unconformity predate Cryogenian injection of fluidized sand, consistent with limited glacial erosion.