Person:

Hoffman, Paul

Microbial structures in Neoproterozoic cap carbonates record the environmental processes present in the aftermath of global glaciation. The Rasthof Formation of northern Namibia is a unique carbonate depositional sequence that formed during post-glacial transgression and highstand following the Chuos glaciation. Carbon isotope profiles from four examined localities reveal that onlap was diachronous over post-glacial, syn-rift topography. The lower Rasthof Formation consists primarily of dark gray thinly (<mm) and thickly (1–4 mm) laminated microbialites that exhibit different rheological responses to the emplacement of syndepositional dikes. The thinly laminated microbialaminite facies commonly host cm-sized syndepositional folds of microbially laminated sediment called roll-up structures. In more thickly laminated facies, layers are deformed into broad decimeter-sized folds, but roll-up structures are absent. Large syndepositional carbonate clastic dikes (0.5–1 m wide) and smaller veins (0.1–0.5 m) cut across bedding in both the thinly and thickly laminated facies, but are conspicuously absent from underlying and overlying beds. These carbonate clastic dikes and veins contain convoluted microbial mats and abundant marine cements. The lack of evidence for wave action or current scouring in the form of bedforms, scour marks, or intraclasts indicates that these microbialaminites formed below storm wave base. The close spatial association of deep-water microbialaminite facies in the Rasthof cap carbonate with carbonate clastic dikes suggests that the emplacement of dikes produced both dm-sized broad folds and cm-scale laterally discontinuous roll-up structures. The emplacement of the dikes, most likely due to the release of fluids into incompletely lithified mats, deformed cement-rich thick laminites into broad folds, while thinly laminated and more slowly lithifying mats were rolled into roll-up structures. Microbialaminite facies in the Rasthof cap carbonate thus not only reflect the depositional and environmental processes that operated in the aftermath of the Sturtian glaciation, but may also provide clues for the formation of roll-up structures found in even older Precambrian carbonates.

Interpretations of major climatic and biological events in Earth history are, in large part, derived from the stable carbon isotope records of carbonate rocks and sedimentary organic matter1,2. Neoproterozoic carbonate records contain unusualand large negative isotopic anomalies within long periods (10–100 million years) characterized by d13C in carbonate (d13Ccarb) enriched to more than +5 per mil. Classically, d13Ccarb is interpreted as a metric of the relative fraction of carbon buried as organic matter in marine sediments2–4, which can be linked to oxygen accumulation through the stoichiometry of primary production3,5. If a change in the isotopic composition of marine dissolved inorganic carbon is responsible for these excursions, it is expected that records of d13Ccarb and d13C in organic carbon (d13Corg) will covary, offset by the fractionation imparted by primary production5. The documentation of several Neoproterozoic d13Ccarb excursions that are decoupled from d13Corg, however, indicates that other mechanisms6–8 may account for these excursions. Here we present d13C data from Mongolia, northwest Canada and Namibia that capture multiple large-amplitude (over 10 per mil) negative carbon isotope anomalies, and use these data in a new quantitative mixing model to examine the behaviour of the Neoproterozoic carbon cycle. We find that carbonate and organic carbon isotope data from Mongolia and Canada are tightly coupled through multiple d13Ccarb excursions, quantitatively ruling out previously suggested alternative explanations, such as diagenesis7,8 or the presence and terminal oxidation of a large marine dissolved organic carbon reservoir6. Our data from Namibia, which do not record isotopic covariance, can be explained by simple mixing with a detrital flux of organic matter. We thus interpret d13Ccarb anomalies as recording a primary perturbation to the surface carbon cycle. This interpretation requires the revisiting of models linking drastic isotope excursions to deep ocean oxygenation and the opening of environments capable of supporting animals9–11.

Triple oxygen isotopes within post-Marinoan barites have played an integral role in our understanding of Cryogenian glaciations. Reports of anomalous View the MathML source values within cap carbonate hosted barites however have remained restricted to South China and Mauritania. Here we extend the View the MathML source anomaly to northwest Canada with our new measurements of barites from the Ravensthroat cap dolostone with a minimum View the MathML source value of −0.75‰. For the first time we pair triple oxygen with multiple sulfur isotopic data as a tool to identify the key processes that controlled the post-Marinoan sulfur cycle. We argue using a dynamic 1-box model that the observed isotopic trends both in northwest Canada and South China can be explained through the interplay between sulfide weathering, microbial sulfur cycling and pyrite burial. An important outcome of this study is a new constraint placed on the size of the post-Marinoan sulfate reservoir (≈0.1% modern), with a maximum concentration of less than 10% modern. Through conservative estimates of sulfate fluxes from sulfide weathering and under a small initial sulfate reservoir, we suggest that observed isotopic trends are the product of a dynamic sulfur cycle that saw both the addition and removal of the View the MathML source anomaly over four to five turnovers of the post-Marinoan marine sulfate reservoir.

Geological evidence indicates that grounded ice sheets reached sea level at all latitudes during two long-lived Cryogenian (58 and ≥5 My) glaciations. Combined uranium-lead and rhenium-osmium dating suggests that the older (Sturtian) glacial onset and both terminations were globally synchronous. Geochemical data imply that CO2 was 102 PAL (present atmospheric level) at the younger termination, consistent with a global ice cover. Sturtian glaciation followed breakup of a tropical supercontinent, and its onset coincided with the equatorial emplacement of a large igneous province. Modeling shows that the small thermal inertia of a globally frozen surface reverses the annual mean tropical atmospheric circulation, producing an equatorial desert and net snow and frost accumulation elsewhere. Oceanic ice thickens, forming a sea glacier that flows gravitationally toward the equator, sustained by the hydrologic cycle and by basal freezing and melting. Tropical ice sheets flow faster as CO2 rises but lose mass and become sensitive to orbital changes. Equatorial dust accumulation engenders supraglacial oligotrophic meltwater ecosystems, favorable for cyanobacteria and certain eukaryotes. Meltwater flushing through cracks enables organic burial and submarine deposition of airborne volcanic ash. The subglacial ocean is turbulent and well mixed, in response to geothermal heating and heat loss through the ice cover, increasing with latitude. Terminal carbonate deposits, unique to Cryogenian glaciations, are products of intense weathering and ocean stratification. Whole-ocean warming and collapsing peripheral bulges allow marine coastal flooding to continue long after ice-sheet disappearance. The evolutionary legacy of Snowball Earth is perceptible in fossils and living organisms.