My group's research focuses on reconstructing the geological context of pivotal environmental transitions in Earth history. These studies begin with geological mapping and stratigraphic analysis, and extend to laboratory studies that incorporate stable and radiogenic isotope geochemistry, geochronology, paleomagnetics, and paleontology. Recently, we have been exploring previously undescribed geological records of the Neoproterozoic Era, ~1000-540 million years ago (Ma), providing original observations and data from unique exposures in Mongolia, Arctic Alaska, Yukon, and the Kalahari desert. The Neoproterozoic witnessed the origin of animals, extreme swings in climate, large perturbations in the chemical composition of the ocean, but the nature and interrelationships of these biological and environmental changes are still poorly constrained. The data being generated from these new localities is bridging this gap and paving the way for a synthesis of the co-evolution of life and the environment through this critical period of Earth history.
Neoproterozoic Earth History
The most important element of any story is time. The geological record contains a time series of events; reconstructing their order is essential for disentangling causes and effects. One of the most provocative stories recently extracted from the geological record comes from Neoproterozoic strata that host sedimentological and paleomagnetic evidence of sea-ice at equatorial latitudes. These data inspired the Snowball Earth hypothesis, which posits that during the Neoproterozoic, a runaway ice-albedo event resulted in global glaciation. Intriguingly, a major diversification of eukaryotic crown groups including the origin of animals is roughly coeval with these glaciations. This apparent coincidence gives rise to questions central to our origins: How extreme were these glaciations? Why did our climatic regulation system fail? Did global glaciation trigger the diversification of life? Did the diversification of life initiate climate catastrophe? To address these questions we first have to build accurate records of what actually happened and how these events unfolded.
The Snowball Earth hypothesis predicts synchronous glaciation at low-latitudes, which end as a result of the syn-glacial build-up of pCO2, leading to a super-greenhouse aftermath. The prediction of synchroneity is testable with U/Pb geochronology on zircons from volcanic rocks inter-bedded with Neoproterozoic glacial deposits. Although “Marinoan” glacial deposits from multiple continents have been dated at 635 Ma, older, “Sturtian” glacial deposits have yielded an array of ages between 750 and 660 Ma. Furthermore, previous studies found evidence for low-latitude deposition only in Marinoan glacial deposits. This led some authors to suggest that the Neoproterozoic glaciations were not Snowball events, but that the Neoproterozoic merely represented a glacial period similar to the Pleistocene.
We began our attempt to calibrate the Neoproterozoic Period on the Kalahari craton, along the Orange River, which marks the border between South Africa and Namibia. There, glacial deposits had been reported to underlie a 741±6 Ma rhyolite flow, representing the oldest age constraint on Neoproterozoic glaciation. We remapped the region and discovered that the conglomerate of the Kaigas Formation (Fm) that is below the dated rhyolite is a debrite that was miscorrelated with glacial deposits of Numees Fm. The Numees diamictite is stratigraphically above the 741 Ma rhyolite, and can be correlated with carbon and strontium isotope (d13C and 87Sr/86Sr) chemostratigraphy to Sturtian glacial deposits around the globe. This age model is important not only because it narrows the age constraints on the Sturtian glaciation, but also because banded iron formation (BIF) is present in the Numees Fm. BIF’s return to the rock record in the Neoproterozoic after a billion year absence is an iconic feature of the Snowball Earth hypothesis, thought to represent the buildup of ferrous iron in anoxic, ice-covered oceans. Work by myself and colleagues in the Kalahari, Death Valley, and NW Canada demonstrates that the reappearance of BIF was not widespread, but focused both in space and time, occurring exclusively in Sturtian-age glacial deposits that formed in grabens with active volcanism [e.g. Fig. S3 of 16] . These geological associations suggest that the combination of lowered sea-level during glaciation, restriction in narrow, actively extending basins, favorable Fe/S ratios in the ocean, and enhanced subaqueous volcanism, conspired to produce this enigmatic facies [15, 33].
After refining the maximum age constraints on putative Sturtian glacial deposits in the Kalahari, we focused our efforts in NW Canada where we identified evidence for grounded marine ice-sheets with interbedded volcanic rocks. With colleagues at Boise State University, we then dated a rhyolite dome that underlies the glacigenic strata at 717.4 ± 0.1 Ma, and dated a tuff within these glacial deposits at 716.5 ± 0.2 Ma, providing the first age constraints on the onset of the Sturtian glaciation . Finally, we dated sills of the Franklin large igneous province (LIP), which host a robust low-latitude paleomagnetic pole, to 716.3 ± 0.5 Ma, indistinguishable to our syn-glacial age, demonstrating that Sturtian glacial deposits extended to the equator , as had been shown previously for the Marinoan glaciation.
The synchroneity of the Franklin LIP and the onset of the Sturtian glaciation suggested a possible link between the two, consistent with the Fire and Ice hypothesis, which proposes that the low latitude breakup of the supercontinent Rodinia, followed by the implacement of LIPS at low-latitude, increased CO2 consumption via enhanced weatherability and plunged the Earth into a global glaciation [e.g. 16]. To test the Fire and Ice hypothesis and to better constrain the duration of the Sturtian glaciation in strata that lack interbedded volcanic rocks, we began a coupled Re-Os geochronology and Osi and Sr isotope chemostratigraphy campaign in NW Canada. With colleagues at MIT and Durham University, we found that the oceanic Osi and Sr isotopes trend toward unradiogenic, mantle-like values going into the Sturtian glaciation, and spike to extremely radiogenic values in the cap carbonate sequence . These data are further consistent with initiation of the Sturtian glaciation via the emplacement and subsequent weathering of flood basalts at low-latitude, followed by glacial scouring and extreme weathering of the continents during deglaciation in a super-greenhouse. Moreover, we dated the Sturtian cap carbonate to 662.4 ± 3.9 Ma (Re-Os isochron). Coupled with the 717.4 ± 0.1 Ma and 716.5 ± 0.2 Ma ages bracketing the onset , these dates represent the first set of age constraints on both the onset and demise of a Neoproterozoic glaciation from a single margin. Together with the recalculation of Re-Os ages from Australia and existing U-Pb zircon ages, these data suggest a minimum duration of ~50 Myr for the Sturtian glaciation, and globally synchronous deglaciation at ca. 662 Ma . We are now building off of these results from NW Canada, and utilizing the unique preservation of weakly deformed organic-rich limestone in Mongolia to construct high-resolution Neoproterozoic-Cambrian 87Sr/86Sr and Osi curves tied to U-Pb and Re-Os geochronology. Our initial results from Mongolia are in line with the data from NW Canada , and suggest major mantle input to the ocean proceeds glaciation, and that the Neoproterozoic increase in 87Sr/86Sr is not a linear trend as previously proposed, but is instead stepwise, with rapid rises after glacial events, consistent with extreme weathering in the super-greenhouse aftermath of global glaciation.
Paleoenvironmental reconstructions rely heavily on geochemical proxy records from marine sediments. Along with dramatic changes in climate and biology, d13C studies through Neoproterozoic strata host some of the largest perturbations to the carbon cycle in the geological record [e.g. 10,16]. Neoproterozoic negative d13C excursions have been recently attributed to the remineralization of a giant dissolved organic carbon (DOC) pool, which would have entailed the consumption of massive volumes of oxidants [e.g. 29,31]. The large DOC model was based on the lack of covariance in organic carbon isotopes (d13Corg) and carbonate carbon isotopes (d13Ccarb) from carbonate samples with very low organic carbon content (TOC). In Mongolia, we discovered a new, large d13Ccarb excursion (from +8‰ to -7‰ back to +10‰ over ~50 m of strata) in relatively TOC-rich carbonate, referred to as the Tayshir excursion . With colleagues at Harvard, we then demonstrated that d13Corg and d13Ccarb covaried through the Tayshir excursion, and argued that a large DOC pool never existed, but rather, the apparent lack of covariance in previous studies was due to masking of detrital and migrated organic matter in TOC-poor rocks . These new data have opened up an alternative framework to reinterpret the d13C record through Earth history that is not in direct conflict with oxidant budgets .
The appearance of macroscopic fossils in the Neoproterozoic has long been assumed to be related to a rise in atmospheric oxygen, however, independent evidence for an oxygenation event has remained elusive [e.g. 35]. Previous studies in NW Canada correlated the first appearance datum (FAD) of Ediacaran fauna with a shift in Fe speciation data that was interpreted to represent an oxygenation of Ediacaran oceans [L]. Our recent mapping has demonstrated that the FAD of Ediacaran fauna occur above a major sequence boundary, and are not associated with the change in Fe speciation data. We also discovered a large negative d13C excursion through these strata that we correlate to the Shuram excursion [M]. We further proposed that the Shuram anomaly was driven by changes in the size and location of the global authigenic carbonate reservoir [31,36]. This anomaly occurs above the FAD of Ediacaran fauna, and directly below the FAD of bioturbation. We are currently exploiting unique late Neoproterozoic to Early Cambrian records in NW Canada and Mongolia to further elucidate the relationships between the appearance of large animals, the evolution of mobility, mixing of the sediment-water interface, changes in the carbon cycle, and the putative rise in atmospheric oxygen. Preliminary paired d13Ccarb and d13Corg results through the Early Cambrian d13Ccarb oscillations in TOC-rich limestone in Mongolia display large lateral gradients and lack covariance, both indicative of authigenesis [consistent with 31]. Coupled with the fact that these strata host phosphorites and some of the richest trace fossil and small-shelly fossil records in the world, our studies in Mongolia promise to enrich our understanding of the coevolution of life and the environment for many years to come.
To compliment our refined environmental records, my group and our collaborators have embarked on a parallel effort to construct a time-calibrated Neoproterozoic paleontological database. We began this work in NW Canada where we dated the Bitter Springs d13C event to 811.5 ± 0.3 Ma, and used this marker to recalibrate the diversification of eukaryotic crown groups . We also remapped outcrops containing unique scale microfossils and reassigned these strata to units that are ~150 million years older than previous estimates , directly above the 811.5 Ma horizon . Further work demonstrated that these microfossils are likely akin to green algae, are composed of phosphate, and may represent the earliest know example of eukaryotic biomineralization . We then extended our reassessment of the Neoproterozoic microfossil record to Death Valley, CA, where a dolostone bed within the glacigenic Kingston Peak Fm had been claimed to represent a syn-glacial microfossil assemblage. Our group’s mapping and geochemical studies show that the fossiliferous horizon are olitstostroms (slump blocks) from a particular horizon in the underlying Beck Spring dolomite, and thus cannot be used to assess the nature of the glaciations . While clearing up the context of these earlier finds, we have discovered new Neoproterozoic microfossil assemblages in Mongolia, Namibia, Death Valley, and the Yukon [24-27,33,34]. These initial efforts suggest that there are major differences in microfossil assemblages before and after the Sturtian glaciation that may be useful not only for biostratigraphy, but also for understanding the impact of global glaciation on the evolution of life.
The breakup of Rodinia and the emplacement of LIPs at low-latitude has long been portrayed as the tectonic context of Neoprotoerozic climate change. The interaction of plumes with continental crust may lead not only to changes in weatherability and CO2 consumption, but also to perturbations in the chemical composition of the ocean, including oxidant and nutrient loads, and also to changes in volatile inputs to the atmosphere. Thus, the construction of a high-resolution tectonic record is necessary to really understand potential links between tectonics and environmental change. Our geological mapping of Neoproterozoic and Paleozoic strata up and down the Cordillera from Arctic Alaska to southern California has also allowed us to reassess the tectonic evolution of the western margin of Laurentia (which occupied central Rodinia). It has been variously suggested that the basal Windemere Supergroup or the Mackenzie Mountains Supergroup (MMSG) record the rifting of Rodinia. In Macdonald et al.  we demonstrate that the MMSG and correlative strata were accommodated by extension exclusively on the NW margin of Laurentia, and suggest that subsidence was related to the passing of the core of Rodinia (Australia-South China-Laurentia-Siberia) over a long-lived plume, which subsequently produced the Gairdner-Gubei-Gunbarrel-Franklin LIPs. We are currently working with colleagues on a new model for the subsequent tectonic evolution of Rodinia that is consistent with thermal subsidence models.
Shock effects including shatter cones, PDFs in quartz grains, impact melt rocks, and pseudotacylites were identified in deeply eroded granites in the Yilgarn Craton (119˚50’E, 27˚10’S). Geological and geophysical relationships suggest that the original structure was at least 30 km in diameter and was formed during the early Proterozoic.