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Project Progress

In 2014, the Macdonald group continued its field-based studies of Meso- and Neoproterozoic successions in North America, Mongolia, and the Congo. These studies aim to cross-calibrate paleontological, geochemical, tectonic, and climatological records through this critical interval and test hypotheses related to their interrelationships. In particular, did biological innovations trigger environmental change or did environmental change stimulate evolution? Fieldwork in northwestern Canada aimed to field-check mapping (e.g. Strauss et al., 2014a), finish a study on the stratigraphy and sedimentology of the 750-730 Ma Callison Lake Formation to better understand the environmental setting of uniquely preserved vase-shaped microfossils (Strauss et al., 2014b), and to document Meso-Neoproterozoic successions in the Wernecke Mountains. The Callison Lake Formation is a particularly important record, not only for its unique microfossil record, but also because it hosts strong geochronological constraints (Strauss et al., 2014b), environmentally diagnostic sedimentary and early diagenetic talc deposits, and the ca. 735 Ma Islay carbon isotope excursion (Rooney et al., 2014; Strauss et al., 2014b). Using geochemical, tectonic, and stratigraphic data, they suggest this carbon isotope excursion was independent of the onset of the ca. 717-660 Ma Sturtian glaciation (Rooney et al., 2014) and was the result of the uplift of broad organic-rich intracontanental seaways that weatherered both organic matter and sulfate into the ocean. In the Wernecke Mountains Macdonald and colleagues further documented contractional structures related to this uplift event, and sampled for geochemistry and paleontology in measured sections of the Meso-Neoproterozoic Pinguicula and Hematite Creek Groups. Preliminary data suggests that these basins host large positive carbon isotope anomalies, and that there may be more structure to the Mesoproterozoic carbon isotope curve than previously suggested. That is, the carbon cycle of the “boring billion” may not be as boring as previously suggested.

In Death Valley, after leading a field trip for NAI, Macdonald and his students continued studies of the regional stratigraphic and geochemical record. Recent work on the Beck Spring Dolomite, which is correlative with the Callison Lake Formation, has revealed a coeval uplift event (Smith et al., in press) suggesting a shared tectonic context for both the deposition of these rocks and the Islay carbon isotope excursion. A continuation of this work is the exploration of the Meso- to Neoproterozoic records of the underlying Crystal Spring Formation. Particularly, Macdonald and colleagues are attempting to better constrain the age of a large positive carbon isotope excursion. Current constraints suggest the presence of a positive carbon isotope excursion >+5 per mil between 1210 Ma and 1080 Ma, culminating with the emplacement of the SW large igneous province.

In Mongolia, the focus has been on further documenting the depositional environments of fossiliferous Cryogeneian successions that are ideally suited to geochemical studies. A new Re/Os age of ca. 660 Ma from the Sturtian cap carbonate in Mongolia is indistinguishable from geochronological constraints on the Sturtian cap carbonate from China and NW Canada (Rooney et al., 2014). Field work in 2014 led to the discovery of new putative Ediacaran calcifying fossils and the final touches on the documentation of large carbon isotopic variability that leads into the Precambrian-Cambrian transition. Finally, in 2014, sampling of core material in the Congo yielded additional age constraints on the Sturtian glaciation, further dispelling the notion of a pre-Sturtian ‘Kaigas’ glaciation. Multiple cores from the Congo and Zambia were shipped back to Harvard and will be available for further geochemical, paleontological, and geochronological studies.

Work in the Cohen lab this year has focused on finding, describing, and better understanding eukaryotic microfossils from lower Neoproterozoic sedimentary successions in northwestern Canada. This work has relied heavily on undergraduate student researchers at Williams College, and has been done in close collaboration with Justin Strauss and Francis Macdonald of the Macdonald lab at Harvard University. Vase-shaped microfossils (VSMs), interpreted as the remains of testate amoebae, are found in Late Tonian sedimentary rocks around the world. A new assemblage of VSMs has recently been described by our NAI team from the Callison Lake Dolostone in Yukon, Canada (Strauss et al., 2014). Dated by a Re-Os isochron age of 739.9 +/- 6.1 Ma, these microfossils are indicative of a marked diversification of eukaryotic life before the Sturtian-age Snowball Earth event, and are roughly coeval with diverse VSM assemblages from the Chuar Group of Grand Canyon, Arizona. Cohen and her colleagues are investigating the taphonomy of the Callison Lake VSMs, analyzing preserved fossils in petrographic thin section, on etched rock surfaces, and from HF macerations. EDS maps are used to analyze the spatial distribution of elements in all three preparation techniques to interpret the pathways by which these informative fossils were preserved. Silica is pervasive in all samples from thin sections and etched surfaces. In most samples, there are carbon accumulations at the edges of fossils, and in some, a high concentration of carbon throughout. In other cases, however, no heightened concentrations of these elements is apparent. Rather, aluminum and other clay minerals were found to be associated with fossil rims. At least three taphonomic processes appear involved: authigenic mineralization, original preservation, and internal molds. Understanding the preservation of VSMs will help us to better determine why these fossils are absent from younger rocks, and give us insight into their potential use as index fossils for the Late Tonian.

Further research in the Cohen lab addresses remarkbale phosphatic sale microfossils from the Tonian (ca. ~800 Ma) Fifteenmile Group, Yukon, Canada. The scales – the most diverse record of eukaryotic organisms before the radiation of animals — are preserved along with microbial and eukaryotic population of organic-walled microfossils. To date, the phosphatic scales have not been recorded from other deposits, raising the question of what preservational circumstances gave rise to the Yukon occurrence. Field work in June 2014 has given us better insight into the sedimentology of the fossiliferous strata, which consist of approximately 40 meters of interbedded organic-rich parallel-laminated limestone, tabular clast or edgewise conglomerate, and calcareous black and grey shale. The absence of wave-generated bedforms, abundance of hemipelagic deposits, and presence of matrix-supported conglomerates interpreted as debrites suggests these strata were deposited well below storm-wave base in a slope environment. Interestingly, all of the autochthonous deep-water and allochthonous redeposited facies contain chert, but it is highly variable in abundance, distribution, and form. Chert deposits include nodules ranging from 1 mm to greater than 4 cm in diameter, many of which exhibit evidence of extremely early formation within sediments. Ongoing work shows that these fossils are easily destroyed via low temperature diagenesis in chert nodules, but less easily destroyed in the host limestone. This observation, along with new sedimentological and environmental interpretations, sheds light on why these unique fossil assemblages have not yet been found elsewhere despite extensive examination; furthermore, this deposit presents a key window into deep-water chert preservation, a taphonomic window that is quite unique among fossiliferous Proterozoic chert deposits that are generally constrained to peritidal settings.

Organic-walled fossils from well-constrained strata of early Neoproterozoic age are rare, especially in comparison with the abundance of younger Tonian, Cryogenian, and Ediacaran assemblages. A third ongoing project in the Cohen lab documents the oldest known organic-walled biota preserved in carbonates from NW Canada. Both eukaryotic and bacterial fossils have been extracted from carbonates of the early Neoproterozoic (pre-811 Ma) Reefal Assemblage of the Fifteenmile Group in the Coal Creek Inlier, Yukon. Fossils include organic filaments, which may represent both bacterial and eukaryotic organisms, and acritarch taxa inferred to be eukaryotic. Finally, Phoebe Cohen is spearheading a project on biological diversity through the Proterozoic Eon. Over the past half-century, the number of fossils described from Proterozoic rocks has increased exponentially. These discoveries have occurred alongside an increased understanding of the Proterozoic Earth system and the geological context of fossil occurrences, including improved age constraints. However, the evaluation of relationships between Proterozoic environmental change and fossil diversity has been hampered by several factors, particularly lithological and taphonomic biases. Cohen and colleagues have compiled and analyzed the current record of eukaryotic fossils in Proterozoic strata to assess the effect of biases and better constrain diversity through time. Their results show that mean within-assemblage diversity increases through the Proterozoic Eon due to an increase in high diversity assemblages, and that this trend is robust to various external factors including lithology and paleogeographic location. In addition, assemblage composition changes dramatically through time. Most notably, robust recalcitrant taxa appear in the early Neoproterozoic Era, only to disappear by the beginning of the Ediacaran Period. Diversity is significantly lower in the Cryogenian Period than in the preceding and following intervals, but the short duration of the nonglacial interlude and unusual depositional conditions may present additional biases in the fossil record. In general, large-scale patterns of diversity are robust while smaller-scale patterns are difficult to discern through the lens of lithological, taphonomic, and geographic variability. These analyses caution against the use of stratigraphic assemblage changes at a single locality to infer large-scale evolutionary trends. Further quantifying and addressing these biases will be an essential step towards a future understanding of the nature and geological context of Proterozoic eukaryotic diversification.

The Bosak lab has been collaborating with Phoebe Cohen, Francis Macdonald, and Sara Prus on the study of fossils preserved in the Cryogenian carbonate strata. Phoebe Cohen described novel organic forms similar to modern red algae (Cohen et al., in press). Kelsey Moore, an undergraduate from Smith College, discovered new agglutinated forms in post-Sturtian cap carbonates from Mongolia and Zambia and pyritized fossil Obruchevella in post-Sturtian cap carbonates from Arctic Alaska. Bosak and Emily Matys, a graduate student, investigated the preservation and indicators of thermal maturity of Recent and agglutinated foraminifera (in collaboration with Dave McNeil, the Geological Survey of Canada) and identified highly ordered organic matter and silica in post-mature samples (McNeil et al., in press). These findings provide insights into the preservation of agglutinated fossils in Cryogenian sediments. In collaboration with D. Lahr (University of Sao Paolo), Bosak also hypothesized links between a large Phanerozoic radiation within Arcellinid testate amoebae and the expansion of vascular plants (Lahr et al., submitted).

Sharon Newman, a graduate student from the Bosak lab, studies microbial fossilization in siliciclastic sediments, with the goal of understanding conditions that preserved microbial mats in shales and on the surfaces of Neoproterozoic siltstones and sandstones. She has developed an experimental system in which to investigate the preservation of cyanobacterial mats and aggregates and fragments of soft-bodied animals. She has identified conditions that preserve thin-sheathed cyanobacterial filaments and is characterizing minerals that coat fossilized cells and determine the influence of Si and Al concentration in the solution on the composition of these minerals. Matt Joss, a Masters student working with Bosak and Rothman is using a similar system to investigate the influence of iron concentration on the preservation of marine organic matter in environments characterized by low concentrations of O₂. Bosak continued her collaboration with Shuhei Ono (MIT) on microbial fractionation of sulfur isotopes and its environmental implications (Ono et al., 2014). Bosak and Ono are supervising Shikma Zaarur, a postdoc, who is determining the magnitude of sulfur isotope fractionation in enrichment and pure cultures of various sulfur cycling microbes that utilize different sulfur and carbon sources.

Giulio Mariotti, a postdoc in the Bosak lab, has experimentally related the macroscopic morphology of large elongated stromatolites to mechanisms that produce sand waves (Mariotti et al., 2014a) and identified a microbially mediated mechanism which accounts for the formation of wrinkle structures (Mariotti et al., 2014b), enigmatic mm-scale ridges and pits that occur in many Ediacaran and Cambrian siliciclastic deposits, often together with trace fossils or impressions of macroscopic organisms and animals. A similar microbially mediated mechanism can also generate linear trails on the surface of sand that share a number of characters with various linear, simple, surface trace fossils that are commonly attributed to animals.

Mariotti, Newman, Summons, Pruss and Bosak took a field trip to the Bahamas to study the formation of ooids, a common type of concentrically laminated, coated carbonate grains. They cored sediments in Pigeon Cay, an oolitic beach and developed a model of ooid production in this environment (Mariotti et al, submitted). This model suggests that a substantial part of carbonate in ooids precipitates in microbially bound sediments and that the growing grains are periodically delivered to the highly agitated areas, polished and rounded. This model explains the high concentration of organic matter in ooids. Shane O’Reilly, a postdoctoral scholar is studying the composition of microbial communities and organic matter in ooids from actively mobilized and microbially stabilized areas of Pigeon Cay to understand the formation and preservation of biosignatures in ooids (in collaboration with Vanja Klepac-Ceraj).

Research on Theme II in the Knoll lab has focused on Mesoproterozoic sedimentary assemblages in Russia, in collaboration with scientists from the geological institute of the Russian Academy of Sciences in Moscow. Based on fieldwork in 2013, the team has been able to establish that, unlike most Mesoproterozoic basins, the ca. 1400 Ma basin of the Volgo-Ural region, just west of the southern Ural Mountains contained oxygenated waters in its deepest parts (Sperling et al., 2014). This documents the variability of redox profiles among Proterozoic basins and underscores the need for statistical methods in the analysis of proxy data used to reconstruct Earth’s redox history. This team has also discovered a rich assemblage of microfossils in these rocks, including a number of taxa interpreted as eukaryotic. A systematic monograph on these fossils is nearing completion. In related research, Knoll and collaborator Emmanuelle Javaux (University of Liege, Belgium) have completed a systematic monograph on the exceptionally rich microfossil assemblages of the 1500-1400 Roper Group, Australia. Together, these deposits provide some of the best available evidence for the timing and nature of the initial eukaryotic radiation in Proterozoic oceans, setting the later Neoproterozoic radiation of diverse crown-group eukaryotes in both phylogenetic and historical context.

Research in the Johnston lab is leading to new insights into environmental change through the later Proterozoic Eon. One of the most exciting findings in Proterozoic research over the last decade has been the finding of anomalously depleted ¹⁷O in sulfate minerals within and just above glaciogenic Neoproterozoic rocks. Given the vast potential of this proxy, the Johnston lab, with NAI support, has installed and is running a next-generation isotope analysis system that enables the precise measure of ¹⁷O. Much of 2014 was spent improving analytical capabilities (see below) and consequently providing new theoretical constraints on the triple oxygen isotope composition of sulfate in the sedimentary record. These analytical improvements will be more fully realized in 2015 when Johnston and students will generate high-precision records of sedimentary sulfates throughout the Precambrian and Phanerozoic. In more detail, the last year witnessed the completion of a laser fluorination line for generation of oxygen gas from sulfates, silicates, and phosphates. In parallel, we installed a new, purpose-designed Thermo 253 Isotope Ratio Mass Spectrometer, linked to gas line described above. This system has already resulted in an improved precision by a factor of 5 (a standard deviation of less than 0.01‰). The scientific application of this system is in progress and already resulted in the submission of the first study (entitled Triple oxygen and multiple sulfur isotope constraints on the evolution of the post-Marinoan sulfur cycle and submitted to EPSL in October). Within unique cap carbonate facies that mark the transition from icehouse to greenhouse climatic states in the Neoproterozoic, unusual seafloor barite beds record anomalous triple oxygen isotope compositions. The generation of this atmospheric signal likely reflects elevated pCO₂, diminished primary production, or both during the Marinoan “Snowball Earth” glaciation. Despite their global occurrence and environmental significance, the depositional history recorded by these barite units remains enigmatic. In the submitted study we present new oxygen and sulfur isotope data from barite beds that mark the top of the Marinoan-aged Ravensthroat cap dolostone in northwest Canada. Within a dynamic 1-box model, they show that the observed isotope systematics do not require unusual perturbations to the marine sulfur cycle, but instead reflect the interplay between sulfide weathering, microbial sulfate reduction and pyrite burial. The isotopic changes recorded in the Ravensthroat barite layer were rapid, however, with ³⁴S enrichment and concomitant removal of the ¹⁷O isotope anomaly occurring over approximately five turnovers of the global marine sulfate reservoir. With conservative estimates on sulfate fluxes from sulfide weathering, this result implies that the post Marinoan marine sulfate reservoir was small (<1% of modern), and potentially responded on timescales that are comparable to those of modern glacial-interglacial intervals. Through the lens of these unique Neoproterozoic seafloor barite beds, here we demonstrate the swiftness of the transition from the Cryogenian to Ediacaran Earth system. Since the submission of this work, new datasets are being generated. These new data are exciting and mean that more advanced models of paleoenvironments are needed. We look forward to submitting these results for publication in 2015. This work, principally carried out by post-doctoral scholar Ben Cowie with PI Johnston, has been entirely supported by NAI funds.

In collaboration with (among others) members of the Knoll lab, John Grotzinger and students have explored the geochemical and petrographic origins of “net-textured pyrite,” unique macroscopic features characterized by millimeter-scale porous structures in a Mesoproterozoic massive sulfide deposit. The deposit is the Black Butte Copper Project, set in a sub-basin of the Belt Supergroup, Montana. Samples of net-textured pyrite come from drill core material in stratiform sedimentary-exhalative-style sulfide horizons in dolomitic shales of the lower Newland Formation. In combination, reflected light microscopy, X-ray micro-computed tomography (micro-CT), and sulfur isotope geochemistry with secondary ion mass spectrometry (SIMS) and multi collector inductively coupled mass spectrometry (MC-ICP-MS suggest an origin from concentrated fluid flow, broadly similar modern low-temperature hydrothermal vent sites such as the Lost City alkaline hydrothermal field. These modern analogs providing exciting environments for geobiological research; at the same time, the structures make it clear that hydrothermal systems influenced both environments and ore deposition in this Mesoproterozoic basin (Bergmann et al., 2014).

Geochemical results exhibit an unexpectedly large range of sulfide-sulfur isotopic composition, from -17‰ (VCDT) to +44‰ with a mean of 9.6‰. If early diagenetic barite δ³⁴S (~15‰, Lyons et al., 2000) represents seawater slightly ³⁴S-enriched by closed-system bacterial sulfate reduction, then the sulfide minerals analyzed here represent a wide range of Rayleigh distillation products. The range of isotopic compositions measured in Black Butte has not been observed with abiotic mixing and removal of thermally-reduced sulfides in modern black smoker systems (~0-5‰). Thus, thermophilic microbial communities may have also oxidized organic material in the surrounding shale to drive sulfate reduction.

The Pearson lab is addressing the question: Are hopanoids primarily remnants of primary producers or of heterotrophic consumers? Do they primarily come from free-living marine communities, or from shallow mats, tidal zone communities, or even terrigenous runoff? We have developed new methodologies and analyses of compound-specific carbon isotope data for hopanoids to infer their sources in modern systems, as proxies for understanding ancient environments. Data from a range of modern systems with different likely sources are pending.

Pearson and colleagues have also been exploring molecular and isotopic biosignatures of organic matter deposited from anoxygenic, photosynthetic communities. By studying the modern analogue system of Mahoney Lake, Canada, they have shown that most of the material reaching the lake bottom comes from the shoreline microbial mats, rather than the planktonic community living in the chemocline. The molecular signature of the mats and sediments bears a striking resemblance to the ooids studied by the Summons group from the Bahamas.

To understand the evolution of Earth’s redox cycles, it is critical to understand the mechanisms that establish, maintain, and record the geologic history of marine euxinia – including its temporal and spatial scales, as well as its biomarker record to sediments. The potential for export and burial of microbial biomass from anoxic photic zones (photic zone euxinia, PZE) remains relatively understudied, despite being of fundamental importance to interpreting the geologic record of bulk total organic carbon and individual lipid biomarkers. In this project we studied the relative concentrations and carbon isotope ratios of lipid biomarkers from the water column and sediments of meromictic Mahoney Lake. Mahoney Lake has been proposed as a modern analogue for Earth systems that may have been sulfidic into the shallow photic zone, including environments in which sulfidic photic zones intercept shallow, sulfide-oxidizing, benthic microbial mats (Hamilton et al., 2014). Pearson and colleagues have found that the fatty acid profiles (Figure 1), as well as n-alcohol and sterol profiles, in the central basin sediments are indistinguishable from material in the shoreline microbial mat (Bovee et al., 2014). This is true both for lipid distribution and for the compound-specific carbon isotope ratios (δ¹³C). In contrast, the lipid profiles and carbon isotope ratios of biomass obtained directly from the region of planktonic PZE do not match the profiles in the basinal sediment. Due to the strong density stratification and the intensive carbon and sulfur recycling pathways in the water column, there appears to be minimal direct export of the sulfur-oxidizing planktonic community to depth. The results instead suggest that basinal sediments are exported from the lake margins, a mat-rich system that integrates an indigenous shoreline microbial community and the degraded remains of laterally-rafted biomass from the PZE community. Material from the lake margins appears to travel down-slope and become deposited in the deep basin; its final composition may be largely heterotrophic in origin. In other projects, we found a similar signature suggesting the importance of heterotrophic bacteria in exported and preserved lipid signatures (Close et al., 2014). This suggests an important role for minerals and other large, sinking particles in aiding the burial of mat-derived organic matter in euxinic systems. Downslope or mineral-aided transport of anoxygenic, photoautotrophic microbial mats may have been a significant sedimentation process in early Earth history.

Finally, the Pearson lab has initiated a new study of the nitrogen isotope composition of porphyrins in Paleozoic and Neoproterozic samples, asking whether these provide a redox indicator. They are analyzing well preserved shales and oil samples from a variety of locations and temporal sources, primarily in the Paleozoic—including the Hirnantian, the Steptoean, and the Famennian. Relatively little is known about the biogeochemical cycling of nitrogen in deep time, which is in part due to a lack of robust observations from the rock record. Using high-throughput preparation of porphyrins by HPLC, followed by oxidation to NO₃- and analysis by the denitrifier method, we are measuring δ¹⁵N values across major biogeochemical transitions. The aim is to compare N-cycle changes to other known redox proxies, illuminating the history of ocean and atmosphere oxygenation. Preliminary results show that the preservation of porphyrins is heterogeneous and appears to depend on source rock lithology, with organic-rich shales being the richest reservoirs. In such samples as measured to date, values of δ¹⁵N generally are negative. Pearson and colleagues propose that such values reflect a fundamentally different nitrogen cycle, linked to changes in global marine redox budgets.

References:
Macdonald, F.A., Schmitz, M.D., Crowley, J.L., Roots, C.F., Jones, D.S., Maloof, A.C., Strauss, J.V., Cohen, P.A., Johnston, D.T., and Schrag, D.P., 2010. Calibrating the Cryogenian, Science, 327: 1241-1243.
Tosca, N.J., Macdonald, F.A., Strauss, J.V., Johnston, D.T., and Knoll, A.H., 2011. Sedimentary talc in Neoproterozoic carbonate successions. Earth and Planetary Science Letters, 306: 11-22.