2011 Annual Science Report

Massachusetts Institute of Technology Reporting  |  SEP 2010 – AUG 2011

Molecular and Isotopic Investigations Across the Neoproterozoic

Project Summary

We gathered new data on molecular and isotopic stratigraphic trends from Cryogenian and Ediacaran sequences from Canada, Oman and Mongolia. These continue to show that the biogeochemical carbon cycle was anomalous, and unlike any other period in Earth history, prior to the advent of complex animal life. While difficult to interpret in a robust way these data are reproducible, reflect real trends and are not the result of 'diagenetic alteration’ as some have proposed.

4 Institutions
3 Teams
3 Publications
1 Field Site
Field Sites

Project Progress

A biomarker study of the oil-bearing Athel Silicilyte, Oman: graduate student Daniel Stolper
Understanding the rise of complex organisms near the end of the Precambrian (p€) eon is one of the major problems in earth history. Critical to understanding this time period is the availability in the rock record of unaltered sediments spanning the transition. The foundation of this project is that the Athel Silicilyte Member of the Athel Formation in the South Oman Salt Basin in Oman overlaps the p€-€ boundary. The Athel Silicilyte is an enigmatic organic and silica-rich rock believed to have formed in a deep-water (possibly restricted) marine basin. Few deeper-water sedimentary rocks are preserved at the p€-€ boundary, and even fewer contain abundant original biomass; but such occurrences are important because some theories about the evolution of complex life at this time propose a fundamental change in deep-water chemistry from reduced to oxidized. The Athel Silicilyte thus presents a potentially unique opportunity to study organic material from deep-water basins at the p€-€ boundary and to test hypotheses about whether deep marine water underwent a change at this time and, if so, how biological systems responded. We analyzed 15 samples from shales bounding the silicilyte and within the silicilyte itself. Biomarkers (biomolecular fossils) from the solvent extractable portion of the rocks were analyzed. Based on the biomarkers, the silicilyte represents a more reducing environment than the bounding shales implying that if a major oxidation of the deep oceans occurred at this time, it was not felt in this particular environment. Despite the more reducing nature of the silicilyte compared to the shales, a peak in biomarkers indicative of silica secreting sponges (which require oxygen) occurs in the silicilyte. This may indicate that sponges at this time were capable of living in more reducing conditions than commonly expected. To better understand how the silica formed, which is crucial to understanding the history of the silicilyte, we are currently performing silicon and oxygen isotopic studies on a small subset of core samples. By combining this information with the biomarkers we hope to construct a simple, plausible model for the formation and redox history of this basin during the p€-€ boundary.

Characterization of the Khufai Formation, Huqf Supergroup, Sultanate of Oman: graduate student Maggie Osburn
The Ediacaran period was a time of critical changes to the biosphere that may have been intimately linked to concurrent changes in global biogeochemistry. While fossil deposits of this age are rare, a rich stratigraphic record of environmental change is preserved. One such record is the Huqf Supergroup of Oman. Dated between 635Ma and the Cambrian boundary, these rocks provided a detailed record of marine chemistry through this time including major isotopic excursions in both carbon and sulfur. The Shuram carbon isotopic excursion – one of the largest deviations in Earth history- was first discovered in Oman before being found in deposits across the planet. While the causes of this isotopic excursion are as yet unknown, one hypothesis implicates major changes in Ediacaran carbon cycling. The Shuram Excursion begins in the uppermost Khufai Formation, reaches a nadir of -12‰ in the Shuram Formation and recovers in the overlaying Buah Formation. The goal of this research is to better characterize the Khufai formation and the initial isotopic decline.

Our approach is multidisciplinary, drawing from the disciplines of stratigraphy, isotope geochemistry, and organic geochemistry. We have first created detailed lithologic logs and developed a sequence stratigraphic model to describe the depositional history. This creates a framework in which geochemical observations can be placed. We are currently employing a number of chemostratigraphic tools including carbon and sulfur isotopes and abundances to try to understand the dynamics of these geochemical cycles during this time period. Organic analysis of biomarker compounds allows for more direct questions about the biosphere by probing they type of microbial communities and the environmental conditions at the time of deposition.

Clumped isotope constraints on the diagenesis of Huqf Supergroup, Sultanate of Oman: Graduate student Maggie Osburn

The Huqf Supergroup of the Sultanate of Oman records a number of important events in latest Precambrian time, including two glaciations in the Abu Mahara Group (ca. 725 – <645 Ma), the enigmatic Shuram carbon isotope excursion in the Nafun Group (ca. <645-547 Ma), and the Precambrian-Cambrian boundary in the Ara Group (ca. 547-540 Ma). This interval contains several extreme stratigraphic excursions in isotope ratios, and a range of hypotheses has been developed, including perturbations of the surficial Earth carbon cycle, and diagenetic processes after deposition. Interpretation of these records requires a more rigorous assessment of diagenetic processes. Work to date by Kristin Bergmann on the diagenetic history of these strata includes developing the clumped isotope thermometry method as a tool for understanding regional diagenesis. She has analyzed stratigraphically constrained samples from a range of paleoenvironments with differing burial histories, from 1 km maximum burial depth to more than 10 km maximum burial depth, to constrain the stability of their isotopic ordering. Results indicate that different parts of the stratigraphic succession have different diagenetic character particularly between the Ara Group and the Nafun Group. In the subsurface and the Huqf outcrop area of central Oman, the post-glacial Abu Mahara cap dolostone shows high temperature, rock buffered diagenesis (Tavg = 176; δ18Ofluid = 15 permil VSMOW; subsurface sample only), the Nafun Group generally experienced lower temperature, fluid buffered diagenesis (Tavg = 69; δ18Ofluid = 1 permil VSMOW) and the Ara Group exhibits higher temperature, rock buffered diagenesis (Tavg = 85; δ18Ofluid = 6 permil VSMOW). Together these data help constrain the differential influence of carbonate diagenesis on carbon and oxygen isotopic records. More detailed analysis on the Shuram excursion interval continues because this excursion as been the subject of much debate and discussion relating to a potential rise in atmospheric oxygen.

Studies of the Cryogenian limestone of the Tayshir member, Tsagaan Oloom Formation, southwestern Mongolia: Francis Macdonald

Derry (2010) proposed that extremely negative carbon isotopic values in Neoproterozoic rocks are the product of alteration from migrating petroliferous brines. If correct, no longer could extreme negative carbon isotope anomalies be used for correlation nor could they be interpreted as perturbations to the global carbon cycle. Derry’s arguments were based primarily on the lack of covariation of carbonate carbon (Ccarb) and organic carbon (Corg) extracted from carbonate rocks, apparent covariation of Ccarb and oxygen (O) isotope values, and inverse covariance of C and strontium (Sr) isotopes. Derry modeled data arrays with a two component mixing model between a carbonate rock and a CO2 charged brine with depleted Ccarb, depleted O, and radiogenic Sr isotopes. To test this idea for other large carbon isotope anomalies we examined successions in northwestern Canada, and southwestern Mongolia, which preserve large carbon isotope anomalies in TOC- (total organic carbon) and Sr-rich strata. We focused on strata with high concentrations of TOC and Sr to assess if the mixing that Derry envisioned is not mixing between brine and carbonate, but is instead mixing between a detrital components in the rock.

C-isotope, Sr-isotope, rare earth element (REE), and trace element (TE) analyses allow us to assess the effects of alteration, detrital contamination and basinal restriction, and where appropriate, integrate data into a global composite. These data suggest: 1) In well-preserved, TOC-rich carbonate rocks, Ccarb and Corg isotopes covary but Ccarb and O isotopes do not (Johnston et al., in press); 2) Corg, Sr , REE, and TE in carbonate rocks are all susceptible to detrital contamination during recrystallization of carbonate grains in proximity to siliciclastic rocks that cannot be cleaned, and thus all such data should be vetted for carbonate content >97%; 3) REE and TE patterns show reproducible patterns in Cryogenian carbonates with % carbonate >97% that are consistent with changes in redox state at the site of deposition. The coincidence between the largest perturbations to the carbon cycle in the geological record, weathering and redox proxies, and climate suggest mechanistic links. Ongoing work focuses on better constraining the timing of these events with U/Pb geochronology and their relationship to the micropaleontological record of the diversification of eukaryotes.

The Neoproterozoic of the Yukon Territory: Team members David Johnston & Francis Macdonald, graduate student Kat Thomas, postdoc Chris Hallmann and PI Roger Summons
In the Yukon Territory of Canada, low metamorphic grade, mixed carbonate and siliciclastic strata are exposed discontinuously in structural inliers. Due to the remote setting and the associated logistical challenges, there have been relatively few studies describing these outcrops, and correlations of Neoproterozoic strata between the inliers have remained speculative. Recent integrated geochronological and chemo- and litho-stratigraphic studies have refined both the absolute and relative age constraints on Neoproterozoic successions in the Yukon (Macdonald et al., 2010a; Macdonald et al., 2010b). These dates and correlations allow us to better unite Neoproterozoic geological, paleontological and geochemical data throughout the northwestern Cordillera.

The late Cryogenian to Ediacaran Windermere Supergroup conformably hosts two glacial diamictites within mixed carbonate and siliciclastic successions. In the Wernecke Mountains, the Rapitan Group is overlain by mixed green shale, siltstone, and platformal dolostone of the Twitya and Keele Formations, an upper glacial deposit, and the ca. 635 Ma Ravensthroat cap carbonate. Contourites in the overlying Sheepbed Formation suggest that an open continental margin was established by the Ediacaran. Although the Sheepbed Formation in the Mackenzie Mountains is composed almost entirely of black shale, in the Yukon it is interspersed with allodapic carbonate beds, which allow us to probe this early Ediacaran succession with a variety of proxies. Upper Ediacaran strata in the Wernecke Mountains host Ediacaran macro-fauna and a large carbon isotope anomaly that is potentially correlative with the Shuram anomaly.

Mixed lithologies span both the Cryogenian and Ediacaran periods and lack significant metamorphism (achnizone to chlorite grade). Work to date suggests minimal meteoric alteration and presence of both Cyrogenian and Ediacaran fossils further distinguish the Neoproterozoic record of the Yukon as the ideal target to probe the redox history of Neoproterozoic oceans. Further, the position of the Sheepbed Formation directly overlying the Ravensthroat cap carbonate captures a critical period in the evolution of Earth’s surface oxidation state.

A multi-proxy approach was used to examine different geochemical signals before the onset of the Sturtian Glaciation in the Windermere Supergroup. While the mechanisms and drivers that lead to the onset of snowball glaciations are yet unresolved, correlations between globally observed chemostratigraphical changes allow for an increasing understanding of the environments present preceding and directly after glaciations. Here, we conducted paired carbonate associated sulfate (δ34SCAS) and pyrite (34Spyr), organic carbon (δ13Corg) and carbonate (δ13Ccarb) along with lipid biomarker analysis of the Coppercap Formation in the Northwest Territories, Canada, which was deposited just prior to the onset of the Sturtian glaciation.

Trimethylarylisoprenoids, carotenoid-derived lipids indicative of purple and green sulfur bacteria, were found throughout the section and indicate persistent sulfidic conditions in the shallow sediments deposited in a syn-rift basin. This is only the second geologic setting in which green and purple sulphur bacteria has been found present together. We observe an average Δ34SCAS-Pyr of ~ 25‰ which is typical for Neoproterozoic deposits. Increased burial of organic carbon and sedimentary sulfide is implicated from an isotopic shift in δ13Ccarb and δ34SCAS, consistent with other studied sections before the Sturtian Glaciation. Euxinic conditions mid-section is evidenced from increased concentrations of arylisoprenoids and which coincides with a ~15‰ increase in δ34SCAS, showing an interplay between more restricted conditions and marine ingressions.

While the Coppercap Formation was deposited in a basin that was only intermittently open to marine communication, the upper section is thought to reflect a more global marine signal. The upper portion of the section contains high concentrations of 2α-methylhopanes and 3β-methylhopanes as well as the trimethylarylisoprenoids, suggesting active bacterial communities in low oxygen marine settings prior to the onset of the Sturtian Glaciation.

Ediacaran records: David Johnston
Our litho- and chemostratigraphic work in the Wernecke Mountains serves to complement previous work in the Mackenzie Mountains (Hoffman and Schrag, 2002; Narbonne et al., 1994; Shen et al., 2008) and Wernecke Mountains (Pyle et al., 2004). Directly overlying the Keele Formation and Ravensthroat cap-carbonate, the carbon cycle records depleted carbonate carbon δ13C values (down to -9‰) with the slow recovery back towards positive values over the 400+ meters of the Sheepbed Formation. In the overlying carbonates of the Gametrail Formation, carbonate values stabilize at ~ 5‰. Although not shown, a large negative carbon isotope anomaly, which we term a Shuram anomaly equivalent, was identified in the Blueflower Formation at this locality and elsewhere in the Wernecke Mountains. Back within the Sheepbed Formation, organic carbon δ13C values increase up-section with a similar trend to what is observed within the carbonate carbon, suggesting a degree of isotopic co-variability. Along with the slow change in δ13Corg, total organic carbon (TOC) contents vary widely, with a majority of the samples containing > 0.2% TOC, with the middle ~ 150 meters of stratigraphy preserving greater TOC values.

Previous iron speciation work on Ediacaran strata from the Mackenzie Mountains (Shen et al., 2008; Canfield et al., 2008) and Newfoundland (Canfield et al., 2007) record a potentially complex story of late Neoproterozoic ocean chemistry. A small sampling of data from Canfield et al. (2008) from the Sheepbed Formation corresponds beautifully with the preliminary data presented here. Post-glaciation, evidence from the Sheepbed Formation presents an intriguing redox history, as it appears that the degree of anoxia oscillates up-section. Throughout the Sheepbed Formation, however, both our preliminary data and the limited data from Canfield et al. (2008) suggest that anoxic and ferruginous conditions prevailed. In contrast to these records, and using a more traditional iron extraction method, Shen and colleagues (2008) argued for anoxia near the base of the Sheepbed Formation, with an apparent oxygenation up-section. Oxygenated deep waters are more consistent with records from the Avalon Peninsula of Newfoundland (see (Canfield et al., 2007)), however precise correlation of the Northwestern Canadian stratigraphy to eastern Canada remains elusive. Overall, all indications are that anoxia was a common feature of paleobasins throughout the world, and that sulfide was scarce. Interestingly, a recent suggestion from detailed study of the Chuar Group suggests that low concentrations of seawater sulfate, once thought to be a requirement for ferruginous conditions, may not be necessary (Johnston et al., 2010).

When this preliminary data is interpreted along side other existing chemostratigraphic studies of the Ediacaran the complexity of pinning the oxidation of the deeper ocean chemistry again becomes apparent. It appears though, when considering that the entire Sheepbed section sampled herein records ferruginous conditions some portion of the paleobasin preserved in Northwestern Canada remained anoxic until the middle Ediacaran (more precisely, the termination of Sheepbed deposition).

A plot of two different biomarker indices vs. depth in the enigmatic Oman Silicilyte Formation. Redox conditions are examined via the ratio of bis and trisnorhopanoids to total c-30 hopanoids. A lager ratio indicates more reducing conditions. Note the peak occurs in the center of the silicilyte. Sponge abundance is examined via the ratio of 24-isopropylcholestane to 24-n-propylcholestane. A larger ratio indicates a larger contribution of sponges. Again the peak occurs in the center of the silicilyte.

Temperatures derived from clumped isotope thermometry are plotted against d18Omineral and d18Ofluid. The d18Ofluid composition is calculated using the measured mineral composition and clumped isotope temperature. In the upper left panel, results from Qarn Alam Salt Dome in northern Oman indicate high temperature diagenesis with low water/rock ratios. In the upper right panel, results from the A4C member of the Ara Group (Precambrian-Cambrian boundary strata) also indicate high temperature diagenesis with low water/rock ratios. In the lower left panel, results from the Khufai Formation indicate moderate temperature diagenesis with high water/rock ratios. In the lower right panel, results from restricted evaporitic carbonates indicate stabilization with restricted fluids in a moderate temperature shallow burial environment.

Exposures of TOC- and Sr-rich Cryogenian limestone of the Tayshir member, Tsagaan Oloom Formation, southwestern Mongolia.

Preliminary chemostratigraphic work on the Sheepbed Formation section chosen for the calibration of high resolution multiple sulfur isotope work. A key describing lithologies is included across the bottom of the figure, and ‘K’ denotes the Keele Formation with ‘Bf’ marking the Blueflower Formation. The Keele and Ravensthroat (not labeled on figure) formations are the Marinoan glaciogenic deposit and cap-carbonate, respectively. F848-F850 are the three measured sections from which samples were collected. Carbon data was collected in the Laboratory for Geochemical Oceanography at Harvard. Iron speciation analyses were performed in the PI’s lab at Harvard. Note that the horizontal scale on the FeP/FeHr plot is reserved.
Vertical lines on iron plots represent calibration values, where FeHr/FeT greater than 0.38 suggests anoxia and FeP/FeHr greater than 0.8 suggests euxinia.

GC-MS data showing peaks for 2α-methylhopanes and 3β-methylhopanes (above) and for trimethylarylisoprenoids showing peaks for green and purple sulphur bacteria (from sample 7Y3-136). Shown right is a proposed environment for the different bacterial communities in a strongly stratified shallow water column.