Symposium Program

Geobiology Symposium 2012

March 23rd and 24th, 2012

Hosted by the Precambrian Research Group and Publican Society,

Department of Earth and Planetary Sciences, McGill University

props.eps.mcgill.ca

 

Welcome

Dear colleagues,

We welcome you to the 2012 Geobiology Symposium and hope you will have a pleasant time inMontréal. This meeting not only offers an opportunity to share and discuss research ideas and resultsbut also to get in contact with other researchers in the field of geobiology to build new relationships.

Please do not hesitate to approach us if you have any request or question, we will be pleased to helpyou.

The 2012 Geobiology Symposium organizing committee,

Grant Cox, Marcus Kunzmann, André Pellerin,Galen Halverson and Boswell Wing

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Agenda

Friday, March 23

Redpath Museum, McGill University5pm - Public keynote lecture by Paul F. Hoffman, Climate science and geology – a tale of threehistories

Saturday, March 24

Redpath Museum, McGill University9 am – 10.30 am - Talks

Nicholas Swanson-Hysell (Invited Speaker), Burgeoning evidence for the primary origin of Neopro-terozoic carbon isotope excursionsMarcus Kunzmann, Zn isotope evidence for immediate resumption of primary productivity aftersnowball EarthAndré Pellerin, Evolutionary Response of S Isotope Fractionation by Sulfate Reducing Microorgan-ismsWilliam Leavitt, The sulfur isotope fractionation of dissimilatory sulfite reductase (Dsr)

10.30 am – 11.00 am - Coffee Break

11 am – 12.30 pm - Talks

Clint Scott (Invited Speaker), A Paleoproterozoic collapse in seawater sulfate and its influenceon the global methane cycleTom Laakso, The Stability of Low Atmospheric Oxygen in the ProterozoicVincent van Hinsburg, The composition of the Early Earth oceans: Constraints from elementpartitioningThi Hao Bui, Sulphur and carbon isotope records across the terrestrial Permian-Triassic (P-T)boundary

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12.30 pm – 1.30 pm - Lunch Break (Lunch will be provided.)

1.30 pm – 2.30 pm - Talks

John Higgins (Invited Speaker), Authigenic carbonates in marine sediments – their global sig-nificance and isotopic composition over Earth historyGrant Cox, Geochemistry of Cryogenian Ironstones - the link to N-MORB and its implicationsEmily Bamforth, Life Before Impact: Biodiversity Patterns Preceding the Cretaceous Mass Extinc-tion (65Ma) Evidence from the Latest Maastrichtian of Central Canada

2.30 pm – 3.30 pm - Coffee Break

3.30 pm – 4.15 pm - Talks

Phoebe Cohen (Invited Speaker), The Record of Eukaryotic Diversification in Proterozoic SeasThomas Maguire, A new model for a biosiliceous Neoproterozoic taphonomic bias

2nd floor, Department of Earth and Planetary Sciences, McGill University5 pm – Poster presentations and Wine & Cheese

Danielle Thomson, Refined stratigraphy and regional correlation of the early Neoproterozoic Wynni-att Formation, Shaler Supergroup, Victoria Island, NWTJohn Prince, An early Neoproterozoic dynamic sulphur cycle: evidence from the Shaler SupergroupKristyn Rodzinyak, Unexpectedly large S isotope fractionation during natural sulfide oxidation atcold temperaturesEric Bellefroid, Iron Isotopes and Rare Earth Element Geochemistry for the Cryogenian TatondukRiver IronstonesLucie Hubert-Theéou, Coupled climate-geochemical modeling of the connections between break-upof Rodinia, flood basalt volcanism, snowball glaciations and strontium cycleFelix Waechter, Chemostratigraphy of the Virgin Spring limestone: Implications for stratigraphiccorrelations in Death Valley, CADylon Trotzuk, The tectonic context of Neoproterozoic stratiform mineralization on the Westernmargin of Laurentia

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Abstracts – Talks

Burgeoning evidence for the primary origin of Neoproterozoic carbon isotope excursions

Nicholas Swanson-Hysell

Winchell School of Earth Sciences, University of Minnesota, USA

The Neoproterozoic Era was a critical interval of climatic and biological change on Earth. An un-derstanding of Earth’s carbon cycle through the Era is crucial for evaluating hypotheses relatedto the extreme variations in climate and evolving redox conditions. The carbon isotope record ofNeoproterozoic carbonates has long been interpreted as a window into the carbon cycle of the Era.However, the extreme magnitude of variation in δ13C values has led to recent proposals that therecord is not reflective of changes to global carbon cycling, but is rather the result of diagenesis. Thisdebate within the community has inspired additional work seeking to test the hypothesis that thecarbon isotope record is giving insight into large-scale changes to the isotopic composition of dissolvedinorganic carbon in the world’s ocean. I will present new data from strata deposited in Ethiopia andSvalbard in the time period leading up to the first Neoproterozoic pan-glacial event that support aprimary origin for high-amplitude changes in carbonate δ13C. Together with new data sets from otherCryogenian and Ediacaran successions, these records strongly suggest that the Neoproterozoic carbonisotope record reflects changing dynamics of carbon cycling on Earth’s surface. Fully explaining thesechanges remains a major challenge as we seek to understand the co-evolution of the surface environmentand life through the Neoproterozoic.

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Zn isotope evidence for immediate resumption of primary productivity after snowball Earth

Marcus Kunzmann1, Galen P. Halverson1, Paolo A. Sossi2, Timothy D. Raub3, Justin L. Payne4 andJason Kirby5

1Department of Earth & Planetary Sciences/GEOTOP, McGill University, Canada,marus.kunzmann@mail.mcgill.ca

2Research School of Earth Sciences, Australian National University, Australia3Department of Earth Sciences, University of St. Andrews, United Kingdom

4School of Earth & Environmental Sciences, University of Adelaide, Australia5CSIRO Land and Water, Centre for Environmental Contaminant Research, Glen Osmond, Australia

Zinc is assimilated in the surface ocean by primary producers and exported to the deep ocean where itis released by re-mineralization. Previous studies of Zn isotopes in the marine environment indicatea consistent biological control. Organisms preferentially assimilate the light isotopes, leaving behindan enriched surface ocean and generating a surface-to-deep isotope gradient akin to that of δ13Cof dissolved inorganic carbon. Since Zn is incorporated in carbonate in trace amounts and withoutsignificant isotopic fractionation, Zn isotope ratios in carbonate rocks deposited in the surface oceanshould track fluctuations in primary productivity.We analyzed Zn, C, and O isotope ratios in a 14 m-thick section of the Nuccaleena Formation, a∼ 635Ma old cap dolostone that drapes Marinoan age glacial deposits in the Adelaide Rift Complex inSouth Australia. Carbon and oxygen isotope composition and sedimentological features mirror otherMarinoan cap dolostones worldwide. The δ66Zn (66Zn/64Zn, versus JMC-Lyon) composition beginswith a decline from 0.47h at the base to a nadir of 0.07h at 5.6m. Above this level, δ66Zn increasesto a maximum of 0.87h at the top of the section. In contrast, the insoluble residue fraction (silt andclay) yields values comparable to previously reported values for siliciclastic rocks, which cluster aroundmean continental crust (0.2–0.3h).The effect of diagenesis on the Zn isotope composition in carbonates is as yet unknown. However,

is seems likely that post-depositional Zn exchange would drive the δ66Zn composition towards thecomposition of the detrital component. Hence, it cannot account for the observed trend of decreasing,than increasing values. We assume the values are primary seawater signatures and propose a two-stagemodel to explain them. During the first stage, the δ66Zn composition evolves toward the bulk of thecontinental crust as the intense weathering input during the post-glacial super-greenhouse climatedominates the ocean Zn budget. The trend of increasing δ66Zn values can be interpreted by aninvigorated biological pump, driven by a high-nutrient flux coupled to continental weathering, whichdepletes 64Zn in the surface ocean and exports it to the deep ocean. Preservation of the biologicalsignal in the δ66Zn profile is a consequence of the hydrological conditions that prevailed after snowballEarth. Partly melt-derived, brackish surface waters subject to intense heating would have capped cold,more saline deep water, suppressing upwelling and homogenization of the marine Zn isotope reservoir.

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Evolutionary Response of S Isotope Fractionation by Sulfate Reducing Microorganisms

André Pellerin1, Nadia Mykyczuk2, Rebecca Austin1, Grant M. Zane3, Lyle Whyte2, Judy D. Wall3,and Boswell Wing1

1Department of Earth and Planetary Sciences, McGill University, Montréal, Canada,andre.pellerin@mail.mcgill.ca

2Department of Natural Resource Science, McGill University, St-Anne de Bellevue, Canada3University of Missouri, Biochemistry Division, Columbia, USA

Microbial sulfur isotope fractionation is controlled by the energy metabolism of sulfate reducingmicroorganisms. It represents a well-defined and precisely measurable characteristic of the phenotypeof the microorganism. As such, it is dependent both on the underlying genotype and on the responseof that genotype to variability in the local environment. Since genotype and environment have bothchanged throughout Earth’s history, the geological record of biogenic S isotopes must reflect theinfluence of both environmental change and molecular evolution. However, the basic interplay betweenmicrobial evolution and S isotope fractionation has not been examined.

We investigated the evolutionary response of S isotope fractionation in the sulfate-reducing bacteriumDesulfovibrio vulgaris Hildenborough (DvH). Two bacteria – the wild type DvH as well as a mutantderived from that strain in which one copy of a gene putatively encoding lactate dehydrogenase wasdeleted and replaced with an antibiotic resistance cassette were used as model organisms. In definedmedia (sulfate and lactate limited) at 33℃ ancestral wild type and mutant DvH exhibit fractionationfactors that reproducibly differ by 0.5h. We serially transferred six replicate lines of the wild typeand six replicate lines of the mutant for 600 generations in batch cultures. Over the course of theexperiments, we assayed fitness through direct competition experiments between the ancestral mutantand descendant wild-type strains (or vice versa). In these competition experiments, we used qPCR tomonitor the relative abundance of the mutant through a unique genetic barcode associated with theantibiotic resistant cassette. After 300 generations, the descendant strains were markedly more fit thantheir ancestors, with relative growth rate increases of nearly 30%. This means that the descendantstrains have a clear selective advantage in the defined media, illustrating that DvH can undergoevolutionary adaptation on laboratory timescales.

Despite the clear evidence for evolutionary changes over the course of our experiment, isotopic assaysof the descendant wild-type and mutant strains reveal that the 0.5h difference in their fractionationfactors is conserved. Preservation of such a small isotope effect implies that the sulfate reducing energymetabolism is remarkably robust to the selective pressures of our experimental setup.

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The sulfur isotope fractionation of dissimilatory sulfite reductase (Dsr)

William D. Leavitt1, Inês C. Pereira2 , André Santos2, Alexander S. Bradley1, Renata Cummins3, andDavid T. Johnston1

1Harvard University, Department of Earth & Planetary Sciences, Cambridge, MA, USA,wleavitt@fas.harvard.edu

2Instituto De Technologia Quimica E Biologica, Oeiras, Portugal3California Institute of Technology, Division of Earth and Planetary Sciences, Pasedena, CA, USA

The sedimentary sulfur isotope record is an integrator of biochemical processes, among the most quan-titatively important of which is microbial sulfate reduction (MSR). Interpretations of the sedimentarysulfur isotope record rely largely on our understanding of the fractionation associated with MSR.This has been empirically determined by numerous cellular-scale studies [1, 2]. Still, a mechanisticunderstanding of the controls on this fractionation has proven elusive. Moreover, metabolic isotopemodels of MSR underpin our interpretations of the modern and ancient sulfur isotope records, yetsuch models require a quantitative understanding for the magnitude of fractionation at each node(i.e. enzymatic step) within the metabolic network [3, 4]. We present data from experimental workwith the purified dissimilatory sulfite reductase (Dsr), from which we can measure and calculate theenzyme-specific isotope fractionation factors (34αDsr,

33αDsr,36αDsr) [2]. In all cases, elemental and

isotopic mass balance was satisfied, and individual sulfur pools were isolated from the bulk solution bysequential precipitation. These are the first enzyme level constraints on isotope fractionation duringMSR, and serve as a template for evaluating the other prominent enzymatic reduction steps within thismetabolic process. This work also provides a set of fundamental boundary conditions for the metabolicfractionation models of MSR [2, 4]. By taking an enzyme-level approach to understanding isotopicfractionation in MSR, we begin to provide the most fundamental constraints on the biogeochemicalfractionation signatures [3]. Akin to the early carbon isotope work on RuBisCO and its importance toour understanding of the carbon cycle [5], the work presented herein will help to unlock the secretsof the sulfur cycle and ultimately allow for the full isotopic interpretation of Earth’s sulfur isotoperecords.

[1] Kaplan & Rittenberg (1964) J. Gen. Micrbio. 34, 195–212. [2] Johnston et al. (2007) GCA 71,3929–47. [3] Hayes (2001) Rev Mineralogy & Geochem 45, 255–77. [4] Bradley et al. (2011) Geobio. 9,446–57. [5] Park & Epstein (1960) GCA 21, 110–26.

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A Paleoproterozoic collapse in seawater sulfate and its influence on the global methane cycle

Clint Scott

Department of Earth and Planetary Sciences, McGill University, Canada

The initial accumulation of atmospheric oxygen is referred to as the Great Oxidation Event (GOE)and is fairly well-constrained to between 2,450 and 2,320 Ma. However, the magnitude and duration ofthe GOE are subject to debate and it is not clear how early Paleoproterozoic oxidation relates to thelong-lived intermediate redox state of the mid-Proterozoic. In order to investigate Paleoproterozoicsurface oxidation, we used a combination of pyrite multiple-sulfur (32S, 33S and 34S) and organiccarbon isotopes from a suite of early Paleoproteorzoic marine black shales. We analyzed the 2,200 to2,100 Ma Sengoma Argillite Formation, deposited during the peak of the Lomagundi carbon isotopeexcursion, and the Upper Zaonega Formation of the Ludikovian Series, Russian Karelia, deposited inthe immediate aftermath of the Lomagundi carbon isotope excursion. Our results suggest that a largemarine sulfate reservoir was an immediate result of the GOE, however, sulfate concentrations collapsedrapidly following the Lomagundi carbon isotope excursion resulting in invigoration of the methanecycle as recorded in the organic C isotope record.

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The Stability of Low Atmospheric Oxygen in the Proterozoic

Thomas A. Laakso and Daniel P. Schrag

Department of Earth and Planetary Sciences, Harvard University

Most studies suggest that oxygen has remained near modern levels throughout the Proterozoic, butwas much less abundant during the preceding billion years. Several decades’ worth of modeling hasshown that a handful of stabilizing feedbacks can explain modern redox conditions; however, it is notobvious these dynamics are also consistent with a billion years of low pO2. We present a simplifiedmodel of the critical biogeochemical cycles that control the redox evolution of Earth’s surface, anduse it to explore the feedbacks required to maintain an atmosphere with low O2, i.e. ∼1% presentatmospheric levels (PAL). We find that stable low oxygen is only possible if the flux of phosphorusto the ocean is great reduced compared to its modern value. Model results show that this limitationholds even if one assumes large differences in Phanerozoic and Proterozoic carbon and sulfur cyclingprocesses. Therefore, a reduced flux of phosphorus to the oceans is the most likely explanation for theProterozoic redox state. We propose this reduction may have been driven by a larger load of ferrousiron in rivers, favoring efficient scavenging of phosphate ions during co-precipitation of ferric oxides.

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The composition of the Early Earth oceans: Constraints from element partitioning

Vincent J. van Hinsberg1,2, Kristoffer Szilas3 and Bernard J. Wood1

1Department of Earth Sciences, University of Oxford, United Kingdom2Department of Earth and Planetary Sciences, McGill University, Canada

3Geological Survey of Denmark and Greenland - GEUS, Copenhagen, Denmark

Although it is still debated at what point liquid water became stable on the surface of the Earth, itis widely accepted that dramatic changes took place over time in the properties and compositions ofthe Early Earth hydrosphere. These changes reflect changes in the sources, sinks and reservoirs ofelements in and on the Earth, which are directly linked to the nature of the atmosphere and crust, andthe plate tectonic style of the young Earth. Moreover, an intimate link has been suggested betweenchanges in element availability in the oceans and the direction of life’s evolution, based on the essentialnature of many trace elements as catalytic constituents in enzymes. An understanding of the evolvingoceans therefore provides a window into both the evolving inorganic Earth and the development of theorganisms that inhabit it.

Direct samples of Early Earth ocean waters are exceedingly rare, and constraining ocean compositionstherefore depends on our ability to accurately read the rock record. Marine sediments, such as BandedIron Formations, cherts and black shales, are an obvious focus in this research, because of theirdirect interaction with ocean water. However, translating marine sediment compositions to actualwater concentrations is complicated, especially owing to possible involvement of organisms that canpreferentially sequester elements.In this contribution, we focus on the high-temperature, inorganic interaction between ocean water

and basalt, which takes place at the mid-ocean ridges and is a dominant source of trace elements to thepresent-day oceans. Combining trace element analysis of minerals taken from well-preserved Archaeanaltered oceanic crust with experimentally characterised mineral-fluid element partitioning allows us toput quantitative constraints on the composition of the oceans at 3.2 Ga, and shows that these wereremarkably similar in trace element composition compared to the modern day.

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Sulphur and carbon isotope records across the terrestrial Permian-Triassic (P-T) boundary

Thi Hao Bui1, Jean-Francois Helie2, and Boswell Wing1

1Earth and Planetary Sciences, McGill University, Montreal, thi.h.bui@mail.mcgill.ca2Département des sciences de la Terre et de l’atmosphère, UQAM, PK-7720, Montreal

The end Permian mass extinction (∼ 252Ma) is known as the greatest biotic crisis in earth history withthe disappearance of more than 90% of marine species and 70% of terrestrial vertebrate families. Tobetter understand the interaction of the carbon and sulphur cycles across the terrestrial P-T boundary,we collected 27 sedimentary rock samples at average resolution of 35 cm and 9 carbonate nodules alonga section of ∼ 10 meters in Karoo Basin, South Africa. We determined carbon and sulphur contents aswell as carbon and sulphur isotope records of above samples.

The organic carbon contents of sedimentary rocks are quite low, less than 0.04 wt %. The averageδ13C value of organic carbon is around -25h throughout the section, but at 59 cm before the P-Tboundary, this value increases to -23.8h and drops sharply to -26.5h over a distance of 110 cm. Thebackground value of -25h is recovered within 19 cm. The δ13C values of carbonate nodules in the≈ 3m preceding the boundary are around -8.5h, while those found in the ≈5 m after the boundaryare around -11.5h. Total sulphur contents of sedimentary rocks are generally less than 0.01 wt%, withthe exception of a sharp peak of ≈0.45 wt % at 5 cm above the boundary. Although their full patternis noisier than that seen in the δ13C record, the δ34S values of Cr(II)-reducible sulphides and differentsulphate species (water-soluble, acid-soluble, and acid-insoluble sulphates) all decrease by at least 8hwithin the 100 cm after the boundary.

The negative shifts of both δ13Ccarbonate and δ13Corganic (∼-3h) at the boundary indicate a decreasein the 13C content of carbon input into the P-T terrestrial system. Likewise, the sharp peak of totalsulphur content coinciding with the boundary suggests a rapid addition of sulphur into the terrestrialenvironment. The shared initial decreases in δ34S values suggest that this sulphur was depleted in 34S.Multiple sulphur isotope compositions (δ34S and ∆33S) of the different sulphate species are generallyequivalent, indicating a shared sulphate source throughout the section. While the δ34S values of theCr(II)-reducible sulphides are compatible with bacterial sulphate reduction, the associated ∆33S valuesare more negative than those typically associated with this process. Although the δ13C and δ34S recordsimply the coherent transfer of 13C- and 34S-depleted material to the PT terrestrial environment, the∆33S values complicate a straightforward identification of the source of this material.

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Authigenic carbonates in marine sediments – their global significance and isotopic composition overEarth history

John Higgins

Department of Geosciences, Princeton University, USA

We propose new framework for interpreting the C isotopic composition of carbonates which focuseson changes in the abundance and isotopic composition of authigenic carbonate in marine sediments.The framework is based on the model developed by Higgins et al. (2009) for the global alkalinity andcarbonate cycles which considered the possibility of two sites of carbonate precipitation in the ocean;well-mixed seawater in the surface ocean and sediment pore-fluids in the near subsurface. The modelpredicts that changes in organic carbon cycling and the identity of the dominant electron acceptors(Fe3+, SO2−

4 , O2) and a decline in the size of the global inorganic carbon pool had a first-order effecton the abundance of authigenic carbonates in marine sediments over Earth history. In particular,increasing oxidation of the ocean and sediments should be accompanied by a decline in the globalimportance of authigenic carbonate precipitation in marine sediments. Here, we expand our analyses toconsider the C isotopic composition of the authigenic carbonates, how it has likely varied over Earthhistory, and what this means for interpreting the C isotopic composition of sedimentary carbonatesthrough time. Our model can explain many of the extreme C isotope excursions observed during theProterozoic and parts of the Phanerozoic without calling on large changes global redox budgets.

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Geochemistry of Cryogenian Ironstones – the link to N-MORB and its implications

Grant Cox1, Galen Halverson1, William Minarik1, Ross Stevenson2, Daniel Le Heron3, FrancisMacdonald4, Justin Strauss4, Paolo Sossi5, Eric Bellefroid1

1McGill University, Montreal, Canada, grant.cox@mail.mcgill.ca2GEOTOP/UQAM, Montreal, Canada

3Royal Holloway University of London, Surrey, U.K.4Harvard University, Cambridge, U.S.A.

5Australian National University, Canberra, Australia

The reappearance in the geological record of sedimentary iron formations after a ∼ 1Ga hiatus [1,2] isa geologically unique feature of the Cryogenian (∼,850Ma to 635Ma). Whereas their close associationwith globally distributed glacial deposits invites interpretation that they are the product of snowballglaciation, they have also been interpreted to be a local product of rifting, similar to modern Red Seametalliferous sediments. Based on major element, REE, and 143Nd/144Nd data from stratigraphically-constrained samples from South Australia (Holowilena), NW Canada (Tindir and Rapitan) andsouthern Namibia (Numees), Cryogenian iron formations can be characterized as a mixture between ahydrothermal source and mid ocean ridge basalt (N-MORB), with very little contribution from thecontinental crust. Similar positive-upward iron isotope profile in each of these basins suggest a commondepositional process for the iron formation. These data indicate that both snowball glaciation and localrifting are prerequisites to Cryogenian iron formation. Notably, the Cryogenian IF patterns are distinctfrom both Archean-Paleoproterozoic banded iron formation and Phanerozoic ironstones.

[1] Isley, A. E. & Abbott, D. H. (1999) Journal of Geophysical Research, 104, 461-477. [2] Klein, C.(2005) American Mineralogist, 90, 1473-1499.

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Life Before Impact: Biodiversity Patterns Preceding the Cretaceous Mass Extinction (65Ma) Evidencefrom the Latest Maastrichtian of Central Canada

Emily Bamforth and Hans C.E. Larsson

Redpath Museum, McGill University, 859 Sherbrooke Street W., Montreal, QC, Canada H3A 1B1emily.bamforth@mail.mcgill.ca

The timing and cause of the Cretaceous mass extinction has been the topic of much debate inpaleontology for the last several decades. The question as to whether or not diversity was decliningprior to the bolide impact event is hindered by the fact that diversity patterns are not consistent acrossdifferent taxonomic groups. In this study, we apply a holistic, multidisciplinary approach to studyingvertebrate biodiversity during the last half-million years leading up the extinction event.

Vertebrate microsites are an invaluable tool in the study of paleoecology. These accumulations of smallvertebrate fossils allow for the quantification of temporal and spatial paleobiodiversity trends. Whencoupled with paleoclimate indictors such as oxygen-18 stable isotope analyses and plant macrofossildata, insight may be gained as to how climatic factors influenced biodiversity in both time and space.Here, we examine the relationships between climate, area and biodiversity during the last four to sixthousand years of the Cretaceous period in central Canada. Data from thirty-six vertebrate microsites,together containing some 9000 vertebrate microfossils, were collected from the base of the latestMaastrichtian (65Ma) Frenchman Formation to the K-Pg boundary clay in Grasslands National Park,SK. Stratigraphic level surveys were performed to assess the relative stratigraphic position of eachmicrosite, and the depositional environment of each was documented.Diversity appears to peak in at least three stratigraphic horizons, with horizons of lower diversity

situated between them. Both the number of sites and the diversity within the sites decreases dramaticallyin the last 10m below the K-Pg boundary. Alpha (within site) diversity fluctuations may be associatedwith the large-scale, repetitive depositional cycles of ironstone, mudstone and sandstone observedthroughout the study area. This study demonstrates that local vertebrate diversity may have beenfluctuating with local environmental conditions until just prior to the K-Pg extinction.

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The Record of Eukaryotic Diversification in Proterozoic Seas

Phoebe A. Cohen

Department of Earth, Atmospheric and Planetary Sciences, MIT, USA

Proterozoic strata record evidence of dramatic changes to the Earth system, including an increasinglyrobust record of eukaryotic diversification. In this talk I will discuss the most recent assessments ofeukaryotic biodiversity in Proterozoic strata, discuss biases in this record, and focus in on new keycomponents of Neoproterozoic diversity. While the Phanerozoic fossil record of diversity has beenthoroughly analyzed for biases relating to rock volume, taphonomy, and sampling, the Proterozoicfossil record has not been subjected to the same analyses – in part because of the huge differencein raw fossil abundance between the two. We can, however, make a start at trying to grasp at therole of biases on the Proterozoic fossil record, which can in turn help us make better correlationsbetween environmental and biotic events. I will briefly discuss these issues and then focus in on twonew records of complex eukaryotic life in the Neoproterozoic: diverse biomineralized scale microfossilsfrom early- to mid-Neoproterozoic strata of the Yukon and macroscopic fossils tentatively assigned tothe red algae from Cryogenian strata of Mongolia. Both of these fossil assemblages shed light on theevolution of eukaryotic life in Neoproterozoic seas and highlight the changing ecological landscapesthat accompanied major climactic shifts on the Neoproterozoic Earth.

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A new model for a biosiliceous Neoproterozoic taphonomic bias

Thomas Maguire

School of Earth and Ocean Sciences, University of Victoria, Canada

Citing biomarker, molecular-clock and micro-RNA evidence, Sperling et al. (2010) postulate a Cryoge-nian origin for silica-biomineralizing demosponges, >100 Myr before the first documented spicules inthe rock record. They suggest that a secular increase in marine Pedogenic Clay Mineral (PCM) deliveryduring the Neoproterozoic supplied more Al(III) to sediment porewaters, thereby stabilizing biogenicsilica (BSi). Subsequent work shows that the secular increase was in micaceous not pedogenic clays,implying no increase in the spicule-preserving Al(III) flux to the sediment. A new model is proposed inwhich a redox-dependent microbial mechanism provides a taphonomic bias against the preservation ofsiliceous sponge spicules before the late Ediacaran.

Under reducing conditions, a variety of microbes exhibit the ability to rapidly (days timescale) respireorganic matter (OM) using structural Fe(III) in smectite clays as a terminal electron acceptor. Thispromotes illitization of clays at early diagenetic temperatures and pressures, and releases significantaqueous Fe(II), Al(III) and Si(IV) through clay dissolution. Under oxic conditions, these reactions areenergetically favourable during diagenesis, resulting in intact delivery of clay minerals to sediments.During the Precambrian, such reactions would have occurred during flocculant settling through thewater column, releasing Fe(II) and Al(III) to seawater rather than to porewaters. Precambrian BSiwas thereby unprotected by Al(III) absorption, and its early diagenetic dissolution was kinetically andthermodynamically favoured, causing the observed taphonomic bias. This scenario is supported bychanges in clay mineralogy across the Cambrian boundary, Neoproterozoic early-diagenetic mineraltrends, and secular change in chert petrology.Important corollaries are that in the Neoproterozoic, marine Si may have been significantly lower

than previously suspected, standard models of chert formation may need revision, clays could havesignificantly contributed to ferruginous conditions, and a feedback to the Neoproterozoic oxygenationevent is implied by increasing floc-associated sequestration of OM under progressively more oxicconditions.

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Abstracts – Posters

Refined stratigraphy and regional correlation of the early Neoproterozoic Wynniatt Formation, ShalerSupergroup, Victoria Island, NWT

Danielle Thomson1 and Robert Rainbird2

1Department of Earth Science, Carleton University, Canada, dthomso2@connect.carleton.ca2Geological Survey of Canada, Ottawa, Canada

The early Neoproterozoic (late Tonian-Cryogenian) Shaler Supergroup of the Amundsen Basin is wellexposed in the Minto Inlier, Victoria Island, NWT. It is correlated with the Mackenzie MountainsSupergroup (Mackenzie Mountains) and Fifteenmile Group (Ogilvie Mountains).The Wynniatt Formation (middle Shaler Supergroup) is composed of >900 meters of sediment

deposited on a broad, distally steepened carbonate ramp within a shallow intracontinental basinthat was periodically restricted from an exterior ocean. Study of the Wynniatt Formation supportsdivision into: 1) lower-carbonate member, an upward-deepening succession of supra- to sub-tidalcarbonates; 2) black-shale member, a recessive interval of dark-grey siltstone and silty shale depositedon a pro-delta; 3) stromatolitic-carbonate member, comprising stacked upward-shallowing cycles ofsub- to supratidal carbonates and, 4) upper-carbonate member, an upward-shallowing successionfrom sub-tidal black calcareous shale to resistant benches of peritidal, cross-bedded grainstone andstromatolitic limestone. Facies stacking patterns reveal cyclical, upward-shallowing parasequences thatdefine at least six third-order sequences. Harmonious sets of third-order base-level rise and fall definethree, second-order sequences (supersequences). Correlative strata in the northern Cordillera startto record northwest-facing rift basins during this time, however our work in the Amundsen Basinsuggests relatively stable tectonics, supporting multi-stage, non-correlative breakup of Rodinia alongthe northwestern margin of Laurentia.

A negative δ13C excursion was recognized in a Wynniatt section from the northeastern Minto Inlierand has been correlated with similar excursions in the Mackenzie and Ogilvie mountains, as well as theglobal Bitter Springs isotopic stage. The Wynniatt anomaly was recorded in rocks collected adjacent toa thick diabase intrusion. We re-sampled this stratigraphic interval from a locality in southwesternMinto Inlier, away from any intrusions, to test its validity. Our data confirms the presence of a negativeanomaly over the same stratigraphic interval, supporting the validity of the excursion and intrabasinaland extrabasinal correlations.

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An early Neoproterozoic dynamic sulphur cycle: evidence from the Shaler Supergroup

John K. G. Prince1, Boswell A. Wing2, Robert H. Rainbird3, Galen P. Halverson2

1Carleton University, Ottawa, Canada, jkgprince@gmail.com2McGill University, Montreal, Canada,

3Geological Survey of Canada, Ottawa, Canada

The Neoproterozoic (1000-542Ma) is a dynamic era of Earth history, punctuated by super continentalbreak-up, global glaciations, and the evolution of metazoan life. During the early to mid-Neoproterozoic,Earth’s redox budget was in a state of flux, the evidence for which is preserved in the isotopic recordsof sulphur and carbon. In order to constrain the initiation of this dynamic behaviour, we describedand sampled 3 outcrop stratigraphic sections and a drill core at ∼ 3m intervals through the MintoInlet Formation of the Shaler Supergroup, which is exposed in the Minto Inlier of NW Victoria Island,NWT.The Minto Inlet Formation is >250m thick and hosts well-preserved Neoproterozoic sulphate-rich

evaporites deposited in a periodically restricted intra-continental marine basin. The Minto InletFormation was deposited between ∼900Ma and ∼800Ma based on detrital zircon geochronology andstratigraphic correlation with well-calibrated sections in the northern Cordillera. We measured multiplesulphur isotopes (δ34S, |Delta33S) of the sulphate fraction in all of the stratigraphically controlleddataset of 67 samples.

Current understanding of the sulphur isotope record suggests that the fraction of S buried as pyriterelative to sulphate evaporites was high, approaching 1 for most of the Proterozoic, through theEdiacaran, and into the early Phanerozoic. We used multiple sulphur isotope measurements of MintoInlet Formation evaporites to constrain models of S fluxes into and out of the ocean prior to the onsetof the Cryogenian (720-635Ma). This approach suggests that the relative burial fraction of S as pyritewas extremely low (≈0.2) during the earliest stages of the deposition of the Minto Inlet Formation.Previously, pyrite burial fractions this low had been recorded only at Permian-Triassic time, more than≈600 Ma after the deposition of the Minto Inlet Formation. Our results indicate large scale variationsin ocean redox conditions and a dynamic sulphur-cycle were initiated during the early-Neoproterozoic.

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Unexpectedly large S isotope fractionation during natural sulfide oxidation at cold temperatures

K.J. Rodzinyak1,2, B.A. Wing1, and R.J. Léveillé2

1Earth and Planetary Sciences, McGill University, Montréal, Québec.(kristyn.rodzinyak@mail.mcgill.ca)

2Canadian Space Agency

The Sample Analysis at Mars (SAM) instrument on the Mars Science Laboratory (MSL) Curiosityrover will be able to measure sulfur isotope ratios of Martian samples in-situ [1]. Microbial sulfatereduction leaves a characteristic ‘biosignature’ behind in the isotopic composition of its metabolicreactants and products. The measurement capabilities of SAM should be able to address questions ofpast Martian habitability for sulfate-reducing microbes.One issue with this approach is that the strongest isotopic signals are often preserved in sulfide

minerals, which may not be long-lived in the oxidizing surface environment of Mars. However, previousresearch suggests that the S isotope composition of the parent sulfides should be indistinguishablefrom their oxidative products [2]. This sulfur isotope consistency implies that potential S isotopebiosignatures may be preserved in Martian sulfates despite the intense oxidation on the surface ofMars.

In order to constrain the sulfur isotope characteristics of oxidative processes in cold dry environments,we studied the sulfur isotope systematics of mineralized sulfate-bearing crusts on sedimentary pyritenodules from Devon Island, Canadian Arctic. These crusts display similarities to rocks studied by theMars Exploration Rovers, including the coexistence of Ca-Mg-Fe sulfates with Fe-oxides and nanophaseferric oxides [3].While we do not yet have a full understanding of the oxidative mechanism behind these unique

fractionations, their existence has clear implications for investigating potential biosignatures on thesurface of Mars. If an isotopic enrichment of 10–20h can be produced by low-temperature sulfideoxidation, preservation of potential records of microbial sulfate reduction may be incompatible withthe present Martian surface environment, or for that matter, much of its climatic history.

[1] Franz, H.B., et al. (2011) LPS XL11, 2800-2801. [2] Seal II, R.R. (2006) Reviews in Min. andGeochem., 633-677. [3] Léveillé, R. (2007) GSA

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Iron Isotopes and Rare Earth Element Geochemistry for the Cryogenian Tatonduk River Ironstones

Eric J. Bellefroid1, Grant M. Cox1, Galen P. Halverson1, Justin V. Strauss2

1Department of Earth & Planetary Sciences/GEOTOP, McGill University, Canada,eric.bellefroid@mail.mcgill.ca

2Department of Earth & Planetray sciences, Harvard University, Cambridge, U.S.A.

Cryogenian iron formations, present on nine different continents, are of particular interest in deter-mining paleo-ocean chemistry as Fe is sensitive to the oceans redox state. While Rare earth element(REE) and yttrium patterns are broadly understood, iron isotope patterns, which show extraordinaryvariability, are controversial. Recently Halverson et al. (2011) argued that the up-section increase inδ57Fe within the Rapitan ironstones is a reflection of a steep iron chemocline in combination with marinetransgression. We present Fe isotope data along with REE + Y data for the Tatonduk River ironstoneswhich show a similar δ57Fe trend. Fe/Ti ratios point to a significant but diluted hydrothermal inputwhile La-Yb ratios, a good redox indicator, show a positive trend with respect to δ57Fe. Therefore, wesuggest that the Tatonduk river ironstones where deposited in a similar fashion to the iron formation ofthe Rapitan group. This implies that the depositional mechanism proposed by Halverson et al. (2011)for Cryogenian ironstones is not restricted solely to the Rapitan group.

Halverson, G., Poitasson, F., Hoffman, P., Nédélec, A., Montel, J. Kirby, J. 2011. Fe isotope and traceelement geochemistry of the Neoproterozoic Rapitan iron formation. Earth and Planetary Letters. 309(1-2), 100-112.

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Coupled climate-geochemical modeling of the connections between break-up of Rodinia, flood basaltvolcanism, snowball glaciations and strontium cycle

Lucie Hubert-Théou1, Galen Halverson1, Yves Goddéris2

1Department of Earth & Planetary Sciences, McGill University, Canada2Géoscience Environnement Toulouse, CNRS-Université Toulouse III-IRD, Toulouse, France

The Neoproterozoic Era (1000-542 million years ago) constitutes a turning point in Earth’s history. Aseries of tectonic (formation and breakup of the supercontinent Rodinia), magmatic (emplacementof larges igneous provinces (LIP)), climatic (total glaciation of the planet) and evolutionary (faunalexplosion at the Neoproterozoic–Cambrian transition) events characterize this time period. Manymulti-disciplinary studies attempt to better identify and understand the magnitude, number, causes,and consequences of these events. These studies address questions that regard the conditions ofoccurrence and the extent of the Cryogenian glacial episodes, or the correlation between mantle plumeand dislocation of Rodinia. Several hypotheses emerge from these studies, including the contestedSnowball Earth theory and the controversial True Polar Wander phenomenon. However few studiesconstrain the global evolution of the Earth-system within the 1000–542 Ma period. A multitude ofgeochemical proxies show strong perturbations during the Neoproterozoic. Among these proxies, thecarbon isotope ratio in marine carbonates records an overall high composition (δ13C≥ 5h), punctuatedby several sharp declines of amplitudes of 10–15h. Strontium isotopes (87Sr/86Sr) are also sensitive toperturbations of the Earth-system, revealing an increase in marine compositions (from 0.7055 to 0.7090),that mirrors the record of declining values in the Paleozoic, with superimposed finer-scale structurethat appears to be closely linked to major climatic and tectonic events. I plan to use GEOCLIM, apowerful coupled climate-carbon model in combination with up-to-date paleogeographies (including aspatio-temporal distribution of Neoproterozoic large igneous provinces and orogens) and geochemicalanalyses of Cryogenian mafic and carbonate rocks, to explore the 850–630 Ma time period. This willelucidate the relationships between the magmatic, tectonic, and climatic events occurring at the end ofthe Precambrian.

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Chemostratigraphy of the Virgin Spring limestone: Implications for stratigraphic correlations in DeathValley, CA

Felix Waechter

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

The glaciogenic Kingston Peak Formation of the Neoproterozoic Pahrump Group in Death Valley, CA,has been assumed to be internally conformable (e.g. Troxel et al., 1987). However, geologic mappingin the Saddle Peak Hills has revealed a new, pronounced angular unconformity of million-year scaleduration within the Kingston Peak Formation. The unconformity occurs at the base of the VirginSpring limestone, and truncates underlying beds of the lowest Kingston Peak member (KP1), theBeck Spring Dolomite, and the Crystal Spring Formation. Thus, KP1 does not represent the onset ofglaciation but is rather genetically related to the Beck Spring Dolomite (Prave, 1994).

The top contact of the Virgin Spring limestone is a sharp erosional surface leading into hundreds ofmeters of glaciogenic strata. These Cryogenian glacial diamictites are highly variable, but uninterrupteduntil the overlying Sentinel Peak member of the Noonday Dolomite, a characteristic Marinoan capcarbonate (Allen & Hoffman, 2007; Peterson et al., 2011) with an inferred 635 Ma age. Whether theglaciogenic Kingston Peak units represent the Sturtian glaciation, the Marinoan glaciation, or both, isan unresolved issue, and has prevented conclusive correlation of the glaciogenic stratigraphy acrossDeath Valley to the Panamint Range, where both glacials, and interglacial strata, are preserved (Prave,1999). A clue for the interpretation of this glacial sequence was discovered in the Saddle Peak Hills,where the uppermost member of the Kingston Peak Formation (KP4) is absent in certain locations, andthe Sentinel Peak member rests unconformably on KP3 instead. Further evidence for a disconformablerelationship between KP3 and KP4 is the occurrence of KP3 diamictite clasts within KP4 strata.The formerly aragonitic, and therefore Sr-rich (1000–3500 ppm) Virgin Spring limestone (Tucker,

1986) yielded 87Sr/86Sr values as low as 0.7067. Fitting these values to the highest resolution 87Sr/86Srseawater curve available for the Neoproterozoic (Macdonald, in prep), confidently dates the VirginSpring limestone as pre-Sturtian. This precludes a previously viable correlation of the Virgin Springlimestone to the interglacial “un-named limestone” in the Panamint Range. The pre-Sturtian age ofthe Virgin Spring limestone, coupled with the disconformable relationship between Kingston Peakmembers KP3 and KP4, suggests that members KP2 and KP3 represent the Sturtian glaciation, andmember KP4 represents the Marinoan glaciation.

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The tectonic context of Neoproterozoic stratiform mineralization on the Western margin of Laurentia

Dylan Trotzuk

Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, USA

The Redstone stratiform copper deposit, located in the arcuate Mackenzie Mountains of the NWT,Canada, contains drill-indicated reserves of 37 million tones averaging 3.92% Cu and 11.3 g/t Ag. Themineralization occurs within Cryogenian strata which represent a tumultuous period in Earth’s historyand globally host many of the world’s richest stratiform copper deposits. Previous mineralizationmodels have called for a single expulsion of copper rich fluid or the development of an evaporitic pumpas the means to mobilize and eventually deposit Cu.

This thesis takes a multifaceted approach to constraining the method of mineralization and the uniqueterrestrial conditions that caused such vast and rich mineralization. Volumetric analyses suggest thatthe single dewatering scenario as explained previously is an insufficient mechanism for mineralization.Chemical analyses, subsidence models, observations of local igneous intrusions and a more refinedunderstanding of the major igneous events of the Cryogenian suggest that exhalation from a nearbymantle large igneous province (LIP) is the only satisfactory mineralizing mechanism that satisfies theunique set of attributes identified.The presence of a major mantle LIP proximal to the Redstone deposit could explain the unique

features of the mineral system and stratigraphic assemblage. The surfacing of this LIP could havedramatically changed the chemistry of the seawater, resulting in the unique chemistry observed.Additionally, through exhalation of extrusive diatremes, the LIP could have supplied the fluid tothe system and provided the impetus to that fluid to force it through the evaporite rich RedstoneFormation. Thus providing the necessary volume of fluid able leach a satisfactory mass of copper fromthe Redbeds to result in the present mineralization.

The recognition of the presence of this major LIP has immense ramifications for the tectonic modelof Rodinia and the possible environmental conditions that led into the Sturtian Glaciation.

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Maps

How to get from the airport to downtown Montreal

Taxis:Taxis are situated at the exit of the arrivals and will take you to downtown Montreal for a fixed fare of$40.

Payment methods include cash, Visa, MasterCard and American Express credit cards; some driversaccept US currency but provincial regulations require customers to pay in Canadian currency.

City buses:The 747 city buses going downtown are situated at the exit of the arrivals. It runs 24 hours a day, 7 daysa week. The trip between Montréal-Trudeau Airport and the Montréal bus terminal at Berri-UQAMmetro station takes approximately 35 minutes outside of rush hour. The service stops at Lionel-Groulxmetro station, and along Boulevard René-Lévesque at Guy, Drummond, Peel, Mansfield, Union, Jeanne-Mance, de l’Hôtel-de-ville, and Saint-Laurent. A map of the downtown stops is shown below. The 747is free for holders of CAM and TRAM monthly passes, and for holders of 1 or 3-day tourist passes.Individual tickets valid for 24 hours across the entire STM network are also available, priced at $8 fora one-way trip. At the airport, tickets are sold at the International Currency Exchange (ICE) counteron the international arrivals level.

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