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Catherine Rose reports on the Fermor Meeting of the Geological Society, London, Burlington House, 19-21 September 2012

The Neoproterozoic Era (1000–543 Ma) was a time of transition between the ancient microbial world and the rise of large, complex animal life of the Phanerozoic. It was marked by the final assembly and rifting apart of the supercontinent Rodinia [22], multiple glaciations reaching equatorial latitudes [8, 13, 9], significant banded iron formation returning to the sedimentary record following a billion year hiatus [13, 14], rising atmospheric oxygen [4, 28], significant disruptions to the carbon cycle [7], diversification of eukaryotic life [16], and the appearance of metazoa in the fossil record [20, 17, 19, 1].

Picture: Participants of the conference Ediacaran field trip standing in front of a rock face with numerous impressions of Ediacaran fauna in Bradgate Park, Leicestershire.

The 2012 Fermor meeting of the Geological Society of London brought together a multidisciplinary group of over 150 climate modelers, geochemists, glaciologists, paleobiologists, stratigraphers, and paleomagnetists to evaluate the current understanding of the extraordinary environments and biotas of the tumultuous Neoproterozoic Era. A highlight at the close of the first day was a seminal talk given by Paul Hoffman for receiving the Wollaston Medal, the highest accolade given by the Geological Society. This riveting talk focused on how scientific understanding of ice ages developed, and highlighted the early discoveries made by physicists and geologists that formed the basis of modern climate science. [A video of this talk with slides can be viewed at www.lifeandplanet.net/relevant-events.html].

Presentations including keynote addresses by leading specialists in their field, as well as two lengthy afternoon poster sessions which highlighted student-driven research, were broadly grouped under four inter-related themes: 1) sequencing the rock record, 2) proxy record data for oceans and atmosphere, 3) co-evolution of life and the Earth system, and 4) modeling the Earth system.

Sequencing the rock record

Determining the timing and duration of the Cryogenian ice ages is important for understanding the behavior of the Earth’s climate through the Neoproterozoic. During recent years, there has been significant progress in improving the constraints on the timing of Cryogenian glaciations. Precise uranium-lead (U-Pb) dates from zircons constrain the age of deposition of glacial deposits in Namibia [10] and cap carbonates in South China [5], and strongly suggest that the younger ‘Marinoan’ glaciation terminated at 635 Ma on two widely separated continental cratons. However, constraining the older ‘Sturtian’ glacial, and the duration of both glaciations, has remained elusive.

A new rhenium-osmium (Re-Os) date from organic-rich shales in northwest Canada [26] indicates that the Sturtian-Rapitan glaciation terminated ∼662 Ma. Combined with existing U-Pb ages, this age proposes that the older Cryogenian glaciation was long-lasting, spanning some 50 million years. The Re-Os date has joined other ages (two Re-Os ages [12], and an unpublished U-Pb age [6]) for the termination of the Sturtian glaciation that are all around 660 Ma, perhaps suggesting that there was a synchronous melt back from a global glaciation. Although this talk was the only one at the conference to present geochronology data in isolation, age constraints were discussed in many other presentations. The meeting co-convener and geochronologist Dr. Dan Condon noted this increased integration of geochronology throughout many research fields as a positive development that would aid the challenging work of sequencing the Neoproterozoic rock record.

Proxy record data for oceans and atmosphere

Many workers interpret the proxy record of Neoproterozoic ocean chemistry to reveal widespread ocean anoxia, with the first record of deep water oxygenation occurring during the middle Ediacaran [3]. However, molybdenum (Mo) isotope and trace metal data from the Doushantuo Formation in China presented at the conference suggest a rise in ocean and atmospheric oxygen at the very beginning of the Ediacaran [18]. Mo is a redox sensitive metal that is preferentially removed in anoxic settings. With widespread ocean anoxia, sulfide minerals and organic matter would sequester Mo over much of the sea floor, reducing Mo concentrations in the global ocean. Thus, rising oxygen levels would drive the shrinking of the total anoxic basinal area, causing Mo concentrations in the ocean to increase. It was proposed that enriched Mo concentrations recorded in the shales of the locally anoxic Doushantuo basin sampled a globally enlarged Mo reservoir, reflecting a broadly oxic ocean [18]. In contrast, sulfur isotopes, Fe-speciation, and trace element data from Ediacaran strata in northwest Canada record persistent anoxic and ferruginous conditions within those sediments [11]. Notably absent from these data is geochemical evidence for a prominent oxygenation event in the mid-Ediacaran, coincident with the rise of the Ediacaran biota at ∼580 Ma, challenging the idea of a dramatic jump in atmospheric oxygen some 2 billion years after the Great Oxidation Event.

While a range of ideas was discussed at the meeting, the driver for an oxygenation event remains elusive. Since oxygen accumulates in the atmosphere when organic carbon is buried, much of this discussion was focused on changes in the cycling of carbon. One hypothesis proposed that refractory organic matter degrades at a faster rate in the presence of labile organic matter as compared to when refractory organic carbon is being oxidized alone [27]. Any increase in free oxygen allows more labile organic carbon to be remineralized before burial, allowing more refractory carbon to be buried and removed from the ocean-atmosphere system. As a result, a positive feedback is established, whereby an increase in oxygen leads to more oxygen accumulating, until the system stabilizes at oxygen levels where refractory carbon can begin to be oxidized [27]. This relationship was presented as a mechanism that may have pushed Earth through a tipping point from a low to high oxygen world. There is still much debate regarding geochemical proxies, particularly as to what extent the data are indicative of local versus global environmental change.

Co-evolution of life and the Earth system

Key questions addressed by paleontologists concern what organism was the common ancestor of extant animal life, and when did it evolve? And if animal multicellularity was an evolutionary response to mid-Neoproterozoic ecosystem change, why did unambiguous fossils of animals appear only much later, either shortly before or during the Ediacaran period? Within the last decade, there has been an increase in the number of documented species of microfossils identified in 800-715 Ma rocks. New specimens of organic-walled microfossil were presented at this conference with perforations interpreted to reflect predation [23]. Together with vase-shaped microfossils, these organic-walled microfossil assemblages indicate the presence of a diverse and ecologically complex biota several million years before the older ‘Sturtian’ glaciation. The innovation of such predation may have triggered ecological and evolutionary responses that help to explain increases in eukaryotic diversity and morphological complexity recorded by fossils and inferred from molecular clocks during the Neoproterozoic [15].

Two intriguing talks presented new discoveries of cm-sized polyp-like structures in Namibia [24] and lobate chambered vesicles in Australia [30], both proposed to be putative sponge-like organisms. The timing of the appearance of these potential sponges to just before and after the ‘Sturtian’ glaciation, respectively, is broadly consistent with molecular clock estimates presented and debated at the meeting. These structures are undoubtedly fascinating, and hint at the prospect that the advent of animals is to be found deeper in time than the Ediacaran, but interpreting them as animals remains a matter of vigorous debate.

Many view the belated appearance of animals in the fossil record as a consequence of limited oxygen availability through the Proterozoic. Large, ornamented Ediacaran microfossils in China have been interpreted to be the eggs and diapause cysts of stem-group animals [31]. Modern animals make cysts when conditions are inimical to egg and embryo growth, such as low ambient oxygen conditions, nutrient limitation, and desiccation. These large microfossils characterize early Ediacaran shelves prone to redox instability and low oxygen levels, but disappear during the mid-Ediacaran (∼560 Ma). It was proposed that these microfossils suggest a relatively early origin but later radiation of both eukaryotes and animals [15], the timing of which may have been controlled by oxygen availability. Some workers argued that the early fossil record of eukaryotes documents sufficient levels of dissolved oxygen to support aerobic metabolism in shallow shelf environments from at least 1600 Ma [2]. Providing adequate circulation can be maintained, both within the ocean and organism, there are no further redox limitations to acquiring a sponge-grade metazoan, since each sponge cell is in diffusive contact with seawater, and therefore has the same oxygen requirements as a single-celled eukaryote [2, 29]. It was proposed that a possible constraint on the evolution of animals was the assembly of the gene regulatory networks necessary for a co-ordinated multicellular existence, rather than oxygen availability [2]. Alternatively, an Ediacaran oxygenation event may have permitted the rise of active predators, which have a very high oxygen demand [29]. Such carnivority would be capable of driving the escalatory arms race critical to the explosion in diversity of animal body plans at the base of the Cambrian. This session of talks highlighted that we are progressively increasing the resolution of the Neoproterozoic fossil record and are continuing the challenging task of bridging the gap between molecular dates and paleontological data.

Modeling the Earth system

Keynote addresses were made assessing the role of atmospheric CO2 before, during and after a snowball glaciation [25, 21]. The snowball earth hypothesis proposes that there must have been an extreme build up of CO2, resulting in super-greenhouse conditions to end the glaciation. However, this scenario has been difficult to reconstruct because the CO2 levels needed to induce melting appeared to be too high, given the proposed duration of the pan-glacial events.

Recent work has focused on the role of clouds and dust to cause deglaciation at plausible CO2 levels, particularly the albedo of aging snow, the effects of seasonal meltwater ponds, and the generation of mud slurries [21]. Although there is no definitive answer, it appears that adding clouds and dust to the model suggests it may have been easier for the snowball to ‘melt’ than has previously been thought. In fact, it may have become so easy to exit a snowball glaciation that it is hard to account for their long durations. Thus, whilst geochronologists made the argument for a long-lived early Cryogenian glaciation, climate modelers grappled with trying to solve the problem of how the glacial could have been shorter in duration. It will continue to be crucial for modeling and physical constraints to progress conjointly and, as Paul Hoffman noted, in order to succeed ‘we need more climate models, not because models give us the right answer, but because they ask the right questions.’

An open forum for each of the four themed sessions wrapped up the proceedings at Burling- ton House. These discussions proved very positive. There was keen discussion regarding possible collaborative research projects that focus our attention to the Tonian (∼1000–800 Ma). The majority confirmed the opinion that this period is little understood, but perhaps holds the key to understanding why the Earth transitioned to the turbulent Cryogenian. Important targets for the collective future research agenda include understanding what initiated the shift to highly variable δ13C records during the Cryogenian, determining what caused the stepwise progression to increasingly lighter δ13C values with each isotopic excursion, and spearheading efforts to improve age constraints in the context of detailed stratigraphic geology. It was clear from the discussion that advancing our understanding of the profound Earth system changes during the Neoproterozoic Era will take continued interdisciplinary efforts. The tone was a united and excited one, both looking at the progress made over the past few years and the progress to come in the years ahead.

The conference concluded with a two-day field excursion to the Ediacaran geology and fauna of Charnwood Forest in Leicestershire, and the Long Mynd in Shropshire. The discovery of Charnia fronds within cleaved volcanic rocks far beneath the Cambrian boundary in Charnwood gave weight to the possibility that the Australian Ediacaran biota, which lay just a few feet below Cambrian fossils, could also be Precambrian. Thus, these outcrops are some of the most historically important fossiliferous Ediacaran sections in the world.

Picture: (a) Mould of a Charniodiscus concentricus from Charnwood Forest, Leicestershire. (b) Mould of Charnia masoni from Charnwood Forest, Leicestershire. (c) Mould of a Cyclomedusa davidi from Charnwood Forest, Leicestershire.

We began the field trip in the core sheds of the British Geological Survey (BGS) at Keyworth where we viewed molds and samples of two-dimensional impressions of fronds and disks, including Charniodiscus concentricus, Charnia Masoni, and Bradgatia linfordensis. These fauna are now described as belonging to the Ediacara biota, but with no consensus on which kingdom, current or extinct, they should be placed. There was much open-ended discussion regarding the role of their complex, often fractal-like morphologies, their feeding strategies, and hold-fast structures. We finished the day at Bradgate Park basking in the low evening light that illuminated many fine examples of some more than 50 known specimens of Ediacaran fossils at this locality. It is clear that these enigmatic fauna require further study. The BGS, in partnership with other institutions, have done excellent work making molds and cataloguing these delicate faunal impressions. Unfortunately more needs to be done to protect the rare outcrops from the predations of graffiti artists and scrambling children.

The following day we headed to the rolling hills and idyllic villages of the Long Mynd to view the alluvial, deltaic and turbiditic facies of the Ediacaran Lightspout Formation. Here, most of the impressions consisted of linear ridges and grooves of microbial mat fabrics Arumberia, string-like filaments, and simple discoidal depressions and bumps of Beltanelliformis minutae. Thanks must go to the graduate students, professors and enthusiasts from the local geological society for such an enjoyable and well organized field trip, particularly Martin Brasier, Alexander Liu, Charlotte Kenchington, Jack Matthews, Philip Wilby, and Jonathan Antcliffe. The excursion was rounded off in the traditional British way with the onset of rain, a good pub lunch, and a pint.

The Fermor meeting allowed a critical examination of the Neoproterozoic in many of its facets. In particular, it stimulated an exchange of views between a diverse group of geoscientists, and it has helped to inform the research agenda on Neoproterozoic Earth history over the coming decade. I would like to thank the organizers, Ian Fairchild, Tim Lenton, Dan Condon, and Graham Shields- Zhou, for their hard work in creating such a positive, invigorating, and fun conference.

Acknowledgements

I am very grateful to Nick Swanson-Hysell, Jon Husson, and Adam Maloof for their thoughtful comments that greatly improved this article.

Picture: Andy Knoll (center) with the organizers of the Fermor conference (from left to right) Dan Condon, Tim Lenton, Graham Shields-Zhou, and Ian Fairchild.

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