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Durham Research Online Durham Research Online Deposited in DRO: 15 March 2018 Version of attached le: Published Version Peer-review status of attached le: Peer-reviewed Citation for published item: Topper, Timothy P. and Greco, Francesco and Hofmann, Axel and Beeby, Andrew and Harper, David A.T. (2018) 'Characterization of kerogenous lms and taphonomic modes of the Sirius Passet Lagerst¤atte, Greenland.', Geology., 46 (4). pp. 359-362. Further information on publisher's website: https://doi.org/10.1130/G39930.1 Publisher's copyright statement: c 2018 The Authors Gold Open Access: This paper is published under the terms of the CC-BY license Use policy The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that: • a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders. Please consult the full DRO policy for further details. Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 http://dro.dur.ac.uk Characterization of kerogenous films and taphonomic modes of the Sirius Passet Lagerstätte, Greenland Timothy P. Topper1, Francesco Greco2,3, Axel Hofmann3, Andrew Beeby4, and David A.T. Harper1 1Palaeoecosystems Group, Department of Earth Sciences, Durham University, Durham DH1 3LE, UK 2Department of Biological, Geological and Environmental Sciences, University of Bologna, 40126 Bologna, Italy 3Department of Geology, University of Johannesburg, Auckland Park 2006, South Africa 4Department of Chemistry, Durham University, Durham, DH1 3LE, UK ABSTRACT However, the presence of clay minerals (or rather their greenschist facies The Sirius Passet Lagerstätte (northern Greenland) is an excep- metamorphic equivalents, such as illite and chlorite) is likely a result of tionally well-preserved early Cambrian faunal community containing diagenesis/metamorphism (Powell, 2003; Butterfield et al., 2007; Page a diverse array of stem-group euarthropods, lobopodians, worms, et al. 2008), and the association of iron-bearing minerals in unweathered sponges, and the iconic Halkieria. Material collected in situ during fossil material may be too sporadic to have been considered a key factor in recent expeditions has yielded a range of fossil specimens that are BST preservation (Gaines, 2014), although resulting from related processes preserved as two-dimensional, reflective films. Here we document in and microbial decomposition (Berner, 1984; Schiffbauer et al., 2014). detail, for the first time from the Sirius Passet, compressed, kerog- The Sirius Passet in Greenland, is one of the most important and enous fossil films characteristic of Burgess Shale–type (BST) preser- potentially the oldest Cambrian Lagerstätten (Budd, 2011). The majority vation. The carbon structure and the taphonomic mode associated of documented fossils from Sirius Passet were collected as talus material, with these films were investigated using Raman spectroscopy. Our and preservation is variable (Peel and Ineson, 2011). Trilobites are pre- analysis confirms that these reflective films are kerogenous, show- served as complete molds showing a veneer of authigenic silica (Strang ing a higher D1 (disordered) band and G (graphite) band intensity et al., 2016a), whereas other less-mineralized taxa are preserved in slight and area, indicating a greater concentration of disordered carbon relief, replicated by silica and clay minerals, with some specimens display- compared to the surrounding matrix. The spectral characteristics of ing three-dimensional phosphatized digestive tracts, and a single taxon the fossils denote moderately ordered kerogenous matter, indicating (Campanamuta mantonae Budd, 2011) preserving three-dimensional that the transitional Buen Formation that hosts the Sirius Passet was muscle fibers (Budd, 2011; Peel, 2016). Remains of organic matter, on thermally altered at a peak temperature of 409 ± 50 °C. Phyllosilicate the other hand, were thought to have been lost, if originally present at minerals are associated with the films, but they are not anatomical all, as a result of the relatively high grade of metamorphism (greenschist or taxon-specific, suggesting that the higher thermal maturation of facies) that the Sirius Passet has experienced (Budd, 2011). Some fossils the kerogen in the Sirius Passet produced a uniform distribution of however, collected both in situ and as talus during more recent expeditions minerals. This is unlike the kerogenous films in the Burgess Shale (2009 and 2011), are seemingly preserved as thin, reflective films and Lagerstätte (British Columbia, Canada) that have been metamor- resemble, at least superficially, fossil films seen in BST deposits (Vinther phosed at a lower temperature of 335 ± 50 °C and typically show an et al., 2011). The composition and mode of preservation of these films anatomically specific phyllosilicate distribution. Preservation as ker- though have been given no attention. Based on 14 specimens, we have ogenous films, however, is not continuous, and the presence of other employed Raman spectroscopy and scanning electron microscopy with taphonomic modes not indicative of BST preservation suggests that energy dispersive spectroscopy (SEM-EDS) elemental mapping to investi- the Sirius Passet represents a unique and complex deposit. gate the composition and the taphonomic modes of fossils from the Sirius Passet Lagerstätte preserved as two-dimensional reflective films. Raman INTRODUCTION spectra from three Burgess Shale (British Columbia, Canada) specimens The extraordinary snapshot of communities preserved in Konservat- (all from the Walcott Quarry Member) were also analyzed for comparison. Lagerstätten has significantly influenced our understanding of the early evolution of animal life in the Cambrian Period. Taphonomic investigations RESULTS have identified several preservational modes in Lagerstätten (Briggs, 2003). In hand specimen, organisms preserved as films from the Sirius Passet In the Cambrian, the most celebrated deposits are those that exhibit Burgess are notoriously difficult to recognize, and are best observed underwater Shale–type (BST) preservation (Gaines, 2014), defined as the exceptional with high-angle reflected white light, where they appear as reflective films preservation of non-mineralizing organisms as two-dimensional carbona- and stand out conspicuously from the non-reflective sedimentary matrix ceous compressions (Butterfield, 1995). This preservational style occurs (Fig. 1). Unlike the Burgess Shale (Butterfield et al., 2007), there appears in fine-grained marine sediments and is characteristic of a number of to be no further distinction of individual morphological features in terms early Paleozoic (Gaines, 2014) and Proterozoic deposits (Anderson et al., of reflectivity; rather, entire fossils are uniformly reflective and polarized 2011). Despite nearly three decades of intensive research into the topic, light appears to have little or no effect. The majority of fossil films are cov- the taphonomic processes that led to BST preservation is still a matter of ered to a varying degree by iron oxide and oxyhydroxide coatings (Fig. 1). debate. Several hypotheses have been proposed, ranging from the simple Raman analysis demonstrates that there was a degree of homogeneity absence of bioturbation and dysoxic or anoxic conditions, to the role of across the host rock and the 14 Sirius Passet fossil specimens, with all clays in inhibiting autolytic decay, early aluminosilicate diagenesis, pyrite spectra exhibiting a strong carbon signal (with the exception of some point mineralization involving sulfate- and iron-reducing bacteria, Fe2+ ions spectra directed at the iron oxide and oxyhydroxide coatings). The major- adsorbing on organic tissues to delay degradation, and the sealing of sedi- ity of point spectra also exhibit vibrational modes at 200 cm–1 and 464 ments by pervasive carbonate cements (see the review by Gaines, 2014). cm–1 attributed to quartz, and a vibrational mode at 262 cm–1 attributed to GEOLOGY, April 2018; v. 46; no. 4; p. 359–362 | GSA Data Repository item 2018107 | https://doi.org/10.1130/G39930.1 | Published online XX Month 2018 ©GEOLOGY 2018 The Authors.| Volume Gold 46 |Open Number Access: 4 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 359 ABD1 G C G D1 D1 G D2 D2 D3 D3 D3 D4 D2 D4 D4 1000 1200 1400 1600 1800 1000 1200 1400 1600 1800 1000 1200 1400 1600 1800 -1 -1 -1 Raman shift cm Raman shift cm Raman shift cm DE 12000 720000 11000 660000 10000 600000 9000 540000 8000 480000 7000 420000 6000 360000 5000 300000 4000 240000 D1 band G band D1 band G band D1 band G bandD1 band G band Figure 1. Kerogenous films (reflective light gray areas) of soft-bodied Fossil films Matrix Fossil films Matrix organisms from the Sirius Passet Lagerstätte (northern Greenland). A: Aaveqaspis inesoni, Geological Museum, University of Copenha- Figure 2. Comparison of Raman spectra from the Sirius Passet gen specimen MGUH32026. B: Overview of specimens preserved (northern Greenland) (A,B,D,E) and Burgess Shale (British Colum- as reflective films of kerogenous remains, including MGUH32027. C: bia, Canada) Lagerstätten (C). A: Mean decomposition of the Raman indeterminate
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