Nanoscale 3D quantitative imaging of 1.88 Ga Gunflint microfossils reveals novel insights into taphonomic and biogenic characters

International audiencePrecambrian cellular remains frequently have simple morphologies, micrometric dimensions and are poorly preserved, imposing severe analytical and interpretational challenges, especially for irrefutable attestations of biogenicity. The 1.88 Ga Gunflint biota is a Precambrian microfossil assemblage with different types and qualities of preservation across its numerous geological localities and provides important insights into the Proterozoic biosphere and taphonomic processes. Here we use synchrotron-based ptychographic X-ray computed tomography to investigate well-preserved carbonaceous microfossils from the Schreiber Beach locality as well as poorly-preserved, iron-replaced fossil filaments from the Mink Mountain locality, Gunflint Formation. 3D nanoscale imaging with contrast based on electron density allowed us to assess the morphology and carbonaceous composition of different specimens and identify the minerals associated with their preservation based on retrieved mass densities. In the Mink Mountain filaments, the identification of mature kerogen and maghemite rather than the ubiquitously described hematite indicates an influence from biogenic organics on the local maturation of iron oxides through diagenesis. This non-destructive 3D approach to microfossil composition at the nanoscale within their geological context represents a powerful approach to assess the taphonomy and biogenicity of challenging or poorly preserved traces of early microbial life, and may be applied effectively to extraterrestrial samples returned from upcoming space missions

Heart Fossilization Is Possible And Informs The Evolution Of Cardiac Outflow Tract In Vertebrates

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Elucidating cardiac evolution has been frustrated by lack of fossils. One celebrated enigma in cardiac evolution involves the transition from a cardiac outflow tract dominated by a multi-valved conus arteriosus in basal actinopterygians, to an outflow tract commanded by the non valved, elastic, bulbus arteriosus in higher actinopterygians. We demonstrate that cardiac preservation is possible in the extinct fish Rhacolepis buccalis from the Brazilian Cretaceous. Using X-ray synchrotron microtomography, we show that Rhacolepis fossils display hearts with a conus arteriosus containing at least five valve rows. This represents a transitional morphology between the primitive, multivalvar, conal condition and the derived, monovalvar, bulbar state of the outflow tract in modern actinopterygians. Our data rescue a long-lost cardiac phenotype (119-113 Ma) and suggest that outflow tract simplification in actinopterygians is compatible with a gradual, rather than a drastic saltation event. Overall, our results demonstrate the feasibility of studying cardiac evolution in fossils.5Coordenacao de Aperfeigoamento de Pessoal de Nivel Superior [01P-03488/2014]Fundacao de Amparo a Pesquisa do Estado de Sao Paulo [2012/05152-0]Conselho Nacional de Desenvolvimento Cientifico e Tecnologico [481983/2013-9]Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Evaluating iron as a biomarker of rhythmites : an example from the last Paleozoic ice age of Gondwana

Microorganisms play a significant role in mineral precipitation, but detecting them in the fossil record is still a challenge. Here we offer an example of how the detection of biological activity in the sedimentary environment can modify a classical depositional model. This study describes the activity of microorganisms in sedimentary structures and the iron mineral formation during the last Paleozoic Ice Age in southwestern Gondwana, recorded by the "Itu rhythmites", Parana Basin, Brazil. The Itu rhythmites have been considered to be varve-type deposits that present alternating dark laminae (clay/silt-size sediments) and light layers (sand/gravel-size sediments) of varied thickness, forming couplets. Earlier studies focused on abiotic processes of these structures. We applied different techniques and analytical approaches were used, such as synchrotron-based techniques and rock magnetic techniques, in order to test the biogenicity of iron minerals contained in putative microbially-induced sedimentary structures. By detecting biominerals in this rock succession, the depositional model had to be reconsidered, taking into account the biological activity, the limitations on the specific conditions for bacterial growth, and for mineral precipitation. Therefore, we offer a new depositional model that considers the role of microorganisms in formation of these laminae, and this model can be considered for other iron-rich rhythmic deposits in other places of the world. Considering the effects of temperature and other factors in the bacterial productivity, the deposition of the latest couplets in the outcrop occurred in different seasons and by different depositional processes, corroborating with the non-periodicity of 1 year per lithological couplet383115CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICO - CNPQCOORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR - CAPESFUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DE SÃO PAULO - FAPESP454609/2014-0; 424367/2016-5AUXPE 2043/20142016/20927-0; 2016/06114-6We acknowledge the comments and suggestions of Kenneth Kodama and Renata Guimarães Netto and, the editorial work of Sunoj Sankaran and Jasper Knight for the valuable criticisms and efforts for the improvement of this manuscript. The authors would like to thank the Brazilian Synchrotron Light Laboratory (LNLS/CNPEM) and the Brazilian Nanotechnology Laboratory (LNNano/CNPEM) for the availability of the facilities (μ-XRF, XRD2, SEM/EDS and TEM) through the approval of the proposals no 20150076, no 20160071, SEM 23265 and ME 22557, respectively; the Geophysical Laboratory from Carnegie Institution of Science (Washington, DC) for the availability of microprobe and Raman spectrometer; the Paleomagnetism Laboratory of University of São Paulo (USPMag), Brazil; the Brazilian Federal Agency for Support and Evaluation of Graduate Education - CAPES Foundation and the PDSE Program for the financial support and scholarship; and the São Paulo Research Foundation – FAPESP (grant no 2016/20927-0 and no 2016/06114-6) for funding this project. The authors also thank CAPES (grant AUXPE 2043/2014) and CNPq (National Council for Scientific and Technological Development, grant no 454609/2014-0 and no 424367/2016-5) and Serrapilheira Institute (project number G-1709-20205