Received: 07 July 2015 | Accepted: 02 May 2016 DOI: 10.1111/gbi.12197 Abstract Preservaion of Pennsylvanian-aged (307 Ma) sot-bodied fossils from Mazon Creek, Illinois, USA, is atributed to the formaion of siderite concreions, which encap- sulate the remains of terrestrial, freshwater, and marine lora and fauna. The narrow range of posiive δ 34 S values from pyrite in individual concreions suggests micro- environmentally limited ambient sulfate, which may have been rapidly exhausted by sulfate-reducing bacteria. Tissue of the decaying carcass was rapidly encased by early diageneic pyrite and siderite produced within the sulfate reducion and methanogenic zones of the sediment, with coninuaion of the later resuling in concreion cementaion. Cross-secional isotopic analyses (δ 13 C and δ 18 O) and mineralogical characterizaion of the concreions point to iniiaion of preservaion in high porosity proto-concreions during the early phases of microbially induced decay. The proto-concreion was cemented prior to compacion of the sediments by siderite as a result of methanogenic producion of 13 C-rich bicarbonate—which varies both between Essex and Braidwood concreions and between fossiliferous and unfossiliferous concreions. This work provides the irst detailed geochemical study of the Mazon Creek siderite concreions and ideniies the range of condi- ions allowing for excepional sot-issue fossil formaion as seen at Mazon Creek. 1 Department of Earth Sciences, University of Toronto, Toronto, ON, Canada 2 Department of Geological Sciences, University of Missouri, Columbia, MO, USA 3 Department of Geology and Geophysics, Yale University, New Haven, CT, USA 4 Department of Earth and Environmental Sciences, Vanderbilt University, Nashville, TN, USA 5 Department of Geology & Geophysics, Louisiana State University, Baton Rouge, LA, USA 6 Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON, Canada Correspondence M. Lalamme, Assistant Professor Department of Chemical and Physical Sciences, University of Toronto Mississauga, Mississauga, ON, Canada. Email: marc.lalamme@utoronto.ca *Present address: Department of Earth Sciences, University of Otawa, Otawa, ON, Canada. ORIGINAL ARTICLE A new model of the formaion of Pennsylvanian iron carbonate concreions hosing excepional sot-bodied fossils in Mazon Creek, Illinois S. Cotroneo 1, * | J. D. Schibauer 2 | V. E. McCoy 3 | U. G. Wortmann 1 | S. A. F. Darroch 4 | Y. Peng 5 | M. Lalamme 1,6 1 | INTRODUCTION The Pennsylvanian (307 Ma) Mazon Creek Konservat Lagerstäte yields excepional sot-issue fossils of terrestrial, shallow marine, fresh, and brackish water lora and fauna (Baird, Shabica, Anderson, & Richard- son, 1985; Feldman et al., 1993; Johnson & Richardson, 1966; LoBue, 2010), and thus preserves a much more complete view of Paleozoic delta plain ecosystems than is typically aforded by the skeletal fos- sil record alone. More than 500 species of plants and animals have been reported, including the enigmaic vertebrate Tullimonstrum (Cle- ments et al., 2016; McCoy et al., 2016), plant leaves (Locatelli, Krajew- ski, Chochinov, & Lalamme, 2016), and an array of jellyish, worms, insects, crustaceans, and arachnids (Baird, Sroka, Shabica, & Kuecher, 1986; Baird, Shabica et al., 1985). The Mazon Creek biota are divided into two primary assemblages (Fig. 1A), the Braidwood in the northeast (terrestrial and freshwater) and the Essex in the southwest (brackish water). The fossils are preserved within siderite (FeCO 3 ) concreions from the lower 3–4 m of the Francis Creek Shale Member of the Car- bondale Formaion (Westphalian D; Fig. 1B) (Baird, 1979, 1997a,b). Previous invesigaions have interpreted the siderite concreions as forming shortly ater burial and found that they are restricted to sites where the Francis Creek Shale Member is at least 15 m thick (Baird, 1979). Rapid sedimentaion and cementaion of the concreions are thought to be essenial processes controlling the quality and quanity Geobiology 2016; 1–13 wileyonlinelibrary.com/gbi © 2016 John Wiley & Sons Ltd | 1