 2006 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org. Geology; April 2006; v. 34; no. 4; p. 277–280; doi: 10.1130/G22198.1; 2 figures; 1 table; Data Repository item 2006058. 277 Constraints on Pennsylvanian glacioeustatic sea-level changes using oxygen isotopes of conodont apatite Michael M. Joachimski Institute of Geology and Mineralogy, University of Erlangen, Schlossgarten 5, 91054 Erlangen, Germany Peter H. von Bitter Department of Natural History, Palaeobiology Section, Royal Ontario Museum, and Department of Geology, University of Toronto, 100 Queen’s Park, Toronto M5S 2C6, Canada Werner Buggisch Institute of Geology and Mineralogy, University of Erlangen, Schlossgarten 5, 91054 Erlangen, Germany ABSTRACT Conodonts from U.S. Midcontinent cyclothems were studied for oxygen isotopes in order to constrain Pennsylvanian glacioeustatic sea-level fluctuations. Pennsylvanian deposits of the Midcontinent United States are composed of cyclic alternations of thin transgressive limestones, offshore gray to black phosphatic shales, and thick regressive limestones, a sequence that is underlain and overlain by nearshore to terrestrial shales with paleosols and coal beds. Glacioeustatic sea-level fluctuations are considered the primary cause for the formation of these cyclothems. Oxygen isotope analyses of conodont apatite from the black (20.1  0.5‰, Vienna standard mean ocean water [VSMOW]) and gray shale units (20.5  0.5‰, VSMOW) show lowest average  18 O values, whereas conodont elements from the regressive (21.0  0.3‰, VSMOW) and transgressive limestone units (21.1  0.6‰, VSMOW) are enriched in 18 O. The maximum change in  18 O of conodonts from the black shale and carbonate units from individual cyclothems is 1.7‰. The 1.7‰ dif- ference in  18 O compares relatively well to Pleistocene interglacial-glacial changes in  18 O of equatorial surface-dwelling foraminifers and suggests that Pennsylvanian glacioeustatic sea-level changes may have been of comparable amplitude. However, since the Pennsyl- vanian glacial maxima are represented by terrestrial sediments and are not documented in the conodont oxygen isotope record, Pennsylvanian glacioeustatic sea-level changes were probably larger than the 120 m fluctuations recorded for the Pleistocene glaciations. Keywords: Pennsylvanian, conodonts, oxygen isotopes, sea level. INTRODUCTION Pennsylvanian to Early Permian time was characterized by the development of high- latitude ice caps in Antarctica and adjacent ar- eas of Gondwana (Veevers and Powell, 1987). The onset of the glaciation has been dated as beginning in the Serpukhovian-Bashkirian age, with ice coverage reaching a first maxi- mum during the Moscovian. Ice masses con- tracted during the Gzelian, but increased again during the Early Permian, reaching a second maximum during the Asselian to Sakmarian. Studies of Pennsylvanian sedimentary depos- its suggest that the glaciation was character- ized by cyclic waxing and waning of Gond- wanan glaciers, which caused high-frequency sea-level fluctuations (Wanless and Sheppard, 1936; Veevers and Powell, 1987; Crowley and Baum, 1991; Heckel, 1977, 1994). During the Pennsylvanian, the interior Unit- ed States was located at 5–10°N latitude (Heckel, 1977; Scotese, 1997), and the ocean covered the greater part of the Midcontinent, ranging from the Appalachian Basin in the east to the Midcontinent Basin in the west. The northern and central part of the Midcon- tinent Basin was relatively shallow, whereas toward the south, the basin deepened into the Arkoma and Anadarko foredeeps of Arkansas and central Oklahoma, respectively. The Mid- continent Basin opened westward and north- westward into the Panthalassa Ocean. The middle portion of Pennsylvanian cyclothems of the Midcontinent United States includes cy- clic alternations of a thin transgressive lime- stone, thin offshore gray to black phosphatic shales, and thick regressive limestones, a se- quence that is underlain and overlain by near- shore to terrestrial shales with well-developed paleosols or coal beds. The transgressive lime- stone represents a deepening-upward sequence and is overlain by the offshore (core) shale, represented by thin gray to black phosphatic shales. The black organic carbon–rich shales have been interpreted to have been deposited in waters deep enough to develop a pycno- cline, which enabled the development of bottom-water anoxia at depths estimated to have been 100 m (Heckel, 1977, 1994). The regressive limestone typically is composed of a thick shallowing-upward carbonate sequence grading from skeletal calcilutites to calcaren- ites, oolites, and peritidal deposits. The over- lying nearshore/terrestrial (outside) shale unit is characterized by shales and sandstones, in- terpreted as deltaic, paralic, and fluvial facies. Paleosols are generally developed at the top of this unit. Glacioeustatic sea-level fluctua- tions have been inferred to be the prime cause of these cyclic sedimentary deposits (Heckel, 1994, 2002). The aim of our study is to provide an esti- mate for the Pennsylvanian glacioeustatic sea- level fluctuations by comparing oxygen iso- tope values of conodonts from different cyclothem units. Conodonts are extinct early jawless vertebrates (Donoghue and Purnell 1999) that possessed a complex feeding ap- paratus of carbonate-fluorapatite elements which have a high potential to preserve the primary oxygen isotope signal due to their dense structure and high crystallinity (Puce ´at et al. 2004).