Millennial-Scale Paleoclimatic Cyclicity Recorded in Paleozoic Marine Deposits
Maya B. Elrick (Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131; email@example.com); Linda A. Hinnov (Earth and Planetary Sciences, Johns Hopkins University, Baltimore MD, 21218; firstname.lastname@example.org)
Paleozoic marine stratigraphic sections have been investigated to evaluate the origin and approximate duration of rhythmically interbedded limestone-marl couplets ("rhythmites"). The Cambrian through Mississippian deposits (~540-360 m.y.) represent deposition during widely varying paleogeographic (western U.S. and Canada, Appalachians), tectonic (passive margin, foreland basin), climatic (greenhouse, incipient icehouse), paleobiologic (pre- and post-terrestrial plant evolution), and eustatic settings. Despite these major differences, the rhythmite successions are strikingly similar in character. Limestone layers (4-10 cm thick) are composed of unfossiliferous, laminated pelleted lime mudstone; marl layers (<4 cm thick) are composed of laminated, unfossiliferous, argillaceous pelleted limestone. The preservation of laminae, lack of fossils, and associated facies relationships indicate that the rhythmites were deposited in anoxic waters below storm-wave base and individual graded laminae (<1 mm thick) represent deposition from distal storm deposits or dilute density currents. Because calcareous pelagic microorganisms did not evolve until the Mesozoic, the fine-grained carbonate material must have been derived from associated shallow carbonate shelves (detrital) and transported seaward via dilute density currents and/or storm-generated currents. Fluctuations in this carbonate component suggests that the interbedding was controlled by changes in storm track location/intensity or marine processes controlling density current generation.
Time series analysis of wt% carbonate was conducted to determine the dominant limestone-marl couplet wavelength (or periodicity). The durations of the wavelengths were calculated by estimating accumulation rates using biostratigraphic controls. The estimated range of accumulation rates for all the studied rhythmite successions suggests that the basic limestone-marl couplet has millennial-scale periodicity (<3000 years). The results from this study combined with other work from Ordovician and Permian evaporites suggests that millennial-scale paleoclimate fluctuations occurred throughout the Paleozoic and are a persistent feature of the Earth's ocean and atmosphere.