EVIDENCE FROM CARBONATE PLATFORMS BEARING ON CLIMATE, SALINITY, DASYCLADALEAN DIVERSITY, AND MARINE ANOXIC EVENTS DURING THE LATE JURASSIC-EARLY CRETACEOUS GREENHOUSE

Abstract

Carbonate platforms are sensitive recorders of environmental change through geologic time. Climatically induced changes in sea level or changes in subsidence are expressed through migration of sediment belts that are recorded in the accumulating sedimentary succession. Changes in nutrient levels are recorded in shifts in the biotic community and, if eutrophic levels are reached, platforms may drown due to breakdown in carbonate production (Follmi et al., 1994). Our group has been focusing on the ∼60 Ma Late Jurassic–Early Cretaceous period, which was originally considered to have had uniform greenhouse conditions (Fischer, 1982). Available climate proxy data, however, such as oxygen isotope data (δ18O) from deep sea cores, the paleontologic data (e.g., Frakes et al., 1992), and the 87Sr/86Sr and carbon isotope data of Jurassic and Cretaceous carbonates (Jenkyns and Wilson, 1999), indicate that there were major cooling and warming events that likely affected global ice volume, sea level, and the accumulating sedimentary and microfossil record of carbonate platforms. Numerous carbonate platforms were developed within the tropical–subtropical belt of the circumequatorial Late Jurassic–Early Cretaceous Tethys Ocean. Many of these platforms have comparable shapes, sizes, facies, subsidence rates, and geologic structure to the present-day Bahamas Banks, a commonly used modern-day analogue (e.g., Bosellini, 2002, and references therein). In addition, the outcropping Tethyan platforms provide important clues to the poorly known subsurface Mesozoic carbonates that underpin the modern Bahamas platform. The Tethyan platforms were characterized by high rates of sedimentation (from 100 m/myr; D'Argenio et al., 1999) and exhibit meter-scale shallowing-upward cycles or parasequences generated during high frequency, small sea-level fluctuations within the Milankovitch band (Strasser, 1991; Schulz and Schafer-Neth, 1997; Lehmann et al., 1999; Strasser et al., 1999; Immenhauser et al., 2004; Husinec and …