Evidence for thermohaline-circulation reversals controlled by sea-level change in the latest Cretaceous

During the early Maastrichtian (ca. 71 Ma), important changes occurred in climate, weathering, sea level, biota, and the global carbon cycle. Global climate was characterized by rapid cooling within the long-term post-Santonian refrigeration of high-latitude surface and intermediate waters (Barrera, 1994; Huber et al., 1995). The intensity of continental weathering increased, as inferred from the rate of change of the seawater 87Sr/86Sr ratio (Nelson et al., 1991; Barrera, 1994), while sea level dropped (Haq et al., 1987). Invertebrates such as rudists and inoceramid bivalves underwent extinction (Johnson and Kauffman, 1996; MacLeod and Huber, 1996), and North American angiosperms declined abruptly in diversity (Johnson, 1992). Benthic and planktic foraminiferal δ13C values in the Southern Ocean decreased markedly at 71 Ma, followed by a sharp rise at 70 Ma (Barrera and Huber, 1990; Barrera, 1994). Here we present the first evidence that a shortlived global oceanographic episode occurred in the early Maastrichtian (ca. 71 to 70 Ma), during which time thermohaline circulation was dominated by a mechanism similar to that of today: cool surface waters at high latitudes sank to form the intermediate and deep waters of the tropics. Prior and subsequent to that episode, the driving mechanism of Maastrichtian thermohaline circulation was different. Our conclusions are based on new foraminiferal δ18O and δ13C data from Deep Sea Drilling Program (DSDP) Sites 305, 463, and 465 in the Pacific Ocean, and published data for Ocean Drilling Program (ODP) Sites 690, 750, and 761 (Barrera and Huber, 1990; Barrera, 1994). Correlation of these sections across latitude was achieved for the first time with the aid of 87Sr/86Sr data. Paleodepths at all sites were between ~800 and 1800 m (Table 1), an interval that in the modern oceans is bathed by intermediate-water masses. Our synthesis thus yields a large-scale areal isotopic characterization of intermediate-depth waters (Fig.1). SAMPLE MATERIAL We analyzed monospecific benthic and shallowdwelling planktic foraminifera; the samples were restricted to a narrow size fraction (planktic species: 200–250 μm; benthic species: 200–300 μm) to minimize biases due to vital effects (Barrera and Keller, 1994). Published isotope records for Sites 305, 463, and 465 are from samples taken at large spacing intervals and generally based on mixed taxa (Douglas and Savin, 1975; Boersma and Shackleton, 1981). Benthic and planktic foraminiferal specimens from all sites appear well preserved under the optical microscope, but show evidence of minor recrystallization of the wall structure under the scanning electron microscope. The consistency of δ18O trends