Palaeogeography, Palaeoclimatology, Palaeoecology, 69 (1989): 245-266 245 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands GEOCHEMICAL AND PALEOENVIRONMENTAL VARIATIONS ACROSS THE CRETACEOUS/TERTIARY BOUNDARY AT BRAGGS, ALABAMA JAMES C. ZACHOS 1, MICHAEL A. ARTHUR 1 and WALTER E. DEAN 2 ~ Graduate School of Oceanography, University of Rhode Island, Narragansett, RI 02882 (U.S.A.) 2U.S. Geologic Survey, P.O. Box 25046, MS 939, Federal Center, Denver, CO 80225 (U.S.A.) (Received March 9, 1988; revised and accepted August 8, 1988) Abstract Zachos, J. C., Arthur, M.A. and Dean, W.E., 1989. Geochemical and paleoenvironmental variations across the Cretaceous/Tertiary boundary at Braggs, Alabama. Palaeogeogr., Palaeoclimatol., Palaeoecol., 69:245 266. The Cretaceous/Tertiary (K/T) boundary in southern Alabama occurs in a sequence of interbedded shallow-marine limestones and marls deposited during a Late Maastrichtian regression and subsequent Danian transgression. The presence of a diverse assemblage of Cretaceous and Paleocene benthic micro- and macrofossils has allowed detailed examination of paleoenvironmental changes in this shallow-marine setting at the time of the K/T boundary extinctions. Although extensive diagenesis has resulted in the recrystallization and cementation of whole rock carbonate, a few molluscan macrofossils have retained their original calcitic structure. The oxygen isotopic record of these macrofossils shows a gradual cooling of 2 3°C over a 3 m.y. period which began in the late Maastrichtian and continued into the Danian. The characteristic depletion in 513C across the K/T boundary displayed by planktonic microfossils from pelagic sequences is not recorded at Braggs. Major changes in whole rock trace and minor element chemistry reflect the decrease in terrigenous mineral fluxes to this location brought about by the rapid Early Paleocene transgression. Introduction During the last decade the majority of geochemical evidence for marine environmen- tal changes across the Cretaceous/Tertiary boundary has come primarily from DSDP (Deep Sea Drilling Project) and land-based pelagic sequences. With this evidence a num- ber of investigators have shown that dramatic changes in oceanic primary productivity oc- curred coeval with the major marine plankton extinctions as well as with the deposition of chemical anomalies suggestive of a bolide impact event (e.g. Hsfi et al., 1982; Perch- Nielsen et al., 1982; Shackleton and Hall, 1984; Zachos and Arthur, 1986). The carbon isotope evidence upon which the productivity records were based suggest that a sharp reduction in oceanic productivity occurred in a relatively short period (< 10,000 yr). An increasing body of paleontological evidence from shallower marine sequences (Kauffman, 1984; Mount et al., 1986), however, suggests that while the extinctions among the planktonic organisms may have been rapid, the extinction of the marine benthic invertebrates may have occurred in a more gradual, step-like manner, raising serious doubts about the timing of and very nature of the extinctions (Officer and Drake, 1983; Hal- lam, 1987; Hut et al., 1987; Officer et al., 1987). Another unresolved aspect of the K/T transi- tion is the nature of possible climate changes. Paleotemperature reconstructions on the basis of oxygen isotope data from DSDP sequences 0031-0182/89/$03.50 ~(:) 1989 Elsevier Science Publishers B.V.