Palaeoceanographic and palaeoclimatic inferences from Late Cretaceous planktonic foraminiferal assemblages from the Exmouth Plateau (ODP Sites 762 and 763, eastern Indian Ocean)

Author(s): Petrizzo, Maria Rose
Author Affiliation(s): Primary:
Universita di Milano, Dipartimento di Scienze della Terra, Milan, Italy
Volume Title: Marine Micropaleontology
Source: Marine Micropaleontology, 45(2), p.117-150. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0377-8398 CODEN: MAMIDH
Note: In English. Includes appendix. 66 refs.; illus., incl. strat. cols., 4 tables, sketch map
Summary: The evolution of planktonic foraminifera during the Late Cretaceous is marked in the Santonian by the disappearance of complex morphotypes (the marginotruncanids), and the contemporary increasing importance and diversification of another group of complex taxa, the globotruncanids. Upper Turonian to lower Campanian planktonic foraminiferal assemblages from Holes 762C and 763B (Ocean Drilling Program, Leg 122, Exmouth Plateau, 47°S palaeolatitude) were studied in detail to evaluate the compositional variations at the genus and species level based on the assumption that, in the Cretaceous oceans as in the modern, any faunal change was associated with changes in the characteristics and the degree of stability of the oceanic surface waters. Three major groups were recognised based on gross morphology, and following the assumption that Cretaceous planktonic foraminifera, although extinct, had life-history strategies comparable to those of modern planktonics: 1 - r-selected opportunists; 2 - k-selected specialists; 3 - r/k intermediate morphotypes which include all genera that display a range of trophic strategies in-between opportunist and specialist taxa. Although planktonic foraminiferal assemblages are characterised by a progressive appearance of complex taxa, this trend is discontinuous. Variation in number of species and specimens within genera has allowed recognition of five discrete intervals each of them reflecting different oceanic conditions based on fluctuations in diversity and abundance of the major morphotypes. Planktonic forms show cyclical fluctuations in diversity and abundance of cold (r-strategists) and warm taxa (k-strategists), perhaps representing alternating phases of unstable conditions (suggesting a weakly stratified upper water column in a mesotrophic environment), and well-stratified surface and near-surface waters (indicating a more oligotrophic environment). Interval 1, middle Turonian to early Coniacian in age, is dominated by the r/k intermediate morphotypes which alternate with r-strategists. These cyclical alternations are used to identify three additional sub-intervals. Interval 2, aged middle to late Coniacian, is characterised by the increasing number of species and relative abundance of k-strategists. After this maximum diversification the k-strategists show a progressive decrease reaching a minimum value in Interval 3 (early to late Santonian), which corresponds to the extinction of the genus Marginotruncana. In the Interval 4, latest Santonian in age, the k-strategists, represented mainly by the genera Globotruncana, increase again in diversity and abundance. The last Interval 5 (early Campanian) is dominated by juvenile globotruncanids and r-strategists which fluctuate in opposite phase. The positive peak (Interval 2) related to the maximum diversification of warm taxa (k-strategists) in the Coniacian seems to correspond to a warmer episode. It is followed by a marked decrease in the relative abundance of warm taxa (k-strategists crisis) with a minimum in the late Santonian (Interval 3), reflecting a decrease in temperature. Detailed analysis of faunal variations allows the Santonian faunal turnover to be ascribed to a cooling event strong enough to cause the extinction of the marginotruncanids. Abstract Copyright (2002) Elsevier, B.V.
Year of Publication: 2002
Research Program: ODP Ocean Drilling Program
Key Words: 12 Stratigraphy, Historical Geology and Paleoecology; Assemblages; Biostratigraphy; Biozones; Cretaceous; Exmouth Plateau; Faunal list; Foraminifera; Indian Ocean; Invertebrata; Leg 122; Marine environment; Mesozoic; Microfossils; ODP Site 762; ODP Site 763; Ocean Drilling Program; Paleo-oceanography; Paleoclimatology; Paleoecology; Paleoenvironment; Paleogeography; Planktonic taxa; Protista; Upper Cretaceous
Coordinates: S195315 S195314 E1121515 E1121514
S203512 S203511 E1121232 E1121231
Record ID: 2002062106
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands