Showing papers by "Richard G. Fairbanks published in 1987"

Tertiary oxygen isotope synthesis, sea level history, and continental margin erosion

Abstract: Tertiary benthic and planktonic foraminiferal oxygen isotope records are correlated to a standard geomagnetic polarity time scale, making use of improved chronostratigraphic control and additional Oligocene isotope data Synchronous changes in both benthic and planktonic δ18O values which occurred in the Oligocene to Miocene (36–52 Ma) are interpreted, in part, to represent ice growth and decay The inferred ice growth events correlate with erosion on passive continental margins as interpreted from seismic and chronostratigraphic records This association is consistent with a link between Oligocene to Miocene erosional events and rapid (>15 m/my) glacioeustatic lowerings of about 50 m High benthic foraminiferal δ18O values suggest the presence of continental ice sheets during much of the Oligocene to Recent (36–0 Ma) Substantially ice-free conditions probably existed throughout the Paleocene and Eocene (66–36 Ma) The mechanisms and rates of sea level change apparently were different between the early and late Tertiary, with glacioeustatic changes restricted to the past 36 my Pre-Oligocene erosion on passive continental margins was caused by eustatic lowerings resulting from global spreading rate changes We apply a model which suggests that large areas of the continental shelves were subaerially exposed during such tectonoeustatic lowstands, stimulating slope failure and submarine erosion The different mechanisms and rates of eustatic change may have caused contrasting erosional patterns between the early and late Tertiary on passive continental margins This speculation needs to be confirmed by examination of data from several passive margins

Benthic Foraminiferal Carbon Isotopic Records and the Development of Abyssal Circulation in the Eastern North Atlantic

Abstract: The North Atlantic at present is ventilated by overflow of the Denmark Strait, Iceland-Faeroe Ridge, Faeroe Bank Channel, and Wyville-Thompson Ridge. The evolution of Cenozoic abyssal circulation of this region was related to tectonic opening and subsidence of these sills. We used δ 1 C records of the benthic foraminifer Cibicidoides to decipher the timing of tectonically controlled changes in bottom-water circulation in the eastern basins (Biscay and Iberian) of the northern North Atlantic. Records from Site 608 (Kings Trough, northeastern North Atlantic) show that from about 24 to 15 Ma (early to early middle Miocene), δ 1 C values in the Kings Trough area were depleted relative to western North Atlantic values and were more similar to Pacific δ 1 C values. This reflects less ventilation of the Kings Trough region as compared to the well-oxygenated western North Atlantic. Comparison of Oligocene δ 1 C records from Site 119 (Bay of Biscay) with western North Atlantic records suggests that the eastern basin was also relatively isolated prior to 24 Ma. At about 15 Ma, δ 1 C values at Site 608 attained values similar to the western North Atlantic, indicating increased eastern basin ventilation in the middle Miocene. This increased advection into the eastern basin predated a major δ 1 8 θ increase which occurred at about 14.6 Ma. Subsidence estimates of the Greenland-Scotland Ridge indicate that the deepening of the Iceland-Faeroe Ridge was coincident with the marked change in eastern basin deep-water ventilation. INTRODUCTION AND PREVIOUS WORK In the Quaternary, high-frequency (lOMO yr.) abyssal circulation changes were climatically controlled (e.g., Curry and Lohmann, 1983, 1985). Cenozoic abyssal circulation changes on the 10to 10-yr. scale, however, may be related either to long-term climatic or to tectonically controlled changes (e.g., Miller and Tucholke, 1983). Reconstructions of tectonic passageways allow evaluation of the causes of abyssal circulation changes. The history of basin development in the Cenozoic North Atlantic is a history of progressive tectonic enlargement and opening of passages that apparently resulted in increased bottom-water circulation (Berggren and Hollister, 1972; Miller and Tucholke, 1983). These tectonic changes had a profound effect upon global abyssal circulation and ocean chemistry, including loci of deposition of organic carbon, silica, and carbonate (Berger, 1970). The history of Cenozoic bottom-water formation in the North Atlantic and its marginal seas has been controversial (cf., Schnitker, 1979, 1980a, b, with Miller and Tucholke, 1983). Today, North Atlantic Deep Water (NADW) is formed by a mixture of Norwegian-Greenland Sea Overflow Water, Labrador Sea Water, and entrained North Atlantic Water (Worthington, 1976; Broecker and Peng, 1983). Based upon evidence of changes in composition of benthic foraminiferal faunas in the eastern North Atlantic, Schnitker (1979) suggested that the first analogue of NADW formed in the middle MioRuddiman, W. F., Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94: Washington (U.S. Govt. Printing Office). 2 Addresses: (Miller and Fairbanks) Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964; (Thomas, present address) Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964. cene, subsequent to the subsidence of the Iceland-Faeroe Ridge. Miller and Tucholke (1983) presented seismic stratigraphic evidence from the eastern and western North Atlantic, which suggested that an analogue to NADW first formed near the end of the Eocene, and this has been substantiated for the western North Atlantic by carbon isotopic studies (Miller and Fairbanks, 1983, 1985). They suggested that Oligocene-Miocene bottom waters formed in the Arctic/Norwegian-Greenland Sea and flowed over sills in the proto-Denmark Straits and Faeroe Bank Channel, although they acknowledged that the Labrador Sea and northern North Atlantic were also possible sources (Figs. 1, 2) (Miller and Tucholke, 1983). We used carbon isotopes to evaluate possible sources of bottom-water supply to the eastern basins of the northern North Atlantic, and specifically to evaluate whether the deep eastern basins were isolated from the western basins. Carbon isotopic comparisons provide strong evidence for the nature and timing of changes in North Atlantic bottom waters. Although changes in lithology and benthic foraminiferal fauna provide supportive evidence for abyssal circulation changes, they are rarely diagnostic by themselves. Benthic foraminiferal δ C analyses have proven to be useful in reconstructing the history of Quaternary abyssal circulation changes (Curry and Lohmann, 1982, 1983, 1985; Boyle and Keigwin, 1982; Shackleton et al., 1983; Mix and Fairbanks, in press; Fairbanks and Mix, in press). Curry and Lohmann (1985) demonstrated that the deep eastern equatorial Atlantic basin was isolated from the western Atlantic during Pleistocene ice ages, but was well mixed during interglacial periods. We have previously applied such tactics to Quaternary δ C records to decipher 10to 10yr. abyssal circulation changes for the western North Atlantic during the Oli-