Showing papers by "Ellen Thomas published in 1987"
TL;DR: In this article, data on the composition of benthic foraminiferal faunas at Deep Sea Drilling Project Site 575 in the eastern equatorial Pacific Ocean were combined with Benthic and planktonic carbon and oxygen isotope records and CaCO 3 data.
Abstract: Data on the composition of benthic foraminiferal faunas at Deep Sea Drilling Project Site 575 in the eastern equatorial Pacific Ocean were combined with benthic and planktonic carbon- and oxygen-isotope records and CaCO 3 data. Changes in the composition of the benthic foraminiferal faunas at Site 575 predated the middle Miocene period of growth of the Antarctic ice cap and cooling of the deep ocean waters by about 2 m.y., and thus were not caused by this cooling (as has been proposed). The benthic faunal changes may have been caused by increased variability in corrosivity of the bottom waters, possibly resulting from enhanced productivity in the surface waters.
47 citations
01 Jan 1987
TL;DR: Miller and Tucholke as mentioned in this paper used the benthic foraminifer Cibicidoides to decipher the timing of tectonically controlled changes in bottom-water circulation in the eastern basins of the northern 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-
39 citations
01 Jan 1987
TL;DR: Baldauf et al. as discussed by the authors used the DSDP Leg 94 sediment from 22 holes at six sites situated between 37° and 53°N in the North Atlantic to evaluate the synchrony of late Pliocene to Holocene data.
Abstract: During DSDP Leg 94 sediment was recovered from 22 holes at six sites situated between 37° and 53°N in the North Atlantic. Paleomagnetic, calcareous nannofossil, foraminiferal, diatom, radiolarian, and dinocyst stratigraphic studies were completed. The excellent magnetostratigraphic results, the near-complete recovery, and the abundant fossil content of the sediment allowed refinement of these stratigraphies, especially for the upper Pliocene-Holocene. Because the Leg 94 sites span middle and high North Atlantic latitudes, it is possible to use them to evaluate the synchrony of late Pliocene to Holocene datums. INTRODUCTION Before Leg 94, Hole 552A (Leg 81, 56°N, Rockall Plateau) represented the most continuous stratigraphic sequence from the high-latitude North Atlantic. Although detailed stratigraphies were completed (Backman et al., 1985), the lack of additional reference sites at other latitudes limited the stratigraphic resolution and made it impossible to evaluate the synchrony of microfossil datums through the middle and high latitudes of the North Atlantic Ocean. During Leg 94, 22 holes at six sites were cored between 37° and 53°N in the North Atlantic Ocean (Fig. 1, Table 1). The nearly continuous recovery, the excellent paleomagnetic record to about 3.5 Ma, and the approximate south-to-north direction of the transect made it possible to evaluate the synchrony of Pliocene and Holocene biostratigraphic events between middle and high latitudes. Unfortunately, the synchrony of Miocene datums generally could not be addressed: either there were no reference holes (lower and middle Miocene sediments were recovered from only two holes), coring was discontinuous, paleomagnetic records were poor (the cores were disturbed, recovery was incomplete, or intensity was low), or microfossil preservation was poor. CHRONOSTRATIGRAPHY Magnetic polarity and microfossil zonations (calcareous nannofossils, foraminifers, diatoms, radiolarians, and dinoflagellates) used for Leg 94 sites are compared in Figure 2 with the chronostratigraphy of Berggren et al. (in press). The calcareous nannofossil zonation of Martini (1971) was used during Leg 94 (Takayama and 70°N Ruddiman, W. E , Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94: Washington (U.S. Govt. Printing Office). 2 Present addresses: (Baldauf, Clement) Ocean Drilling Program, Texas A&M University, College Station, Texas, 77843; (Thomas) Deep Sea Drilling Project, Scripps Institution of Oceanography, La Jolla, CA 92093; (Takayama) Department of Geology, College of Liberal Arts, Kanazawa University, Kanazawa, 920 Japan; (Weaver) Institute of Oceanographic Science, Wormley, Godalming, United Kingdom; (Backman) Department of Geology, Stockholm University, Stockholm, Sweden; (Jenkins) Department of Earth Sciences, Open University, Milton Keynes, England; (Mudie) Geological Survey of Canada, Atlantic Geoscience Centre, P.O. Box 1006, Dartmouth, Nova Scotia B2Y 4A2, Canada; (Westberg-Smith, present address) c/o A-020, Scripps Institution of Oceanography, La Jolla, CA 92093. 60
38 citations
TL;DR: Tephra layers in two Antarctic ice cores were correlated on the basis of their chemical compositions and estimated ages as discussed by the authors, and it was shown that the tephra was produced about 3,200 years ago from a major explosive eruption of Candlemas Island in the South Sandwich Islands.
Abstract: Tephra layers in two Antarctic ice cores are correlated on the basis of their chemical compositions and estimated ages. We believe the tephra was produced about 3,200 years ago from a major explosive eruption of Candlemas Island in the South Sandwich Islands. This is the first time that tephra layers have been correlated between two widely separated ice cores and demonstrates that tephra layers may serve as stratigraphic markers for correlating, and perhaps in some cases, dating Antarctic ice cores.
37 citations
01 Jan 1987
TL;DR: Doherty et al. as discussed by the authors studied the deep-sea benthic foraminifers from the North Atlantic Ocean and found that the faunal associations changed from glacial to interglacial intervals during the last 150,000 yr.
Abstract: Deep-sea benthic foraminiferal faunas were studied from Sites 608 (depth 3534 m, 42°50'N, 23°O5'W) and 610 (depth 2427 m, 53°13'N, 18°53'W). The sampling interval corresponded to 0.1 to 0.5 m.y. at Site 608 and in the sections of Site 610 from which core recovery was continuous. First and last appearances of benthic foraminiferal taxa are generally not coeval at the two sites, although the faunal patterns are similar and many species occur at both sites. Major periods of changes in the benthic faunas, as indicated by the numbers of first and last appearances and changes in relative abundances, occurred in the early Miocene (19.217 Ma), the middle Miocene (15.5-13.5 Ma), the late Miocene (7-5.5 Ma), and the Pliocene-Pleistocene (3.5-0.7 Ma). A period of minor changes in the middle to late Miocene (10-9 Ma) was recognized at Site 608 only. These periods of faunal changes can be correlated with periods of paleoceanographic changes: there was a period of sluggish circulation in the northeastern North Atlantic from 19.2 to 17 Ma, and the deep waters of the oceans probably cooled between 15.5 and 13.5 Ma, as indicated by an increase in δ 1 8 θ values in benthic foraminiferal tests. The period between 10 and 9 Ma was probably characterized by relatively vigorous bottom-water circulation in the northeastern Atlantic, as indicated by the presence of a widespread reflector. The faunal change at 7 to 5.5 Ma corresponds in time with a worldwide change in 6*C values, and with the Messinian closing of the Mediterranean. The last and largest faunal changes correspond in time with the onset and intensification of Northern Hemisphere glaciation. INTRODUCTION Deep-sea benthic foraminifers from the North Atlantic Ocean have been studied more intensely than similar faunas from other oceans. Pioneering studies of Quaternary faunas were made by Brady (1884—material from the HMS Challenger), Cushman (1923), and Phleger, Parker, and Peirson (1953). Streeter (1973) used the data from the latter authors in a quantitative study and concluded that deep-sea faunal associations of benthic foraminifers are related to the water masses below which they are living, and that the faunal associations changed from glacial to interglacial intervals during the last 150,000 yr. Streeter and Shackleton (1979) compared the faunal composition during the last 150,000 yr. with the oxygen isotopic ratios from tests of Uvigerina species. Schnitker (1974; 1980) recognized a similar correlation between water masses and benthic faunal composition in the western North Atlantic, and concluded that Epistominella ex/gMα-dominated faunas are presently typical of the deepocean basins north of 35 °N. South of 35 °N, faunas have abundant Nuttallides umbonifera; faunas typified by Uvigerina and Hoeglundina species occur at somewhat shallower depths along the continental margins and the Mid-Atlantic Ridge. During interglacial periods the faunas resembled the modern ones, but during glacial periods the Uvigerina-Hoeglundina faunas migrated basinward and became dominant in the deep basins north of about 35°N, while a mixed Uvigerina peregrina-Epistominella exigua fauna lived more to the south, and a 1 Ruddiman, W. F., Kidd, R. B., Thomas, E., et al., Init. Repts. DSDP, 94: Washington (U.S. Govt. Printing Office). 2 Present address: Lamont-Doherty Geological Observatory, Palisades, N.Y. 10964. mixed Epistominella exigua-Nuttallides umbonifera fauna dominated south of 23 °N. In high-resolution studies of benthic foraminiferal faunas of the Rockall Plateau area, Schnitker (1984) concluded that orbital parameters induced fluctuations in the faunal composition of benthic foraminifers in the late Miocene and during the glacial Pliocene and Pleistocene, The same author (Schnitker, 1982) also described even higher-frequency oscillations in the faunal composition of Quaternary benthic foraminifers (650-2500 yr.). Older faunas (Paleogene-Recent) from material recovered on DSDP Leg 12 at sites in the North Atlantic and Bay of Biscay were discussed by Berggren (1972). He concluded that there was a major change in deep-sea benthic foraminiferal faunal composition during the middle Miocene, but he did not present data on the exact timing or the sequence of first and last appearances of taxa. Schnitker (1979) studied Paleogene to Recent faunas from the Bay of Biscay (DSDP Leg 48), and concluded that there were major faunal changes at Paleocene/Eocene boundary, at the Eocene/Oligocene boundary, at the Oligocene/Miocene boundary, at the end of the middle Miocene, and in the Pliocene. According to Miller (1983), however, Paleogene faunal changes in the Bay of Biscay occurred over the period from early middle Eocene to earliest Oligocene, and did not coincide with the Eocene/Oligocene boundary. Murray (1984; and this volume) described and quantitatively analyzed benthic deep-sea faunas from the North Atlantic, including the Rockall Plateau area (DSDP Leg 81). He compared Recent faunas with their older counterparts, and deduced bottom-water movements from the changes in faunal composition with time. Berggren and Schnitker (1983) reviewed the Cretaceous to Recent environmental history of the northeastern North Atlantic,
30 citations