English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Tools annual

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Presents the access of premium content as premium feature

Premium Content

Global: Zendy Plus annual

Global: Zendy Free Plan

Ocean Drilling Program (ODP) Sites 865 and 866, at Allison and Resolution guyots, respectively, were logged during Leg 143. Shipboard analysis of recovered core materials indicated rhythmic repetition of shallowing-upwards facies, however, recovery at both sites averaged less than 16% (l%-2% in the shallow-water limestones). The original depth of the recovered core material was not known with certainty, which precluded visual quantification of Milankovitch cycles. The geophysical logs, which provided continuous records of lithologic changes, were analyzed in an attempt to detect Milankovitch periodicities. Age controls were not sufficient to provide time constraints for the logged interval from 102.5 to 867.0 mbsf of Site 865; however, the entire depth interval is of late Albian age. Logs exhibit strong cyclicity in the depth interval from 250 to 490 mbsf that is mainly controlled by variations in porosity and, to a lesser extent, clay content. Sedimentation rate increases slightly downhole. Because of the high, although variable, accumulation rate (about two times the Albian sedimentation rate at Resolution Guyot), the spectral analysis revealed high-amplitude frequencies corresponding to eccentricity and obliquity. The cycle frequency corresponding to a Milankovitch period of 413 k.y. dominates all spectra; the frequency corresponding to 123-95 k.y. has high-amplitude for the interval from 330 to 490 mbsf. Vertical resolution was not high enough to resolve frequencies corresponding to precession. At Site 866, the logged interval, from 78.0 to 1679.4 mbsf, corresponds to a time interval of about 33 m.y. Two depth intervals showed strong cyclicity in the geophysical logs: 430-670 and 935-1135 mbsf. Sedimentation rate is highly variable and increases downhole from about 30 m/m.y. in the middle-to-late Albian section to about 80 m/m.y. in the Barremian section. Vertical resolution was not sufficient to resolve frequencies corresponding to precession; the 41-k.y. obliquity peaks are resolvable only in the lower portion of Hole 866A. The 413-k.y. eccentricity cycle dominates all spectra. The 123to 95-k.y. cycles, which are probably the alternations between wackestone and more coarse-grained packstones, likely represent the combined influence of obliquity and eccentricity orbital perturbations on sea level. MILANKOVITCH CYCLES IN SEDIMENTS Since 1976 (Hays et al., 1976), analyses of time series of climatesensitive indicators have provided geological evidence that a major portion of climate variation is driven by changes in insolation caused by perturbations in Earth's orbital parameters. Such quasi-periodic climate variations were predicted originally by Milankovitch (1941). Exactly how changes in the insolation patterns are translated into climatic changes remains the subject of extensive research. Nevertheless, the periods of orbital eccentricity (413 and 123-95 k.y.), obliquity (41 k.y.), and precession (23-19 k.y.) have been observed in analyses of a variety of sediment types and in many stratigraphic intervals. Percentage of calcium carbonate content and oxygen isotope ratios (e.g., Briskin and Harrell, 1980) are parameters frequently used for detecting Milankovitch frequencies in Quaternary pelagic sediments. Oxygen isotope ratios for benthonic marine foraminifers, as well, reflect global ice volume; numerous published studies clearly demonstrate that Quaternary glacial regimes are related to orbital periodicities (Imbrie et al, 1984). Because the magnitude of the change in insolation caused by the precession-obliquity-eccentricity cycles is so small (Berger, 1977), models that seek to explain climate cycles in terms of variations in insolation include climatic-oceanic feedback mechanisms that amplify the magnitude of the resultant climate changes (Berger et al., 1984; Sundquist and Broecker, 1985; Barron et al., 1985; Prell and Kutzbach, 1987; Sarnthein et al., 1988). In particular, most theoretical studies to date have placed great emphasis on ice-sheet amplification of the relatively weak Milankovitch signals (Berger et al., 1984; Start and Prell, 1984; Ruddiman and Mclntyre, 1984). Confirmation of the effects of climatic forcing in sediments of pre-Quaternary age extends over a range of sedimentary systems, 1 Sager, W.W., Winterer, E.L., Firth, J.V., and Sinton, J.M. (Eds.), 1995. Proc. ODP, Sci. Results, 143: College Station, TX (Ocean Drilling Program). 2 Department of Geology and Geophysics, University of Hawaii, 2525 Correa Rd., Honolulu, HI 96822, U.S.A. both siliciclastic and carbonate, and depositional environments. For example, mean periodicities of 20,40, and 100 k.y. have been derived for carbonate-marl deposition in the Cretaceous and lower Tertiary of the Apennines (e.g., Arthur and Fischer, 1977), for pelagic sediments drilled in the open ocean (e.g., Dean et al., 1977; Arthur, 1979; McCave, 1979), for Triassic shallow marine carbonates in the northern Alps (e.g., Schwarzacher, 1964; Fischer, 1964), and for Eocene (Bradley, 1929) and Triassic lacustrine sediments (e.g., van Houten, 1964; Olsen et al., 1978) in the U.S. Parameters most useful in the study of these sediment types include variations in porosity or grain size, the relative abundance and mineralogy of clays, and variations in one or more of the biogenous components. Previous studies of geologic time series from Cretaceous deep-ocean sediments (e.g., McCave, 1979; Arthur, et al., 1984; Cotillon and Rio, 1984; de Graciansky andGillot, 1985; Herbert and Fischer, 1986) revealed the same, or similar, high power in the eccentricity and precession peaks, regardless of the ocean of origin, indicating that this is truly a global signature that persists through geologic time. Although the volume of polar ice caps during the Cretaceous is not known accurately (Kemper, 1987), Hardie et al. (1986) suggest that the growth patterns of ice caps with even greatly reduced volume would cause eustatic fluctuations on the order of several meters, sufficient to cause emergence cycles at carbonate platforms. The persistence of Milankovitch periodicities through the Cretaceous, a time of warm seas and possibly minimal ice cover (Barron and Washington, 1985), implies that, whereas the presence of ice sheets may be an important factor in the modulation of global climate, their presence is not required for such modulation to occur (Fisher et al., 1985; Herbert et al., 1986). Jarrard and Arthur (1989) first explored the feasibility of using spectral analysis of downhole geophysical logs to detect cyclic changes in mineralogy and porosity in Pleistocene sediments from Sites 645 in the Labrador Sea and 646 in Baffin Bay. Their analysis revealed periodicities of roughly 20, 40, and 100 k.y. in the sonic, resistivity and U/Th logs. They reasoned that fluctuations in bottomwater currents in response to Milankovitch cycles caused the ob-

Milankovitch Cycles from Fourier Analysis of Logs from Sites 865 and 866