McKenzie, J.A., Davies, PJ., Palmer-Julson, A., et al., 1993Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 13315. SEDIMENTARY RHYTHMS AND CLIMATIC FORCING OF PLEISTOCENE-HOLOCENEMIXED CARBONATE/SILICICLASTIC SEDIMENTS OFF THE GREAT BARRIER REEF1Craig R. Glenn,2 Dick Kroon,3 and Wuchang Wei4ABSTRACTSediments recovered from the upper-slope and outer-shelf Great Barrier Reef transect (Ocean Drilling Program Leg 133, Sites819 through 821) contain a high-resolution, 1.5-m.y. record of mixed siliciclastic and carbonate sedimentation. More than 30multimeter-scale couplets of fining- and coarsening-upward sediments having varying proportions of carbonate and authigenicglauconite are present. These couplets stack into cyclic bundles observed in carbonate and magnetic susceptibility data that, in turn,vary systematically with respect to systems tract development deduced from sequence-stratigraphic analysis. At Site 821, thesebundles and their variations clearly track relative changes in sea level for the past ~ 1.48 m.y. The data presented suggest that thesevarious levels of cyclicity are expressed as the result of high sedimentation rates along the margin and that the dominant control ontheir development has been climate variations that modulated variations in sea level and terrigenous influx. The average durationof many of these cycles is in the Milankovitch waveband and is related to both fourth- (102-443 ka) and fifth-order (34-88 ka)cycles of sea-level change. These cycles are examined with respect to their lithology (color, percentage of total foraminifers, bioclasts,siliciclastics, authigenic glauconite, nannofossils, and tunicates), sequence stratigraphy, downhole log characteristics, magneticsusceptibility, and percentage of carbonate. Time series and Fourier transform results from high-resolution magnetic susceptibilityand sonic velocity data suggest strong orbital forcing of the sedimentary cycles, although with differing intensities for differentdepositional packages and time frames. Magnetic susceptibility/carbonate cycles are interpreted as pulses of deposition offine-grained siliciclastics during times of maximum flooding (along downlapped surfaces) and subsequent highstand progradation.These pulses dilute carbonate and contain fine-grained magnetically susceptible detrital minerals.INTRODUCTIONThroughout the past 2 m.y. (cf. Hays et al., 1976), and probablyduring much of the Phanerozoic record at least (e.g., Berger et al.,1984; Einsele et al., 1991; Fischer and Bottjer, 1992), the majormechanism for changing global climate on time scales between10,000 and 500,000 yr appears to have been changes in the distribu-tion of solar energy by latitude and by season, as dictated by threemaster orbital variables. These are 41-k.y. obliquity cycles of changesin the inclination (tilt) of the EartiYs rotational axis, 100- and 413-k.y.eccentricity cycles of the changing shape of the Earth's orbit aroundthe sun (from circular to more elliptical), and 19- and 23-k.y. preces-sion cycles. The last-mentioned are related to changing distancebetween the Earth and the sun at any given season and are modulatedby the waxing and waning of the cycles of eccentricity. The effects ofall orders of orbital forcing now have been well documented in thedeep-marine record (cf. Imbrie et al., 1984, 1989) and in terrestrialrecords as well (cf. Glenn and Kelts, 1991, for summary).This study analyzes the nature of cyclic marine sedimentationrecovered from the Great Barrier Reef (GBR) transect of OceanDrilling Program (ODP) Leg 133 (Sites 819 through 821). For ourpurposes, the cycles and rhythms discussed here have been groupedinto packages representing particular orders of duration (Table 1). Ourapproach is similar to that of Roof et al. (1991) for the Gulf of Mexico(Site 625) in that we attempt to analyze data collected not from thedeep sea but, rather, from a relatively shallow-water, mixed carbon-ate/siliciclastic depositional setting lying adjacent to a passive conti-nental margin. Our data set differs significantly from Roof et al.'s andother data sets, however, in that we are analyzing sedimentary cyclic-McKenzie, J.A., Davies, PJ., Palmer-Julson, A., et al., 1993. Proc. ODP, Sci. Results,133: College Station, TX (Ocean Drilling Program).2 Department of Geology and Geophysics, School of Ocean and Earth Science andTechnology, University of Hawaii, Honolulu, HI 96822, U.S.A.3 Department of Geology and Geophysics, University of Edinburgh, Edinburgh, EH93JW, United Kingdom.4 Scripps Institution of Oceanography, La Jolla, CA, 92093, U.S.A.ity from a section that was deposited at very high rates (>IOOO m/m.y.)and contains a complicated, mixed assemblage of both open-marineand shallow-water-dwelling organisms, sand- to clay-sized siliciclas-tic materials, fining- and coarsening-upward cycles, and authigenicmineral phases.Here, we document and interpret the cyclicity present in seismic,sedimentologic, geochemical, magnetic, and downhole-logging datain terms of their causal mechanisms. These cycles are examined withrespect to their lithology (color, percentage of total foraminifers,bioclasts, siliciclastics, authigenic glauconite, nannofossils, and tuni-cates), sequence stratigraphy, downhole log characteristics, magneticsusceptibility, and carbonate content. We first describe the generalnature of the cycles and then apply statistical approaches to thehigh-resolution portions of the data set to examine their underlyingperiodicities. Variations of climate at low latitudes, such as off thenorthern GBR, are thought to be dominated by eccentricity andprecessional cycles. This is due to both the diminished effect ofobliquity changes and the increased importance of monsoons at lowlatitudes. Through regulation of the heating intensity of continentalmasses, orbital precession influences rates of precipitation on 19- and23-k.y. periods by altering the intensity of low-pressure cells thatdrive monsoonal winds (Rossignol-Strict, 1983; Pokras and Mix,1985; Prell and Van Campo, 1986; Prell and Kutzbach, 1987). Thisprecessional control on low-latitude precipitation is modulated by theeccentricity cycle. As outlined below, however, our data suggest thatevidence for eccentricity, precession, and obliquity cycles is presentin the GBR cores and that forcing by all of these orbital rhythmsimpacts sedimentation primarily by influencing rates and magnitudeof sea-level change and, secondarily, by modulating