Coarse-fraction sediment (>63 µm) studies for Leg 151 Sites 908 and 909 have enabled us to reconstruct the paleoclimate since the early Miocene (18.0 Ma) of the Fram Strait, in order to better understand this gateway between the North Atlantic and the Arctic Ocean. Study of the grain-size distribution patterns (subfractions 63-125 µm, 125-250 µm, 250-500 µm, 500-1000 µm, and >IOOO µm plus the coarse-component composition) suggests possible seasonal ice-rafting starting as early as 14.0 Ma. Also shown is a clear signal of ice-rafting events between 10.8 and 8.6 Ma, 7.2 and 6.8 Ma, 6.3 and 5.5 Ma, and since 5.0 Ma. Evidence indicates that the initiation of the East Greenland Current took place at 10.8 Ma and is related to the reorganization and shutdown of deep-water connections in the Middle and Central American Seaways during the latest middle to late Miocene. During the Pliocene-Pleistocene, the polar front moved over Site 908, covering the area with sea ice and pack ice during the Pleistocene (between 1.9 and 1.2 Ma). The West Spitsbergen Current episodically reached the Fram Strait during the Pliocene-Pleistocene, suggesting that the Norwegian Sea was never completely covered by sea ice and pack ice in that era.
Well-developed Campanian to Maestrichtian pelagic cyclic sediments were recovered from Hole 762C on the Exmouth Plateau, off northwest Australia, during Ocean Drilling Program Leg 122. The cycles consist of nannofossil chalk (light beds) and clayey nannofossil chalk (dark beds). Both light and dark beds are strongly to moderately bioturbated, alternate on a decimeter scale, and exhibit gradual boundaries. Bioturbation introduces materials from a bed of one color into an underlying bed of another color, indicating that diagenesis is not responsible for the cyclicity. Differences in composition between the light and dark beds, revealed by calcium carbonate measurement and X-ray diffraction analysis, together with trace fossil evidence, indicate that the cycles in the sediments are a depositional feature. Diagenetic processes may have intensified the appearance of the cycles.Spectral analysis was applied to the upper Campanian to lower Maestrichtian cyclic sediments to examine the regularity of the cycles. Power spectra were calculated from time series using Walsh spectral analysis. The most predominant wavelengths of the color cycles are 34-41 cm and 71-84 cm. With an average sedimentation rate of 1.82 cm/k.y. in this interval, we found the time durations of the cycles to be around 41 k.y. and 21 k.y., respectively, comparable to the obliquity and precession periods of the Earth's rotation, which strongly suggests an orbital origin for the cycles.On the basis of sedimentological evidence and plate tectonic reconstruction, we propose the following mechanism for the formation of the cyclic sediments from Hole 762C. During the Late Cretaceous, when there was no large-scale continental glaciation, the cyclic variations in insolation, in response to cyclic orbital changes, controlled the alternation of two prevailing climates in the area. During the wetter, equable, and warmer climatic phases under high insolation, more clay minerals and other terrestrial materials were produced on land and supplied by higher runoff to a low bioproductivity ocean, and the dark clayey beds were deposited. During the drier and colder climatic phases under low insolation, fewer clay minerals were produced and put into the ocean, where bioproductivity was increased and the light beds were deposited.
Changes in coarse fraction, calcium carbonate, and total organic carbon (TOC) accumulation rates and wt% define four different Neogene climatic and oceanographic regimes for the deep-water Fram Strait area (Site 909). These changes are not as well developed in intermediate water depth at Site 908, possibly indicative of differences in current velocities at the two sites. Stage I, upper lower to middle Miocene (Site 909, 838-1062 meters below seafloor [mbsf]; 17.5-10.85 m.y.), sediments consist of a fining-upwards sequence of mass-wasted sediments, interbedded with laminated and bioturbated sediments. Tectonic influences are the dominant control on these sediments, which were deposited before the initiation of sustained bottom-water flow through Fram Strait. Stage II, from middle to upper Miocene (Site 909, 838-368 mbsf; 10.85-5.7 m.y.; Site 908, 185-167 mbsf; 6.2-5.7 m.y.), shows the greatest variation in bulk and coarse fraction accumulation rates. It consists of two step-wise increases in bulk accumulation rate, followed by a rapid decrease. In each sequence, after the increase begins, the number of layers with high nonbiogenic carbonate increases, and an increase in the amount and variability of the coarse fraction follows. When the accumulation rate decreases, the coarse component decreases first, followed by a decrease in nonbiogenic carbonate and then the total accumulation rate decrease follows. The changes are most likely a result of the initiation and/or major increase of glaciation and the build-up of ice sheets, accompanied by changes in bottom-water strength. The first dropstones were recovered during Stage III, upper Miocene to middle Pliocene (Site 909, 368-200 mbsf; Site 908, 167-104 mbsf; 5.7 to 2.8 m.y.), during which the coarse fraction accumulation rates are low (
INTRODUCTION Northwestern Australia from the Exmouth Plateau to the Scott Plateau forms one of the oldest oceanic margins in the world (155 Ma), with a relatively low sediment influx and a large biogenic component (Fig. 1). It is an ideal margin for comprehensive and integrated sedimentologic, biostratigraphic, paleobathymetric, and subsidence studies. Two Ocean prilling Program (ODP) legs are planned in this area: Leg 122, to drill a transect of three or four sites across the Exmouth Plateau, and Leg 123, to drill one site on the northern Exmouth Plateau and one site on the Argo Abyssal Plain (Fig. 2). The Exmouth Plateau off northwestern Australia is about 600 km long and 300-400 km wide with water depths .ranging from 600 to 4000 m (Fig. 1). ,. The plateau consists of rifted and deeply subsided continental crust i covered by a Phanerozoic sedimentary sequence about 10 km thick. It is • separated from the Northwest Shelf by the Kangaroo Syncllne, and is bound I to the north, west and south by Jurassic oceanic crust of the Argo, ; Gascoyne and Cuvier abyssal plains. The Canning and Carnarvon Basin { sediments extend over the plateau from the east and abut the Pilbara Precambrian block (Exon and Willcox, 1978, 1980; Exon et al., 1982; Exon and Williamson, 1986). The Argo Abyssal Plain is an extremely flat, about 5.7 km deep, abyssal plain located north of the Exmouth and Wombat plateaus and west of the Rowley Shoals and Scott Plateau (Fig. 1). It is underlain by the oldest (Late Jurassic M-26) oceanic crust known in the Indian Ocean, crust that is slowly being consumed by the convergence of Australia and the Sunda arc. Legs 122 and 123 Scientific Prospectus page 2 terrigenous material. This and the paleo-waterdepths (10-4000 m) make it a prime target for detailed studies of biostratigraphy, sediment facies, paleoenvironment and stratigraphic evolution owing to subsidence and sea. level fluctuations during the entire period from the Late Triassic through the Quaternary. This prospectus was compiled using the drilling proposals by von Rad, Exon, Symonds and Willcox (1984, ODP proposal 121/B); Arthur (SOHP deep stratigraphic test proposal, 1985, ODP proposal 211/B); von Rad, Exon, Williamson and Boyd (1986, ODP proposal 121/B revised); Gradstein (1986, ODP proposal 240/B); and Mutter and Larson (1987 ODP proposal 288/B); and Langmulr and Natland (ODP proposal 267/F, 1986). We also relied heavily on the pre-site survey information in the cruise reports of R/V Sonne cruise SO-8 (von Stackelberg et al., 1980) and Rig Seismic cruises 55 and 56 (Falvey and Williamson, 1?86; Exon and Williamson, 1986). BACKGROUND Tectonic and stratigraphic background The geological development of the Exmouth Plateau has been discussed
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