OLR (1988) 35 (6) D. Submarine Geology and Geophysics 549

correlate with glacial intervals, but the CaCO3 content appears much less variable before 490,000 BP. The frequency of eolian grain size fluctuations is higher than the 100,000-yr glacial cycle, and the mid-Brunhes event at 300,000 BP was apparently during a 50,000-yr interval of less active atmospheric circulation. Following that event, wind intensity shows less variability. The eolian record would place the start of increases in paleoclimate signals at 875,000 BP, 25,000 yr before changes are indicated by 8~eO and CaCO3a/o amplitude increases. Rea: Dept. of Geol. Sci., Univ. of Michigan, Ann Arbor, MI 48109, USA. (hbf)

88:3441 Chyba, C.F., 1987. The cometary contribution to the

oceans of primitive Earth. Nature, Lond., 330(6149):632-635.

Recent compilations of the lunar impact record, combined with the mass-scaling law for crater diameters in the large-body (gravity-scaling) regime, allow an estimate of the total mass incident on the Moon during the period of heavy bombardment. Using only those craters and basins still extant, the calculation provides a lower limit independent of cometary flux models, extrapolations of the lunar impact record, or assumptions about the cometary mass spectrum. Results imply that the Earth would have acquired an exogenous ocean of water between ~4.5 and ~3.8 Gyr ago if comets comprised ~> 10% by mass of the impacting population. Lab. for Planetary Studies, Cornell Univ., Ithaca, NY 14853, USA.

88:3442 Davies, P.J., P.A. Symonds, D.A. Feary and C.J.

Pigram, 1987. Horizontal plate motion: a key aliocyclic factor in the evolution of the Great Barrier Reef. Science, 238(4834): 169%1700.

The Great Barrier Reef complex of Australia thins dramatically and becomes younger from north to south. These variations are a consequence of the Cenozoic northward movement of the Indian- Australian Plate. The temperate climatic conditions off northeast Australia during the Early Tertiary were progressively replaced by tropical conditions. The present-day south-to-north facies distribution along the eastern Australian margin mimics the Cenozoic vertical facies sequence through the north- ern part of the Great Barrier Reef region. @1987 by AAAS. Bur. of Mineral Resour., G.P.O. Box 378, Canberra 2601, Australia.

88:3443 Grunow, A.M., D.V. Kent and I.W.D. Dalziel, 1987.

Mesozoic evolution of West Antarctica and the

Weddeli Sea Basin: new paleomagnetic con- straints. Earth planet. Sci. Letts, 86(1):16-26.

Paleomagnetic data from the Antarctic Peninsula and recent results from the Ellsworth-Whitmore Mountains block suggest that since the Middle Jurassic these West Antarctic blocks have undergone little relative movement and have rotated relative to the East Antarctic craton. New data from Lower Cretaceous rocks from the Thurston Island region suggest that the Antarctic Peninsula, Ellsworth- Whitmore Mountains and Thurston Island blocks define a single entity which we call Weddellia; some motion between these blocks is possible within the limits of the paleomagnetic data. Between Middle Jurassic and Early Cretaceous, Weddellia remained attached to West Gondwanaland while East Ant- arctica moved southward relative to Weddellia. From Early Cretaceous to Mid-Cretaceous, Wed- dellia rotated clockwise 30 ° and moved sinistrally ~2500 km relative to East Antarctica, to its present position. Early to Mid-Cretaceous is suggested as the time of the main if not initial opening of the Weddell Sea. Lamont-Doherty Geol. Observ., Palisades, NY 10964, USA.

88:3A.A. A. Oppo, D.W. and R.G. Fairbanks, 1987. Variability in

the deep and intermediate water circulation of the Atlantic Ocean during the past 25,000 years: Northern Hemisphere modulation of the Southern Ocean. Earth planet. Sci. Letts, 86(1):1-15.

Results using//'3C measured in benthic Foraminifera as a water mass tracer show the Southern Ocean was dominated by Pacific water, with little NADW contribution during the last glaciation, and docu- ment the direct physical connection between inso- lation-controlled Northern Hemisphere oscillations and changes in the chemical and physical properties of Southern Ocean water. ~3C records from the deep tropical Atlantic document the transition from domination of the Atlantic by Southern Ocean water during the last glaciation to domination by NADW during the Holocene. During the last glaciation the boundary between NADW and Southern Ocean water was north of its modern position, allowing entry of more Southern Ocean water into the eastern Atlantic through the Romanche Fracture Zone. Carbon isotope records from the Caribbean and Mediterranean seas suggest the influence of Medi- terranean Overflow Water extended to the Carib- bean during the last glaciation, and therefore was a much more important water mass in the interme- diate-depth Atlantic than it is today. Lamont- Doherty Geol. Observ., Palisades, NY 10964, USA.