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

88:3445 Pudsey, C.J., J.W. Murray and P.F. Ciesielski, 1987.

Late Pliocene to Quaternary sedimentation on the South Orkney shelf, Br. Antarct. Surv. Bull, 77:81-97.

Seven gravity cores (650-1540 m) contain diatoms ranging from Late Pliocene to Late Pleistocene/Re- cent. The sediments are diatomaceous muddy sands with abundant ice-rafted debris. Modal sand size is coarser at more exposed sites, reflecting action by bottom currents. Assemblages of planktonic and benthic Foraminifera in the core tops differ from those further downcore, indicating a more corrosive water mass today than in the past. Grain-size and composition of the terrigenous fraction are rather uniform downcore, indicating that source areas and transport processes have not varied greatly during the time represented. The sedimentary record is incomplete however, with most of the Pleistocene missing in an least one core. Dept. of Geol. Sci., Univ. of Birmingham, P.O. Box 363, Birmingham B15 2TT, UK.

88:3446 Quevauviller, Philippe and Isabel Moita, 1986.

Holocene history of a transgressive system: the continental shelf of the North Alentejo (Portugal). Bull. Inst. G~ol. Bassin Aquitaine, 40:85-95. (In French, English abstract.) Inst. de Geol. du Bassin d'Aquitaine, Univ. de Bordeaux I, 351, cours de la Liberation, 33405 Talence Cedex, France.

88:3447 Shaw, H.R., 1987. The periodic structure of the

natural record, and nonlinear dynamics. Eos, 68(50): 1651-1665.

'Natural processes are complex; interactions are not linearly related to each other; and feedback abounds....This paper addresses how nonlinear dy- namics can contribute to interpretations of the geologic record and evolutionary processes.' The co-occurrence of rare events, even once, is 'strong evidence for their coupling,' which is a 'more robust conclusion than those studies that look for simple cause-effect correlations between exact periodici- ties.' The evolution of nonlinear periodicity is illustrated by the sine circle map. USGS, Menlo Park, CA 94025, USA. (fcs)

88:3448 ten Haven, H.L., J.W. de Leeuw, J. RullkOtter and

J.S.S. Damst6, 1987. Restricted utility of the pristane/phy~ne ratio as a palaeoenvironmental indicator. Nature, Lond., 330(6149):641-643.

The acyclic C,9 and C20 isoprenoid hydrocarbons, pristane (Pr) and phytane (Ph), respectively, have been widely assumed to be diagenetic products of the phytyl side chain of chlorophyll, although alternative precursors have been suggested. The ratio of these compounds is usually interpreted as an indicator of the oxicity of the environment of deposition. Recent advances in organic geochemistry in combination with geological constraints suggest the Pr/Ph ratio cannot be used as an indicator for oxygen levels. In hypersaline deposition environ- ments the rationale behind a low Pr/Ph ratio is easier to understand, and application of the Pr/Ph ratio can be expected to be successful. Inst. of Petroleum and Organic Geochem., KFA Julich, P.O. Box 1913, D-5170 Julich, FRG.

D180. Paleontology (see also E-BIOLOGICAL OCEANOGRAPHY)

88:3449 Chen, Wenbin, 1987. Radiolaria in cores from the

Okinawa Trough. Acta oceanol, sin. (English version), 6(3):397-404. Second Inst. of Oceanogr., State Oceanic Admin., Hangzhou, People's Republic of China.

D200. Gravity, geodesy, magnetism 88:3450

Borisova, G.P., 1986. Estimation of the paleointensity of the geomagnetic field by a complex of thermal methods. Phys. solid Earth (a translation of Fiz. Zemli), 22(10):840-845.

The intensity of the ancient geomagnetic field was determined for samples of effusive rocks using a modification of the methods of Wilson and Van Ziil, which takes into account the differences in mag- netization before and after thermal treatment. The intensity values could be correlated with the 'mag- netic hardness' of the samples, which can vary with the age of magnetization. Leningrad Dept., IZMIRAN, USSR. (hbf)

88:3451 Counil, J.-L. and Jos6 Achache, 1987. Magnetization

gaps associated with tearing in the Central America subduction zone. Geophys. Res. Letts, 14(11):1115-1118.

Vector magnetic anomaly maps derived at sea-level from Magsat data show a large positive anomaly above the Middle America Trench. Relative highs