(Table 1) Ooze/chalk cycles: frequency of occurrence at DSDP Leg 74 Sites

Shallow- to deep-water environments are represented by the sediments and rocks recovered from the Walvis Ridge- Angola Basin transect. These calcareous oozes, chalks, limestones, and volcaniclastic sedimentary rocks are used to define and correlate four lithostratigraphic units. The sediments were deposited in cycles which represent recurring tectonic or Oceanographic events and may be related to climatic fluctuations and orbital perturbations. Turbidites are the most common and easily identified sedimentary cycle. They are Late Cretaceous to Paleocene in age and are repeated in intervals ranging from thousands to tens of thousands of years. They are also found interbedded between basalt layers. Turbidites are easily distinguished from the other cycles present by their sedimentary structures, mineral composition, alteration products, and physical properties (GRAPE) data. Large-scale turbidites, debris, or slump breccias are found at or just above the Cretaceous/Tertiary boundary and indicate an event of considerable energy possibly related to intense tectonic activity. Diagenetic cycles, interpreted as small-scale dissolution cycles or sequences produced by biogenic activity, occur in early Paleocene chalks. The recurrence intervals average -20,000 y. but have a wide range of values. Variations in CaCO3 content, color, gradational boundaries, and trace fossil content characterize these sediments. These cycles reflect bottom-water conditions. Ooze-chalk cycles occur in upper Oligocene to upper Paleocene sediments and represent conditions that once existed at the sediment/water interface where they obtained their diagenetic potential. These oscillations are repeated over tens of thousands of years and may have no modern analogs. Color variations in sediments at the Cretaceous/Tertiary boundary indicate local fluctuations in oxygen content within the sediments or the water column. This situation lasted for several hundred thousand years and is not repeated elsewhere in the sequence. Large dissolution cycles are recorded in the sediments at Site 527 that are of middle Miocene and early Oligocene to middle Eocene age. During this time the seafloor at this site appears to have been located at or subsided to a depth occupied by a fluctuating CCD and lysocline.

Geochemistry of organic carbon at DSDP Sites 75-530 and 75-532

CHN analyses of sediments and rocks sampled during DSDP Leg 75 in the South Atlantic have provided concentrations of organic carbon and atomic C/N ratios of organic matter from two sites. High values of organic carbon were measured in sediments deposited during Neogene and Cretaceous times at Site 530 in the Angola Basin; sediments deposited at other times contain less than 0.5% organic carbon. Development of the Benguela Current and associated upwelling-supported biological productivity is recorded in late Miocene to Holocene sediments which contain 1 to 7% organic carbon. These sediments include debris flows and turbidites composed of predominantly biogenic materials originally deposited on the Walvis Ridge and on the African continental margin. Organic-carbon-rich black shales containing up to 17% organic carbon occur in late Albian to Coniacian turbidite sequences. These Cretaceous black shale layers are commonly several centimeters thick and are separated by bioturbated fine-grained organic-carbon-poor turbidites which are usually much thicker. At Site 532 on the Walvis Ridge, biogenic sediments deposited between late Miocene and Holocene times contain 1 to 9% organic carbon. Fluctuations in the intensity of high biological productivity associated with the Benguela Current are preserved in alternating light and dark layers of sediments. C/N ratios of organic matter in sediments from both sites are typical of marine sources

Alkenones and SST of surface sediments (Appendix)

We have analysed alkenones in 149 surface sediments from the eastern South Atlantic in order to establish a sediment-based calibration of the U37K' paleotemperature index. Our study covers the major tropical to subpolar production systems and sea-surface temperatures (SST's) between 0° and 27°C. In order to define the most suitable calibration for this region, the U37K' values were correlated to seasonal, annual, and production-weighted annual mean atlas temperatures and compared to previously published culture and core-top calibrations. The best linear correlation between U37K' and SST was obtained using annual mean SST from 0 to 10 m water depth (U37K' = 0.033 T + 0.069, r**2 = 0.981). Data scattering increased significantly using temperatures of waters deeper than 20 m, suggesting that U37K' reflects mixed-layer SST and that alkenone production at thermocline depths was not high enough to significantly bias the mixed-layer signal. Regressions based on both production-weighted and on actual annual mean atlas SST were virtually identical, indicating that regional variations in the seasonality of primary production have no discernible effect on the U37K' vs. SST relationship. Comparison with published core-top calibrations from other oceanic regions revealed a high degree of accordance. We, therefore, established a global core-top calibration using U37K' data from 370 sites between 60°S and 60°N in the Atlantic, Indian, and Pacific Oceans and annual mean atlas SST (0-29°C) from 0 m water depth. The resulting relationship (U37K' = 0.033 T + 0.044, r**2 = 958) is identical within error limits to the widely used E. huxleyi calibrations of and attesting their general applicability. The observation that core-top calibrations extending over various biogeographical coccolithophorid zones are strongly linear and in better accordance than culture calibrations suggests that U37K' is less species-dependent than is indicated by culture experiments. The results also suggest that variations in growth rate of algae and nutrient availability do not significantly affect the sedimentary record of U37K' in open ocean environments.