Unusual Oligocene Braarudosphaera-rich layers of the South Atlantic and their palaeoceanograpghic implications.

The origin of the Oligocene Braarudosphaera layers over large ocean basins has remained a mystery since the question was raised a lmost three decades ago by the Deep Sea Drilling Project. A micropalaeontological study, coupled with oxygen and carbon isotope data, from Sites 363 and 522 in the S Atlantic Ocean have shed some new light on this question. Our Braarudosphaera abundance data correlate with oxygen isotope curves, suggesting that Braarudosphaera preferred cold waters. Variations in the relative abundance of sphenoliths (warm-water taxa), and longer Coccolithus pelagicus placoliths, also indicate colder water conditions during times of elevated Braamdosphaera deposition. Upwelling and mid-ocean overturn of cold, nutrient-rich, low-salinity water are suggested to be the most likely palaeoceanographic mechanisms for the deposition of Braarudosphaera layers. Introduction The genus Braarudosphaera includes calcareous nannoplankton species with a coccosphere of pentaliths, each comprising with five elements of basic trapezoidal shape. Braarudosphaera has a long stratigraphic range, from the Early Cretaceous to Recent. It is represented in the modem oceans by the type species, Braarudosphaera bigelowii, which is covered by twelve flat pentaliths. Modern occurrences of braarudosphaerids (dominated by B. bigelowii) are mostly associated with nearshore, low-salinity marine waters (Bukcy, 1974; Okad.a, 1992 ). High concentrations of modern representatives have been found in areas of relatively low salinities, such as the GulfofMaine (31.8-32.7%o: Gran&Braarud, 1935) and Gulf ofPanama (26.8-32.5o/oo) where it reached 1500 cells/litre of water (Smayd.a, 1966). Bukry (1974) noted that their affinity for low-salinity waters is well illustrated by tl1eir abundance in the Black Sea (salinity 17 -18%o) , and tl1eir absence in the Red Sea (salinity 37-41%o). They have sometimes been reported from open-ocean sites with normal salinities, but only at very low abundances, e.g. Okada & Honjo, 1973. Some braarudosphaerids have also been reported from the Persian Gulf (Martini, 1967) where surface salinities are high (34-40%o). During Deep Sea Drilling Project (DSDP) Leg 3, in the S Atlantic Ocean, strong seismic reflectors were found in the Oligocene and the horizons were recovered in cores. Detailed examination of the cores revealed chalk intervals almost exclusively composed of Braarudosphaera (Maxwell et al., 1970). These layers immediately aroused great interest, in geophysics as unusual extensive acoustical reflectors, and in sedimentology because of tlleir significance in the study of sediment lithification and diagenesis (Wise & Hsii, 1971; Wise & Kelts, 1972). They also posed an intriguing palaeoceanographic question as to why this typically nearshore nannoplankton genus became so abundant in the open ocean. Origin of Braarudosphaera layers Salinity-reduction hypotheses Several hypotl1eses have been proposed to explain these 17 unusual Braarudosphaera horizons. Most of these hypotheses address the problem from the standpoint of low-salinity conditions in the S Atlantic during the Oligocene, a view based on modern occurrences of this genus. Mechanisms suggested to reduce surface-water salinities over large areas include: an increase in the regional rainfall (Bukry, 1974); periodic fluxes ofOligocene glacial melt-waters to reduce near-surface salinity (Hsii in Bukry, 1978); high influx of fresh water from the Orange River into the Cape Basin, coupled with upwelling of nutrient -rich, intermediate or deep, low-salinity water (Bolli et al., 1978); and initiation of the Benguela Current was postulated as the trigger for this upwelling process in the area of Leg 40 in tl1e mid-latitude S Atlantic by Siesser (1978) and Melguen (1978).