The Ceará Rise in the western equatorial Atlantic is an area of low primary productivity (30 gC/m2a, Rühlemann et al., 1995). Sediment cores from this locality show variable CaCO3-content throughout the late Quaternary. Our investigation aimed to answering the question whether the observed variations of CaCO3 reflect variable primary productivity
or variable dissolution influenced by periodic admixture of AABW (Antarctic Bottom Water). Depth profiles of 230Th-excess (230Th from the water column) and CaCO3 were measured on two cores GeoB 1523-1 (3° 49.9' N, 41° 37.3' W; water depth: 3291m) and EW 9209 1JPC (5° 54.4' N, 44°11.7' W; water depth: 4056m) from the Ceará Rise. The CaCO3-fluxes corrected for sediment redistribution by normalization to 230Thex are very similar in both cores. Since the lysocline has always been deeper than 3,500m during the last 200 ka (1ka 1000 years) (Curry & Lohmann, 1990), we assume that the CaCO3-flux supplied to the deeper locality EW9209 1JPC is comparable to the one into the shallower sediment of GeoB 1523-1. Supposing that the degree of CaCO3 dissolution increases linearly with depth below the lysocline level and that the CCD has remained constant at 4,600m depth (Berger, 1977), we evaluated the variations of the depth of the lysocline for the last 200 ka. As shown in Fig. 1, the lysocline level shallowed during d18O-stage 4 and during d18O substages 5.2 and 6.5 (Martinson et al., 1986). The observed periodicity of the lysocline variation at the Ceará Rise exhibits a correlation with the variability of the Composite Preservation Index of the CaCO3 in the Southern Ocean (Howard and Prell, 1994).
Whether the periodicity in the record of CaCO3 has
been a consequence of changing admixture of AABW or of variable biological productivity is still controversial and probably it is reasonable to invoke both mechanisms for explaining the fluctuations of the carbonate content. For example, the level of biological productivity, as deduced from the BaBio-, 231Pa- or 10Be- fluxes in core EW9209 1JPC remains constant in core sections corresponding to d18O-stage 4 where we observe the strongest dissolution signal, suggesting enhanced supply of AABW. However, the lysocline remained at the same level during the minima of carbonate in d18O stage 6.3 (150 ka B.P.) with a reduced level of productivity, and in d18O stage 2 with a higher degree of productivity, as suggested by the Ba-deposition. These contradictory results still require an explanation.
Our evaluation of the alkalinity during the Holocene yields values similar to the ones measured during GEOSECS (1973), whereas during glacial stages alkalinity attains maximum values up to 65 µequiv./L higher than today.
Berger, W. H., Nature 269, 301-304 (1977).
Curry, W.B. & Lohmann, G.P, Paleoceanography 5, 487-505 (1990).
Howard, W. R. and Prell, W. L., Paleoceanography 9 453-482 (1994).
Martinson, D.G. et al. Quaternary Research 27, 1-29 (1987).
Rühlemann, C. et al., Mar. Geol. submitted (1995).
Fig. 1: Depth of the Lysocline versus age in ka B.P.