Showing papers by "Augusto Mangini published in 1990"

Response of manganese in the ocean to the climatic cycles in the Quaternary

Abstract: Profiles of the 230Th concentration in Mn crusts from the central Pacific Ocean measured at extremely high depth resolution reveal that the growth rates of Mn crusts are influenced by climate. Based on a “constant flux model” the sections of maximum 230Th concentration correspond to periods of slow growth during glacial stages. Fast growth occurred during interglacial stages 1, 5, and 7, probably due to a larger supply of Mn oxides from the water column. High-resolution profiles of 230Th and 10Be in sediment cores from high biological productivity areas display radioisotope maxima in the interglacial stages and minima during glacial periods, the ratio of the fluxes of 10Be/230Th being ≥4. The only exceptions are observed at 135 and 270 kyr B.P., where the ratio of the fluxes is as low as 0.1 to 0.3. We presume that this “230Th anomaly” reflects short periods of time at the end of glacials when precipitation of MnO2 occurred. This hypothesis is confirmed by peaks of Mn observed in sediment cores mainly at the transitions from glacial to interglacial stages. The standing crops of Mn in these layers suggest release of Mn2+ from the sediments during glacial stages and buildup of Mn in the water column to concentrations of up to 10 µmol/L.

High Resolution 10Be and 230Th Stratigraphy of Late Quaternary Sediments from the Fram Strait (Core 23235)

Abstract: We obtained a high resolution stratigraphy of sediment Core 23235 (Fram Strait, 78°51,55′N, 01°18,59′E, 2456 m water depth) from the measurement of the concentration of 10Be and 230Th in depth profiles. The average sedimentation rate is approximately 3±0.3 cm/kyr. On a length of 806 cm the Core 23235 covers a time span of approximately 270 kyr. The profiles of the concentration of 10Be and 230Th have maxima during interglacials and minima during glacial periods. During the interglacials the fluxes of 10Be (up to 4.35±0.17 · 106 at cm-2 yr-1 and of 230Th (up to 26.70±6.37 dpm cm-2 yr-1) greatly exceed the depositional fluxes expected from the production of of 10Be in the atmosphere and 230Th in the water column. In contrast, during glacial stages 2 and 6 the 10Be fluxes are similar to or slightly lower than production fluxes, while the 23Th fluxes are significantly lower than the predicted flux. The only exception to this trend occurred during a short period of stage 6, at about 135 ka, which has the highest 230Th flux in the core record. At this ‘230Th flux anomaly’ the ratio of the production exceeding isotope fluxes of 10Be and 230Th has a value of 0.3:1, whereas during interglacial conditions this ratio approximates 4 : 1. We presume that the ‘230Th anomaly’ reflects a very short period of time during glacial stage 6, when renewal of deep water took place (it is generally assumed that no deep water was formed in the North Atlantic during stage 6). As a consequence of sudden oxygen supply to the water column at 135 ka, 230Th was scavenged from the water column with Mn-oxides more efficiently than 10Be.