Beate Dittrich-Hannen

Minimum 10Be exposure ages of early Pliocene for the Table Mountain plateau and the Sirius Group at Mount Fleming, Dry Valleys, Antarctica

Using accelerator mass spectrometry (AMS), we have measured 10Be and 26Al in quartz from granites and sandstones from Table Mountain and Mount Fleming, Antarctica. Our data show that the plateau surface at Table Mountain had formed by early Pliocene time at the latest. Granites fringing but within the Sirius Group at Table Mountain give a minimum exposure age of 2.6 Ma for this deposit. A sandstone clast on the Ferrar dolerite surface just outside and below, and thus postdating, the Sirius Group has a minimum age of 2.9 Ma. Two samples from the Sirius Group at Mount Fleming have 10Be concentrations that have reached secular equilibrium. This deposit is at least 4.8 m.y. old. The Sirius Group at Mount Fleming cannot have been deposited after 3.0–2.5 Ma, as implied by biostratigraphic data. Our dates contradict the hypothesis that in the Pliocene East Antarctica was deglaciated and the climate was significantly warmer and wetter. The preservation of these surfaces indicates a continuous cold desert in the dry valleys since the beginning of the Pliocene. The high 10Be concentrations we have measured cannot be reconciled with uplift of the Transantarctic Mountains at a rate of 1 km/m.y. during the past 3 m.y.

Beryllium 10, thorium 230, and protactinium 231 in Galapagos microplate sediments: Implications of hydrothermal activity and paleoproductivity changes during the last 100,000 years

Biogenic particle fluxes from highly productive surface waters, boundary scavenging, and hydrothermal activity are the main factors influencing the deposition of radionuclides in the area of the Galapagos microplate, eastern Equatorial Pacific. In order to evaluate the importance of these three processes throughout the last 100 kyr, concentrations of the radionuclides 10Be, 230Th, and 231Pa, and of Mn and Fe were measured at high resolution in sediment samples from two gravity cores KLH 068 and KLH 093. High biological productivity in the surface waters overlying the investigated area has led to 10Be and 231Pa fluxes exceeding production during at least the last 30 kyr and probably the last 100 kyr. However, during periods of high productivity at the up welling centers off Peru and extension of the equatorial high-productivity zone, a relative loss of 10Be and 231Pa may have occurred in these sediment cores because of boundary scavenging. The effects of hydrothermal activity were investigated by comparing the 230Thex concentrations to the Mn/Fe ratios and by comparing the fluxes of 230Th and 10Be which exceed production. The results suggest an enhanced hydrothermal influence during isotope stages 4 and 5 and to a lesser extent during isotope stage 1 in core KLH 093. During isotope stages 2 and 3, the hydrothermal supply of Mn was deposited elsewhere, probably because of changes in current regime or deep water oxygenation. A strong increase of the Mn/Fe ratio at the beginning of climatic stage 1 which is not accompanied by an increase of the 230Thex concentration is interpreted to be an effect of Mn remobilization and reprecipitation in the sediment.

Sediment redistribution versus paleoproductivity change: Weddell Sea margin sediment stratigraphy and biogenic particle flux of the last 250,000 years deduced from 230Thex, 10Be and biogenic barium profiles

High resolution 230Thex and 10Be and biogenic barium profiles were measured at three sediment gravity cores (length 605–850 cm) from the Weddell Sea continental margin. Applying the 230Thex dating method, average sedimentation rates of 3 cm/kyr for the two cores from the South Orkney Slope and of 2.4 cm/kyr for the core from the eastern Weddell Sea were determined and compared to δ18O and lithostratigraphic results. Strong variations in the radionuclide concentrations in the sediments resembling the glacial/interglacial pattern of the δ18O stratigraphy and the 10Be stratigraphy of high northern latitudes were used for establishing a chronostratigraphy. Biogenic Ba shows a pattern similar to the radionuclide profiles, suggesting that both records were influenced by increased paleoproductivity at the beginning of the interglacials. However, 230Thex0 fluxes (0 stands for initial) exceeding production by up to a factor of 4 suggest that sediment redistribution processes, linked to variations in bottom water current velocity, played the major role in controlling the radionuclide and biogenic barium deposition during isotope stages 5e and 1. The correction for sediment focusing makes the ‘true’ vertical paleoproductivity rates, deduced from the fluxes of proxy tracers like biogenic barium, much lower than previously estimated. Very low 230Thex0 concentrations and fluxes during isotope stage 6 were probably caused by rapid deposition of older, resedimented material, delivered to the Weddell Sea continental slopes by the grounded ice shelves and contemporaneous erosion of particles originating from the water column.

10Be and Ba concentrations in West African sediments trace productivity in the past

Particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles. In order to determine the influence of these processes on the depositional fluxes of10Be and barium we have performed high-resolution measurements on sediment core GeoB 1008-3 from the Congo Fan. Because the core is dominated by terrigenous matter supplied by the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic10Be and Ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10Be and Ba, which are assumed to be proportional to the flux of Al2O3. The meanBa/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous10Be concentrations of 9.1 × 109 atoms/g Al2O3 are either due to input of particles with high10Be content by the Congo River or due to scavenging of oceanic10Be by riverine particles. The maxima of biogenic10Be and Ba concentrations coincide with maxima of the paleoproductivity rates. Time series analysis of the10Be and of Ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic10Be. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba to biogenic particle fluxes to the fact that the ocean residence time of Ba is approximately 10 times longer than that of10Be.

Solar Variability Traced by Cosmogenic Isotopes

Cosmogenic Isotopes By J U R G B E E R 1 , S T E P H A N T. B A U M G A R T N E R 1 , B E A T E D I T T R I C H H A N N E N 2 , J U R G H A U E N S T E I N 3 , P E T E R K U B I K 4 , C H R I S T I A N L U K A S C Z Y K 1 , W E R N E R M E N D E 5 , R I T A S T E L L M A C H E R 5 AND M A R T I N S U T E R 2 1 Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-8600 Diibendorf, Switzerland Institute of Particle Physics, ETH-H6nggerberg, CH-8093 Zurich, Switzerland Physics Institute, University of Bern, CH-3012 Bern, Switzerland Paul Scherrer Institute, c/o Institute of Particle Physics, ETH-H6nggerberg, CH-8093 Zurich, Switzerland institute of Meteorology, Freie Universitat Berlin, Germany