ALBERT

Description: The first full water column hafnium isotopic compositions of Atlantic seawater have been obtained at seven locations from the Labrador Sea to the Drake Passage. Despite subpicomolar concentrations in seawater, a precision of the Hf isotopic measurements of 〈0.7 epsilon-Hf units was achieved. An overall epsilon-Hf range between -3.1 in the Labrador Sea and +4.4 in Antarctic bottom water was determined, the distribution of which broadly reflects continental weathering inputs. Within particular water column profiles, significant differences of up to 4 epsilon-Hf units occur. Combined with Nd isotope data of the same samples, it is evident that the Hf isotopic composition of seawater is too radiogenic for a given Nd isotopic composition and that the largest difference between expected and measured Hf isotopic compositions in seawater occurs near the oldest continental crust in the Labrador Sea. This corroborates the previous proposition, which was mainly based on ferromanganese crust data, that the Hf isotopic composition of seawater is controlled by incongruent weathering of continental crust and possibly, to some extent, by hydrothermal contributions. Hafnium concentrations in the ocean do not increase along the deep ocean conveyer indicating an oceanic residence time of only a few hundred years, which is significantly shorter than previously assumed. The Hf isotopic composition of past seawater can therefore serve as a proxy for short distance, basin scale mixing processes and the regime and intensity of nearby continental weathering processes.

Description: We present dissolved neodymium and hafnium concentrations and radiogenic isotope compositions of surface and deep-water masses from the Bay of Biscay. Neodymium isotope signatures in surface waters of the Bay of Biscay are dominated by local weathering inputs from the surrounding continental margin. Subsurface Eastern North Atlantic Central Water (ENACW) shows a distinct Nd isotope signature (εNd≅‑12.0) at the southwestern-most station and is significantly diluted by mixing with more radiogenic waters on its way north along the European margin. Furthermore, the Nd isotope data clearly show a declining fraction of Mediterranean Outflow Water (MOW) at intermediate depths on its way north indicating that only 40% to 60% of MOW still present in the mixture at the Galician margin arrive at the stations further north in the Bay of Biscay. A potentially seasonal variability of the flow path of MOW is identified when comparing the results of the Nd isotope compositions and salinity data of this study with those of earlier studies from the area. In agreement with Nd isotope and concentration analyses Hf isotope composition of MOW is affected by large-scale inputs of terrigenous material into the Mediterranean as can be deduced from elevated Hf concentrations at the Galician margin. Hf isotope signatures of all water masses of the Bay of Biscay, moreover, are overprinted by local weathering inputs and do not reflect water mass mixing. However, combined dissolved Nd and Hf isotopes serve as a useful indicator of local weathering influences on signatures expected from long distance admixture.

Description: 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.

Description: Today the western tropical Atlantic is the most important passage for cross-equatorial transfer of heat in the form of warm surface water flowing from the South into the North Atlantic. Circulation changes north of South America may thus have influenced the global thermohaline circulation system and high northern latitude climate. Here we reconstruct late Quaternary variations of western equatorial Atlantic surface circulation and Amazon lowland climate obtained from a multiproxy sediment record from Ceará Rise. Variations in the illite/smectite ratio suggest drier climatic conditions in the Amazon Basin during glacials relative to interglacials. The 230Thex-normalized fluxes and the 13C/12C record of organic carbon indicate that sea level fluctuations, shelf topography, and changes of the surface circulation pattern controlled variations and amplitude of terrigenous sediment supply to the Ceará Rise. We attribute variations in thermocline depth, reconstructed from vertical planktic foraminiferal oxygen isotope gradients and abundances of the phytoplankton species Florisphaera profunda, to changes in southeast trade wind intensity. Strong trade winds during ice volume maxima are associated with a deep western tropical Atlantic thermocline, strengthening of the North Brazil Current retroflection, and more vigorous eastward flow of surface waters.

Description: We present biogenic opal flux records from two deep-sea sites in the Scotia Sea (MD07-3133 and MD07-3134) at decadal-scale resolution, covering the last glacial cycle. Besides conventional and time-consuming biogenic opal measuring methods, we introduce new biogenic opal estimation methods derived from sediment colour b*, wet bulk density, Si/Ti-count ratio, and Fourier transform infrared spectroscopy (FTIRS). All methods capture the biogenic opal amplitude, however, FTIRS - a novel method for marine sediment - yields the most reliable results. 230Th normalization data show strong differences in sediment focusing with intensified sediment focusing during glacial times. At MD07-3134 230Th normalized biogenic opal fluxes vary between 0.2 and 2.5 g/cm2/kyr. Our biogenic opal flux records indicate bioproductivity changes in the Southern Ocean, strongly influenced by sea ice distribution and also summer sea surface temperature changes. South of the Antarctic Polar Front, lowest bioproductivity occurred during the Last Glacial Maximum when upwelling of mid-depth water was reduced and sea ice cover intensified. Around 17 ka, bioproductivity increased abruptly, corresponding to rising atmospheric CO2 contents and decreasing seasonal sea ice coverage.

Description: A sediment core, collected from the western part of the continental slope of the Ross Sea at 2380 m water depth, records events of the last two climatic cycles (250 kyr). A 230Thex-based chronology was obtained and boundaries of the isotope stages were set assuming that biological productivity was enhanced during periods of less ice cover. Then, 230Thex0, organic carbon, biogenic silica and biogenic Ba distributions were compared to the glacial-interglacial stage boundaries and corresponding ages of the delta18O record of Martinson et al. [Martinson et al., 1987, doi:10.1016/0033-5894(87)90046-9]. Sediment accumulation rates ranged between 1.2 cm kyr**-1 in the isotope stage 6 and 3.8 cm kyr**-1 during the Holocene. Variations in the concentrations and fluxes of organic carbon, biogenic Ba, biogenic silica and Mn gave information on palaeoclimate changes. Processes of sediment redistribution in the Ross Sea margin were enlightened from a comparison of the measured and expected fluxes of 230Thex. Calculation of the focusing-corrected accumulation rates of biogenic Ba enabled us to evaluate the export palaeoproductivity. Corrected accumulation rates of biogenic components and calculated palaeoproductivities were low, compared to the Antarctic Polar Front in the Atlantic sector, throughout the last two climatic cycles. Glacial-interglacial changes of sea ice cover and ventilation of the Ross Sea were probably major causes of variations in biogenic particle flux and distribution of redox-sensitive elements within the sediment column.