Description: Die radiogene Isotopenzusammensetzung bestimmter gelöster Metalle im Meerwasser (Neodym, Blei, Hafnium) kann als Tracer in der Ozeanographie und Paläo- Ozeanographie genutzt werden. Diese Metalle haben Verweilzeiten im Ozean, die vergleichbar sind mit der Zeit, die das Wasser der Weltozeane braucht, um einmal komplett ausgetauscht zu werden (ca. 1500 Jahre). Die typischen Isotopensignaturen bestimmter Wassermassen bleiben daher über lange Distanzen erhalten (Tausende von Kilometern). Änderungen der isotopischen Zusammensetzung dieser Metalle im Meerwasser geschehen auf Zeitskalen von wenigen 10er Jahren bis hin zu Millionen von Jahren als Funktion von Veränderungen der Zlrkulation und der Mischung von Wassermassen, der Eintragsquellen von erodiertem kontinentalem Material, von VerwitteIungsprozessen oder geographischen VerändeIungen wie der Öffnung oder Schliessung von Verbindungen zwischen Meeresbecken oder Gebirgsbildungen. Die Effekte dieser Prozesse überlagern sich und müssen voneinander getrennt werden, um aus den Zeilserien der radiogenen Isotopensignale verlässliche Angaben über Paläozirkulation oder Verwitterungseintrag in der Vergangenheit abzuleiten. Tiefsee-Eisen-/Mangankrusten, die chemische MeeIwasser-Präzipilate daIstellen und die bei ihrem sehr langsamen Wachstum (wenige mm pro Million Jahre) die radiogenen Isotopensignaturen des Tiefenwassers einbauen, stellen ideale Archive dar, um die Evolution der radiogenen Isotopensignaturen im Ozean der letzten 60 Millionen Jahre zu rekonstIuieren.

Description: Be and Nd isotope compositions and metal concentrations (Mn, Fe, Co, Ni, and Cu) of surface and subsurface ferromanganese hardground crusts from Ocean Drilling Program Leg 194 Marion Plateau Sites 1194 and 1196 provide new insights into the crusts' genesis, growth rates, and ages. Metal compositions indicate that the hardgrounds, which have grown on erosional surfaces in water depths of 〈400 m because of strong bottom currents, are not pure hydrogenetic precipitates. Nevertheless, the ratios between cosmogenic 10Be and stable 9Be in hardgrounds from the present-day seafloor at Site 1196 between 1 x 10–7 and 1.5 x 10–7 are within the range of values expected for Pacific seawater, which shows that the hardgrounds recorded the isotope composition of ambient seawater. This is also confirmed by their Nd isotope composition (Nd between –3 and 0). The 10Be/9Be ratios in the up to 30-mm-thick and partly laminated hardgrounds do not show a decrease with depth, which suggests high growth rates on the present-day seafloor. The subsurface crust at Site 1194 (117 m below the seafloor) grew during a sedimentation hiatus, when bottom currents in the late Miocene prevented sediment accumulation on the carbonate platform during a sea level lowstand. The age of 8.65 ± 0.50 Ma for this crust obtained from 10Be-based dating agrees well with the combined seismostratigraphic and biostratigraphic evidence, which suggests an age for the hiatus between 7.7 and 11.8 Ma.

Description: Ocean and atmosphere circulation and continental weathering regimes have undergone great changes over thousands of years as well as tens of millions of years. During the glacial stages of the Pleistocene, ocean circulation was generally more sluggish and deep water circulation in the Atlantic had a shallower flow. At the same time, weathering on the continents was enhanced by glacial erosion, particularly in high northern latitudes, which increased the input of erosional detritus into the ocean. In addition, atmospheric pressure gradients were larger, leading to higher wind speeds and increased supply of aeolian dust to the ocean. Prior to the onset of Northern Hemisphere glaciation and pronounced glacial/interglacial cyclicity at ∼3 m.ya., global climate was warmer than at present. There is also evidence for a more vigorous thermohaline circulation during the early Pliocene.

Description: Marine records of the radiogenic isotope composition of the elements neodymium (Nd), lead (Pb), hafnium (Hf), strontium (Sr), and osmium (Os) allow the reconstruction of past ontinental weathering inputs on different time scales as a function of their respective oceanic residence times. Sr and Os have oceanic residence times significantly longer than the global mixing time of the ocean and are efficiently mixed on a global scale. Their isotope composition changes on long time scales as a function of plate tectonics and major orogenies, which allows their use as precise stratigraphic tools for the entire Phanerozoic. In contrast, Hf, Pb, and in particular Nd, have residence times on the order of or shorter than the global mixing time of the ocean, which results in distinct isotopic signatures of water masses and allows the reconstruction of past water mass mixing and weathering inputs on both long and short time scales. Here applications of these isotopes systems with a focus on the shorter residence time tracers are reviewed (without claiming to be comprehensive) and problems and potential solutions are discussed. Keywords: Radiogenic isotopes, paleo-oceanography, ocean circulation, water mass mixing, continental weathering

Description: Arctic Ocean circulation as we know it today is an exceptional situation compared with the geological past. This was shown from geochemical analyses of a unique marine sediment core recovered in the central Arctic Ocean. A major transition from oxygen-poor sediments to well oxygenated sediments 17.3 million years ago indicates that the Fram Strait, which is the only deep water connection of the Arctic Ocean with the Atlantic Ocean, already opened at this time and allowed the establishment of a well-ventilated ocean basin. Isotope geochemical results suggest that the Arctic deep circulation was strongly influenced by sea ice formation during most of the past 15 million years and was not predominantly controlled by inflowing Atlantic waters, as is the case today.

Description: Black shale sediments from the Barremian to Aptian South Atlantic document intense and widespread burial of marine organic carbon during the initial stages of seafloor spreading between Africa and South America. The enhanced sequestration of atmospheric CO2 makes these young ocean basins potential drivers of the Early Cretaceous carbon cycle and climate perturbations. The opening of marine gateways between initially restricted basins and related circulation and ventilation changes are a commonly invoked explanation for the transient formation and disappearance of these regional carbon sinks. However, large uncertainties in paleogeographic reconstructions limit the interpretation of available paleoceanographic data and prevent any robust model-based quantifications of the proposed circulation and carbon burial changes. Here, we present a new approach to assess the principal controls on the Early Cretaceous South Atlantic and Southern Ocean circulation changes under full consideration of the uncertainties in available boundary conditions. Specifically, we use a large ensemble of 36 climate model experiments to simulate the Barremian to Albian progressive opening of the Falkland Plateau and Georgia Basin gateways with different configurations of the proto-Drake Passage, the Walvis Ridge, and atmospheric CO2 concentrations. The experiments are designed to complement available geochemical data across the regions and to test circulation scenarios derived from them. All simulations show increased evaporation and intermediate water formation at subtropical latitudes that drive a meridional overturning circulation whose vertical extent is determined by the sill depth of the Falkland Plateau. Densest water masses formed in the southern Angola Basin and potentially reached the deep Cape Basin as Walvis Ridge Overflow Water. Paleogeographic uncertainties are as important as the lack of precise knowledge of atmospheric CO2 levels for the simulated temperature and salinity spread in large parts of the South Atlantic. Overall temperature uncertainties are up to 15 °C and increase significantly with water depth. The ensemble approach reveals temporal changes in the relative importance of geographic and radiative forcings for the simulated oceanographic conditions and, importantly, nonlinear interactions between them. Progressive northward opening of the highly restricted Angola Basin increased the sensitivity of local overturning and upper ocean stratification to atmospheric CO2 concentrations due to large-scale changes in the hydrological cycle, while the chosen proto-Drake Passage depth is critical for the ocean dynamics and CO2 response in the southern South Atlantic. Finally, the simulated processes are integrated into a recent carbon burial framework to document the principal control of the regional gateway evolution on the progressive shift from the prevailing saline and oxygen-depleted subtropical water masses to the dominance of ventilated high-latitude deep waters.