ALBERT

Description: During the last decades, the Chilean margin offshore Maule (34±S −36±S) had been reported as a highly locked and seismically quiet zone. The stress-accumulated state finished on the 27th of February 2010, when a megathrust earthquake (with Mw = 8.8) ruptured » 400 km of the Nazca-South America plate boundary. Unfortunately, up to now little was known about the seismic structure offshore Maule. In the frame of the third phase of the project SFB 574 “Volatiles and Fluids in Subduction Zones” of the Christan-Albrechts University of Kiel, seismic data was analyzed in order to obtain detailed images of the deep structure of the margin and of the outer rise. Here are presented constraints on the forearc and the subduction zone structure of the rupture area derived from seismic refraction and wide-angle data. The results show a wedge shaped body » 40 km wide with typical sedimentary velocities interpreted as a frontal accretionary prism (FAP). Landward of the imaged FAP, the velocity model shows an abrupt velocity-contrast suggesting a lithological change, which is interpreted as the contact between the FAP and the paleo accretionary prism (backstop). The backstop location is coincident with the seaward limit of the aftershocks, defining the updip limit of the co-seismic rupture and the seismogenic zone. Furthermore, the seaward limit of the aftershocks coincides with the location of the shelf break in the entire earthquake rupture area (33.5±S−38.5±S), which is interpreted as the location of the backstop along the margin. Published seismic profiles at the northern and southern limit of the rupture area also show the presence of a strong horizontal velocity gradient imaging the seismic backstop at a distance of » 30 km from the deformation front. The seismic wide-angle reflections from the top of the subducting oceanic crust constrain the location of the plate boundary offshore, dipping » 10±. The projection of the epicenter of the Maule earthquake onto our derived interplate boundary yielded a hypocenter around 20 km depth. This implies that the earthquake nucleated somewhere within the seismogenic zone, neither at its updip nor at its downdip limit. The second part of this thesis focuses on the dependency between the incoming plate’s bend faulting, lithospheric hydration and shallow outer rise seismic activity. To support the interpretation, are presented Vp and Vs seismic models obtained from wide angle seismic data and the derived 2D Poisson’s ratio distribution at the outer rise. The oceanic lithosphere shows a high degree of hydration, due to the water infiltration through the bending-related faults exposed to seawater. This process is presumably intensified bythe existence of a seamount in the area. It is concluded that the water infiltrates deep into the lithosphere, triggering shallow earthquakes in the outer rise and likely serpentinization in the mantle, estimated to be about 10%.

Description: For the past 50 years it has been assumed that the principal pathway for the deep limb of the Atlantic Meridional Overturning Circulation (AMOC) is the Deep Western Boundary Current (DWBC). However, recent observations of Lagrangian floats have shown that the DWBC is not necessarily a unique, dominant, or continuous pathway for these deep waters. A significant portion of the deep water export from the subpolar to the subtropical gyres follows a pathway through the interior of the Newfoundland and subtropical basins, which is constrained by the western boundary and the western flank of the Mid-Atlantic Ridge. The hypothesis that deep eddy-driven recirculation gyres are a mechanism for partitioning the deep limb of the AMOC into the DWBC and this interior pathway is investigated here. Eulerian and Lagrangian analyses of the output of ocean general circulation models at eddy-resolving, eddy-permitting, and non-eddy permitting resolutions are used to test this hypothesis. Eddy-driven recirculation gyres, simulated in the eddy-resolving and eddy-permitting models and similar to recirculations inferred from hydrographic data, are shown to shape the export pathways of deep water from the subpolar to the subtropical gyres.

Description: Reconstructing the climate of mid-Cretaceous is a special challenge for climate modelling. High atmospheric C02 concentrations and a vastly different geography seem to have caused a global temperature increase. At the same time the geologic record provides ambiguous data about the strength and spatia.l distribution of this warming. In the following, 4 experiments with the new, fully coupled "Kiel Climate Model" (KCM) are realised to gain new insights into the mid-Cretaceous climatic conditions and to to investigate their influence on the ocean circulation. Furthermore, the changed boundary conditions (C02 =1200 ppmv, geography) and their respective impacts are examined independently. The experiments show a significant global warming in near surface temperatures of approximately 9°C and of about 6°C for sea surface temperatures, leading to ice free polar caps in the annual mean. Additionally they indicate that greatly increased trace gas concentrations are needed to reproduce distinct warming for tropical and subtropical regions. Changes in geography provide mainly high latitude warming. The ocean surface circulation is dominated by wind-driven gyre circulation comparable to present clay. A predominant westward flow through the Tethys Sea and a largely decreased southern circumpolar current are simulated. Changes in model geography lead to a clear decline in upper ocean salinities and a strong increase with depth for the deeper ocean. The warm, fresh surface layer prevents a deep mixing in the water column and there is no global meridional overturning circulation observable. As a result the model shows a strengthening in vertical ocean stratification for mid-Cretaceous contrarily to prior studies.

Description: Modeling of the seepage history of DNAPLs is investigated as a new, non-invasive site investigation tool in order to elucidate the possible position of still unknown DNAPL source zone at many investigated industrial sites. Therefore, the spatio-temporal spreading behavior of the DNAPL TCE is studied with the multiphase modeling software TMVOC in small and large scale 2D multiphase scenarios with varying parameter sets concerning groundwater flow, composition of aquifers and aquitards and subsurface morphologies, as depressions and trenches. The small scale models were calibrated by laboratory experiments conducted at La Sapienza University, Rome. They exhibited that even groundwater pore velocities of vw = 0.05 m/d have a strong impact on the spreading behavior and the position of a DNAPL body. Downstream inclined percolation path ways, enhanced dissolution rates and lateral transportation in downstream direction are the most dominant impacts. Small scale layering of the subsoil with horizontal lenses of impermeable materials affects the distribution pattern only slightly at vw 〉 5 m/d, which are common flow velocities in many gravelly aquifers in Europe. Upscaling of the models to field scale problems exhibited potential transportation length in downstream direction of several hundreds of meters, assuming a moderate spill rate of ca. 3 kg/day over an area of several square meters. Investigating real subsurface morphology including real material parameters provided by the ModelPROBE reference site Chimica di Bianchi in Rho, Italy, revealed that the DNAPL TCE will be transported out of moderate depressions (slope of 2.5°) even at groundwater flow velocities of vw ≤ 1 m/d, which is in the range of documented groundwater flow velocities at the reference site. Moreover, the documented material classes, which comprise the aquitard at the site, are not in general impermeable for percolating DNAPLs. Only pure clays with a hydraulic conductivity of kf ≤ 10-9 m/s are long-term barriers for vertical DNAPL percolation. The conducted investigations deliver a reasonable explanation for the often unknown position of DNAPL source zones at former industrial sites and are, as far as it is known, the first large scale scenarios of DNAPL spreading behavior in real subsurface morphology. Based on the conducted research it can be concluded that at the reference site Chimica di Bianchi the main mass of DNAPLs was not at the assumed hot spot, which was encapsulated in the 1980s, but probably migrated considerable distances in downstream direction, passing through or following partly the topography of the aquitard. But the applicability of multiphase modeling as additional non-invasive site investigation tool is still challenging due to software restriction concerning size and resolution of the models and handling of heterogeneous permeability fields.

Description: Mid-ocean ridges and volcanic passive continental margins are prime regions to explore active and extinct hydrothermal systems. In both settings, a large number of hydrothermal vents have already been discovered by direct observations and/or geophysical surveys. The growing interest in these systems results from their relevance for different fields of marine sciences. For example, commercially interesting ore deposits form as a byproduct of hydrothermal venting at the seafloor, unique ecosystems evolve around submarine vent sites, and hydrothermal systems driven by sill intrusions into organic sediments are related to hydrocarbon maturation and even venting of greenhouse gases into the atmosphere. Numerical simulations of hydrothermal fluid flow can help to gain a quantitative understanding of the subsurface physicochemical processes that control these systems. This thesis contributes to a better understanding of hydrothermalism in oceanic and continental settings by presenting a newly developed hydrothermal flow model and two case studies of hydrothermal flow at mid-ocean ridges and volcanic passive margins. To explore the effects of bathymetric relief on hydrothermal fluid flow in submarine settings, a systematic study has been carried out using 375 simulations. These simulations show that temperature-induced pressure variations in the subsurface result in the deviation of hydrothermal plumes towards bathymetric highs in submarine settings. The plume deviation from its origin is directly related to the surface slope and depth of the heat source. A case study for the fast-spreading East Pacific Rise at 9° 30’N shows that bathymetric effects help to focus venting directly onto the ridge axis – only if bathymetry is taken into account can across axis fluid flow be reconciled with exclusive on-axis venting. A second case study for the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge shows that also here venting is likely to occur at local bathymetric highs. The effects of hydrothermal convection triggered by sill intrusions in continental settings have been explored in a case-study for the Gjallar Ridge area on the Norwegian margin. This area is affected by a swarm of sill intrusions originated from North-Atlantic continental break-up during the Paleocene-Eocene transition as well as pre-break-up faults resulting from extensional tectonics. The structures are interpreted using 3D multichannel seismic data in combination with a structural and thermal reconstruction of the margin using TECMOD software. The reconstructed temperature is used as initial condition for sediments prior to sill injection and the detailed thermal history of sediments is modeled by a 2D fluid flow simulation. The simulation results show that high-temperature venting (〉200°C) occurs less than 1000 years following sill emplacement. The faults play strong roles for transferring the fluids to far-off regions. As a result of circulating hot fluids, the maturity of sedimentary rocks is greatly enhanced, especially where the hot fluids are trapped below impermeable sills during their ascent, thereby suggesting potential zones for future hydrocarbon explorations. Furthermore, solution strategies for modeling hydrothermal fluid flow by finite element, finite volume and semi-Lagrangian methods are explained in particular in order to find out how the temperature equation is solved. Different schemes of fully-implicit, Crank-Nicolson and exponential for temperature diffusion and finite volume and semi-Lagrangian for temperature advection are evaluated. The results suggest that the most accurate method for solving temperature diffusion is Crank-Nicolson. However, other methods such as fully implicit and exponential are still valid. The mass conserving finite volume method is the most accurate method for solving temperature advection; however, limited time-stepping is its major drawback and thus semi-Lagrangian method is usually preferred. Therefore, the definition of optimum method is linked to the accuracy of interest and complexity of the media.

Description: The isotopic composition of Phanerozoic marine sediments provides important information about changes in seawater chemistry. In particular, the radiogenic strontium isotope (87Sr/86Sr) system is a powerful tool for constraining plate tectonic processes and their influence on atmospheric CO2 concentrations. However, the 87Sr/86Sr isotope ratio of seawater is not sensitive to temporal changes in the marine strontium (Sr) output flux, which is primarily controlled by the burial of calcium carbonate (CaCO3) at the ocean floor. The Sr budget of the Phanerozoic ocean, including the associated changes in the amount of CaCO3 burial, is therefore only poorly constrained. Here, we present the first stable isotope record of Sr for Phanerozoic skeletal carbonates, and by inference for Phanerozoic seawater (δ88/86Srsw), which we find to be sensitive to imbalances in the Sr input and output fluxes. This δ88/86Srsw record varies from ∼0.25‰ to ∼0.60‰ (vs. SRM987) with a mean of ∼0.37‰. The fractionation factor between modern seawater and skeletal calcite Δ88/86Srcc-sw, based on the analysis of 13 modern brachiopods (mean δ88/86Sr of 0.176±0.016‰, 2 standard deviations (s.d.)), is -0.21‰ and was found to be independent of species, water temperature, and habitat location. Overall, the Phanerozoic δ88/86Srsw record is positively correlated with the Ca isotope record (δ44/40Casw), but not with the radiogenic Sr isotope record ((87Sr/86Sr)sw). A new numerical modeling approach, which considers both δ88/86Srsw and (87Sr/86Sr)sw, yields improved estimates for Phanerozoic fluxes and concentrations for seawater Sr. The oceanic net carbonate flux of Sr (F(Sr)carb) varied between an output of -4.7x1010mol/Myr and an input of +2.3x1010mol/Myr with a mean of -1.6x1010mol/Myr. On time scales in excess of 100Myrs the F(Sr)carb is proposed to have been controlled by the relative importance of calcium carbonate precipitates during the “aragonite” and “calcite” sea episodes. On time scales less than 20Myrs the F(Sr)carb seems to be controlled by variable combinations of carbonate burial rate, shelf carbonate weathering and recrystallization, ocean acidification, and ocean anoxia. In particular, the Permian/Triassic transition is marked by a prominent positive δ88/86Srsw-peak that reflects a significantly enhanced burial flux of Sr and carbonate, likely driven by bacterial sulfate reduction (BSR) and the related alkalinity production in deeper anoxic waters. We also argue that the residence time of Sr in the Phanerozoic ocean ranged from ∼1Myrs to ∼20Myrs.

Description: IODP Expedition 307 made it for the first time possible to investigate the entire body of a cold-water coral carbonate mound. Here we provide new insights into the long-term history of Challenger Mound on the European continental margin off Ireland. This study is based on age determinations (230Th/U, 87Sr/86Sr) and geochemical signals (Mg/Li and Ba/Ca) measured in the scleractinian cold-water coral Lophelia pertusa from IODP Site 1317 in the Porcupine Seabight. The paleoceanographic reconstructions reveal that coral growth in the Porcupine Seabight was restricted to specific oceanographic conditions such as enhanced export of primary production and Bottom-Water Temperatures (BWT) between ∼8–10 °C, related to the water mass stratification of the Mediterranean Outflow Water (MOW) and Eastern North Atlantic Water (ENAW). The geochemical signals from the coral skeletons can be explained by the close interaction between cold-water coral growth, sea-surface productivity and the surrounding water masses - the boundary layer between MOW and ENAW. Enhanced sea-surface productivity and the build-up of a stable water mass stratification between ENAW and MOW caused enhanced nutrient supply at intermediate water depths and facilitated a steady mound growth between∼3.0 - 2.1 Ma. With the decrease in sea-surface productivity and related reduced export productivity the food supply was insufficient for rapid coral mound growth between∼1.7 - 1 Ma. During the late Pleistocene (over the last∼0.5 Myr) mound growth was restricted to interglacial periods. During glacials the water mass boundary between ENAW/MOW probably was below the mound summit and hence food supply was not sufficient for corals to grow.