Mineralogy and diagenesis of slope sediments at DSDP Leg 84 holes

The distribution and composition of minerals in the silt and clay fraction of the fine-grained slope sediments were examined. Special interest was focused on diagenesis. The results are listed as follows. (1) Smectite, andesitic Plagioclase, quartz, and low-Mg calcite are the main mineral components of the sediment. Authigenic dolomite was observed in the weathering zones of serpentinites, together with aragonite, as well as in clayey silt. (2) The mineralogy and geochemistry of the sediments is analogous to that of the andesitic rocks of Costa Rica and Guatemala. (3) Unstable components like volcanic glass, amphiboles, and pyroxenes show increasing etching with depth. (4) The diagenetic alteration of opal-A skeletons from etching pits and replacement by opal-CT to replacement by chalcedony as a final stage corresponds to the typical opal diagenesis. (5) Clinoptilolite is the stable zeolite mineral according to mineral stability fields; its neoformation is well documented. (6) The early diagenesis of smectites is shown by an increase of crystallinity with depth. Only the smectites in the oldest sediments (Oligocene and early Eocene) contain nonexpanding illite layers.

Calcareous nannofossil zonation, ages and sedimentation rates at DSDP Hole 76-534

A zonation is presented for the oceanic late Middle Jurassic to Late Jurassic of the Atlantic Ocean. The oldest zone, the Stephenolithion bigotii Zone (subdivided into a Stephanolithion hexum Subzone and a Cyclagelosphaera margerelii Subzone), is middle Callovian to early Oxfordian. The Vagalapilla stradneri Zone is middle Oxfordian to Kimmeridgian. The Conusphaera mexicana Zone, subdivided into a lower Hexapodorhabdus cuvillieri Subzone and a Polycostella beckmannii Subzone, is the latest Kimmeridgian to Tithonian. Direct correlation of this zonation with the boreal zonation established for Britain and northern France (Barnard and Hay, 1974; Medd, 1982; Hamilton, 1982) is difficult because of poor preservation resulting in low diversity for the cored section at Site 534 and a lack of Tithonian marker species in the boreal realm. Correlations based on dinoflagellates and on nannofossils with stratotype sections (or regions) give somewhat different results. Dinoflagellates give generally younger ages, especially for the Oxfordian to Kimmeridgian part of the recovered section, than do nannofossils.

Cenozoic oxygen isotopic values for benthic foraminifera from DSDP and ODP low and high latitude marine sediment cores

The climate during the Cenozoic era changed in several steps from ice-free poles and warm conditions to ice-covered poles and cold conditions. Since the 1950s, a body of information on ice volume and temperature changes has been built up predominantly on the basis of measurements of the oxygen isotopic composition of shells of benthic foraminifera collected from marine sediment cores. The statistical methodology of time series analysis has also evolved, allowing more information to be extracted from these records. Here we provide a comprehensive view of Cenozoic climate evolution by means of a coherent and systematic application of time series analytical tools to each record from a compilation spanning the interval from 4 to 61 Myr ago. We quantitatively describe several prominent features of the oxygen isotope record, taking into account the various sources of uncertainty (including measurement, proxy noise, and dating errors). The estimated transition times and amplitudes allow us to assess causal climatological-tectonic influences on the following known features of the Cenozoic oxygen isotopic record: Paleocene-Eocene Thermal Maximum, Eocene-Oligocene Transition, Oligocene-Miocene Boundary, and the Middle Miocene Climate Optimum. We further describe and causally interpret the following features: Paleocene-Eocene warming trend, the two-step, long-term Eocene cooling, and the changes within the most recent interval (Miocene-Pliocene). We review the scope and methods of constructing Cenozoic stacks of benthic oxygen isotope records and present two new latitudinal stacks, which capture besides global ice volume also bottom water temperatures at low (less than 30°) and high latitudes. This review concludes with an identification of future directions for data collection, statistical method development, and climate modeling.

(Table 1) Comparisons of dynamic and static Young's moduli of basalt at DSDP Holes 69-504B, 70-504B and 83-504B

Compressional- and shear-wave velocity logs (Vp and Vs, respectively) that were run to a sub-basement depth of 1013 m (1287.5 m sub-bottom) in Hole 504B suggest the presence of Layer 2A and document the presence of layers 2B and 2C on the Costa Rica Rift. Layer 2A extends from the mudline to 225 m sub-basement and is characterized by compressional-wave velocities of 4.0 km/s or less. Layer 2B extends from 225 to 900 m and may be divided into two intervals: an upper level from 225 to 600 m in which Vp decreases slowly from 5.0 to 4.8 km/s and a lower level from 600 to about 900 m in which Vp increases slowly to 6.0 km/s. In Layer 2C, which was logged for about 100 m to a depth of 1 km, Vp and Vs appear to be constant at 6.0 and 3.2 km/s, respectively. This velocity structure is consistent with, but more detailed than the structure determined by the oblique seismic experiment in the same hole. Since laboratory measurements of the compressional- and shear-wave velocity of samples from Hole 504B at Pconfining = Pdifferential average 6.0 and 3.2 km/s respectively, and show only slight increases with depth, we conclude that the velocity structure of Layer 2 is controlled almost entirely by variations in porosity and that the crack porosity of Layer 2C approaches zero. A comparison between the compressional-wave velocities determined by logging and the formation porosities calculated from the results of the large-scale resistivity experiment using Archie's Law suggest that the velocity- porosity relation derived by Hyndman et al. (1984) for laboratory samples serves as an upper bound for Vp, and the noninteractive relation derived by Toksöz et al. (1976) for cracks with an aspect ratio a = 1/32 serves as a lower bound.

Hydrogeological investigations in the upper Ouémé catchment in Benin, West Africa

The scope of this PhD thesis was the hydrogeological conceptualisation of the Upper Ouémé river catchment in Benin. The study area exceeds 14,500 km**2 and is underlain by a crystalline basement. At this setting the typical sequence of aquifers - a regolith aquifer at the top and a fractured bedrock aquifer at the bottom - is encountered, which is found in basement areas all over Africa and elsewhere in the world. The chosen regional approach revealed important information about the hydrochemistry and hydrogeology of this catchment. Based on the regional conceptual model a numerical groundwater flow model was designed. The numerical model was used to estimate the impact of climate change on the regional groundwater resources. This study was realised within the framework of the German interdisciplinary research project IMPETUS (English translation: "Integrated approach to the efficient management of scarce water resources in West Africa"), which is jointly managed by the German universities of Bonn and Cologne. Since the year 2000 the Upper Ouémé catchment was the principal target for investigations into the relevant processes of the regional water cycle. A first study from 2000 to 2003 (Fass, 2004, http://nbn-resolving.de/urn:nbn:de:hbz:5n-03849) focused on the hydrogeology of a small local catchment (~30 km**2). In the course of this thesis five field campaigns were underdone from the year 2004 to 2006. In the beginning of 2004 a groundwater monitoring net was installed based on 12 automatic data loggers. Manual piezometric measurements and the sampling of groundwater and surface water were realised for each campaign throughout the whole study area. Water samples were analysed for major ions, for a choice of heavy metals and for their composition by deuterium, oxygen-18 and tritium. The numerical model was performed with FEFLOW. The hydraulic and hydrochemical characteristics were described for the regolith aquifer and the bedrock aquifer. The regolith aquifer plays the role of the groundwater stock with low conductivity while the fractures of the bedrock may conduct water relatively fast towards extraction points. Flow in fractures of the bedrock depends on the connectivity of the fracture network which might be of local to subregional importance. Stable isotopes in combination with hydrochemistry proved that recharge occurs on catchment scale and exclusively by precipitation. Influx of groundwater from distant areas along dominant structures like the Kandi fault or from the Atacora mountain chain is excluded. The analysis of tritium in groundwater from different depths revealed the interesting fact of the strongly rising groundwater ages. Bedrock groundwater may possibly be much older than 50 years. Equilibrium phases of the silicate weathering products kaolinite and montmorillonite showed that the deeper part of the regolith aquifer and the bedrock aquifer feature either stagnant or less mobile groundwater while the shallow aquifer level is influenced by seasonal groundwater table fluctuations. The hydrochemical data characterised this zone by the progressive change of the hydrochemical facies of recently infiltrated rainwater on its flow path into deeper parts of the aquifers. Surprisingly it was found out that seasonal influences on groundwater hydrochemistry are minor, mainly because they affect only the groundwater levels close to the surface. The transfer of the hydrogeological features of the Upper Ouémé catchment into a regional numerical model demanded a strong simplification. Groundwater tables are a reprint of the general surface morphology. Pumping or other types of groundwater extraction would have only very local impact on the available groundwater resources. It was possible to integrate IMPETUS scenario data into the groundwater model. As a result it was shown that the impact of climate change on the groundwater resources until the year 2025 under the given conditions will be negligible due to the little share of precipitation needed for recharge and the low water needs for domestic use. Reason for concern is the groundwater quality on water points in the vicinity of settlements because of contamination by human activities as shown for the village of Dogué. Nitrate concentrations achieved in many places already alerting levels. Health risks from fluoride or heavy metals were excluded for the Upper Ouémé area.

Geochemistry of lavas and dikes from the upper oceanic crust of Hess Deep Rift, eastern Pacific Ocean

Strontium isotopes are useful tracers of fluid-rock interaction in marine hydrothermal systems and provide a potential way to quantify the amount of seawater that passes through these systems. We have determined the whole-rock Sr-isotopic compositions of a section of upper oceanic crust that formed at the fast-spreading East Pacific Rise, now exposed at Hess Deep. This dataset provides the first detailed comparison for the much-studied Ocean Drilling Program (ODP) drill core from Site 504B. Whole-rock and mineral Sr concentrations indicate that Sr-exchange between hydrothermal fluids and the oceanic crust is complex, being dependent on the mineralogical reactions occurring; in particular, epidote formation takes up Sr from the fluid increasing the 87Sr/86Sr of the bulk-rock. Calculating the fluid-flux required to shift the Sr-isotopic composition of the Hess Deep sheeted-dike complex, using the approach of Bickle and Teagle (1992, doi:10.1016/0012-821X(92)90221-G) gives a fluid-flux similar to that determined for ODP Hole 504B. This suggests that the level of isotopic exchange observed in these two regions is probably typical for modern oceanic crust. Unfortunately, uncertainties in the modeling approach do not allow us to determine a fluid-flux that is directly comparable to fluxes calculated by other methods.

Organic carbon in waters, suspended matter, and upper layer bottom sediments of the Kara Sea and Obskay Guba

Along three sections in the Kara Sea and Obskaya Guba concentrations of dissolved and particulate organic carbon (DOC and POC, respectively) in waters , as well as of organic carbon in bottom sediments (Corg) in September-October 2007 were determined. DOC varied from 6.3 to 2400 µg/l, POC - from 0.84 to 12.2 mg/l. For all samples the average DOC was 200 µg/l (n = 78; sigma = 368), the average POC - 2.7 mg/l (n = 92; sigma = 2.7). Concentrations of Corg in dried samples of upper layer bottom sediments varied from 0.13 to 2.10% (aver. = 0.9%; n = 21; sigma = 0.49%). It is shown that distribution of different forms of organic matter (OM) is an indicator of supply and scattering of particulate matter in the Kara Sea and that DOC and POC of the Kara Sea are formed under impact of runoff of the Ob and Yenisei Rivers. It is found that distribution of OM in bottom sediments is closely related to their grain size composition and to the structure of currents in the area. Variations in Corg concentration in bottom sediment cores from the zone of riverine and sea water mixing represent variability of OM burial.

Rb-Sr and K-Ar data for Upper Vendian clayey rocks of the Russian Platform

Geochemical behavior of Rb-Sr and K-Ar systems in Upper Vendian clayey rocks of the Russian Platform is under consideredation. The use of additional data on grain size fractions of sedimentary rocks recovered from boreholes drilled in the Gavrilov Yam area made it possible to confirm the previous conclusion on two stages of epigenetic matter transformation (approximately 600 and 400 Ma ago). Distortions are related to transformation of sediments due to interaction in the water-rock system. Interaction degree was more intense in the upper part of the sedimentary section relative to its lower strata. These conclusions are substantiated by materials from boreholes that characterize different types of Vendian sections and different tectonic zones.

A low-latitude Upper Pleistocene oxygen isotope stack

Below oxygen isotope stage 16, the orbitally derived time-scale developed by Shackleton et al. (1990) from ODP site 677 in the equatorial Pacific differs significantly from previous ones (e.g. Kominz and Pisias, 1979 doi:10.1126/science.204.4389.171; Morley and Hays, 1981 doi:10.1016/0012-821X(81)90034-0, Imbrie et al. 1984), yielding estimated ages for the last Earth magnetic reversals that are 5-7% older than the K/Ar values (Mankinen and Dalrymple, 1979 doi:10.1029/JB084iB02p00615; Berggren et al., 1985; Harland and Armstrong, 1989) but are in good agreement with recent Ar/Ar dating (Baksi et al., 1991; 1992 doi:10.1126/science.256.5055.356; Spell and McDougall, 1992 doi:10.1029/92GL01125). These results suggest that in the lower Brunhes and upper Matuyama chronozones most deep-sea climatic records retrieved so far apparently missed or misinterpreted several oscillations predicted by the astronomical theory of climate. To test this hypothesis, we studied a high-resolution oxygen isotope record from giant piston core MD900963 (Maldives area, tropical Indian Ocean) in which precession-related oscillations in delta18O are particularly well expressed, owing to the superimposition of a local salinity signal on the global ice volume signal (Rostek et al., 1993 doi:10.1038/364319a0). Three additional precession-related cycles are observed in oxygen isotope stages 17 and 18 of core MD900963, compared to the SPECMAP composite curves (Imbrie et al., 1984; Prell et al., 1986 doi:10.1029/PA001i002p00137), and stage 21 clearly presents three precession oscillations, as predicted by Shackleton et al. (1990). The precession peaks found in the delta18O record from core MD900963 are in excellent agreement with climatic oscillations predicted by the astronomical theory of climate. Our delta18O record therefore permits the development of an accurate astronomical time-scale. Based on our age model, the Brunhes-Matuyama reversal is dated at 775 +/- 10 ka, in good agreement with the age estimate of 780 ka obtained by Shackleton et al. (1990) and recent radiochronological Ar/Ar datings on lavas (Baksi et al., 1991; 1992; Spell and McDougall, 1992). We developed a new low-latitude, Upper Pleistocene delta18O reference record by stacking and tuning the delta18O records from core MD900963 and site 677 to orbital forcing functions.

In-situ Iron isotope measurements and mineral compositions from ODP Hole 206-1256D and IODP Hole 335-U1256D

In-situ Fe isotope measurements have been carried out to estimate the impact of the hydrothermal metamorphic overprint on the Fe isotopic composition of Fe-Ti-oxides and Fe-sulfides of the different lithologies of the drilled rocks from IODP Hole 1256D (eastern equatorial Pacific; 15 Ma crust formed at the East Pacific Rise). Most igneous rocks normally have a very restricted range in their 56Fe/54Fe ratio. In contrast, Fe isotope compositions of hot fluids (> 300 °C) from mid-ocean-ridge spreading centers define a narrow range that is shifted to lower delta 56Fe values by 0.2 per mil - 0.5 per mil as compared to igneous rocks. Therefore, it is expected that mineral phases that contain large amounts of Fe are especially affected by the interaction with a fluid that fractionates Fe isotopes during exsolution/precipitation of those minerals. We have used a femtosecond UV-Laser ablation system to determine mineral 56Fe/54Fe ratios of selected samples with a precision of < 0.1 per mil (2 sigma level) at micrometer-scale. We have found significant variations of the delta 56Fe (IRMM-014) values in the minerals between different samples as well as within samples and mineral grains. The overall observed scale of delta 56Fe (magnetite) in 1256D rocks ranges from - 0.12 to + 0.64 per mil, and of delta 56Fe (ilmenite) from - 0.77 to + 0.01 per mil. Pyrite in the lowermost sheeted dike section is clearly distinguishable from the other investigated lithological units, having positive delta 56Fe values between + 0.29 and + 0.56 per mil, whereas pyrite in the other samples has generally negative delta 56Fe values from - 1.10 to - 0.59 permil. One key observation is that the temperature dependent inter-mineral fractionations of Fe isotopes between magnetite and ilmenite are systematically shifted towards higher values when compared to theoretically expected values, while synthesized, well equilibrated magnetite-ilmenite pairs are compatible with the theoretical predictions. Theoretical considerations including beta-factors of different aqueous Fe-chlorides and Rayleigh-type fractionations in the presence of a hydrous, chlorine-bearing fluid can explain this observation. The disagreement between observed and theoretical equilibrium fractionation, the fact that magnetite, in contrast to ilmenite shows a slight downhole trend in the delta 56Fe values, and the observation of small scale heterogeneities within single mineral grains imply that a general re-equilibration of the magnetite-ilmenite pairs is overprinted by kinetic fractionation effects, caused by the interaction of magnetite/ilmenite with hydrothermal fluids penetrating the upper oceanic crust during cooling, or incomplete re-equilibration at low temperatures. Furthermore, the observation of significant small-scale variations in the 56Fe/54Fe ratios of single minerals in this study highlights the importance of high spatial-resolution-analyses of stable isotope ratios for further investigations.