Geochemical investigation and determination of hiatuses on Voering Plateau sediment records

The sediments recovered on ODP Leg 104 have been reported to be characterized by hiatuses. The hiatuses were defined by biostratigraphy and were believed to be caused by erosion related to temporary changes of bottom current composition and velocity. They have been associated with major paleoenvironmental changes, reorganization of global deep water production, and increased bottom water flows. Because of the importance of hiatuses for ongoing research, we decided to closely investigate the sedimentation history for the most significant Pliocene and Miocene biostratigraphic hiatuses by sedimentologic and geochemical means. The sedimentologic studies include clay mineral distributions, grain size data, and organic carbon concentrations. The geochemical studies include determination of 87/86Sr ratios, 10Be and Ir concentrations. The results of the sedimentologic studies suggest either that paleoenvironmental changes associated with hiatuses are not represented in the preserved sediments, or that the hiatuses are an artifact of interpretation of the biostratigraphic data. Strontium isotopes indicate continuous sedimentation for the interval investigated at Site 642, an interpretation confirmed by the steady decline in 10Be. 87/86Sr ratios in the interval from above and below proposed hiatuses H 2.2/2.3 and H2.1/2.2 at Site 643 display stronger changes with depth than expected by steady sedimentation. Ir data for this same interval indicate reduced sedimentation rates. Combining both, sedimentologic and geochemical evidence, the proposed hiatuses could not be confirmed and may represent preservation artifacts.

Global data compilation of benthic data sets II

In this study we present a global distribution pattern and budget of the minimum flux of particulate organic carbon to the sea floor (J POC alpha). The estimations are based on regionally specific correlations between the diffusive oxygen flux across the sediment-water interface, the total organic carbon content in surface sediments, and the oxygen concentration in bottom waters. For this, we modified the principal equation of Cai and Reimers [1995] as a basic monod reaction rate, applied within 11 regions where in situ measurements of diffusive oxygen uptake exist. By application of the resulting transfer functions to other regions with similar sedimentary conditions and areal interpolation, we calculated a minimum global budget of particulate organic carbon that actually reaches the sea floor of ~0.5 GtC yr**-1 (>1000 m water depth (wd)), whereas approximately 0.002-0.12 GtC yr**-1 is buried in the sediments (0.01-0.4% of surface primary production). Despite the fact that our global budget is in good agreement with previous studies, we found conspicuous differences among the distribution patterns of primary production, calculations based on particle trap collections of the POC flux, and J POC alpha of this study. These deviations, especially located at the southeastern and southwestern Atlantic Ocean, the Greenland and Norwegian Sea and the entire equatorial Pacific Ocean, strongly indicate a considerable influence of lateral particle transport on the vertical link between surface waters and underlying sediments. This observation is supported by sediment trap data. Furthermore, local differences in the availability and quality of the organic matter as well as different transport mechanisms through the water column are discussed.

Hydrogen, carbon, and oxygen isotope ratios of interstitial fluides of ODP Leg 104 holes

Carbon, hydrogen, and oxygen isotope ratios determined on 32 squeezed interstitial fluid samples show remarkable variations with depth. For the most part these variations are related to diagenetic and alteration reactions taking place in the sediments, and in the underlying basalts. delta13C SumCO2 depth distributions at Sites 642 and 643 are the result of mixing of original SumCO2 of the paleo bottom water with SumCO2 released by remineralization of organic matter. At Site 644, where sulfate exhaustion occurs, the processes of methanogenesis by CO2 reduction and anaerobic methanotrophy strongly influence the delta13C SumCO2 distribution. Hydrogen and oxygen isotopes roughly covary, and become enriched in 16O and1H with depth. This effect is most pronounced at Sites 642 and 643, possibly due to the influence of the directly underlying basalts. Isotope depletions at Site 644 are much lower, corresponding to the greater sediment depth to basement. The alternative, that the O, H isotope shifts are due primarily to autochthonous diagenetic and exchange reactions, is not supported by the data available.