Geochemistry of pore water and sediments offshore Nicaragua and Costa Rica

Four volcanic ash-bearing marine sediment cores and one ash-free reference core were examined during research cruise RV Meteor 54/2 offshore Nicaragua and Costa Rica to investigate the chemical composition of pore waters related to volcanic ash alteration. Sediments were composed of terrigenous matter derived from the adjacent continent and contained several distinct ash layers. Biogenic opal and carbonate were only minor components. The terrigenous fraction was mainly composed of smectite and other clay minerals while the pore water composition was strongly affected by the anaerobic degradation of particulate organic matter via microbial sulphate reduction. The alteration of volcanic matter showed only a minor effect on major element concentrations in pore waters. This is in contrast to prior studies based on long sediment cores taken during the DSDP, where deep sediments always showed distinct signs of volcanic ash alteration. The missing signal of ash alteration is probably caused by low reaction rates and the high background concentration of major dissolved ions in the seawater-derived pore fluids. Dissolved silica concentrations were, however, significantly enriched in ash-bearing cores and showed no relation to the low but variable contents of biogenic opal. Hence, the data suggest that silica concentrations were enhanced by ash dissolution. Thus, the dissolved silica profile measured in one of the sediment cores was used to derive the in-situ dissolution rate of volcanic glass particles in marine sediments. A non-steady state model was run over a period of 43 kyr applying a constant pH of 7.30 and a dissolved Al concentration of 0.05 ?M. The kinetic constant (AA) was varied systematically to fit the model to the measured dissolved silica-depth profile. The best fit to the data was obtained applying AA = 1.3 * 10**-U9 mol of Si/cm**2/ s. This in-situ rate of ash dissolution at the seafloor is three orders of magnitude smaller than the rate of ash dissolution determined in previous laboratory experiments. Our results therefore imply that field investigations are necessary to accurately predict natural dissolution rates of volcanic glasses in marine sediments.

Geochemical characterizations of the Cretaceous formations at DSDP Holes 93-603B and 11-105

Geochemical characterizations of the Cretaceous formations at Site 603 are quite comparable with those at Site 105. In the Blake-Bahama and the Hatteras formations, the petroleum potential is medium (<5 kg HC/t of rock) to very low (<0.5 kg HC/t of rock), and the organic matter is mainly of type III origin, that is, terrestrial. At the top of the Hatteras Formation, there is a condensed series, which chiefly contains organic matter of type II origin, with up to 20 wt.% total organic carbon content in Core 603B-34 and 25 wt.% in Core 105-9. This accumulation corresponds to the Cenomanian/Turonian boundary event. An examination of dinoflagellates in the kerogen concentration assigns dates to the samples studied by organic geochemistry. The Cenomanian and Turonian age of the organic-matter-rich black claystones indicates a low rate of sedimentation, about 1 m/Ma. Furthermore, the occurrence of type II organic matter indicates an anoxic environment with insufficient oxygen renewal to oxidize the sinking hemipelagic organic matter. This organic enrichment is not related to local phenomena but to sedimentation over an extended area, because deposits are well known in various areas with different paleodepths in the North Atlantic.

Geochemical analysis of sediments from three coring sites in the Mediterranean

In a gravity core from the eastern Mediterranean Sea, a chemically and mineralogically distinct, 5.5-cm-thick layer is present above sapropel S-1 and overlain by hemipelagic marls. Calcite is completely absent in this exotic layer, dolomite is present only in small amounts, and the Cr concentrations are significantly enhanced. The layer was deposited primarily under reducing conditions, but the distributions of redox-sensitive elements show that a large part of the exotic layer is now oxidised by a downward-progressing oxidation front. Sediments from within the nearby anoxic, hypersaline Urania Basin are similar to those from the exotic layer, in particular in S-, C-, and O-isotope distributions of pyrite and dolomite, as well as increased Cr concentrations. Mud expulsion due to expansion of gas-rich mud is proposed to explain the presence of the exotic layer outside the Urania Basin. The deposition of an anoxic layer above S-1 shielded the sapropel from oxidation which resulted in the rare occurrence of a complete preservation of S-1 and provides the first minimum age for the start of anoxic mud accumulation in the Urania Basin.

Carbonate sedimentology of the Propeller Mound, Northeast Atlantic

High resolution studies from the Propeller Mound, a cold-water coral carbonate mound in the NE Atlantic, show that this mound consists of >50% carbonate justifying the name "carbonate mound". Through the last ~300,000 years approximately one third of the carbonate has been contributed by cold-water corals, namely Lophelia pertusa and Madrepora oculata. This coral bound contribution to the carbonate budget of Propeller Mound is probably accompanied by an unknown portion of sediments buffered from suspension by the corals. However, extended hiatuses in Propeller Mound sequences only allow the calculation of a net carbonate accumulation. Thus, net carbonate accumulation for the last 175 kyr accounts for only <0.3 g/cm2/kyr, which is even less than for the off-mound sediments. These data imply that Propeller Mound faces burial by hemipelagic sediments as has happened to numerous buried carbonate mounds found slightly to the north of the investigated area.