Hydrocarbon gar in sediments of the Costa Rica ubduction zone

At the active continental margin off Costa Rica substantial amounts of hydrocarbon gases are encountered in sediments. The molecular composition (C1-C3) of free hydrocarbon gas as well as the isotopic composition (d13C of methane and ethane and D of methane) was analysed on core samples (ranging between 50 and 380 m depth) collected at sites 1040-1043 which was drilled during ODP Leg 170. In addition, the molecular composition of the C1-C3 hydrocarbons and the d13C composition of C1 and C2 hydrocarbons was determined on adsorbed gas from selected depth intervals at Site 1041 (50-380 mbsf). The molecular composition, and stable carbon and hydrogen isotope signature of low molecular weight hydrocarbons from core sediments and gas pockets indicate that most of the gas was generated by microbial CO2-reduction. Beside d13C values of about -80 per mil for methane (which is typical for microbially- generated methane) extremely light d13C values of -55 per mil were measured for ethane. The carbon isotope composition of methane and ethane, as well as the C1/(C2+C3) ratio display distinct trends with increasing depth. Gas mixing calculations indicate that the percentage of thermally-generated ethane increases from 10% at about 75 mbsf to almost 80% at 380 mbsf. The fraction of thermogenic methane in this depth interval is calculated to range from 0.03 to 1.8% of the total methane. The small contribution of thermogenic methane would increase the d13C value by <1 per mil. Therefore, the increase of d13C of methane (by about 12 per mil) with depth cannot be explained by gas mixing alone. Instead, the observed d13C trend is caused by successive isotope depletion of the methane precursor within the sedimentary organic matter due to progressing microbial gas generation.

Geochemistry of tephra from the Taupo Volcanic Zone

The Taupo Volcanic Zone (TVZ), central North Island, New Zealand, is the most frequently active Quaternary rhyolitic system in the world. Silicic tephras recovered from Ocean Drilling Programme Site 1123 (41°47.16'S, 171°29.94'W; 3290 m water depth) in the southwest Pacific Ocean provide a well-dated record of explosive TVZ volcanism since ~1.65 Ma. We present major, minor and trace element data for 70 Quaternary tephra layers from Site 1123 determined by electron probe microanalysis (1314 analyses) and laser ablation inductively coupled plasma mass spectrometry (654 analyses). Trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie-lines between three sediment cores (1123A, 1123B and 1123C) recovered from Site 1123. These tephra tie-lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the Site 1123 composite record. Trace element fingerprinting of tephras identifies ~4.5 m and ~7.9 m thick sections of repeated sediments in 1123A (49.0-53.5 mbsf [metres below seafloor]) and 1123C (48.1-56.0 mbsf), respectively. These previously unrecognised repeated sections have resulted in significant errors in the Site 1123 composite stratigraphy and age model for the interval 1.15-1.38 Ma and can explain the poor correspondence between d18O profiles for Site 1123 and Site 849 (equatorial Pacific) during this interval. The revised composite stratigraphy for Site 1123 shows that the 70 tephra layers, when correlated between cores, correspond to ~37-38 individual eruptive events (tephras), 7 of which can be correlated to onshore TVZ deposits. The frequency of large-volume TVZ-derived silicic eruptions, as recorded by the deposition of tephras at Site 1123, has not been uniform through time. Rather it has been typified by short periods (25-50 ka) of intense activity bracketed by longer periods (100-130 ka) of quiescence. The most active period (at least 1 event per 7 ka) occurred between ~1.53 and 1.66 Ma, corresponding to the first ~130 ka of TVZ rhyolitic magmatism. Since 1.2 Ma, ~80% of tephras preserved at Site 1123 and the more proximal Site 1124 were erupted and deposited during glacial periods. This feature may reflect either enhanced atmospheric transport of volcanic ash to these sites (up to 1000 km from source) during glacial conditions or, more speculatively, that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea-level changes. Only 8 of the ~37-38 Site 1123 tephra units (~20%) can be found in all three cores, and 22 tephra units (~60%) are only present in one of the three cores. Whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume. Instead it is postulated that tephra preservation at Site 1123 is 'patchy' and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents. At this site, at least 5 cores would need to have been drilled within a proximity of 10's to 100's of metres of each other to yield a >99% chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1.65 Ma.

Downward particle fluxes at the oligotrophic and mesotrophic site of the EUMELI program

A two year record of downward particle flux was obtained with moored sediment traps at several depths of the water column in two regions characterized by different primary production levels (mesotrophic and oligotrophic) of the eastern subtropical North Atlantic Ocean in the framework of the EUMELI program. Settling particles were collected with multisample conical sediment-traps moored at 1000 and 2500 depths in the water column. Time-series samples were obtained between February 1991 and November 1992. During this time, sampling intervals varied from 8 to 10 d and were synchronized at all depths and also between the oligotrophic and mesotrophic moorings. Sediment-trap sampling procedures were consistent with JGOF and described elsewhere. The data shown here are mass, particulate organic carbon (POC), particulate inorganic carbon (PIC), coccolithophore, opal, and lithogenic downward fluxes obtained during the entire sediment-trap deployments at both sites.

Heat-flow studies in the Peru Trench subduction zone

New heat-flow values were obtained in the central Peru Trench area during site surveys and drilling of Ocean Drilling Program (ODP) Leg 112 by measuring temperatures with ordinary surface heat-flow probes and in the drill holes and by estimating from bottom-simulating reflectors resulting from gas hydrates. The values determined by these methods are consistent with each other within the limits of error. When combined with existing data, heat-flow distribution from the trench to the coast was delineated. Heat flow is lower than 40 mW/m**2 at the bottom of the trench and 40 to 50 mW/m**2 on the landward slope. The low heat flow at the trench bottom can be explained partly by a high sedimentation rate. Heat flow is variable about where the Mendana Fracture Zone meets the trench. This low heat flow might result from hydrothermal circulation in the fracture zone, which some scientists believe is a new propagating rift. On the landward slope, no significant difference in heat flow is recognized between the northern side and the southern side of the fracture zone, in spite of differences in the age of the subducting plate and the tectonic history. Heat flow on the landward slope may be slightly higher than that in most other subduction zones.

Petrography and geochemical composition of the Atlantis II Fracture Zone

SeaBeam echo sounding, seismic reflection, magnetics, and gravity profiles were run along closely spaced tracks (5 km) parallel to the Atlantis II Fracture Zone on the Southwest Indian Ridge, giving 80% bathymetric coverage of a 30- * 170-nmi strip centered over the fracture zone. The southern and northern rift valleys of the ridge were clearly defined and offset north-south by 199 km. The rift valleys are typical of those found elsewhere on the Southwest Indian Ridge, with relief of more than 2200 m and widths from 22 to 38 km. The ridge-transform intersections are marked by deep nodal basins lying on the transform side of the neovolcanic zone that defines the present-day spreading axis. The walls of the transform generally are steep (25°-40°), although locally, they can be more subdued. The deepest point in the transform is 6480 m in the southern nodal basin, and the shallowest is an uplifted wave-cut terrace that exposes plutonic rocks from the deepest layer of the ocean crust at 700 m. The transform valley is bisected by a 1.5-km-high median tectonic ridge that extends from the northern ridge-transform intersection to the midpoint of the active transform. The seismic survey showed that the floor of the transform contains up to 0.5 km of sediment. Piston-coring at two locations on the transform floor recovered more than 1 m of sand and gravel, which appears to be turbidites shed from the walls of the fracture zone. Extensive dredging showed that more than two-thirds of the crust exposed in the transform valley and its walls were plutonic rocks, principally gabbros and residual mantle peridotites. In contrast, based on dredging and seafloor morphology, only relatively undisrupted pillow basalt flows have been exposed on crust of the same age spreading away from the transform. Magnetic anomalies are well defined out to 11 m.y. over the flanking transverse ridges and transform valley, even where layer 2 appears to be absent. The total opening rate is 1.6 cm/yr, but the arrangement of the anomalies indicates that the spreading for each ridge is asymmetric, with the ridge flanks facing the transform spreading at a rate of 1.0 cm/yr. Such an asymmetric spreading pattern requires that both the northern and southern ridges migrate away from each other at 0.2 cm/yr, thus lengthening the transform at 0.4 cm/yr for the last 11 m.y. To the north, the fracture zone valley is oriented differently from the present-day transform, indicating a paleospreading direction change at 17 m.y. from N10°E to due north-south. This change placed the transform into extension for the 11-m.y. period required for simple orthogonal ridge-transform geometry to be reestablished and produced a large transtensional basin within the transform valley. This basin was split by continued transform slip after 11 m.y., with the larger half moving to the north with the African Plate.

Alkenone accumulation and SST reconstruction for the Benguela upwelling zone

A feature of Pliocene climate is the occurrence of "permanent El Niño-like" or "El Padre" conditions in the Pacific Ocean. From the analysis of sediment cores in the modern northern Benguela upwelling, we show that the mean oceanographic state off Southwest Africa during the warm Pliocene epoch was also analogous to that of a persistent Benguela "El Niño". At present these events occur when massive southward flows of warm and nutrient-poor waters extend along the coasts of Angola and Namibia, with dramatic effects on regional marine ecosystems and rainfall. We propose that the persistent warmth across the Pliocene in the Benguela upwelling ended synchronously with the narrowing of the Indonesian seaway, and the early intensification of the Northern Hemisphere Glaciations around 3.0-3.5 Ma. The emergence of obliquity-related cycles in the Benguela sea surface temperatures (SST) after 3 Ma highlights the development of strengthened links to high latitude orbital forcing. The subsequent evolution of the Benguela upwelling system was characterized by the progressive intensification of the meridional SST gradients, and the emergence of the 100 ky cycle, until the modern mean conditions were set at the end of the Mid Pleistocene transition, around 0.6 Ma. These findings support the notion that the interplay of changes in the depth of the global thermocline, atmospheric circulation and tectonics preconditioned the climate system for the end of the warm Pliocene epoch and the subsequent intensification of the ice ages.