Peter Stoffers

Trapping efficiencies of sediment traps from the deep Eastern North Atlantic:: the 230Th calibration

Bottom-tethered sediment traps deployed in the deep eastern North Atlantic between 54°N 20°W and 33°N 20°W (L1, L2, L3), at the European continental margin at 49°N (OMEX) and off the Canary Islands (ESTOC) were investigated for the determination of 230Th trapping efficiencies. The ratios of 230Th flux measured in the traps (Fa) to the expected 230Th flux from the production rate of 230Th in the overlying water column (Fp) ranged between 0.09 and 1.26. For the traps with deployment periods >300 days the interannual variation of Fa/Fp ratios (different years but same location and water depth) were up to 10%, suggesting that the average 230Th flux to the sediment traps did not vary significantly. The influence of lateral advection on the 230Th flux was taken into account either by applying a mass balance of 230Th and 231Pa or by assuming a constant removal rate of 230Th from the water column, an assumption based on similar 230Th concentration-depth profiles observed at most locations investigated. 230Th trapping efficiencies were between 9 and 143%, showing a trend of increasing efficiencies with increasing water depth. No relation was found between current velocities and 230Th trapping efficiencies. Our investigations suggest that the observation of constant or even increasing particle flux rates with increasing water depths in several sediment trap arrays investigated may be a result of sediment trap biases. The correction for the trapping biases is important for the understanding of the regional differences in the particle flux in the eastern North Atlantic.

Sr-Nd-Pb isotope evidence against plume-asthenosphere mixing north of Iceland

Iceland straddles the mid-Atlantic spreading axis, between the Kolbeinsey Ridge to the north and the Reykjanes Ridge to the south. Published geochemical data from the Reykjanes Ridge show evidence for mixing between a MORB component and the Iceland plume. Available data from the Kolbeinsey Ridge suggest that similar mixing may not be occurring there. To investigate in detail the relationship between the Iceland plume and MORB along the Kolbeinsey Ridge, we have collected and analysed samples between the Tjo¨rnes and Spar fracture zones (ca. 67°–69°N). The 16 Kolbeinsey Ridge samples show limited isotopic variation and are characterised by relatively unradiogenic Pb (206Pb/204Pb= 17.912 to 18.053, 207Pb/204Pb= 15.404 to 15.453 and 208Pb/204Pb= 37.543 to 37.690, 87Sr/86Sr= 0.70280 to 0.70298, 143Nd/144Nd= 0.51307 to 0.51323). On the basis of their Rb, Sr, Nd, Sm, U, Th and Pb concentrations, the basalts are N-type MORB. Sr and Nd isotope ratios show significant systematic variations with latitude, becoming more enriched (87Sr/86Sr increases, 143Nd/144Nd decreases) towards Iceland, apparently supporting the classical model of plume-asthenosphere mixing. However, the Pb isotopes show no such relationship, and are thus inconsistent with this mixing model. On the basis of Pb and Sr isotope data it is possible to exclude the Iceland source as an end-member in the genesis of the Kolbeinsey Ridge basalts, implying that Iceland plume material does not flow northward along the Kolbeinsey Ridge. The isotopic variations within the Kolbeinsey data set can be attributed to heterogeneities in the MORB source. The boundary between the plume and MORB sources appears to coincide with the Tjo¨rnes Fracture Zone. This fracture zone may, by analogy with the Australia-Antarctic Discordance, overlie a zone of mantle convergence. The topographic anomalies over the Kolbeinsey and Reykjanes Ridges imply that hot, less dense material underlies them both. The absence of an Icelandic plume signature in the Kolbeinsey geochemistry, however, leads us to propose an asymmetrical shape for the plume, generated by a southerly component of flow in the Kolbeinsey MORB source. A similar flow direction has previously been proposed for the whole North Atlantic on the basis of independent mantle mass-balance calculations

Hydrothermal silica chimney fields in the Galapagos Spreading Center at 86°W

Silica chimneys were discovered in 1985 at 86°W in the rift valley of the Galapagos Spreading Center at 2600 m depth (“Cauliflower Garden”). The inactive chimneys lack any sulfides and consist almost entirely of amorphous silica (up to 96 wt.% SiO2, opal-A); Fe and Mn oxides are minor constituents. Oxygen isotope data show that formation of the silica chimneys took place at temperatures between 32°C (+29.9‰ δ18O) and 42°C (+27.8‰ δ18O).Th/Udating reveals a maximum age of 1440 ± 300y. Amorphous silica solubility relations indicate that the silica chimneys were formed by conductive cooling of pure hydrothermal fluids or by conductive cooling of a fluid/seawater mixture. Assuming equilibrium with quartz at 500 bars, initial fluid temperatures of more than 175°C (i.e., a concentration of > 182 ppm SiO2) were required to achieve sufficient supersaturation for the deposition of amorphous silica at 40°C and 260 bars. If the silica chimneys originate from the same or a similar fluid as higher-temperature ( < 300°C) sulfide-silica precipitates found nearby (i.e., 2.5 km away), then subsurface deposition of sulfides may have occurred.

Hydrographic structure of brine-filled deeps in the Red Sea—new results from the Shaban, Kebrit, Atlantis II, and Discovery Deep

Abstract Hydrographic profiles with CTD and water samples from four brine-filled deeps in the Red Sea (Shaban, Kebrit, Atlantis II and Discovery Deep) were taken during the Meteor 31/2 expedition in February/March 1995. In the Shaban Deep area two smaller basins containing high saline brine were found in addition to the already known southern basins. Corresponding salinities (150.7%. Cl in the upper brine section) and near equal brine levels (around 1325 m water depth) suggest subbottom connections below the sills separating the northern and southern basins. An insignificant decrease in temperature (to about 24.1 †C) has occurred in the upper brine since detection of the Shaban Deep in 1981 and no changes were found for the brine-seawater level. No significant changes have been observed in the Kebrit Deep over the last 23 years in temperature (24.3 †C), chlorinity (154.0 ± 0.3%. Cl), and brine level (1466 ± 1 m). The transition from high saline brine to normal seawater salinity probably extends over a depth of only a few decimetres or less. Strong changes, however, were registered for the Atlantis II Deep. The temperature increase, known from numerous earlier investigations, continued. The gradient of increase was the steepest during the last 2.5 years and a temperature of 71.7 °C was reached in the lower brine of the SW basin. The structure of the lower transition zone, between about 1990 m water depth and high saline brine, has also changed significantly, now containing two convective layers with nearly constant temperatures (61 and 55 °C, respectively). The neighbouring Discovery Deep also shows evidence of increased hydrothermal activities. While the temperature was nearly constant (near 44.7 °C) until 1977, it has since increased to almost 50 °C and the brine level has risen by about 17 m.

Submarine volcanoes and high-temperature hydrothermal venting on the Tonga arc, southwest Pacific

Submarine hydrothermal vents and associated seafloor mineralization on the Tonga arc have been found for the first time, in the summit calderas of two shallow-water volcanoes, greatly extending the known areas and diversity of seafloor hydrothermal activity in the western Pacific region. The highest temperature vents (245–265 °C) occur at water depths of 385–540 m near the summit of one volcano at 24°S. The vents are spatially related to basaltic dike swarms exposed at a summit cone and in the caldera walls. Clusters of large (to 10 m high) barite, anhydrite, and sulfide chimneys on the summit cone are vigorously discharging clear hydrothermal fluids with temperatures on the seawater boiling curve. There is abundant evidence of phase separation, which can be seen as flame-like jets of steam (H2O vapor) at the chimney orifices. Pyrite, marcasite, sphalerite, and chalcopyrite line the interiors of the highest temperature vents, similar to black smoker chimneys on the mid-ocean ridges.

Mantle dynamics, element recycling, and magma genesis beneath the Kermadec Arc‐Havre Trough

New geochemical and isotopic data are presented for lavas from three sites in the Havre Trough-Lau Basin back arc and six volcanoes along the Kermadec arc. The back arc basalts range from MORB-like to arc-like in composition and contain a variable contribution from the underlying slab. The least contaminated MORB-like back arc lavas from 24°–29°S are low degree partial melts of a source with Pacific MORB isotopic characteristics. A transition occurs at 30°S between the strongly depleted northern Kermadec (and Tonga) arc lavas and the mildly depleted southern Kermadec arc lavas. This transition does not correlate with changes in the back arc extension rate or width but may reflect inhibited mantle wedge replenishment behind the shallower-dipping northern Kermadec-Tonga slab. Northern Kermadec lavas require mixing between two components: (1) depleted Havre Trough mantle and (2) fluid derived from altered MORB crust with a slight input of sediment lead. Inter-volcano differences in fluid compositions probably reflect local variations on the subducting slab rather than mineralogical variation in the mantle wedge. Southern Kermadec lavas require an additional component: (3) Pacific sediment melt. This sediment melt is only detected where the subduction rate is 650°C before passing through the sub-arc melt generation zone.

Elemental mercury at submarine hydrothermal vents in the Bay of Plenty, Taupo volcanic zone, New Zealand

Hot springs in active geothermal areas such as Yellowstone National Park, the Geysers geothermal field in California, and the Taupo volcanic zone in New Zealand are notably enriched in the trace metals Au, Ag, As, Sb, and Hg. Such near-surface hot springs have formed many of the worlds important deposits of gold and silver and some of the largest deposits of mercury. The majority of these are associated with continental geothermal systems in subaerial environments. Here we report the discovery of active mercury-depositing hot springs in a submarine setting, at nearly 200 m water depth, within the offshore extension of the Taupo volcanic zone of New Zealand. These vents contain the first documented occurrence of elemental mercury on the sea floor and provide an important link between offshore hydrothermal activity and mercury-depositing geothermal systems on land. The discovery has implications for mercury transport in sea-floor hydrothermal systems and underscores the importance of submarine volcanic and geothermal activity as a source of mercury in the oceans.

First observations of high-temperature submarine hydrothermal vents and massive anhydrite deposits off the north coast of Iceland

High-temperature (250°C) hydrothermal vents and massive anhydrite deposits have been found in a shallow water, sediment-filled graben near 66°36′N in the Tjornes Fracture Zone north of Iceland. The site is located about 30 km offshore, near the small island of Grimsey. The main vent field occurs at a depth of 400 m and consists of about 20 large-diameter (up to 10 m) mounds and 1–3 m chimneys and spires of anhydrite and talc. A north–south alignment of the mounds over a 1-km strike length of the valley floor suggests that their distribution is controlled by a buried fault. Widespread shimmering water and extensive white patches of anhydrite in the sediment between the mounds indicates that the entire 1-km2 area occupied by the vents is thermally active. A 2-man research submersible JAGO was used to map the area and to sample vent waters, gases, and chimneys. Actively boiling hydrothermal vents occur on most of the mounds, and extensive two-phase venting indicates that the field is underlain by a large boiling zone (200×300 m). The presence of boiling fluids in shallow aquifers beneath the deposits was confirmed by sediment coring. The highest-temperature pore fluids were encountered in talc- and anhydrite-rich sedimentary layers that occur up to 7 m below the mounds. Baked muds underlie the talc and anhydrite layers, and pyrite is common in stockwork-like fractures and veins in the hydrothermally altered sediments. However, massive sulfides (pyrite–marcasite crusts) were found in only one relict mound. Subseafloor boiling has likely affected the metal-carrying capacity of the hydrothermal fluids, and deposition of sulfides may be occurring at greater depth. Although the mounds and chimneys at Grimsey resemble other deposits at sedimented ridges (e.g. Middle Valley, Escanaba Trough, Guaymas Basin), the shallow water setting and extensive boiling of the hydrothermal fluids represent a distinctive new type of seafloor hydrothermal system.

Discovery and Description of Giant Submarine Smectite Cones on the Seafloor in Eyjafjordur, Northern Iceland, and a Novel Thermal Microbial Habitat

ABSTRACT With the submersible JAGO and by scuba diving we discovered three remarkable geothermal cones, rising 33, 25, and 45 m from the seafloor at a depth of 65 m in Eyjafjordur, northern Iceland. The greatest geothermal activity was on the highest cone, which discharged up to 50 liters of freshwater per s at 72°C and pH 10.0. The cones were built up from precipitated smectite, formed by mixing of the hot SiO2-rich geothermal fluid with the cold Mg-rich seawater. By connecting a rubber hose to one outflow, about 240 liters of pure geothermal fluids was concentrated through a 0.2-μm-pore-size filter. Among 50 thermophilic isolates, we found members of Bacillus and Thermonema and a new unidentified low-G+C gram-positive member of theBacteria as well as one member of theArchaea, Desulfurococcus mobilis. Analysis of small-subunit rRNA genes PCR amplified and cloned directly from environmental DNA showed that 41 out of 45Bacteria sequences belonged to members of theAquificales, whereas all of the 10Archaea sequences belonged to theKorarchaeota. The physiological characteristics of isolates from different parts of the cones indicate a completely freshwater habitat, supporting the possibility of subterranean transmittance of terrestrial organisms.

First seamount age evidence for significantly slower African plate motion since 19 to 30 Ma

Resolving the time‐space (and compositional) evolution of volcanism along long-lived South Atlantic hotspot trails is important to understanding the connection between hotspot volcanism and mantle plumes. 40 Ar= 39 Ar ages are reported here for rocks dredged from a line of five individual seamounts along an290 km northeast to southwest line extending from the vicinity of Saint Helena Island, and also for Circe Seamount. These seamounts were created in a midplate setting and could have formed rapidly (1 Myr). The St. Helena Seamount ages reveal a remarkably linear migration rate of volcanism of 20 1m m=yr for at least the past 19 Myr, which is interpreted as the absolute motion of the African plate. Because this is much slower than estimated for earlier African plate migration it also represents the first evidence based on seamount ages for a significant deceleration (33%) of the African plate since at least 19 Ma. However, this change could have occurred as early as 30 Ma when the limited data for the Tristan=Gough hotspot chain are also considered. This deceleration supports a relationship between African plate speed and the upsurge of hotspot volcanism on the African continent at25 Ma. We suggest that the increased number of oceanic African hotspots between19 and 30 Ma points to a link also between major changes in plate motion and the onset and continuation of oceanic hotspot volcanism. Our study supports the assumption that chains of individual, rapidly (?) formed seamounts have considerably more potential of providing clear insights into how mantle plumes interact with overriding lithosphere than do those consisting of uninterrupted, more massive lines of hotspot volcanism.