Description: Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 μmol Si cm−2 yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Description: Magmatic sill intrusions into organic-rich sediments cause the release of thermogenic CH4 and CO2. Pore fluids from the Guaymas Basin (Gulf of California), a sedimentary basin with recent magmatic activity, were investigated to constrain the link between sill intrusions and fluid seepage as well as the timing of sill-induced hydrothermal activity. Sampling sites were close to a hydrothermal vent field at the northern rift axis and at cold seeps located up to 30km away from the rift. Pore fluids close to the active hydrothermal vent field showed a slight imprint by hydrothermal fluids and indicated a shallow circulation system transporting seawater to the hydrothermal catchment area. Geochemical data of pore fluids at cold seeps showed a mainly ambient diagenetic fluid composition without any imprint related to high temperature processes at greater depth. Seep communities at the seafloor were mainly sustained by microbial methane, which rose along pathways formed earlier by hydrothermal activity, driving the anaerobic oxidation of methane (AOM) and the formation of authigenic carbonates. Overall, our data from the cold seep sites suggest that at present, sill-induced hydrothermalism is not active away from the ridge axis, and the vigorous venting of hydrothermal fluids is restricted to the ridge axis. Using the sediment thickness above extinct conduits and carbonate dating, we calculated that deep fluid and thermogenic gas flow ceased 28 to 7kyr ago. These findings imply a short lifetime of hydrothermal systems, limiting the time of unhindered carbon release as suggested in previous modeling studies. Consequently, activation and deactivation mechanisms of these systems need to be better constrained for the use in climate modeling approaches.

Description: Silicon isotopic compositions (δ30Si) of modern and ancient siliceous sedimentary rocks provide valuable information on conditions in depositional environments, but interpretations are hampered by the lack of experimentally validated fractionation factors. Here, we present new constraints on the magnitudes of kinetic and equilibrium isotope effects during chemical precipitation of amorphous silica in batch-reactors at low temperature (10-35°C) and near-neutral pH (7.5-8.5), as analogue for non-biogenic chert formation. Instantaneous fractionation factors, derived from δ30Si-values of the total dissolved (SiTD) silica and mass balance computations with αinst=(δ30Sippt+1000)/(δ30SiTD+1000), decrease with progressive precipitation and reduced reaction rates. This suggests that silica deposition in the batch-reactors is kinetically-dominated at the start of the experiments but approaches a metastable equilibrium after ca. 400hours. Modelled kinetic fractionation factors range from 0.9965 at 10°C, to 0.9976 at 20°C and 0.9993 at 35°C and pH8.5, whereas equilibrium isotope effects are smaller and range from 0.9995 at 10°C, to 1.000 at 20°C and 1.0005 at 35°C. Our results suggest that large isotope effects are only expressed in natural systems where dissolved and precipitated silica are not equilibrated, implying that the kinetic conditions of non-biogenic silica precipitation provide important constraints on silicon isotope ratios of siliceous rocks, with particular relevance for those preserved in the Archean chert record

Description: Detectible δ30Si variations in present-day chemical silica deposits have stimulated the application of silicon isotopes to infer environmental conditions from ancient equivalents. Interpretations of δ30Si signatures remain problematic in view of potential post-depositional changes, of which magnitudes and underlying mechanisms are largely unknown. A critical issue in the interpretation of isotope data from cherts concerns the extent to which early-diagenetic processes modify original δ30Si signatures. Here, we report δ30Si variations in opal-A, opal-A/CT and opal-CT from fossil sinter deposits in an active discharge apron in the Geysir geothermal field, Iceland. Opal-A samples show an average δ30Si of − 0.7 ± 0.2‰, while opal-CT samples are isotopically lighter with an average δ30Si of − 2.0 ± 0.4‰, implying a sizable shift of approximately 1.3‰ between the different phases. This shift can be explained by repetitive dissolution/re-precipitation processes, diffusive transport or temperature differences during phase transitions. On average, the fossil opal-A tends to be less negative in δ30Si than amorphous silica that recently precipitated from the hydrothermal water. The difference can be attributed to primary variability in isotopic fractionation that accompanies precipitation out of spring water at the surface, or to a post-depositional release of surface 28Si at the onset of diagenetic formation. Our results corroborate the perception that original silicon isotope signatures of silica, acquired during chemical precipitation from a saturated solution, may not be preserved in the geological record, and that post-depositional changes must be taken into account when interpreting data from ancient chert deposits.

Description: During opening of a new ocean magma intrudes into the surrounding sedimentary basins. Heat provided by the intrusions matures the host rock creating metamorphic aureoles potentially releasing large amounts of hydrocarbons. These hydrocarbons may migrate to the seafloor in hydrothermal vent complexes in sufficient volumes to trigger global warming, e.g. during the Paleocene Eocene Thermal Maximum (PETM). Mound structures at the top of buried hydrothermal vent complexes observed in seismic data off Norway were previously interpreted as mud volcanoes and the amount of released hydrocarbon was estimated based on this interpretation. Here, we present new geophysical and geochemical data from the Gulf of California suggesting that such mound structures could in fact be edifices constructed by the growth of black-smoker type chimneys rather than mud volcanoes. We have evidence for two buried and one active hydrothermal vent system outside the rift axis. The vent releases several hundred degrees Celsius hot fluids containing abundant methane, mid-ocean-ridge-basalt (MORB)-type helium, and precipitating solids up to 300 m high into the water column. Our observations challenge the idea that methane is emitted slowly from rift-related vents. The association of large amounts of methane with hydrothermal fluids that enter the water column at high pressure and temperature provides an efficient mechanism to transport hydrocarbons into the water column and atmosphere, lending support to the hypothesis that rapid climate change such as during the PETM can be triggered by magmatic intrusions into organic-rich sedimentary basins.

Description: Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56 μmol Si cm−2 yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Description: Rift-related magmatism in the Guaymas Basin, Gulf of California induces hydrothermal activity within the basin sediments. Mobilized fluids migrate to the seafloor where they are emitted into the water column changing ocean chemistry and fuelling chemosynthetic ecosystems. New seismic and geochemical data from the northern rift arm of the Guaymas Basin document the variety of fluid expulsion phenomena from large-scale subsurface sediment mobilization related to contact metamorphosis to focused small-scale structures. The geochemical composition of emitted fluids depends largely on the age of the fluid escape structures with respect to the underlying intrusions. Whereas, old structures are dominated by methane emission, young vent sites are characterized by hot fluids that carry a wide range of minerals in solution. The overall high geothermal gradient within the basin (mainly between 160 and 260 °C/km) leads to a thin gas hydrate stability zone. Thus, deep hydrothermal fluid advection affects the gas hydrate system and makes it more dynamic than in colder sedimentary basins.

Description: Various plant compartments of a single bell pepper plant were studied to verify the variability of boron isotope composition in plants and to identify possible intra-plant isotope fractionation. Boron mass fractions varied from 9.8 mg/kg in the fruits to 70.0 mg/kg in the leaves. Boron (B) isotope ratios reported as δ11B ranged from -11.0‰ to +16.0‰ (U ≤ 1.9‰, k=2) and showed a distinct trend to heavier δ11B values the higher the plant compartments were located in the plant. A fractionation of Δ11Bleaf-roots = 27‰ existed in the studied bell pepper plant, which represents about 1/3 of the overall natural boron isotope variation (ca. 80‰). Two simultaneous operating processes are a possible explanation for the observed systematic intra-plant δ11B variation: 1) B is fixed in cell walls in its tetrahedral form (borate), which preferentially incorporates the light B isotope and the remaining xylem sap gets enriched in the heavy B isotope and 2) certain transporter preferentially transport the trigonal 11B-enriched boric acid molecule and thereby the heavy 11B towards young plant compartments which were situated distal of the roots and typically high in the plant. Consequently, an enrichment of the heavy 11B isotope in the upper young plant parts located at the top of the plant could explain the observed isotope systematic. The identification and understanding of the processes generating systematic intra-plant δ11B variations will potentially enable the use of B isotope for plant metabolism studies.

Description: This study aims to explore the extent and controls of silicon isotope fractionation in hot spring systems of the Geysir geothermal area (Iceland), a setting where sinter deposits are actively formed. The δ30Si values of dissolved silica measured in the spring water and sampling sites along outflowing streams, covering a temperature range between 20 and 100 °C, were relatively constant around +0.2‰, whereas the δ30Si signatures of associated opaline sinters from the streambeds were between −0.1‰ and −4.0‰, becoming progressively more negative in the downstream parts of the aprons. Here, the deposited sinters represent some of the most 30Si depleted abiotically produced terrestrial materials documented to date. Compared to the data reported for Icelandic basalts, considered to be the source of the silicon, the δ30Si values of the fluids and sinter deposits are higher and lower, respectively. The resulting values for apparent solid–water isotope fractionation (Δ30Sisolid–water) decreased with decreasing temperature from ca. −0.7‰ at ∼80 °C to −3.7‰ at ∼20 °C, locally down to −4.4‰. This temperature relationship was reproducible in each of the investigated hot spring systems and is qualitatively consistent with recent findings in laboratory experiments on kinetic fractionation for a flowing fluid. However, the apparent fractionation magnitudes observed in the field are ca. −2‰ more negative and thus significantly larger. We infer that solid–water silicon isotope fractionation during deposition of amorphous silica from a flowing fluid correlates inversely with temperature, but is essentially a function of the precipitation rate, such that the fractionation factor decreases with increasing rate. As an important corollary, the effective fractionation behavior during precipitation of silica from saturated solutions is a system-dependent feature, which should be taken into account when using silicon isotopes for paleo-environmental reconstructions.

Description: SO241 set out to test the hypothesis that rift-related magmatism is able to increase carbon emissions from sedimentary basins to the extent that they can actively force climate. To this end we investigated a study area in the Guaymas Basin in the Gulf of California which is one of very few geological settings where rift-related magmatism presently leads to magmatic intrusions into a sediment basin. During the cruise we collected 1100 km of 2D seismic lines to image the extent and volume of magmatic intrusions as well as the extent of metamorphic overprinting of the surrounding sediments and associated subsurface sediment mobilization. We selected three typical seep sites above magmatic intrusions for detailed geochemical studies using gravity corers, multicorers and TV grab. With these samples we will be able to determine the pore water composition to assess the amount and composition of hydrocarbon compounds that are released from these systems. Detailed ocean bottom seismometer measurements at a seep site in the center of the Guaymas Basin will provide further insights into effects of magmatic intrusions on carbon release and diagenetic overprinting of the sediments. It will be possible to reconstruct its long-term seepage history from big carbonate blocks that we have collected with a TV-grab. The northeastern margin of the Guaymas Basin is known for the presence of gas hydrates. During the cruise we collected several seismic lines, which show a clear but unusually shallow BSR indicating high heat flow in the region. Using the seismic data we discovered a previously unknown geological structure on the flank of the northern rift segment: a large mound that seems to consist entirely of black smoker deposits. It seems to be the result of a recent intrusion into the underlying sediments and changes the view how such systems function. The structure was investigated with a comprehensive geochemical, geothermal, and video surveying program which revealed at least seven vents that are active simultaneously. These vents inject methane and helium-rich vent fluids several hundred meters up into the water column. These findings suggest that large-scale magmatism, for example during the opening of an ocean basin under the influence of a hot spot, can be an effective way of liberating large amounts of carbon high up into the water column. The data collected during SO241 will allow us to constrain the amount of carbon that can escape into the atmosphere during LIP emplacement and their relevance on a global scale can be assessed. In addition to reaching the main objectives of the project we discovered a large landslide complex that was probably associated with a tsunami.