OS43A-0983
Manganese Cycles as a Stratigraphic Tool in the Arctic Ocean: Reevaluation of Core AO96/12- 1PC Using the new Itrax XRF Core Scanner at Stockholm University
Cyclical variation in the manganese content in key core 96/12-1PC taken during the Arctic Ocean-96 cruise on the Lomonosov Ridge have been interpreted to represent climatically controlled changes in the input of manganese from the Siberian hinterland and/or variations in the intermediate and deep water ventilation of the Arctic basins. It was therefore suggested that these manganese cycles represent warm phases that correlate to marine oxygen isotope stages and, thus, could be used for chronostratigraphic correlation between cores from the central Arctic Ocean where traditional isotope stratigraphy is difficult or impossible to establish due to the lack of calcareous microfossils. Analysis performed on 96/12-1PC using the new Itrax XRF core scanner at Stockholm University confirms these manganese cycles, and first measurements on neighboring cores suggest that these cycles may be correlated over large distances. Moreover, the XRF results confirm the previously suggested strong correlation between sediment color and manganese content. These manganese cycles therefore likely can be used for a first order chronostratigraphic correlation on at least a regional scale in the central Arctic Ocean. However, our XRF results does not rule out that local variations in cycle thicknesses or occurrences well may occur. The hypothesis of that brown manganese rich sediment cycles represent warm phases that can be correlated to marine oxygen stages will be tested further on material to be obtained on the southernmost part of the Lomonosov Ridge during the LOMROG expedition in August-September 2007.
OS43A-0984
Version 2.0 of the International Bathymetric Chart of the Arctic Ocean: A new Database for Oceanographers and Mapmakers
The International Bathymetric Chart of the Arctic Ocean (IBCAO) was first released to the public after its introduction at the American Geophysical Union (AGU) Fall Meeting in 1999 (Jakobsson et al., 2000). This first release consisted of a Digital Bathymetric Model (DBM) on a Polar stereographic projection with grid cell spacing of 2.5 x 2.5 km derived from an accumulated database of all available bathymetric data at the time of compilation. The IBCAO bathymetric database included soundings collected during past and modern expeditions as well as digitized isobaths and depth soundings from published maps. Compared to previous bathymetric maps of the Arctic Ocean, the first released IBCAO compilation was based upon a significantly enhanced database, particularly in the high Arctic. For example, de-classified echo soundings acquired during US and British submarine cruises between 1958 and 1988 were included as well as soundings from icebreaker cruises conducted by Sweden and Germany at the end of the last century. Despite the newly available data in 1999, there were still large areas of the Arctic Ocean where publicly available data were completely absent. Some of these areas had been mapped by Russian agencies, and since these observations were not available to IBCAO, depth contours from the bathymetric contour map published by the Head Department of Navigation and Hydrography (HDNO) (Naryshkin, 1999) were digitized and incorporated in the database. The new IBCAO Version 2.0 comprises the largest update since the first release; moreover, the grid spacing has been decreased to 2 x 2 km. Numerous multibeam data sets that were collected by ice breakers, e.g. USCGC Healy, R/V James Clarke Ross, R/V Polarstern, IB Oden, now form part of the database, as do the swath bathymetric observations acquired during the 1999 SCICEX expedition. The portrayal of the Eastern Arctic Basin is vastly improved due to e.g. the Arctic Mid Ocean Ridge Expedition 2001 (AMORE) and Arctic Gakkel Vents 2007 (AGAVE) expedition while mapping missions aboard the USCGC Healy have revealed the "real" shape of the sea floor of the central Lomonosov Ridge and in areas off Northern Alaska in the Western Arctic. This paper presents an overview of the new data included in Version 2.0 as well as a brief discussion on the improvements and their possible implications for IBCAO users. Jakobsson, M., Cherkis, N., Woodward, J., Macnab, R. and Coakley, B., 2000. New grid of Arctic bathymetry aids scientists and mapmakers. EOS, Transactions American Geophysical Union, 81: 89, 93, 96. Naryshkin, G., 1999. Bottom relief of the Arctic Ocean. In: H.D.o.N.a. Oceanography and A.-R.R.I.f.G.a.M.R.o.t.W. Ocean (Editors). Russian Academy of Sciences, pp. Bathymetric contour map.
OS43A-0985
Arctic Gakkel Ridge hydrothermal plume study by in-situ redox and particle size measurements.
Throughout the Arctic Gakkel Vents Expediton (AGAVE cruise), Eh electrodes (redox sensor) were mounted on all vehicles, i.e., CTD/rosette, PUMA and JAGUAR AUVs and mini-ROV CAMPER. The electrodes voltages were logged through either SBE 9+ auxiliary channel (CTD) or RS-232C ports (PUMA and CAMPER) or self-recorded by an independent logger (JAGUAR). The LISST (Laser In-Situ Scattering and Transimssiometry)-Deep particle size analyzer was attached on the CTD/rosette with an independent data logger and a battery pack. Redox sensor has been used widely over different tectonic and oceanographic settings to detect hydrothermal emission. Negative shifts of redox voltage in the course of vehicle track lines as well as CTD casts provide an indication of "close range" from the source. None of CTD cast in the peridotite site (~85 deg N, 7.5 deg E) showed any redox negative shift. There were various magnitude of redox negative shift in different height from the bottom recorded in CTD casts and AUV and CAMPER track lines in the volcano site near the eastern end of bared central high (~85.5 deg N, 85 deg E). Although the redox negative shifts varied from almost a mV to almost a hundred mV, the redox data collected during the cruise could not confirm the existence of high temperature vents in the volcano site.
OS43A-0986
Volcanic Structure of the Gakkel Ridge at 85°E
We present an initial volcanologic interpretation of a magmatically-robust segment of the ultra-slow spreading (3- 7 mm/yr) Gakkel Ridge at 85°E in the eastern Arctic Basin based on surveys conducted during the July 2007 Arctic GAkkel Vents Expedition (AGAVE). A previous expedition (2001 AMORE) and seismic stations in the area found evidence for active hydrothermal circulation and seismicity that suggested volcanic activity may be ongoing at 85°E. We examine multi-beam bathymetric data, digital imagery, and rock and sediment samples in order to determine the nature of volcanic accretion that is occurring in this environment including the distribution of flow types and their relationship to features of the axial valley. Raw multi-beam bathymetric data was logged by the Kongsberg EM 120 1°x1° multi-beam echo sounder aboard the icbreaker IB Oden. Digital imagery was recorded on five video and still cameras mounted on the CAMPER fiber-optic wireline vehicle, which was towed 1-3m above the seafloor. Digital imagery was recorded on thirteen CAMPER drift-dives over interesting bathymetry including: a volcanic ridge in the axial valley named Duque's Hill, and Oden and Loke volcanoes that are part of the newly discovered Asgard volcanic chain. Talus, lava flows, and volcaniclastics were sampled with the clamshell grabber and slurp suction sampler on CAMPER. A variety of lava morphologies are identified in the imagery including large basalt pillows with buds and other surface ornamentation, lava tubes, lobates, sheet flows, and a thick cover of volcaniclastic sediment over extensive areas suggestive of explosive volcanic activity.
OS43A-0987
Sea-ice and River Water Inventories Along the Laptev Sea Continental Slope Derived From Stable Oxygen Isotope Composition Within the Water Column
Water masses from the shelf areas are an important contribution to the Arctic Ocean halocline and recent observations have shown that the circulation pattern within the shelf can strongly influence the contribution of river water to the halocline of the Arctic Ocean (e.g. Guay et al. 2001; Dmitrenko et al. 2005). Understanding the processes and the exchange of water masses between the Arctic Ocean basin and the shelf areas are important in respect to halocline stability and potential climatic changes. We use stable oxygen (δ 18O) isotopes of the water to study the exchange of water masses between the Arctic Ocean basin and the shelf areas. The vast Siberian shelf areas are significantly influenced by river runoff and sea-ice processes and δ 18O is an ideal tracer to distinguish between these different freshwater sources. River water can be identified by its highly depleted δ 18O signal relative to marine waters. Sea-ice melting and formation can be identified since salinity is strongly influenced while the δ 18O signal of the remaining water remains nearly unaltered. Considerable inter annual differences are observed between Sea-ice and river water inventory values, which are derived from δ 18O and salinity values in the Eurasian Basin, along the continental margin of the Laptev Sea. Data are from samples taken during NABOS 2005 and 2006 as well as in 1995 (Frank, 1996). Since atmospheric conditions have been shown to be responsible for different transport paths of river water, a simple transport model bases on NCEP wind data is applied. This approach can explain the main features between years with significantly different wind patterns and vorticities. Nevertheless it cannot explain the observed inter annual differences between river water and brine water distributions observed also between years with rather similar vorticities. Also a correlation with annual discharge rates of the contributing rivers cannot account for the differences due to residence times exceeding one year. These results indicate an additional independent and thereby highly complicated system of freshwater storage and release on the continental shelves.
OS43A-0988
Spatial Variability of Land-Sea Carbon Exchange at a Coastal Area in Barrow, Alaska
Relatively cold and low salinity sea water of the Arctic Ocean was considered to be a sink for atmospheric CO2 (Takahashi et al., 1997) because the solubility of CO2 in seawater increases as temperature decreases, and the arctic sea water transports CO2 to greater depths. However, carbon exchange in the Arctic sea is not well evaluated yet, because available data is very limited (Semiletov et al., 2007). Also, terrestrial inflows, such as thawing permafrost and coastal erosion, also affect oceanic air-sea CO2 exchange especially in the Arctic (ACIA., 2004) creating a variety of regional carbon cycles (Semiletov et al., 2007). Our aim is to quantify an air-sea CO2 exchange of a spatially wide coastal sea area, in Barrow, Alaska and to extrapolate the future carbon cycle in response to climate change. Boat cruises for pCO2 measurements operated from July 29 to August 5, 2007. The surveyed area was mainly divided into three parts: Chukchi Sea, Beaufort Sea, and Elson Lagoon. Conductivity of sea surface (CS) and sea surface temperature (SST) were also measured together with pCO2. The result showed distinct differences in pCO2 among three areas. Average delta pCO2 (dpCO2) (a difference between an atmospheric CO2 and pCO2), CS, and SST were -114.9 ppm, 47.0 mScm-1, and 8.0 C at Chukchi Sea, -53.1 ppm, 43.5 mScm-1, and 8.9 C at Beaufort Sea, and 43.7 ppm, 41.1 mScm-1, and 9.5 C at Elson Lagoon. Relatively high dpCO2 value in the Beaufort Sea implies a large terrestrial input from Elson Lagoon where dpCO2 value is positive. This is supported by lower CS in the Beaufort Sea and Elson Laggon than in the Chukchi Sea. Sea currents from Pacific Ocean, which continuously flow through the Chukchi Sea, are thought to carry warmer water. However, SST was lower in the Chukchi Sea than in the Beaufort Sea. This may be because a prevailing wind from north east creates Ekman transport causing an upwelling along the Chukchi Sea coast and this upwelling carries deep cold water to the surface (personal communication with Dr.Okkonen of UAF). pCO2 is very sensitive to temperature and total carbon material in the water. A simple temperature-pCO2 equation suggested by Takahashi (Takahashi et al., 1993) showed an air-sea CO2 exchange in Beaufort Sea could be reversed from a sink to a source if SST increases by 3.7 C under a constant chemical condition. Moreover, pCO2 is very sensitive to a total carbon matter in water because of the high Revelle factor. Ongoing thawing of the permafrost and increasing coastal erosion (Aguirre et al., 2006) changes not only terrestrial carbon cycle (Oechel et al., 1994) but also oceanic carbon cycle as well in the Arctic.
OS43A-0989
Macrofauna of shallow hydrothermal vents on the Arctic Mid-Ocean Ridge at 71N
Deep-sea hydrothermal vents are usually associated with a highly specialized fauna and since their discovery in 1977, more than 400 species of animals have been described. Specialized vent fauna includes various animal phyla, but the most conspicuous and well known are annelids, mollusks and crustaceans. We have investigated the fauna collected around newly discovered hydrothermal vents on the Mohns Ridge north of Jan Mayen. The venting fields are located at 71°N and the venting takes place within two main areas separated by 5 km. The shallowest vent area is at 500-550 m water depth and is located at the base of a normal fault. This vent field stretches approximately 1 km along the strike of the fault, and it is composed of 10-20 major vent sites each with multiple chimney constructions discharging up to 260°C hot fluids. A large area of diffuse, low- temperature venting occurs in the area surrounding the high-temperature field. Here, partly microbial mediated iron-oxide-hydroxide deposits are abundant. The hydrothermal vent sites do not show any high abundance of specialized hydrothermal vent fauna. Single groups (i.e. Porifera and Mollusca) have a few representatives but groups otherwise common in hydrothermal vent areas (e.g. vestimentifera, Alvinellid worms, mussels, clams, galathaeid and brachyuran crabs) are absent. Up until now slightly more than 200 species have been identified from the vent area. The macrofauna found in the vent area is, with few exceptions, an assortment of bathyal species known in the area. One endemic, yet undescribed, species of mollusc has been found so far, an gastropod related to Alvania incognita Warén, 1996 and A. angularis Warén, 1996 (Rissoidae), two species originally described from pieces of sunken wood north and south of Iceland. It is by far the most numerous mollusc species at the vents and was found on smokers, in the bacterial mats, and on the ferric deposits. A single specimen of an undescribed tanaidacean has also ben found. The crinoid Heliometra glacialis is dominating large areas surrounding the vent fields. Calcareous sponges were common in the area. Calcareous sponges normally represent only a minor fraction of the sponge fauna and it was therefore a big surprise that eight out of a total of 13 species reported here are calcareans. Annelids were the most speciose group with more than 80 identified species, followed by crustaceans. Possible explanations for the lack of typical vent fauna is discussed.
OS43A-0990
Towed and AUV Technologies for Arctic Operations
The 2007 AGAVE expedition included a suite of vehicles specifically designed and adapted for under-ice operations. These include the towed vehicle CAMPER, and the Puma and Jaguar autonomous underwater vehicles. The CAMPER (CAMera and samPlER) towed system was designed for high resolution imaging and sampling of the seafloor. It utilizes the oceanographic standard 0.68" electro-optical cable with three fibers and three copper conducting cables. The system consists of five cameras - a bottom looking high definition camera, a forward looking high definition camera system, a forward looking obstacle avoidance camera, a pan and tilt driving camera and a camera for instrumentation- and associated HMI and HID lighting systems. Sampling is accomplished by two hydraulically actuated systems. One of the samplers is a clam shell bucket design that is primarily intended for sampling rocks and sessile biological organisms. The other sampler uses a suction based mechanism and is intended for sampling fluids and motile biological organisms. The two AUVs, Puma and Jaguar, are identical in their base design but were outfitted with different sensor suites. These vehicles are a two hull design with most of the weight in the lower hull and most of the floatation in the top hull yielding a design that is passively stable in pitch and roll. Three thrusters, one for the vertical axis and two main thrusters allow for decoupled control in the vertical and horizontal axes. The 6 kWH battery packs are good for 24 hour operations. Both vehicles are equipped with acoustic modems, four channel long baseline receivers, doppler velocity logs, a high resolution depth sensor, three axes north seeking fiber optic gyroscopes and a pumped CTD. In addition, Puma, which is intended primarily for mid-water column work, also supports an Eh sensor, an optical backscatter sensor and an experimental long range optical backscatter sensor. The Jaguar vehicle, with a focus on near bottom measurements, is equipped with a 230 kHz multibeam, a digital still camera and strobe, a magnetometer and an Eh sensor. Several drift dives were carried out with the CAMPER towed vehicle at two different sites on the Gakkel Ridge this summer and yielded considerable high definition and video imagery as well as geological and biological samples. Several dives were also carried for mapping the mid-water column and the seafloor with the two AUVs and these successfully returned water column CTD, Eh, optical backscatter, magnetic and multibeam data.
OS43A-0991
New Arctic Gravity Field Grids with Applications for Oceanography and Bathymetry
New Arctic Gravity Project 5' grids have been compiled, incorporating significant new data sources in addition to the extensive holdings of airborne, surface, submarine and satellite gravimetry which went into the earlier releases of the ArcGP grids. The new data sources include Russian surface gravimetry over the over the entire Siberian shelf and adjacent ocean regions, provided by VNIIO, as well as new cruise data from submarines and surface vessels, such as the 2007 LOMROG cruise to the Lomonossov Ridge. New high-resolution satellite gravity data, derived from ICESat laser altimetry, has also been incorporated in regions of poor surface data quality. A corresponding precise geoid model of the Arctic has additionally been derived based on GRACE satellite data. The geoid model, combined with ocean sea surface heights derived from ICESat and ERS, may provide independent estimates of ocean mean dynamic topography (MDT), and thus provide independent means for deriving overall Arctic Ocean circulation. We compare MDT from a number of oceanographic models to the satellite/ArcGP estimates, illustrating the large differences between a number of current Arctic Ocean oceanographic models. These comparisons are done as part of a recently completed ESA study - ArcGICE - aimed at quantifying errors in geoid, MDT and tides in support of Arctic sea-ice thickness mapping. Applications of the ArcGP data also include the estimation of bathymetry in regions of poor bathymetric data coverage. We show examples from the Morris Jesup Rise/Lomossov Ridge areas, where inversion of ArcGP gravity and new LOMROG gravimetry is used together with existing bathymetry track data for detecting errors in the IBCAO bathymetry.
OS43A-0992
Diversity of Microorganisms Associated With low Temperature Iron Deposits at the 71°N Hydrothermal Vent Field Along the Arctic Mid-Ocean Ridge
Rust coloured mounds and chimney-like deposits of the newly discovered71°N hydrothermal vent fields at the south-western part of the Mohns Ridge have been investigated. Iron is the fourth most abundant element in the Earth's crust and thus represents one of the most abundant redox active metals widely available for microbial energy generation. Microbial Fe-oxidation is a widespread process in the deep-sea environments, but only recently have studies begun to elucidate these processes and describe the phylogenetic and physiological diversity of the microbial communities that mediate them. Therefore studying the process by which iron is oxidised and how this influence these cold deep-sea communities is of significant importance. We have studied the microbial communities present in these low-temperature rust coloured deposits in order to elucidate the phylogenetic and physiological diversity of the microbial populations inhabiting these deep-sea environments. Polyphasic characterisations by using geochemical and biological analyses have been performed. The deposited material has a highly porous microtexture of branching, twisted filaments resembling stalks of the iron- oxidising Gallionella sp, but numerous other unidentified filamentous structures were also found to be present. Phylogenetic analysis of clone libraries has so far demonstrated that the bacterial community is dominated by members of the Proteobacteria, Planctomycetes and Chloroflexi. The archaeal community consists of both Crenarchaeota and Euryarchaeota. The Crenarchaeota sequences affiliates with other reported uncultivated Deep-Sea archaeal sequences. To further investigate the ecological impact of these iron mounds and their interaction with microorganisms cultivation experiments have been applied. We are specifically focusing on enrichment of iron oxidizing bacteria. Preliminary results indicates that iron oxidizers related to the newly described Mariprofundus ferrooxidans as well as iron reducers related to Rhodoferax ferrireducens are present in these environments, but we have not yet been able to obtain pure isolates form this site. Strains isolated from these systems will provide an opportunity to explore the phylogenetic diversity of neutrophilic Fe metabolism and to establish model systems for molecular metabolic studies. Description of microbial communities existing under such characteristic sub-seafloor environments is important when understanding the role these communities play in the major biochemical processes.
OS43A-0993
Geochemical Characterization of Hydrothermal Plume Fluids From Peridotite- and Basalt- Dominated Regions of the Ultra-Slow Spreading Gakkel Ridge
Geochemical characterization of hydrothermal plumes initially located during the 2001 AMORE cruise to the Gakkel Ridge was undertaken as part of the 2007 Arctic Gakkel Vents Expedition (AGAVE). One peridotite- and one basalt-dominated area were targeted for this exploration to constrain the range of venting environments found on the Gakkel Ridge, the ultra-slow spreading endmember of the global mid-ocean ridge. CTD hydrocasts at the 7 E peridotite-hosted site relocated the plumes found initially on the AMORE cruise. The target plume was located between 2800 and 2950 meters and exhibited a localized signal in temperature and light scattering. While shipboard analysis of dissolved gases was unavailable at the 7 E site, samples were preserved for manganese and helium measurements. No Eh signal was found at the 7 E site. The 85 E basalt-hosted site has experienced recent volcanic activity and was more extensively studied relative to the 7 E site during the AGAVE cruise. CTD casts detected numerous temperature, light scattering, and Eh plumes at 85 E indicative of multiple hydrothermal sources. Three of the plumes sampled exhibited methane concentrations ranging from 20 nM to greater than 250 nM and hydrogen concentrations ranging from 10nM to 100nM. In situ Eh measurements recorded negative excursions of at least 25 mV in each plume. Associated manganese and particle chemistry samples collected at both sites will be analyzed in time for this meeting.
OS43A-0994
Benthic Oxygen Dynamics and Organic Matter Turn Over in the Deep Arctic Ocean
We used autonomous bottom landers equipped with a suite of instruments and sensors to study the benthic degradation rates of organic matter in-situ in the Fram strait (between Greenland and Spitsbergen). We occupied stations on a depth gradient reaching from the 1200 m, over 2500 m to the deepest area in the Arctic Ocean (Molloy Deep 5600 m). Preliminary results of oxygen uptake and efflux of CT give lower degradation rates at the intermediate depth stations than at the deeper and shallower. Even at 2500 m dissolved oxygen concentrations in the bottom water were surprisingly variable and oscillated with up to 10 µM over time scales of hours and with up to 30 µM over monthly time scales.
OS43A-0995
Microbial Diversity in Samples of High Temperature Vent Chimneys From the 71 °N Hydrothermal Fields at the Arctic Mid-Ocean Ridge
To get a first insight into the diversity of microorganisms present in the recently discovered active hydrothermal fields along the Mohns Ridge in the Norwegian-Greenland Sea, 16S rDNA clone libraries were constructed with DNA extracted from the walls of active smoker pipes from different locations. Enrichments targeting different physiological groups of microorganisms were prepared both under aerobic, micro-aerobic, and strictly anaerobic conditions. Different combinations of substrates and electron acceptors, pH, and temperatures were used. The enrichment cultures were monitored by use of PCR in combination with denaturing gradient gel electrophoresis (PCR-DGGE) and partial 16S rDNA sequencing. Species dominating in the enrichments were isolated, and their 16S rRNA genes were analyzed. The clones obtained from DNA amplified with primers specific for Archaea represented members of the orders Archaeoglobales, Thermococcales, Desulfurococcales, and Thermoproteales, as well as some unidentified groups. Three major fractions of the clones showed highest similarity to hyperthermophiles belonging to the families Pyrodictiaceae and Desulfurococcaceae, and an unidentified group which was given the name "Arctic Ridge Hydrothermal Vent Archaea" (ARHVA). The major fraction of the clones obtained by use of PCR primers specific for Bacteria affiliated with various genera of Aquificales. Clones representing Proteobacteria, Deferribacteres, Bacteroidetes, Deinococcus- Thermus, Chloroflexi and Firmicutes were also detected. Many clones were relatively distantly related to sequences in the GenBank database. Different types of both thermophiles and hyperthermophiles were enriched and isolated. The isolates were phylogenetically affiliated to Thermotogales, Thermales, Nautilales, Aquificales, Archaeoglobales, Thermococcales, and Desulfurococcales. The cultivation experiments documented the presence of microorganisms mediating various metabolic processes including fermentation, reduction of sulfate, ferric iron, nitrate, oxygen and elemental sulfur. The information gained from the clone libraries and enrichment cultures revealed that already well-characterized phylogenetic groups, as well as clusters unique for these hydrothermal fields, were present.
OS43A-0996
Low-Temperature Hydrothermal Deposits at the 71„aN Vent Fields at the Arctic Mid-Ocean Ridge: Architecture, Microtextures, and Geochemistry
Vast amounts of low-temperature siliceous Fe-deposits are formed distal to a high-temperature hydrothermal venting areas at the southwestern part of the Mohns Ridge. The low-temperature deposits occur as small yellow to rust coloured mounds and chimney-like structures along faults and fissures in the rift valley floor, for distances up to several kilometres. The mounds and chimneys have a stratified structure of millimetres to centimetres thick laminated layers that are alternating with centimetre sized hollow spaces. The individual layers have in general a highly porous microtexture of most frequently branching, twisted filaments resembling stalks of the iron-oxidising Gallionella sp, which have different thickness and colour of the mineral coating in the separate lamina. Other filamentous structures of apparently biogenic as well as abiotic origin are also present, and dominate in some lamina. The deposited material is loosely consolidated due to infilling of the interspaces between the filaments in a number of thin, discrete lamina. The filaments and the interstitial material have a similar composition of mainly FeO and silica, and electron diffraction reveal 2-line ferrihydrite. The REE composition of the siliceous ferrihydrite show a similar pattern to that of the basaltic crust, indicating that they formed from low-temperature hydrothermal fluids, derived from interactions between the basaltic crust and circulating seawater. Mn-accumulation is observed within lamina most commonly at the surface of deposits. Sediment particles of basaltic glass, high- temperature vent minerals like baryte, and diatoms within lamina that are present in discrete layers show that they sequentially formed at the surface of the deposits at the different stages of growth. This together with the architecture of alternating layers of biogenic filaments and hollow spaces suggest that the formation of these deposits were controlled by biofilms of Fe-oxidising microorganisms that developed at the interface between the reduced vent fluid and the surrounding oxidized seawater. Successive nucleation and precipitation of ferrihydrite and amorphous silica on the microbial stalks results in the formation of the mounds and chimneys.