James P. Cowen

Earthquake-induced changes in a hydrothermal system on the Juan de Fuca mid-ocean ridge

Hydrothermal vents on mid-ocean ridges of the northeast Pacific Ocean are known to respond to seismic disturbances, with observed changes in vent temperature. But these disturbances resulted from submarine volcanic activity; until now, there have been no observations of the response of a vent system to non-magmatic, tectonic events. Here we report measurements of hydrothermal vent temperature from several vents on the Juan de Fuca ridge in June 1999, before, during and after an earthquake swarm of apparent tectonic origin. Vent fluid temperatures began to rise 4–11 days after the first earthquake. Following this initial increase, the vent temperatures oscillated for about a month before settling down to higher values. We also observed a tenfold increase in fluid output from the hydrothermal system over a period of at least 80 days, extending along the entire ridge segment. Such a large, segment-wide thermal response to relatively modest tectonic activity is surprising, and raises questions about the sources of excess heat and fluid, and the possible effect on vent biological communities.

Scavenging rates of dissolved manganese in a hydrothermal vent plume

Abstract The biogeochemical scavenging of dissolved manganese (Mn) from hydrothermal plumes was investigated using radiotracer ( 54 Mn) techniques. The measured scavenging rate constant, k 1 , was lowest in the buoyant plume ( −1 ), increasing to ∼2 y −1 in the non-buoyant plume at distances of 20 km from the ridge valley axis. The direct biological contribution to the dissolved Mn scavenging rate (i.e the fraction suppressed by the addition of a metabolic poison) also increased over the same distances, being minor or absent at plume depths in the proximal plume, yet the major component at distal plume stations. These and other data suggest that the capacity for scavenging dissolved Mn onto particles evolves with increasing age of the plume, suggestive of a microbial response to changing conditions within the plume. Estimated maximum scavenging rates of dissolved Mn onto particles ( R DMn = k 1 [DMn])were noted at plume depths for all stations, a function of very high dissolved Mn concentrations in the case of the buoyant plume and proximal non-buoyant plume. R DMn values, integrated over plume depths, ranged from 3.4 to 1.7 mM m −2 y −1 for the non-buoyant plume at on-axis and off-axis stations, respectively. The application of the data to the dispersal of hydrothermal constituents and to plume aging is discussed.

The effect of hydrothermal processes on midwater phosphorus distributions in the northeast Pacific

Abstract The distributions of dissolved, particulate and sedimentary phosphorus were measured in the region of the Juan de Fuca Ridge to determine the impacts of hydrothermal processes on the phosphorus cycle in the oceans. Significant negative dissolved phosphate anomalies, ranging from 0 to 60 nmol/l, were observed in the water column at depths between 1900 and 2300 db. The largest anomalies (

Morphological and compositional evidence for biotic precipitation of marine barite

Barite formation in the surface oceans is generally assumed to be dominated by abiotic precipitation. Acceptance of this pathway is largely the result of the absence of a pelagic marine organism known to precipitate the ovoid to rounded-rectangular barite crystals typically observed in marine waters and sediments. Barite crystals observed in net-tow particles and on substrates retrieved from the seafloor (both in the central North Pacific) were examined by scanning electron microscopy and energy dispersive X-ray spectrometry. Three distinct crystalline forms of bar&e were observed: ovoid and hexagonal crystals several microns in diameter, and aggregates of submicron-sized crystals. Ovoid and hexagonal-type crystals contained between 0 and 26 mole percent SrS04. The microcrystalline barite contained no detectable Sr (CO.05 percent). Hexagonal-type crystals were precipitated by an unusual benthic foraminifera. Comparison of the morphology and composition of the barite crystals observed in this study to crystals precipitated by a variety of biotic and abiotic processes suggests a biotic origin for the ovoid barite crystals, the most common form of barite observed in this region.

Autotrophic ammonia oxidation in a deep-sea hydrothermal plume

Direct evidence for autotrophic ammonia oxidation is documented for the first time in a deep-sea hydrothermal plume. Elevated NH(4) (+) concentrations of up to 341+/-136 nM were recorded in the plume core at Main Endeavour Field, Juan de Fuca Ridge. This fueled autotrophic ammonia oxidation rates as high as 91 nM day(-1), or 92% of the total net NH(4) (+) removal. High abundance of ammonia-oxidizing bacteria was detected using fluorescence in situ hybridization. Ammonia-oxidizing bacteria within the plume core (1.0-1.4x10(4) cells ml(-1)) accounted for 7.0-7.5% of the total microbial community, and were at least as abundant as methanotrophs. Ammonia-oxidizing bacteria were a substantial component of the particle-associated communities (up to 51%), with a predominance of the r-strategist Nitrosomonas-like cells. In situ chemolithoautotrophic organic carbon production via ammonia oxidation may yield 3.9-18 mg C m(-2) day(-1) within the plume directly over Main Endeavour Field. This rate was comparable to that determined for methane oxidation in a previous study, or at least four-fold greater than the flux of photosynthetic carbon reaching plume depths measured in another study. Hence, autotrophic ammonia oxidation in the neutrally buoyant hydrothermal plume is significant to both carbon and nitrogen cycling in the deep-sea water column at Endeavour, and represents another important link between seafloor hydrothermal systems and deep-sea biogeochemistry.

Methane in aging hydrothermal plumes

Methane concentration and the stable carbon isotopic composition ( 13 C-CH4) were measured in aging hydrothermal plume waters originating at the Endeavour Segment, Juan de Fuca Ridge. CH4 concen- trations decreased systematically from 600 nM at on-axis stations to 11 nM at a distal station 15 km off axis; background CH4 concentrations are 2 nM. CH4 behaves nonconservatively in plume waters and does not correlate with conservative parameters such as temperature anomaly (), but is highly correlated with NH4 and total dissolvable Mn. 13 C-CH4 values for plume depth samples varies inversely and significantly (R 2 0.89) with methane concentrations normalized to . Some 13 C-CH4 values (1.8 and 10.9 ‰) measured at the 15 km off-axis station are among the heaviest yet reported from a natural marine environment. Less than 5% of original hydrothermal methane remains in the plume at this station. The data are consistent with extensive microbial methane oxidation. A narrow range of fractionation factors (rc 1.0072 to 1.0077) was calculated for the Endeavour samples. These fractionation factors are less than those reported by Coleman et al. (1981), but fall near the trend line of their rc versus temperature data when extrapolated to plume water temperature (2 °C). Copyright

In situ observations of the onset of hydrothermal discharge during the 1998 Submarine Eruption of Axial Volcano, Juan de Fuca Ridge

A volcanic eruption at the summit of Axial Volcano on January 25, 1998, instantaneously created extensive and vigorous hydrothermal discharge. Moorings 2 km apart along the eruption fissure recorded temperature increases of ∼0.6°C up to 115 m above bottom within hours of initial seismic activity. Water temperatures at the mooring sites remained high for about 5 days, then declined steadily over the next 2 weeks. A response cruise 18 days after the eruption found hydrothermal temperature anomalies of ∼0.1°C over the eruption site, and a more intense and much thicker plume 20 km downstream of the eruption. We estimate the steady-state heat flux required to produce this distal plume, evidence of discharge conditions perhaps 1-13 days after the eruption, as 60-230 GW. The Axial eruption thus produced the largest vent field heat flux yet measured, but these high levels lasted less than 3 weeks.

Rapid dike emplacement leads to eruptions and hydrothermal plume release during seafloor spreading events

The creation of ocean crust by rapid injection of magma at mid-ocean ridges can lead to eruptions of lava onto the seafloor and release of “event plumes,” which are huge volumes of anomalously warm water enriched in reduced chemicals that rise up to 1 km above the seafloor. Here, we use seismic data to show that seafloor eruptions and the release of hydrothermal event plumes correspond to diking episodes with high injection velocities and rapid onset of magma emplacement within the rift zone. These attributes result from high excess magma pressure at the dike source, likely due to a new influx of melt from the mantle. These dynamic magmatic conditions can be detected remotely and may predict the likelihood of event plume release during future seafloor spreading events.

Microbial diversity within basement fluids of the sediment-buried Juan de Fuca Ridge flank

Despite its immense size, logistical and methodological constraints have largely limited microbiological investigations of the subseafloor basement biosphere. In this study, a unique sampling system was used to collect fluids from the subseafloor basaltic crust via a Circulation Obviation Retrofit Kit (CORK) observatory at Integrated Ocean Drilling Program borehole 1301A, located at a depth of 2667 m in the Pacific Ocean on the eastern flank of the Juan de Fuca Ridge. Here, a fluid delivery line directly accesses a 3.5 million years old basalt-hosted basement aquifer, overlaid by 262 m of sediment, which serves as a barrier to direct exchange with bottom seawater. At an average of 1.2 × 104 cells ml−1, microorganisms in borehole fluids were nearly an order of magnitude less abundant than in surrounding bottom seawater. Ribosomal RNA genes were characterized from basement fluids, providing the first snapshots of microbial community structure using a high-integrity fluid delivery line. Interestingly, microbial communities retrieved from different CORKs (1026B and 1301A) nearly a decade apart shared major community members, consistent with hydrogeological connectivity. However, over three sampling years, the dominant gene clone lineage changed from relatives of Candidatus Desulforudis audaxviator within the bacterial phylum Firmicutes in 2008 to the Miscellaneous Crenarchaeotic Group in 2009 and a lineage within the JTB35 group of Gammaproteobacteria in 2010, and statistically significant variation in microbial community structure was observed. The enumeration of different phylogenetic groups of cells within borehole 1301A fluids supported our observation that the deep subsurface microbial community was temporally dynamic.

Branched aliphatic alkanes with quaternary substituted carbon atoms in modern and ancient geologic samples

A pseudohomologous series of branched aliphatic alkanes with a quaternary substituted carbon atom (BAQCs, specifically 2,2-dimethylalkanes and 3,3- and 5,5-diethylalkanes) were identified in warm (65°C) deep-sea hydrothermal waters and Late Cretaceous black shales. 5,5-Diethylalkanes were also observed in modern and Holocene marine shelf sediments and in shales spanning the last 800 million years of the geological record. The carbon number distribution of BAQCs indicates a biological origin. These compounds were observed but not identified in previous studies of 2.0 billion- to 2.2 billion-year-old metasediments and were commonly misidentified in other sediment samples, indicating that BAQCs are widespread in the geological record. The source organisms of BAQCs are unknown, but their paleobiogeographic distribution suggests that they have an affinity for sulfides and might be nonphotosynthetic sulfide oxidizers.