University, Diss., 2004--Kiel.

University, Diss., 2004--Kiel.

Hydrothermal vents are deep-sea hot springs. Vents are home to luxuriant assemblages of animals that colonize the warm venting fluids. High biomass is fed by microbes that use hydrogen sulphide and other reduced chemicals in the vent fluid as an energy source to fix inorganic carbon. Individual vents may persist for a few years to several decades. The specialized animals must find new vents, cope with changing fluid conditions and foster their offspring. The composition and structure of vent communities vary in space and time. My research at Axial Volcano, a seamount on the Juan de Fuca Ridge (JdFR) in the northeast Pacific, aims to find pattern in this variation and to propose viable hypotheses of the mechanisms driving the patterns. Axial is an ideal location as it supports mature vent fields (venting for over 15 years) and young, developing vents initiated by a volcanic eruption in 1998. Thus, I was able to study both temporal and spatial variation in vent communities at the same site and relate patterns of developing assemblages to patterns observed at longer-lived vents. Pattern detection is the first critical step in any community ecology study as it justifies and focuses the search for process. I have refined existing statistical methods and developed novel techniques to test for pattern in vent species distributions and abundances. I modified an existing null model approach and showed that species distributions among sixteen vents differ from random in a long-lived (>15 years) vent field. I also developed a novel null model to confirm that initial patterns of community assembly seven months following the Axial eruption differ from random recruitment of species and individuals to new vents. My description of the community response to the Axial eruption is the first quantitative report of patterns of vent colonization and succession. My work documents that new vents are colonized quickly (within months) and that initial assemblages are variable. However, rapid community transitions and species replacements within the first few years cause new assemblages to resemble mature vents by 2.5 years post-eruption. Three habitat factors correlate with the development of nascent vent assemblages: the recruitment timing of the tubeworm Ridgeia piscesae post-eruption, vent age and vent fluid hydrogen sulphide content. I also describe a new polynoid polychaete discovered colonizing the new vents in high densities. My major contribution to vent community ecology is revealing species patterns through extensive sampling and rigorous statistical methods. These patterns are a necessary step towards understanding the processes that structure vent communities: they direct future research effort towards the key species and generate hypotheses to be experimentally tested. My work also elucidates how vent species respond to habitat destruction and creation, which is critical information for effectively managing Canada's only hydrothermal vent Marine Protected Area on the JdFR. / Graduate

This work seeks to explore current ecological theory through application to communities inhabiting hydrothermal vents. This thesis aims to: (1) add to and synthesise knowledge of species and their distributions at the intra- and intersegment scale; and (2) evaluate vent community patterns and speculate on processes. Samples used are submersible grabs of low temperature (<60°C) tubeworm assemblages on basalt and sulphide surfaces. Species abundances and distributions on three segments of the Juan de Fuca Ridge (Axial, Cleft, and CoAxial) are described. Community descriptors such as species density, Simpson's and the Shannon-Wiener diversity indices, evenness, species richness, species abundance-distribution models, species percent-average relative abundance and density are used. Vent community structure is compared among segments using these descriptors, visual descriptions, pairwise correlations, Friedman tests of distributions, cluster and correspondence analysis, rarefaction, complementarity, a test for saturation, and Whittaker's beta diversity. Vent community composition on Axial, north Cleft, and CoAxial is similar at the segment and inter-segment scale. The limpet Lepetodrilus fucensis is the most abundant species at all sites. Differences among communities are best seen temporally, not spatially. Senescent communities can be distinguished from active vent assemblages. Pioneer communities, however, are statistically indistinguishable from intermediate communities when sampled two or more years post-eruption. Axial and Cleft species dispersion fits the core-satellite hypothesis. The exceptions are the polynoids Branchinotogluma sp., Lepidonotopodium piscesae, and Levensteiniella kincaidi, which are widespread and present in low local abundances. Both local and mesoscale regional mechanisms explain observed local diversity. Spatial isolation, not habitat differences, influences between-habitat diversity (beta diversity) on Axial, Cleft, and all three segments combined. Meiofauna are important for species richness estimates, identifying differences among structurally similar communities, and understanding input/output between vents and the deep-sea. Measurements such as species richness and diversity indices may be poor at distinguishing among vent communities because vents are species poor and uneven. The Michaelis-Menten, Jackknife 2, and Chao 2 nonparametric vent species richness estimators perform well with small samples. Vent communities should be compared to habitats of similar diversity and evenness as well as disturbance and productivity regimes. Candidate comparison communities include communities in early successional states, selected taxocenes such as carabid beetles on fungi, or high disturbance and/or low diversity systems like the rocky intertidal, organically polluted sediments and oxygen minimum zones below upwelling regions in the deep-sea. / Graduate

Sediments from the northern end of Juan de Fuca Ridge are Late Quaternary in age and contain widely correlatable cycles of turbidity current and hemipelagic sedimentation. Sediments from the Ridge were examined for their mineralogy, structure, components of the sand fraction, rates of sedimentation and grain size distribution to establish processes of sedimentation, stratigraphy, correlation and local hydrothermal relationships. Ten gravity and Phleger core sites along two profiles of the Ridge were examined in detail, one section was perpendicular to West Valley, the main spreading centre, and one section was within and parallel to West Valley. Sediment from Cascadia Basin was compared to the results of the Ridge study. Changes in sedimentation defined by core X-radiograph structure, components of the sand fraction and grain size distribution, indicated cycles of relatively coarse sediment overlain by finer bioturbated sediment with a repeated stratigraphic relationship in all but one Juan de Fuca Ridge core. Changes in sediment composition are attributed to brief, episodic, continent derived turbidity current deposition followed by lengthy periods of hemipelagic sedimentation for each cycle. Differences in composition exist between sediment of ridges and valleys, with a greater winnowed foraminiferal-hemipelagic and a lesser turbidity current influence in the former area. Radiocarbon dated foraminiferal-rich intervals from ridge sediments were exclusively Late Pleistocene with Middle Ridge sediment having an inferred 9000-9500 B.P. Late Pleistocene-Holocene boundary. Similar sedimentation cycles between Middle Ridge and valley localities enabled correlation of ridge and valley stratigraphy and the Late Pleistocene-Holocene boundary. A stratigraphic relationship based on the episodic deposition of continent derived turbidites exists between the northern end of Juan de Fuca Ridge and the continental Pacific Northwest. Pulses of turbidity current sedimentation coincide with initial interglacial warming trends during the Late Pleistocene. Holocene sedimentation for Juan de Fuca Ridge is of hemipelagic origin with rare local turbidity current deposition. Hydrothermal minerals were not detected. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Graduation date: 1988

This study applies Bayesian inversion to receiver functions (RF) to estimate local shear wave velocity (Vs) structure of the crust and upper mantle beneath two ocean bottom seismometers (OBS) offshore, and two land-based seismometers onshore Vancouver Island, British Columbia, Canada. We use passive seismic data recorded on NC89, a permanent NEPTUNE (North-east Pacific Time-series Undersea Networked Experiments) OBS located on the continental slope, and on a temporary autonomous KECK foundation OBS, KEBB, located at the Endeavour segment of the Juan de Fuca Ridge (JdFR). The two land based seismometers (OZB and PGC) are located on Vancouver Island and are part of the Canadian National Seismograph Network (CNSN). The introduction of NEPTUNE has helped to fill a gap in offshore seismic monitoring, however; due to high noise levels and a relatively short deployment time, few useful events have been recorded (to date) for RF analysis. In this study, we utilize three-component, broadband recordings of large (M6+), distant (30 -100 degrees) earthquakes to compute RFs due to locally generated P (compressional) to S (shear) converted waves. RFs are then inverted using a non-linear Bayesian approach which yields optimal profiles of Vs, Vp (compressional wave velocity), and strike and dip angles, as well as rigorous uncertainty estimates for these parameters. Near the JdFR a thin sediment layer (<1 km) is resolved overlying a 2 km thick oceanic crust. The crust contains a large velocity contrast at the depth of an expected axial magma chamber. The oceanic crust thickens to 10 km at the continental slope where it is overlain by 5 km of sediments. At the coastal station (OZB) a low velocity zone is imaged at 16 km depth dipping approximately 12 degrees NE. Evidence for this low velocity zone is also seen beneath southern Vancouver Island (PGC) at a depth consistent with previous studies. Determining such models at a number of locations (from the spreading ridge to the coast) provides new information regarding local structure and can aid in seismic hazard analysis. / Graduate

Thesis (M. Sc.)--Carleton University, 2008. / Includes bibliographical references (p. 157-168). Also available in electronic format on the Internet.

Three gastropods occupy a range of habitats along gradients in hydrothermal flux at Juan de Fuca Ridge vents. I examined how these species co-exist and identified mechanisms driving their abundances. First, I measured temperatures and spatial patterns in adult densities of the three species at three distances from vents to test if thermal regime relates to their habitat selection. Lepetodrilus fucensis and Depressigyra globulus were most dense in-vent (0-25 cm) at variable temperatures (10+5°C): 2100 and 240 incl. dm -2 (respectively). Provanna variabilis was most abundant far-vent (51-75 cm: 60 ind. dm 2) at stable temperatures (3±0.5°C). Thermal conditions are key in their habitat selection: behavioural experiments showed that these gastropods select fluid temperatures<18°C. L.fucensis and D. globulus preferred 5-15°C, while P. variabilis preferred 4-12°C. The next studies sought to explain how Lepetodrilus fucensis reaches order of magnitude higher densities in comparison to other gastropods. First. I quantified L. fucensis recruitment and sex ratio patterns to identify innovative life history traits. I meaured size structure and density at in- and far-vent locations. Early postlarval juveniles occupied far-vent at remarkable densities (2419 ind. dm-2). To test for sex ratio biases, I sexed animals from different habitats and sizes. Populations nearest vents hosted the largest females (>6.0 mm), while peripheral habitats were male-biased. A transplant experiment showed that female survivorship and gonad fullness were significantly lower than males in far-vent locations. Sex ratio biases are driven by two mechanisms: females maximize their reproductive output by selecting optimal habitats and suffer relatively higher mortality in low flux. Next, I hypothesized that the Lepetodrilus fucensis gill symbiosis is a key adaptation. I used multiple approaches to determine if the prevalence of the association and relationship to the limpets condition support this hypothesis. FISH probes specific to the 16S rRNA molecule of a gamma-Proteobacteria hybridized where bacteria were present. Direct sequencing using symbiont-specific primers gave a single unambiguous sequence. indicating high specificity. Light and TEM micrographs of gill tissue from a range of species also showed that the symbiosis is ubiquitous. In addition, the gills of in-vent animals had high surface area. dense symbiont populations and healthy tissues, while far-vent animals showed the reverse trend, suggesting that the symbiosis benefits L. fucensis. Carbon fixation by gill tissues was stimulated by inorganic sulfide and related to the abundance of bacteria on the gill. These data indicate a persistent and specific symbiosis that is dependent on access to sulphide. I further examined feeding by Lepetodrilus fucensis to determine if the bacteria contribute to their host's nutrition. The morphology of feeding structures were compared among Lepetodrilus species. L. fucensis exhibited specialized features: the gill is enlarged. the lamellae are free of the mantle. do not narrow and are stabilized by ciliary junctions. The radula and stomach of L. fucensis are also reduced. Shipboard observations confirmed suspension feeding by L. fucensis. In addition, the symbiont may be ingested because its phylotype was well-represented in food material on the gill. The limpet's morphological specializations are consistent with dependence on suspension feeding and/or symbiont farming; however, L. fucensis also grazes. a mechanism likely important in peripheral locations. Lepetodrilus fucensis populations are partitioned by size and sex along environmental gradients near vents. Peripheral populations are dominated by recruits and adults tend to be male: grazing is likely their primary feeding mode. Larger animals form stacks in venting fluids and are female-biased. These populations access suspended particles for food and sulphide, which generates dense symbiont populations for ingestion. Multiple feeding modes sustain high L. fucensis densities in a space-limited environment and may be an innovative strategy that drives its remarkable abundances.

The 1980 Vancouver Island Seismic Project (VISP) was conducted to investigate lithospheric structure associated with the underthrusting oceanic Juan de Fuca plate and the overriding continental America plate. The principal components of the survey were: (l) an onshore-offshore refraction line, which was approximately perpendicular to the continental margin (line 1), and (2) a refraction line which ran along the length of Vancouver Island approximately parallel with the continental margin (line IV). Lines I and IV were originally interpreted by Spence el a.1. (1985) and McMechan and Spence (1983), respectively. However since the original interpretations of these lines, deep multichannel seismic reflection data have been obtained on southern Vancouver Island as part of the 1984 LITHOPROBE project and off the west coast of the island during a marine survey in 1985. This study was undertaken to resolve differences between the subsurface structures proposed in the original interpretations of lines I and IV and those suggested by the more recently acquired deep reflection data. The vertical two-way traveltimes to prominent reflectors, observed in the onshore-offshore deep reflection data, were used as a constraint in constructing velocity models which are consistent with both the reflection and refraction data. The traveltimes and amplitudes observed in the VISP refraction data were modeled using a two-dimensional raytracing and asymptotic ray theory synthetic seismogram routine. The principal difference between the model originally interpreted for line I and the revised model involves the introduction of a twice repeated sequence of a low velocity zone (≈ 6.4 km/s) above a thicker high velocity zone (≈ 7.1 km/s) for the underplated region directly above the subducting Juan de Fuca plate in place of the single high velocity block underlain by a thick low velocity zone. The revised model for line IV is significantly different from the originally interpreted model. The two low-high velocity zones of line 1 are continued along the length of the island at depths between 10 and 35 km. Below this, the structure of the subducted plate is included to maintain consistency with the revised model developed for line 1. Additional features of the revised onshore-offshore model corresponding to line 1 include an oceanic lithosphere that dips approximately 3° beneath the continental slope, then 14° to 16° beneath the continental shelf and Vancouver Island, and an average velocity for the upper oceanic mantle of 8.22 km/s. Two separate two-dimensional models were needed to explain the data collected along line IV as a result of considerable azimuthal coverage due to a 30° change in profile direction. The revised models developed for line IV are consistent with the revised model developed for line 1. The velocity in the upper 10 km ranges from 5.5 km/s to approximately 6.7 km/s. Below 10 km the velocity structure is consistent with that interpreted for line 1 and shows some variations along strike of the subduction zone. Several possible interpretations can be made for the origin of the sequence of layers directly above the subducting plate beneath Vancouver Island. The two favored interpretations are: (1) a. three stage tectonic process consisting of: stage 1 — offscraping of sediment from the top of the subducting plate forms the uppermost low velocity layer in the sequence; stage 2 — an imbricated package of mafic rocks derived by continuous accretion from the top of the subducting oceanic crust forms the first high velocity layer; and stage 3 — stages 1 and 2 repeat themselves with stage 2 currently occurring; or (2) remnant, pieces of oceanic lithosphere left stranded above the current subducting plate during two previous episodes of subduction in which the subduction thrust jumped further westward isolating the remnant. The revised model along line IV indicates that this process of subduction underplating could have been a pervasive feature of this convergent margin. / Science, Faculty of / Physics and Astronomy, Department of / Graduate