Richard A. Lutz

Volcanic eruption of the mid-ocean ridge along the East Pacific Rise crest at 9°45-52'N: direct submersible observations of seafloor phenomena associated with an eruption event in April, 1991

In April, 1991, we witnessed from the submersible Alvin a suite of previously undocumented seafloor phenomena accompanying an in-progress eruption of the mid-ocean ridge on the East Pacific Rise crest at 9°45′N–52′N. The volume of the eruption could not be precisely determined, although comparison of pre- and post-eruption SeaBeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were < 10 m high. Effects of the eruption included: (1) increased abundance and redistribution of hydrothermal vents, disappearance of numerous vent communities, and changes in characteristics of vent fauna and mineral deposits within the eruption area since December, 1989; (2) murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates; (3) destruction of a vent community by lava flows, mass wasting, and possible hydrovolcanic explosion at a site known as ‘Tubeworm Barbecue’ in the axial summit caldera (ASC) at 9°50.6′N; (4) near-critical temperatures of hydrothermal vent fluids, ranging up to 403°C; (5) temporal variations over a 2 week interval in both temperatures and chemical/isotopic compositions of hydrothermal fluids; (6) unusual compositions of end-member vent fluids, with pH values ranging to a record low of 2.5, salinities ranging as low as 0.3 wt% NaCl (one-twelfth that of seawater), and dissolved gases reaching high concentrations (> 65 mmol/l for both CO2 and H2S); (7) venting at temperatures above 380°C of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices; (8) disorganized venting of both high-temperature fluids (black and gray smoke) and large volumes of cooler, diffuse hydrothermal fluids directly from the basaltic seafloor, rather than from hydrothermal mineral constructions; (9) rapid and extensive growth of flocculent white bacterial mats (species unknown) on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid; and (10) subseafloor downslope migration of magma normal to the ridge axis in a network of small-scale (1–5 m diameter) lava tubes and channels to distances at least 100–200 m outside the ASC. We suggest that, in April, 1991, intrusion of dikes in the eruption area to < 200 m beneath the ASC floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor-rich hydrothermal fluids through the overlying rubbly, cavernous lavas. Low salinities and gas-rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid-vapor phase separation (boiling) and phase segregation. Hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and/or hydrovolcanic explosions. These fissures and pits were lined with white mats of a unique fast-growing bacteria that was the only life associated with the brand-new vents. The prolific bacteria, which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids, may thrive on high levels of gases in the vapor-rich hydrothermal fluids initially escaping the hydrothermal system. White bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic ‘blizzard’ up to 50 m above the bottom. The bacterial proliferation of April, 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling, and/or by grazing as other organisms are re-established in the biologically devastated area.

LARVAL ECOLOGY OF MARINE BENTHIC INVERTEBRATES: PALEOBIOLOGICAL IMPLICATIONS

1. Modes of larval development play important roles in the ecology, biogeography, and evolution of marine benthic organisms. Studies of the larval ecology of fossil organisms can contribute greatly to our understanding of such roles by allowing us to race effects on evolutionary time scales.

Temporal and spatial patterns of biological community development at nascent deep-sea hydrothermal vents (9°50'N, East Pacific Rise)

The April 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9°45′N and 9°52′N on the East Pacific Rise provided a unique opportunity to follow temporal changes in biological community structure from the “birth” of numerous deep-sea hydrothermal vents. In March l992, DSV Alvin was used to deploy an on-bottom observatory, the Biologic–Geologic Transect, to monitor faunal succession along a 1.37 km segment of the axial summit caldera between 9°49.61′N and 9°50.36′N (depth ∼2520 m). Photo- and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the Transect were performed in March 1992, December 1993, October 1994, and November 1995. Photographic and chemical time-series analyses revealed the following sequence of events in low-temperature venting areas. (1) Immediately following the 1991 eruption, hydrogen sulfide and iron concentrations in diffuse fluids were extremely high (>1 mmol kg-1) and microbially derived material blanketed active areas of venting in the form of thick microbial mats. (2) Mobile vent fauna (e.g. amphipods, copepods, octopods, and galatheid and brachyuran crabs) and non-vent fauna (e.g. nematocarcinid shrimp) proliferated in response to this increased biological production. (3) Within 1 yr of the eruption, areal coverage of microbial mat was reduced by ∼60% and individuals of the vestimentiferan tube worm Tevnia jerichonana settled gregariously in areas where diffuse flow was most intense. (4) Two years after the eruption, maximum levels of H2S decreased by almost half (from 1.90 to 0.97 mmol kg-1) and dense thickets of the vestimentiferan Riftia pachyptila dominated vent openings previously inhabited by Tevnia jerichonana. (5) Three years after the eruption, maximum hydrogen sulfide levels declined further to 0.88 mmol kg-1 and mussels (Bathymodiolus thermophilus) were observed on basaltic substrates. (6) Four years after the eruption, galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0.72 mmol kg-1. Also by this time mussels had colonized on to tubes of Riftia pachyptila. (7) Between 3 and 5 yr after the eruption, there was a 2- to 3-fold increase in the number of species in the faunal assemblages. In the absence of additional volcanic/tectonic disturbance, we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5–10 yr following the eruption. We also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase. We hypothesize that the above series of events (1–7) represents a general sequence of biological successional changes that will occur at newly formed low-temperature deep-sea hydrothermal vents along the northern East Pacific Rise and contiguous ridge axes. Megafaunal colonization at deep-sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life-history strategies of individual species, physical oceanographic processes, and the dynamic hydrothermal environment. Our observations indicate that the successful sequential colonization of dominant megafaunal vent species, from Tevnia jerichonana to Riftia pachyptila to Bathymodiolus thermophilus, also may be strongly influenced by temporal changes in geochemical conditions. Additional evidence demonstrating the close link between diffuse vent flux, fluid geochemistry, and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow. These correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast-spreading mid-ocean ridges should consider not only the interplay of species-specific life-history strategies, community productivity, and physical oceanographic processes, but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species.

Chemical speciation drives hydrothermal vent ecology.

The physiology and biochemistry of many taxa inhabiting deep-sea hydrothermal vents have been elucidated; however, the physicochemical factors controlling the distribution of these organisms at a given vent site remain an enigma after 20 years of research. The chemical speciation of particular elements has been suggested as key to controlling biological community structure in these extreme aquatic environments. Implementation of electrochemical technology has allowed us to make in situ measurements of chemical speciation at vents located at the East Pacific Rise (9° 50′ N) and on a scale relevant to the biology. Here we report that significant differences in oxygen, iron and sulphur speciation strongly correlate with the distribution of specific taxa in different microhabitats. In higher temperature (> 30 °C) microhabitats, the appreciable formation of soluble iron-sulphide molecular clusters markedly reduces the availability of free H2S/HS- to vent (micro)organisms, thus controlling the available habitat.

Ecology of deep‐sea hydrothermal vent communities: A review

Studies of the many active and inactive hydrothermal vents found during the past 15 years have radically altered views of biological and geological processes in the deep sea. The biological communities occupying the vast and relatively stable soft bottom habitats of the deep sea are characterized by low population densities, high species diversity, and low biomass. In contrast, those inhabiting the generally unstable conditions of hydrothermal vent environments exhibit high densities and biomass, low species diversity, rapid growth rates, and high metabolic rates. Biological processes, such as rates of metabolism and growth, in vent organisms are comparable to those observed in organisms from shallow-water ecosystems. An abundant energy source is provided by chemosynthetic bacteria that constitute the primary producers sustaining the lush communities at the hydrothermal sites. Fluxes in vent flow and fluid chemistry cause changes in growth rates, reproduction, mortality, and/or colonization of vent fauna, leading to temporal and spatial variation of the vent communities. Vent populations that cannot adapt to modified flow rates are adversely affected, as is evidenced by high mortality or lower rates of colonization, growth, or reproduction. Substantial changes in biota have been witnessed at several vents, and successional cycles have been proposed for the Galapagos vent fields. Dramatic temporal and spatial variations in vent community structure may also relate to variations in larval dispersal and chance recruitment, as well as biotic interactions.

Dispersal barriers and isolation among deep-sea mussel populations (Mytilidae: Bathymodiolus ) from eastern Pacific hydrothermal vents

Deep‐sea hydrothermal vent species are widely dispersed among habitat islands found along the global mid‐ocean ridge system. We examine factors that affect population structure, gene flow and isolation in vent‐endemic mussels of the genus Bathymodiolus from the eastern Pacific Ocean. Mussels were sampled from localities including the Galapagos Rift (GAR, 0°48′ N; 86°10′ W) and the East Pacific Rise (EPR, 13° N to 32° S latitude) across a maximum distance of 4900 km. The sampled range crossed a series of topographical features that interrupt linear aspects of the ridge system, and it encompassed regions of strong cross‐axis currents that could impede along‐axis dispersal of mussel larvae. Examinations of mitochondrial DNA sequences and allozyme variation revealed significant barriers to gene flow along the ridge axis. All populations from the GAR and EPR from 13° N to 11° S were homogeneous genetically and appeared to experience unimpeded high levels of interpopulational gene flow. In contrast, mussels from north and south of the Easter Microplate were highly divergent (4.4%), possibly comprising sister‐species that diverged after formation of the microplate ≈ 4.5 Ma. Strong cross‐axis currents associated with inflated bathymetry of the microplate region may reinforce isolation across this region.

Time-series temperature measurements at high-temperature hydrothermal vents, East Pacific Rise 9°49′–51′N: evidence for monitoring a crustal cracking event

Abstract Temperature measurements of hydrothermal vent fluids provide an important indicator of the physical and chemical state of mid-ocean ridge crest hydrothermal and magmatic systems. Changes in vent fluid temperature and chemistry can have dramatic effects on biological communities that inhabit these unique ecosystems. In an attempt to understand temporal variability of ridge crest hydrothermal activity as it relates to geological processes at the ridge axis, six high-temperature hydrothermal vents on the East Pacific Rise crest between 9°49′N and 9°51′N were instrumented and sampled repeatedly during five years following a submarine volcanic eruption in 1991. Bio9 vent, located on the floor of the axial trough near 9°50.2′N, has the most complete record of fluid temperatures from 1991 to 1997, including a continuous temperature record of nearly three years (1994–1997). Bio9 vent fluids were 368°C in 1991, increased to an estimated temperature ≥388°C after a second volcanic event in 1992, and thereafter declined over the next ∼2 years reaching a temperature of 365°C in December 1993. Continuous temperature records and point measurements made by Alvins thermocouple probe show Bio9 vent fluids were stable for ∼15 months at 365±1°C, until March 26, 1995. On March 26, an abrupt 7°C increase occurred over a period of eight days at this vent, and a maximum temperature of 372±1°C persisted for 14 days. The vent fluid cooled gradually over ∼3.5 months to 366±1°C, and for several months at the end of the recording period the temperature increased a few degrees. A continuous record of fluid temperature at this vent between November 1995 and November 1997 shows a 5±1°C increase for the two-year period. The abrupt temperature increase at Bio9 vent, and coincident changes in faunal community structure, and geochemistry of vent fluids from this area suggest that a crustal event occurred, either in the form of a cracking front in the crust or intrusion of a small dike. Based on the results of a microseismicity experiment conducted around the Bio9 vent in 1995 [Sohn et al., Trans. Am. Geophys. Union 78 (1997) F647; Sohn et al., Nature (in press)], and the identification of a small earthquake swarm which occurred on March 22, 1995 we conclude that the temperature anomaly measured at Bio9 four days following the swarm was caused by a cracking front penetrating into hot crustal rocks beneath the vent.

Fluid venting in the eastern Aleutian subduction zone

Fluid venting has been observed along 800 km of the Alaska convergent margin. The fluid venting sites are located near the deformation front, are controlled by subsurface structures, and exhibit the characteristics of cold seeps seen in other convergent margins. The more important characteristics include (1) methane plumes in the lower water column with maxima above the seafloor which are traceable to the initial deformation ridges; (2) prolific colonies of vent biota aligned and distributed in patches controlled by fault scarps, over- steepened folds or outcrops of bedding planes; (3) calcium carbonate and barite precipitates at the surface and subsurface of vents; and (4) carbon isotope evidence from tissue and skeletal hard parts of biota, as well as from carbonate precipitates, that vents expel either methane- or sulfide-dominated fluids. A biogeochemical approach toward estimating fluid flow rates from individual vents based on oxygen flux measurements and vent fluid analysis indicates a mean value of 5.5 + 0.7 L m -2 d -1 for tectonics-induced water flow ( Wallmann et al., 1997b). A geophysical estimate of dewatering from the same area (von Huene et al., 1997) based on sediment porosity reduction shows a fluid loss of 0.02 L m -2 d-1 for a 5.5 km wide converged segment near the deformation front. Our video-guided surveys have documented vent biota across a minimum of 0.1% of the area of the convergent segment off Kodiak Island; hence an average rate of 0.006 L m -2 d -1 is estimated from the biogeochemical approach. The two estimates for tectonics-induced water flow from the accretionary prism are in surprisingly good agreement.

Distinct patterns of genetic differentiation among annelids of eastern Pacific hydrothermal vents

Population genetic and phylogenetic analyses of mitochondrial COI from five deep‐sea hydrothermal vent annelids provided insights into their dispersal modes and barriers to gene flow. These polychaetes inhabit vent fields located along the East Pacific Rise (EPR) and Galapagos Rift (GAR), where hundreds to thousands of kilometers can separate island‐like populations. Long‐distance dispersal occurs via larval stages, but larval life histories differ among these taxa. Mitochondrial gene flow between populations of Riftia pachyptila, a siboglinid worm with neutrally buoyant lecithothrophic larvae, is diminished across the Easter Microplate region, which lies at the boundary of Indo‐Pacific and Antarctic deep‐sea provinces. Populations of the siboglinid Tevnia jerichonana are similarly subdivided. Oasisia alvinae is not found on the southern EPR, but northern EPR populations of this siboglinid are subdivided across the Rivera Fracture Zone. Mitochondrial gene flow of Alvinella pompejana, an alvinellid with large negatively buoyant lecithotrophic eggs and arrested embryonic development, is unimpeded across the Easter Microplate region. Gene flow in the polynoid Branchipolynoe symmytilida also is unimpeded across the Easter Microplate region. However, A. pompejana populations are subdivided across the equator, whereas B. symmitilida populations are subdivided between the EPR and GAR axes. The present findings are compared with similar evidence from codistributed species of annelids, molluscs and crustaceans to identify potential dispersal filters in these eastern Pacific ridge systems.

Evolutionary relationships among deep-sea mytilids (Bivalvia: Mytilidae) from hydrothermal vents and cold-water methane/sulfide seeps

A protein electrophoretic survey of mytilids inhabiting deep-sea hydrothermal vents and cold-water methane/sulfide seeps revealed electromorph patterns diagnostic of 10 distinct species. From hydrothermal vents located at sites on the Galápagos Rift, the Mid-Atlantic Ridge, and the Mariana Back Arc Basin, we detected four species of mytilids. Six additional species were detected from three cold-water seep sides in the Gulf of Mexico. The patchy distribution and temporal stability of seeps may provide a greater opportunity for mytilid diversification and persistence than vent sites Neis genetic distances (D) between species were relatively large (range: 0.528 to ∞) both within and among habitat types. This pronounced degree of genetic differentiation suggests a relatively ancient common ancestor for the group. Phylogenetic trees were generated using distance Wagner and parsimony analyses of allozyme and morphological characters. The tree topologies obtained from both methods support: (1) the hypothesis that a seep ancestor gave rise to the deep-sea hydrothermal vent mytilids, (2) a historical progression from shallow-water to deep-water habitats, and (3) a co-evolutionary progression from external to internal localization of bacterial symbionts. Whether the seep mytilid taxa constitute paraphyletic or polyphyletic groups remains unresolved. Our phylogenetic hypotheses also provide a benchmark for the phylogeny of mytilid bacterial symbionts.