Abstracts 2008

• Chuck Fisher
• Julie Huber
• Debbie Smith
• Doug Wiens

Chuck Fisher

Science Community Lecture: Chemoautotrophic symbioses: Making the Best of a Potentially Toxic Environment

Many marine invertebrates live in sulfide-rich habitats and have evolved mechanisms that allow them to survive in the presence of what would otherwise be toxic levels of sulfide. Some marine invertebrates have made a major evolutionary jump and acquired chemoautotrophic symbionts that can both detoxify sulfide and bring the benefits of autotrophy to the association. This type of symbiosis has arisen polyphyletically: it has evolved numerous times in a variety of distantly related animal groups. In all cases, the chemoautotrophic symbionts require both oxygen and a reduced chemical energy source, most often sulfide; however, because sulfide and oxygen will not coexist for significant periods of time in the same space, animals with chemoautotrophic symbionts must either live in areas where the two chemical substrates are constantly mixing, or separate the acquisition of sulfide and oxygen spatially or temporally. The hosts in these symbioses have evolved a variety of behaviors, life histories, anatomies, and/or physiological mechanisms to protect them from the toxic effects of sulfide, supply reduced sulfur compounds and oxygen to their symbionts, and allow them to utilize chemoautotrophic primary production as a main source of nutrition. In this talk, chemoautotrophic symbioses from a phylogenetically diverse group of hosts using a variety of different approaches to flourish in diverse marine environments, will be compared and contrasted.

One group, the vestimentiferan tubeworms, will be discussed in more detail since they have evolved what might be considered extreme adaptations to the chemoautotrophic life style and are the most intensively studied chemoautotrophic symbiont-containing group of animals. All known species of vestimentiferans have many features in common, including their rather unique anatomy, physiology, and blood properties; however, species living in different environments have also evolved important differences, including very different life histories and mechanisms of interacting with their environment. In the ephemeral but energy-rich hydrothermal vent environments, vestimentiferans like Riftia pachyptila can support extremely high rates of chemoautotrophy and grow very quickly to reproductive size. In cold seep environments, species like Lamellibrachia luymesi will grow much more slowly, but have adaptations for obtaining sulfide from sediments that allow them to live for hundreds of years. In either setting, these mouthless and gutless annelids can dominate the communities, and, in fact, can alter the chemical environment and provide habitat for numerous other species of vent and seep animals.

General Public Lecture: Life at the Edge: Evolutionary Adaptation to Extreme Environments

The deep mid-ocean ridge system is home to some of most extreme animal habitats found on Earth. In addition to the high pressure, low temperature, and lack of light characterizing the deep-sea in general, hydrothermal vent environments are characterized by toxic chemicals and temperatures that would be hostile to most other animal life. Perhaps even more challenging than the chemicals, dissolved gases, and temperature with which vent animals contend are the extreme gradients in time and space of all of these parameters. For example, temperatures around some vent animals can fluctuate from ice-water to hot-tub levels over seconds and can exceed 350° C only inches away. Specially adapted animals not only tolerate these conditions, they often thrive under them. As a result, the biomass of many deep ocean ridge ecosystems rivals that of coral reefs or tropical rain forests.

Although many vent animals appear quite similar to well-known shallow water species at first glance, when we take a closer look at their internal anatomy and physiology, we find amazing adaptations that allow these animals to flourish in their extreme habitat. These include clams with special feet and blood used to "mine" sulfide from cracks in the mid-ocean ridge lavas, mussels and snails that have giant gills filled with special bacteria to provide them with food, and shrimp that have lost their normal eyes and instead use patches on their backs to "see" the faint light of active hydrothermal vents. Even the most bizarre looking of the vent fauna have evolved from much more "normal" looking ancestors, and their evolutionary history is easily traced from their DNA. This group includes one of the hydrothermal vent "poster-children", the giant tubeworms of the East Pacific Rise. These tubeworms were at first thought to be so different from other animals that they were described in a new Phylum, created by taxonomists just for these tubeworms. However we now know from studies of their DNA that they are evolutionary cousins of the bristle worms, a large and well know group of marine animals.

During this presentation, video footage and high-resolution pictures obtained using submersibles and remotely operated vehicles will be used to introduce the audience to the mid-ocean ridge system, the deep-sea, and life in the ephemeral environment of hydrothermal vents. Numerous examples will be used to illustrate how evolution produces the wide variety of specially adapted animals that inhabit the deep-sea hydrothermal vents of the world.

Julie Huber

Science Community Lecture: Microbial Ecology of Subseafloor Crustal Communities

The subseafloor crustal environment represents one of the largest, yet least accessible and poorly studied environments on our planet. The subseafloor microbial community living within oceanic crust offers many opportunities to study both exciting and cutting-edge aspects of marine microbial ecology, including limits of life, molecular evolution, microbial diversity and biogeography, functional genomics of complex communities, origins of life, and biofilm formation. Yet the crustal biosphere remains undersampled, and our knowledge of what microbes are present and how they are distributed in this dynamic geochemical environment over time and space is fragmentary.

This talk will focus on subseafloor microbial communities at geographically and geochemically distinct deep-sea hydrothermal seamounts across the Pacific Ocean. All three locations, Axial, Loihi, and the Mariana Arc, are recently eruptive seamounts located above 2000 m and host diffusely venting fluids with high concentrations of carbon dioxide. However, their geological and chemical setting differs greatly; Axial is a mid-ocean ridge seamount with fluids dominated by high concentrations of hydrogen sulfide, Loihi is a mid-plate hotspot seamount with extremely high concentrations of dissolved iron (FeII), and the Mariana seamounts are at a convergent plate boundary and host a variety of fluids, including those with very low pH and high concentrations of particulate sulfur.

Using seafloor video footage and photos, we will explore these environments, from exploding volcanoes to lakes of molten sulfur, highlighting the diversity of habitats and the microbial communities they host. We will apply a variety of molecular and geochemical tools on deep-sea crustal fluids from these seamounts as a window into the subseafloor microbial community. The subseafloor biosphere is a unique and ubiquitous habitat on Earth and remains one of the most challenging environments to study. Much remains to be discovered in this exciting and complex ecosystem.

General Public Lecture: Pushing the Limits: Microbial Life at Deep-Sea Hydrothermal Vents

The world's oceans are teeming with microscopic life forms, encompassing a staggering amount of diversity. Although they are largely invisible to the naked eye, microbial communities of untold diversity dominate nearly every corner of our oceans, from the deepest marine sediments to the sun-drenched coral reefs. Despite their crucial role in elemental cycling and earth's evolution, the marine microbial world remains vastly undersampled, and our understanding of these microbial communities severely limited.

The deep-sea is one of the least explored parts of the microbial world, and until only 30 years ago, scientists believed that because it was devoid of sunlight and plants, there was no life there. The discovery of thriving animal and microbial communities at the Galapagos spreading ridge in 1977 changed our fundamental perceptions of life on planet Earth. The chemistry of hydrothermal vents creates many habitats for microbial and animal communities, all of which are intimately linked to and dependent on the geochemistry of their environment. These organisms encounter many conditions that we humans consider too extreme- too hot, too toxic, too little oxygen- but microbes seem to find a way and continue to push the limits of life.

An impetus for studying life at deep-sea hydrothermal vents is that life may have originated and evolved near hydrothermal systems, and that organisms currently living in these likely analogues of early habitats may still harbor characteristics of early life. In addition, microbes unique to the hydrothermal vents could provide insight into metabolic processes, strategies for growth, and survival of life on solar bodies with a water history, such as Mars and Jupiter's moon Europa. With a sample size of only one, the search for life beyond Earth must begin with life as we know it on Earth. Using examples from research expeditions around the globe, we will explore this extreme life on Earth at deep-sea hydrothermal vents.

Debbie Smith

Science Community Lecture: A graveyard of core complexes at the Equatorial Mid-Atlantic Ridge

Oceanic core complexes are large domes in which lower crustal and upper mantle rocks are exposed at the seafloor. Their tops show corrugations that run parallel to the spreading direction of the ridge. Core complexes form at mid-ocean ridges through slip on detachment faults rooted below the spreading axis, but why they form remains controversial. To date, most studies of core complexes have been based on isolated inactive domes that have moved away from ridge axes due to seafloor spreading. A recent survey of the slow-spreading Mid-Atlantic Ridge (MAR) near 13°N reveals a segment in which linked detachment faults extend for 75 km along one flank of the spreading axis. The detachment faults are apparently all currently active and at various stages of development. A graveyard of core complexes extends away from the axis for at least 100 km. Within the surrounding region there is a strong correlation between detachment faults at the ridge axis and high rates of earthquakes. Examination of seismicity and seafloor morphology farther north along the MAR suggests that active detachment faulting is occurring in many segments and that detachment faulting is more important in the generation of ocean crust at this slow-spreading ridge than previously suspected. This talk will examine the characteristics and occurrences of core complexes along the MAR, based on recent results and observations from sea-going expeditions. The relationship between core complex formation and hydrothermal venting and the evolution in our ideas about how slow-spreading ridges work will also be discussed.

General Public Lecture: Strange Seafloor Domes Not All That Strange Anymore

Scientists study long-overlooked features for clues to how Earth's crust is formed

Strange rumbling from the seafloor was detected in 2001 by underwater hydrophones near the equatorial Mid-Atlantic Ridge, part of the 40,000-mile undersea volcanic mountain chain that winds around the globe. Scientists have come to expect shakes, rattles, and rolls along mid-ocean ridges, where Earth's tectonic plates are spreading apart and magma from deep within the planet rises and erupts at the surface. But the seismic activity detected was coming from an area 35 miles west of the ridge—an area that should have been all quiet.

Looking to solve this mystery, however, we may have serendipitously found clues that shed new light on an even larger one: How the ocean's crust forms and evolves to shape the face of our planet. It turns out that a quirky seafloor feature, which scientists used to consider rare, may be rather common and play a key role in the evolution of Earth's surface. These features, which are shaped like domes, were formed by very long-lived extensional faults and bring up deep-seated rocks to the seafloor. They are called core complexes. One entire flank of the ridge about 60 miles (100 kilometers) long was covered with core complexes in different stages of development, from cradle to corpse the further you got from the ridge.

This talk will follow how we went from chasing the source of the strange deep-sea grumbling to the gold mine of core complexes.

Doug Wiens

Science Community Lecture: Imaging mantle flow and melt production beneath backarc spreading centers and island arcs

The mantle underlying volcanic island arcs associated with subduction is the locus of some of the most important geological processes on the planet. These processes include production of crust to form new sea floor at back-arc spreading centers and the production of new silicic crust at the volcanic islands. Mapping the seismological structure of this mantle 'wedge', between a subducting plate and the overriding lithosphere, is essential for understanding magma production, transport, and mantle flow patterns in this complex region. Deployments of ocean bottom seismographs at several island arcs provide data for imaging the structure of the mantle wedge. Most of the region, extending from the volcanic arc into the backarc spreading center, is characterized by extremely low seismic velocities and high attenuation of seismic energy. The low velocity region beneath the spreading center extends nearly the entire width of the backarc basin, suggesting a broad region of magma production at depths of 30-90 km. Variations in seismic velocity, spreading center elevation, and major element compositions of the back arc basin basalts suggests that upper mantle temperature varies by about 100°C between different backarc regions, with the Lau backarc mantle showing the warmest temperatures and the Mariana backarc showing the coldest.

The lowest seismic velocities beneath the volcanic arc generally delineate an inclined region extending above the subducting slab from about 150 km depth up to the arc volcanoes. This low velocity region may result from small quantities of partial melt induced by fluids given off by the subducting slab. If magma is widely present in the upper mantle beneath the arc or backarc it is at melt contents of 2 percent or less. The slow velocity anomalies associated with the volcanic arc and backarc spreading center mantle are separate at the shallowest depths, but merge at depths of greater than 100 km. This suggests that any direct geochemical interchange of melt between the arc and backarc spreading center occurs at depths of greater than 80 km.

The mantle flow pattern beneath island arcs influences many processes, such as the path of melt from the slab source region and the distribution of geochemical anomalies. Seismic anisotropy can provide direct evidence for the pattern of solid flow in the mantle, since constituent olivine minerals align during flow. The orientation of the fastest seismic velocity marks the direction of maximum extensional strain, which is approximately flow-parallel. Fast anisotropy directions parallel to the volcanic arc, indicating arc-parallel flow within the mantle wedge, are found beneath most island arcs. In some cases this pattern is confirmed by geographical patterns of geochemical anomalies. Geodynamical modeling suggests that along-strike flow may be facilitated by temporal or spatial changes in slab dip, slab rollback, and a low viscosity region in the mantle wedge beneath the volcanic arc and backarc.

General Public Lecture: Repaving the earth's surface

Unlike the other terrestrial planets or the moon, most of the earth is completely resurfaced approximately once every 100 million years. The earth's mid-ocean ridge system produces about 2 square miles of new ocean floor each year, made up of 8 cubic miles of newly formed basalt and gabbro rock. How does this amazing repaving machine work?

Any repaving project requires some viscous material such as cement or asphalt to solidify and form a new surface. For the mid-ocean ridge system, the paving material is basaltic melt. This melt forms from rising hot currents flowing in the earth's mantle, which is made up of solid peridotite rock but can still flow slowly. The melting temperature of the peridotite decreases as the rock becomes shallower, so that eventually the rock partially melts. This basaltic melt is less dense than the surrounding rock and so makes it way up to the surface in small voids between mineral grains and in magma veins and channels. Most of the melting to form mid-ocean ridge basalts occurs within a zone several hundred miles wide beneath the ridge at depths of 10-50 miles into the mantle.

When the melt reaches the crust, it collects in magma chamber at a depth of 1 or 2 miles beneath the ridge. The top of the magma chamber is a thin region with nearly pure molten rock, whereas the deeper parts of the magma chamber are made up mostly of gabbroic rock that has crystallized out of the magma, with only a small portion of molten rock present. Some magma will now erupt onto the seafloor, forming underwater volcanos and lava flows, but other magma is cooled in place by water circulating through cracks in the seafloor. Water heated by the magma is ejected into the oceans through hot springs, providing heat and energy to sustain a diverse population of organisms in the dark depths of the ocean.

Although we understand the basic processes outlined here, there are still many questions. How much of the upper mantle beneath the ridge is molten? What is the shape and size of the melt producing region? How fast does the melt rise up to the surface? What causes melt produced over a wide region to collect in a small magma chamber beneath the ridge? How deep does the hot water circulation penetrate beneath hot springs along the ridge? Modern technology now allows us to explore mid-ocean ridges deep beneath the ocean surface to find the answers to these and other questions. I will discuss my own research which involves placing seismographs on the ocean bottom to record earthquakes associated with eruptions and to image the mantle magma production system deep beneath the seafloor. I will also show short, seafloor exploration video clips of scientists collecting rock samples and sampling seafloor hot springs with manned submersibles. This exploration of the deep ocean floor is slowly revealing how the earth's repaving machine works.