Fluid Seeps at Continental Margins: A Report of a Workshop Defining Critical Research Issues Affecting Geology, Biology, the Oceans and the Atmosphere

Compiled By:

J. Casey Moore
Earth Sciences
UC Santa Cruz
Santa Cruz, CA 95064

Summary

During the last 15 years fluids seeps have been discovered along many continental margins, although hydrocarbon seeps have been known at some localities for many years. Seeps, both dominated by hydrocarbons and aqueous fluids, are now known to be a general feature of the geohydrogeology of continental margins. These fluid systems support biological communities, alter the rocks they flow through, reflect processes at depth, and affect the chemistry of the ocean and atmosphere.

Because of the importance of fluid seeps in earth sciences we organized a field trip, symposium, and workshop on "Fluid Seeps at Active Continental Margins" at the AAPG Pacific Section Meeting in spring of 1999. With the background of the field trip and symposium, a workshop outlined critical issues for research on seeps at the earth's surface, on associated biological communities, on the geochemical impact of seeps, and on seeps in the subsurface and in the stratigraphic record. Here we report the deliberations of this workshop.

Quantifying the flux of fluids from the lithosphere into the hydrosphere and atmosphere emerged as one of the most important research goals of this workshop. This endeavor requires extensive surveying of continental margins, establishing observatories, and calibration of remotely sensed images in terms of geomorphology, biology, fluid chemistry, and fluid flow rate. Statistical models will be required to extend our necessarily limited investigations to a global perspective. Once the distribution of and flux from seeps is known we will have to ascertain how the fluids interact with and alter the lithosphere below. Evaluating fluxes and understanding fluid interactions will allow us to estimate the influence of fluid seeps on biological resources and on the chemistry of the ocean and atmosphere, as well as the role of seeps as geological hazards.

In addition to the major question of their flux, seeps offer exciting opportunities for fundamental research on chemosynthetic microbiology and biological mediation of mineral precipitation. To understand the dynamics of seeps we need to study their subsurface plumbing, measure the pressure and density variations that may drive them, and model the fluid flow. Careful studies of seeps in the stratigraphic record are necessary to evaluate their influence on the geologic and paleontologic record. Better industrial and international cooperation, and accelerated information exchange through a web site could accelerate research on fluid seeps.

Introduction

Offshore and coastal oil and gas seeps have been known many years in hydrocarbon provinces such as Southern California and the Gulf of Mexico. In the 1980s and 1990s submersible and remotely operated vehicle (ROV) investigations discovered numerous seep sites along the continental margins globally (Figs. 1,2). These results indicate that seep phenomena are not restricted to hydrocarbon provinces but are a general feature of the geohydrologic system of continental margins.

Seep systems are complex with processes of interest to both fundamental and applied science (Fig. 3). Seep fluids (liquids and gases) include, natural gas (Fig. 4; hydrocarbon gas, carbon dioxide, nitrogen, hydrogen sulfide, and other gases) and oil. Obviously seeps can be direct indicators of hydrocarbon migration. Seeps support chemosynthetic biological communities (Figs. 5,6) and, through their carbonate precipitates (Fig. 7), form hardgrounds for non-chemosynthetic organisms. Submarine and subaerial seeps provide a view from the shallow (Fig. 8) into the deep-seated subsurface, and link the lithosphere, biosphere, hydrosphere and atmosphere. Some submarine seeps hydrologically connect to terrestrial groundwater systems.

In order to synthesize the current understanding of seeps at continental margins and their influences on geology and the environment, we sponsored a field trip, symposium, and workshop on this topic at the Pacific Section Meeting of the American Association of Petroleum Geologists in Monterey CA, April 28 through May 1, 1999. The field trip guide with related papers (Garrison et al., 1999) and symposium abstracts (AAPG, 1999) are published. This report outlines the results of a one day workshop that looked forward to the critical issues for research on seeps systems.

Seeps are distinguished by lower temperatures and lower flow rates than hydrothermal vents at spreading centers. Seeps are associated with consolidating sedimentary basins, especially at continental margins, whereas vents generally lie over young oceanic crust and are driven by temperature contrasts between magma bodies and seawater. Here we focus on submarine and terrestrial seeps at continental margins.

Critical Issues for Future Research on Fluid Seeps

The critical issues for future research on fluid seeps are organized under the manifestations of seeps at the sea bed or land surface, the biological influence of seeps, the geochemical effects of seeps on the oceans and atmosphere, and seep phenomena in the subsurface and as preserved in the stratigraphic record. In each case we outline the importance of the issue, the state of knowledge in the area, and what needs to be done to advance our knowledge.

A. Fluid Seeps at the Earth's Surface

Group Leaders: Daniel Orange and Sara Foland

The earth's surface provides an interface for the observation of fluid migration (Fig. 3). The surface of the lithosphere also is the interface at which humans interact directly with seeps, whether observing subaerial springs and travertine deposits or avoiding seeps in siting offshore platforms. This interface provides a boundary across which we can measure flux, observe effects on biology, trace effects upward into the ocean and atmosphere, and infer influences downward into the subsurface. Hence it is the starting point for our consideration of fluid seeps.

A1) What is the Distribution of Seeps and the Flux From Them?

Importance: Quantification of the flux of seep fluids from the lithosphere is required to ascertain their influence on the oceans and atmosphere (see C below). We need to know the distribution of seeps and understand any process that a particular seep type may indicate. This will allow us to evaluate the impact of seepage, including geologic hazards.

State of Knowledge: We have learned to recognize seeps through the hardgrounds and biological communities that they form, as well as by the direct indicators of fluids such as oil slicks, and chemical and thermal signals. We know that seepage is both focused and diffuse and we have widely scattered measurements of rates. Our global inventory is growing (Figs. 1, 2), but continental margins remain largely unknown with respect to seep geology.

What Needs to be Done: We need to initiate more systematic surveys on a representative sampling of continental margins to quantify the types and locations of seeps. The systematic surveys will rely on remote sensing technology (various types of acoustic surface and subsurface imaging techniques) with ground-truthing by direct observation (submersibles, ROVs and examination of terrestrial systems). Mapping needs to detect oceanographic / lithologic patterns at scales relevant for interpretation of biological phenomena. Flux measurements should be made in some smaller portion of the imaged area, both where direct indicators of focussed flow occurs and where it does not (diffuse flow areas). Because it will be impossible to make these surveys and flux measurements everywhere, reasonable statistical models need to be used to extrapolate the measurements to global estimates of flux.

A2) How Does the Geomorphic Expression of Seeps Vary With Fluid Flux, Composition and Concentration?

Importance: A variety of apparently seep-related geomorphic features exist on the seafloor (Figs. 2) and are identifiable in acoustic images. We need to calibrate these acoustic images in terms of the processes forming the various seep features and their associated flux rates, fluid chemistries, fluid compositions, and faunal associations. Then, we will be able to use the efficient acoustic surveying tools to evaluate seepage phenomena along large areas of continental margins.

State of Knowledge: Recognized fluid-induced geomorphic features include seep precipitates (carbonates and hydrates), pockmarks, piping/rills, brine pools, and mud volcanoes. The geomorphic signature of these features has been well described at scales ranging from kilometers to meters. Although fluid expulsion features have been identified, we have little understanding of the processes that form them. We know that fluid seeps vary with the chemistry of the expulsed fluids. For example fluid seeps with predominantly methane verses sulfide chemosynthetic sources have been distinguished. Moreover, some threshold of chemical flux is necessary to reach the threshold of production of macrofauna at the earth's surface.

What Needs to be Done: Each of these geomorphic seep types needs to be investigated to ascertain the key processes in their formation and the geologic framework in which they occur. All structural or stratigraphic controls must be documented. We need to identify the fauna, the precipitates, and measure the chemistry and rates of fluid flow at various geomorphic seep types. The role of transients and periodic activity needs to be documented. Accordingly, seeps must be instrumented for extended periods, with sufficiently high sampling rate that transients and periodic events can be documented. All the information needs to be categorized by geomorphic seep type so that the relatively easily acquired visual images can be effectively interpreted.

A3) What Geologic Hazards Result From Seepage?

Importance: Fluid seepage and associated phenomena may control a large range of geologic hazards. We need to better understand how these processes work to better protect humans from natural disasters (e.g. large submarine landslides) or avoid hazards as human technology moves into environments where seeps are common, such as into deepwater along continental margins. Some important questions include: Why does slope failure occur in some places and not others? What is the relation of fluid expulsion and slope failure to hydrate stability and sea level? What is relationship of seeps to hazards such as liquid sands (overpressure) and corrosion associated with anaerobic oxidation of methane. Is the upper limit of hydrate stability a zone of increased fluid flux and increased probability for initiation of slope failure?

State of Knowledge: The link between fluid expulsion and slope failure (seepage-induced spring sapping) has been modeled and demonstrated in a number of settings. The head gradients measured at such seeps are insufficient to trigger failure, suggesting either transient phenomena or that failure is driven by other mechanisms. We know that hydrate accumulates on the seafloor and in the shallow subsurface at some localities. We suspect that human activity I these areas, such as placement of a drilling platform, could de-stabilize these hydrate accumulations. Circumstantial cases have been made for development of basal shear plane of submarine landslides near the base of the gas hydrate stability zone.

What Needs to be Done: The various types of geologic hazards (e.g. landslides, shallow overpressures) need to be systematically studied in relation to surface manifestations of seepage and imaging of the subsurface so that we can develop the capacity to predict the hazardous areas and avoid them. Working with industry, we could use well and geotechnical data in combination with seafloor studies and mapping to examine the relationship between seeps and shallow overpressures ("liquid sands"), seeps and corrosion, and seeps and slope failure. We need better information on the distribution of gas hydrates in the subsurface and an effort to develop good tools to remotely sense the concentration of hydrate.

B. Biological Interaction with Fluid Flow Systems

Group Leaders: Ian MacDonald and Jim Barry

Importance: Biological interaction with fluid flow systems can be broadly defined as any persistent colonization of a fluid flow regime on the seafloor or in the subbottom fluid volume. This can involve direct chemosynthetic or secondary utilization of fluid components by a microbial community (e.g., Figs. 5,6). Various annelids, crustaceans, and bivalves have evolved strategies for exploiting microbial productivity through symbiotic partnership or browsing free-living cells. Such species either feed on the chemosynthetic components or, less directly, benefit from habitat or attachment substrata generated by protracted geochemical alteration in a fluid-discharge regime.

The accumulation of authigenic carbonates and other seep-related, biologically mediated minerals is significant in regulation of flow rates and flow pattern and may be a significant term for global balances of ocean / atmospheric chemistry. Thus, understanding how bacteria and higher organisms make a living from fluid flow has broad significance. There are other cross-cutting applications for seep biology as well. For example, assemblages of species that depend on seep processes are widely used as robust tracers of locations where fluid seepage is ongoing (see A2 above). Also, anomalous aggregations of sessile organisms are interpreted as an indication of past seepage. Such interpretations are key to identifying paleo-seeps in the sedimentary record (See D2, below). Because seepage is a persistent feature of continental margins, the biology of seep and vent biota is potentially important in understanding evolution and diversity of life on earth and other planets. Biology is also important for seep studies because it provides a point of entry for public interest in seep science.

What is Known: Seep biology and ecology is a rapidly maturing field, but it remains influenced by the sequence of scientific discovery. Prior to the seminal discovery of "chemosynthetic communities" at hydrothermal vent fields in the early 1980's, exploitation of chemosynthetic microorganisms by more evolved taxa was entirely unknown. Although analogous seep communities were discovered in the Gulf of Mexico, off Oregon and Japan, and elsewhere within less than five years, the focus of basic research has remained primarily on hydrothermal vent examples.

In this context, many details of biological adaptations and characteristics enabling chemosynthetic life are now known. Parameters that distinguish seep fauna from vent fauna have been the subject of many recent and ongoing studies, which have produced some reliable results. Notably, temporal patterns of recruitment, growth, and reproduction/propagation appear to be strongly influenced by the different temporal and spatial scales of venting versus seepage. Generally, venting and associated geochemical alteration of the seafloor is characterized by higher volumes and more rapid fluctuations than seeping. Seep fauna are consequently characterized by slower growth and lower metabolic rates than their vent analogs, at least where data exists to make direct comparison. Generally, the degree specialization in more evolved organisms commonly associated with chemosynthetic microogranisms is less at seeps than at vents. Molecular genetics has rapidly transformed ideas about the evolution of vent and seep fauna, but the sampling grid is still far too coarse and haphazard to define rigorously the biological variability of these global systems.

What Needs to be Done: Interestingly, some of the greatest uncertainty, and most rapid discovery, concerns chemosynthetic microbiology. Despite the pivotal role of chemosynthetic symbionts, for example, none have been cultured in isolation from host tissue and their molecular characterization still faces serious obstacles. Likewise, the extents of the habitat for free-living chemosynthetic organisms s is subject to very vigorous debate and must be defined at the surface and in the subsurface.

Many researchers have noted the great advantage that seep fauna have over human researchers. They (the seep organisms) sample their environment constantly, often preserving a record of processes or events that would otherwise go completely undetected. Certainly this is the case in the fossil record, a record that we must learn to interpret (see D2, below). Overall, however, care should be taken to interpret seep biology from a biologically defensible basis. The approach needs to be interdisciplinary, with sampling of multiple processes.

Additional, specific approaches are briefly outlined as follows: 1) Microbial mediation of fluid / precipitate chemistry should make broader use of molecular, biomarker, and isotopic techniques. 2) Molecular taxonomy should study larval biology / ecology of key species as well as adaptive behavior and physiology. 3) Microelectrodes suggest an approach for measuring chemical species at biologically significant scales. 4) Seafloor observatories and associated in situ analyzers (video, chemical, fluid flow, heat flow, seismicity, sulfate reduction, methane flux) are needed. 5) Animal-substrate interactions should quantify the role of animal behavior / physiology in substrate modification and bioturbation (e.g. ventilation of sediment column by clam bio-irrigation).

C. Geochemisty of Seeps: Influence on the Oceans and Atmosphere and Clues to Fluid Origins

Group Leaders: Miriam Kastner, Keith Kvenvolden, and Bruce Luyendyk

Importance: Seeps are a defined link in geochemical cycles between the lithosphere, biosphere, hydrosphere, and atmosphere (Fig. 3). Seeps rich in greenhouse gases (Fig. 4) (methane and carbon dioxide) can also indirectly affect the biosphere by causing atmospheric changes leading to global warming.

Hydrocarbon seeps, which consist of oil and/or hydrocarbon gases, provide from the surface a view of shallow and deep subsurface processes. These seeps have been used in petroleum prospecting and have led to the discovery of major oil and gas fields. An inventory of hydrocarbon seeps is important to understand the magnitude and effect of petroleum pollution resulting from mankind's activities. Hydrocarbon gas seeps also affect the atmosphere by adding an unknown component of greenhouse gases.

Water and gas and/or mixed seeps likewise provide a window to deep-seated processes and insights on the hydrology and physical state at depth. Water and gas seeps in the ocean transport solutes through various transfer loops in all types of continental margins. The various transport loops influence volatile element cycling, (i.e., rare gases, C, N, S. halogens, and others), their geochemical mass balances, and global fluxes. The transfer of greenhouse gases to the atmosphere is mostly through volcanic activity, and the transfer of material and volatile elements to the deep mantle is through subduction. Meteroic water seeps must play an important role in the global hydrologic cycles and in the transport of solutes and pollutants from the continents into the ocean. Submarine, together with subaerial seeps, play a key role in coupling between the hydrologic and atmospheric fluid cycles vis-‡-vis the carbon cycle.

State of Knowledge: Seep fluids are chemically diverse, consist of liquid, gas, and oil, and of various mixtures of these end members. The more common gases in seeps are CH₄, CO₂, N₂, and H₂S. Methane dominated seeps have been studied most extensively. Seeps of shallow and/or deep sources, cold or warm, saline to fresh, have been characterized. The relative global importance and distribution of each type is, however, unknown.

Seeps alter the water and sediment chemistries as well as the sediment physical properties. At seepage sites where especially CH₄, CO₂, and H₂S discharge, the water chemistry, particularly O₂ and dissolved inorganic carbon (DIC) concentrations, and thus the pH, are strongly affected, and the d¹³C of the DIC altered. This is reflected in the d¹³C of benthic foraminifera calcite shells and in the wide range of authigenic carbonates that occur as chimneys, cements, or slabs. Other important seep related authigenic minerals are barite, sulfides (mostly Fe sulfides), tar, and gas hydrates. Especially the carbonates and barite could be useful guides for mapping paleo-seeps sites. Precipitates may also seal seeps and change patterns of fluid flow.

Locations of prolific hydrocarbon seepage are known on the continents, for example in the Caspian Sea region, and major seeps in the marine environment are also known to occur, for example, offshore from southern California and in the Gulf of Mexico. Except at a few regions, little is known about the rate of seepage, and the flux of hydrocarbons from seeps is difficult to measure because many seeps are small, episodic, and ephemeral. Even on land the integrated rate of hydrocarbon seepage is not known.

What Should be Done, Research Goals: The potential effects seeps have on ocean and atmospheric chemistry, thus on global change and on biology, requires that the impact of marine seep emissions on the oceans, the atmosphere, and the biosphere be quantified and modeled. Research goals include:
(1) The global impact of seep gases on the atmospheric greenhouse gas budget and atmospheric contamination and on oceanic margin chemistry needs to be determined.
(2) The effects of marine seep discharge on the ocean biosphere must be understood. and
(3) The transport of meteroic water and pollutants into the ocean through continental margins seep systems must be quantified.

In order to achieve the above goals, data on regional and global inventories of the numbers and distributions of seeps of each type, including hydrocarbon seeps, and flux estimates for the key constituents that are being emitted are required (see A1 above). These data would be essential for geochemical mass balance studies. Before this occurs, reliable methods need to be determined and developed to detect seeps, characterize their discharge products, and quantify flow rates from them.

Observations need to be monitored over time, and methods to be able to acquire continuous time series data sets that will determine the life history of the various types of seeps and their plumbing systems need to be developed. Examples of key questions to be answered are: Is seepage steady state on a short (observation) time scale? Is it episodic, and does it correlate with episodic tectonic and volcanic events? What starts seepage and what ends it? Can stratigraphic data be found that bear on the life history of seeps (see D2, below)?

Such accurate data taken over appropriate time scales are needed to build a calibrated statistical model for the flow hydrology of the various seep types. The input data required for such a model are the seep fluxes, the sources of the seep fluids, and the plumbing systems. Generalized 2D, 3D, and 4D seep models are needed that take into account inputs and outputs and the important seep variables such as seep fluid chemical composition and geologic setting.

What Should be Done, Strategy: A strategy for the study of seeps should include seep detection, characterization of emission products, quantification of fluxes, and study of time variations. An interdisciplinary approach must be used as the issues cross borders of geochemistry, physical oceanography, geophysics, hydrology, and biology. Expertise will be required in remote sensing, acoustics, fluid dynamics, and tracer chemistry.

As part of determining a global inventory of seeps, remote detection systems need to be employed. Detection methods need to be selected that are valid and cost-effective. In addition to methodologies for interrogation of individual seeps, obtaining information of composition and rate of flow for both terrestrial and marine locations will be essential. Likely methods include satellite remote sensing, sonar backscatter, geochemical sniffer surveys, detecting anomalies in temperature and water chemistry, and utilizing ocean floor monitoring systems. Biologic communities can be used to detect various types of seeps.

Besides usual methods to characterize seep distribution and products a special effort is required to understand the plumbing systems of seeps. Diagnostic tracers need to be identified that provide information on fluid sources. These could include radiocarbon, deuterium, radium, tritium, and helium isotopes. Contaminants in meteoric water such as CFCs or SF6 can be used as tracers. Research should be directed toward the identification of new tracers.

Quantification of emissions can be developed using direct capture of seep emissions, calibrated sonar backscatter, chemical tracers and sniffers, optical measurements of bubble populations, and flow rate measurements on the sea floor, and resistivity measurements to name some current technology.

An approach to establish time series of seep behavior could include (1) remote sensing by repeated satellite imagery and sonar surveys, (2) direct water column sampling with sniffer and CTD measurements, for example; complementary data are necessary to establish causal relationships between seepage and seismicity or rainfall for instance, and (3) in situ monitoring of discharge rates, seep products, and physical properties. Systems should be developed to be placed both directly over a seep and to monitor the behavior of a seep field at a distance from the focused discharge.

D. Subsurface Plumbing of Seeps and Paleoseeps in the Stratigraphic Record

Group Leaders: Kathy Campbell, Peter Eichhubl, Bob Garrison Janet Yun

D1) Subsurface Plumbing of Seeps

Importance: Identification of fluid migration pathways in the subsurface provides insight into hydrogeologic processes in sedimentary basins and continental margins (Fig. 8). The geometry of flow pathways is necessary to understand how fluids move from the deep subsurface to the surface during sediment dewatering, compaction, and deformation. In the subsurface, these pathways can be controlled by stratigraphy, structure, and diagenetic features, including gas hydrate. Fluid conduits can be identified at subsurface depths ranging from kilometers to less than a meter by using seismic reflection profiling of varying frequencies and well information. Combining surface seep distribution with subsurface data on flow paths can determine plumbing geometries, and lateral and vertical extent of fluid systems. These geometric observations combined with pressure and temperature at depth may lead to insights on what drives fluid systems. For example we need to understand the relative importance of advection verses convection and have some understanding of residency times of fluids. Investigations of subsurface plumbing and dynamics directly link to studies of the petroleum system, migration issues, and explicit concerns of the hydrocarbon industry.

State of Knowledge: In the academic community, fluid flow in the subsurface has been the focus of intensified research in the past decade. The link between subsurface plumbing with surface manifestations of seepage is a topic of interest in both academia and the petroleum industry, yet it is not well-understood. Topics of active research include the effects of structural discontinuities (faults, fractures) on basin and reservoir scale fluid flow, non-linear dynamics of basinal fluid flow, and extent of mass transfer.

Shallow gas and gas hydrate have been identified in the subsurface at all types of margins through imaging with seismic data and by direct sampling in boreholes. Gas hydrates also occur locally in seafloor outcrops. Because of their tendency to melt and produce copious amounts of free gas, gas hydrates are of great interest in the petroleum industry, and in communities interested in geologic hazards and global change.

What Needs to be Done: The connection of subsurface fluid flow to surface seepage is, with exception of terrestrial hydrothermal systems, commonly missing in studies of seeps. This gap in knowledge is in part due to an insufficient amount of geophysical and well data for a given site. We need to study the of interaction of seepage fluids with near-surface fluids (sea water, meteoric water), effects of near-surface cementation on sealing of flow pathways, and the use of geochemical and fossil criteria indicative of near-surface conditions in plumbing systems that are applicable to outcrop studies. We should also focus on studying microbial/rock interaction in shallow and deep fluid migration settings and possible evidence in the geologic record. In addition to defining geometries and lithologies in the subsurface, measurements of temperature, pressure and fluid composition will ultimately provide the data with which the seep system dynamics can be conceptualized and modeled.

Gas hydrates are important in subsurface plumbing because they comprise huge reservoirs of gas (mostly methane) and they may form barriers to migration. We need to determine the distribution of gas hydrates in order to understand the plumbing of the system. In addition to their effects of plumbing, a primary question in the gas hydrate field is how much exists? We need to determine the distribution of free-gas and gas hydrates in sediment, initially through direct sampling and more globally by use of geophysical proxies calibrated at the sites of direct sampling. The hydrate and free gas distributions must be related to subsurface stratigraphy and structure, and surface seeps, including surface accumulations of gas hydrate.

D2: Seeps in the Stratigraphic Record

Importance: Subaerially exposed paleoseep sites provide a temporal record of various types of fluid seeps (e.g. hydrocarbon, brine) and their associated biota. They also make possible fine-scale visualizations of fluid flow systems in three dimensions, define the temporal evolution of the fluids, and allow reconstructions of the paleohydrology in individual seep systems in sedimentary sequences undergoing burial and deformation. These subaerial outcrop studies provide a resolution that exceeds anything possible using submersibles, ROV's, or seismic profiling in the study of modern seeps. Study of paleoseeps also may allow us to determine whether different tectonic regimes of seepage (e.g. forearc faults, serpentinite diapirs, salt tectonics, etc.) produce different products.

The origins of seep biota and changes in vent-seep communities through time have fundamental implications for the possible origin and evolution of life in extreme environments. In addition, the stratigraphic record of seep fossils elucidates the biogeographic distribution of chemosynthetic and other seep-associated taxa in the past. Hence, this record enables us to test current models/predictions of how vent-seep faunas have migrated among sites (e.g. biologic versus tectonic controls).

State of Knowledge: For the past decade, individual researchers or research groups have worked on specific paleoseep deposits, or on isolated local problems in a given system. This work has provided a scattered record of vent-seep occurrences in the geologic past as well as a general overview of changes in vent-seep biota through time in different geologic settings. Among significant findings of these studies are the following:

1. Evolution of seep/vent faunas: To date, approximately 17 macrofossil-rich hydrocarbon seep systems have been recognized in Phanerozoic strata of Jurassic to Pliocene age (Fig. 1). Economic geologists, stratigraphers and paleontologists have recognized and exploited many additional hydrothermal and epithermal deposits (not shown on map) that also contain macroinvertebrate and microbial microfossils, or biomarker signatures for archaebacteria . With regard to macrofossil-rich localities, Paleozoic to early Mesozoic hydrothermal vent and cold-seep faunas appear to be dominated by brachiopods, whereas the Late Jurassic to Holocene interval has more diverse, "modern" seep faunas, especially chemosymbiotic bivalves. This shift in dominant taxa of vent-seep settings through time mirrors brachiopod-to-bivalve trends seen in other marine sedimentary environments (e.g., reefs, shelves, etc.), except that the community change came later in undersea spring/seep settings. The Permo-Triassic has an apparent "gap" in hydrothermal vent and cold seep deposits and faunas, and little is known about the Precambrian record of hydrothermal vent and cold seep deposits. Fossil cold seep deposits seem to be concentrated in the Lower Cretaceous and upper Eocene to Miocene, for reasons that remain unclear.
2. Many paleoseep carbonate bodies have geometries and internal structures that are very similar to modern marine seep carbonates, suggesting the potential to reconstruct time-averaged hydrological, chemical, and biological conditions.
3. Similar cement petrographic sequences exist in many paleoseep carbonates implying similar geochemical processes in the evolution of individual systems, from birth to death and burial.
4. Ancient seep sites occur in a variety of geologic/tectonic settings including passive, convergent and transform margins, paleo-submarine slides, and serpentinite diapirs.
5. Many paleoseeps show pronounced structural control of fluid flow, manifested by alignment of seep carbonates and faunas along fractures and faults. Some paleoseeps have strong evidence for substantial horizontal (e.g bedding-parallel conduits) as well as vertical fluid flow.

What Needs to be Done: A long-term goal should be characterization of the entire geologic record of paleoseep occurrences and their faunas through interdisciplinary studies and tabulation of individual occurrences. These investigations should focus on a number of key questions.

1. Are the gaps in seep occurrences in the geologic record real and, if so, do they reveal something about larger plate tectonic cycles? For example, does the apparent Permo-Triassic gap represent a shut-down of vent-seep systems during assembly of Pangea, and, if so, which "refugia" paleoenvironments harbored vent-seep faunas at that time?
2. Do times of apparent concentrated seepage (e.g Early Cretaceous) coincide with times of atmospheric/oceanographic change, and thus have a possible genetic relationship, or are they simply artifacts of local tectonic conditions that were "ripe" for fluid seepage?
3. How are the products of fluid flow (e.g., carbonates, fauna) linked to the interplay of biological-chemical-tectonic processes through time?
4. Are there definitive signatures for ancient microbe-mineral interactions? Are there ways to recognize paleoseepage that did not produce carbonates or seep faunas? Are there criteria for the recognition of paleo-gas hydrates?
5. Do different types of paleoseep deposits develop in different tectonic/geologic settings? Here we need to determine the variations in the control of subsurface structure on fluid seepage and effects of diagenetic processes on seep chemistry.
6. What can be learned about the paleohydrology of seep systems through careful study of individual fossil seep sites? What characterisitics of paleoseeps are most revealing about ancient flow conditions? Do paleoseep systems at individual sites show consistent patterns of change over time that may reflect inception and shutdown of fluid flow?
7. Fresh water fluid seeps on passive margins are a major type of modern cold seepage. Was this also true in the past? How can we recognize this important category of cold seeps?

Answering the above questions will require refinement of paleoseep identification methods, and continuing identification and systematic studies of paleoseep sites throughout the geologic record. Most of these studies will probably be on outcrops, with supplementary information from boreholes and high resolution seismic records.

General Perspectives Affecting Critical Issues

A number of items will assist studies of fluid seeps across many of the topics discussed above. These general concerns include defining locations that are especially prospective for seep studies as well as various actions that will foster more effective industrial/academic and international cooperation.

Prospective Locations for Investigations of Seeps

Several active seep regions have been identified already that at first could be investigated further in order to get a first order idea of the magnitude of the problem. Examples are: the Santa Barbara Channel, the Sea of Okhotsk, Azerbaijan and the Caspian Sea, the Black Sea, and the Gulf of Mexico margin. Subduction zones and accretionary prisms are regions of widespread and various seep fields. Potential study areas are Cascadia and Nankai margins, and studies should also be pursued in the Aleutian and Middle American trench systems which are known to have abundant active seeps.

Suggestions for Better Academic/ Industry and International Cooperation

Understanding fluids and fluid flow is the stock and trade of the oil industry. Therefore academic scientists need to seek more stable, long-term interactions with industrial partnerships that recognize a synergy of interests between them and petroleum companies. Academic researchers need to take observations of fluid systems at the surface, at outcrops, and in the subsurface and compare them to focused investigations of petroleum migration undertaken by colleagues with applied interests. Academic scientists need to directly engage their petroleum industry colleagues and visa versa. Scientists in the oil industry should attempt to make data and other resources available to colleagues in universities and at research institutes.

It is essential that a study of fluid seepage be tied to existing global initiatives such as the MARGINS/InterMARGINS and directly linked with the Ocean Drilling Program (ODP). A successful seeps initiative is intimately dependent on ODP because seeps only provide a 2D view at the surface. To understand the geochemistry and hydrology of seeps the 3D and 4D linkages to the deep-seated sources can only be established through borehole studies, especially including ocean drilling. We also need to interact with established programs focusing on groundwater discharge in the coastal zone that are part of the International Geosphere Biosphere Program.

Independently and in parallel there is also need to develop individual projects in a coherent fashion and link active state of the art research on seeps into the global research community. We need to increase international collaborations and communications and to promote jointly funded research opportunities among groups of research scientists and students. These groups should be interdisciplinary and include paleontologists, stratigraphers, sedimentologists, structural geologists, organic and inorganic geochemists, and marine geologists/geophysicists. They would identify key studies needed and evolve appropriate research strategies for study of different type of seeps. Among specific steps to encourage such collaborative endeavor are:
1) Create a website to serve as a repository of information on seeps/paleoseeps, as a clearinghouse for new data, and as a newsletter for ongoing activities.
2) Establish funded research conferences (including support for international travel) that focus on key subtopics.
3) Establish an International Geological Correlation Project (IGCP) specific to the global record of seeps/paleoseeps, a project that promotes the exchange of information and sponsors conferences at different field localities around the world every two or three years.
4) Establish a new research initiative, similar to the LExEn program (Life in Extreme Environments) sponsored by the NSF and NASA, that focuses on paleoseeps, and seek sponsorship/funding from NSF, PRF, NERC, and other funding agencies.

Acknowledgements

We acknowledge the UC Energy Institute for providing funding for our symposium and workshop. We thank Monterey Bay Aquarium Research Institute for use of their luxurious conference facility for the workshop and the Pacific Section of the American Association of Petroleum Geologists for sponsorship of the field trip and symposium.

References

AAPG, 1999, AAPG Pacific Section Meeting Abstracts: American Association of Petroleum Geologists Bulletin, v. 83, p. 681-706.

Garrison, R. E., I. Aiello, and J. C. Moore, eds., 1999, Late Cenozoic fluid seeps and tectonics along the San Gregorio Fault zone in the Monterey Bay Region, California, v. GB-76: Bakersfield CA, American Association of Petroleum Geologists, Pacific Section, 156 p.

SCOR/LOICZ Working Group 112:, 1999, Magnitude of submarine groundwater discharge and its influence on coastal oceanographic processes. Working Group 112 of The Scientific Committee on Oceanic Research (SCOR) and The Land-Ocean Interactions in the Coastal Zone (LOICZ) programme element of the International Geosphere-Biosphere Programme (IGBP) http://www.jhu.edu/~scor/WG112.html

Workshop Participants: Emails

Participants Mailing Addresses:

Figure Captions

Figure 1. Global distribution of modern and ancient fluid seeps. Modern seep and pockmark distribution from Hovland and Judd, 1988, with additions. (K. Campbell, personal communication, 1999).

Figure 2. Map of Monterey Bay region showing the locations major fault zones and fluid seep localities. Note localities of ancient seepage features (Fig 6) near Santa Cruz. Red lines are active faults, dashed where inferred. Black lines are inactive faults. S.G. F.Z. and M.B. F.Z. indicate San Gregorio and Monterey Bay fault zones, respectively. Image created my Norman Maher, Monterey Bay Aquarium Research Institute. © Monterey Bay Aquarium Research Institute, 1999.

Figure 3. Schematic cross of seepage system showing the role of faults stratigraphy, and salt in focusing fluid flow. Seafloor manifestations of seepage include direct expulsions of oil and gas, gas hydrates, biological communities and precipitates of various types.

Figure 4. Large clams contain "chemosynthetic" symbiotic bacteria that survive on fluid-borne hydrogen sulfide. White barrel is device for sampling fluids. Tube worms in background are also chemosynthetic. Locality offshore Oregon.

Figure 5. Bacterial mat (Beggiatoa sp.) from Santa Barbara Basin. Width of view is about 40 cm. © Monterey Bay Aquarium Research Institute, 1999. Photo supplied by Peter Eichhubl.

Figure 6. Gas plume, probably mostly methane, from Santa Barbara Basin. Width of view is about 80 cm. © Monterey Bay Aquarium Research Institute, 1999. Photo supplied by Peter Eichhubl.

Figure 7. Cylindrical to conical carbonate seep structures from Smooth Ridge in Monterey Bay (Fig. 2). A. Carbonate seeps structures on seafloor. Diameter of structure closest to camera is about 50 cm. B. Detail of seep structure; diameter of sample about 30 cm. These features probably form in the shallow subsurface and are exhumed by submarine erosion. Photos courtesy of Monterey Bay Aquarium Research Institute, © Monterey Bay Aquarium Research Institute, 1999.

Figure 8. Bedding plane exposure of carbonate seep structures in upper Miocene outer shelf deposits near Santa Cruz CA (Fig 2). Note similarity of cylindrical structures to examples from Monterey Bay (Fig. 5). Width of view about 8 m. Photo courtesy of Bob Garrison.