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The North Sea's fattyness is, after its saltiness, a peculiar property, … It should be assumed here that in the ocean as on land there exists, here and there, seepages of running oily liquids or streams of petroleum, naptha, sulphur, coal-oils and other bituminous liquids.
Translated from Erich Pontoppidan, 1752
This chapter begins with a review of the pioneering work undertaken on the Scotian Shelf, off eastern Canada, by L. H. King and his colleagues at the Bedford Institute of Oceanography. However, having ‘cut our teeth’ in the North Sea, the pockmarks and seeps here have become the standard against which we compare those of other areas. Consequently it is appropriate to review our early studies of North Sea pockmarks. This provides a historical perspective on pockmark research, and indicates how this early work led us to the conclusions that pockmarks and seabed seeps are important geological phenomena and indicators of processes associated with seabed fluid flow. In some cases the sites we visited early on have been the subjects of further work. This is also reviewed here.
By the end of this chapter it becomes clear that seeps and pockmarks, along with the associated carbonates and biological communities, are components of the important hydrocarbon cycle.
Seabed fluid flow involves the flow of gases and liquids through the seabed. Such fluids have been found to leak through the seabed into the marine environment in seas and oceans around the world - from the coasts to deep ocean trenches. This geological phenomenon has widespread implications for the sub-seabed, seabed, and marine environments. Seabed fluid flow affects seabed morphology, mineralization, and benthic ecology. Natural fluid emissions also have a significant impact on the composition of the oceans and atmosphere; and gas hydrates and hydrothermal minerals are potential future resources. This book describes seabed fluid flow features and processes, and demonstrates their importance to human activities and natural environments. It is targeted at research scientists and professionals with interests in the marine environment. Colour versions of many of the illustrations, and additional material - most notably feature location maps - can be found at www.cambridge.org/9780521819503.
Migration is like Einstein's watch. Observations concerning its operation can be made, but since opening the system is not permitted, only hypotheses about its operation, consistent with these operations, can be made. The movement of hydrocarbons in the deep and shallow subsurface is a complex balance of processes. We can draw conclusions based only on our observations. We may never know if these conclusions are correct for any given situation.
Martin D. Matthews, 1996
Wide varieties of seabed features are formed as fluids migrate towards and emerge from the seabed; these include pockmarks and mud volcanoes. The nature and distribution of these features are dependent upon the supply of fluids, and the formations (rock and/or sediment) through which they migrate. To understand the formation of seabed fluid flow features and processes, various physical forces, including external triggers, must be considered.
A look at the exploration history of the important oil areas of the world proves conclusively that oil and gas seeps gave the first clues to most oil-producing regions. Many great oil fields are the direct result of seepage drilling.
Some marine geohazards, with the potential to affect offshore operations, are associated with seabed fluid flow. The petroleum industry, in particular, has learned from experience that these geohazards must be recognised and understood, and that safeguards must be in place if serious consequences are to be avoided. But seabed fluid flow is not always a negative thing. It provides resources such as methane and metals, and acts as a guide to others, particularly petroleum. However, the marine environment is sensitive and biological communities associated with vents and seeps are uncommon and fragile – they require protection.
Although no single, unequivocal proof exists for the hypothesis we present, a broad range of evidence suggests it. We expect reluctance by scientists to support such an idea at this stage without further testing, but if the hypothesis holds, it will require a major shift in thinking about what drives Quaternary climate change.
James Kennett, 2002, in an interview with Lifland about the reaction to his book: Methane hydrates in Quaternary climate change: The Clathrate Gun Hypothesis (Lifland, 2002)
Once through the seabed, fluids of any origin contribute to, and in some cases, significantly affect, the marine environment. The chemical, physical, and biological nature of seawater may be affected by seabed fluid flow. Even in the deep oceans, emissions from hydrothermal vents, cold seeps, and mud volcanoes are influential. However, a proportion of some components passes through the hydrosphere and enters the atmosphere. Methane is one of these components. Contributions to atmospheric methane by the oceans, and in particular the seabed, are poorly understood, but may be important not only to today's climate, but also to global climate change over geological time-scales.
The importance of gas in transmitting, marking and altering sediments and of its traces as clues to depositional, paleo-ecological and diagenetic history is not generally appreciated.
Gas generated beneath the seabed is buoyant, and tends to migrate towards the surface. Geological conditions may impede its progress, so shallow gas accumulations are formed. The exact nature of these accumulations depends on the type of sediment they are held in. Also, in certain pressure and temperature conditions, migrating gas may be sequestered by the formation of gas hydrates. Evidence is provided by various forms of acoustic signal recorded on seismic profiles, and is supported by the results of drilling, as well as the occurrence of natural gas seeps at the seabed.
Gas beneath the seabed is not a geological curiosity, but is common-place and widespread.
A major realization at this ‘meeting of minds’ was that many carbonate mounds, bioherms or reefs described in geological literature were in fact the product of chemosynthesis. Similarly, biologists became aware that chemosynthesis was deeply rooted in the geological past and may be traceable as far back as the origin of life itself.
Beauchamp and von Bitter, 1992
Mineral precipitation occurs at the seabed in two circumstances in association with seabed fluid flow: microbial utilisation of fluids, and in response to changes in physical and chemical conditions. This chapter starts with an investigation of the nature and origin of methane-derived authigenic carbonate (MDAC). This leads to a discussion of the influences of fluid flow on the formation of other types of marine carbonate including those associated with biological activity, such as stromatolites and deep-water coral reefs. Non-carbonate minerals are formed as a result of precipitation and fluid-flow processes. We discuss the metalliferous deposits formed by hydrothermal activity at ocean-spreading centres, and by cool-water submarine seepage. Finally there is a discussion of the possible modes of formation of ferromanganese nodules, including the hypothesis that they were formed as a result of seabed fluid flow.
The development of an observational science like geology depends upon the personal experience of individual workers. I suggest that, with certain reservations, the best geologist is he who has seen the most rocks.
H. H. Read
It is impossible to review all the data, reports, and publications of examples of seabed fluid flow. This chapter, organised as a round-the-world tour, comprises descriptions of examples of features associated with seabed fluid flow: pockmarks, seeps, mud volcanoes, and gas hydrates. The chapter provides the basic information required to undertake more analytical studies of these features, the processes that are responsible for them, and their implications.
‘Seabed fluid flow’ encompasses a wide range of fluids (gases and liquids) that pass from sediments to seawater, involving natural processes that modern science would pigeon-hole into a wide range of disciplines, mainly in the geosciences, biosciences, chemical sciences, environmental sciences, and ocean sciences; they also impinge on (or are affected by) human activities. With our background, it is inevitable that the most prominent fluid in this book is methane. There is a vast literature on hydrothermal vents, and a growing interest in submarine groundwater discharge with which we do not wish to compete. However, we recognise the importance of considering all forms of seabed fluid flow so that similarities and differences in the processes may be considered. We have attempted to assimilate all forms, manifestations, and consequences of seabed fluid flow of whatever origin.
It is impossible, in a single volume, to do justice to such a multidisciplinary subject. The pace of research has progressively increased since our own interests in pockmarks and seeps began. Of particular significance is the move of the petroleum industry from the continental shelves into the deeper waters of the continental slope and rise; this has rejuvenated research in deep-seabed processes, and has resulted in rethinking many old ideas not least because of the discovery of many deep-water features associated with seabed fluid flow.
Discovery commences with the awareness of anomaly, i.e. with the recognition that nature has somehow violated the paradigm-induced expectations that govern normal science.
This chapter introduces the concept that seabed fluid flow is a widespread and important natural process. It has important consequences for subseabed and seabed geological features, and also for marine biological processes, and the composition of the oceans. Seabed fluid flow provides both hazards and benefits for human activities, and it is recognised that some sites are precious and need protection.
Given the immense amount of organic matter in the sediments of the Earth's crust, it is evident that the geological consequences of this subterranean generation of gas must also be immense.
H. Hedberg, 1974
The fluids available to flow through the seabed include various types (liquid and gaseous, organic and inorganic) derived from microbiological and geological processes – some close to the seabed, some distant both in space and time. To provide a foundation for subsequent chapters in which we look at the fate of these fluids, this chapter provides a brief review of the nature and origins of the fluids, and identifies some areas where controversy still persists.
… geology concerns fluids every bit as much as it does solids. That is, geology usually conjures the idea of just the rock the earth is made of; but this rock is a locus of major interactions between fluids and solids. Fluids firstly concern the environments of sedimentary deposit, which differ according to location: salt ocean water, continental fresh water, atmospheric air, to name a few. Then the movements of the water tables in the beds running along accidents, evidenced by water springs, for example, are important phenomena. The oil and gas fields that petroleum explorers look for are yet another illustration of how fluids participate in geological processes.
Seabed fluid flow occurs in a wide range of oceanographic environments (coastal, continental shelf, slope and rise, and the deep ocean) and geological (plate tectonics) contexts (convergent, divergent, and transform plate boundaries, and intraplate settings). In this chapter each situation in which seabed fluid flow occurs is described. In some cases only a brief description is necessary. However, in others, particularly those that are less well known, more detail is appropriate.