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.

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.

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.

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.

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.

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.

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.

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.

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.

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.