marine geology

marine geology The oceans cover 71 per cent (approximately 360 million km²) of the Earth's surface. Marine geology is the study of the nature and history of the earth beneath the sea. The origins of marine geology lie in the development of submarine telegraphy in the latter half of the nineteenth century. For the first time it became imperative to find out the depth and nature of the sea bed, knowledge essential for routing and operating submarine telegraph cables. Early cable laying ships were the first to make routine soundings and recorded the shoal topography of the Mid-Atlantic Ridge between America and Europe. Increasing interest in the oceans led the British Admiralty to sponsor the first large-scale truly oceanographic expedition and during 1872–6 HMS Challenger circumnavigated the globe making the first systematic collection of oceanographic and geological data, ultimately visiting all the oceans and travelling over 100 000 km. However, our knowledge of the shape and structure of the sea floor, and the processes that shape it, progressed only modestly from the late nineteenth century until the Second World War. The development of the sediment corer was a major innovation, allowing the recovery of sediment sequences from the sea floor and analysis of the geological history contained within them. Several geophysical techniques for investigating the sea bed and its substructure, such as sonar, gravity, magnetic, and seismic methods, which had largely been experimental in the 1920s and 1930s, were developed during the Second World War and became major geological surveying tools during the subsequent great expansion of oceanographic research, which continues to the present day. In the late 1960s, these tools played a major role in revolutionizing the Earth sciences by providing evidence for plate tectonics. For the first time, the major features of the oceans basins could be explained as the edges of plates, where new crust is created (at mid-ocean ridges) and destroyed (at ocean trenches).

Today, marine geologists use a variety of tools to investigate the ocean floor. These are largely either imaging techniques (geophysical mapping and profiling or observation using cameras) or direct sampling.

The depth of the ocean floor is conventionally measured using echo-sounders, in which depth is determined by measuring the time between the emission of a sound pulse and the detection of the echo from the sea floor. If the speed of sound in the water is known, then depth can be calculated. However, modern survey vessels commonly use swath bathymetry systems that use a fan of sound beams to produce highly accurate contoured strips of sea floor whose width may be up to twice the water depth. Acoustic plan-view images of the sea floor can also be obtained using sidescan sonar, in which an array of transducers is used to illuminate the sea floor with sound on either side of the survey vehicle. The sound is backscattered from the sea bed according to the topography, sediment type, and small-scale bottom roughness. The resulting acoustic image provides the equivalent of aerial photographs of the sea floor (Fig. 1). Swath bathymetry (which provides topographic maps) and sidescan sonar (which provides ‘aerial’ views) can be combined using computational tools such as geographical information systems (GIS) to provide detailed geological information. High-resolution views of the sea floor can be obtained by fixing sidescan sonar apparatus to deep towed instrument platforms that operate within a few hundred metres of the sea floor. Although deep-towed sidescan sonar will have a higher resolution than near-surface towed vehicles, the width of coverage will be narrower and the survey speed much slower. A range of instruments can be attached to deep-towed vehicles for high-resolution work. These can include geochemical ‘sniffers’ which can locate hydrothermal plumes and hence proximity to hydrothermal vent fields, where metal-rich, often superheated, water exits from the sea bed after having passed through oceanic crust in circulation systems driven by magmatic heat.

The substructure of the sea floor can also be investigated using sound waves, by using a frequency of operation that allows the sound to penetrate the sea floor and to be reflected back from interfaces between different types of sediment or rock. This forms the principle of seismic reflection profiling, in which a picture of the subsurface structure is built up along the track of the ship. Surveys using closely spaced track lines can be used to build up three-dimensional pictures of the geological structure beneath the sea floor.

The sea floor can be directly sampled using dredges, corers, and grabs (Fig. 2). Dredges are metal frames attached to chain-mail type bags, which, when dragged over rocky topography, break off pieces of rock which are collected in the bag. Corers recover continuous sequences of sub-sea floor sediment. There are various types. Most cores taken today are box cores or piston cores. Box corers are lowered on to the sea floor and recover shallow blocks of sea bed, preserving the sediment– water interface. A piston corer consists of a tubular barrel driven into the sediment by a large weighted head. The barrel contains a piston which reduces the internal friction, effectively sucking the sediment into the barrel. Piston corers can take cores up to 50 m in length. Analysis of cores recovered by this method has revolutionized our understanding of recent climate change.

The ability to core long intervals beneath the ocean floor became a reality with the birth of the Deep Sea Drilling Project (DSDP) in 1968. This project was conceived by scientists from American oceanographic research centres, who set up a parent organization, Joint Oceanographic Institutions for Deep Earth Sampling (JOIDES), and aimed to investigate Earth history by drilling the deep ocean floor using the dynamically positioned drill-ship Glomar Challenger. On its first cruise cores were recovered from 140 metres below the sea floor in the Gulf of Mexico and contained traces of oil, providing the first direct evidence that oil occurred in ocean depths deeper than the continental shelves. Later the same year, Glomar Challenger drilled a transect of holes across the flanks of the Mid-Atlantic Ridge between Dakar, Senegal, and Rio de Janeiro, Brazil. The recovered cores conclusively showed that the age of the Earth's crust directly beneath the overlying sediment pile increases systematically with distance from the mid-ocean ridge crest, validating the theory of plate tectonics and continental drift. On DSDP Leg 13, gypsum, a mineral produced by the evaporation of sea water (an evaporite) was recovered from a drill site in 2000 m water depth in the western Mediterranean, in sediments 5 million years old. Gypsum is typically found in lagoons along arid coasts. The only reasonable explanation for this is that the Gibraltar gateway to the Atlantic became closed and the Mediterranean Sea evaporated to dryness, resulting in a gigantic desert basin over 2 km deep between Europe and North Africa, although there were periodic marine incursions. Some 700 000 years later, the Gibraltar gateway subsided a little and the connection to the Atlantic was fully re-established, restoring marine conditions to the Mediterranean Basin.

Such was the success of DSDP that in 1975 an international phase was initiated. Research organizations in Britain, France, West Germany, the USSR, and Japan participated in the project. The International Phase of Ocean Drilling (IPOD) was superseded by a new international research programme: the Ocean Drilling Program (ODP), which commenced operations at sea in 1985 with a new drillship, JOIDES Resolution, which had improved capabilities. In 1997, ODP provided striking evidence for what was behind one of the great mysteries of Earth history: the extinction of the dinosaurs and many other organisms 65 million years ago. Geologists on land have recovered continental cores from this time containing iridium (an element abundant in extraterrestrial material), shocked quartz and soot, probably derived from global wildfires. This pointed to global devastation, probably caused by an asteroid, estimated to be 10 km across, crashing into the Earth at a location that has since been identified as the Yucátan Peninsula in Mexico. On ODP Leg 171B, JOIDES Resolution recovered a core from this time interval at a drill site 560 km east of Florida and 2000 km from the now-buried Yucátan impact crater. These sediments of 65 million years ago contained a layer 15 cm thick of ejecta, which included glassy spherules condensed from the vapour cloud produced by the impact and dust and ash produced by the resulting fireball. Today, the Ocean Drilling Program continues to be the world's leading international marine geological research program and comprises a partnership of 22 countries.

Marine geology is one discipline of oceanography but it under pins the other disciplines in that the record of the sea floor reflects the chemical and physical evolution of the oceans above. Study of the deep sea floor and the record it preserves is therefore essential to understanding Earth history and the oceans themselves.

R. G. Rothwell

Bibliography

Open University Course Team (1998) The ocean basins: their structure and evolution (2nd edn). Butterworth-Heinemann, Oxford.
Seibold, E. and and Berger, W. H. (1996) The sea floor: an introduction to marine geology (3rd edn). Springer-Verlag, Berlin.
Summerhayes, C. P. and and Thorpe, S. A. (1996) Oceanography—an illustrated guide. Manson Publishing, London. (Chapters 8, 9, 10, and 20 in particular.)