Contributors:

Pamela Hallock, Department of Marine Science, University
of South Florida, 140 7th Avenue South, St. Petersburg, FL 33701

Anne Boersma, Microclimates, Inc., 540 Gate Hill Road,
Stony Point, NY 10980

Introduction

Deep-sea sediments, those found at depths greater than about 500 m, cover roughly two-thirds of the Earth. Not surprisingly, there are many kinds of deep-sea sediments. Fortunately, for someone learning about them, the predominant deep sediment is carbonate ooze, which covers nearly half the ocean floor. Even more fortunate for the marine geology student, by understanding a few simple concepts about the processes of deep-sea sedimentation, one can predict with a high degree of accuracy the kind of sediment found in any part of the ocean .

The basic principles to understand are source, means of transport, rate of supply, and potential for dissolution or change on the sea floor. The basic sources of the sediments found in the deep sea are erosion from land , eruption of volcanoes, production by pelagic organisms , and cosmic fallout. Means of transport, which applies mostly to sediments eroded from land, refers to whether the sediments were dispersed out over the oceans by wind, were transported to the deep sea by gravity flows, were conveyed far from shore by surface currents before settling out of suspension, or were carried and dropped by melting ice. Rates of supply for sediments eroded from land or erupted by volcanoes declines with distance from a source. Rates and types of production by pelagic organisms vary with nutrient supplies and temperature in the surface waters of the ocean. Potential for dissolution or change depends upon the chemistry of the water in the deep sea and in the deep-sea sediments themselves.

Summary of Deep Sea Sediments

Deep sea sediments may be grouped into several fairly simple categories. The descriptions are modified from Anderson. 1988
Terrigenous slide, slump, debris flows
turbidites
glacial marine
brown clays
Origin on land with proximity to glaciers, deserts, rivers, volcanoes or mounain belts determining the amount of terrigenous sediment available.
Pelagic brown clays
biogenic carbonate oozes Globigerina
Pteropod
Coccolithophore
biogenic siliceous oozes Diatom
Radiolaria
Biogenic sediments are controlled by nutrient supply, temparature, salinity, oxygen and carbon dioxide content and by the pH of the surface waters of the ocean. These same factors influence the deep waters through control of the CCD. The physical and chemical factors are influenced by positions of the continents relative to the major climatic belts of the planet.
Authigenic manganese nodules
phosphate nodules
zeolites
extraterrestrial
Authigenic sediments are controlled by locations and extent of black smoker activity on the ridges, by where upwelling zones are found and by super-slow processes that only produce abundant sediment if all other types are essentially non existent.
Other volcanic
wind-borne
extraterrestrial

Development of the Basic Tools

Francis Bacon, in 1620, noted the geographic evidence for continental movement, that is, the way the continents on either side of the Atlantic appear to fit together. Wegner, between 1912 and 1930, assembled evidence from fossils, rock types and structures indicating that during the Triassic, the continents were united into a single supercontinent that he called Pangaea. Unfortunately, Wegener lacked a credible geophysical mechanism to explain how and why the continents moved apart. As a result, his synthesis was discredited for more than 30 years.

Studies of successions of invertebrate fossils played a major role in the development of the science of geology in the 19th century. The terms Paleozoic, Mesozoic and Cenozoic refer to "ancient", "middle" and "recent" life. Deep-sea oozes recovered by the Challenger expedition (1872-76) contained abundant shells and skeletons of foraminifera and radiolaria, but these microfossils were not considered useful for correlation because it was assumed that deep-sea environments were unchanging through time. However, in the oil fields of the United States, Russia, and elsewhere in the early 1900's, geologists quickly recognized that fossil foraminifera are extraordinarily useful in determining the relative ages of rocks and in correlating rock formations from place to place. Thanks to their tiny size, hundreds of such microfossils are often found in small pieces of limestone or chalk.

Laying of the trans-Atlantic telegraph cables, which began in the mid-1800's, required knowledge of ocean depths. Depth measurements, tediously carried out using lead-weighted lines, resulted in the discovery of the mid-Atlantic ridge and other bathymetric features. During the first scientific expedition to survey the deep ocean, the Challenger expedition (1872-1876), the basic types of deep-sea sediments were described and classified by Sir John Murray. However, it was advances in engineering and geophysics in the first half of the 20th century that provided the technology necessary for the explosion in paleoceanographic research in the second half of the century.

Our earliest records of the ocean floor were sounding taken with a lead and line. In order to plumb the depths of the ocean, lengths of piano wire were used to lower a weight to the bottom. The Challenger expedition which ran from 1872 to 1876 took 492 deep sea soundings. Modern oceanography in the United States began with the establishment of Scripps Institution of Oceanography as a biological research station in 1903, and Woods Hole Oceanographic Institution in 1930.

Submarine warfare in World War I necessitated the development of electronic echo sounding, which was then applied to ocean depth measurements by the German Meteor expedition in 1925. This cruise carried two acoustic depth recorders which were used to make soundings every two to three nautical miles, giving new detail on the configuration of the sea floor. With the military expansion of practical oceanography in World War II, studies of transmission of sound in the ocean resulted in extensive use of fathometers in bathymetric surveys of the ocean.

Physical and chemical studies of radioactive and stable isotopes, including their detection and measurement, during WWII also provided breakthroughs that were directly applicable to paleoceanographic research. Rates of decay of radioactive isotopes were recognized as potential radiometric clocks for determining the ages of rocks and sediments. In 1947, H. Urey determined that stable oxygen isotopes fractionate at different temperatures during precipitation and evaporation of water. Emiliani, who is credited with founding "paleoceanography" as a field of research, recognized that the fractionation of isotopes of oxygen that are incorporated into the CaCO2 shells of marine organisms should record the oceanic temperature at which the shells formed. He proceeded to demolish the idea that the deep ocean environment has been constant through Earth history. Using 18O/16O isotope ratios in shells of benthic foraminifera, he showed that bottom water temperatures in the mid Cenozoic were several degrees warmer than at present.

The Tectonic Revolution

By the 1950's, the basic tools were available for marine geologists to begin the most important revolution in scientific thinking since Darwin's Theory of Evolution. Fortunately, research funding was also available, thanks in part to Cold War concerns about submarine warfare. The leadership and scientific vision of geoscientists Revelle of the Scripps Institution of Oceanography and Ewing of the Lamont Geological Observatory were instrumental in directing interest and resources to deep-sea geology. Ewing initiated and Heezen and Tharp developed and published the widely-used, detailed maps of the ocean floors . Soviet scientists were also actively involved in mapping ocean-floor features and sediment distributions.

The tectonic revolution in the Earth sciences really began in 1961 when Dietz of the U.S. Coast and Geodetic Survey proposed a theory of how the sea floor is created and destroyed, which he called "sea-floor spreading." A year later Hess of Princeton University proposed that plate formation and continental movement is driven by convection currents within the mantle. Hess postulated that ocean crust forms volcanically at ocean ridge crests, cools and subsides with distance from the ridge, and ultimately is dragged downward into oceanic trenches. The theory of plate tectonics developed quickly in subsequent years.

A Brief History of Ocean Drilling

The composition, distribution and age of ocean sediments could be studied without the context of the Theory of Plate Tectonics, but understanding and interpreting processes and patterns would be much more difficult. Also, without the stimulus to support or disprove this new theory, the resources that have been dedicated to ocean drilling over the past 35 years would not have been allocated. The contributions made by the scientists and administrators that developed and pursued the idea of drilling in the deep ocean and by the political leaders who made the financial resources available must also be recognized.

The first scientific drilling operations in the deep sea began in 1961 in 945 m water depth off southern California, drilling 1,315 m into the sea floor. Immediately thereafter, a second site in 3,558 m of water, known as the Experimental Mohole, was drilled off Baha California. This hole penetrated 183 m of sediment and 13 m of basalt, failing to reach the Mohole but demonstrating the feasibility of recovering scientifically valuable cores at depths well beyond the reach of the Kullenberg piston corer.

In 1964, four United States oceanographic institutions joined together as JOIDES, Joint Oceanographic Institutions for Deep Earth Sampling, proposing that the U. S. National Science Foundation (NSF) support drilling off Jacksonville, Florida. Six sites were continuously cored to sub-bottom depths of more than 1 km, revealing significant oceanographic changes on the east Florida margin since the Late Cretaceous. Well preserved planktic and benthic microfossils from the cores were instrumental in developing the biostratigraphic zonation schemes used today.

JOIDES then initiated the Deep Sea Drilling Project (DSDP), which originally proposed 18-months of ocean drilling in the Atlantic and Pacific Oceans. The NSF funded modification of a drilling vessel under construction; it was modified specifically for scientific ocean drilling, core recovery and analysis. The resulting Glomar Challenger spent 15 years drilling the ocean basins and providing geologic data to solidify the theory of plate tectonics, to develop the discipline of paleoceanography, and to greatly advance scientific understanding of Earth history and processes.

In the 1970's, other U.S. and international institutions joined JOIDES. In 1985, the Ocean Drilling Project (ODP) succeeded DSDP with dedication of a larger, more sophisticated drillship, the JOIDES Resolution . The ODP continues past its original 10-year mission. The scientific discoveries of DSDP and ODP have affected everything from oil and mineral exploration to predicting earthquakes and global-climate fluctuations. Yet those discoveries would not have been possible without such astonishing engineering feats as hole re-entry cones, advanced piston corers, and stabilization techniques that allow drilling in stormy Antarctic seas, which is further testimony to the interdisciplinary nature of the Earth sciences. Furthermore, these discoveries would not have been possible if the United States, Germany, France, Canada, Japan, the United Kingdom, and the European Science Foundation had not dedicated the monetary resources needed to undertake this level of scientific research.