11. Sedimentary Processes in the North Atlantic

The sediments of the North Atlantic north of 45°N do not seem to have attracted widespread attention prior to 1940, when Bramlette and Bradley published the results of their studies of cores obtained between Ireland and Newfoundland by the cable ship Lord Kelvin (Bramlette and Bradley, 1940). Since that time, however, a wealth of information concerning the sediments of this region has been gathered through the activities of ships sent out by institutions on both sides of the Atlantic. In a preliminary report, such as the present volume, it would not be appropriate to review all of this information in detail. Much of it remains unpublished and that which is published is widely scattered through the geologic literature. However the general nature and composition of oceanic sediments are fairly well known, and reviews of these topics are available (Murray and Renard, 1891; Shepard, 1948; Beall and Fischer, 1969; Weser, 1971; Pimm, Garrison and Boyce, 1971; etc.). There would seem to be little value in yet another descriptive catalogue. The purpose of the present contribution is simply to place the results of deep sea drilling on Leg 12 into the overall context of our present knowledge of North Atlantic sediments. Accordingly, we have tried to present only the broad outlines of present information in the hope that those more knowledgeable and with more time at their disposal will one day take the opportunity to assemble a more detailed and extensive presentation. Each site drilled during Leg 12 of the Deep Sea Drilling Project was drilled in order to obtain information relating to certain specific problems. The sediments sampled at each site have been described and discussed in relation to these problems in the foregoing site reports (Chapters 3 through 10, this volume). The sediments sampled at each site are summarized in Figure 1. We have endeavored to refrain from repeating detailed descriptions of the sediments. Rather, in this summary chapter we have tried to view the sediments of the northern North Atlantic in terms of the broad processes of sediment distribution. Nearly all the sediments of the northern North Atlantic between the Gibbs Fracture Zone and the latitude of Iceland have been transported long distances by various mechanisms before being deposited, and the source of the sediment is often difficult to determine. In some fields of sedimentology, for example recent fluviatile sedimentation, the broad provenance of the sediments, the processes of sediment dispersion and deposition and the environment of deposition are well defined. Thus a detailed analysis of the sediment deposits can lead to considerable insight and understanding of their development. Virtually all oceanic sediments however are polygenetic and individual sources of sedimentary material exert a correspondingly less significant influence over the nature of the resultant sedimentary deposit than do the agents of sediment transportation and deposition. In many cases even the traditional distinction between terrigenous and pelagic sediments is of little value in our attempts to understand the pattern of sediment distribution. As will be immediately apparent from the site reports, the details of the nature and composition of the sediments of the northern North Atlantic are complex. This complexity is to be expected. The North Atlantic is surrounded by land masses contributing sediments derived from virtually every known rock type and ranging from Precambrian to Recent in age. This region of the ocean, because of its geometry and geographic location, is oceanographically and biologically complex leading to a corresponding complexity of distribution of pelagic material. Thus it follows that the key to understanding sedimentation in the northern North Atlantic lies in viewing the sediments in terms of the processes of transportation and deposition. Once a working model has been developed it may become possible, through a detailed study of more drilled cores from more sites, to unravel the many strands brought together by the processes of transportation and thus by successive refinements, to develop the type of comprehensive model which can now be constructed for less subtle and less complex sedimentary situations. Sediments are transported from their source either by currents while suspended in water, or by winds while airborne and then by currents. A less important contribution comes from chemical deposition from the bottom water. The currents can either be driven by forces independent of the sediment load (for example, horizontal gradients in the potential density due to salinity and temperature, wind stresses, tides, etc.) or by gravitational forces depending on the density increases caused by the sediment load. Depending on these factors and on the prime source of the sediments we can distinguish three categories of sediment transportation mechanisms. (a) Turbidity currents driven by excess density due to suspended sediment (usually, but not always, terrigenous). (b) Deep ocean and bottom currents which are part of the normal ocean circulation, carrying sediment of all types (mostly fine). (c) Surface and near surface currents, mostly wind driven, which transport the living biomass which is concentrated in the euphotic zone, and fine terrigenous sediment. They also control ice-rafted sediments. It should be emphasized that this list is by no means all inclusive, but serves only to present the broad scale, major processes as a basis for discussion. We have not discussed, for example, secondary processes of sediment distribution, such as small scale reworking.