The Geology of the Atlantic Ocean

Ancient human remains and artifacts occur in Africa and Asia where evolution of man has been traced through many stages from early hominoids about 4 × 10⁶ years ago (Kalb et al., 1982; Totten, in preparation). Oldest dates for human fossils and artifacts in North America and South America are about 100,000 years (Carter, 1980) with no evidence of evolution from ancestral stock. This means that man first became acquainted with the Atlantic Ocean along the coasts of Africa and Europe. Overwhelming evidence indicates that the earliest immigrants arrived in North America from Asia via the land bridge that existed across Bering Strait during a time of low sea level associated with glaciation. This first immigration may have been during the early Wisconsinan (Würm) glacial stage prior to about 100,000 years ago. Artifacts at this time were so simple that humans must have subsisted mostly on small game, fish, and gathering of plants (Willey, 1966, p. 29–37). Nevertheless, humans spread through the un-glaciated parts of North America and on to South America (Müller-Beck, 1967; Haynes, 1970; Willey, 1971, p. 28; McNeish, 1976; Dumond, 1980) as representatives of the Neanderthal Mousteroid culture. Paucity and uncertain identification of artifacts limits knowledge about this early colonization, and some pre-12,000-year datings have been revised to much younger dates (Bischoff and Resenbauer, 1981).

Most of the Earth’s interior is far beyond access by visual observation or direct sampling, and therefore concepts of its nature must be based upon indirect geophysical measurements. The simplest geophysical measurement of the Earth is its surface dimension. At least as long ago as 550 B.C., Pythagoras believed the Earth to be a sphere, because the sun and moon were observed to be so. About 350 B.C. Aristotle, tutor to Alexander III of Macedon, agreed, noting that the Earth casts a circular shadow during eclipses of the moon. Moreover, at the many seaports of the Mediterranean coasts there must have been innumerable observations that only the sails of ships were visible at a distance, their hulls being concealed by the curve of the Earth between the seaports and the ships. This probably was the reason why the Pharos (lighthouse) at Alexandria built in 280 B.C. was so high (135 m). Presumably, on the basis of such information, Aristotle guessed the circumference of the Earth to be 400,000 stadia; at 1 stadium = 185.2 m, this 74,000-km circumference is 85 per cent too large.

Isotopic dates of anorthosites from the Moon and meteorites indicate that the age of the Solar System is about 4.6 b.y. Modal lead ages of volcanic rocks and ores suggest that the age of the Earth is 4.55 to 4.6 b.y.; thus the major melting and crustal-forming events closely followed accretion of the Earth and the other planetary bodies. Until 1.0 b.y. ago plate growth may have been accommodated mostly by vertical or horizontal displacements and buckling and shearing of plates (Kröner, 1981). According to Condie (1982), continental rifting began about 2.0 b.y. ago and became widespread about 1.0 b.y ago, when continental fragmentation occurred and oceanic basins were produced and destroyed by sea-floor spreading. This cycle was important for the Pan-African system in northern Africa, Brazil, and other areas around the North Atlantic Ocean. However, most of the Pan-African mobile belts are ensialic (only sial), suggesting that this was a time of transition between dominantly ensialic tectonics and modern plate tectonics. Continental fragmentation followed by sea-floor spreading was the dominant tectonic style with the opening of the North Atlantic (Iapetus oceanic basin) 700 to 500 m.y. ago (Fig. 101). This Paleozoic sea-floor spreading episode was followed by closing of the Iapetus basin and then by renewed sea-floor spreading that initiated the present ocean-floor cycle during the Mesozoic Era (Fig. 101, 103) and produced the present structural configuration of the Atlantic Ocean. The position and morphology of the transition from continental to oceanic crust was controlled by the construction of the mega-continent Pangaea, and the subsequent breakup of this landmass.

This chapter is intended as a concentration of base data on stratigraphy, petrography, and fossil content of drift (post-rift) sediments in the Atlantic region. Some regional generalizations about the role of tectonic and other geological processes are drawn; broader generalizations are summarized in the chapter on Evolution of the Ocean Floor. Before details are discussed, a general description of the drift supersequence is needed. This includes differences from sediments of the earlier synrift supersequence, separation of the two super sequences by a broad unconformity, and the control over drift sedimentation exerted by continental-margin subsidence, sea-level changes, and climate changes.

Thickness, depths, ages, and structural features of sedimentary strata on the ocean floor were presented in the previous chapter on the basis of geophysical measurements and drill holes. This chapter is intended to provide information on physical, chemical, and biological compositions of the sediments and inferences about sources, pathways, and rates of deposition of the sediments and their components. Most of this information is better obtained from surface sediments, because vastly more surface samples and short cores are available than samples from deeper and older sediments that are obtained from drill holes and at places where tectonic movements or ocean-floor erosion has exposed older strata in outcrop. Finally, a knowledge of modern surface sediments is needed in order to be able to recognize abnormal properties of older sediments that may indicate changes in sources and depositional conditions.

In any large synthesis such as this one, there comes a place where one should stand back from the detailed data to reach generalizations about their significance, the “big picture.” In fact, of course, such generalizations often are reached before completion of detailed analysis of data on the basis of assessments developed while data are being accumulated or being organized. The generalizations to be discussed here are a mixture of assessments made prior to and during detailed studies, and revisions made after completion of detailed studies. As with all generalizations, these ones are subject to further change as new information becomes available through wider application of old techniques, invention and use of new techniques, or development and testing of new paradigms. This section of our synthesis is not a summary of the main part of the book, because it does not attempt to include all of the subject material, which was summarized previously anyway in words or diagrams. It is not a conclusions section, because it draws upon some information not previously presented. We hope that the section will provide philosophy that is useful to readers, a means of integrating data from loosely related fields of inquiry, and points of departure for studies by those who object to our generalizations.

In his attempts to understand the world that he occupies, man has had two sets of questions: what? and how? Prior to recorded history and throughout nearly all of the time since then man has asked who created the universe around him. Much later, and especially during the past century, the question has been how was the universe created. To answer how requires knowledge of the subsets what and when, whose difficulty was so great that they were neglected and by default the question who long dominated. Eventually, the question why may be asked.