Abyssal Memories: Reading the Ocean’s History from the Fossil Record on the Deep-Sea Floor –  Bern, 19.11.1993

USA/Deutschland

Wolfgang H. Berger

Balzan Preis 1993 für Paläontologie mit besonderer Berücksichtigung der ozeanographischen Aspekte

Für sein Werk als Pionier der Paläozeanographie, wobei er mikropaläontologische Methoden zur Erforschung der geologischen Geschichte unserer Ozeane und ihrer Auswirkungen auf das Klima anwandte.

Among the important revolutions in human thought that have occurred within the last several centuries, concerning the nature of the universe and man’s place within it, we must surely count the realization of the true meaning of fossils. The heroes of this story worked in the first half of the last century: natural philosophers such as Jean Baptiste Lamarck, who understood fossils as the remains of ancestors of living organisms, Georges Cuvier, who showed that extinction is a fact of life, Aristide d‘ Orbigny, who discovered the memories of great catastrophes in the fossils of the Vienna Basin. Soon thereafter Charles Darwin and Alfred Russell Wallace established evolution as a viable proposition by providing a common-sense mechanism (Natural Selection). In fact, the new ideas introduced by these pioneers were powerful enough to help drive an expensive 3-year expedition across the world’s seas: the famous CHALLENGER EXPEDITION (1872-1875) which marks the beginning of oceanography as a scientific discipline. The search for „living fossils“ on the deep-sea floor was an important motivation for this adventure.

In the course of the CHALLENGER expedition, the Scotsman John Murray made the discovery that at least one half of the deep-sea floor is covered with the remains of shell-bearing plankton, mainly microscopic unicellular algae and protozoans. Almost all of this plankton lives in the uppermost sunlit layer of the ocean, with some living in the dark zones below. Also, there is a small proportion of benthic protozoans contributing shells to the sediment. Murray realized that the sea floor collects a memory of the conditions of growth in surface waters (temperature and productivity), in the shape of shells of the organisms that once lived and reproduced there. However, he had but small samples from the surface of the sea floor, so that he could not use this insight for the reconstruction of ancient conditions.

Systematic recovery of sediment sequences from the deep-sea, using steel-tubes lowered on a wire, did not occur for another fifty years. It was accomplished during the METEOR EXPEDITION (1925-1927), in the central and southern Atlantic. Fossils from these sediments (the planktonic foraminifers) were studied by Wolfgang Schott, who was able to contrast conditions during glacial times with the conditions of the present ocean. He introduced the concept of quantitative paleontology, by exactly counting the relative abundance of each species present. A change in the relative abundances of the species reflects with great precision corresponding changes in temperature and productivity of over­ lying waters. Forty years after the expedition it was possible to demonstrate, on the basis of these data (using simple computer algorithms), that glacial ocean temperatures had been much the same as today in the great desert region in the North Atlantic known as the „central gyre“, but that surface waters had been much colder than today close to Africa and along the equator. Also, indications for increased productivity along NW Africa could be detected in these data, as well as the first hint that the deep circulation during glacial time was quite different from the one which we see today.

Evidence on deep circulation carne from the study of preservational patterns: Schott had pointed out that the boundary between the two main deep-water masses, Antarctic Bottom Water and North Atlantic Deep Water, also marks a preservational boundary, such that samples from above the level of the boundary show good preservation while those from below that level do not. In times past, the boundary between well-preserved and poorly preserved fossils moved up and down along the sloping sea floor, reflecting changes in the depth level of the boundary between the two main deep-water masses. It subsequently turned out that there is a counterpoint pattern of preservation on the deep-sea floor in the Pacific basin, so that the deep preservation boundaries in the Atlantic and Pacific move in see-saw fashion. These movements, in essence, reflect the changing intensity of the flushing of the Atlantic basin by North Atlantic Deep Water.

Comparisons between different basins became possible through the exploits of the circumglobal SWEDISH DEEP SEA EXPEDITION(1947-1949), using the research vessel ALBATROSS. The most important technical innovation that provided for the success of the expedition was the „Kullenberg“ piston corer. This is a clever modification of the steel tube principle, which excludes water from the tube but admits sediment. This device allowed the taking of long cores, some 7 meters long in fact, during this expedition  (Later, the same principle would be used by scientists at Lamont Geological Observatory to take cores up to four times longer.) Thus, the ALBATROSS retrieved a multitude of sedimentary sequences from all ocean basins, with memories reaching back between 500,000 to one million years.

The marine geologists working on the cores recovered by the ALBATROSS established the new field of „ocean history“ as read from deep-sea sediments, what we now call „paleoceanography“. Many fundamental concepts were established by these pioneers, chiefly with regard to glacial-interglacial fluctuations in oceanic and climatic conditions: the Swedish-American geochemist Gustaf Arrhenius produced evidence for great changes in productivity in the eastern tropical Pacific, and argued for corresponding changes in the strength of trade winds as a result of changing ice cover in northern latitudes; the ltalian­American paleontologist and isotope chemist Cesare Emiliani established the cy­clicity of the ice-age oxygen isotope record, and argued for the importance of orbital forcing in climatic change (in the sense proposed by the Serbian astronomer Milutin Milankovitch); the Swedish geochemist Eric Olausson proposed that carbonate preservation patterns contained clues to changes in deep-sea circulation; the American paleontologists Frances Parker and Fred B. Phleger produced evidence for large-scale shifts in climatic zones in the last several hundred thou­ sand years. I was fortunate, as a student in the 1960’s, to profit directly from the advice of Parker and Phleger, with both of whom I worked closely then and many years afterward.

Several things happened late in the 1960’s that helped establish the field of paleoceanography as we know it today. First, the study of deep-sea sediments increasingly became the turf of geologists working at oceanographic institutions. This trend reflected the fact that cores were collected by ships run by such institutions, and that affiliation with universities (and hence in-house education of young marine geologists) became commonplace. The effect was to channel the imagination of geologists into paths compatible with the existing knowledge about the physics, chemistry, and biology of the ocean. It became possible to ask intelligent what-if questions: What if sea ice greatly expanded over the North Atlantic? What if the North Pacific had a deep-water source (rather than only the Atlantic)? What if the ocean were much less productive than today? How exactly would such changes be reflected in the sedimentary (=fossil) record?

Second, there was a great surge of interest in deep-sea sediments as a result of the initiation of deep-sea drilling by the GLOMAR CHALLENGER. This venture, funded by the U.S. National Science Foundation, was administered by several leading oceanographic institutions, with Scripps Institution of Oceanography as manager. It was what was happening in the Earth Sciences to a growing number of ocean-going geologists and paleontologists: the last time in human history that enormous blank regions on the world’s map suddenly became accessible for detailed exploration. Of course, geophysicists had done much by remote sensing already, using sound waves reflected from the sea floor, and mapping the subtle variations in magnetic properties of the ocean bottom. However, now it became possible to retrieve the very stuff of the ocean’s memory: the fossils deep within, each telling about the living conditions of the past. Many miles of core started to pile up at the core repository at Scripps: the diaries of the ocean were readily available for inspection. (This process continues at Lamont and at Texas A&M as this is written, under the auspices of the Ocean Drilling Program.)

Third, detailed stratigraphic work became possible through the application of the recently developed magnetic reversal time-scale to deep-sea sediments, in combination with intensive study of the stratigraphic ranges of the fossils found in deep-sea sediments: foraminifers, nannofossils, radiolarians and (to a lesser extent) diatoms. This work had begun before the deep drilling project, in fossil-rich sections exposed in uplifted regions, being of great interest in the context of petroleum exploration. However, it now expanded rapidly within the academic environment, as the necessity arose to find the ages of the sediments recovered by drilling. Assigning such ages is a skill which is won through patient study and long experience, and a single-minded attention to detail. Without such age assignments the memories of the ocean cannot be properly ordered: they are but disjoint recollections. I was fortunate to be associated with several pioneers in this field (M. N. Bramlette, W. R. Riedel, F. L. Parker) and to have the benefit of dose collaboration with recognized leaders for many fruitful years.

Other important developments, in the early seventies, concerned the improvement of analytical equipment, and the introduction of mathematical statistics to the interpretation of fossil assemblages. Also, a new emphasis on climatic change arose, stemming from concerns about the impact of human activities on climate (that is, the expectation that the increase in carbon dioxide in the atmosphere will produce global warming). It soon became clear to a number of earth scientists (including climatologists) that predictive powers of climate models could be checked by demanding „post-diction“ of past conditions. In addition, the nature of climatic change could be studied over many thousands of years, using deep-sea sediments, under quite different background conditions (including times without ice caps). These aims gave great impetus to the field.

My own interests, strongly influenced by my advisors Fred Phleger and M.N. Peterson, turned to the question of how exactly the ocean writes its diaries: how do organisms live before they become fossils, what happens to the shells on the sea floor, what exactly can we read from the record. The task is perhaps comparable to finding the meaning of a hieroglyphic script by watching the scribes at work and noting the circumstances under which they use the symbols. Comparing the number and types of living organisms in the water column with the flux of shells to the sea floor, it was possible to obtain estimates of life spans and productivity of the plankton. Comparing the states of preservation of the shells in different parts of the sea floor, it was possible‘ to make inferences about the carbonate chemistry of bottom waters (and of interstitial waters).

As a participant in one of the early „legs“ of the Deep Sea Drilling Project I was suddenly obliged to apply all I had learned regarding sedimentology, oceanography, and paleontology, on short notice. There is no time for lengthy consultations when the sediment comes on board. I remember vividly the discoveries we made, bent over the microscope, finding minerals that had not been described from the sea floor before, and hesitating (as relatively inexperienced post-docs) to claim their presence. During this expedition, also, we found (as had others) a strange sequence of calcareous and non-calcareous sediments in a number of sites. A portion of the sequencing could be explained by assuming that the sea floor had subsided, as required by sea floor spreading (which by then most of us young people considered a fact, no matter what older colleagues thought). However, to explain all of the sequence, it had to be assumed that the depth limit of preservation of calcareous fossils (which is near 4.5 km on average) had been at much different levels in the past. Later it turned out that these fluctuations in the level of preservation are somewhat similar in all ocean basins. It appeared that the deep ocean was starved of carbonate whenever large tropical

shelves were available to receive calcareous sediments. In these studies I had the good fortune to profit from the extensive knowledge and deep insights of my former teacher and now colleague E.L. Winterer.

Paleoceanography today is inevitably breaking up into many subfields, by periods studied, and by emphasis on the type of reconstruction attempted. Thus, there is a large community of Quaternary ocean historians, whose chief hunting grounds are the last 2 to 3 million years. In this period, which we might consider „our time“ as human beings, ice caps grow and decay in far northern latitudes, on Canada and over Scandinavia. The ocean responds to the associated climatic changes, and this is recorded in the fossil deposits. It is now well established that these changes are governed by astronomical forcing, exactly as claimed by Milankovitch in the 1930’s and later by Emiliani in his classic study on Pleistocene temperatures (1955). What is interesting and intriguing, however, is that the response of the ocean changes in unpredictable ways, at certain times within this period.

Also, there is a large community of paleoceanographers studying the „Neogene“, that is, the last 20 million years or so. This period contains the story of the buildup of the great ice caps, first on Antarctica, and then in the north. Also, it saw the closure of the Tethys, a mighty sea-way running from north of India west to the eastern Pacific. The Mediterranean is the modest remnant of that sea way. In the Neogene, the ocean became „familiar“ in its overall circulation patterns, and in the types of organisms living in it.

Yet others specialize in the investigation of the ancient ocean, when it was much warmer than now, and the circulation patterns were fundamentally different. Less oxygen was then dissolved in deep waters; nitrate (an important nutrient) quite probably was less stable, and the productivity of the ocean was greatly reduced on the whole. We do not yet understand the conditions in that old ocean which lies beyond 40 million years ago. The fossils are quite different from what lives today – the record is written in an ancient language. It is as though, familiar with modem English, we attempt to read the Beowulf saga in the original. Perhaps not impossible, but difficult indeed.

And thus the tasks ahead differ for the various subfields of our discipline: More physics and dynamics for the Quaternary studies, where reconstruction is adequate, but understanding of the system is still lacking. But more exploration and better description for the more ancient memories of the ocean, which are written in symbols no longer in use, and on tablets only partly preserved. Much effort, it is safe to predict, will be expended on high-resolution studies in the future, that is, the reconstruction of ocean conditions on time-scales between one year and one thousand years. Such studies are restricted to privileged regions close to the coast. Because many coastal processes are not orderly, high-resolution records tend to be difficult to interpret. However, they contain the precious details that we need to gather in order to understand the ocean’s role in climatic change. This knowledge, in tum, is crucial for wise decisions regarding our climatic future.