12. Cenozoic History from Deep-Ocean Drilling

Our planet has plenty of ice in high latitudes (Fig. 12.1). In fact, the central theme of climatic evolution in the Cenozoic (the time after the demise of ammonites and dinosaurs) is an overall cooling (starting with the Eocene and culminating in the northern ice ages) and an associated overall drop of sea level (making ice on land takes water; sea ice does not change sea level: the water stays in the ocean). Sea ice, like land ice, does affect albedo, though, by providing a base for snow.

The overall cooling trend has been known for some time from the study of fossils on land. It is reflected in the evolution of animals and plants whose offspring exists today. Modern evidence comes from oxygen isotopes of benthic foraminifers in deep-sea sediments recovered by drilling (Fig. 3 in Preface). It has become general knowledge, along with an appreciation for increased climate variation with ice buildup. Cooling occurred largely in steps, presumably a reflection mainly of sudden and large changes in albedo whenever snow fields or sea ice underwent notable change in extent, invading new areas (Fig. 1.​7).

Changes in ocean productivity in the Cenozoic presumably resulted from intensification of the wind along with a change in the nature of the thermocline. The thermocline, the boundary between warm and cold water, became shallower and better defined as the cold water layer at depth grew thicker. A shallow and distinct thermocline means decreased productivity over large oceanic areas, with low nutrient content in surface waters. In contrast, coastal ocean productivity increased noticeably in the Neogene, especially the late Miocene. There was an increase in the range of productivity, with large areas becoming deserts and opposite extremes focused within upwelling regions. The result of a strong and shallow thermocline presumably is an enhanced depth stratification of plankton, a development noted in sediments since the beginning of the Neogene right after the somewhat chaotic Oligocene.

Strengthened winds (polar ice increases the temperature gradients that drive winds) presumably increase upwelling, which (it is thought) lowers diversity locally. Thus, while we can be sure that there is a relationship between an overall cooling and a change in diversity of plankton and other organisms, we have problems specifying details on a regional level. The available information is not unambiguous, with the desert specialists (the nannofossils) having an evolutionary trajectory quite different from those of diatoms and foraminifers. In both of these forms, abundance and diversity seem to increase during the Cenozoic, a statement not applicable to nannofossils and to radiolarians (Fig. 12.2).

Evidence for the great cooling trend in deep-sea sediments was first provided by the oxygen isotopes in benthic foraminifers sampled by deep-sea drilling (Fig. 12.3). The relevant graph by Ken Miller (Rutgers University) and associates (published in 1987) has been referred to hundreds of times. A similar compilation by J. Zachos and associates (global, rather than Atlantic focused; published in 2001) shows that the major features recognized in the Atlantic by Miller et al. are in fact global in nature. Both compilations show two remarkable ramps of cooling lasting for more than 10 million years – one in the Eocene and the other within the Miocene. In each case, at the end of the gradual cooling, there is a striking acceleration, as though the cooling reached a critical level resulting in vast ice buildup and albedo change (positive feedback). We might guess that an end-of-Eocene ice buildup took place rather readily on Antarctica, as the place where one of the poles is located centrally on an enormous continent with cold winters during the dark season. The continental ice buildup of the late Neogene, around the Arctic Sea, may be considered a lot more difficult and hesitant; the solid base for land ice is being removed from the pole. A mid-Miocene ice buildup is commonly assigned to Antarctica, the IRD (ice-rafted debris) in northern latitudes being as yet rare or missing at the time.

The essential trends and steps within the general trends were worked out soon after the Deep-Sea Drilling Project started, by two teams combining the skills of paleontology and isotopic geochemistry. The pioneers were the US geologists R. Douglas and S. Savin and the NZ-US geologist J. Kennett with the British geophysicist and geochemist N. Shackleton (later Sir Nicholas). Their observations showed that cooling was especially vigorous in high latitudes, being obvious in benthic species (bathed by high-latitude bottom water) and in planktonic species from the subantarctic realm. Also, their data indicated that the cooling occurred in a few major steps, suggesting the operation of nonlinear feedback mechanisms (i.e., the uneven response to forcing and the action of positive feedback are indicated, such as expected when freezing is involved).

An important aspect of the overall cooling trend, and one that touches on the search for causes of the cooling, is the stratigraphy of strontium isotopes. Chemically, strontium is an element homologous to calcium, that is, it acts somewhat like calcium and is present in calcium carbonate, therefore (even if not very abundantly). This means it can be found in calcareous fossils, whether they are remains of benthic organisms or pelagic ones, shallow or deep. The ratio between the isotopes ⁸⁷Sr and ⁸⁶Sr (both less than 10% of naturally occurring Sr) reflects (among other items) changes in the sources supplying strontium to the sea. The common sources are erosion of continents (i.e., river input) and supply from volcanic emissions. The long-term trend in the ratio shows a steadily increasing influence of continental erosion relative to volcanic activity, as expected for cooling (Fig. 12.4). The trend distinctly accelerated about 40 million years ago and again 16 million years ago. Erosion of continents, of course, is tied to uplift and regression (water-covered continents are not eroded). But there is evidence that volcanic activity changed also throughout the Cenozoic, complicating the interpretation of shifts in the strontium isotope ratios.

When focusing on cooling, a change in volcanism is not helpful: It potentially produces a change in ratios of Sr isotopes that is independent of cooling. (“Potentially” independent because ice buildup results in “loading” of continental crust which presumably stimulates volcanic activity, which would have opposed any evidence for increased erosion of continental crust.) As far as the observed trend, it pretty much is as expected for cooling. It accelerates from the end of the Eocene to about 16 Ma, an interesting period when the overall tendency of cooling may have given way to signals from increased volcanism (J.P. Kennett published Neogene DSDP ash layer abundances in 1982, p.384). The overall trend expected from cooling then starts up again at about 7 Ma, a time of major uplift in the Asian highlands (Tibet) and a volcanically relatively quiet period. Sea level apparently dropped low enough to help isolate the Mediterranean from the world ocean. (Thus, cooling pulses may have been an important cause for a repeated drying up of that sea at the end of the Miocene.) Toward the end of the Neogene, some 4 million years ago, there are indications of major volcanic activity again in DSDP cores.

Deep-ocean drilling (Fig. 12.5), beginning with the Deep-Sea Drilling Project (DSDP, Glomar Challenger) and continued with ODP (and a new vessel, dubbed JOIDES Resolution), profoundly changed the understanding of geologic history for the last 100 million years. The grand steps of cooling in the Cenozoic and the anaerobic periods in the Cretaceous were among the most stupendous findings of scientific deep-ocean drilling. The steps were a surprise for most geologists, as was the fact that the cooling is mainly a high-latitude phenomenon. These were, however, only two of several major discoveries based on using the technology of drilling, a technology borrowed from the oil industry and adapted for academic use.

Without assignment of ages and environmental interpretation after inspection of microfossils and nannofossils, the available information would not have yielded the relevant insights: To some degree, the fossils (Figs. 4.​7 and 12.6) allow keeping track of the changing environment, thanks to evolution, that is, thanks to the fact that they are the remains of organisms adapted to their habitat. Much more reliably, however, they allow assignment of geologic ages because they differ from one period to the next. Age control is greatly refined and turned into absolute age numbers, using correlations into radioactively dated land sections, correlated with sediments on the seafloor based on magnetic reversal stratigraphy and to some extent on isotope stratigraphy, that is, changes in physical and chemical changes in sediment properties, providing probable ages for biological changes observed.

The major cooling steps identified tend to be associated with major changes in both the marine flora and fauna. A very big change was at the end of the Eocene (Fig. 12.6). There was a general dying off of ancient benthic species, as the water became too cold for them (or else shallow-water forms on shelves found themselves without any water). Planktonic species also suffered: a rich Eocene flora and fauna disappeared, and a rather dull Oligocene one appeared, with comparatively little diversity. At times, in the Oligocene, there were enormous blooms of Braarudosphaera, a nannoplankton genus. It is a form now mainly found in stressful conditions in shallow water. The deposits of these strange blooms have long been a source of puzzlement to geologists.

One possible response to questions arising with regard to the Oligocene blooms is to invoke vertical water column instability in the Oligocene (which is bad for phytoplankton production). Well-developed stratification may have developed only sporadically, thus favoring the blooming of stress-indicator species. There is an interesting message in the Oligocene blooms: (1) Biostratigraphy may routinely reflect strange events and (2) the biostratigraphy of ancient deep-sea sediments may reflect times of unusual production, rather than recording regular conditions. While the resulting sequences might not affect dating, they might render questionable the reconstruction of “typical” past conditions.

The cooling steps are commonly referred to as products of ice-related feedback (calling on albedo changes from the spreading of snow and ice). If the assumption is correct, then an important aspect of albedo feedback is the associated sea-level change. Sea-level change is a complicated issue (see Chap. 6). It may respond to tectonic forcing without any involvement of ice, making interpretations difficult. In any case, however, cooling steps tend to be associated with a drop in sea level. Even modest changes in this level have potentially large effects because much land is close in elevation to the level of the sea (Fig. 2.​1). Whenever sea level stands high, sediments are trapped in great expanses of shelf seas, and the deep sea receives relatively less sediment. When sea-level stands are low, the reverse is true. The same pattern holds for carbonate rocks: A high sea level results in buildup of limestones on the continents; a low sea level moves carbonate to the deep sea, where it accumulates as shell sand and – silt replete – with plankton fossils. Changes in ocean productivity, though, can greatly complicate the pattern observed.

When correlating the cooling steps into land sections, we tend to find evidence for mountain building. In answer to the question “whence the stepwise cooling,” the suggestion comes up, therefore, that mountain building and loss of shelves are to blame (Fig. 12.7). As with ice-driven changes, albedo is part of the story. Shelf sediments are commonly light colored; the sea is not. In addition, the wearing down of the mountains removes the greenhouse gas carbon dioxide from the atmosphere. One might expect, as well, that less water vapor (a powerful greenhouse gas) is delivered by a sea that has lost its shelves. The cooling itself then develops its own dynamics: A cold ocean takes up carbon dioxide, for example, diminishing carbon in the atmosphere (and presumably in other reservoirs communicating with the ocean and the atmosphere).

Carbon burial in upwelling regions is another aspect of those internal dynamics, a phenomenon especially important in the Neogene with its striking history of changes in upwelling and increased burial of land-derived sediments, some presumably replete with organic matter, as well. In addition to minding evidence for the uplift of the Himalayas (Fig. 12.7), then, we must contemplate the masses of erosion products in the Bay of Bengal and elsewhere (and the composition of those masses) when discussing global cooling.

The high-latitude cooling indicated in the oxygen isotope trends shows two major steps in the last 50 million years before the last one that is associated with the start of northern ice ages (Fig. 12.3). The first one roughly coincides with the AFS and the collision of the Indian subcontinent with Asia, a collision that initiated the closure of the ancient tropical seaway “Tethys.” The AFS is near the end of the Eocene and thus was preceded by millions of years of cooling (presumably related to mountain building and attendant erosion). The second large cooling step is in the middle Miocene, associated with a fundamental change in deep-sea sedimentation and preceded by evidence for carbon dioxide drawdown (signaling involvement of the carbon cycle).

We do not know why the steps occurred and exactly when they did. Perhaps we see the effect of reaching a critical threshold along a trend. Or perhaps, the physical geography changed at that very time, barring or opening certain ocean passages and thus redirecting the heat carried by ocean currents and the nutrients they transfer (Fig. 12.8). Of course, the two types of possible causes – internal feedback and change of geography – are not mutually exclusive. We know that both are at work. The question is how important they are in each given situation.

The time when summer snow could first stay on the ground after sufficient cooling (first on Antarctica and then on continents surrounding the Arctic) must have been crucially important. Similarly the appearance and varying extent of sea ice carrying a snow cover must have provided significant positive albedo feedback to cooling (note the full reversibility of this positive snow feedback). Details in the timing may have been influenced by the sequence of closing gateways in the Tethys and opening gateways in the Southern Ocean. Without these changes in geography, ice ages presumably would have come anyway, thanks to mountain building. However, they might have come at somewhat different times and with different dynamics.

One way of looking at the problem of where in time a step should be placed is to note the signs of a corresponding large reorganization of the ocean’s geochemistry (and hence planetary climate). According to strontium stratigraphy, one such reorganization took place around 40 Ma close to the end of the Eocene (labeled “AFS” in Fig. 12.4, for “Auversian facies shift.”) The “Auversian” is an obsolete technical term for a time span in the late Eocene, defined in the region of Auvergne in France. In Auversian time (in modern terms, Bartonian-Priabonian time), the deposition of calcareous sediments in the deep ocean greatly increased, a phenomenon usually referred to as a drastic drop in the CCD at the end of the Eocene. Also, from the Oligocene on the types of deep-sea sediments were rather strictly segregated, with Murray-type carbonate ooze accumulating on the elevated parts of the deep-sea floor, and siliceous sediments rich in the remains of diatoms and radiolarians restricted to zones of upwelling. It is equally possible, of course, that the main change occurred at the end of the Oligocene. The fact that some time is needed for the Drake Passage to become highly effective (through deepening) suggests the Paleogene-Neogene boundary as the correct choice for the event defining an oceanographically effective passage between the Patagonia and the Palmer Peninsula.

If we place the opening of the Drake Passage (a rather poorly known event from the point of view of plate tectonics) at the end of the Eocene, we may choose the beginnings of a long-term process. In fact, the effective opening of the passage is a matter of controversy and learned discussion. Today the Drake Passage is responsible for the major mixing in the sea in high Southern latitudes and for the existence of the largest of all currents there, the one circling Antarctica, flowing from west to east (formally, the “Antarctic Circumpolar Current”). The appearance of the current may have been rather sudden (as far as geologic history), but simple geologic reasoning suggests that there must have been at least one element favoring gradualism: a gradual deepening and widening of the tectonically opening Drake Passage. In fact, the response of benthic foraminifers, displaying gradual turnover of species uncomfortable in the cooling bottom water of the late Eocene into the early Oligocene, may well reflect the expected gradual trend linked to the deepening of Drake Passage after opening.

Moving from one time scale to another can be confusing. There are really three such scales that need to be considered: First, the human scale, which includes decades and even centuries, that is, personal career changes, for example, and grandchildren. Also, the “Renaissance” (a period in human history) belongs into this scale. The second important scale is the intermediate one, which is focused on one to several millennia and includes the mixing time of the ocean and (at the long end) ice ages and Milankovitch Theory and radiocarbon dating. The third scale is the very long geological time scale, measured in units of a million years. As we go back in time to exemplify processes, we are likely to encounter geological time. There is one great exception: Impact time presumably is very short (Chap. 13). Many of the “official” boundaries defining epochs presumably are linked to impacts large and small (This is a matter of discussion among experts).

The Drake Passage and the associated Circumpolar Current are features that belong to all scales, but the source of silicate for upwelling may be seen differently, depending on the time scale employed. On the geologic scale, the current maintains a reasonably high value of the nutrient in the ocean, feeding high-production systems. On the human scale (the one at issue in much of the climatology and oceanography of global warming), the complicated network of subsurface currents carrying nutrients is at focus in the attempt to understand productivity. Human-relevant information from marine geology is being contributed on the intermediate and the short scales, not so much on the long scale.

Thus, the actual histories of the Cretaceous and the Cenozoic are of great interest when reconstructing marine resources, but as far as analogs have very limited application to some of the major problems facing humankind. The study of history, however, is bound to increase understanding of processes relevant to those major problems, as mentioned.

In many cases, we become aware of “tipping points,” situations of reaching critical levels along a trend. For example, at some crucial level of sea-level drop (e.g., from mountain building), shelf seas can no longer serve as carbonate repositories. Instead, karst-making begins, which implies delivery of carbonate from the shelves to the ocean, for deposition at depth. (The dissolution of shallow-water carbonates on an exposed shelf produces a landscape called “karst.”) The shortage of silicate in the world ocean (chert becoming less abundant) suggests that wind-driven upwelling starts vigorously, presumably around Antarctica, robbing the rest of the sea floor of opaline deposits. A corresponding shift in sediment types into siliceous ooze and mud around Antarctica is seen in deep-sea sediments of late Paleogene to the early Neogene age (Fig. 12.9).

Diatoms may be considered the typical Neogene plankton microfossil, in preference to foraminifers or nannofossils (Fig. 12.2). This observation suggests that the cooling trend must have greatly affected diatom abundance. It is clear that cooling started in the early Eocene (Fig. 12.3). One infers that the appearance of Drake Passage may have intensified any ongoing cooling, following B. Haq. The capture of silica by diatoms of the Antarctic margin presumably contributed to the purity of Oligocene deep-sea carbonates over much of the seafloor (a purity noted by K.J. Hsϋ in the DSDP Leg 3 report). Today’s sediments around Antarctica are exceedingly rich in diatoms and other siliceous fossils, as has been pointed out more than a century ago by many workers (Fig. 10.​11). The high silica deposition is a presumably legacy of an open Drake Passage and the presence of ice on Antarctica. The latter generates enormous circumpolar winds, winds that drive the great ring current and generate its deep mixing aspect (i.e., bringing up silicate and other nutrients for recycling). Without the Drake Passage, world-wide productivity might be lower. It certainly would be differently distributed geographically.

According to Haq, the difference between the present ocean and the Eocene one of about 45 million years ago reflects strongly the closing of tropical gateways and the opening of passages around Antarctica and in high latitudes in general (Fig. 12.8). In detail, each of the changes associated with openings and closings holds a wealth of interesting stories. One of the most fascinating, for example, is the drying out of the ancestral Mediterranean at the end of the Miocene between about 6 and 5 million years ago, as discovered during Leg 13 of the Deep-Sea Drilling Project (with W.B.F. Ryan and K.J. Hsü as cochief Scientists).

Huge amounts of salt were deposited at that time in the Mediterranean Tethys basin, so much in fact that the salinity of the global ocean must have been reduced by several percent (“La crisi di salinità” of Italian geologists). In the case of the Mediterranean desiccation, one assumes that a drop in sea level owing to ice buildup was at least partially responsible for isolating the ancient sea from the world ocean. Messinian gypsum-bearing rocks are seen on land, as well, uplifted. Drying the ancient Tethys basin (that is, the ancestral Mediterranean Sea) must have raised sea level elsewhere, by some 10 m, briefly reversing the general regression and perhaps thereby slowing ice buildup, which arrived around 3 million years ago in northern high latitudes on land, rather than during the time of the crisi salinità, as one might expect if cooling accompanied regression.

As concerns the heat budget, one corollary of the cooling leitmotif of the Neogene ocean is a displacement of the intertropical convergence zone (ITCZ, the heat equator) to north of the geographic equator. There are several reasons for this. One is the whitening of the Antarctic continent. This has the effect of increasing wind speeds in the southern hemisphere and pushing southern climatic zones northward. Another is the northward movement of large continental masses which set up monsoonal regimes favorable to the northward transfer of heat. The uplift of Tibet and the growth of the Himalayas as a Neogene consequence of the Paleogene collision of the Indian subcontinent with the Eurasian Plate had climatic consequences: a strengthening of monsoons and an increase in weathering. Another factor favoring northern heat piracy is the peculiar geographic configuration of margins in the Atlantic and to a lesser degree in the Pacific basin, configurations that provide for the northward deflection of westward-flowing equatorial currents. As a consequence, the Gulf Stream in the Atlantic and the Kuroshio in the Pacific are strengthened. The end result is that the southern hemisphere loses out on heat: Glaciers in Southern New Zealand at present are in walking distance from the seashore. At that latitude, vineyards occur around Bordeaux in the northern hemisphere, not glaciers.

One important aspect of this planetary heat asymmetry is the fact that the North Atlantic tends to deliver deep water to the ring current around Antarctica. On the whole, the North Atlantic receives shallow water in return. Thus, the northern Atlantic system represents a heat pump, with warm (shallow) water in and cold (deep) water out. Besides heat, nutrients are involved, making the North Atlantic “anti-estuarine.” It entered this state in the middle Miocene, as seen in the “silica switch” of the Swiss-American geologist G. Keller (Princeton) and the USGS geologist J. Barron (California) (see Fig. 12.10). Incidentally, they favor an opening of the Drake Passage as linked to the Miocene silica switch, based on evidence from hiatus stratigraphy. Their suggestion certainly emphasizes the importance of the two events, the mid-Miocene “silica switch” and the “Drake Passage opening.”

Significantly, the “silica switch” occurs close to a major cooling step, presumably accompanied by an ice buildup and a substantial drop in sea level. The step may be the largest one in the Miocene. It is called “Mi3” in the oxygen isotope stratigraphy of K.G. Miller and colleagues. B.U. Haq and associates, in their 1987 Science article on sea-level variations through geologic time, placed a major drop in sea level at the end of the middle Miocene, based largely on seismic information in sediment stacks on continental margins but also on subsurface data from drilling.

The cooling step and major sea-level drop in the middle Miocene (presumably a large icing over in Antarctica) were associated with major upwelling, as seen in the record of margins in upwelling regions, for example off Namibia. The foraminifers of that time display a major excursion of carbon isotopes (toward positive values) indicating the burial of organic carbon (Fig. 12.11). The carbon isotope excursion is the largest in the last 50 million years. It was discovered in the earliest drilling sites, even in the 1970s. The second largest in this time span is at the Eocene-Oligocene boundary, in the Paleogene, an isotopic event somewhat reminiscent of the one in the Monterey in being closely followed by cooling. Choosing “the last 50 million years” for a discussion of cooling avoids consideration of the (negative) carbon isotope excursion at the Paleocene-Eocene boundary, which was associated with major warming. A large release of methane from the deep seafloor to the environment has been postulated for that time (the position of the event and the warming are in the name PETM, Paleocene-Eocene Thermal Maximum).

The French-American marine geologist and biostratigrapher Edith Vincent (then at S.I.O.) used the label “Monterey Event” for the Miocene carbon isotope excursion. The label implies drawing a parallel of the carbon isotope excursion to the evidence for increased upwelling at the time of the origin of the Californian Monterey Formation. Some of the organic carbon buried globally (and generating the characteristic carbon isotopic signal in benthic deep-sea foraminifers prior to the great middle Miocene cooling) could conceivably be of terrigenous origin, as emphasized by the marine geologist L. Diester-Haass (Saarbrϋcken). But much of the signal presumably has a marine origin, judging from the sediments delivered by upwelling in the eastern Pacific margin (i.e., the Monterey Formation). Much of the sediments of the Monterey Formation are finely layered and also have plentifully phosphate and chert content, the latter presumably generated by diatoms and other siliceous export from the sunlit zone and suggesting high marine productivity (Fig. 12.12).

Strong upwelling apparently started only about 10 million years ago in most coastal systems, but high-coastal production presumably started considerably earlier. Additional upwelling intensity likely set in with each cooling step. Thus, ocean productivity presumably did benefit from the cooling all through the Cenozoic, especially in post-Eocene time, and earlier than suggested by strong coastal upwelling.

We have earlier referred to the drying up of the Mediterranean at the end of the Miocene. It was not to be the culmination of Neogene cooling; the northern ice ages were. Presumably the Mediterranean desiccation was indeed facilitated by a sea-level drop caused by ice buildup, some of it on the northern hemisphere. But in the time that followed the last desiccation pulses (apparently including the earliest Pliocene), the planet experienced a relatively calm and warm period, lasting some 2 million years. This warmish time in the Pliocene was terminated by the onset of the northern ice ages, some 3 million years ago.

The onset of northern ice ages looks like expected, in principle, from the general Cenozoic cooling trend (see Hodell et al., 2002. ODP Leg 177 synthesis). For the onset to materialize, one would think, continued uplift had to keep moving the climate toward an increasingly cold state, so that at times, even the mighty Gulf Stream failed to bring enough heat northward to stop an ice age from developing. In the late Pliocene, when the signs were right (low summer insolation as postulated by Milankovitch Theory), the ice started growing on Canada. The climate then entered a 3 million year period (lasting throughout the Pleistocene) when relatively small changes in summer sunlight in high northern latitudes were translated into large-scale changes in ice mass and sea level (see Chap. 11). That northern ice buildup started around 3 million years ago within the late Pliocene was documented by the Woods Hole geologist and biostratigrapher W.A. Berggren during one of the early DSDP legs, based on ice-rafted debris (Fig. 12.13). The event not only resulted in cooling within the Gauss Chron but also provoked the onset of major climate fluctuations, as suggested in a sudden increase of isotope variations in benthic foraminifers (Cibicides spp.) and in planktonic ones seen off Antarctica, as documented by D. Hodell and associates, by deep-ocean drilling (see in J.P. Kennett and D.A. Warnke, eds., 1992, The Antarctic Paleoenvironment: A Perspective on Global Change. AA Res. Ser. 56).

Several distinguished marine geologists have insisted that the timing of the onset of the ice ages is linked to closing the Panama Strait, which ended the loss of heat to the Pacific by westward currents from the Atlantic. Supposedly, moisture for making snow was then available to high northern latitudes, moisture brought northward by warm Caribbean waters (e.g., Gulf Stream, Fig. 5.​14). The implication is that availability of moisture is limiting to northern polar ice growth, not just cooling. The concept, as an explanation for the origin of northern ice ages, has been questioned by W.H.B. and G. Wefer, whose “Panama Paradox” hypothesis calls for attaining a critically low temperature within the long-term cooling trend rather than a link to the closing of the Panama Straits, as the crucial element of the onset of the ice ages. Clearly, however, moisture is involved also, whenever making snow and ice, as evidenced by modern distribution patterns of ice in Scandinavia. Thus, the question about the role of moisture supply in starting the ice ages must be considered unresolved.

To interpret the meaning of a given sample from the seafloor, the sediment has to be placed in its original depth and location at the time of deposition. This is done by “backtracking” the path that a site has taken as it aged. Without taking proper account of this path, the sediment sequences in drill cores cannot be interpreted correctly, especially if substantial changes in depth or in latitude are involved. For example, when a site moves down the East Pacific Rise in the Neogene and northward across the equator, it necessarily collects a number of different types of sediment in sequence (calcareous ooze, siliceous clay, siliceous calcareous ooze, siliceous clay, Red Clay). The resulting vertical stack of different sediments (leftmost panel, Fig. 12.14) can look like evidence for changes in the sedimentary record of the ocean’s climate system, when in fact it was produced by plate motion.

Horizontal backtracking (a focus of the work of Y. Lancelot, 1937–2016) refers to geographic position, using the appropriate pole of rotation for reconstruction of latitude and longitude. Vertical backtracking (Fig. 12.14) addresses the reconstruction of depth by considering the subsidence of a piece of seafloor, most commonly from a cooling of the crust and uppermost mantle. Vertical backtracking is a necessary procedure whenever depth-sensitive properties are involved, such as carbonate content, sedimentation rates of oozes, or preservation of calcareous fossils, on time scales for which tectonic subsidence matters.

The position of the CCD, by definition, involves both depth and carbonate content of deep-sea sediments, and its path through the Cenozoic, therefore, can only be accurately described when taking the general subsidence of the seafloor into account. To obtain the path of the CCD (an important element of all conditions that relate to the marine carbon cycle), we must reconstruct the depth levels at which the CCD was crossed for a large number of drilling sites. Reconstructions vary with the quality of dating of the basement and of sediments, but there is general agreement (since 1985) on the major features of the Cenozoic behavior of the CCD. The most notable features are the drastic drop of the CCD at the end of the Eocene and the short-lived but remarkably substantial excursion to shallow depths during the “carbonate crash” of Mitchell Lyle (ODP Leg 138) at the end of the middle Miocene (Fig. 12.15). The first feature is linked to the end-of-Eocene Auversian facies shift (the “AFS”); the second (the “Crash”) is tightly linked to a cooling step and a sea-level drop producing a large hiatus.

An overall parallelism in CCD variation between Atlantic and Pacific basins is remarkable. It suggests that the geochemistry of the marine carbon cycle is global in nature rather than carrying only information from each ocean basin. Comparison of sea-level reconstructions and δ¹⁸O stratigraphy suggests that periods of high sea level are characterized by a shallow CCD and by warm high latitudes and periods of low sea level by a deep CCD and cold high latitudes (and cold deep waters). Why should a relatively warm ocean have a shallower CCD than a cold one? Is not cold water less favorable to the preservation of carbonate than warm water? The fuzziness of possible answers to such simple questions reflects the depth of ignorance surrounding the discoveries having to do with the history of the CCD.

A simple hypothesis linking sea level to CCD fluctuations is the concept of basin-shelf fractionation. As mentioned earlier, shelves covered by seawater are carbonate traps. As they remove CaCO₃ from the ocean, they tend to starve the deep seafloor of carbonate and hence raise the CCD. Conversely, bared shelves supply carbonate to the deep sea. In this very simple bookkeeping hypothesis, variations in temperature, while important in the thermodynamics of carbonate deposition, are incidental, while changes in sea level are close to basic forcing – and to geological time scales (Fig. 6.​1).

There is good reason to believe that the mass balance hypothesis of CCD fluctuations in fact does not account for all the relevant observations. It is compatible with the CCD drop at the end of the Eocene but not necessarily with the other major feature in the CCD history, that is, the carbonate crash (CC). The CC is reminiscent of the dissolution event observed at each onset of a glacial period during the last million years. It may be due to increased production and provision of organic matter producing carbonic acid upon oxidation. Available data bearing on the questions are still quite incomplete.