Changes in oceanic circulation and the Earths climate

Two of the most significant influences on the Earth's climate are the concentration of greenhouse gases in the atmosphere (Sections 5.7, 13.1.1), and the extent, distribution, and bottom topography of the oceans. The configuration of the ocean basins affects the transport of heat in the oceans, by surface currents and deep-water circulation, thereby affecting the temperature and moisture content of the atmosphere over oceanic areas. Surface currents are essentially wind driven, and, therefore, largely determined by the circulation of the atmosphere. The rotation of the Earth, and the concentration of incoming solar radiation within the tropics, produces surface easterly (trade) winds at low latitudes, westerlies at intermediate latitudes, and easterlies at high latitudes (greater than 60°).

If the Earth's surface was entirely covered by an ocean, the resulting westerly directed, equatorial ocean current, and the intermediate latitude easterly directed, circumpolar currents, would bracket irregular "gyres," circulating clockwise in the northern hemisphere, and anticlockwise in the southern hemisphere. In this situation the world-encircling equatorial and circumpolar currents would tend to inhibit the transfer of heat by surface currents from low to high latitudes, and the temperature gradient between the equator and the poles would be accentuated. As a consequence, sea ice might form in the polar oceans. However, land masses with north-south trending shorelines in low and intermediate latitudes, will deflect the equatorial and circumpolar currents, to the right in the northern hemisphere, and to the left in the southern hemisphere, thereby intensifying the gyres, and transferring heat from the tropics to higher latitudes by means of western boundary currents. In this scenario the temperature gradient is reduced. A classic example at the present day is the Gulf Stream of the western North Atlantic, which warms the air above the ocean in the extreme North Atlantic, thereby ameliorating the climate of Iceland and northwest Europe. The opening or closing of gateways for the equatorial or circumpolar currents, as a result of continental drift, can, therefore, have pronounced effects on the Earth's climate (Smith & Pickering, 2003).

During the past 200 Ma the supercontinent of Pangea has progressively rifted apart. The resulting fragments have drifted across the face of the globe, such that a continuous tropical seaway, the neo-Tethys, was formed, and subsequently closed, and a southern ocean gradually opened up around Antarctica (Figs 13.2-13.7). By the mid-Cenozoic, a complete southern circumpolar current came into existence, which isolated and insulated Antarctica, and was probably instrumental in triggering the first major build-up of the Antarctic ice cap (Kennett, 1977).

At the beginning of the Mesozoic Era, 250 Ma ago, the supercontinent of Pangea extended from pole to pole (Fig. 13.2), without extensive polar landmasses in either hemisphere. Strong western boundary currents off the eastern shores of Pangea would have transported warm water to high latitudes, preventing the formation of ice-sheets and warming the east-facing coasts relative to the west. The interior of the supercontinent would have had strong seasonal extremes. By 160 Ma (Fig. 13.3) a low latitude east-west seaway had started to open up, between what is now North America

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Fig. 13.2 Possible circulation pattern of surface ocean currents during the early Triassic (245 Ma). Figs 13.2-13.7. Continental reconstructions, and present and paleo-shorelines, are from Smith et al., 1994 (Copyright © Cambridge University Press, reproduced with permission). Land areas shaded. Any indication of a surface equatorial counter current has been omitted, because of the uncertainties surrounding its existence and location in the geologic past. The currents shown in Figs 13.3,13.5,13.6, and 13.7 are based in part on Haq, 1989.

Fig. 13.2 Possible circulation pattern of surface ocean currents during the early Triassic (245 Ma). Figs 13.2-13.7. Continental reconstructions, and present and paleo-shorelines, are from Smith et al., 1994 (Copyright © Cambridge University Press, reproduced with permission). Land areas shaded. Any indication of a surface equatorial counter current has been omitted, because of the uncertainties surrounding its existence and location in the geologic past. The currents shown in Figs 13.3,13.5,13.6, and 13.7 are based in part on Haq, 1989.

Fig. 13.4 As for Fig. 13.2, for the Early Cretaceous (130 Ma).
Fig. 13.6 As for Fig. 13.2, for the mid-Paleocene (60 Ma).

and northwest Africa, as a consequence of the first phase of rifting of the supercontinent. This was initiated about 180 Ma ago. Thus the "Tethyan embayment" in Pangea (Fig. 13.2) was extended to the west to facilitate a circum-global equatorial current. This meant that some tropical waters were heated to a higher temperature before turning northwards and southwards to warm higher latitudes. In this way the whole Earth became warmer and the temperature gradient from the equator to the poles was further reduced.

The separation of Antarctica from Africa, which started about 165 Ma ago, was the first stage in the break-up of Gondwana (Fig. 13.4). This was followed at about 125 Ma by the rifting apart of South America and Africa, which started in the south and propagated northwards. This, coupled with the complex fracture zone pattern in the equatorial Atlantic region, due to transform faulting, meant that the gateway between the North and South Atlantic did not open up until about 95 Ma (Fig. 13.5) (Poulsen et al., 2001). The initial changes in the deep-water circulation, resulting from the opening of this gateway, may explain the "anoxic event" that produced the widespread black shales in adjacent areas at that time (Poulsen et al., 2001). By 95 Ma India had separated from Antarctica and a major Southern Ocean was opening up south of Africa and India. However, in the late Cretaceous, and even in the early Cenozoic (Fig. 13.6), the circum-equatorial current still existed, and the surface water in the high latitude oceans was still very much warmer than it is today.

Throughout the Cenozoic, Africa, India, and Australia continued to drift northwards, away from Antarctica, thereby enlarging the southern and Indian Oceans, and ultimately forming the Alps and the Himalayas as a result of the collision of Africa and India with Eurasia (Section 10.4.1). By 30 Ma (Fig. 13.7), the Tethyan seaway was effectively closed, and the Southern Ocean completely encircled Antarctica, as a result of the opening of gateways south of Tasmania, and in the Drake Passage, south of South America. These results of continental drift, gave rise to major changes in the near surface oceanic circulation. There was no longer a complete circum-equatorial current, and a pronounced circum-polar current was established in the Southern Ocean. Thus the equatorial water became less warm, and Antarctica was insulated from the warmer water circulating in the major southern hemisphere gyres of the Pacific, Atlantic, and Indian Oceans. A change in oxygen isotope ratios in the tests of planktonic and benthic microfossils (Shackleton & Kennett, 1975), and the first major build-up of ice on Antarctica, coincided with these developments, and appear to mark a transition from a Greenhouse to an Icehouse Earth. The change in oxygen isotope values is particularly pronounced and well documented, and is essentially coincident with the Eocene-Oligocene boundary (Fig. 13.8). This is also the time of the opening of the gateway south of Tasmania (Exon et al., 2002). The full opening of the Drake Passage is less well constrained, but was probably shortly after this (Livermore et al., 2004). Oxygen isotope ratios and a drop in sea level of 40 m suggest that during the early Oligocene the volume of ice in Antarctica built up to perhaps as much as one-half of its present volume. This and subsequent increases in ice volume, and changes in sea level, gave rise to an emergence of land areas, and a major reduction in the area of shallow seas on continental crust (cf. Figs 13.6, 13.7).

Following a period of warming and deglaciation in the late Oligocene (Fig. 13.8), additional major increases in the volume of ice on Antarctica, and associated drops in sea level, are thought to have occurred in the mid-Miocene and at the end of the Miocene. The drop in sea level associated with the increase in ice volume at 6 Ma may explain the isolation, and subsequent desiccation, of the Mediterranean Sea, as a result of the exposure of the sill at the Strait of Gibraltar (Van Couvering et al., 1976), and would have restricted the flow of water through the ocean gateway between North and South America. However, additional tectonic movements were required before a complete land bridge formed, about 3 Ma ago; as determined from the interchange of mammals between North and South America (Marshall, 1988). The gradual formation of the Isthmus of Panama would have led to the intensification of the Gulf Stream, and ultimately, perhaps, to the formation of the northern hemisphere ice-sheets (Haug & Tiederman, 1998).

The warm waters of the Gulf Stream would have given rise to more warm and moist air, and hence more precipitation, in relatively high latitudes in the North Atlantic area. The geographic distribution of ice sheets is determined not only by cold ambient temperatures, but also by the availability of precipitation. The Plio-Pleistocene ice-sheets of the northern hemisphere were restricted to Greenland, northern North America, and northwest Europe for this reason. Similarly, the occurrence of tropical rain forests is determined not only by high temperatures, but also by the delivery of

70 J_ I I I I 1 I I I I 1 I I j j 1 I j j I 1 I j I ■!■■■■■ ,

Fig. 13.8 Global deep sea oxygen isotope record compiled from measurements on benthic fauna from numerous Deep Sea Drilling Project and Ocean Drilling Project cores. Fitted curve is a smoothed five point running mean. The temperature scale relates to an icefree ocean and only applies therefore to the time prior to the onset of large scale glaciation in Antarctica (approx. 35 Ma). Much of the subsequent variability in the §'8O records reflects changes in Antarctic and Northern Hemisphere ice volume. When seawater evaporates, molecules containing the lighter isotope 16O evaporate more readily. Thus when atmospheric water vapor precipitates as snow in polar regions, '8O depleted water becomes sequestered in the polar ice caps and the proportion of '8O in seawater increases (part of figure 2 in Zachos et al., 2001, reproduced from Science 292, 686-93, with permission from the AAAS).

precipitation by warm equatorial currents. The tropical coal forests of the Carboniferous, for example, formed at the western end of the equatorial Tethyan embay-ment in the embryonic supercontinent of Pangea (Fig. 3.9). At the present day the most extensive areas of tropical rain forest are in the Amazon basin and the archipelago of southeast Asia, areas warmed by the main westward directed equatorial currents of today's oceans. The progressive cooling of the Earth's climate during the last 50 Ma, particularly in higher latitudes, led to a general reduction in the amount of precipitation, and an increase in aridity. Thus formerly forested areas in high latitudes were turned to tundra, and in temperate latitudes to grassland. As a consequence of the major cooling about 6 Ma, even some low latitude, tropical forests were converted to savannah. This is thought to have had a profound effect on mammalian, and, ultimately, human evolution.

During "Greenhouse Earth" conditions the oceans are warm throughout, with very little deep-water circulation. As a consequence the bottom waters become deoxygenated and there is the potential for the preservation of organic material and hence the formation of black shale deposits (Sections 3.4, 5.7). In as much as there is vertical mixing, it is probably triggered by regional changes in the salinity, and hence the density, of seawater in the tropics. Weak circulation, and the preservation of organic matter, meant that there was much less upwelling of nutrients compared to the present oceans. Thus, the overall fertility of the Cretaceous oceans was low, but the potential for the ultimate formation of oil, from Cretaceous marine source rocks, was high. In "Icehouse Earth" conditions cold, dense water forms in polar regions, sinks, and flows towards the equator, thereby creating a relatively vigorous, and certainly very significant, deep water circulation. The cooling that marked the transition from a Greenhouse to an Icehouse Earth, at about the Eocene-Oligocene boundary, probably enabled sea ice to form around the margin of Antarctica for the first time. During the formation of the sea ice much of the salt content of the seawater is expelled, increasing the density of the seawater beneath the ice. This cold, dense water would then sink to the ocean floor, and flow northwards, as it does at the present day.

One of the enigmas of the late Cenozoic cooling of the Earth is the relatively sudden build-up of ice in Antarctica in the Mid-Miocene (Fig. 13.8). One interesting and remarkable possibility is that it was caused by a change in the topography of the sea floor in the extreme North Atlantic, as a consequence of tectonic processes

(Schnitker, 1980). It is likely that the Greenland-Iceland-Faroes Ridge had subsided sufficiently at this time for cold water from the Arctic to spill over this sill and sink towards the ocean floor. Although cold and saline, it is not as dense as the Antarctic bottom water moving northwards. As a consequence, the Arctic water travels south at an intermediate depth, and is ultimately deflected towards the surface off Antarctica. Here it is "warm," relative to the surrounding seawater, and creates more moisture laden air, and hence enhanced precipitation over Antarctica. This model again emphasizes the importance of precipitation, in addition to sub-zero temperatures, in facilitating the build-up of an ice sheet.

albedo. However, the formation of mountain belts may affect the climate in a more substantial way, by changing the rate of weathering at the Earth's surface, which in turn affects the amount of carbon dioxide in the atmosphere.

The weathering of carbonates exposed on land, by a weak carbonic acid solution, formed by the dissociation of carbon dioxide from the atmosphere, or soil, in rainwater, produces calcium and bicarbonate ions that are then transported to the ocean by rivers. In the oceans the weathering reaction is reversed: calcium carbonate is secreted by organisms, to produce their tests, which, if preserved after the death of the organism, form carbonates on the sea floor.

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