Role of Ocean Gateways in Climatic Change
Woods Hole Oceanographic Institution and Brown University
With its low albedo and large capacity to store heat, the ocean represents one of the most important components of the global climatic system. With a specific heat per unit mass of 4 times that of air, water (i.e., the ocean) has a thermal capacity over 1000 times that of the atmosphere. The energy imbalance from incoming solar radiation between the summer and winter hemispheres and between high and low latitudes results in motions (i.e., transport) in the atmosphere and oceans. The result is a net transport of energy from summer to winter hemispheres and from low to high latitudes. There is little variation at low latitudes in temperature and energy transport. The ocean, with its high thermal capacity, acts as a buffer for such energy changes.
The ocean, at the same time, plays an important role in the meridional transport of energy. The question may then be asked: Has it always been so? The answer is unequivocally no. Present continent-ocean geometry is the result of a long [200 million year (m.y.)] history of changing spatial configurations. The role of gateways (and barriers) is fundamental for recreating the scenario of past circulation histories (and their concomitant effect on paleoclimate). We are only at an early stage in understanding this history, but we can outline the following tentative scenario.
The underlying assumption is the general view that the global climatic cooling characteristic of the Cenozoic is due to natural, earthbound causes, namely the poleward shift of continental masses and the gradual development of a meridional (Cenozoic) rather than latitudinal (Mesozoic) oceanic circulation pattern. Late Neogene (last 15 m.y.) climatic oscillations are, in turn, no doubt the result of the superposition on this global cooling trend of short-term periodic fluctuations in orbital parameters (i.e., the astronomical theory of glaciation, which is less a theory than a mechanism).
The discussion below is presented in two parts. The first deals with a brief summary of the role of the oceans (and their geometry) in heat transfer and thus, ultimately, as a source of climatic change. The second summarizes the main paleogeographic events that have had an influence on oceanic circulation and their effect, where discernible, on climatic evolution.
OCEANS, CLIMATES, AND GATEWAYS
The oceans occupy nearly three quarters of the surface of the Earth and receive over 80 percent of the total solar radiation absorbed at the surface. With a specific heat (0.93 cal g−1 °C−1) considerably larger than that of land (0.60–0.19 cal g−1 °C−1), seawater yields up its stored heat much more reluctantly than do land areas. With an overturn (i.e., mixing) rate of 500–1500 yr, the oceans may be viewed as vast heat reservoirs that act as a conservative element in maintaining climatic stability.
The Sun is the ultimate source of ocean currents. Uneven absorption of shortwave radiation over small areas on the ocean surface results in density differences that, in turn, set in motion, slow, lateral water-mass movements. Heat energy is transported and distributed accordingly. Deeper waters are warmed by the slower processes of convective mixing. In polar regions such mixing is ultimately complete, whereas elsewhere the thermocline denotes the level to which mixing is effective. In spite of the large amount of radiation that reaches the surface of the ocean, vertical heat transfer is limited to the surficial layers because nearly 90 percent of the heat energy is used in evaporation.
The ocean accounts for nearly a third of the poleward transfer of heat energy [the remaining two thirds is accounted for by atmospheric transport in the form of sensible (50 percent) and latent (15 percent) heat]. This is accomplished primarily in the form of wind-driven surface currents. In a typical ocean basin, surface circulation assumes the form of an anticyclonic gyre centered on a semipermanent (seasonally shifting) area of stable high pressure within the basin. Equatorward the gyre is constrained by the seasonally migrating intertropical convergence zone, poleward by the high-latitude easterlies and longitudinally by landmasses. Gyres commonly consist of four components: a westerly equatorial current, a polar easterly current, a poleward ascending western boundary current, and an equatorial descending eastern boundary current. The intensity and volume of a given current gyre is dependent on wind stress and such geostrophic factors as the Coriolis force, the piling up of waters on the western margins of ocean basins, and the constrictions or deflections by continental geometry.
Heat energy is absorbed by surface waters as a function of latitude. The equatorial zone is the basic source of most of the heat energy in oceanic gyres, although additional radiation is added over the entire extent of a meridional current. In subequatorial belts, westward-moving gyre currents absorb heat at the rate of about 0.1°C for each degree of longitude. Because of this lengthy exposure to high insolation, the maximum surface temperatures are found in the western equatorial extremities of ocean basins. Eastwardly moving currents on the polar sides of gyres lose heat at about the same rate. Meridional currents moving poleward or equatorward cool and warm, respectively, at an average of 0.3°C for each degree of latitude.
At present, the most important oceanic currents from a climatic point of view are the poleward moving extremities of the western boundary currents (such as, North Atlantic Current, Kuroshio, and Brazil Current). These currents transport warm surface waters into regions of low temperatures and thus enhance high-latitude atmospheric heating and evaporation.
By way of contrast, the presence of the Antarctic Circumpolar Current in the southern hemisphere prevents penetration of westerly boundary current beyond 40° S and serves to isolate the Antarctic continent from major sources of precipitation.
This brief summary gives an idea of the role that the oceans play in transporting heat energy and their role as a stabilizing force in maintaining the climatic status quo.
At the same time, it will be apparent that the oceans contain the potential for playing a significant role in climatic change given the right set of conditions. The fact that the geologic record provides ample evidence of significantly different climates in the past (when compared with that of today) suggests significant changes in the ocean currents and atmospheric circulation patterns of the past.
Gateways (and barriers) represent the means for transporting (or blocking) heat energy from one area of the globe to another. Identifying these gateways (barriers) and their geometry and refining the time of their inception is of fundamental importance to delineating the scenario of the past climatic history of the Earth.
Global paleography during the Triassic suggests that the major continental landmasses were essentially fused together into a single supercontinent, Pangaea, which was of bipolar extent. Our knowledge of detailed ocean-continent paleogeography during this time, however, is such as to preclude interpretations of the possible role of ocean gateways on climate. The Tethys Sea extended westward as a triangular re-entrant of Panthalassa—the global ocean—between Africa-Arabia to the south and southeast Asia to the north. A westward flowing equatorial current may be deflected into northern and southern high latitudes on encountering the eastern margins of Pangaea. Relative cooling would have occurred at higher latitudes so that these currents would have descended equator-ward as cool eastern boundary currents along the western margins of North and South America.
The great abundance and extensive latitudinal distribution of evaporites in the mid-Triassic and the mid-late Triassic development of coral reefs (including the earliest hermatypic hexacorals, at present restricted to shallow warm waters circumscribed by the 30° parallel in the northern and southern hemisphere) along the northern margin of the Tethys Seaway (− 30° N) attest to warm (i.e., tropical) climates extending to 60° latitude in both hemispheres. The effects of increased continentality (fusion of Pangaea in the Permian) was no doubt an important factor in the climate of the Triassic.
Jurassic paleogeography did not differ significantly from the Triassic with one major exception (see below). The latitudinal extension of the continents from pole to pole essentially blocked circumglobal oceanic circulation at high southern latitudes. The Tethys was a large triangular re-entrant of Penthalassa—the global ocean—into the eastern continents from the east. Evidence of distinct temperature-linked gradients is ambiguous during the Jurassic [although floral evidence suggests that a “tropical” belt with cycadlike plants and ferns can be distinguished from “temperate” belts with conifers and ginkgos (Barnard, 1973)]. The essentially latitudinal circulation pattern (deflected into higher latitudes around the polar extremities of Pangaea) would appear to have contributed to the equitable climate of the Jurassic.
A significant circulation event occurred during the late Jurassic—the opening or extension of a seaway through Central America, linking the “Atlantic” and “Pacific” Oceans for the first time in the equatorial region. Coupled with the gradual opening of the North Atlantic by seafloor spreading, this allowed the development of a circumglobal equatorial current system, which has continued, essentially unabated, until relatively recent time [about 3 million years ago (Ma)].
Trans-Atlantic flow of warm, equatorial waters from the Tethys into the equatorial Pacific via the Central America Seaway resulted in a circumglobal equatorial current that served as the main dispersal agent of tropical faunal and floral elements throughout the Late Mesozoic and Cenozoic (Berggren and Hollister, 1974, 1977).
The effect of this event on climate during the late Jurassic is less clear, although the expansion of extensive evaporite deposits into higher latitudes and the concomitant decrease of coal formation suggest increasing aridity. Although there are some discrepancies between data from the rocks (evaporites, corals, bauxites, carbonates), fossils (reefs, microfossils, flora, dinosaurs), and oxygen isotopes, a synthesis of present data suggests warm climates—although on the average the Jurassic may have been slightly cooler than the Triassic (Frakes, 1979). For instance, the latitudinal extent of Jurassic evaporites suggests that it was drier than the Triassic on average. On the other hand, the temporal distribution of evaporites suggests that the early Jurassic, as well as the Late Triassic, was probably cooler and more humid than the middle Triassic.
The significance of the expansion of carbonates into higher latitudes in the Late Jurassic as a climatic indicator is equivocal; it may be a function of eustatic sea-level changes (Hallam, 1975). Although a link between transgressions and
an increase in global temperature (resulting in an expansion of the tropical belt and concomitantly, carbonate deposition) appears to be valid in the later Mesozoic and Cenozoic, it is not possible to determine this relationship on the scale treated here.
Data from paleobiogeography and the distribution of rock types suggests that the Cretaceous was a time of high sea levels, shallow epicontinental seas, and warm, dry climates in which tropical conditions comparable with those of today extended under optimum condition beyond 45° N and 70° S latitude (Frakes, 1979). Beyond this, climatic zones were warm to cool-temperate. Mean annual temperatures have been estimated to have been between 10–15°C warmer than the present day, with a temperature gradient about half that of the present.
Oxygen isotope data (Savin, 1977) suggest a general warming trend during the Early Cretaceous (Valanginian-Albian), followed by a gradual but significant cooling in the Late Cretaceous (Campanian-Maestrichtian). Superimposed on this general trend is a shorter-term temperature decrease during the Albian or Cenomanian, followed by a temperature rise in the Turonian and/or Coniacian.
The Early Cretaceous warming may have brought bottom-water temperatures in the central and northwestern Pacific up to an Albian maximum of about 17°C and the Late Cretaceous cooling to a minimum of about 11°C. Concomitant surface temperatures were about 28°C (Albian) and about 19°C (Maestrichtian) (Douglas and Savin, 1975; Frakes, 1979).
The continents were clustered together and continued to extend nearly from pole to pole during the Cretaceous. A single gyre system continued to operate in each hemisphere to about 50–60° latitude, with wide, deep, and sluggish equatorial currents being substantially heated by the long fetch (over 180° of arc) in low latitudes. Poleward transport of this heat in the gyres to the latitude of the descending subtropical circulation cells and the subsequent evaporation nutured the humid, somewhat cooler zones near the poles. Thus the presence of coals at 70° S (New Zealand) and to within a few degrees of the pole in the northern hemisphere (Alaska) may have their explanation in the warm, wide, and deep western boundary-ocean currents that could have flowed unimpeded along the western margins of the Pacific into high latitudes.
The role of ocean pathways in the development of the climatic history of the Cretaceous is difficult to assess at this time simply because they are difficult to identify. One of the major paleogeographic events of the Cretaceous was the separation of Africa and South America and the formation of a permanent marine connection between the North and South Atlantic Oceans during the Turonian Age, about 95 Ma (Reyment and Tait, 1972a, 1972b). However, the existence of barriers to deep-water circulation in the Arctic and Antarctic regions as well as the Rio Grande Rise and Walvis Ridge in the South Atlantic would appear to have nullified any significant contribution to climatic change that the initiation of an oceanic gateway for surface circulation between the North and South Atlantic might have had.
The continued fragmentation of the world ocean led to an increasingly inefficient latitudinal thermal energy exchange. Paleobiogeographic and oxygen isotopic studies yield a complementary picture of a long-term global temperature decline, development of a thermally stratified ocean, and enchanced zonal climatic differentiation during the Cenozoic (Berggren and Hollister, 1974, 1977; Frakes, 1979). This climatic decline followed a faunally and florally (Haq and Lohmann, 1976; Haq et al., 1977) and oxygen isotopically (Boersma and Shackleton, 1977; Vergnaud-Grazzini et al., 1978, 1979) recognizable “climatic optimum” in the early Eocene.
The opening of the northeast Atlantic (between Greenland and Norway) began in the early Eocene at about the same time as the separation of Australia and Antarctica (Berggren and Hollister, 1974, 1977). The widespread development of biogenic siliceous deposits in a circumequatorial belt during the Middle Eocene has been linked with the presence of persistent wind-driven (trade winds), diverging ocean-surface currents and associated upwelling, as well as temporally related volcanism in the northeast Atlantic associated with the early phase of rifting and seafloor spreading (Berggren and Hollister, 1974, 1977; Drewry et al., 1974). The high-latitude source of the intensification of oceanic surface and deep-water circulation has been the subject of considerable debate. Although vertebrate faunal evidence suggests elimination of a subaerial barrier in the Norwegian-Greenland Sea at 50 Ma (McKenna, 1972), recent geophysical data suggest a later, early Oligocene date (about 35 Ma) for the separation of Greenland-Spitsbergen (Talwani and Eldholm, 1977). The picture is complicated by the fact that the Greenland-Iceland-Faeroe Ridge may have prevented significant exchange of water from the polar regions to the North Atlantic until the Late Oligocene, about 23 Ma (Tucholke and Vogt, 1979).
An alternate source for the origin of cold, deep water formation—the “big flush”—may lie in the high southern latitude, probably from the Weddell region of the Antarctic (Schnitker, 1980). These cold bottom waters could have moved northward into the North Atlantic through the nascent fracture zones developing in the subsiding Rio Grande Rise.
In the southern hemisphere the northward transit of India diverted the westward flowing Pacific-equatorial current into the counterclockwise subtropical gyre, which penetrated southward, past eastern Australia into the nascent gateway that was opening up between Australia and Antarctica. Isolation of the warm Pacific gyres at these high southerly latitudes led, through adiabatic cooling and evaporation, to precipitation on the Antarctic continent, which eventually led to full continental glacial conditions. This was the beginning of the
gradual thermal isolation of the Antarctic continent, which was essentially completed by mid to late Oligocene time (about 25 Ma) with the subsidence of the Tasman Rise and the opening of the Drake Passage and which has continued to this day in the form of the circum-Antarctic current as the gateway has continued to widen during the Neogene.
Oxygen isotope studies indicate a progressive cooling from the Middle Eocene to Early Oligocene time in the sub-Antarctie (Shackleton and Kennett, 1975; Kennett and Shackleton, 1976; L.D. Keigwin, University of Rhode Island, personal communication), North Atlantic (Vergnaud-Grazzini et al., 1978, 1979), South Atlantic (Boersma and Shackleton, 1977), and mid-high latitude Pacific (Douglas and Savin, 1975; Savin et al., 1975). Superimposed on this trend is a pronounced cooling event seen near the Eocene-Oligocene boundary in the Pacific and sub-Antarctic regions, whereas in the Atlantic the cooling appears to be more gradual and of lesser magnitude (Kiegwin, 1980). The relationship of the opening of the Greenland-Norwegian Sea to the Arctic and the possible initial opening of the Drake Passage at about this time (about 38 Ma, J.Sclater, Massachussetts Institute of Technology, personal communication) to the climatic “event” at the Eocene-Oligocene boundary remains unclear because of the lack of precision in dating these various events, but they may be expected to have played a role in enhancing the latitudinal thermal gradients resulting from a greater latitudinal transport of cooler waters originating at high latitudes.
At about the same time, about 40 Ma, the significant generation of cold bottom waters in the Antarctic (and possibly the North Atlantic) resulted in the formation of the psychrospheric fauna, which today lives at temperatures less than 10°C. The appearance of this fauna in the Atlantic Ocean and Tethys Sea at about this time suggests that the Rio Grande Rise has been breached, allowing cold, dense waters to move along a north-south meridional corridor, enhancing the transition from a latitudinal thermospheric circulation to a meridional thermohaline circulation. This transition has been one of the major threshold events in the evolution of the present climate of the world. From this time on, thermally differentiated water masses have moved in a basically meridional pattern, transferring heat across latitudes and establishing a basically latitudinally controlled, zonal climatic gradient of significant proportions.
With the opening of the Drake Passage by mid- to late-Oligocene time the circumpolar current had been established, thermally isolating the Antarctic Continent from the influence of warmer waters to the north (Kennett et al., 1972, 1975). This event appears to be closely linked chronologically with the extension of glaciers on Antarctica to sea level, as shown by drilling in the Ross Sea (Hayes and Frakes, 1975).
During the Late Cretaceous and Paleogene, northward movement and rotation of Africa continued to close the Tethys Sea in the east. In the west, right-lateral motion between Africa and Europe narrowed the western junction of the Tethys with the Atlantic between Spain and Morocco. The junction of Arabia and Asia, severing the marine gateway connection between the Indo-Pacific and Atlantic Oceans, occurred during the Early Miocene, about 18 Ma (Van Couvering and Miller, 1971; Berggren, 1972a; Berggren and Hollister, 1974). The transition from a thermospheric to two-layered psychrospheric structure in the Tethys Sea appears to have been effected by Late Eocene time, about 40 Ma (Benson and Sylvester-Bradley, 1971). The effect on climate of the closure of the east-west Tethyan gateway and the subsequent evolution of the Tethys into an eastern (Indo-Pacific Ocean) and western (Mediterranean Sea) part is not clear at this point.
A major decline in isotopic temperature occurs in the Middle Miocene, about 14 Ma (Savin, 1977; Woodruff et al., 1980), and it has been linked with the establishment of the East Antarctic Ice Sheet (Shackleton and Kennett, 1975; Savin, 1977). This cooling event is difficult to link with the development or termination of a structural barrier to flow of water masses because of the disparity in the data that exist on the timing of various events in the North Atlantic. However, the concomitant effects of the severance of the Tethys Seaway and the subsidence of the Iceland-Faeroe Ridge (linking the Arctic polar sea and the Atlantic Ocean) have been linked in a scenario that connects this thermal event with the development of Antarctic glaciation and the evolution of “modern” ocean circulation (Schnitker, 1980). The scenario is as follows:
Closure of Tethys in the east (18 Ma) created an evaporation basin that contributed increasing amounts of warm, saline waters to the North Atlantic.
Continued fragmentation of Tethys elsewhere (northward drift of Australia separated the equatorial Indian Ocean from the equatorial Pacific) closed off the deep-water connection and reduced the flow of the equatorial current system.
Restriction of the Central American Seaway by the gradual emergence of the Isthmus of Panama gradually reduced the flow of the equatorial current between the Atlantic and Pacific. Deflection of the North Equatorial Current northward increased the strength of the Gulf Stream.
The breakup of Tethys and its low-latitude circumglobal flow resulted in a temporary increase in surface-water temperatures in the late early Miocene.
The gradual subsidence of the Iceland-Faeroe Ridge allowed North Atlantic surface waters into the Norwegian during the early Miocene. By mid-Miocene time subsidence had progressed to the point where reflux of cold bottom water became significant. The Arctic Ocean was then linked with the world ocean as a heat sink.
Early North Atlantic deep water, resulting from Norwegian Sea overflow, traversed the length of the Atlantic and inserted itself as an intermediate water mass in the circumAntarctic current system.
Relatively “warm” and saline North Atlantic deep water would rise to the surface near the Antarctic rather than the cold, low-salinity local water. This warm, saline water contained heat that could be converted to latent heat by evaporation. The resulting high evaporation rates supplied moisture
to Antarctica sufficient to push it across the threshold value needed to build up and maintain a stable continental ice cap.
During the latest Miocene (about 5.5–5.0 Ma) the west Tethys Sea was isolated from the Atlantic Ocean. In the Pliocene, marine connections were established and the Mediterranean Sea gradually evolved from a thermohaline to its present thermospheric condition. The effect of the closure of the marine gateway linking the Tethys and Atlantic and the subsequent desiccation of the West Tethys Sea on climate is not clear. In general, the increased continentality would have resulted in enhanced climatic seasonality, although the formation of extensive evaporite deposits (i.e., sabkhas) indicate hot arid conditions as one component of the overall climate. The subsequent opening of the Gilbraltar gateway and the evolution of a thermospheric watermass in the Mediterranean has resulted in art “inland sea” in which there is an excess of surface evaporation over local inflow from rivers and precipitation. The Mediterranean would seem to have a buffering effect on climate, contributing (together with the circumMediterranean Alpine chain to the north and the broad, expansive desert areas to the south) to the “Mediterranean climate” to which it gives its name.
Perhaps one of the most significant late Neogene paleogeographic events was the elevation of the Isthmus of Panama, which resulted in the termination of the marine gateway linking the Atlantic and Pacific Oceans, at 3.5 Ma (Berggren and Hollister, 1974). This event had a dramatic effect on oceanic circulation patterns (interruption of the Atlantic to Pacific equatorial current system), paleobiogeography of marine (change from global to disjunct distribution patterns) and terrestrial (re-establishment of north-south migration route— “The Great American Faunal Interchange”) organisms. The effect of this barrier on climate is not fully clear, but the proximity in time of its formation to the initiation of northern hemisphere polar glaciation (about 3 Ma; Berggren, 1972b) may indicate a significant role—pumping increased volumes of warm, high-salinity waters (Gulf Stream) into high latitudes where evaporation led to precipitation over the region of eastern Canada and Greenland, cooling, and eventually to the development of the polar ice cap (Luyendyk et al., 1972; Berggren and Hollister, 1974).
During the last 100,000 yr eustatic sea-level fluctuations have led to alternate opening and closing of the Straits of Bab el Mandab in the southern Red Sea and the temporal isolation (and partial evaporation) of the Red Sea (Berggren, 1969). This cyclic process may be expected to have occurred earlier in the Pleistocene as well, but stratigraphic evidence is lacking. What effect these events would have had on climate is not clear, but they may be expected to have been minor and of local extent.
The history of climate over the past 200 m.y. is one of changing oceanic currents that are in turn the result of changing ocean-continent geometries. During the Mesozoic, the conti nents of the globe were grouped into a single landmass of essentially bipolar extension. This configuration resulted in a highly efficient, poleward heat transport by large, sluggish gyre systems that were substantially warmed by equatorial transit across nearly half the globe.
The gradual breakup of Pangaea and the consequent dismemberment of Panthalassa into the several connected, but distinct, oceans of today resulted in a decline in the equitable climatic conditions of the Mesozoic. The initial event in the sequence of events leading to the present climatic conditions may have been the mid-Mesozoic (about 140 Ma) development of a circumequatorial, Tethyan seaway that would have had the effect of establishing additional stable gyre systems, thereby diminishing the efficiency of the gyre system in poleward heat transport. As more oceans and gyres have been created and low-latitude seas have become less extensive (through continental drift), global climatic equatability has declined, accelerating significantly during the Cenozoic. In general terms, Mesozoic circulation was latitudinal (and meridional transport of heat energy was relatively efficient), whereas Cenozoic circulation has been predominantly longitudinal (meridional), although meridional heat transport has become increasingly less efficient. The role of gateways has been of fundamental importance in this history of declining climatic equatability because they have served as the conduit through which new current systems are introduced into a new ocean-continent geometry.
I thank J.P.Kennett and A.M.Ziegler for their critical reviews of an earlier draft of the manuscript. This study has been supported by the Submarine Geology and Geophysics Branch of the Oceanography Division of the National Science Foundation, Grant No. OCE-7819769. This is Woods Hole Oceanographic Institution Contribution No. 4759.
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