The Jurassic Climate
University of Birmingham, England
All the reliable evidence that we can muster points strongly to the conclusion that the Jurassic climate was appreciably more equable than that of the present day, with tropical-subtropical conditions extending far into the present temperate belts and temperate conditions occurring in polar regions. There is no evidence of polar ice caps, and, at least partly for this reason, the ocean surface stood at a higher level with respect to the continents. This in itself must have contributed to the higher equability of continental climates. In addition, there appears to have been in general more extensive aridity on the continents whose distribution was, of course, appreciably different from that of today.
Unfortunately, with the present state of knowledge it is difficult to go beyond broad qualitative statements. For the Jurassic, we are denied the excellent information of oceanic surface-and bottom-water temperatures obtained from oxygen isotope analysis of microfossils in deep-sea drilling cores of Cretaceous and Cenozoic strata. Furthermore, Jurassic fossils have been extinct too long to have close modern relatives whose climatic tolerances are precisely known. In the following sections of this paper the principal climatic criteria are briefly outlined and the evidence for climatic changes through space and time discussed. A fuller account of some topics, with additional references, is given by Hallam (1975).
By far the best climatic indicators among sedimentary rocks found in the Jurassic are evaporites and coals (Frakes, 1979). Substantial deposits of evaporites (notably gypsum, anhydrite, and halite) indicate conditions of both warmth and aridity, whereas coals indicate swampy conditions in generally humid regimes, though there is no particular temperature connotation. On the other hand, the abundance and extent of deposition of limestones is not particularly reliable. In particular, the greater spread of limestone facies in the late Jurassic, far from signifying increased temperature, is probably no more than a consequence of the greater extent of epicontinental seas at that time (Hallam, 1975).
Laterites and bauxites have been widely considered to be good indicators of humidity, because of the intensity of chemi-
cal weathering required for their formation. However, in southern Israel there is a horizon of reworked laterites, including pisolitic conglomerates, sandwiched between evaporite-bearing Upper Triassic and Lower Jurassic deposits and attributed to the basal Jurassic. Following the development of a karstic surface after an episode of regression, leading to the subaerial exposure of hypersaline mud flats, so-called flint clays were generated by chemical weathering in the vadose zone (Goldbery, 1979). Whereas swampy conditions might have occurred locally, no drastic regional increase of humidity after the late Triassic, followed by a return to aridity, apparently needs to be invoked.
Aeolian sandstones with large-scale dune cross bedding should be good indicators of desert conditions, but doubt has been thrown on the aeolian origin of at least part of the wellknown Lower Jurassic Navajo Sandstone of the Rocky Mountain states. Stanley et al. (1971) recorded ripple-marked and wavy-bedded horizons, shale seams associated with widespread truncation planes, and dolomitic carbonate lenses, suggesting a subaqueous origin for the deposits containing them. Some fossil organisms are good temperature indicators, notably hermatypic corals in marine and ferns in terrestrial deposits. Reef-building corals suggest a minimum water temperature of about 20°C, and there are abundant Jurassic ferns whose living relatives cannot tolerate frost. The occurrence of genera or, better, species of a wide variety of organisms over a broad range of latitude is in itself a strong argument for climatic equability.
Considerable attention has been paid to oxygen isotope paleotemperature determinations on Jurassic belemnites obtained from rocks currently exposed on the continents. There is such a wide disparity in the results of various workers, however, presumably as a consequence of significant postdepositional alteration, that I have argued at some length that they contribute little to a further understanding of Jurassic climates (Hallam, 1975). Unfortunately, there is only a negligible record of Jurassic microfossils in deep-sea drilling cores, from which one might expect to obtain more reliable results.
CLIMATIC CHANGES IN SPACE
The fact that rich Jurassic terrestrial fern and gymnosperm floras are known from both polar regions is a strong argument in favor of general warmth and equability, and this is strongly supported by the wide distribution of fern genera whose modern relatives are intolerant of cold (Barnard, 1973). Thus the basal Jurassic Dictyophyllum ranges from 50° N to 60° S, and a number of Middle Jurassic genera are almost as widespread, from 40° N to 50° S. These distributions imply a tropical-subtropical climate extending far beyond the present limits. According to Vakhrameev (1964), the plant record indicates that winter temperatures in Siberia probably never fell below 0°C. Equability is also indicated by the wide latitudinal range of large reptiles (Colbert, 1964) and ceratodontid lungfishes. The latter, whose living relatives are confined to the tropical-subtropical zone, are more or less worldwide in distribution (Schaeffer, 1971).
If the continents enjoyed a warm, equable climate, the same should be true of the marine realm; and indeed the majority of invertebrate genera are cosmopolitan in distribution. While substantial carbonate buildups partly composed of corals are confined to deposits in what are inferred on palaeomagnetic grounds to have been low latitudes, such as the Pliensbachian of Morocco, the Oxfordian of the Swiss Jura and southern Poland, and the Bajocian and Oxfordian of the Paris Basin (Figures 17.1 and 17.2), reef-building corals are also found as far as 60° N paleolatitude, in Sakhalin, some 30° beyond the present limits (Beauvais, 1973). The absence of such corals from comparably high latitudes in the North Atlantic region and southern hemisphere is quite probably due to factors other than low temperature,
The bivalves are instructive to study, both because they are the most abundant and diverse macroinvertebrate group in the Jurassic and because they include many extant families and even genera whose climatic tolerance is well known. In marked contrast to the present-day situation, there is no sharp reduction in diversity with increasing latitude, and many genera and even some species have a wide latitudinal range. A particularly good example is provided by the pectinid genus Weyla, largely confined to the eastern Pacific margins, with the same species extending all the way from Chile to southern Alaska. The best candidates for a stenothermal tropical group, restricted to a belt within 30° of the Jurassic equator, are a minority of thick-shelled genera including rudists (Hallam, 1977). There is also a group of distinctive foraminifera more or less confined to the zone of the Tethys and thought to be a stenothermal tropical group (Gordon, 1970).
There has been considerable controversy about the environmental cause of the Tethyan and Boreal provinciality exhibited by ammonites and belemnites, with the Boreal Realm (or superprovince) being confined to the northern part of the northern hemisphere. The majority opinion is that ambient
temperature was the primary control, but there are difficulties with such a simple interpretation. One of the most important objections is that a drastic change from one faunal realm to the other may take place within a mere degree of latitude, yet the evidence of the climatically more sensitive plants indicates strongly that latitudinal temperature gradients were much more modest than today. It seems necessary to invoke a complex of factors, including paleogeographic configurations, environmental stability, and perhaps such latitudinally related factors as changing patterns of diurnal illumination and constancy of food resources throughout the year (Hallam, 1975). Climate is unlikely to have been the dominant control in all events.
With regard to the distribution of arid and humid belts, the best evidence comes from evaporite and coal deposits (Figures 17.1 and 17.2). According to Gordon (1975), evaporites range between 45° N and 45° S, but the deposits are concentrated in a narrower zone 10–20° from the palaeoequator. Nearly all of the evaporites are confined to the western parts of Laurasia and Gondwana. Among the more substantial deposits in North America are the Lower Lias Argo Salt of the Scotia Shelf and the probably Middle Jurassic Louann Salt and equivalents around the margins of the Gulf of Mexico, while important deposits of gypsum and anhydrite occur in the Bajocian, Callovian, and Oxfordian of the United States Western Interior. There are also evaporites in the Andean Jurassic, in the Oxfordian and Kimmeridgian of northern Chile, Turning to the Old World, there are thick sequences of pre-Bathonian evaporites around the northwestern, northern, and eastern margins of Africa and in the Jurassic of the southern U.S.S.R., southern Iran, and Arabia (Hallam, 1975; Leeder and Zeidan, 1977). Thinner deposits occur also in the basal and late Jurassic of western, southern, and central Europe.
By far the most abundant coal measures occur over a wide area of the Soviet Union, especially in the Lower and Middle Jurassic, and there are also important Lower Jurassic coals in eastern Australia. In Europe thin Lower Jurassic coal beds are known in the so-called Gresten facies of northern Austria and the Mecsek Mountains of Hungary, also in the basal Liassic of southern Scandinavia and the Middle Jurassic of the northern North Sea. In the New World, coals are much rarer, but thin coal seams occur in the Lower and Middle Jurassic of southern Mexico as well as in the Upper Jurassic of Montana, the Dakotas, Alberta, and British Columbia (Jansa, 1972).
The overall geographic distribution of evaporites and coals bears quite a close resemblance to that of the Triassic, and so Robinson’s (1971) inference of a western arid belt and two eastern humid belts seems to apply also to the Jurassic. Hence Robinson’s climatic model is relevant. She suggests that winds reaching the eastern parts of Laurasia and Gondwana, on either side of the Tethyan Ocean, might have brought monsoon-type summer rains to areas of middle and low latitude, while a dry, hot season would occur in winter as winds blew offshore. The central and western parts of the two supercontinents would have tended to have a much less humid climate because the dominant easterly winds would have traveled over land for a considerable distance or, blowing toward the equator without the intervention of mountains, could not readily have jettisoned their moisture. Coal occurrence is largely restricted to the eastern, peninsular parts of the landmasses in middle to high latitudes, where the temperature was more and the rainfall less strictly seasonal. It is worth adding that coals form less readily in the tropical than in the temperate zone (Frakes, 1979).
CLIMATIC CHANGES THROUGH TIME
There is no convincing evidence of any notable global temperature change through the course of the Jurassic. The best evidence available concerns the areal distribution of terrestrial plant provinces in Eurasia. Vakhrameev (1964) drew a boundary between a northern, possibly temperate, Siberian Province and a southern, presumed subtropical, Indo-European Province. He detected a slight northward shift of this boundary from the early to the mid Jurassic and an appreciably greater northward shift from the mid to the late Jurassic (Figure 17.3). The implied slight warming trend through the period continues into the Cretaceous.
More can be said about humidity-aridity distribution. From the greater spread of evaporite facies in the late Jurassic, for instance into Chile and the southern parts of the Soviet Union, Frakes (1979) inferred an overall trend toward a drier climate from early in the period. This is confirmed by the occurrence of xerophytic plants in the late Jurassic of the latter area (Vakhrameev, 1964). However, the regional picture may be more complicated. Thus, in Israel the Lower Jurassic contains evaporites, while the Middle and Upper Jurassic contains coals, so that the climatic trend through time is inferred by Goldberg and Friedman (1974) to have been the reverse of what is assumed to be the general picture. Yet there are abundant late Jurassic evaporites in southern Europe and the Middle East. Goldberg and Friedman stressed the importance of regional climatic change and draw an analogy with the Gulf of Mexico margins. In southern Texas, for instance, a dry climate is recorded by gypsum deposits in the Laguna Madre, whereas the moister climate of Louisiana is reflected by salt-marsh deposits. Perhaps local swampy conditions in an area of moderately dry climate can promote the formation of thin coals, in which case the validity of coal distribution as a climatic indicator needs to be more closely investigated.
Frakes (1979) argued for a continuation of the global trend toward greater aridity into the Cretaceous. Yet evaporite-bearing deposits in the Jurassic of the western interior of the United States are succeeded by coal-bearing deposits in the Lower Cretaceous. On the other hand, the facies change from the Upper Triassic to the Lower Jurassic in western Europe supports Frakes’s postulation of a global change toward increased humidity. Thus the Keuper red beds contain evaporites and a suite of clay minerals, in which kaolinite is absent, suggestive of postdepositional magnesium enrichment in hypersaline water (Jeans, 1978). Substantial quantities of kaolinite, suggesting intensive leaching on a land experiencing a warm, humid climate, first appear in the topmost Triassic (Rhaetian) marginal marine deposits and continue into the Lias (Will, 1969). A humid climate is confirmed by the occurrence in northern Europe of Rhaeto-Liassic plant beds including coals and perhaps also by the more widespread occurrence of Liassic ironstones (Hallam, 1975).
With regard to the oceans, much interest has been provoked by Fischer and Arthur’s (1977) model of cyclic alternations, lasting about 32 million years and ranging back to the Triassic, between what they term polytaxic and oligotaxic episodes. Polytaxic episodes are characterized by high organic diversity, higher and more uniform oceanic temperatures, with continuous pelagic deposition, widespread marine anoxicity, and eustatic sea-level rises. In contrast, oligotaxic episodes, such as at present, are characterized by lower marine temperatures with more pronounced latitudinal sedimentation, marine regression, and a lack of anoxicity. During polytaxic episodes, warm, globally equable climates result in reduced oceanic convection, causing expansion and intensification of the oxygen minimum layer, while colder climatic intervals give rise to increased circulation rates and better oxygenation of ocean waters.
Whereas there may well be some merit in the Fischer and Arthur model for the Cretaceous and Cenozoic, for which we have an ample record from deep-ocean cores, the evidence they cite for the Jurassic, such as oxygen isotope data from belemnites, is dubious, and I see no grounds for their invocation of an oligotaxic episode in Bathonian-Callovian times. I am rather inclined to believe that the whole of the Jurassic was a polytaxic episode, at least with regard to climate and oceanic circulation.
Perhaps the greatest advance in the future will come from paleoclimatic modeling of the type outlined by Gates (Chapter 2). The geographic location of the continents and oceans is accurately known, and reasonably accurate estimates can be made of the spread of epicontinental seas, which toward the end of the period was much greater than today. A fair approximation to mean annual temperature distributions in different zones of latitude can be achieved by utilizing data on fossil distributions, though it may prove more difficult to quantify temperature, seasonality, and rainfall. Reasonable estimates can also be made about the location of mountain belts.
One of the questions of most obvious interest is the extent to which the climatically equable world of the Jurassic, with its eastern humid and western arid belts, is a function primarily of the different geography of the time, compared with today. In addition, it would be instructive to enquire into the climatic effects of a more or less progressive rise of sea level through most of the period, with a concomitant flooding of continental low-lands and the creation of a continuous, low-latitude oceanic girdle in the latter part of the period following opening of the oldest, central sector of the Atlantic.
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