Abstract

The Late Cretaceous and Cenozoic high latitude land vegetation bequeathed a sensitive paleobotanical and palynological record of regional and global environmental change. Foliar physiognomy provides the most reliable indicators. This record is frequently available for northern localities, augmented by wood and palynomorph data. Southern data are provided mainly by palynomorphs, with some foliar, cuticular, and wood information. The northern high latitude vegetation was mainly deciduous, whereas evergreen taxa locally predominate in the south. Major northern clades were all derived from lower latitudes; in contrast, Antarctica was a center of evolutionary innovation and dispersal. Differences in northern and southern vegetation are a function of continental configurations, interrelated with continentality (winter-summer temperature range), seasonality, moisture/aridity regimes, sea-level cycles, and overprinted by biotic stress or selective mechanisms. Vast land areas encircled the North Pole (to within 85°N), enhancing climatically driven northward and southward migrations, whereas an Antarctic continent continuously occupied the South Polar latitudes, had relatively restricted dispersal corridors, and became increasingly isolated as the other Gondwana fragments spread northward.

Introduction

Fossils of plants that lived at high latitudes provide a sensitive and unparalleled record of the complex interplay of global climatic change and polar conditions through time. High latitude plants, presently portrayed by extant polar desert, tundra, and taiga floras, require adaptations for stringent "icehouse" conditions. An icehouse world is, however, infrequently encountered in earth history. More typical "greenhouse" conditions necessitate other strategies for plant survival (Spicer, 1989a; Spicer and Chapman, 1990). In the Mesozoic and Cenozoic greenhouse world, forests thrived near the poles despite the seasonal stress of polar light cycles (the near congruity of rotational and magnetic poles is assumed here).

Light and particularly temperature are the principal controlling factors for polar vegetation, whereas precipitation is generally the prime factor at lower latitudes (Ziegler, 1990). Realistic quantitative estimates of these physical conditions can be ascertained from fossil floras. These estimates are crucial data for climatic modeling and for understanding and predicting global climate change. Environmental parameters (mean annual temperature, MAT; mean annual temperature range, MAR; coldest month mean temperature, CMM) that can be determined from fossil floras, and their validity and limitations are reviewed by Spicer and Parrish (1990a). Parameters derived from fossil floras reflect vegetational response to the environment and are obtained by careful analysis and interpretation of leaf physiognomy (size and margin morphology, Bailey and Sinnott, 1915; Wolfe, 1971, 1979, 1985; Wolfe and Upchurch, 1987) and overall floral composition (Wolfe, 1979). Such information is available for northern high latitude floras, especially from upper Cretaceous to Eocene strata from northern Alaska. In southern high latitudes, fossil leaf assemblages are less common, and environmental interpretations are not yet well developed. Ongoing work on leaf floras of southern basins (e.g., Daniel et al., 1990; Parrish et al., 1991) should help correct this imbalance in data types, but in the meantime much of the southern data are derived, by necessity, from palynomorphs.

Useful qualitative information can be obtained from wood anatomy, tree-ring data, and palynomorphs. Palynomorph assemblages contribute a broad-brush view of the regional vegetation or, if locally derived, can provide a more detailed picture. Generalized climatic conditions can be inferred from palynomorph and leaf fossils by analogy with presumed modern counterparts (nearest living relatives), but this method can be risky because it assumes evolutionary stasis. For conservative taxa (conifers, ferns), such conclusions may be reasonably reliable.

The northern and southern high latitude (-60 to 90^°) vegetational history from the middle Cretaceous through the Cenozoic is presented in this overview, along with its suggested relationship to global change, in particular climatic change.

Vegetational changes evolved along different pathways in the northern and southern regions, although basic physiologic constraints of polar conditions are similar. Physiognomic parallels at both poles (e.g., highly dissected ginkgo leaves, the broad-leaved conifers Podozamites and Agathis-type, Sphenopteris-like ferns) illustrate this latter point.

Paleogeographic Framework

The difference in continental configurations between the northern and southern polar regions is the overriding cause of differences in the evolution of their respective floras. These differences are illustrated (Figures 9.1 and 9.2) in the paleogeographic reconstructions of Smith et al. (1981). During the Late Cretaceous and Cenozoic, vast land areas encircled the North Pole (to within 85°N), facilitating climatically driven northward and southward floral migrations, whereas an Antarctic continent continuously occupied the South Polar position, had relatively restricted dispersal corridors, and became increasingly isolated as other Gondwana fragments spread northward.

Figure 9.1

Cretaceous high latitude plant fossil localities on 100 and 80 Ma paleocontinental reconstructions (polar Lambert equal-area projections) of Smith et al. (1981): (a) Northern Albian-Cenomanian localities, 1: Smiley (1966, 1967, 1969a,b), Scott and Smiley (more...)

Figure 9.2

Cenozoic high latitude plant fossil localities on 40 and 10 Ma paleocontinental reconstructions (polar Lambert equalarea projections) of Smith et al. (1981): (a) Northern Paleocene-Eocene localities, 1: Wolfe (1980); 2: Wolfe (1966), Wolfe and Poore (more...)

Summary Of High-Latitude Vegetational Changes

Significant botanical events, and vegetational types and trends for northern and southern high latitudes are outlined in Figures 9.3 to 9.6, plotted alongside "global change" information. These charts are based on studies and fossil localities cited below and in Figures 9.1 and 9.2.

Figure 9.3

Vegetational trends and global change for the Cretaceous northern high latitudes. Sources of data are in the text. Sea-level curve for Figures 9.3 to 9.6 from Haq et al. (1987); time scale for Figures 9.3 and 9.4 from Harland et al. (1990).

Figure 9.6

Vegetational trends and global change for the Cenozoic southern high latitudes.

Southern Cretaceous

Albian-Cenomanian and Early Angiosperms

By the middle Cretaceous, angiosperms were well established in the southern high latitudes. At least 8 an giosperm pollen taxa were present near the tip of the Antarctic Peninsula (about 66°S) by the Albian-Cenomanian (Dettmann and Thomson, 1987; Baldoni and Medina, 1989). There, and slightly further north in southern South America (Volkheimer and Salas, 1975; Archangelsky, 1980; Romero and Archangelsky, 1986), palynomorphs and leaves represent monocotyledonous and several dicotyledonous families, including "higher" (nonmagnoliid) forms that produced triaperturate pollen. Vegetation was predominantly conifer forest with podocarps (Podocarpus and Microcachrys-type) and araucarians, pteridosperms, and diverse fern understory and ground cover.

Figure 9.4

Vegetational trends and global change for the Cretaceous southern high latitudes. Isotope data for Figures 9.4 and 9.6 from Barrera et al. (1987), Stott and Kennett (1990), and Stott et al. (1990).

For a small riparian collection (seven leaf taxa) of mainly microphyllous and deciduous angiosperms from Livingston Island, South Shetland Islands (59 to 65°S), a MAT of 13 to 20°C was suggested (Rees and Smellie, 1989; Rees, 1990), although values are tentative at best for samples with fewer than 20 species (Wolfe, 1971). Associated foliage includes various conifers and cycads. Several types of fern foliage and Equisetites may have formed ground cover, as in northern high latitudes. Well-defined growth rings in conifer wood attests to strong seasonality, but the lack of rings in angiosperm woods (Chapman and Smellie, 1992) is difficult to explain.

Angiosperms had penetrated close to 80°S by the Albian-Cenomanian in New Zealand (in Motuan and Ngaterian Stages; Raine, 1984). They have an older record in southeastern Australia (Aptian Koonwarra beds, ~60°S) based on pollen (Dettmann, 1986a) and foliage (with flowers) of a small, rhizomatous, perennial angiosperm (Taylor and Hickey, 1990).

Douglas and Williams (1982) interpreted the Albian-Cenomanian climate in southern Australia as wet warm temperate, with moderate seasonality, probably seasonally dry, and no widespread winter freezing. Cooler temperatures are indicated by Parrish et al. (1991) based on reevaluation of the Albian vegetation, though not as cold as suggested by oxygen isotope results (Gregory et al., 1989; MAT less than 0°C). Parrish et al. (1991) noted that the Albian floras include both deciduous, thin-cuticled leaves, commonly occurring as mats (e.g., Phyllopteroides), plus more abundant microphyllous conifers and small-leaved bennettitaleans with thick cuticles and specialized stomata for reducing water loss. The latter could have retained their leaves throughout the winter if temperatures were low enough to slow metabolic processes significantly. In their rift valley continental setting, these floras apparently experienced greater MAR than those in coastal sites, and thus with colder winters could contain evergreen taxa.

In contrast, coastal Cenomanian floras from South Island, New Zealand (Daniel et al., 1990) are physiognomically more comparable with coeval floras of Alaska. Many major groups, including angiosperms, conifers, ferns, and cycadophytes, are broad leaved, have mostly thin cuticles, and appear deciduous.

On the coast of East Antarctica, in Prydz Bay, Albian palynomorph assemblages in Ocean Drilling Program (ODP) site 119 drillhole samples (Truswell, 1990) record podocarpaceous conifer vegetation with ferns (especially schizaeaceous types) and various hepatics (liverworts). Fungal material is common. Recycled palynomorphs in Recent muds of the Weddell Sea (Truswell, 1983; Truswell and Anderson, 1985) indicate this vegetational type probably extended around much of the East Antarctic coastal margin.

Albian conifer wood from James Ross Island (~66°S) has uniform rings, absence of false rings, and large earlywood cell size, which indicate little water stress and no evidence for frost (Francis, 1986). Narrow latewood indicates sudden dark-induced dormancy, similar to coeval northern high latitude wood.

All the southern high latitude fossil occurrences fall on the marginal areas of Gondwanaland fragments. Palynomorph assemblages indicate the predominance of temperate, podocarp-araucarian-fern forest vegetation, with lycopod and fern moorland vegetation in some areas (Douglas and Williams, 1982; Dettmann, 1986a,b; Dettmann and Thomson, 1987; Truswell, 1990; Dettman et al., 1992). The nature of the vegetation in the more inland, continental craton of East Antarctica remains unknown.

Turonian-Coniacian-Santonian

Palynological data spanning the Turonian-Coniacian-Santonian interval is available from James Ross Island (Baldoni and Medina, 1989) and from the Australasian high latitude sector. The podocarp-araucarian-fern forest association continued through this interval in southern high latitudes. In the Turonian-Coniacian, the abundance and diversity of angiosperms remained low (Dettmann, 1989), while cryptogams were still an important part of the vegetation. This was, however, an important transitional time when the established southern podocarpaceous conifer forest vegetation was diversifying, and new angiosperm families that henceforth typified southern vegetation started to appear. The southern high latitude region was a locus of evolutionary innovation from the Turonian to the end of the Cretaceous. Among the conifers, Lagarostrobus originated in the Turonian (Dettmann, 1989). This important forest tree dominated southern high latitude forests from the Santonian through the Paleocene, and produced pollen (Phyllocladidites mawsonii ) identical to that of the Huon pine (L. franklinii), now restricted to wet, cool temperate, maritime western margins of Tasmania. Dacrydium-type conifers originated in the Coniacian (Dettmann, 1989). Based on the angiosperm pollen record, Ilex (Aquifoliacaeae) originated in southern high latitudes. Its earliest fossil record was in the Turonian of the Otway Basin, southeastern Australia (Martin, 1977; Dettmann, 1989). Proteaceae, now a major southern family, also evolved at about this time (Dettmann, 1989).

Campanian-Maastrichtian

Evolutionary innovation in southern floras reached its zenith during the Campanian. Evidence is provided by a rich palynomorph record from James Ross, Seymour, and adjacent islands near the tip of the Antarctic Peninsula (e.g., Dettmann and Thomson, 1987; Dettmann and Jarzen, 1988; Askin, 1989, 1990b), from New Zealand (e.g., Couper, 1960; Mildenhall, 1980; Raine, 1984), and from the Otway and Gippsland Basins of southeastern Australia (e.g., Stover and Partridge, 1973; Dettmann and Jarzen, 1988; Dettman et al., 1992). The Nothofagaceae (we accept the family status, discussion and references cited in Dettmann et al., 1990) originated in southern high latitudes in the early Campanian, with ancestral pollen types widespread from Australasia to the Antarctic Peninsula. N. brassii, fusca, and menziesii types differentiated soon after (Dettmann et al., 1990), with the southern South America-Antarctic Peninsula area the center of Nothofagus diversification. In the Proteaceae, several types of both Proteoideae and Grevilleoideae occur in Campanian rocks, with earlier arrival of ancestral Proteaceae from northern Gondwana (Dettmann, 1989). The Antarctic Peninsula and New Zealand-southeastern Australia areas were both early Campanian diversification sites for various proteaceaeous groups (Beauprea type, Macadamia type, Gevuina-Hicksbeachia type, Knightia type, Xylomelum type; Dettmann and Jarzen, 1988; Pocknall and Crosbie, 1988; Dettmann, 1989). Gunnera (Gunneraceae) immigrated southward from northern Gondwana via the Antarctic Peninsula in the Campanian (Jarzen and Dettmann, 1990). Members of Myrtaceae may have entered via this route, while Winteraceae appeared in the New Zealand-southeastern Australian area (Dettmann, 1989). Loranthaceae appeared near the end of the Campanian in the Antarctic Peninsula area (Askin, 1989). The conifer Dacrycarpus also appeared in the high southern latitudes during the Campanian (Dettmann, 1989).

Diversification continued through the Maastrichtian, with many endemic angiosperm pollen taxa of unknown botanical affinities appearing in the fossil record. This may represent parallel (to northern) development of a herbaceous, "weedy" strategy during the general cooling trend. The latest Maastrichtian on Seymour Island (~66°S) included a short warm interval characterized by members of Bombacaceae, Olacaceae (Anacolosa type), and Sapindaceae (Cupanieae tribe) (Askin, 1989).

A rich Campanian leaf assemblage from King George Island (Zamek locality), South Shetland Islands, suggests a broad-leaved forest community, including evergreen types (with thick, coriaceous leaves) and deciduous Nothofagus growing in a subhumid mesothermal climate (Birkenmajer and Zastawniak, 1989). Abundant ferns, especially gleicheniaceous types (Cao, 1989), grew on adjacent moist lowland areas at the end of the Cretaceous.

Conifer and angiosperm wood from the James Ross Basin have wide, uniform rings and low latewood-to-earlywood ratios, indicating high productivity, sudden dark-induced dormancy, and no water stress (Francis, 1986). Rainfall of 1000 to 2000 mm/yr was suggested. From the late Maastrichtian (and Early Paleocene) on Seymour Island, dispersed plant cuticle from evergreen foliage of araucarians, Cupressaceae, and podocarp conifers, plus numerous angiosperm taxa, including Myrtaceae and Lauraceae, indicates a wet climate, MAT approximately 8 to 15°C, MAR probably <16°C, and CCM >1°C (G. R. Upchurch, University of Colorado, personal communication, 1990).

Podocarpaceous conifer (especially Lagarostrobus)-Nothofagus-Proteaceae forest grew throughout much of the southern high latitudes. Presumed paleoclimates were humid, warm-cool temperate (Cranwell, 1969; Mildenhall, 1980; Francis, 1986; Dettmann and Thomson, 1987; Dettmann and Jarzen, 1988; Askin, 1989, 1990a; Truswell, 1991).

Comparison of Northern and Southern High-Latitude Vegetation

Cenozoic Vegetational Changes

In both hemispheres, after a cooler earliest Paleocene, climatic warming into the Eocene was reflected by concomitant changes in high latitude vegetation. Late Paleocene and Eocene floras exhibit increased diversity and complexity and, in both northern and southern high latitudes, include typically warmer-climate (mesothermal) taxa and communities. In the Cenozoic, global climates ultimately were controlled by tectonism that occurred in southern mid- to high latitudes, culminating in the isolation of Antarctica, inception of the Antarctic Circumpolar Current, development of the modern cryospheric ocean, buildup of Antarctic ice, and deterioration into icehouse conditions (references in Kennett and Barker, 1990). Vegetational changes in the north and south correspond to tectonic events and climatic changes (Figures 9.5 and 9.6). Whereas broad dispersal corridors surrounding the North Pole allowed climatically driven northward and southward migrations, post-Eocene Antarctica was completely isolated, resulting in stepwise extinction of taxa with each climatic deterioration. Important sequential cooling steps identified in the Cenozoic southern oceans (summarized by Kennett and Barker, 1990) occurred in the Middle Eocene, near the Eocene-Oligocene boundary, in the middle Oligocene, the Middle Miocene, the early Late Miocene, the latest Miocene, and the Late Pliocene, with some intervening, short-lived warming trends.

Figure 9.5

Vegetational trends and global change for the Cenozoic northern high latitudes. Time scale for Figures 9.5 and 9.6 from Berggren et al. (1985) and Harland et al. (1990).

Southern high latitude floras suffered the loss of many taxa (including mesothermal types) during and at the end of the Eocene. Subsequent loss of most remaining taxa occurred through the Oligocene and Miocene, with an extremely depauperate flora surviving to the Pliocene in coastal refugia. Along ice-free margins, present-day Antarctica supports a cryptogamic flora (e.g., mosses, lichens) with only two vascular plant species found in sheltered areas of the Antarctic Peninsula.

A similar diversity decline occurred in northern high latitudes during and at the end of the Eocene with the southward migration of many taxa and permanent disappearance from high latitudes of mesothermal forms. Superimposed on the poleward and equatorward migrations with warming or cooling trends were significant vegetational changes such as the increase of Pinaceae and Salicaceae in high latitudes and development of an evergreen, mixed conifer forest community (including Pinus, Picea, and Tsuga). During the final cooling stages, the taiga forest community developed and, subsequently, the tundra.

Acknowledgments

This review is largely based on a presentation at a symposium, arranged by the Geophysics Study Committee, at the 1990 annual meeting of Geological Society of America. We appreciate the helpful comments of our reviewers, Drs. Charles J. Smiley and Thomas N. Taylor, and Stephen R. Jacobson for an earlier review. R.A.A gratefully acknowledges support of a National Science Foundation grant (DPP 9019378), and R.A.S. the support of a NERC grant (GR3/7939).

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