Jack A. Wolfe

North American nonmarine climates and vegetation during the Late Cretaceous

Abstract Analyses of physiognomy of Late Cretaceous leaf assemblages and of structural adaptations of Late Cretaceous dicotyledonous woods indicate that megathermal vegetation was an open-canopy, broad-leaved evergreen woodland that existed under low to moderate amounts of rainfall evenly distributed through the year, with a moderate increase at about 40–45°N. Many dicotyledons were probably large, massive trees, but the tallest trees were evergreen conifers. Megathermal climate extended up to paleolatitude 45–50°N. Mesothermal vegetation was at least partially an open, broad-leaved evergreen woodland (perhaps a mosaic of woodland and forest), but the evapotranspirational stress was less than in megathermal climate. Some dicotyledons were large trees, but most were shrubs or small trees; evergreen conifers were the major tree element. Some mild seasonality is evidenced in mesothermal woods; precipitational levels probably varied markedly from year to year. Northward of approximately paleolatitude 65°N, evergreen vegetation was replaced by predominantly deciduous vegetation. This replacement is presumably related primarily to seasonality of light. The southern part of the deciduous vegetation probably existed under mesothermal climate. Comparisons to leaf and wood assemblages from other continents are generally consistent with the vegetational-climatic patterns suggested from North American data. Limited data from equatorial regions suggest low rainfall. Late Cretaceous climates, except probably those of the Cenomanian, had only moderate change through time. Temperatures generally appear to have warmed into the Santonian, cooled slightly into the Campanian and more markedly into the Maastrichtian, and then returned to Santonian values by the late Maastrichtian. The early Eocene was probably warmer than any period of the Late Cretaceous. Latitudinal temperature gradients were lower than at present. For the Campanian and Maastrichtian, a gradient of about 0.3°C/1° latitude is inferred. Equability was high: a mean annual range of temperature of about 8°C is inferred for paleolatitude 51–56°N during the Campanian. Most Late Cretaceous plants evolved in a climate characterized by absence of freezing and low to moderate amounts of precipitation. A brief, low-temperature excursion and a major, long-lasting increase in precipitation occurred at the Cretaceous-Tertiary boundary. In megathermal climates, these events selected for plants that could exist in rainforest environments. In mesothermal climates, deciduousness and contamitant structural adaptations were selected. The events at the Cretaceous-Tertiary boundary had a major and long-lasting impact on the evolution of land plants and their ecosystems. Low precipitation at low to middle Late Cretaceous latitudes is suggested to be the result of high levels of atmospheric CO2, which, in turn, are probably related to inability of warm, saline oceans to store large amounts of carbon. Conditions appear to have rapidly changed at the Cretaceous-Tertiary boundary, when oceanic circulation and stratification may have been fundamentally altered. After the boundary, the oceans were apparently able to store much greater amounts of carbon, and the oceans withdrew large amounts of CO2 from the atmosphere. In turn, more precipitation fell at low to middle latitudes; the resulting high-biomass vegetation formed a second major carbon reservoir to keep atmospheric CO2 low relative to the Late Cretaceous. Changes in oceanic and atmospheric circulation probably resulted from some factor external to the ocean-atmosphere system.

Tertiary climatic fluctuations and methods of analysis of tertiary floras

Abstract On theoretical grounds, an analysis of the physiognomy of a Tertiary leaf assemblage is more direct and reliable than a circuitous floristic analysis in assigning thermal regimes to fossil assemblages. Using primarily foliar physiognomy and secondarily floristic composition, it can be shown that: (1) some middle latitude Tertiary assemblages probably lived under meteoroligically tropical climates; (2) a major and rapid climatic deterioration occurred in the Oligocene; and (3) a major climatic fluctuation probably occurred in the Late Eocene. These analyses thus substantiate the conclusions of several other paleobotanists regarding climatic fluctuations. Recent criticisms of these analyses are shown to be invalid and to be based largely on misinterpretations.

Tertiary climates and floristic relationships at high latitudes in the northern hemisphere

During the Paleocene and Eocene, climates were characterized by a low mean annual range of temperature (a maximum of 10–15°C), a moderate to high mean annual temperature (10–20°C), and abundant precipitation; strong broad-leaved evergreen vegetation extended to almost lat. 60°N during the Paleocene and to well above 61°N during the Eocene. Poleward of the broad-leaved evergreen forests were forests that were broad-leaved deciduous; these deciduous forests, however, were unlike extant broad-leaved deciduous forests in general floristic composition and physiognomy. Coniferous forests probably occupied the northernmost latitudes. At the end of the Eocene, a major climatic deterioration resulted in a high (> 30°C) mean annual range of temperature and a low mean annual temperature (< 10°C). Vegetation represented temperate broad-leaved deciduous and coniferous forests. The Oligocene and Neogene climatic trends represent a decrease in both mean annual range of temperature and mean annual temperature. Tundra vegetation did not appear until late in the Neogene. The present distribution of broad-leaved evergreens concomitant with the principles of plant physiology indicates that present winter light conditions at high latitudes could not support broad-leaved evergreen forest. A possible solution to the problem is to increase winter light by lessening the inclination of the earths rotational axis.

Fossil forms of Amentiferae

Review of the procedures used in determining fossil plant organs indicates that the many Cretaceous records of extant genera of “Amentiferae” based on leaves should be rejected as theoretically unreliable. Palynological data, in combination with some valid megafossil data, indicate that most recognizable members of “Amentiferae” are no older than the later part of the Late Cretaceous. Juglandales appear to be derivatives of the ancient Normapolles complex and unrelated to other “Amentiferae.” A preliminary account of some of the comparative foliar morphology of extant “Amentiferae” indicates that some—particularly Betulaceae and Fagaceae—are closely related to Hamamelidales but that other families—notably Rhoipteleaceae, Juglandaceae, Didymelaceae, and Leitneriaceae—are unrelated to this order.

A Pliocene flora and insect fauna from the Bering Strait region

Abstract A flood-plain forest has been preserved beneath a lava flow that invaded the Inmachuk River Valley in the northern part of the Seward Peninsula, Alaska, during the Pliocene Epoch. The fossil flora is of great biogeographic interest because of its position (Fig. 1) in a tundra region about 250 km east of Bering Strait, 75 km south of the Arctic Circle, and 65 km west of the northwestern limit of spruce-birch forest. It provides insight into the history of the development of the circumpolar boreal forest (taiga). A rich arthropod fauna casts light on the phylogeny of several modern insect genera and on the origin of modern tundra faunas. A potassium-argon analysis of the overlying basaltic lava provides our first radiometric age estimate (5.7±0.2 million years) for the Clamgulchian Stage, a Late Tertiary time-stratigraphic unit based on fossil plants and widely recognized in Alaska ( Wolfe and Hopkins 1967) and northeastern Siberia.

Alaskan Cretaceous-Tertiary floras and Arctic origins

Cretaceous floras in Alaska, when compared to those at mid-latitudes, generally indicate later appearances in Alaska of major clades and major leaf morphologies. Compared to mid-latitude floras, Alaskan Late Cretaceous floras contain few major clades. The Alaskan clades diversified but at a low taxonomic level. Migrational pathways into high latitudes were probably along streams. Similar patterns characterized the Alaskan Tertiary, although some southward migrations of lineages occurred during the Neogene. Review of other Arctic paleontological data from Ellesmere Island, previously used to suggest that the Arctic was a major center of origin during the Late Cretaceous, indicates that the ages of supposedly substantiating dinoflagellate floras were misinterpreted. When the dinoflagellate data are interpreted ac- cording to standard methodology, first occurrences of genera and species groups on Ellesmere are, like the Alaskan occurrences, later than first occurrences at middle latitudes.

Late Tertiary floral assemblage from upland gravel deposits of the southern Maryland Coastal Plain

A diverse flora has been discovered in a dark clay lens in upland gravel in southern Maryland near Brandywine. More than 49 taxa have been identified in the assemblage, which includes leaves, seeds, fruits, pollen, and a Taxodium (bald cypress) trunk. The vegetation is dominated by deciduous trees and vines. Four taxa are now absent from North America but survive elsewhere; one is extinct. A late Miocene age and warm-temperate climate are inferred from the flora. The clay lens probably represents a cutoff distributary in the extensive braided stream system that covered the area and is unique in Maryland. Similar dark clays have been described from Miocene sands and gravels in New Jersey. The Brandywine flora is the first direct evidence of the Miocene age of part of the fluvial upland deposits of Maryland. On the basis of the age inferred from the flora, the Brandywine deposit is correlated with the St. Marys Formation or the Eastover Formation, which are upper Miocene shelly marine units south and southeast of Brandywine.

Paleoecologic, paleoclimatic, and evolutionary significance of the Oligocene Creede flora, Colorado

Application of multivariate statistical techniques, especially correspondence analysis, results in the recognition of four major communities for the Creede plant assemblages: fir-spruce forest, fir-pine forest, pine-juniper forest or woodland, and mountain mahogany chaparral. Physiognomy of the Creede assemblages indicates a mean annual temperature of Chamaebatiaria , a shrub that now inhabits dry, open environments, had an ancestral taxon characteristic of the Creede fir-forest communities and strongly indicates a major shift in habitat for this lineage during the Neogene. A second genus, Luetkea , which is now herbaceous, is represented by a probable woody ancestral taxon that was common in the Creede forest communities. Consideration of the adaptive and morphologic histories of the Creede lineages suggests that physiologic adaptation may precede morphologic change. The Creede forest communities have no modern homologues.

Tectonic uplift of a middle Wisconsin marine platform near the Mendocino triple junction, California

An uplifted wave-cut marine platform eroded across bedrock of the Franciscan Complex at Point Delgada, northern California, is overlain by 0.5 to 5 m of wave-worked pea gravel, which is in turn directly overlain by fluvial gravel and silt deposited as alluvial fans. Woody plant debris at the base of the fluvial deposits includes cones of Brewer spruce ( Picea breweriana ), which today are found only at higher elevations and latitudes. Fossil wood debris from this horizon yields a 14C date of 44,800 ± 1,300 yr. The 44,800 yr radiocarbon age for the base of the fluvial deposits establishes an approximate age for the immediately underlying marine gravels and wave-cut platform. We tentatively correlate this terrace with the middle Wisconsin high sea-level stand at −37 m, dated at about 45,000 yr B.P. If this age is correct, the tectonic uplift since middle Wisconsin time has been 44 m, and the average rate of uplift has been at least 1.0 m/1,000 yr. This relatively high rate is probably related to interaction among the Pacific, Gorda, and North American plates at the Mendocino triple junction.

Radiometric dating of ash partings in Alaskan coal beds and upper Tertiary paleobotanical stages

New K-Ar and fission-track ages from volcanic ash partings in coal beds on the Kenai Peninsula, Alaska, substantiate an 8-m.y. age for the paleobotanical Homerian-Clamgulchian Stage boundary. An age estimate for the Seldovian-Homerian Stage boundary indicates that the Clamgulchian Stage spans an interval of at least 3.3 m.y. Direct radiometric correlations can now be made between these paleobotanical stages and radiometrically dated time-stratigraphic units in other parts of the world. Ash partings in coals can provide radiometric age control for stratigraphic correlations in terrestrial sections that may otherwise be difficult, if not impossible, to date or correlate by other means. The possibility of detrital contamination in ash-parting samples for dating purposes exists, but careful use of this coal-bed correlation technique can aid in the assessment of coal and petroleum resources in coal-bearing, terrestrial sedimentary sequences.