Judith Totman Parrish

Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States A comparison

Panfish, J.T. and Peterson, F., 1988. Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States--A comparison. In: G. Kocurek (Editor), Late Paleozoic and Mesozoic Eolian Deposits of the Western Interior of the United States. Sediment. Geol., 56: 261-282. Wind directions for Middle Pennsylvanian through Jurassic time are predicted from global circulation models for the western United States. These predictions are compared with paleowind directions interpreted from eofian sandstones of Middle Permsylvanian through Jurassic age. Predicted regional wind directions correspond with at least three-quarters of the paleowind data from the sandstones; the rest of the data may indicate problems with correlation, local effects of paleogeography on winds, and lack of resolution of the circulation models. The data and predictions suggest the following paleoclimatic developments through the time interval studied: predominance of winter subtropical high-pressure circulation in the Late Pennsylvanian; predominance of summer subtropical high-pressure circulation in the Permian; predominance of summer monsoonal circulation in the Triassic and earliest Jurassic; and, during the remainder of the Jurassic, influence of both summer subtropical and summer monsoonal circulation, with the boundary between the two systems over the western United States. This sequence of climatic changes is largely owing to paleogeographic changes, which influenced the buildup and breakdown of the monsoonal circulation, and possibly owing partly to a decrease in the global temperature gradient, which might have lessened the influence of the subtropical high-pressure circulation. The atypical humidity of Triassic time probably resulted from the monsoonal circulation created by the geography of Pangaea. This circulation is predicted to have been at a maximum in the Triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west.

Upwelling and Petroleum Source Beds, With Reference to Paleozoic

Upwelling zones are areas of persistent high organic productivity in the oceans and represent one type of setting for the deposition of petroleum source beds. Upwelling currents are driven by winds associated with the major features of atmospheric circulation, and the locations of ancient upwelling zones can be predicted from global atmospheric circulation models. Qualitative circulation models for the past are now made possible by the availability of good reconstructions of past continental positions and paleogeography. Atmospheric circulation was modeled on paleogeographic reconstructions for the following Paleozoic stages: Franconian (Late Cambrian), Llandeilo-Caradoc (Late Ordovician), Wenlock (Late Silurian), Emsian (Early Devonian), Visean (Early Carboniferous), Wes phalian (Late Carboniferous), and Kazanian (Late Permian). Four types of persistent upwelling currents are recognized. Coastal upwelling currents (e.g., the Peru Current) are the most familiar and are the types usually cited as petroleum source-bed settings. However, three additional types of upwelling have also been important in the past, especially during times of high sea-level stands. They are associated with divergence under stable atmospheric low-pressure systems and comprise: (1) symmetrical divergence at the equator, (2) symmetrical divergence in wide oceans at about 60° lat. (e.g., around Antarctica), and (3) radial divergence in more restricted oceans at about 60° lat. (e.g., around the southern tip of Greenland). To the extent that the data allow reliable conclusions, the distribution of Paleozoic petroleum source beds appears to correspond closely to the distribution of predicted upwelling zones. Coupled with sea level models, the upwelling model can be particularly powerful, because while sea-level models predict likely times of source-bed deposition, the upwelling model contributes information on the sites of source-bed deposition.

The Pangaean megamonsoon - evidence from the Upper Triassic Chinle Formation, Colorado Plateau

The Upper Triassic Chinle Formation was deposited at an exceptional time in Earths paleogeographic and paleoclimatic history. During the Triassic, the supercontinent Pangaea was at its greatest size, in terms of both aggregated continental crust and exposed land area. Moreover, the exposed land was divided symmetrically about the paleoequator between the northern and southern hemispheres. These conditions were ideal for maximizing monsoonal circulation, as predicted from paleoclimate models. The Chinle was deposited between about 5?to 15?N paleolatitude in the western equatorial region of Pangaea, a key area for documenting the effects of the monsoonal climate. This study summarizes sedimentologic and paleontologic data from the Chinle Formation on the Colorado Plateau and integrates that data with paleoclimatic models. The evidence for abundant moisture and seasonality attest to the reversal of equatorial flow and support the hypothesis that the Triassic Pangaean climate was dominated by monsoonal circulation. The Chinle Formation contains continental lithofacies deposited in fluvial channels, crevasse splays, lakes, bogs, marshes, and lacustrine deltas that reflect abundant precipitation and shallow water tables. Paleosols and ichnofossils indicate that water tables and lake levels fluctuated episodically. Interbedded lacustrine carbonates and marginal-lacustrine siltstones and mudstones indicate longer-term but regular, episodic fluctuations in lake level. Fine-scale laminations in lacustrine carbonates suggest a seasonal influx of clastic sediment, and thus precipitation, to the basin. Uppermost Chinle strata consist of lacustrine and marginal-lacustrine mudstones interbedded with minor eolian sand sheets and eolian dunes; thus, the later Triassic reflects continued precipitation, but was marked by more pronounced and extended dry seasons.

CARBONIFEROUS PALEOGEOGRAPHIC, PHYTOGEOGRAPHIC, AND PALEOCLIMATIC RECONSTRUCTIONS

Abstract Two revised paleogeographic reconstructions of the Visean and Westphalian C-D stages are presented based on recent paleomagnetic, phytogeographic, stratigraphic, and tectonic data. These data change the positions of some continental blocks, and allow the definition of several new ones. The most important modifications that have been incorporated in these reconstructions are: (1) a proposed isthmus linking North America and Siberia across the Bering Strait; and (2) the separation of China and Southeast Asia in six major blocks, including South China, North China, Shan Thai-Malaya, Indochina, Qangtang, and Tarim blocks. Evidence is presented that suggests that at least the South China, Shan Thai-Malaya, and Qangtang blocks were derived from the northern margin of Gondwana. Multivariate statistical analysis of phytogeographic data from the middle and late Paleozoic allow definition of a number of different phytogeographic units for four time intervals: (1) the Early Devonian, (2) Tournaisian—early Visean, (3) Visean, and (4) late Visean—early Namurian A. Pre-late Visean—early Namurian A floral assemblages from South China show affinities with northern Gondwana floras suggesting a southerly position and provides additional support for our reconstruction of South China against the northern margin of Gondwana. There is a marked decrease in the diversity of phytogeographic units in the Namurian and younger Carboniferous. This correlates closely with the time of assembly of most of Pangaea. The general pattern of Carboniferous phytogeographic units corresponds well with global distribution of continents shown on our paleogeographic reconstructions. In addition, we have constructed paleoclimatic maps for the two Carboniferous time intervals. These maps stress the distribution of rainfall, as this should be strongly correlated with the floras. There is marked change in the rainfall patterns between the Visean and Westphalian C-D. This change corresponds with the closing of the Appalachian-Ouachita ocean between Laurussia and Gondwana, and reflects the removal of a low-latitude moisture source that probably gave rise to monsoonal conditions along the northern margin of Gondwana in the Visean and earlier times. As well, the presence of a substantial heat source at high elevation in the Late Carboniferous significantly influenced the distribution of climatic belts.

Late Cretaceous–early Tertiary palaeoclimates of northern high latitudes: a quantitative view

Analyses of plant community structure, vegetational and leaf physiognomy, and growth rings and vascular systems in wood provide qualitative and quantitative data that can be combined to define non-marine palaeoclimatic parameters with better resolution than is available from other, principally sedimentological, methods. Application of these techniques to Cenomanian through Paleocene floras from high palaeolatitudes (75°-85°N) indicates a polar light regime similar to that of the present. Plant data suggests Cenomanian sea level mean annual air temperatures (MATs) of 10 °C, and MATs of 13 °C, 5°C and 6-7 °C in the Coniacian, Maastrichtian, and Paleocene respectively. Evapotranspirational stresses at sea level were low and precipitation was in most part uniform throughout the growing season in the Cenomanian, with possible seasonal drying occurring by the Maastrichtian. Maastrichtian winter freezing was likely, but periglacial conditions did not exist at sea level. Permanent ice was likely above 1700 m at 75°N in the Cenomanian, and above 1000 m at 85°N in the Maastrichtian. These near-polar data provide critical constraints on global models of Late Cretaceous to early Tertiary climates.

Late Cretaceous terrestrial vegetation: A near-polar temperature curve

Quantitative estimates of terrestrial paleotemperatures near the North Pole were derived from the physiognomy of the vegetation, including leaf-margin analysis, from rocks of latest Albian and Late Cretaceous age of the North Slope of Alaska. During the latest Albian and Cenomanian, mean annual temperature was approximately 10 ±3 °C. During the Coniacian, mean annual temperature may have been 2-3 °C warmer than during the Albian-Cenomanian but no higher than 13 °C. During the Campanian and Maastrichtian, mean annual temperature would have been about 2-8 °C. Although the ranges for the individual estimates overlap, differences among the floras indicate that the relative changes in mean annual temperature did occur.

Paleobotanical evidence for cool north polar climates in middle Cretaceous (Albian-Cenomanian) time

Mid-Cretaceous (Albian-Cenomanian) floras are abundant and diverse on the North Slope of Alaska. The older floras consist of conifers, cycadophytes, ferns, ginkgophytes, and sphenophytes (horsetails). Angiosperms appeared in latest Albian time and rapidly diversified. The preserved floras consist entirely of deciduous plants, with the exception of a microphyllous conifer, ferns, and sphenophytes. Deciduousness is evidence for strong seasonality, which for these floras might be variations in either light or temperature or both. Cool temperatures are suggested by the prevalence of toothed leaves among the angiosperms and the presence of large-leaved conifers. The paleobotanical evidence points to a mid-Cretaceous climate that was no warmer than cool temperate on the North Slope of Alaska.

Late Jurassic Climates, Vegetation, and Dinosaur Distributions

The Jurassic and Cretaceous are considered to have been warmer than today on the basis of various climate data and model studies. Here, we use the available global record of climate‐sensitive sediments, plants, and dinosaurs to infer broadscale geographic patterns for the Late Jurassic. These provide a context for our more detailed accounts of the Morrison and Tendaguru Formations in North America and East Africa. At the global scale, evaporites predominated in low latitudes and coals in mid‐ to high latitudes. We ascribe these variations to a transition from drier to wetter conditions between the equator and poles. Plant diversity was lowest in equatorial regions, increasing to a maximum in midlatitudes and then decreasing toward the poles. Most dinosaur remains are known from low‐latitude to marginally midlatitude regions where plant fossils are generally sparse and evaporites common. Conversely, few dinosaur remains are known from mid‐ to high latitudes, which have higher floral diversities and abundant coals. Hence, there is an obvious geographic mismatch between known dinosaur distributions and their primary food source. This may be due to taphonomic bias, indicating that most dinosaur discoveries provide only a small window on the diversity and lifestyles of this group. On the basis of our global‐ and local‐scale studies, we suggest that dinosaur preservation was favored in environments toward the drier end of the climate spectrum, where savannas rather than forests predominated. A holistic approach, incorporating climate and vegetation as well as geography, is required to better understand patterns of dinosaur ecology and evolution.

Plant taphonomy in incised valleys: Implications for interpreting paleoclimate from fossil plants

Paleoclimatic interpretations of the Upper Triassic Chinle Formation (Colorado Plateau) based on plants conflict with those based on the sedimentary rocks. The plants are suggestive of a humid, equable climate, whereas the rocks are more consistent with deposition under highly seasonal precipitation and ground-water conditions. Fossil plant assemblages are limited to the lower members of the Chinle Formation, which were deposited within incised valleys that were cut into underlying Lower to Middle Triassic and older rocks. In contrast, the upper members of the formation, which were deposited across the fluvial plain after the incised valleys were filled, have few preserved fossil plants. The taphonomic characteristics of the plant fossil assemblages, within the stratigraphic and hydrologic context of the incised valley-fill sequence, explain the vertical and lateral distribution of these assemblages. The depositional, hydrological, and near-surface geochemical conditions were more conducive to preservation of the plants. Fossil plant assemblages in fully terrestrial incised-valley fills should be taphonomically biased toward riparian wetland environments. If those assemblages are used to interpret paleoclimate, the paleoclimatic interpretations will also be biased. The bias may be particularly strong in climates such as those during deposition of the Chinle Formation, when the riparian wetlands may reflect local hydrologic conditions rather than regional climate, and should be taken into account when using these types of plant assemblages in paleoclimatic interpretations.

Paleoclimatic significance of Mid-Cretaceous floras from the middle Clarence Valley, New Zealand

Evidence from leaf-margin analysis and vegetational physiognomy indicates that a mid-Cretaceous flora from the South Island of New Zealand grew at a mean annual temperature of 10 degrees C. This compares with an identical estimate derived from a coeval flora in northern Alaska. Both floras grew at about the same latitude and in coastal plain settings. This study provides the first direct comparison of non-marine near-polar climates in the pre-Tertiary record.