Alfred M. Ziegler

Paleozoic Base Maps

This paper contains 50 maps which have been designed for use by the geologic community in preparing paleogeographic, biogeographic, climatologic, and tectonic reconstructions of the Paleozoic periods. Seven maps for each of seven Paleozoic intervals are included, plus a suture map showing the outlines of the Paleozoic continents in their present positions. The intervals chosen are the Late Cambrian (Franconian), Middle Ordovician (Llandeilian-earliest Caradocian), Middle Silurian (Wenlockian), Early Devonian (Emsian), Early Carboniferous (Visean), Late Carboniferous (Westphalian CD), and Late Permian (Kazanian). The paleomagnetic information used to orient the continents is given. For each interval, three types of maps are included, one locality map with place names labelled, four paleogeographic maps with our interpretation of the distribution of mountains, lowlands, shallow seas, and deep oceans, and two outline maps for those who prefer to make their own paleogeographic interpretations. Several projections are used-Mercator, Mollweide, and stereographic polar-to suit the various requirements of paleogeographic work.

Rainfall patterns and the distribution of coals and evaporites in the Mesozoic and Cenozoic

Abstract Paleoclimatologists have adequately demonstrated over the last twenty years that the distribution of climatically sensitive sediments can be related to present-day climatic zones that are roughly parallel to latitude. For example, coals were deposited at paleolatitudes that correspond to the present-day rainy zones, at the equator and at about 55°N and 55°S, indicating that those zones were present in the past. From this, it can be concluded that atmospheric circulation has not been radically different from its present configuration, despite some apparently great differences in some climatic parameters, such as the equator-to-pole temperature gradient. Nevertheless, the zonal model for predicting the distribution of climatically sensitive sediments is not wholly adequate. Although general atmospheric circulation may not have been radically altered from its present pattern, it would have differed in some respects because the paleogeography was different from the present geography. Therefore, we have constructed what we believe are more realistic maps of the distribution of rainfall through time. These maps are based on maps of atmospheric circulation that have been successfully used to predict the distribution of some petroleum source beds and phosphorites. We have compared the predicted distribution of rainfall to the distribution of coals and evaporites for each of seven geologic stages. The circulation-rainfall maps were more successful than the zonal model for predicting the distribution of evaporites and coals for times when the paleogeography would have maximized the development of a monsoonal circulation, which most effectively disrupts the general zonal pattern. For times when a monsoonal circulation would not have been important, the zonal model adequately predicted the distribution of these rocks.

Permian phytogeographic patterns and climate data/model comparisons

The most recent global “icehouse‐hothouse” climate transition in earth history began during the Permian. Warmer polar conditions, relative to today, then persisted through the Mesozoic and into the Cenozoic. We focus here on two Permian stages, the Sakmarian (285–280 Ma) and the Wordian (267–264 Ma; also known as the Kazanian), integrating floral with lithological data to determine their climates globally. These stages postdate the Permo‐Carboniferous glaciation but retain a moderately steep equator‐to‐pole gradient, judging by the level of floral and faunal differentiation. Floral data provide a particularly useful means of interpreting terrestrial paleoclimates, often revealing information about climate gradations between “dry” and “wet” end‐member lithological indicators such as evaporites and coals. We applied multivariate statistical analyses to the Permian floral data to calibrate the nature of floral and geographical transitions as an aid to climate interpretation. We then classified Sakmarian and Wordian terrestrial environments in a series of regional biomes (“climate zones”) by integrating information on leaf morphologies and phytogeography with patterns of eolian sand, evaporite, and coal distributions. The data‐derived biomes are compared here with modeled biomes resulting from new Sakmarian and Wordian climate model simulations for a range of CO2 levels (one, four, and eight times the present levels), presented in our companion article. We provide a detailed grid cell comparison of the biome data and model results by geographic region, introducing a more rigorous approach to global paleoclimate studies. The simulations with four times the present CO2 levels (4×CO2) match the observations better than the simulations with 1×CO2, and, at least in some areas, the simulations with 8×CO2 match slightly better than those for 4×CO2. Overall, the 4×CO2 and 8×CO2 biome simulations match the data reasonably well in the equatorial and midlatitudes as well as the northern high latitudes. However, even these highest CO2 levels fail to produce the temperate climates in high southern latitudes indicated by the data. The lack of sufficient ocean heat transport into polar latitudes may be one of the factors responsible for this cold bias of the climate model. Another factor could be the treatment of land surface processes and the lack of an interactive vegetation module. We discuss strengths and limitations of the data and model approaches and indicate future research directions.

Tracing the tropics across land and sea: Permian to present

The continuity through the past 300 million years of key tropical sediment types, namely coals, evaporites, reefs and carbonates, is examined. Physical controls for their geographical distributions are related to the Hadley cell circulation, and its effects on rainfall and ocean circulation. Climate modelling studies are reviewed in this context, as are biogeographical studies of key fossil groups. Low-latitude peats and coals represent everwet climates related to the Intertropical Convergence Zone near the Equator, as well as coastal diurnal rainfall systems elsewhere in the tropics and subtropics. The incidence of tropical coals and rainforests through time is variable, being least common during the interval of Pangean monsoonal climates. Evaporites represent the descending limbs of the Hadley cells and are centred at 10° to 40° north and south in latitudes that today show an excess of evaporation over precipitation. These deposits coincide with the deserts as well as seasonally rainy climates, and their latitudinal ranges seem to have been relatively constant through time. Reefs also can be related to the Hadley circulation. They thrive within the regions of clear water associated with broad areas of downwelling which are displaced toward the western portions of tropical oceans. These dynamic features are ultimately driven by the subtropical high-pressure cells which are the surface signature of the subsiding branches of the Hadley circulation. Carbonates occupy the same areas, but extend into higher latitudes in regions where terrestrial surface gradients are low and clastic runoff from the land is minimal. We argue that the palaeo-latitudinal record of all these climate-sensitive sediment types is broadly similar to their environments and latitudes of formation today, implying that dynamic effects of atmospheric and oceanic circulation control their distribution, rather than temperature gradients that would expand or contract through time.

Simulations of Permian Climate and Comparisons with Climate‐Sensitive Sediments

We use a climate model to simulate two intervals of Permian climate: the Sakmarian (ca. 280 Ma), at the end of the major Permo‐Carboniferous glaciation, and the Wordian (ca. 265 Ma). We explore the climate sensitivity to various levels of atmospheric CO2 concentration and to changes in geography and topography between the two periods. The model simulates large seasonality and high aridity in the continental interiors of both hemispheres for both periods. The northern summer monsoon weakens and the southern monsoon strengthens between the Sakmarian and the Wordian, owing to changes in geography and topography. The northern middle and high latitudes cool in winter, between the Sakmarian and Wordian, associated with northward shift of the continents. This high‐latitude cooling strengthens the winter westerlies and shifts the maximum storm‐track precipitation south. In the Southern Hemisphere, the winter westerlies weaken from the Sakmarian to the Wordian. Starting the simulations with no permanent ice fields (i.e., by assuming that the late Sakmarian postdates deglaciation) and imposing increased levels of atmospheric CO2 four times the present level, we find no tendency for reinitiation of major glaciation. Some permanent snow fields do develop in high southern latitudes, but these are primarily at high elevation. However, the combination of low CO2 levels (such as present‐day levels) and a cold summer orbital configuration produces expanded areas of permanent snow. The results are based on statistics derived from the final 5 yr of 20‐yr simulations. Paleoenvironmental indicators such as coal, evaporite, phosphate, and eolian sand deposits agree qualitatively with the simulated climate. The extreme cold simulated in high latitudes is inconsistent with estimates of high‐latitude conditions. Either the interpretation of observations is incorrect, the model is incorrect, or both; a possible model deficiency that leads to cold conditions in high latitudes is the relatively weak ocean‐heat transport simulated by the heat diffusion parameterization of the upper ocean model.

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.

Permian climates: Evaluating model predictions using global paleobotanical data

The most recent global icehouse‐hothouse climate transition in Earth history occurred in the Permian. Warmer polar conditions relative to today existed from the middle Permian through the Mesozoic and into the Cenozoic. We focus here on one particularly well-correlated middle Permian stage that postdated the deglaciation, the Wordian (267‐264 Ma), integrating floral and lithological data to determine Wordian climates globally. Paleobotanical data provide the best means of interpreting terrestrial paleoclimates, often revealing important information in the continuum between “dry” and “wet” end-member lithological indicators such as evaporites and coals. New statistical analyses of Wordian floras worldwide have enabled a greater understanding of original vegetation patterns and prevailing climate conditions. The derived climate interpretations are compared with new Wordian atmospheric general circulation model simulations. The model matches the data well in the tropics and northern high latitudes, but predicts colder conditions in southern high latitudes. We discuss possible reasons for this discrepancy.

Coal, climate and terrestrial productivity: the present and early Cretaceous compared

Abstract At the present time, rainfall sufficient and consistent enough for swamp formation and peat preservation occurs in the equatorial and temperate belts. Evaporite deposits and aeolian sandstones occur in the consistently dry areas, while the intermediate areas of seasonal rainfall tend not to have any of these climatically significant sediments. In the Cretaceous, the temperate belts are well represented by coals as today, but the tropical belt is not, and the same can be said of other Mesozoic and early Cenozoic periods. Instead, the tropics are represented by low diversity floras and sediments, like the Nubian Sandstone, which can be interpreted as representative of climates in which precipitation was markedly seasonal. The inference is that the Intertropical Convergence Zone was less latitudinally confined during the warmer ‘greenhouse’ periods, and that this might have been due to weaker polar fronts.

World peat occurrence and the seasonality of climate and vegetation

Abstract The climatic controls on present-day peat formation are examined in order to develop a uniformitarian tool for refining paleoclimate studies, and to create a retrodictive model for the coals of the geologic record. A peat database has been compiled for the world and the resulting patterns are compared with aspects of precipitation and temperature drawn from a gridded data set. Precipitation continuity through the annual cycle is important for biologic productivity in the terrestrial realm. It is expressed in terms of the number of months with average precipitation above 40 mm. Temperature is limiting at higher latitudes where the potential growing season may be defined by the number of months with mean temperature exceeding 10°C. A map predicting the growing season has been constructed from these rainfall and temperature statistics which compares very well with actual vegetation persistence through the year as measured directly from satellite instruments. However, biologic production will not result in peat accumulation unless groundwater levels are maintained through the growing season. Our peat prediction map therefore shows the percentage of time that precipitation occurs during the potential growing season. Present-day peat accumulations are well predicted by this map.

Geosynclinal Development of the British Isles during the Silurian Period

The British geosynclinal complex was bounded on both northwest and southeast by continental margins during the Lower Paleozoic. The northern margin, in Scotland and the northern parts of Ireland, consisted of a mountain range, the Scottish Highlands, which was technically active and was positioned between a stable continental foreland to the northwest, and an uncoupled oceanic crustal plate to the southeast. The southern margin, in Wales and the Welsh Borderland, was characterized by a horst and graben structure and was gradually subsiding during most of the Silurian period. The Scottish margin produced tremendous quantities of immature graywacke sands which accumulated in a trench adjacent to the source area, on the site of the present Southern Uplands. A pattern developed about the middle of the Silurian in which deposits in this trench became deformed and uplifted shortly after deposition and, in turn, contributed sediments to the trench which was progressively displaced to the south toward the Lake District. In this way, the marine area was gradually overwhelmed by crustal foreshortening, deformation, and sedimentation, and by the end of the Silurian, continental conditions were established.