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Abstract The application of any General Circulation Model (GCM) simulation is only as valuable as: the quality of the original input boundary conditions, the computer model used and its track record for producing quality paleoclimate simulations, and the extent to which the simulation results have been tested successfully with the geologic record. We utilize a global simulation of the Kimmeridgian (Late Jurassic) to focus on an area of investigation. The simulation was tested against the geologic record. The results replicate the paleoclimate with a consistency that is acceptable, if not impressive. Therefore, the results can be utilized to map the distribution of climatically sensitive sediments within a given area. The study area includes the western part of the Tethys Sea, which was a zonally oriented tropical sea at a paleolatitude of about 0°-25°N and isolated from the Panthalassa Ocean by an isthmus. The sea was characterized by warm tropical surface water and generally strong net evaporation creating conditions of low oxygen content and elevated salinities in the surface water mass. Much of the margin receives insufficient precipitation to maintain lasting soil moisture or generate runoff to the sea in any season, precluding development of a lush vegetative cover. The general lack of runoff improves conditions for reef growth and carbonate deposition on the continental shelves. Much of the margin possesses wind-driven coastal upwelling. As a positive correlation exists between upwelling and high primary productivity on the margins of today’s World Ocean, we predict that a similar relationship occurred in the Kimmeridgian Western Tethys Sea. The high salinities contributed to a probable negative water balance, such as the Mediterranean Sea today. This circulation would keep the main axis of the basin generally oxygenated, except for isolated, restricted local basins. In this study, model results correlate well with published regional lithofacies maps where data are available. This complement offers encouraging proof that this model generally replicates the real Late Jurassic paleoclimate by creating the proper physical conditions under which the biota existed and sediments were deposited.
Abstract By the earliest Cretaceous, a meridionally oriented rift system began splitting Northern Gondwana into the respective continents of South America and Africa. The system terminates abruptly against the Falkland-Agulhas transform on the south and the St. Paul-Romanche transform to the north, which give the boundaries to the present-day South Atlantic Ocean. This 5000 km long system created an elongated, segmented, complex series of rift valleys that were the settings for lakes ranging in age from Neocomian through Barremian. Various geologic factors defined the major segmentations of the margin and ultimately controlled basin dimensions. Early in the history of these basins, the lakes occupying some basins became anoxic, allowing organic-rich sediments to accumulate. These source rocks and their generated oils have been shown through geochemistry and biomarker studies to change character north of the Rio Grande Rise-Walvis Ridge complex toward the interior of Northern Gondwana. The southern rift lake basins that evolved into the Santos, Campos, and Espirito Santo basins on the South American margin and the Angola, Congo, Cabinda basins on the African margin generated oils from source rocks originally deposited in saline to brackish water anoxic lakes. In the more continental interior basins of Sergipe-Alagoas, Potiguar (South America), and Gabon (Africa) the organic-rich sediments were deposited in freshwater lakes that were dysaerobic to anoxic. These relationships imply evaporative conditions in the south and a net positive water balance in the interior of Northern Gondwana, far-removed from a convenient moisture source. However, when the rift lakes are plotted on paleoclimate maps derived from a general circulation model (GCM) simulation this apparent paradox is readily explainable. The southern saline-brackish lakes are in an arid region. In contrast, the freshwater lakes are in a region affected by a massive, seasonal system of monsoonal and trade wind-dominated precipitation that covers most of the northern portion of the continent. This exemplary integration of geochemistry and paleoclimate modeling elucidates the absolute requirement of multidisciplinary approaches to resolving regional questions related to geological processes that control the formation of sedimentary rocks.
Abstract An understanding of climate models and their applications requires a background in climate and climate dynamics. The fundamentals of the energy balance which drives the Earth's climate system and a description of the circulation of the oceans and atmosphere are provided here as general background.
Abstract Three basic requirements are needed to simulate paleoclimates: (1) knowledge of the major forcing factors which influence the climate, (2) a climate model which incorporates all the major processes and feedbacks needed to predict climate and climate change, and (3) sufficient observations of the state of the climate system to ensure that the predicted climates can be validated or compared with data. The primary task of climate modeling is to replace the complex natural system by a hierarchy of simplified ones which can be used as quantitative tools to investigate climate and climate change. The process of simplification is essential, as the incorporation of every molecule and their interactions is impossible. Simplification can take many forms. First, the system can be simplified by limiting the time scale of interest in the model. By specifying longer time scale processes as part of the "external" component of the model (e.g. assuming that the deep ocean is fixed or that the ice caps are not dynamic), the computational effort is limited to a finite "internal" set of variables. Second, there is a clear trade-off between spatial resolution and computational effort. Typical global scale climate models limit the horizontal resolution (e.g. 4.5° latitude by 7.5° longitude grids) or the vertical resolution. Typically, the computational time required varies as a function of the cube of the horizontal resolution in global models. Third, the physical processes incorporated in climate models can be simplified. Processes which can be computed from physical laws can often be parameterized
Abstract A primary problem is to identify the principle causes of climatic change which occur over the spectrum of time scales represented in Earth history. The simple energy balance model described in Chapter 2 indicates that, in the most general terms, the climatic forcing factors fall within three major categories: (1) the amount and distribution of solar radiation received at the top of the atmosphere, (2) the composition of the atmosphere and its effect on the Earth's radiation budget, and (3) the nature of the surface of the Earth. Numerous terrestrial and extraterrestrial factors have been proposed to explain the climatic record of the Earth. Following the general classification of forcing factors above a long list of forcing factors can be compiled, including solar evolution, solar variability, variations in the Earth's orbit, the influence of galactic clouds of dust and gas, extraterrestrial impacts, greenhouse gases, volcanism, changing continental positions, sea level variations, and mountain building. These climatic forcing factors may have jointly or independently influenced past environments. In many cases, it is unclear whether a single cause explains a recorded climate change (e.g. a major glaciation) or whether each time period has been the product of a different set of factors. These types of climatic forcing factors must be specified in climate models as the "external" drivers which influence the climatic simulation. For most of Earth history the nature of the forcing factors is poorly known and the specification of all of the factors which may have been important during a
Abstract The continental configurations of Pangaea are markedly different from the present day. Continental size and aggradation were at a maximum, with considerable exposed land area stretching from pole-to-pole. A single world ocean, Panthalassa with a semi-enclosed Tethyan Sea, dominated the marine environment. The large differences in continental configuration should yield perhaps extreme climatic conditions, very different from the present climate. The expectations for an extreme climate are borne out by the geologic record. The Permo-Carboniferous is a time of extensive polar ice caps (e.g. Crowell, 1983). The distribution of red beds and evaporites (e.g. Schwarzbach, 1963; Frakes, 1979) are indicative of extreme continental and near-shore marine climates. The Permo-Triassic presents substantial evidence for a warm and more arid climate. The times of Pangaean continental geometry, because of its extreme geographic configuration, and the evidently extreme climatic conditions, have become the focus of a number of climate model studies (see reviews by Crowley, 1994; Barron and Fawcett, 1994). Model studies range from qualitative models based on analogy with the present day (Parrish, 1982; Patzkowsky and others, 1991), to Energy Balance Climate Models (EBMs) that address climate change as a function of the balance of incoming solar energy, outgoing infrared radiation and poleward heat transport (Crowley and others, 1989; Baum and Crowley, 1991; Crowley and Hyde, 1991; Hyde and others, 1990), to fully resolved models of the atmospheric general circulation or ocean circulation (Hunt, 1984; Kutzbach and Gallimore, 1989; Kutzbach and others, 1990) to analyses of the sensitivity of Pangaean continental geometries to orbital variations (Kutzbach, 1994). Climate simulations with highly idealized continental geometries (Barron and others, 1984; Hay and others, 1990) also aid in the interpretation of the climate of supercontinents.
Abstract The Cretaceous is a logical period for investigation because the warm, equable Cretaceous is the largest well-documented contrast from the present day climate (Barron, 1983). Mid-Cretaceous temperatures have been reconstructed within two limits (Fig. 5.1) with globally averaged surface temperatures at least 6°C higher than the present day. The lack of evidence for any permanent polar ice, the warmth of the oceanic deepwaters (~15°C), and the evidence for reduced seasonality of the continents have focussed substantial attention on the nature of warm climates in Earth history.
Abstract The record from the early Eocene (approximately 59 to 52 million years ago) is of particular interest for understanding climate and climate change during warm time periods. The Eocene is well-documented as the warmest interval of the Cenozoic (e.g. Shackleton and Boersma, 1981; Shackleton, 1986; Oberhansli and Hsu, 1986). For example, Eocene deciduous forests of a subtropical nature have been found at latitudes above 60° in latitude and vertebrate fauna requiring frostless winters are found at latitudes above 70° (Estes and Hutchison, 1980; Wolfe, 1980; McKenna, 1980). A careful examination of the early Eocene record would be warranted purely from the viewpoint of understanding the characteristics of one of the most recent periods of extreme planetary warmth. However, the characteristics of the early Eocene provide an even more important justification for comprehensive study. First, several authors (Berner and others, 1983; Kasting and Richardson, 1985; Owen and Rea, 1985) have proposed an explanation of Eocene warmth which is based on increased levels of atmospheric carbon dioxide. Second, comparison of the climate model results, wind velocities determined from eolian material in deep sea cores, the character of oceanic isotopic paleotemperatures, and a host of additional isotopic, floral and faunal indicators supports a hypothesis that the role of the Eocene ocean in transporting heat poleward may be significantly greater than at present (Barron, 1987; Sloan and Barron, 1992). The possibility of a warm climate dominated by oceanic poleward heat transport (i.e. a weaker atmospheric role) has important implications for the prediction of future
Abstract The major limiting factor in marine organic productivity is the nutrient supply within the photic zone (nitrates, phosphates, and silica). Nutrients are supplied to the photic zone from coastal erosion and river input and by remineralization of nutrients below the photic zone. Consequently, two are characterized by high productivity, coastal waters and divergences or upwelling zones. Sediments associated with upwelling tend to be rich in biologically produced opaline silica, marine organic carbon and phosphorus (Suess and Thiede, 1983: Thiede and Suess, 1983). The rich biology, high productivity, and the concentration of nutrients has generated substantial interest in upwelling regions. The tendency for higher accumulation of organic carbon, in particular, has fostered considerable attention on the prediction of past locations of upwelling as potential sites of petroleum source rock generation. Parrish (1982) pioneered the prediction of past upwelling using a qualitative model based on analogies with present day climate. Scotese and Summerhayes (1986) developed a parametric model based on the methods utilized by Parrish (1982). Barron (1985) and Kruijs and Barron (1990) utilized global numerical models of the climate system to predict the conditions required for upwelling. Much of the text that follows stems from Barron (1985) and Kruijs and Barron (1990). The prediction of upwelling locations will follow a two step approach. First, the physical conditions which create oceanic divergences will be described and second, the predictive capabilities of models will be assessed. The assessment of the predictive capabilities of numerical models will be examined through four steps : (1)
Abstract Water plays a central role in nearly all Earth processes. Water is a necessary and limiting factor for life. Precipitation is the major limiting factor for terrestrial productivity. Water is the primary agent of erosion and sediment transport and it is an essential element of geochemical cycles because of its solvent properties. Gases dissolved in rain water interact with soils and rocks. The resultant chemical weathering is closely linked to rainfall amount and vegetation cover. Water is also a major element of the climate system through the radiation balance and energy transfer. Despite its significance, quantitative tools to reconstruct and to understand changes in the moisture budget through Earth history are limited. Most of the available quantitative tools focus on the measurement of temperature. Vegetation, continental weathering products, desert sediment features and evaporite minerals are the primary sources of information used to reconstruct precipitation and evaporation conditions for Earth history (Frakes, 1979). Model studies are equally one-sided.
Abstract The development of lakes largely depends on two simple factors, the presence of a depression and a positive water balance. Lacustrine sedimentation reflects basin tectonics, drainage area, topography, rates of subsidence, nutrient levels, nutrient recycling, light availability, turbidity, oxygenation, depth, run-off, evaporation, precipitation and sediment supply. Climate is the major control on the water balance and on the characteristics of the water. Factors (e.g. Katz, 1990) leading to high productivity (e.g. nutrient input, nutrient recycling, solar input) and high organic carbon preservation (oxygen availability as a function of temperature, salinity and rates of supply, depth and sedimentation rate) may lead to the formation of petroleum source rocks. Given the importance of climate in regulating the water balance (the distribution of lakes) and in regulating temperature, salinity, wind mixing and lake stratification, climate models may have a role in the prediction of lacustrine environments. In particular, GCMs may aid in the prediction of the distribution of lacustrine petroleum source rocks. Climatic characteristics that govern lake occurrence and lake stratification have been investigated with a GCM by Barron (1990). Essentially five factors are considered as a guide to the prediction of lacustrine sedimentation: (1) positive atmospheric water balance is a requirement to determine the potential distribution of lakes, (2) seasonal temperature variations in excess of 40°C are taken as one criteria for seasonal lake turnover, (3) temperatures that seasonally vary through the value of 4°C, the density maximum for freshwater, are taken as a second criteria for seasonal turnover resulting in higher
Abstract Severe storms are recognized as important agents in sedimentary transport and in generating sedimentary structures and textures. Reef destruction, offshore sand transport, significant erosion and suspension, changes in meandering river paths, debris flows, major fluvial scour events, millimeter to meter storm layers, lag deposits, hummocky cross-stratification, coastline modification, and extensive supratidal sedimentation have been cited as storm-related features (for example, Perkins and Enos, 1968; Kreisa, 1981; Dott and Bourgeois, 1982; Perlmutter, 1982; Duke, 1985). The role of severe winter storms, hurricanes, and alternative mechanisms in the formation of hummocky cross-stratification is contested (Marsaglia and Klein, 1983; Duke, 1985, 1986; Klein and Marsaglia, 1986; Swift and Nummedal, 1986). The nature of the atmospheric circulation and the arrangement of continents in the past are both significant influences on storm generation. A detailed physically based consideration of climatic and geographic controls on severe storms will result in a greater understanding of the distribution and formation of storm-related deposits. The results given here summarizing the work of Barron (1985), are based on modern observations, physical processes deduced from these observations, and climate model simulations for past geographies.
Abstract Tremendous effort has been focussed on reconstructions of the distribution of organisms through time. One of the major assumptions in paleoclimatic study is that the distribution and character of life on Earth is closely tied to the environment. In some cases, the interpretation of the environmental conditions associated with the distribution and character of organisms has been the subject of debate. Climate models may provide an independent assessment of the environmental conditions which govern organism distribution and may also provide added insight into biogeographic interpretation. Here a case study on the tropical life during the mid-Cretaceous is offered as evidence for the unique role of climate models in biogeographic reconstruction and interpretation. A second case study, by Moore and Ross (1994) describes Kimmeridgian-Tithonian climate model experiments which provide insights into dinosaur and ammonoid biogeography.
Abstract The latitudinal and seasonal distribution of solar energy are governed by orbital element variations (described in Chapter 3) in eccentricity, obliquity or tilt, and precession of the equinoxes. The cyclic variations in Pleistocene glaciation have a well documented correspondence with Milankovitch orbital periodicities (Hays and others, 1976). Rhythmic sedimentary variations are not limited to the Pleistocene, and periodicities within a 10 4 to a 10 6 are not uncommon (e.g. Gilbert, 1895; Fischer, 1980; 1981; de Boer, 1982; Schwarzacher and Fischer, 1982). Interestingly, the magnitude of the variations in solar insolation are relatively small, on the order of seven percent variations in the amplitude of the seasonal cycle at mid-latitudes for large differences in orbital elements. Either the climate-sedimentary system must be highly sensitive to insolation variations of this form, or the magnitude of the forcing is amplified through positive feedbacks. A key objective is to be able to predict the distribution and character of rhythmic sediments.
Abstract The following references are a suggested supplement for this map exercise: Barron (1985, p. 448-452), Barron (1989, p. 601-604), Barron and others (1989, p. 157-174), Cecil (1990, p. 533-536), Demaison and Moore (1980, p. 1179-1209), Ivanhoe and Leckie (1993, p. 87-90), Klemme and Ulmishek (1991, p. 1809-1810, 1844-1846), Kruijs and Barron (1990, p. 195-200), Sloan and Barron (1990, p. 489-492), and Tissot and Welte (1984, p. 509-512). This exercise utilizes a series of paleoclimate maps designed to provide experience in their use and interpretation. For the most part, they are seasonal maps for December/January/February and/or June/July/August. Some will have varying atmospheric CO 2 concentrations or other boundary conditions, so check to make sure the correct map is being utilized. Besides an index map, they include sea level pressure, surface temperature, precipitation, runoff, precipitation-minus-evaporation (P-E), surface wind vectors, and wind vectors at 500 mb. Some questions will require the use of multiple maps.
Abstract In this chapter the principles developed in Chapter 9 are applied to a unique geometric set of lakes, more specifically a regional Early Cretaceous system of meridionally-oriented rift lakes. Northern Gondwana existed as a separate but disintegrating, compound continent for no more than about 50 my, from around the Jurassic-Cretaceous boundary to the mid-Cretaceous. By the Cenomanian (early Late Cretaceous), the continents of South American and Africa were completely separated from one another and the Atlantic became a continuous ocean. This case history is devoted to examining a meridionally-oriented string of lakes that formed in the interior of Northern Gondwana during the rifting phase. The focus will be on the African margin from the Walvis Ridge northward to the present-day Niger Delta, a distance which spans a latitude of about 22° (2400 km). An isopach map showing the distribution of syn-and post-breakup sediments on the Atlantic margins of both continents is given in Figure 14.1. This Early Cretaceous rifting event created a series of parallel, elongate depressions in which paleoclimatic conditions favored the development of lakes (Gerrard and Smith, 1982; Ojeda, 1982; Asmus and Baisch, 1983; Reyre, 1984; McHargue, 1990). The lakes occupied what became the present-data Atlantic margin of South America and Africa. They range in age from Neocomian (Berriasian, Valanginian, Hauterivian) to Barremian (Early Cretaceous) 145.5 to 124.5 my (Harland and others, 1990). Most contain organic-rich lacustrine source rocks which vary regionally in richness and kerogen content. The basal rocks in each basin are composed of siliciclastics with
Abstract This chapter deals with interpreting stratigraphy from a suite of paleoclimate maps generated from a global paleoclimate simulation and comparing the results with the geologic record. The CCM results can be viewed globally or, by focussing on a particular set of latitude/longitude coordinates, regionally. In this chapter we utilized the principles developed thus far in this short course to focus on a region occupied by a seaway, the western part of the Tethys Sea in the Kimmeridgian stage (154.7-152.1 Ma; Harland and others, 1990). The paleolatitudes range from 5°S to 35°N and the paleolongitudes from 50°W to 30ºE. It is important to remember that while we are examining only a small area of the simulation, the detail and accuracy are not enhanced from the global results. The surface grid still is approximately 4.5° latitude by 7.5° longitude.
Abstract One of the most difficult aspects of paleoclimatic studies, in both the interpretation of past environments and in describing cause and effect relationships, is an inability to consider the complex response of sediments and life to the climate system. In general, there has been a strong tendency to focus on only on a limited set of variables (e.g. surface temperature). In addition, if we identify an important causal relationship, such as the relationship of upwelling to petroleum source rock formation, often we attempt to try to fit all cases into this single cause-effect relationship. An ability to consider multiple variables simultaneously, and to incorporate multiple causal relationships has been elusive. The nature of GCMs lends itself to multivariable analyses. The output from these model experiments can be considered as an opportunity to overlay several variables in order to understand climate-sediment relationships, to formulate biogeographic interpretations, or to expand interpretations beyond a single variable. One case study would be to consider upwelling predictions of petroleum source rock distribution, like those in Chapter 7. The primary basis of the high productivity/upwelling calculations is knowledge of the surface winds and the positions of the continents. An assumption of such studies is that upwelling will result in increased productivity, and that in coastal regions, this upwelling will promote oxygen deficiencies and higher organic carbon deposition rates. However, upwelling derived by knowledge of wind stress alone will give an overestimate of the regions of upwelling (areas conducive to wind-driven upwelling). High river input, causing input
Abstract The concept of time is central to the geologic sciences. The sedimentary record represents a spectrum of time scales, from nearly instantaneous events to persistent conditions lasting perhaps millions of years. The very nature of the record highlights change. Climate model results are not consistent with this emphasis on time and change, because they represent only a snapshot in time. Essentially, the model experiments describe an equilibrium climate for a specific set of external conditions (geography, topography, composition of the atmosphere, and solar input). The external conditions are fixed. A time evolving series of simulations, in which carbon dioxide slowly varies or sea level advances and retreats as an underlying external boundary condition, can only be completed with highly simplified models due to the computational constraints. This constraint limits the comparison of climate models with geologic observations because they are unable to address the fundamental property of the record - global change.