Michael A. Arthur

Bio: Michael A. Arthur is an academic researcher from Pennsylvania State University. The author has contributed to research in topics: Cretaceous & Total organic carbon. The author has an hindex of 72, co-authored 166 publications receiving 19941 citations. Previous affiliations of Michael A. Arthur include Scripps Research Institute & University of Rhode Island.

Carbon Isotope Fluctuations in Cretaceous Pelagic Limestones: Potential Stratigraphic and Petroleum Exploration Tool

Abstract: Significant short-term carbon isotope fluctuations are present in Cretaceous pelagic limestones from widely distributed onshore sections in the Circum-Atlantic-western Tethyan region. More than 1,000 closely spaced samples were analyzed during this study. At least seven major ^dgr13C excursions can be correlated from section to section. The most important "heavy events" occur near the Aptian-Albian and Cenomanian-Turonian boundaries, whereas "light events" are near the Jurassic-Cretaceous, Albian-Cenomanian, Turonian-Coniacian, and Cretaceous-Tertiary boundaries. The association of "events" with stage boundaries and the consistent correlation of "events" between stratigraphic sections provides a significant new tool for time-rock correlation independent of stan ard biostratigraphic techniques. The temporal association of these carbon isotope events with stage boundaries (faunal and floral events), global eustatic sea-level variations, and oceanic "anoxic events" demonstrates the potential usefulness of carbon isotope studies in interpreting variations in paleo-oceanic circulation. Furthermore, the association of carbon isotope variations with anoxic events is potentially useful for evaluation of the precise timing and the magnitude of preservation of organic matter in deep-sea and continental-margin sediments. Thus, isotopic studies may aid in estimating potential hydrocarbon resources in largely unexplored oceanic basins or along continental margins.

MARINE BLACK SHALES: Depositional Mechanisms and Environments of Ancient Deposits

Abstract: Organic-carbon-rich strata or "black shales," including dark gray to black, laminated, carbonaceous mudrocks characterized by impoverished benthonic faunas, or devoid of metazoan life, have long intrigued geol­ ogists, both because of their widespread distribution at certain times in the past and their early recognition as potential hydrocarbon source rocks. One of the major obsessions of many early workers, to the mid1900s, was the application of uniformitarian principles to depositional models for black shales. Thus, although it was recognized that some black shale units were unusually widespread, many early workers sought less extensive, modern analogues for environments of formation of dark-colored, rela-

Chemical weathering ,a tmospheric co 2 , and climate

Abstract: There has been considerable controversy concerning the role of chem- ical weathering in the regulation of the atmospheric partial pressure of carbon dioxide, and thus the strength of the greenhouse effect and global climate. Arguments center on the sensitivity of chemical weathering to climatic factors, especially temperature. Laboratory studies reveal a strong dependence of mineral dissolution on temperature, but the expression of this dependence in the field is often obscured by other environ- mental factors that co-vary with temperature. In the field, the clearest correlation is between chemical erosion rates and runoff, indicating an important dependence on the intensity of the hydrological cycle. Numerical models and interpretation of the geologic record reveal that chemical weathering has played a substantial role in both maintaining climatic stability over the eons as well as driving climatic swings in response to tectonic and paleogeographic factors.

Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present

Abstract: Since 65 million years ago (Ma), Earth's climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 10(5) to 10(7) years, rhythmic or periodic cycles driven by orbital processes with 10(4)- to 10(6)-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 10(3) to 10(5) years. Here, recent progress in defining the evolution of global climate over the Cenozoic Era is reviewed. We focus primarily on the periodic and anomalous components of variability over the early portion of this era, as constrained by the latest generation of deep-sea isotope records. We also consider how this improved perspective has led to the recognition of previously unforeseen mechanisms for altering climate.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Abstract: The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Stable isotopes in ecosystem studies

Abstract: The use of stable isotopes to solve biogeochemical problems in ecosystem analysis is increasing rapidly because stable isotope data can contribute both source-sink (tracer) and process information: The elements C, N, S, H, and all have more than one isotope, and isotopic compositions of natural materials can be measured with great precision with a mass spectrometer. Isotopic compositions change in predictable ways as elements cycle through the biosphere. These changes have been exploited by geochemists to understand the global elemental cycles. Ecologists have not until quite recently employed these techniques. The reasons for this are, first, that most ecologists do not have the background in chemistry and geochemistry to be fully aware of the possibilities for exploiting the natural variations in stable isotopic compositions, and second, that stable isotope ratio measurements require equipment not normally available to ecologists. This is unfortunate because some of the more intractable problems in ecology can be profitably addressed using stable isotope measurements. Stable isotopes are ideally suited to increase our understanding of element cycles in ecosystems. This review is written for ecologists who would like to learn more about how stable isotope analyses have been and can be used in ecosystem studies. We begin with an explanation of isotope terminology and fractionation, then summarize isotopic distributions in the C, N, and S biogeochemical cycles, and conclude with five case studies that show how stable isotope measurements can provide crucial information for ecosystem analysis. We restrict this review to studies of natural variations in C, N, and S isotopic abundances, cxcluding from consideration ~5N enrichment studies and hydrogen and oxygen isotope studies. Our focus on C, N, and S derives in part from our

An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics

Abstract: Past episodes of greenhouse warming provide insight into the coupling of climate and the carbon cycle and thus may help to predict the consequences of unabated carbon emissions in the future.

The impact of climate change on the world's marine ecosystems.

Abstract: Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed. Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation. The impacts of anthropogenic climate change so far include decreased ocean productivity, altered food web dynamics, reduced abundance of habitat-forming species, shifting species distributions, and a greater incidence of disease. Although there is considerable uncertainty about the spatial and temporal details, climate change is clearly and fundamentally altering ocean ecosystems. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.