Showing papers by "Malcolm B. Hart published in 1996"

Biotic Recovery from Mass Extinction Events

Abstract: Containing papers by leading authorities on several of the major extinction events of the geological record, the volume brings together new data on a wide range of floral and faunal groups. Several of the papers describe the recovery and recolonization processes following the extinction events while others discuss the problems of'survivor taxa', "disaster taxa' and "progenitor species'. The examples chosen come from geological successions in North America, South America, Europe, Asia and the Indian Subcontinent.

Cretaceous Bio-Events

Abstract: Bio-events occur at local, regional, and global scales, reflecting short-term, extraordinary, environmental changes. They can be classified as Diversification Bio-Events (punctuated evolution, population blooms, colonization and immigration bio-events), or Diversity Reduction Bio-Events (mass mortality, ecosystem shock, extinction and emigration bio-events). High-resolution (cm-dm scale) stratigraphic, geochemical, and paleobiological analyses demonstrate that many regional and most global bio-events are complex, multicausal phenomena. They may consist of two or more, closely spaced levels (“steps”) of biological response to rapid environmental changes — perturbations and their feedback loops — in ocean-climate systems. This is especially true for regional to global mass extinctions, which also tend to be ecologically graded, affecting more tropical, more stenotopic taxa/ecosystems first and most profoundly, and more poleward and/or eurytopic biotas later, and to a lesser degree. Comparisons of the stratigraphic expression of local, regional, and global bio-events are presented for the Cenomanian-Turonian (Ce-Tu: middle Cretaceous) Greenhorn Cyclothem at the Pueblo, Colorado reference section, where seven regional and one global bio-event (the C-T mass extinction) intervals are well defined. Regional and global bio-events are documented for the Americas, Europe, North Africa, and India. Global bio-event intervals include: the Jurassic-Cretaceous mass extinction interval; the Early Aptian Selli Bio-Event; the Late Aptian mass extinction interval; the Middle-Late Albian substage boundary bio-events; the Albian-Cenomanian stage boundary bio-events; the Cenomanian-Turonian boundary mass extinction interval; the Turonian-Coniacian stage boundary bio-events; the Coniacian/Santonian stage boundary bio-events; the Santonian/ Campanian stage boundary bio-events; the 68 Ma (Middle Maastrichtan) extinction interval; and the Cretaceous-Tertiary boundary mass extinction interval.

Recovery of the food chain after the Late Cenomanian extinction event

Abstract: Abstract The Late Cenomanian bio-event is accepted as a globally synchronous extinction event that is characterized by significant biological changes which are coincident with isotopic and geochemical anomalies. The extinction event is characterized by changes on the macro-, meio-, micro- and nanno-scale and, in many examples, it has been suggested that these are ‘step-wise’. In the United Kingdom successions the steps are emphasized by depositional non-sequences, but in all cases the biological changes are in the same order. In microfaunal terms this involves a marked reduction in the benthonic foraminifera followed closely by a restriction of the planktonic foraminifera to surface-dwelling morphotypes. The event is also characterized by short-lived floods of Heterohelix sp. (small, biserial planktonic foraminiferids), Bulimina sp. (small, benthonic foraminiferids often characteristic of low-oxygen environments, calcispheres (?calcified dinoflagellates) and radiolaria. Following the extinction events, in the very latest Cenomanian, the recovery phase begins. Almost immediately the calcareous nannoplankton flora is restored but the dinoflagellates do not recover until much later in the Turonian. The benthonic foraminifera recover slowly with the fauna of the Early Turonian being of low diversity, with long-ranging taxa — including a large (?deep water), internally complex, agglutinated genus (Labyrinthidoma). The planktonic foraminifera recovered quickly with the Praeglobotruncana, Dicarinella, and Marginotruncana faunas appearing in succession within 100 000–200 000 years of the end of the extinction event. Using a model of the normal food chain it is possible to identify the order in which the basic building blocks needed for ecosystem recovery are put in place following the extinction event.

Foraminiferal recovery after the mid-Cretaceous oceanic anoxic events (OAEs) in the Cauvery Basin, southeast India

Abstract: Abstract The Cauvery Basin, SE India, is one of the best exposed late Mesozoic to Tertiary basins in India. The study of foraminiferal assemblages from the core samples obtained from two 120m deep wells in the basin records the occurrence of two mid-Cretaceous anoxic events in the basin. Abrupt increases in planktonic: benthonic ratios and reductions in benthonic diversity are recorded in the late Albian and in the late Cenomanian-early Turonian. These events coincide with worldwide oceanic anoxic events (OAEs). The OAEs had an impact on the microfauna of the basin, with approximately 35–45% of the benthonic species not surviving and major morphological changes occurring in the planktonic community. The late Cenomanian-early Turonian event was more significant than the late Albian event, with considerable readjustments occurring in the planktonic foraminifera. The genus Rotalipora disappeared and the genus Hedbergella was largely replaced by Whiteinella. Marginotruncana appeared for the first time and the Dicarinella population expanded considerably. Praeglobotruncana was the most tolerant genus, undergoing least change. The planktonic foraminifera evolved from small, weakly ornamented forms with poorly developed keels, into robust, well ornamented forms with well developed keels. The pattern of evolution of planktonic foraminifera suggests a recolonization of deeper water environments after the late Cenomanian-early Turonian anoxic event.