Loess biostratigraphy: new advances and approaches in mollusk studies

Since the beginning of the 20th century, loess biostratigraphy has been strongly influenced by investigations of mollusks, these shells constituting the major fossil remains found in loess sequences. The earlier studies mostly involved identifying species and determining biozones or assemblages with regard to the presence or absence of key species, using the classic concept of biozones. Although this provided accurate elements with which to define an indicative biostratigraphy, the time resolution was not always sufficiently precise to connect those studies with the more recent and high resolution analyses that are now routine in Quaternary investigations. This paper reviews mollusk studies carried out in Northern Hemisphere loess sequences and shows that a consideration of them. as both biostratigraphic and palaeoclimatic indices, enhances their potential and opens up particularly interesting areas of research. The first example demonstrates that the last glacial mollusk assemblages in North America show compositional similarities to those in Europe. The climatic interpretation, however, appears more restricted by local conditions, The second example shows that climatic conditions can be used to infer variations in the composition of biozones and, thus, address the significance of the distributional pattern of key species. The third example demonstrates the value of high-resolution studies and the potential of comparing the results of mollusk analyses with other proxies as an underpinning of the biological interpretation. Finally, the need for more high-resolution investigations in both North America and Asia is stressed. (C) 2001 Elsevier Science B.V. All rights reserved.