An end-member algorithm for deciphering modern detrital processes from lake sediments of Lake Donggi Cona, NE Tibetan Plateau, China

Muddled or mixed? Inferring palaeoclimate from size distributions of deep-sea clastics

Abstract One of the outstanding problems of palaeoclimate reconstruction from physico-chemical properties of terrigenous deep-sea sediments stems from the fact that most basin fills are mixtures of sediment populations derived from different sources and transported to the site of deposition by different mechanisms. Conventional approaches to palaeoclimate reconstruction from deep-sea sediments do not distinguish between provenance and dispersal-related variations, and therefore often fail to recognise the true significance of variations in sediment properties. We formulate a set of requirements that each proposed palaeo-environmental indicator should fulfil, and focus on the intrinsic coupling between grain size and chemical composition. A critical review of past achievements in grain-size analysis serves to introduce a conceptual model of spatio-temporal grain-size variation based on dynamic populations. Each dynamic population results from a characteristic combination of production and transport mechanisms that corresponds to a distinct subpopulation in the data analysed. The mathematical–statistical representation of the conceptual model can be obtained by means of the end-member-modelling algorithm EMMA. Applications of the model to several ocean basins are discussed, as well as methods to examine the validity of grain-size-based palaeoclimate reconstructions. Palaeoclimate reconstructions of a high- and low-latitude basin illustrate the common degree of complexity of deep-sea grain-size records.

Muddled or mixed? Inferring paleoclimate from size distributions of deep-sea clastics

Petrography and paleo-environment of continental carbonate deposits with particular reference to the upper cretaceous and lower eocene of languedoc (Southern France)

The contributions of the earth sciences, particularly geomorphology and sedimentary petrography, to the interpretation and environmental reconstruction of archaeological contexts is called "geoarchaeology." The physical context provides a palaeo-environmental legacy liable to patterning and interpretation just as artifacts imply prehistoric cultural activity. Through field study and laboratory analyses, the geoarchaeologist elaborates the micro-, meso-, and macro-environments of a site, and provides input for erecting prehistoric human activity patterns in time and space. The categories of information that must be examined in the field, the range of interpretive problems that confront the field worker, and the laboratory techniques which corroborate field interpretations can be illustrated by examining archaeology in an alluvial site. These considerations emphasize the need to involve the geomorphologist in all aspects of the design and execution of an archaeological excavation.

Geoarchaeology: The Geomorphologist and Archaeology

Using an area of recent lacustrine sedimentation as a reference model, several methods of treating grain-size distribution of sands were evaluated for their ability to discriminate between, or identify, depositional environments. The techniques of Passega (1957), Mason and Folk (1958), Friedman (1961), Sahu (1964), and Klovan (1966) were applied to sediment samples from known depositional settings. When analyzed as though from unknown environments, none of the methods could reliably identify the true depositional environment of the samples as established on the basis of hydrographic, topographic, geographic and sedimentologic criteria. Of the methods, only factor analysis (Klovan, 1966) produced mappable results consistent with known energy conditions at the depositional sites. Differences in energy conditions which may exist between lacustrine and marine environments may be such that the techniques might not be expected to be environmentally sensitive. However, the ultimate goal of all such "grain-size-environment" studies is to predict depositional environments of ancient sand bodies for which little or no information concerning the depositional setting is available. Because factor analysis was able to produce useable results even under the restricting conditions of a lacustrine setting it may be expected that it will perform even better in the marine realm. The lack of success of the methods may result from any or all of the following factors: (1) Standard bivariate plots cannot adequately express the complex processes producing particular grain-size distributions. (2) Grain-size parameters are merely descriptors of grain-size distributions. As such they may not contain all available information, nor perhaps of the right kind, to enable environmental determination. (3) Combinations of amount and type of kinetic energy (which may be primarily responsible for the grain-size distribution of a sand) may produce similar distributions in different environments. The hypothesis that grain-size distributions can identify depositional environments is probably not universally valid. It is concluded that grain-size distributions reflect depositional processes rather than environment and that the two need not be the same.

J. T. Solohub

Papers

Evaluation of Grain-Size Parameters in Lacustrine Environments