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Size and shape of small sedimentary particles: a proxy for the palaeoenvironment?

donderdag, 13 maart, 2008 - 16:00
Campus: Brussels Humanities, Sciences & Engineering campus
Faculteit: Science and Bio-engineering Sciences
D
2.01
Daisy Tysmans
doctoraatsverdediging

There is increasing interest in understanding how sedimentary particles undergo size and shape changes as a result of an evolving environment, in particular under the influence of climate changes and inherent transport and sedimentation processes. Improved insights concerning these issues allow to better understand these complex interactions and contribute to the reconstruction of past environmental conditions. Despite considerable research efforts, the understanding of particle behavior and more specifically size and shape evolutions within this context is still rather incomplete.

Theoretically, analysis of the size and shape of sediments can yield information about the climate or the environmental condition in which they were formed. The use of grain size distributions has become established, but the shape of sediments is frequently disregarded. Most likely because the measurement of grain shape is not as straightforward as defining grain sizes. Thanks to the evolution of microelectronics Dynamic Image Analysis could develop (Xu et al., 2003). Dynamic Image Analysis defines the size and shape of particles by analyzing digital images.

The loessic environment responds to global climatic variations. Loess sequences can be considered as high resolution sedimentary archives (Derbyshire et al., 1997, Antoine et al., 2001, 2003). They yield information about the palaeoclimate, the processes of formation, transportation and sedimentation as well as about the post sedimentary processes in the loess sequence.

In this work loess sediments are studied. Their grain size and grain shape are defined with Automated Dynamic Image Analysis. Based on both parameters an attempt is made to reconstruct the environment in which they were formed. This work showed that based on their grain size and grain shape distribution at least fluvial, estuarine and aeolian sediments can be differentiated from each other with the two-dimensional Kolmogorov-Smirnov test. In the aeolian environment, several sedimentation phases could be distinguished. Differences in sedimentary structures are reflected in the signature of both grain size and grain shape. Cold conditions with stronger, more turbulent wind dynamics, drier environments and a reduced vegetation cover seem to result in the sedimentation of larger and more circular grains. Slightly warmer conditions, with weaker, less turbulent winds, less dry environments and a slightly increased vegetation density seem to result in the sedimentation of smaller and less circular grains. Grain shape differences are always small. It seems that aeolian transport is extremely selective regarding both size and shape of the grains it transports.

However, more work is needed to draw firm conclusions about the ability of grain size and grain shape to be a proxy. It was demonstrated that they record palaeoenvironmental differences and are influenced by different processes. However, the straightforward interpretation of their signatures as an environmental proxy needs further investigation.