Sonntag, 24. Mai 2009

What lake sediments are good for

47,8 million years ago, deep inside the paneuropean tropical forest, the common noises of this rich ecosystem were overlaid by a terrifing bang. A hughe explosion, and subsequentely a large cloud raised up to the sky, bringing death and distruction to the inhabitants of this world, and leaving a large hole in the landscape.
For at least a million years unlucky animals felt in the lake that filled the crater, even a road-killed oppossum, that conserved by the particular geological setting, million years after will be worshipped by a bunch of primates. The discovery of D. masillae has (re)brougth (in positive but also negative sense) to the public the fossil lagerstätte of Messel, a heritage not only notable for his paleontological, but also geological value.

Lakes rappresents basins that can record and preserve biological and sedimentological changes happening in terrestrial environments.
Exceptionally good sedimentation “traps” are maar-lakes – lakes created by a phreatomagmatic eruption and subsequentely filled by precipitation or runoff. Maar-lakes are surrounded by a wall of volcanic material – ash and tuff- deposited during their formation. The slopes so tends to fall in direction of the center, restricting the catchement on the near, the lake surrounding area. This has important effects on the possible inwash in the lake – fossils found in the lake sediments rappresent a very restricted and local fauna and flora taphonomic assocation.
This maybe also explain the low biodiversity of found fish and plants fossils in the Messel Maar - the steep cliffs were lacking vegetation, and fish couldn´t colonise the lake easy because of the missing connections to other rivers and lakes.
Maar lakes normally – apart of their circular
form- possess steep walls and cliffs, that suddenly fall from the shore to the bottom of the lake. Because of their depth, they show a strong temperature gradient between superficial and bottom water layers, and so different water densitys, preventing deep reaching currents and so most time a mixing of the different layers. The bottom water is also impoverished by oxygen, creating a zone where colonisation by higher animals is practically impossible, the so called Monimolimnion. In this setting, sediments can deposit undisturbed and carcasses that reach the bottom are not disarticulated by scavengers.

This stratification of the water column can be disturbed in temperate zones by low temperatures during spring and autumn (in winter the surface is frozen), the cooled superficial water can reache the same density of the cold bottom water, enabling a mixing of the water bodies. In contrast, in tropic zones - or like Messel , were temperature variations during the year are much lesser
, the Monimolimnion can – if not disturbed by geological or exceptionally climatic events- be preserved for long periods, and so the sedimentation is more stable and preservation af animals more efficient.

The well known modern european maar lakes of the Eifel and the Auvergne are examples of dimictic (2x mixing per year) lakes of the temperate zone, the fossil maar lakes of Messel an
d surrounding areas are thought to resemble monomictic (1 or lesser mixing per year) lakes of today tropic zones.
The first recorded the biotic, sedimentological and climatic changes of the last million to 100.000 years, the second recorded parts of an ecosystem of the Eocene, 47 million years ago.

Even if the settings of this examples are different, they show
some similarities, primary in geological structure, and so studying “recent” shapes maybe can give clues to interpret fossil forms and vice versa. A typical sediment of a maar succession was observed by the Messel drilling project in 2001. In addition of the “oil-shales” – the fossil bearing bituminous pelites- and/or lacustrine sediments of the upper and lower Messel-formation (0-240m), volcaniclastic sequences (240-373m) as well as rocks of the diatreme breccia (373-433m) breccia produced by rising and exploding magma that breaks the circumstanding rocks) were discovered. The bottom of the lacustrine sediments rappresent coarser erosion material from the vegetation-lacking walls, followed by detritus-and organic material rich mud (mostly including fossil remnants of monocellular algae), deposited in a 300 up to 600m in diameter lake surrounded by a dense forest. With the infilling of the Messel-lake, and surrounding other lakes - that today rappresents the Messel-formation, probably a bog developed, and finally “dry” land covered by forest.
By the way, even if bogs are also a good archive, especially for pollen and spores, the found plant fossils rappresents a restricted flora, including plants adapted for bog conditions (for example low nutrient content), and are so much lesser suitable for paleoenvironments reconstructions.

Lakes are in geological perspectives short-living features, for Messel a sedimentation period of only one milli
on years is presumed. Neverthenless their rappresent not only an important paleontological, but also a geological archive, in distant past but also in/for the last 1000 years.

Schematic geological profile of the Messel-Maar:

Brigth brown: Tephra-layers and volcaniclastic material (tephra - wall)
Black: Bituminous pelites, stratified with sandy layers (Messel Formation)
Green: Conglomerates, sandstones and coarse sediments with volcaniclastic material
Dark green: Permian Sediments (Moret Formation)
Pink: Carboniferous sediments, cristalline rocks, granits and amphibolites
(after GRUBER & MICKLICH 2007)


GRUBER, G. & MICKLICH, N. (2007): Messel – Schätze der Urzeit. Hessisches Landesmuseum Darmstadt & WBG
NITZSCHE, T.; ROLF, C. & deWALL, H. (2006): Origin of magnetic anomalies in volcaniclastic units of the Messel maar-diatreme (Germany). Z. Dt. Ges. Geowiss. 157/3: 373-385

1 Kommentar:

Dinorider d'Andoandor hat gesagt…

Thanks for this post David, now I understand many things about this site and its excellent fossils depite having no previous knowledge on geology stuff.