Evidence for the evolution of an oxygen minimum layer at the beginning of S-1 sapropel deposition in the eastern Mediterranean

The sediments of the eastern Mediterranean basin contain a series of organic-rich sapropels intercalated with organic-poor nannofossil oozes. Until recently the timing of the onset of sapropel formation was not known accurately because of the low resolution achievable by conventional radiocarbon dating. Compilation of all available ¹⁴C-AMS dates show that the base of S-1 (the most recent sapropel) was initiated 8800 years B.P. (¹⁴C age corrected by 400 years for reservoir effect) under a 500 m water column and moved progressively into deeper water reaching depths of 3500 m at 8200 years B.P. The linear correlation between the age of S-1 onset and water depth

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suggests that formation of sapropels moved into deeper water at a rate of 1000 m/200 year. A model is suggested in which export production which sank below the well-mixed surface layers (500 m) was respired consuming dissolved oxygen in the Levantine deep water until a threshold value was reached when sapropels began to be preserved in the sediment. This resulted in a progressively deepening oxygen minimum zone with time until eventually the entire deep water in the basin was oxygen depleted. Assuming that the threshold value for sapropel formation was complete anoxia, it was calculated that primary productivity in the basin during the deposition of S-1 was a factor of 5 greater than that found at present.     

Facies and geometry of tidal channel-fill deposits (Arcachon Lagoon, SW France)

The Arcachon Lagoon has an important network of tidal channels and well developed tidal flats covered by the marine grass Zostera marina. Based on 66 piston cores taken from the Graveyron tidal channel, and observations on the neighbouring channels, this paper documents the facies and geometry of the channel-fill deposits. In the inner lagoon (studied area) the tidal channels are 80 to 150 m wide and have a meandering morphology with sandy point bars 2 to 5 m thick. The channel-fill does not consist of the classic inclined heterolithic bedding typical of many channel-fills (Reineck, 1958), but of cross-stratified sandy deposits characterized by the absence of slack-water clay-drapes. These unusual facies characteristics are due to the low turbidity of the lagoonal waters which is caused by the lack of significant river inflow and the dense coverage of Zostera marina on the tidal flats. The overall geometry of the channel-fill deposits is characterized by a narrow sand-ribbon shape, a few kilometres long, 80 to 150 m wide and 1 to 5 m thick. This sand ribbon is made of elliptical sand bodies, deposited as point bars, that coalesce longitudinally along the channel axis. This narrow shape is due to the fact that the lateral migration of the channel is virtually nil (reduced to a few metres). In spite of their characteristic meandering morphology, these channels do not deposit extensive tabular sand sheets of amalgamated point bars like the tidal creeks on the North Sea tidal flats. Two factors are thought to control this lack of channel migration. (1) The tidal flats adjacent to the tidal channels are made of 3- to 5-m-thick cohesive muddy sediments covered by Zostera marina that prevents the erosion of the channel banks. This first mechanism is supported by the observation that the tidal creeks that drain the muddy tidal flats covered by Zostera marina do not migrate laterally, whereas those that drain the sandy tidal flats devoid of a dense coverage of marine grass do have active lateral migration. (2) The tidal channels are not fed by any river and therefore do not receive any fluvial sand influx during the winter floods. Their morphology is in equilibrium with the tidal discharge and represents a stable stage in the development of the channel. This second mechanism is supported by the fact that the only tidal channels that actively migrate laterally in the lagoon receive sandy fluvial influx from the River Leyre located in the southeastern corner of the lagoon.