Lateral versus downward coccolith flux in a canyon setting at the southwestern Cretan margin (eastern Mediterranean Sea)

A total of eight sediment traps (June 2005-May 2006) were deployed along six moorings at the southwestern Cretan margin to obtain time series results. The continental slope here (between Crete and Gavdos islands) is cut by major submarine canyon systems that drain into the deep Gortys-Gavdos Troughs. From west to east, four moorings were deployed at different depths along the axis of the Samaria Canyon and on the Gavdos rise, one mooring along the axis of the Paximades Channel, and one mooring on the northern side of the eastern South Cretan Trough. The major canyon systems coincide with subaerial canyons onshore, pointing to a strong tectonic control. Geophysical analyses of the area (Alvez et al., 2007) revealed a Pliocene-Holocene sedimentation pattern that coincides with the modern canyons’ locations, showing a greater thickness in regions coinciding with modern depocenters. The Pliocene-Holocene sedimentary unit is represented by alternations of fine-grained turbidites and coarse-grained mass-wasting deposits, which are typical of confined deep-water basins. The majority of the sediment traps were located in close proximity to the bottom (15 m above the sea floor) in order to detect activity in the canyon systems. At these mooring sites, coccolithophore fluxes were analyzed as part of the sediment-forming material. Coccolithophores are a major phytoplankton group in the eastern Mediterranean and contribute significantly to carbonate sedimentation in pelagic and hemipelagic settings. In this study, we analyzed the combined flux of intact coccospheres and single coccoliths. The coccosphere/ coccolith ratio is an important ecological indicator in water samples (Cachão et al., 2000), and in sediment traps, fluxes of coccospheres represent the primary signal of export production, while fluxes of single coccoliths may indicate that there also was lateral transport. A detailed analysis of coccolithophore export production (Malinverno et al., 2009) from the eight sediment traps by means of intact coccosphere flux revealed a consistent pattern of seasonal variation in species’ distribution and total coccolithophore export. A major export peak in late spring to early summer, decreasing flux during summerfall, and an overall low flux in winter correspond to the seasonal variations in sea-surface temperature, surfacemixed layer depth, and precipitation that are associated with varying total surface primary production, as detected through remote sensing in the surface waters. In our dataset, the coccosphere/coccolith ratio decreases with the collection depth as a function of coccosphere disruption. Malinverno et al. (2009) also showed that robust coccospheres (Emiliania huxleyi), which are more easily preserved, increase in relative abundance in the coccosphere assemblage at increasing collection depths. In addition, minimum values of the coccosphere/ coccolith ratio occur at distinct intervals at some mooring sites. These intervals coincide with peak mass fluxes and likely represent events of redeposition of material mobilized from the bottom at the margins of the canyon system.