TRACKING PAST FLUVIAL INPUTS OFF LYBIA
In December 2011, a set of sediment cores was retrieved from the Lybian margin (Gulf of Sirte) onboard the Dutch research vessel Pelagia during cruise 64PE349. I am collaborating with two colleagues for this project: Anne Osborne (GEOMAR Kiel) and Rik Tjallingii (GFZ Potsdam).
Preliminary report cruise 64PE349 (available on request).
Abstract: Terrestrial evidence for episodes of Sahara greening comes from lake and spring deposits, fossils of elephants and giraffes, and the distribution of savannah-type pollen. Marine records from the Mediterranean and Atlantic can give a more continuous account of changes from a dry and dusty Sahara to a humid and more vegetated northern Africa. In the deep ocean sediments of the eastern Mediterranean in particular, organic rich layers termed ‘sapropels’ are concurrent with African Humid Periods at 10-5 ka and 130-120 ka, and also correspond to increases in northern hemisphere insolation. As sapropels are also associated with negative trends of 18O in planktonic foraminiferal
calcite, links have been made between an increase in freshwater delivery to the Mediterranean and the formation of these layers. A study comparing three sites in the eastern Mediterranean showed, surprisingly, that the magnitude of change in 18O was greater in the Ionian Sea (ODP site 971A, inset Figure 1) than in a location closer to the outlet of
the Nile [Rohling et al., 2002]. The presence of major river channels in other parts of North Africa has been revealed by satellite-based radar, which is able to penetrate the overlying sands. In eastern Libya these channels are up to 5km wide. These waterways are likely to have formed during the Messinian (Late Miocene), however recent work comparing the Nd isotope geochemistry of freshwater snail shells with ODP site 971A suggests that the channels were reactivated during the last interglacial (130-120 kyr) [Osborne et al., 2008]. ODP site 971A is located in the Ionian Sea, some 600 km from the hypothesised outlet of the paleorivers. Sediment cores obtained from closer to the Libyan coast should better record the timing and magnitude of changes in river runoff, and will also be less influenced by any changes in Mediterranean surface water circulation or the addition of freshwater from more distal sources.