Laminated diatomaceous sediments of the Red Sea, their composition and significance as recorders of abrupt changes in productivity and circulation during the Late Quaternary

Abstract

Laminated sediments from the Shaban Deep, a brine-filled basin in the northern Red Sea, were investigated with special emphasis on the major biogenic components (especially diatoms) within the sediments. First, diatom assemblages from plankton tows distributed in the northern Red Sea were compared with surface sediment samples from brine and non-brine locations. Fragile forms that dominate the plankton assemblage are subject to strong dissolution leaving the sedimentary record enriched in heavily silicified species. Detailed backscattered electron imagery was used in order to determine laminae composition and genesis with emphasis on the diatomaceous component between the Last Glacial Maximum (~22 ka) and the Late Holocene (last 2000 years). Sediment fabric includes six types: a) a laminated structure with alternating light (mainly coccoliths) and dark (diatom frustules) layers; b) a pocket-like structure attributed to the sinking of particles within fecal pellets and aggregates; c) a matrix of tightly packed diatoms; d) homogenous intervals (turbidity deposition); e) silt accumulations which origin may lie in agglutinated foraminifers; and f) pyrite layers with pyrite formation possibly initiated at the seawater-brine interface. Sedimentation models were produced that take into account the importance of different diatom assemblages as tracers of changes in stratification/mixing in the northern Red Sea. The models include: 1) An annual sedimentation cycle for the last 15 kyr with coccolithophorids reflecting the summer season while diatoms represent fall-winter conditions. 2) A LGM-Deglaciation model that concentrates on the shift from mixed water column conditions of the LGM to stratified conditions of the Deglaciation. No annual cycle could be established for this time frame due to high carbonate dissolution and thus the lack of the summer signal in the sediments.