Carbonate preservation and climatic changes in the central Red Sea during the last 380 kyr as recorded by pteropods

Abstract

Numerical abundances of pteropods and planktic foraminifera, and the mode of pteropod preservation, were determined in core KL 11 taken from the central part of the Red Sea. The abundance of pteropods (shells/g dry sediment) was compared to that of planktic foraminifera (% Pt/Pf + Pt) — a technique that permits detection of changes in carbonate preservation for the last 380 kyr. The numerical abundance of pteropods is influenced by the properties of the water column, and preservation is influenced by the bottom water. During the last ∼ 200 kyr (except during isotope stage 5.5) carbonates are in general well preserved. During this period the abundance pattern of the pteropods and planktic foraminifera is very similar and follows the climatic signal of the Red Sea with high numbers during the interglacial stages, changing to very low numbers during glacial maximum conditions. The similar abundance trends of the two groups, and between them and the δ 18O curve, indicates abundance is strongly linked to salinity. From isotope stage 6 to the bottom of the core the pteropods occur in low numbers, unlike the planktic foraminifera, which continue to display the high-amplitude glacial-interglacial cyclicity. The deviations, mainly during interglacial stages, between the abundance pattern of the two groups and the low % Pt/Pf + Pt values, indicate a significant change in carbonate preservation. Distinctive carbonate dissolution intervals are recognized in the Red Sea, correlating to large scale deep water dissolution events of the ‘mid-Brunhes dissolution cycle’ in the Indian Ocean. The anti-estuarine circulation pattern of the Red Sea prevents a direct connection between the deep water masses of the two oceans and rules out the likelihood that changes in the deep water circulation caused these carbonate dissolution events. The numerical variations between nonmigratory epipelagic and migratory mesopelagic pteropods were used to evaluate changes in the structure of the water column. Abundance maxima of mesopelagic pteropods, as in the recent Red Sea, indicate an aerated water column with ≥0.5 ml O 2/l oxygen concentrations at the minimum zone. Mesopelagic abundance maxima coincide commonly with negative monsoonal index values indicating a more aerated water column connected to increasing aridity in the Red Sea region. Abundance maxima of epipelagic pteropods indicate a strongly stratified water column, at times causing severe depletion in oxygen at intermediate water depths. Epipelagic peak events coincide often with positive monsoon index values implying an overall milder and more humid climate in the Red Sea, probably associated with enhanced precessional-controlled southwest monsoon activity.