Dust transport and synoptic conditions over the Sahara-Arabia deserts during the MIS6/5 and 2/1 transitions from grain-size, chemical and isotopic properties of Red Sea cores

摘要

The Red Sea, situated between the Sahara and Arabia deserts, is a natural trap for fine-grained particles transported by winds and floods from weathered materials in these deserts, derived from the Proterozoic crustal granitic and basaltic terrains and Phanerozoic carbonates. The accumulated fine particles provide valuable information on the climate conditions in the source regions, and modes of atmospheric circulation over the Red Sea region. Here, we report on the grain-size, mineralogy, chemical composition, and Nd and Sr isotope ratios of fine-grained insoluble residues (IR) from acid leaching, that were recovered from two deep-sea cores KL23 and KL11 drilled in the northern and central Red Sea, respectively. We focus on the MIS6/5 and MIS2/1 transitions. KL23 samples are closer to the "Sahara granitoids" field and display significant variations during the MIS6/5 transition, between εNd of -7 to -2 and ~(87)Sr/~(86)Sr of 0.7096 to 0.7114. Smaller variations occurred during the MIS2/1 transition, between εNd of -5 to -6 and ~(87)Sr/~(86)Sr of 0.7104 to 0.7118. The KL11 values are closer to the "Blue Nile" field, which comprises mixtures of granitoid and basalt derived particles. During the MIS6/5 transition their εNd varies from -4 to -1 and in ~(87)Sr/~(86)Sr from 0.7059 to 0.7091. During periods equivalent to sapropels S5 and S1, both cores appear to converge to similar isotope ratios of εNd~-2 and ~(87)Sr/~(86)Sr ~ 0.7095, indicating derivation from the granitoid terrains of the Arabian-Nubian Shield occurring adjacent to the Red Sea and possibly distributed by runoff.In terms of synoptic conditions, the data indicate that during glacials, dust was blown to the northern Red Sea from the northern Sahara desert in association with the Mediterranean winter cyclones, and to the central Red Sea from the Ethiopian plateau by southern winds that are associated with monsoonal circulation. During interglacials, especially sapropel intervals, monsoonal rains caused erosion and flooding at the Arabian-Nubian Shield margins of the Red Sea. During the Heinrich events associated with the studied intervals (H11 and H1), dust from mixed basaltic-granitic terrains was blown from all of the regional sources in the Red Sea region, reflecting severe regional aridity and transport of dust by regional winds, when monsoonal activity and Mediterranean cyclone activity were weak.