G. Trommer

Sensitivity of Red Sea circulation to monsoonal variability during the Holocene: An integrated data and modeling study

Received 20 October 2009; revised 6 March 2010; accepted 30 June 2010; published 4 November 2010. [1] We used an oceanic general circulation model to evaluate the sensitivity of the hydrography and circulation of the Red Sea in response to reduced sea level and modified atmospheric conditions during the Holocene. With Holocene sea level close to the modern level, the Red Sea was sensitive to changes in atmospheric conditions, and it only shows a relatively mild response to sea level change. Changes in the monsoon system influence the exchange flow through the Strait of Bab el Mandab, the meridional overturning circulation of the Red Sea, and its hydrography. Forced by humid conditions the (modeled) Red Sea temperature increased by ∼1.5°C, while when arid conditions were imposed, the temperature decreased by ∼2.5°C. Similar heating and cooling events during the early and late Holocene are seen in a sea surface temperature record from the northern Red Sea (derived from the temperature sensitive TEX86 molecular biomarker), which suggests that humid conditions prevailed during the early Holocene and more arid conditions prevailed during the late Holocene. The gradual decline in Red Sea temperature between these two time periods suggests a gradual decline in the summer monsoon strength. This monsoon trend and the resulting changes in the Red Sea circulation are supported by the distribution of crenarchaea fossil lipids in Red Sea sediments from this period. Monsoon‐ driven changes in the exchange flow through the Strait of Bab el Mandab affected the crenarchaea population structure, and therefore, their molecular fossil distribution in the sediments of the Red Sea potentially provides an index for the summer monsoon strength during the Holocene.

Millennial‐scale variability in Red Sea circulation in response to Holocene insolation forcing

In order to assess how insolation-driven climate change superimposed on sea level rise and millennial events influenced the Red Sea during the Holocene, we present new paleoceanographic records from two sediment cores to develop a comprehensive reconstruction of Holocene circulation dynamics in the basin. We show that the recovery of the planktonic foraminiferal fauna after the Younger Dryas was completed earlier in the northern than in the central Red Sea, implying significant changes in the hydrological balance of the northern Red Sea region during the deglaciation. In the early part of the Holocene, the environment of the Red Sea closely followed the development of the Indian summer monsoon and was dominated by a circulation mode similar to the current summer circulation, with low productivity throughout the central and northern Red Sea. The climatic signal during the late Holocene is dominated by a faunal transient event centered around 2.4 ka BP. Its timing corresponds to that of North Atlantic Bond event 2 and to a widespread regionally recorded dry period. This faunal transient is characterized by a more productive foraminiferal fauna and can be explained by an intensification of the winter circulation mode and high evaporation. The modern distribution pattern of planktonic foraminifera, reflecting the prevailing circulation system, was established after 1.7 ka BP.