The Climate Impact of the Messinian Salinity Crisis

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Murphy, Lisa Nicole
Kirk-Davidoff, Daniel B
This study uses an atmospheric general circulation model to examine the regional and global climate response to the Messinian Salinity Crisis (MSC) roughly 6 Ma. During this time, the tectonic collision of the African and European plates isolated the Mediterranean Sea (MedSea) from the Atlantic Ocean. MedSea level is estimated to have fallen between 1000-2000 m and desiccation may have lasted for 90 kyr. Our results show that the substantial MedSea depression generates planetary-scale atmospheric waves responsible for significant climate effects throughout the Northern Hemisphere. A notable deepening of the Aleutian Low and a significant equator-ward shift in the Atlantic jet stream are evident. Cyclical patterns in Messinian sediments suggest alternating wet and dry climate during the MSC. These cycles have been attributed to variations in the Earth's precession. This is the first study to detail how reduced MedSea level alters orbitally-driven climate change during the Late Miocene. Reduced MedSea level results in wetter conditions to the Northeast, in particular the Alps, consistent with proxy data. This signal is robust under all precession signals and is supported by evidence of greater weathering of the Alps during the MSC. Desiccation and lowered MedSea level results in greater precipitation over the Guinea Coast region of North Africa. Greater runoff from this region is supported by proxy evidence of higher monsoon intensity and enhanced total organic carbon accumulation throughout the Messinian. We couple our model to an online aerosol model to examine the response of dust to varying orbital parameters and to MedSea desiccation. Modeling dust source and transport changes in response to decreased dustiness during precession minimum shows that warmer tropical North Atlantic SSTs, attributed to increased insolation in the absence of dust, enhances evaporation and favors more precipitation over the western tropical North Atlantic. This stresses the importance of allowing dust to respond to climate change and including prognostic dust in paleo-simulations that examine changes in the West African monsoon. Enhanced dust loading over the tropical North Atlantic Ocean occurs when the Mediterranean is desiccated. This reduces the net radiative flux at the surface, which cools SSTs north of the Equator and shifts the ITCZ towards the Southern Hemisphere, consistent with theories that link African dust with extended Sahel droughts. Greater ocean productivity results from nutrient rich iron-laden dust waters, which is consistent with increased benthic foraminiferal accumulation rates off the African coast between 5.8 Ma and 5.25 Ma. The dustier Northern Hemisphere inhibits convective precipitation in the tropical North Atlantic and large-scale precipitation over Eastern Europe and into Central Asia, in agreement with proxy evidence of greater aridity in these regions between 6.2 and 5 Ma. Our results show that a desiccated Mediterranean has a significant impact on Northern Hemisphere sea-ice formation during precession maximum, which agrees with &delta<super>18</super>O proxies. Sea ice growth spreads southward, especially in the Labrador and Bering Seas. Interestingly, proxy data studies show discontinuous sea-ice in the Labrador Sea and south of Greenland, as well as concurrent ice-rafting in both the northwest Pacific and Gulf of Alaska sites in the late Miocene, a few million years prior to Northern Hemisphere glaciation.