Palaeoceanography of the interglacial eastern Mediterranean Sea

Abstract

The sensitivity of the present interglacial climate to the ongoing anthropogenic-driven increase of atmospheric greenhouse gas poses a fundamental concern to modern society. The Mediterranean region is responding with a distinct change towards drier and warmer conditions, which affects also the hydrography of the entire basin and is likely to continue in the near future. Notably, changes occurring in the atmosphere/sea surface are rapidly transferred to the deep sea via an effective thermohaline circulation, led by processes acting in the eastern Mediterranean. This thesis rises from the need to advance our understanding of the sensitivity of the eastern Mediterranean region to perturbations in the atmosphere/ocean system during interglacial periods. In addition, it exploits the sensitivity of the basin’s hydrography and climate to both high- to mid-latitude and tropical/subtropical systems in order to provide insights on large-scale teleconnections during periods of climate change. Investigations presented here specifically focus on the last two intervals of peak interglacial conditions, at times of substantial hydrological perturbations when extensive hydrographic changes led to the widespread deposition of organic-rich sediments (sapropels). Results for the last interglacial period indicate that ~124 ka BP, a large monsoon-fuelled freshwater injection along the North African margin substantially lowered the sea surface salinity (from 39 to 33 practical salinity units) in the Aegean Sea, a key site of deep-water overturning for the eastern Mediterranean. This change, in turn, triggered a rapid (~40 years) collapse of the Aegean deep-intermediate-water ventilation, promoting euxinic conditions – hostile to aerobic life – which expanded toward the photic layer within ~650 years, and subsequently propagated to the open eastern Mediterranean. Holocene Aegean records also reveal the occurrence of a monsoon-related freshwater disturbance – albeit far less intense – in the eastern Mediterranean during the early to middle Holocene interval of insolation/monsoon maximum (10 to 5.5 ka BP), which led to a less pronounced hydrographic response, with the “sole” stagnation of the deep-waters. Besides these dramatic changes in the basin’s hydrography that are mainly tied to monsoon-forced perturbations in the eastern Mediterranean freshwater budget, the regional climate in the past – as well as today – appears intimately linked to variations in the high latitude atmospheric forcing, which dominates the winter regime. Accordingly, eastern Mediterranean records are here exploited as invaluable archives of past variability in the meridional extent of high latitude atmospheric circulation, i.e., the atmospheric polar vortex. Specifically, early Holocene records from the Aegean Sea bear witnesses of a particularly sharp Aegean cooling event between 8.3 and 8.15 ka BP, embedded within an underlying cooling episode of 8.6-8.0 ka BP. In both timing and duration, this sharp event agrees with the so-called “8.2 event” reported from several records from the wider North Atlantic region, which has been related to a brief weakening of the Atlantic meridional overturning circulation. Taken together, these results point to the exceptional sensitivity of the eastern Mediterranean Sea interglacial climate to climate forcing, with rapid responses to climate forcing, and propagation of the signals through the water column down to the deep sea and, in turn, affecting the entire ecosystem.