Last interglacial pressure pattern over the Mediterranean from aeolian sediments: A case study from Apulia

Abstract

In this work, we reconstruct a change in the wind regime and sea level pressure pattern during marine isotope stage (MIS) 5.5.To obtain these results, we studied two elements of the landscape south of the city of Gallipoli (Apulia region, southern Italy), namely, the Carmine-Li Foggi palaeo-littoral ridge (CLlr) and a field of domal-longitudinal fossil dunes present in the Il Campo locality, both dating back to MIS 5.5, through the use of geological and geomorphological surveys together with geochronological analyses by Uranium series and amino acid racemization/epimerization. A detailed study was carried out on the aeolian units within CLlr (AU1 and AU2) and the fossil dunes of the Il Campo locality (AU3) to reconstruct the palaeo-directions of the winds and, possibly, the sea level pressure patterns. AU1 was the first aeolian unit to settle. The accumulation of AU1 at the northern and southern edges of CLlr and its internal structure indicate effective aeolian transport both towards the N and S. Based on the principle of actualism,the AU1 unit allows us to recognise a “first aeolian phase” characterised by the alternation of winds from the S and NW. It involves an autumn–winter pressure pattern characterised by an Atlantic footprint similar to the current pattern: the Atlantic depressions entering the Mediterranean caused, as occurs today over Apulia, the alternation of winds from the S and NW before and after the passage of the low-pressure minima, respectively. In spring–summer, the pressure fields were likely very similar to the current pressure fields, with the Azores high dominating the central–western Mediterranean and the low pressure dominating the eastern Mediterranean, with winds not able to significantly impact sand transport, just as occurs today; this summer regime left the prevailing imprint of the Atlantic winter regime and therefore NW–S bimodality.AU2 and AU3 settled later than AU1 in a “second aeolian phase” of MIS 5.5. AU2 is characterised by south-eastward-dipping foresets. AU3 consists of domal and linear dunes oriented in the NW–SE direction. We interpret AU2 and AU3 as coastal dunes originating in a prevailing NW wind regime.Based on the principle of actualism and on literature data, we conclude that AU2 and AU3 settled under a pressure pattern which in fall–winter was similar to the current pattern, that is, characterised by Atlantic cyclones transiting over the Mediterranean, and the consequent NW–S bimodality of the winds. In the spring–summer, instead, the pattern was characterised by a pressure gradient from west (high) to east (low) higher than the current pressure gradient; this gradient caused stronger NW winds compared to today over the Apulia, according to a similar (but increased) mechanism which today, on the Aegean Sea, causes the development of the Etesio-Meltemi summer winds.Such strong summer winds from the NW, which were effective for sand transport, coupled with the NW–S bimodality in fall–winter, caused the overall prevalence of NW winds during the “second aeolian phase”. This summer pressure gradient over the Mediterranean from west (high) to east (low) greater than today was caused by a summer strengthening of the east Mediterranean low-pressure branch, in turn caused by the northward shift of the intertropical convergence zone (ITCZ), which was widely documented during the sapropel S5 event and verified during MIS 5.5 after sea level had risen to the highstand.