A drier Mediterranean region at the Miocene to Pliocene transition

Between 5.97-5.33 Ma, kilometre-thick evaporite units were deposited in the Mediterranean Basin during the so-called Messinian Salinity Crisis (MSC). The MSC was marked by a strongly negative hydrological budget, with a net evaporative loss of water exceeding precipitation and riverine runoff. Knowledge about the contemporary changes in continental and marine circum-Mediterranean environments are still limited to qualitative descriptions in terms of temperature and humidity. Here we reconstruct continental mean annual temperatures (MAT) using branched glycerol dialkyl glycerol tetraether (brGDGT) biomarkers for the time period corresponding to MSC Stage 3 (5.55-5.33 Ma) and compare them with values obtained from Δ₄₇ clumped isotope geochemistry measured on paleosol carbonate nodules found at few locations in the Mediterranean basin. Additionally, for the same time interval, we estimate sea surface temperatures (SSTs) of the Mediterranean Sea using the isoprenoidal GDGT-based TEX₈₆ proxy. The excellently preserved organic biomarkers were extracted from outcrops onshore (Malaga, Sicily, Cyprus) and offshore (DSDP core holes 124 and 134 from the Balearic abyssal plane, hole 374 from the Ionian Basin and hole 376 drilled west of Cyprus) covering a rather vast portion of the Mediterranean Basin.

Calculated MATs for the 5.55 to 5.33 Ma time interval show values around 16 to 19 ºC for the Malaga, Sicily and Cyprus outcrops. The MAT values calculated for DSDP Leg 13 holes 124, 134 and Leg 42A holes 374 and 376 are lower, around 13 to 16 ºC. Comparing the brGDGT-MAT values with Δ₄₇-MAT values from carbonate nodules, shows high congruence between both approaches. For the northern Mediterranean Δ₄₇-MAT is 20 ºC and brGDGT-MAT is 19 ºC. For Cyprus Δ₄₇-MAT is 21 ºC and brGDGT-MAT is 18 ºC. Given the very different type of the used paleoproxies, the similarity of the obtained MAT values provides a strong indication of their (cross)validity in sampled sections. Additionally, the measured δ¹⁸O values of ~33‰ (VSMOW) for the carbonate nodules used for the Δ₄₇-MAT indicate highly evaporative conditions for the two onland sites where these were collected (Northern Apennines and Cyprus).

For samples where the branched and isoprenoid tetraether index was lower than 0.4 we could calculate TEX₈₆ derived SSTs averaging around 27 ºC for all sampled locations, reaching the higher end of the values obtained in the Mediterranean region for the pre-MSC through U^k₃₇ and TEX₈₆ derived SSTs of Tzanova et al. (2015), Vasiliev et al. (2019) and Kontakiotis et al. 2022. Independent of pitfalls that may arise in using molecular biomarkers as temperature proxies, both SST estimates independently hint towards much warmer Mediterranean Sea water during the latest phase, Stage 3 of the MSC. These elevated temperatures coincide with higher δ²H values measured on alkenones and long chain n-alkanes (both records indicating more arid and/or warmer conditions than today between 5.55 and 5.33 Ma). We conclude that between 5.55 to 5.33 Ma the temperatures in the Mediterranean region was similar to present-day conditions, yet the region has suffered from excess evaporation as indicated by combined high δ¹⁸O values from carbonate nodules δ²H values from biomarkers.