Sorbas’ basin secrets unveiled: First record of Sea Surface Temperature and Sea Surface Salinity in the Western Mediterranean prior to the onset of the Messinian Salinity Crisis

Between 5.97 and 5.33 Ma, the Mediterranean area was fundamentally impacted by the Messinian Salinity Crisis (MSC), a pivotal event that led to the transformation of the Mediterranean Sea into an extensive evaporitic basin, caused by rapidly changing environmental conditions. The MSC was caused by a combination of tectonic and climatic factors, resulting in severe connectivity restriction between the Mediterranean-Paratethys system and the Atlantic Ocean. The Sorbas Basin, thanks to its proximity to the Atlantic gateway and astronomically dated sedimentary succession, represents a key element in understanding the nature of palaeoceanographic transformations affecting the Western Mediterranean, preceding and announcing the MSC. Here, we present the first sea surface temperature (SST) and sea surface salinity (SSS) estimates recorded in Sorbas (i.e., Western Mediterranean) for the time interval between 7.3 to 6.0 Ma. The studied section includes the lower Abad Member (cyclic alternations of homogenous marls and indurated layers) and the upper Abad Member (cyclic alternations of sapropels, diatomites and marls). SSTs were estimated using TEX₈₆ and U^K´₃₇ biomarker-based proxies, with cross validation at distinct levels. To further constrain the SSS changes, we combined the TEX₈₆ and U^K´₃₇ based SST estimates with δ¹⁸O values measured at the same stratigraphic levels on the planktonic foraminifera Orbulina universa. The temperature estimates vary between 17 and 27 °C, with a pronounced cold (17 °C) peak at 7.1 Ma, following the restriction of the Betic and Rifian corridors. This cooling is followed by a generally warmer period lasting until 6.27 Ma, when a colder trend emerges and lasts until 6.18 Ma. A marked and sharp cooling from 27 to 18 °C is observed at 6 Ma, preceding the onset of the MSC. The SST-δ¹⁸O- calculated salinity ranges between 34 and 44 for most of the levels. However, several levels around 7.0, 6.74, 6.52 and 6.06 Ma, generated SSS values as low as 20, provoked by exceptionally low, yet not fully understood , δ¹⁸O_{O. universa} component in our SSS calculation. The low values might be associated with a significant local influx of fresh water, considering the basin's restricted nature. When comparing our results to coeval records existing for the Eastern and Central Mediterranean (Agios Myron, Kalamaki and Monte dei Corvi), we notice a correlation of warmer and colder peaks across the Mediterranean, albeit with minor leads and lags. Importantly, the Sorbas SST values are well within the range of SSTs in the Eastern and Central Mediterranean. The SSS values of Sorbas are also within the range reported in the Eastern Mediterranean with the exception of those levels presumably affected by fresh water input. In the absence of a full explanation for the associated low-δ¹⁸O_{O. universa} values, we observe the dominance of the C_37:4 alkenone component, exclusively associated with fresh to brackish water environments, at some of these levels, strongly suggesting the occurrence of repeated fresh water influx into the basin.