[M-GI28] Drilling Earth Science

[MGI28-08] Changes in the Atlantic-Mediterranean Marine System During the Messinian Salinity Crisis Based on Os isotopic ratios

*Takeru Kaji¹, Erika Tanaka², Yusuke Kuwahara^1,3, Rachel Flecker⁴, Emmanuelle Ducassou⁵, Trevor Williams⁶, IODP Expedition 401 Scientists⁷, Yasuhiro Kato^1,3 (1.The University of Tokyo, 2.Kochi University, 3.Chiba Institute of Technology, 4.University of Bristol, 5.Université de Bordeaux, 6.Texas A&M University, 7.IODP)

Keywords:Messinian Salinity Crisis, Os isotope ratio, Mediterranean Sea, Ocean circulation

The Messinian Salinity Crisis (MSC, 5.97–5.33 Ma) was an event in which the salinity of the Mediterranean Sea rapidly increased, leading to the formation of massive salt deposits (> 1.2 × 10^6 km³) [1]. One of the factors of this dramatic event was the gradual closure of the late Miocene Atlantic-Mediterranean connections at the south Spain and North Morocco. The restriction of seawater exchange between the Mediterranean Sea and the Atlantic caused excessive evaporation of Mediterranean seawater [2]. Although the paleoceanographic conditions around the gateway during the MSC have not fully been understood yet, it is known that, since the Pliocene, a highly saline water mass flowing from the Mediterranean into the North Atlantic called the Mediterranean Outflow Water (MOW), plays a role in the formation of Atlantic Mediterranean Water and influences the maintenance of North Atlantic Deep Water [3]. Therefore, elucidating the oceanographic changes around the gateway before and after the MSC could not only clarify past hydrological changes in the Mediterranean but also provide insights into the impact of the Mediterranean Sea on the Atlantic thermohaline circulation and global-scale geochemical cycles.
To provide new insights into the seawater exchange between the Atlantic Ocean and the Mediterranean Sea during the MSC, this study applies osmium (Os) isotopic analysis to ocean drilling core samples (U1610, U1611) which were collected at both sides of the Strait of Gibraltar during International Ocean Discovery Program (IODP) Expedition 401. The seawater Os isotope ratio (187Os/188Os) reflects the balance of inputs via continental weathering and volcanic activity, which is widely used as an indicator for Earth’s surface geochemical cycles. In particular, the residence time of Os (~10^4 years) is longer than oceanic circulation timescales (~10^3 years) but shorter than the timescale of the MSC (~10^5 years). Therefore, this method is highly effective in evaluating Mediterranean-Atlantic seawater exchange during the MSC [4]. The sediment core samples analyzed in this study are composed of carbonate-rich or detrital materials from the late Miocene to the earliest Pliocene [5]. Here we will report the results of Os isotope ratio analysis for the study sites and compare them with seawater Os isotope ratios reconstructed from the central and eastern Mediterranean Sea [4], and the Pacific Ocean [6–8]. Based on our results, we also discuss the timing of restriction and reconnection between the Atlantic and Mediterranean around the MSC.

[1] Roveli et al. (2014) Marine Geology, 352, 25-58. [2] Meijer & Krijgsman (2005) Earth and Planetary Science Letters, 240(2), 510-520. [3] Rogerson et al. (2012) Reviews of Geophysics, 50(2). [4] Kuroda et al. (2016) Paleoceanography, 31(1), 148-166. [5] Flecker et al. (2024) Expedition 401 Preliminary Report: Mediterranean–Atlantic Gateway Exchange. International Ocean Discovery Program. [6] Holm-Nielsen et al. (2009) Bioresource technology, 100(22), 5478-5484. [7] Ravizza (1993) Earth and Planetary Science Letters, 118(1-4), 335-348. [8] Pegram & Turekian (1999) Geochimica et Cosmochimica Acta, 63(23-24), 4053-4058.