Permanent closure of the Tethyan Seaway in the northwestern Iranian Plateau driven by cyclic sea-level fluctuations in the late Middle Miocene

Abstract

The opening and closure of marine gateways have been suggested to substantially affect ocean current circulations and global climatic changes. In the Miocene, the closure of the Tethyan Seaway, which connected the proto-Mediterranean Sea with the Indian and Atlantic Oceans, blocked thermohaline exchanges between them and possibly caused a major climate change in the Middle Miocene Climatic Transition. However, the timing, process, and forcing mechanism of the closure of this seaway are controversial. To address this question, we focus on the chronology and sedimentary facies of Miocene strata in the Zagros foreland basin in the Western Iranian Plateau. Based on magnetostratigraphy and biostratigraphy, we establish a chronology of 17 to 10 Ma for the studied Miocene strata and constrain the timing of the permanent closure of the northwestern segment of the Tethyan Seaway to 12.8 Ma. By comparison with global sea-level fluctuations, we suggest that middle Miocene marine transgressions and regressions in the studied region were mainly driven by global climatic change through its control on the growth and decay of ice sheets and thus sea level changes. Our detailed depositional sequence analysis exhibits 100-kyr Earth's eccentricity cycles of marine transgressions and regressions between 15.0 and 12.8 Ma implying they were controlled by astronomical factors. Moreover, the Tethyan Seaway experienced stepwise evolution changing from partially opened seaway, restricted marine connection, intermittent connections, to permanent closure in its northwestern segment during the early and middle Miocene.