International Ocean Discovery Program
Expedition 401 Scientific Prospectus

Mediterranean–Atlantic Gateway Exchange¹

¹Flecker, R., Ducassou, E., and Williams, T., 2023. Expedition 401 Scientific Prospectus: Mediterranean–Atlantic Gateway Exchange. International Ocean Discovery Program. https://doi.org/10.14379/iodp.sp.401.2023

Abstract

Marine gateways play a critical role in the exchange of water, heat, salt, and nutrients between oceans and seas. The advection of dense waters helps drive global thermohaline circulation, and because the ocean is the largest of the rapidly exchanging CO₂ reservoirs, this advection also affects atmospheric carbon concentration. Changes in gateway geometry can therefore significantly alter both the pattern of global ocean circulation and associated heat transport and climate, as well as having a profound local impact.

Today, the volume of dense water supplied by Atlantic–Mediterranean exchange through the Gibraltar Strait is amongst the largest in the global ocean. For the past 5 My, this overflow has generated a saline plume at intermediate depths in the Atlantic that deposits distinctive contouritic sediments in the Gulf of Cadiz and contributes to the formation of North Atlantic Deep Water. This single gateway configuration only developed in the early Pliocene, however. During the Miocene, a wide, open seaway linking the Mediterranean and Atlantic evolved into two narrow corridors: one in northern Morocco, the other in southern Spain. Formation of these corridors permitted Mediterranean salinity to rise and a new, distinct, dense water mass to form and overspill into the Atlantic for the first time. Further restriction and closure of these connections resulted in extreme salinity fluctuations in the Mediterranean, leading to the formation of the Messinian Salinity Crisis salt giant.

Investigating Miocene Mediterranean–Atlantic Gateway Exchange (IMMAGE) is an amphibious drilling proposal designed to recover a complete record of Atlantic–Mediterranean exchange from its Late Miocene inception to its current configuration. This will be achieved by targeting Miocene offshore sediments on either side of the Gibraltar Strait during International Ocean Discovery Program (IODP) Expedition 401 and recovering Miocene core from the two precursor connections now exposed on land with future International Continental Scientific Drilling Program (ICDP) campaigns. The scientific aims of IMMAGE are to constrain quantitatively the consequences for ocean circulation and global climate of the inception of Atlantic–Mediterranean exchange, to explore the mechanisms for high-amplitude environmental change in marginal marine systems, and to test physical oceanographic hypotheses for extreme high-density overflow dynamics that do not exist in the world today on this scale.

Plain language summary

Today, Mediterranean–Atlantic seawater exchange takes place exclusively through the Gibraltar Strait. Around 8 million years ago, however, there were another two gateways: one in northern Morocco and the other through southern Spain. Both connections have subsequently closed and been tectonically uplifted and preserved on land. This process contributed to a major episode of global cooling in at least two ways:

1. Initial restriction of seawater exchange through these marine corridors caused the saltiness of the Mediterranean to increase and generated a dense water body that flowed out into the Atlantic, changing the pattern of global ocean circulation and drawing CO₂ dissolved in surface waters down into deeper parts of the ocean.
2. Extreme restriction of the pre-Gibraltar Strait connections raised salinity in the Mediterranean substantially, leading to the precipitation of more than 1 km of salt on the Mediterranean Sea floor. This phenomenon, known as a salt giant, occurs episodically in Earth’s history but no salt giant is forming today. The extraction of large volumes of salts from seawater changes ocean chemistry, which has knock-on consequences for the global carbon cycle and hence is a driver of climate change.

The chemical and physical properties of the sediments preserved in and on either side of the fossilized corridors are key to understanding and quantifying the global cooling caused by changes to Atlantic–Mediterranean exchange 5–8 million years ago. IODP Expedition 401 will recover records of exchange preserved offshore in the Atlantic and Mediterranean, and subsequent onshore drilling with ICDP will target the fossil gateway records that are now preserved on land in northern Morocco and southern Spain. The Investigate the Miocene Mediterranean–Atlantic Gateway Exchange (IMMAGE) project is the first Land-2-Sea drilling project.