339: Mediterranean Outflow

Figure 1: Expedition 339 sites that were drilled (yellow dots) in the Gulf of Cádiz and West Iberian margin. Note that the drilled sites are located around the margins of the continents, in shallower water depths. Deeper water is denoted by darker blue colors, whereas lighter blue to light green colors indicate shallower water depths. Land is denoted by the light to dark brown colors. Figure from Expedition 339 Preliminary Report

The Strait of Gibraltar is a significant gateway that currently connects the Atlantic Ocean to the Mediterranean Sea. From November 2011 to January 2012, the Integrated Ocean Drilling Program (IODP) Expedition 339 science team drilled a total of seven sites, consisting of five sites in the Gulf of Cadiz (GC) and two sites off the West Iberian margin (Table 1 & Figure 1). The major objective of this drilling expedition was to study what happened when the Strait of Gibraltar opened about 5 million years ago, causing warm and salty water to flow into the North Atlantic Ocean. Scientists chose to drill in this region because it is an important spot to study the movement of Mediterranean Outflow Water (MOW) through the Strait of Gibraltar and how it affects the world’s ocean currents and weather patterns. It is also an area of interest for understanding the effects of tectonic activity on the evolution of the Strait of Gibraltar and how sediments collect around the continents. Climate plays a huge role in influencing the changes in the MOW and ocean currents over time. From about 6 million years ago, when the connections between the Atlantic Ocean and the Mediterranean Sea in Spain and Morocco closed off, to 5.3 million years ago when the Strait of Gibraltar opened back up, the way the Earth’s tectonic plates moved had an even bigger impact on how sediment built up and how the ocean currents changed.

Some of the Expedition 339 crew members had the opportunity to watch the International Space Station (ISS) pass overhead on the night of Dec 4th, 2011, even though it appeared as a tiny moving dot in the sky. As reported by Helder Pereira, while drilling at Site U1386, the Joides Resolution (JR) research vessel was spotted by the ISS as it was orbiting the Earth! Interestingly, while they were watching, the ISS was also observing them and took a beautiful picture of the Iberian Peninsula. Can you spot the JR inside the orange circle located a few miles southeast of Faro (Algarve, Portugal) in the lower bottom right of Figure 3?

Figure 2: Smaller inset figure of the Earth with an arrow that is pointing to the general location where Expedition 339 drilled. Larger location sketch with the main water-masses, deep ocean currents, and surface currents along the continental margin (modified from Hernández-Molina et al., 2006). Figure from Expedition 339 Preliminary Report

Expedition 339 had five broad scientific objectives, which are summarized below:

  1. Understand the opening of the Strait of Gibraltar gateway and when the Mediterranean Outflow Water (MOW) began.  Many scientists have different hypotheses on how the Strait of Gibraltar opened around 5 million years ago. Some believe it was due to tectonic changes, while others think it was caused by erosion of the land. The reopening ended the isolation of the Mediterranean Sea and the global effects of the Messinian Salinity Crisis (this was a time in which the Mediterranean Sea was isolated from the Atlantic Ocean, and drying up of the sea left behind major salt deposits and highly saline waters). It took some time for the deep MOW to flow out, and the exact timing of such outflow is relatively unknown. The first objective of Expedition 339 was to drill through sediment layers on the seafloor to determine the age of those sediments in the Gulf of Cádiz. Scientists also wanted to examine changes in contourite deposits, or sediment deposits that happen offshore, and MOW bottom water changes (temperature, salinity)  during the geologic period from the end of the Miocene Period (~5 million years ago) to the early to middle Pliocene Period (~3 million years ago).
  2. Determine the ancient circulation pattern of MOW and how such different circulation changes might affect global climate: Today, warm and salty (more than 300,000 tons of excess salt) water flows into the North Atlantic from the Mediterranean Sea through the Strait of Gibraltar gateway every second! A density increase in North Atlantic Deep Water could affect the ocean’s thermohaline circulation (circulation of deep waters driven by density differences in those waters) and could have implications for climate change. To better understand these processes, researchers are actively studying the millennial to long-term changes of Mediterranean outflow and its effects on thermohaline circulation. The second objective was to date the unconformities (where there is missing time in the sedimentary record) and discontinuities identified on seismic records. Such identification  can help to assess the sedimentary record’s  link to past circulation variation and events and understand the forces driving bottom water circulation changes over different timescales.
  3. Establish a marine reference section of Pleistocene (2.58–0.017 million years ago) climate (rapid climate change): This objective intersects with the principal objective of Site U1385 [APL-763] which was to drill into the Iberian margin, offshore Portugal to recover a sediment record that could be studied to infer  Pleistocene climate changes. In turn, this sediment record can be correlated with polar ice cores and terrestrial sediments and other climate records  to better understand climate change during the past ~2.58 million years.
  4. Identify external controls on the sediments from the Gulf of Cádiz contourites and West Iberian margin: The fourth objective of this expedition was to study how sea level change and the size of the Strait of Gibraltar impacted sediment deposits in the Gulf of Cádiz and the West Iberian margin. This will help to distinguish between regional changes caused by climate and sea level, and to develop a better understanding of the sediments and under what conditions they were deposited  in the area. The researchers plan to drill and analyze sediment cores, date them, and correlate them to create a pattern of the sediments  and the hiatuses, or times of no sediment deposition, between them. They also want to evaluate how the Mediterranean Outflow Water flux and how global sea level changes affected the Gibraltar sill, as well as  the circulation of the North Atlantic. By analyzing the composition of sediments and how fast they accumulated through time, scientists hope to better understand the sediment supply and flux for the sediment deposits in the gulf.
  5. Investigate the tectonic activity in the region, and how it controlled the deposition of sediments in the region: This objective relates to using the sediments that were drilled to infer the age of tectonic events and the changes that resulted from this events in the Gulf of Cadiz and Iberian margin. The scientists also wanted to investigate diapirs, or the movement of low-density rocks such as salts, into older, more dense rocks. 
Figure 3: The Iberian Peninsula at night. This photo was taken on Dec 4th, 2011 at 00:13:44 GMT. Spacecraft nadir point: 36.6° N, 13.9° W; Photo center point: 40.5° N, 5.0° W; Spacecraft Altitude: 206 nautical miles (382 km). Figure from The JR seen from space!

The expedition successfully met all five of the scientific objectives, and recovered about 5447 meters (3.38 miles!) of cores. The region was drilled for the first time for scientific purposes and the results confirmed some pre-expedition hypotheses and also provided new information and ideas not initially anticipated. For instance, the scientists discovered a larger-than-anticipated petroleum system with huge hydrocarbon potential, presenting a new and significant opportunity to explore oil and gas reserves in the region!

Figure 4: General circulation pattern of the Mediterranean Outflow Water (MOW) pathway in the North Atlantic (modified from Iorga and Lozier, 1999). Red circles filled with yellow indicate the relative location of the sites. AB = Agadir Basin, BAP = Biscay Abyssal Plain, BB = Bay of Biscay, EP = Ex- tremadura Promontory, GaB = Galicia Bank, GoB = Gorringe Bank, HAP = Horseshoe Abyssal Plain, MAP = Madeira Abyssal Plain, MI = Madeira Island, PAP = Porcupine Abyssal Plain, RC = Rockall Channel, SAP = Seine Abyssal Plain, St.V = Cape São Vicente, TAP = Tagus Abyssal Plain. Figure from Expedition 339 Preliminary Report

Results from Expedition 339 opened the door for even more post-expedition research. Some studies focused on the reconstruction of the Mediterranean-Atlantic water exchange after the opening of the Gibraltar Strait 5.3 million years ago to understand the behavior of the Mediterranean Outflow Water during this period (Bahr et al., 2014) and to study the past ocean conditions (García-Gallardo et al., 2017). Studies published using the recovered sediments observed that MOW varied in strength and location during different historical periods, and that MOW resembles water from the Levantine Basin in the Eastern Mediterranean (Kaboth et al., 2016). A new detailed record from 416,000 years ago reveals that changes in MOW strength and depth during the Late Pleistocene age (~0.12–0.017 million years ago) were caused by the climate; the main factor controlling these changes being the rainfall pattern around the Mediterranean region (Nichols et al., 2020). Also, pollen and biomarker data from Site U1385 (Figure 1) was used to study the unique climate during Marine Isotope Stage (MIS) 13, a period around 533,000 to 478,000 years ago, when the climate was cool and humid, resulting in forest expansion in the Iberian Peninsula (Oliveira et al., 2022).


Bahr, A., Jiménez-Espejo, F.J., Kolasinac, N., Grunert, P., Hernández-Molina, F.J., Röhl, U., Voelker, A.H.L., Escutia, C., Stow, D.A.V., Hodell, D., and Alvarez-Zarikian, C.A., 2014. Deciphering bottom current velocity and paleoclimate signals from contourite deposits in the Gulf of Cádiz during the last 140 kyr: an inorganic geochemical approach. Geochemistry, Geophysics, Geosystems, 15(8):3145–3160. https://doi.org/10.1002/2014GC005356

García-Gallardo, Á., Grunert, P., Van der Schee, M., Sierro, F.J., Jiménez-Espejo, F.J., Alvarez Zarikian, C.A., and Piller, W.E., 2017. Benthic foraminifera-based reconstruction of the first Mediterranean-Atlantic exchange in the early Pliocene Gulf of Cadiz. Palaeogeography, Palaeoclimatology, Palaeoecology, 472:93–107. https://doi.org/10.1016/j.palaeo.2017.02.009

Kaboth, S., Bahr, A., Reichart, G.-J., Jacobs, B., and Lourens, L.J., 2016. New insights into upper MOW variability over the last 150 kyr from IODP 339 Site U1386 in the Gulf of Cadiz. Marine Geology, 377:136–145. https://doi.org/10.1016/j.margeo.2015.08.014

Nichols, M.D., Xuan, C., Crowhurst, S., Hodell, D.A., Richter, C., Acton, G.D., and Wilson, P.A., 2020. Climate-induced variability in Mediterranean Outflow to the North Atlantic Ocean during the late Pleistocene. Paleoceanography and Paleoclimatology, 35(9):e2020PA003947. https://doi.org/10.1029/2020PA003947

Oliveira, D., Desprat, S., Yin, Q., Rodrigues, T., Naughton, F., Trigo, R.M., Su, Q., Grimalt, J.O., Alonso-Garcia, M., Voelker, A.H.L., Abrantes, F., and Sánchez Goñi, M.F., 2020. Combination of insolation and ice-sheet forcing drive enhanced humidity in northern subtropical regions during MIS 13. Quaternary Science Reviews, 247:106573. https://doi.org/10.1016/j.quascirev.2020.106573

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