Mediterranean heat injection to the North Atlantic delayed the intensification of Northern Hemisphere glaciations

The intensification of Northern Hemisphere glaciations at the end of the Pliocene epoch marks one of the most substantial climatic shifts of the Cenozoic. Despite global cooling, sea surface temperatures in the high latitude North Atlantic Ocean rose between 2.9–2.7 million years ago. Here we present sedimentary geochemical proxy data from the Gulf of Cadiz to reconstruct the variability of Mediterranean Outflow Water, an important heat source to the North Atlantic. We find evidence for enhanced production of Mediterranean Outflow from the mid-Pliocene to the late Pliocene which we infer could have driven a sub-surface heat channel into the high-latitude North Atlantic. We then use Earth System Models to constrain the impact of enhanced Mediterranean Outflow production on the northward heat transport in the North Atlantic. In accord with the proxy data, the numerical model results support the formation of a sub-surface channel that pumped heat from the subtropics into the high latitude North Atlantic. We further suggest that this mechanism could have delayed ice sheet growth at the end of the Pliocene.

Latest Miocene restriction of the Mediterranean Outflow Water: a perspective from the Gulf of Cádiz

The Mediterranean-Atlantic water mass exchange provides the ideal setting for deciphering the role of gateway evolution in ocean circulation. However, the dynamics of Mediterranean Outflow Water (MOW) during the closure of the Late Miocene Mediterranean-Atlantic gateways are poorly understood. Here, we define the sedimentary evolution of Neogene basins from the Gulf of Cádiz to the West Iberian margin to investigate MOW circulation during the latest Miocene. Seismic interpretation highlights a middle to upper Messinian seismic unit of transparent facies, whose base predates the onset of the Messinian salinity crisis (MSC). Its facies and distribution imply a predominantly hemipelagic environment along the Atlantic margins, suggesting an absence or intermittence of MOW preceding evaporite precipitation in the Mediterranean, simultaneous to progressive gateway restriction. The removal of MOW from the Mediterranean-Atlantic water mass exchange reorganized the Atlantic water masses and is correlated to a severe weakening of the Atlantic Meridional Overturning Circulation (AMOC) and a period of further cooling in the North Atlantic during the latest Miocene.

Mediterranean Water Properties at the Eastern Limit of the North Atlantic Subtropical Gyre since 1981

A high-quality hydrographic CTD and Argo float data was used to study the property changes along the westward branch of the Mediterranean Outflow Water (MOW) in the northeast Atlantic between 1981 and 2018. In this period, the temperature and salinity are marked by periods of cooling/freshening and warming/salinification. Since 1981, the MOW properties at the core decreased by −0.015 ± 0.07 °C year−1 and −0.003 ± 0.002 year−1. The different phases of the North Atlantic Oscillation (NAO) influence the main propagation pathways of the MOW into the North Atlantic basin, thus affecting the trends determined within different NAO-phases. The temperature and salinity show a strong correlation with NAO, with NAO leading the properties by 8 and 7 years, respectively, indicating a delayed response of the ocean to different forcing conditions. A decrease in oxygen concentration (−0.426 ± 0.276 μmol kg−1 year−1) was calculated for the same period; however, no connection with the NAO was found.

Hotter and Weaker Mediterranean Outflow as a Response to Basin-Wide Alterations

Time series collected from 2004 to 2020 at an oceanographic station located at the westernmost sill of the Strait of Gibraltar to monitor the Mediterranean outflow into the North Atlantic have been used to give some insights on changes that have been taking place in the Mediterranean basin. Velocity data indicate that the exchange through the Strait is submaximal (that is, greater values of the exchanged flows are possible) with a mean value of −0.847 ± 0.129 Sv and a slight trend to decrease in magnitude (+0.017 ± 0.003 Sv decade−1). Submaximal exchange promotes footprints in the Mediterranean outflow with little or no-time delay with regards to changes occurring in the basin. An astonishing warming trend of 0.339 ± 0.008°C decade−1 in the deepest layer of the outflow from 2013 onwards stands out among these changes, a trend that is an order of magnitude greater than any other reported so far in the water masses of the Mediterranean Sea. Biogeochemical (pH) data display a negative trend indicating a gradual acidification of the outflow in the monitoring station. Data analysis suggests that these trends are compatible with a progressively larger participation of Levantine Intermediate Water (slightly warmer and characterized by a pH lower than that of Western Mediterranean Deep Water) in the outflow. Such interpretation is supported by climatic data analysis that indicate diminished buoyancy fluxes to the atmosphere during the seven last years of the analyzed series, which in turn would have reduced the rate of formation of Western Mediterranean Deep Water. The flow through the Strait has echoed this fact in a situation of submaximal exchange and, ultimately, reflects it in the shocking temperature trend recorded at the monitoring station.

Benthic foraminifera as tools to distinguish contourites and turbidites

<p>Contourites occur where along-slope bottom currents induce large accumulations of sediments in the deep sea (Faugères and Stow, 2008). Distinguishing among contourites and other depositional facies on continental slopes is fundamental for paleoenvironmental reconstructions like bottom current velocities. Nonetheless, reliable and easily applicable diagnostic criteria to properly differentiate between contourites and other coarse-grained and/or graded deep-water deposits such as turbidites are still sparse (e.g., de Castro et al., 2020). The differentiation and interpretation of these deposits is particularly complex in areas where downslope and along-slope sedimentary processes co-occur.</p><p>The SW Iberian Margin represents an ideal natural laboratory to study the complex interaction of downslope and along-slope processes. Persistent bottom current activity of Mediterranean Outflow Water (MOW) since the early Pliocene (García-Gallardo et al., 2017) resulted in the deposition of thick contourite drift bodies in the Gulf of Cádiz (Hernández-Molina et al., 2014). At the same time, downslope transport, channeled through submarine canyons, occurs frequently. Extensive turbidite intervals - intercalated between contouritic layers and often reworked by bottom currents - have been identified in several Pleistocene and Pliocene sediments in this area (Stow et al., 2013).</p><p>The aim of this study is to define diagnostic criteria to differentiate normally graded contourites and turbidites as well as reworked turbidites based on microfaunal analyses. Benthic foraminiferal assemblages along Pleistocene contouritic (~0.5 Ma) and turbiditic (~0.9 Ma, ~1.1 Ma) sequences in the Gulf of Cádiz (IODP Site U1389) are evaluated to test if their faunal composition provides a reliable tool to distinguish these deposits and the underlying sedimentary processes.</p><p> </p><p> </p><p> </p><p>References:</p><p>de Castro, S., Hernández-Molina, F.J., de Weger, W., Jiménez-Espejo, F.J., Rodríguez-Tovar, F.J., Mena, A., Llave, E., Sierro, F.J., 2020. Contourite characterization and its discrimination from other deep‐water deposits in the Gulf of Cadiz contourite depositional system. Sedimentology. https://doi.org/10.1111/sed.12813</p><p>Faugères, J.C., Stow, D.A.V., 2008. Contourite Drifts. Nature, Evolution and Controls. Dev. Sedimentol. 60, 257–288. https://doi.org/10.1016/S0070-4571(08)10014-0</p><p>García-Gallardo, Á., Grunert, P., Voelker, A.H.L., Mendes, I., Piller, W.E., 2017. Re-evaluation of the “elevated epifauna” as indicator of Mediterranean Outflow Water in the Gulf of Cadiz using stable isotopes (δ13C, δ18O). Glob. Planet. Change 155, 78–97. https://doi.org/10.1016/j.gloplacha.2017.06.005</p><p>Hernández-Molina, F.J., Llave, E., Preu, B., Ercilla, G., Fontan, A., Bruno, M., Serra, N., Gomiz, J.J., Brackenridge, R.E., Sierro, F.J., Stow, D.A.V., García, M., Juan, C., Sandoval, N., Arnaiz, A., 2014. Contourite processes associated with the Mediterranean Outfl ow Water after its exit from the Strait of Gibraltar: Global and conceptual implications. Geology 42, 227–230. https://doi.org/10.1130/G35083.1</p><p>Stow, D.A.V., Hernández-Molina, F.J., Llave, E., Bruno, M., García, M., Díaz del Rio, V., Somoza, L., Brackenridge, R.E., 2013. The Cadiz Contourite Channel: Sandy contourites, bedforms and dynamic current interaction. Mar. Geol. 343, 99–114. https://doi.org/10.1016/j.margeo.2013.06.013</p>