Impact of Mediterranean Outflow Water on the Pliocene North Atlantic

The opening and closing of ocean gateways play an important role amongst climate forcing mechanisms: along with paleogeography, surface and deep-water circulation are altered, and hence global heat transport. One of the best known examples for the climatic significance of oceanic currents is the Gulf Stream that generates a moderate climate in northern Europe. Another important component of North Atlantic circulation patterns is the warm and saline Mediterranean Outflow Water (MOW) that enters the North Atlantic via the Gibraltar Strait. Its onset, that is strongly related to the opening of the Gibraltar Strait during the Late Miocene and Early Pliocene (c. 5.2 Ma), and early history are poorly constrained and its impact on oceanography and climate since the Pliocene are largely unexplored. The herein presented project is part of the research goals of Expedition 339 of the Integrated Ocean Drilling Program (IODP) that explores the environmental significance of this warm and saline water mass during the Pliocene and Pleistocene and its role in global climate as a component of North Atlantic circulation. New sample material from several drill-sites in the Gulf of Cadz and along the Western Iberian Margin have been gathered between November 2011 and January 2012 in the course of the expedition. The herein proposed project focuses on the Pliocene history of MOW, its impact on local paleoceanography and its potential contribution to the transition from the Early-Middle Pliocene climate optimum into cooler conditions of the Late Pliocene. Based on micropaleontology and geochemistry, an integrated data-set of paleoenvironmental proxies will be established to evaluate variations in MOW at different time-scales from 0.5 ka to 10 ka. Quantitative analysis of benthic foraminiferal assemblages is used for the reconstruction of MOW velocity and intensity. d18O and Mg/Ca from planktic and benthic foraminiferal shells are evaluated for changes in water- temperature and salinity. d13C, TOC, S and CaCO3 are applied as proxies for productivity and bottom-water ventilation. This set of proxies allows to address two major topics: (1) Paleoceanographic changes related to the opening of the Gibraltar Strait and the onset of MOW during Late Miocene Early Pliocene. While the opening of the Gibraltar Strait is stratigraphically well-constrained to 5.23 Ma, the onset and early history of MOW remains poorly understood. The new proxy records will help to evaluate the impact of the new water source on the local Early Pliocene paleoenvironment. (2) Evaluation of variability in MOW intensity and its potential relation to the Middle Pliocene Thermal Optimum (MPTO) and Late Pliocene climate deterioration. The Early-Middle Pliocene world with its warm-house climate and elevated CO 2 -levels is often referred to as a potential analogue to future climate change. However, the underlying mechanisms of the MPTO as well as the subsequent climate cooling are strongly debated. A strong focus of the project will thus lie on the variability of MOW intensity, the impact of these changes on North Atlantic circulation patterns and the potential impact on MPTO and the Late Pliocene climate transition. Due to the integration with IODP expedition 339 and the close cooperation with the highly regarded science party, the outcome of the project will document for the first time the role of MOW in North Atlantic circulation since the Pliocene. It will also improve our understanding of the interplay of ocean circulation and climate in general and thus serve as a valuable source of information for modeling future climate change.

The opening of the Gibraltar Strait at 5.3 million years (myr) ago marked the beginning of renewed water mass exchange between the Mediterranean Sea and the Atlantic Ocean. Today, the injection of warm and salty Mediterranean Outflow Water (MOW) into the Atlantic promotes heat transport to high northern latitudes (as does e.g., the Gulf Stream). The reconstruction of past variability of MOW strength thus provides an important key for our understanding of northern hemisphere climate dynamics. FWF-project P 25831-N29 explored the little known early history of this oceanic circulation pattern and evaluated its behavior in different climatic states of the Pliocene (c. 2.6 to 5.3 myr before present). Our analyses relied on sediment cores recovered by the International Ocean Drilling Program (Expedition 339) along the southern Iberian Margin, west of Gibraltar, at 500 to 700 meters water depth.Our results show that Mediterranean-sourced waters reached the study area periodically shortly after the opening of the Gibraltar Strait 5.3 myr ago. The continuous presence of MOW can only be verified at 44.5 Myrs ago. The paleoclimate at this time was considerably warmer than today, and water mass exchange remained weak throughout this earliest phase of MOW development. Increasing Arctic glaciation finally led to the transition into a cooler paleoclimatic state with distinct alterations of warm and cold periods. Our data indicate that cooler paleoclimatic conditions and increasing aridity in the Mediterranean region initiated a long-term strengthening of MOW at 3.2 myr ago. Particularly intensified water mass exchange occurred during pronounced ice ages between 2.8 and 2.6 Myrs ago and suggest a key-role for MOW in North Atlantic heat transport in these cold periods.The outcome of this project provides new insights into the previously little known early history of MOW. It also adds crucial information on key-aspects of MOWs role in North Atlantic heat transport, and ultimately to the dynamic interplay between ocean circulation and climate change. Furthermore, our research efforts resulted in the improvement and new development of methods for the reconstruction of water temperature and the strength of oceanic currents. The successful conclusion of the project thus provides a wide range of starting points for prospective (paleo)climatic and paleoceanographic studies.