Assessing marine aerosol delivery to infer paleo-wind changes using mid-latitude stalagmites from Northern Spain

Wind is a pivotal component of the atmospheric system and plays a key role in the local and global weather and climate system. For example, wind delivers air masses loaded with humidity, which can rain out when elevated along a mountain range. Depending on the wind strength it can result in a mild breeze or a severe storm. Wind distributes dust, ash, pollen, contaminants and many other particles in the atmosphere, which all play their role in the Earth System. Furthermore, wind erosion or loess deposition are examples of the interaction with the geosphere. There are countless examples of interactions between wind and other components of the Earth system. However, little is known about wind in the past since it is not recorded directly in any geological climate archive. How can we reconstruct the climate of the past or infer wind related processes in the past if we do not know how wind actually behaved? Therefore, indirect approaches are necessary to reconstruct the wind in the past. In this doctoral thesis we assess the potential of using marine contributions recorded in speleothems (in particular stalagmites) as a novel paleowind proxy. It may sound counterintuitive to use cave deposits, which are protected by wind, however, stalagmites do record signals from outside the caves. A classical source to sink approach is applied where the fate of marine aerosols is investigated. Marine aerosols are primarily produced on the ocean by wind (the stronger the wind, the higher the marine aerosol load in the atmosphere) and are transported on land where they deposit. They are infiltrated into caves by drip water where they can form speleothems. Therefore, stalagmites might record changes in wind over the oceans. This hypothesis is tested by investigating each step starting with the marine aerosol transport and deposition on land (Chapter 1), arrival in the cave (Chapter 1 & 2), incorporation into stalagmites (Chapter 3) and finally brought into context with climate on longer time scales (Chapter 4). Hereby we focus on Sodium (Na) as a marine aerosol tracer due to its chemical conservation, low potential of contamination and favorable capability to be measured. Stalagmites are increasingly used as paleoclimate archives due to their conservation potential of climate proxies, absolute and precise chronologies by radiometric dating methods (radiocarbon and Uranium decay series), relatively easy accessibility in caves (no costly drilling techniques or expeditions to remote places) and detectability of geochemical signals in the laboratory. However, it is necessary to understand all processes affecting the geochemical signal in detail before a proxy can be used to reconstruct climate. With multiproxy approaches complex mechanisms can be brought into perspective to each other and interrelationships can be studied. Mid-latitude stalagmites from Northern Spain grown in caves within the first few kilometers from the coast provide a great target to test the feasibility of a wind proxy due to its influence by the westerly wind belt, low potential of contamination since wind primarily derives from the undisturbed open Atlantic Ocean and geochemical conservation of local and global climate signals allowing multiproxy approaches to contrast wind under different climate conditions. Monitoring the marine aerosol delivery (Chapter 1) confirms the marine origin and reveals a strong relationship with wind intensity. Coastal caves within the first few hundred meters from the coast are most sensitive to changes in wind intensity over the seasonal cycle, however, the strong decrease of marine contributions with distance from the coast has to be considered when sea-level changes over time shifting the coastline. Marine aerosol deposition at cave locations situated >3 km from the coast is still controlled by wind intensity but a shifting coastline due to sea-level change should not impact the marine aerosol delivery since deposition remains constant. Therefore, unshielded caves situated a bit inland are most feasible targets to reconstruct paleowind intensity. An extensive cave monitoring study is performed in La Vallina cave to assess the infiltration of marine contributions into caves (Chapter 2). Drip water measurements confirm the arrival of marine contributions in the cave. Furthermore, potential stalagmite formation, hydrological behaviors, sources or sinks along the percolation flow path, the role of vegetation on top of the cave and other important karst and cave processes are studied with the obtained data set of the cave monitoring. Before a paleowind proxy can be applied the incorporation of marine contributions into stalagmites is investigated (Chapter 3). It turns out that trace element incorporation is a complex mechanism influenced by different processes and features in stalagmites. Therefore, we conclude that wind cannot (yet) be reconstructed with stalagmites. Fluid inclusions, crystal defects and other impurities such as detrital particles overprint the marine contribution making it impossible to extract the marine component ultimately controlled by wind. In the last chapter a classical multi-proxy climate reconstruction is performed to demonstrate how midlatitude stalagmites from Northern Spain resolve local and global climate signals. This reconstruction focuses on the Penultimate Glacial phase, also known as MIS 6, and reveals climate variations on different time scales. On orbital time scales the climate reacts to solar forcing and exhibits melt phases of the Eurasian Ice Sheet during high solar insolation phases. On millennial time scales abrupt climate events are recorded which are explained by melt water releases affecting the ocean circulation reducing the heat transport to Europe, thus triggering abrupt cold events. Such climate feedback mechanisms are revealed in other climate archives confirming our interpretation. We demonstrate how this new highresolution record provides an absolutely dated reference chronology and can be used as a target to tune other records from the North Atlantic region lacking absolute chronologies.