Marine sedimentary rocks deposited across the Neoproterozoic Cryogenian Snowball interval, similar to 720-635 million years ago, suggest that post-Snowball fertilization of shallow continental margin seawater with phosphorus accelerated marine primary productivity, ocean-atmosphere oxygenation, and ultimately the rise of animals. However, the mechanisms that sourced and delivered bioavailable phosphate from land to the ocean are not fully understood. Here we demonstrate a causal relationship between clay mineral production by the melting Sturtian Snowball ice sheets and a short-lived increase in seawater phosphate bioavailability by at least 20-fold and oxygenation of an immediate post-Sturtian Snowball ocean margin. Bulk primary sediment inputs and inferred dissolved seawater phosphate dynamics point to a relatively low marine phosphate inventory that limited marine primary productivity and seawater oxygenation before the Sturtian glaciation, and again in the later stages of the succeeding interglacial greenhouse interval.

The catastrophic Indian Ocean tsunami of December 2004 raised urgent questions about the paleotsunami history in the region. Numerous studies have since been conducted to gain better understanding of the magnitude, frequency and impact of past tsunamis, especially around the coasts of the Indian Ocean.

Southwest Thailand directly faces the Sunda Arc trench where the earthquake that generated the Indian Ocean tsunami took place. The lack of historical documents and of suitable geological archives makes paleotsunami research a challenge in this area however. Phra Thong Island, on the Andaman Coast of southwest Thailand, with its marshy swales has proven an exception and is one of the few suitable locations for these types of studies. Apart from the 2004 Indian Ocean tsunami layer, three to four more distinct paleotsunami layers, separated by organic rich soil horizons have been previously identified and dated using radiocarbon and Optically Stimulated Luminescence techniques. Despite these efforts, several outstanding issues have to be resolved: (1) the correlation of tsunami/paleotsunami layers over larger distances remains ambiguous, particularly older layers; (2) age attributions for several of the paleotsunami layers differ at different locations; and (3) alterations of soil and sand layers by postdepositional processes are still poorly understood

This thesis addresses these issues and aims to develop geochemical proxies that allow characterising each tsunami/paleotsunami layer in three different swales. As shown here, X-ray fluorescence elemental geochemistry, combined with loss-on-ignition analysis and mineralogical identification, is a promising tool for identifying paleotsunami layers. The specific geochemical signature of each sand layer can then be used for correlations over larger distances. This approach has the potential to provide a means for estimating past tsunami inundation distances, and thus their magnitude.