May 19 – Licentiate Thesis Presentation by Sandra Gdaniec

231Pa and Th-isotopes as tracers of deep water ventilation and scavenging in the Mediterranean Sea

Friday, 19 May 2017
10h15, Nordenskiöldsalen, Geohuset, Stockholm University

Per Andersson, Swedish Museum of Natural History 
Matthieu Roy-Barman, Laboratoire des Sciences du Climat et de l'Environnement 
Carl-Magnus Mörth, Stockholm University

Docent Helen Coxall, Stockholm University 
Professor Per Roos, University of Denmark



231Pa and Th-isotopes are used as tracers of biogeochemical processes such as scavenging, particle transport and ocean circulation. 231Pa and 230Th are produced uniformly in seawater from the decay of dissolved U. After production, 231Pa and 230Th are removed by adsorption onto settling particles (scavenging) and subsequently buried in marine sediments. 230Th is more particle reactive compared to 231Pa. Consequently, 230Th will be removed from the open ocean by adsorption onto settling particles, while 231Pa tend to be laterally transported by currents and removed by scavenging in areas of high particle flux. 232Th indicates lithogenic supply via rivers and resuspension of sediments, which provides additional information about processes involved in the cycling of particle reactive elements in the ocean.
231Pa,  230Th and 232Th were analyzed in unfiltered seawater samples and in suspended particles collected in the Mediterranean Sea during the MedSeA-GA04-S cruise and used to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean margin environment. Observed nuclide distributions indicate the impact of deep water formation processes, where observed differences can be linked to the type of deep water formation process that occurs in respective basin. Essentially all in-situ produced 230Th is buried in Mediterranean Sea sediments. Despite lower affinity of 231Pa for marine particles, most 231Pa is also scavenged and deposited in Mediterranean Sea sediments. The efficient scavenging of 231Pa produces a relatively low fractionation between 231Pa and 230Th in terms of the fractionation factor FTh/Pa. The study of 231Pa, 230Th and 232Th in the Mediterranean Sea has important implications for our understanding of processes that control their water column distributions and how their behavior can be utilized to trace chemical flux in modern and past ocean environments.

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