Thorium and protactinium isotopes as tracers of marine particle fluxes and deep water circulation in the Mediterranean Sea

²³¹Pa, ²³⁰Th and ²³²Th were analyzed in unfiltered seawater samples (n=66) and suspended particles (n=19) collected in the Mediterranean Sea during the MedSeA-GA04-S cruise along the GEOTRACES section GA04S and used to investigate mechanisms controlling the distribution and fractionation of Pa and Th in an ocean margin environment. ²³¹Pa and ²³⁰Th are particle reactive radionuclides and are often used as tracers of processes such as boundary scavenging, particle transport and ocean circulation. The depth profiles of total ²³¹Pa and ²³⁰Th concentrations in the Mediterranean Sea displayed non-linear shapes. Higher total ²³²Th concentrations were observed at the straits and in deep waters pointing at lithogenic sources. Fractionation factors F_Th/Pa ranged from 1.4 to 9. Application of a box-model illustrated that 94% of the ²³¹Pa and almost all of the ²³⁰Th (99.9%) produced in the Mediterranean Sea is removed to the sediment by scavenging. The negligible export of ²³⁰Th to the Atlantic Ocean, leads to a reevaluation of the mean settling speed of the filtered particles, which is now estimated to 500-1000m/y. The low F_Th/Pa fractionation factors are attributed to the efficient scavenging and lack of transport of ²³¹Pa to the Atlantic Ocean.