Changes in 231Pa/230Th signatures in the bottom water of the ocean

There has recently been much development in the description and modeling of the particulate 231Paxs/230Thxs ratio in the ocean as proxy for the meridional ocean circulation, especially to study the situation in the glacial Atlantic Ocean. Many studies have investigated the effects of ventilation, mass flux and particle composition on 231Paxs/230Thxs ratios. The interpretations as paleoproxy rely usually on the assumption that the isotopic signal stored in the sediment is determined by the composition of suspended or sinking particles when these arrive at the respective water depth. This composition is controlled by exchange with the deep water column [1]. At depths >2500m, Scholten et al. [2] found agreement between 231Paxs/230Thxs ratios in suspended material and surface sediments. Chase et al. [3] observed no difference in activity ratio between surface sediment and a fluff layer present on top of their cores. However, there are several processes that may cause the ratio in surface sediments to differ from the ratio in sinking particles. Bottom currents transport and redistribute the sediment and fractionate grain size [4] and isotopes [5,6]. As a result of this transport and of early diagenetic reactions in the sediment, surface sediments may have a chemical composition and reactivity that is different from sinking particles. We will discuss the possible bias that these processes can give to the signals measured in bottom waters and stored in the sediment. Whereas Pa/Th ratios have mostly been studied in relation to deep water formation in the North Atlantic, the formation of deep water in the Weddell Sea (Weddell Sea Bottom Water, WSBW) also effects the distribution of 230Th and 231Pa. We will present some new water column data confirming the strong effect of ventilation on the distribution of 231Pa and 230Th in the Atlantic sector of the Southern Ocean. Both nuclides accumulate at intermediate depth in the Weddell Sea while concentrations are appreciably lower in the newly formed Weddell Sea Bottom Water. [1] Thomas et al. (2006) Earth Planet. Sci. Lett. 241, 493-504. [2] Scholten et al. (2008) Earth Planet. Sci. Lett. 271, 159-169. [3] Chase et al. (2003) Deep-Sea Res. II 50, 739-768. [4] McCave, I. N. and Hall, I. R. (2006) Geochem. Geophys. Geosyst. 7, Q10N05. [5] Kretschmer et al. (2010) Earth Planet. Sci. Lett. 294, 131-142. [6] Kretschmer et al. (2011) Geochim. Cosmochim. Acta 75, 6971-6987