Heidelberg 2015 – wissenschaftliches Programm
UP 13.10: Poster
Donnerstag, 26. März 2015, 10:45–19:45, G/Foyer
Conceptual model: possible changes of the seawater uranium isotopic composition through time — •Hannah Nowitzki, Norbert Frank, and Jens Fohlmeister — Universität Heidelberg, Germany
U behaves in seawater like a conservative element. More than 99% of the oceanic U content is 238U, whereas 234U is only present in trace amounts. As the residence time of U is significantly longer than the mixing time of the ocean, the ocean is well mixed with respect to U and its isotopic composition (Dunk 2002). Moreover, living corals incorporate U without isotopic fractionation. Therefore, the past seawater isotopic evolution of (234U/238U) can be accessed via U/Th age-dating of corals and the subsequent calculation of the initial (234U/238U) value.
The isotopic (234U/238U) composition of seawater during the last 360ka scatters around the modern seawater value (δ234U ≈ (145±15) %, Henderson 2002). As these variations in the δ234U value are rather small, a ’constant seawater isotopic composition hypothesis’ is often used to validate U/Th ages of fossil corals. However, some authors find that the variability of the isotopic composition exceeds the expected range and suggest that it provides valuable information on variations in continental weathering and global run-off fluctuations or sea-level changes.
This work will attempt to compare literature data of the seawater U isotopic composition to the results of a conceptual box-model of the oceanic U budget.