Variability of δ234U in the Mediterranean Sea, Amazon Estuary, and Atlantic Ocean

• Main Author: Border, Evan C.
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Published: 2020
• Subjects: 500; 500 Natural sciences and mathematics; 530; 530 Physics; 550; 550 Earth sciences; North Atlantic
• Online Access: https://archiv.ub.uni-heidelberg.de/volltextserver/28744/; https://archiv.ub.uni-heidelberg.de/volltextserver/28744/1/Border_Evan_Dissertation_PDF_A.pdf; https://doi.org/10.11588/heidok.00028744; https://nbn-resolving.org/urn:nbn:de:bsz:16-heidok-287445

It has long been assumed that the world's oceans are homogeneous in δ234U, even on a sub-‰ scale, however this has not been comprehensively investigated outside of the North Pacific using modern high-precision MC-ICP-MS. In this study, the δ234U of water samples from across the Mediterranean Sea, Amazon Estuary, and North Atlantic is presented to show that the oceanic uranium system is much more variable and dynamic than previously believed and variable on a ‰-scale. Mediterranean water masses are elevated in δ234U compared to the Atlantic by ~1-2‰, allowing for the estimation of the δ234U of riverine and groundwater inputs to the basin. Analysis within the Amazon Estuary shows that there are significant geographical differences in the non-conservative behavior of U, and that the Amazon has little to no effect on the δ234U of the nearby Atlantic. The upper Atlantic is on average 1‰ lower in δ234U than the deep Atlantic, indicating that oceanic δ234U is not in steady-state but rather decreasing. Results point towards the offset seen in the upper Atlantic possibly being the result of inputs from the Indian Ocean (which has yet to be investigated at such high-precision), indicating that such oceanic δ234U variability may be a widespread phenomena.