Investigating the use of 232 Th/ 230 Th as a dust proxy using co-located seawater and sediment samples from the low-latitude North Atlantic

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Rowland, George H., Ng, Hong Chin, Robinson, Laura F., McManus, Jerry F., Mohamed, Kais J., McGee, David
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Thorium; Dust; Sediment; Seawater; Isotope; North Atlantic
• Online Access: https://hdl.handle.net/1983/46a5b811-01f5-4afc-a707-cbe5334a6553; https://research-information.bris.ac.uk/en/publications/46a5b811-01f5-4afc-a707-cbe5334a6553; https://doi.org/10.1016/j.gca.2017.07.033; https://research-information.bris.ac.uk/ws/files/124158731/232Th_230Th_GHR_et_al_post_PeerRev_PreProofs.pdf; http://www.scopus.com/inward/record.url?scp=85027862791&partnerID=8YFLogxK

The thorium isotope ratio 232 Th/ 230 Th can be measured in seawater and sediment samples, and has been used as a proxy to reconstruct lithogenic fluxes to the oceans for the modern day and the Pleistocene. There has not yet been a systematic study testing the proxy using the 232 Th/ 230 Th ratio in seawater and the ratio recorded in the underlying sediment. In this study we use co-located core-top sediments and seawater samples from five seamount sites spanning the tropical North Atlantic to investigate the link between seawater and sediment 232 Th/ 230 Th ratios across a range of water depths. Our results indicate that a broad correlation exists between seawater and sedimentary 232 Th/ 230 Th ratios. Both seawater and sedimentary 232 Th/ 230 Th ratios record a signal consistent with decreasing lithogenic input east to west, from Africa to South America. However, calculated 232 Th fluxes for the core-top sediment samples indicate a strong dependence on depth, with up to a factor of ∼4 difference from shallow (<600 m) to deep sites (>2900 m). This depth dependence is likely caused by either a deficit of 230 Th burial at depth compared to the production in the overlying water column, through addition of 232 Th, or by a combination of the two. By comparing seawater and sedimentary 232 Th/ 230 Th ratios we derive an apparent fractional solubility of 232 Th of 29 ± 3%, in reasonable agreement with the upper end of existing estimates.