Past Ocean Reconstruction
Deep ocean circulation has been considered to be playing a significant role in earth’s climate system as an atmospheric CO2 reservoir and global heat conveyor. To study how it operates, chemical tracers, such as the carbon isotope composition (δ 13C) and the cadmium content (Cd/Ca) of benthic foraminifera, have been employed. However, these proxies provide information about water mass distributions rather than flow rates which ultimately control heat transport and carbon storage of deep water. Therefore, we apply another proxy in the U-series toolbox, the ratio of 231Pa/230Th, to reconstruct the rates of past ocean circulation.
Naturally occurring in seawater, 231Pa (half-life 32.5 kyr) and 230Th (half-life 75.2 kyr) are produced through the alpha-decay of soluble U at a constant activity ratio of 0.093. Since both radio-nuclides are particle-reactive, they are adsorbed onto particles once produced in seawater with subsequent removal from seawater as particles settle to sediments. However, with a higher particle reactivity, 230Th is more readily removed from seawater, while less particle reactive 231Pa can be advected by deep ocean circulation to high productivity areas to be subsequently removed. In the North Atlantic where North Atlantic Deep Water (NADW) is formed, 231Pa/230Th ratios in sediments are expected to be lower than their production ratio 0.093. As NADW travels southward, high values of 231Pa/230Th are expected to be found under Southern Ocean sediment. If NADW slows down, less 231Pa can be transported southward leading to lower sedimentary 231Pa/230Th ratios in Southern Ocean. In this way, variations of deep flow rates in the past can be reconstructed with varying 231Pa/230Th records in sediments.
Fig. Schematic of 231Pa/230Th fractionation in the ocean (Illustration by Feifei Deng, adapted from Henderson, 2000)
However, it is quite challenging to apply 231Pa/230Th as an ocean circulation tracer since distributions of 231Pa and 230Th are also influenced by other factors such as particle flux and composition. Therefore, in order to use 231Pa/230Th to reconstruct past ocean circulation, a thorough understanding of distribution patterns of 231Pa and 230Th in the modern ocean has to be achieved.
Participating in GEOTRACES program, we obtain seawater samples from the central and southwestern Atlantic Ocean and determine concentrations of 231Pa and 230Th with MC-ICP-MS in our laboratory. By providing 231Pa and 230Th dataset in broader oceanic settings, we expect to understand how various factors influence the 231Pa and 230Th distributions in the ocean, which will ultimately help us use 231Pa/230Th to reconstruct past ocean circulation and climate change.
- Measuring 231Pa and 230Th concentrations in seawater samples collected in the central and southwestern Atlantic Ocean during GEOTRACES cruises JC057 and D361;
- Assessing factors that might have an influence on the oceanic distributions of 231Pa and 230Th;
- Assessing how reliable the collection, storage and analytical techniques are for the measurement of 231Pa and 230Th under GEOTRACES inter-calibration framework.
Dr. Alex L. Thomas (University of Edinburgh)
2. Thomas, A.L., Henderson, G. M., and Robinson, L. F. (2006). Water column behaviour of Pa and Th isotopes in the south-western Indian Ocean: Implications for (231Paxs/230Thxs)0 use as a paleoproxy. Earth Planet Sci. Lett.,241: 493-504
3. Henderson, G. M., and R. F. Anderson (2003), The U-series toolbox for paleoceanography, in Reviews in Mineralogy and Geochemistry “Uranium Series Geochemistry”, edited by Bourdon, B. et al., pp. 493-531, Mineral. Soc. Of Am., Washington, D. C.