Variations in the particulate flux of ²³⁰Th and ²³¹Pa and paleoceanograhpic applications of the ²³¹Pa/²³¹Th ratio

Abstract

Fractionation between 230Th (t1/2 = 7.5x104yr) and 231 Pa (t1/2 = 3.2x104yr), the two longest-lived radionuclides produced from the decay of natural uranium, is found widely within the oceans. This large scale fractionation was investigated in samples of sinking particulate matter collected with sediment traps and in deep-sea sediment cores. New analytical methods for uranium, thorium, and protactinium isotopes by Inductively-Coupled Plasma Spectrometry (ICP-MS) with a conventional Meinhard concentric glass nebulizer were developed. Composite samples from sediment traps deployed a year or longer in diverse geographic regions of the ocean were analyzed to examine assumptions underlying the use of the 230Th-normalized flux method and the 231Pa/230Th ratio for reconstruction of the fluxes of sedimentary components in the present and past ocean. The compiled results demonstrated that over most of the ocean, the flux of 230Th into the sediments balances approximately its production rate in the overlying water column, with an accuracy better than 30%; whereas 231Pa tends to migrate towards the margins or other regions of high particle flux. Thus the 231Pa/230Th ratio is sensitive to regional differences in the scavenging intensity. In addition to this, the influence of particle composition on the 231 Pa/230Th ratio was investigated, but it was not possible to reach a definitive conclusion. The thesis describes aspects of the large scale geochemical fractionation between the two elements within the Atlantic Ocean during the Holocene and the Last Glacial Maximum and attempts to answer questions concerning the causes of the large scale fractionation resulting from a differential partitioning of the two elements between a vertical flux with sinking particles and a horizontal flux to boundaries due to ocean circulation. A quantification of the influence on the fractionation of the two nuclides by advection resulting from the thermohaline circulation was made, and it is shown that almost half of the 231 Pa production in the water column is exported from the Atlantic Ocean to the Southern Ocean. This export is important in the budget of 231Pa in deep-sea sediments of the Southern Ocean. This suggests that the variations in 231Pa/230Th ratio reflect variations not only in particle flux, but also in the intensity or pattern of the thermohaline circulation. Reconstruction of the 230Th-normalized biogenic opal and carbonate paleo-fluxes suggests no significant changes in surface production in the southern Indian Ocean between the Last Glacial Maximum and the Holocene period.