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

Funding for this research was provided by the National ScienceFoundation (Grants OCE-8817836, OCE-8922707, OCE-9016494, and OCE-9200780). Funds from the Woods Hole Oceanographic Institution and from theGordon Research Conferences for travel to scientific meetings are also gratefullyacknowledged.

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution May 1994

Fractionation between 230Th (t1/2 = 7.5x104yr) and 231 Pa (t1/2 =3.2x104yr), the two longest-lived radionuclides produced from the decay ofnatural uranium, is found widely within the oceans. This large scalefractionation was investigated in samples of sinking particulate matter collectedwith sediment traps and in deep-sea sediment cores. New analytical methods foruranium, thorium, and protactinium isotopes by Inductively-Coupled PlasmaSpectrometry (ICP-MS) with a conventional Meinhard concentric glass nebulizerwere developed. Composite samples from sediment traps deployed a year orlonger in diverse geographic regions of the ocean were analyzed to examineassumptions underlying the use of the 230Th-normalized flux method and the231Pa/230Th ratio for reconstruction of the fluxes of sedimentary components inthe present and past ocean. The compiled results demonstrated that over most ofthe ocean, the flux of 230Th into the sediments balances approximately itsproduction rate in the overlying water column, with an accuracy better than 30%;whereas 231Pa tends to migrate towards the margins or other regions of highparticle flux. Thus the 231Pa/230Th ratio is sensitive to regional differences inthe scavenging intensity. In addition to this, the influence of particle compositionon the 231 Pa/230Th ratio was investigated, but it was not possible to reach adefinitive conclusion. The thesis describes aspects of the large scale geochemicalfractionation between the two elements within the Atlantic Ocean during theHolocene and the Last Glacial Maximum and attempts to answer questionsconcerning the causes of the large scale fractionation resulting from a differentialpartitioning of the two elements between a vertical flux with sinking particlesand a horizontal flux to boundaries due to ocean circulation. A quantification ofthe influence on the fractionation of the two nuclides by advection resulting fromthe thermohaline circulation was made, and it is shown that almost half of the231 Pa production in the water column is exported from the Atlantic Ocean to theSouthern Ocean. This export is important in the budget of 231Pa in deep-seasediments of the Southern Ocean. This suggests that the variations in231Pa/230Th ratio reflect variations not only in particle flux, but also in theintensity or pattern of the thermohaline circulation. Reconstruction of the 230Th-normalizedbiogenic opal and carbonate paleo-fluxes suggests no significantchanges in surface production in the southern Indian Ocean between the LastGlacial Maximum and the Holocene period.