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.
