A New Proxy of the 230Th/231Pa Fractionation in the Pacific Water Column During the Past?

Norbert Frank Heidelberger Akademie der Wissenschaften, Im Neuenheimer Feld 366, 69120 Heidelberg, Germany


Anke Scholz PSI-Zuerich, CH-5232 Villingen, Switzerland

Anton Eisenhauer Geochemisches Institut, Bollschmittstr. 1, 37077 Göttingen, Germany

Augusto Mangini Heidelberger Akademie der Wissenschaften, Im Neuenheimer Feld 366, 69120 Heidelberg, Germany

The activities of 231Pa and 230Th in two hydrogenous manganese crusts from the Northern Equatorial Pacific (VA13-2: 146°W, 9°25´N, 4830m; 73DK6: 170°10´W, 9°24´N, 2860m) were measured with a high depth resolution (50µm) to a total depth of 800µm. From the depth distribution of 230Thexcess and 231Paexcess we reconstruct the initial 230Th/231Pa activity ratios and we estimate mean growth rates which are 5.92±0.36 mm My-1(VA13-2) and 2.6±0.13 mm My-1 (73DK6), respectively. The initial 230Th/231Pa activity ratios are 10.4±2 (VA13-2) and 7.7±0.6 (73DK6). This result corroborates earlier published 230Th and 231Pa data from
different hydrogenous manganese nodules. The comparison of these values with those of the water column indicates
differential adsorption of Th and Pa on different solid phases. Assuming that these values are representative we suggest that Th is more efficiently adsorbed on top of hydrogenous manganese encrustations than is Pa.

The time corrected 230Th/231Pa activity ratios show
significant variability in both samples (Fig. 1). Crust VA13-2 shows a very constant initial Th/Pa activity ratio of 10 over the past 150 ky, intercalated are two sections with values of > 40, reflecting a low 231Pa activity. A reasonable explanation for this result is an enhanced export of 231Pa to areas of high biological productivity during periods of enhanced upwelling, for example to the western margin of the American continent. The shallower crust 73DK6 records a smaller (approx. 5) and although variable initial Th/Pa ratio over the past 300ky, but due to the poor time resolution we cannot distinguish peaks of this ratio.

We conclude that hydrogenous Mn crusts are monitoring radionuclide fractionation, as well as changes of the scavenging process of the radionuclides Th and Pa in seawater during the past.

Fig. 1: Initial [230Th/231Pa] activity ratio of Mn crusts VA13-2 and 73DK6 versus time.