Thorium-230 Stratigraphy of Alpha Ridge Sediment (Arctic Ocean)
Abstract
The Alpha Ridge (central Arctic Ocean) is characterized by very uniform sedimentary deposition essentially linked to vertical particulate rain. This property led us to investigate the behavior of U-series isotopes (Th-230 and Pb-210) in such a setting, i.e., one without significant sedimentary advection. Two sites cored with a 70 cm-long multicorer during the 2005 Hotrax Expedition and located about 20 nautical miles apart and at different water depths (core 11: 2644 m and core 12: 1585 m) were selected for the purpose of this study. Lead-210 profiles are practically identical in both cores, with high activities at the surface (>30 dpm/g), followed by a first minimum at 1 cm (<10 dpm/g), and then increasing values down to approximately 7 cm (about 15 dpm/g), and a gradual decrease below this core depth. This pattern suggests significant bioturbation, at least down to 8-10 cm, and some Pb-210 diffusion below. At three distinct depths the Th-230 activities are above supported Th-230 values (approx. 1.2 dpm/g): from 0 to 8 cm (with a maximum ranging 25-30 dpm/g), 15 to 20 cm (up to 7 dpm/g) and 26 cm to core bottom (34 and 38 cm, respectively in cores 12 and 11). Here again, despite their large bathymetric difference, the two sites yielded almost identical 230Th-profiles. 230Th-activities are highly correlated with the CaCO3 content, allowing for the decay of the excess-thorium 230 (230Thxs) downcore. Assuming a linear initial relationship between CaCO3 and Th-230xs, the assignment of the lowermost part (below 37 cm) of core 11 to the oxygen isotope 5e seems probable. Maximums in organic carbon and carbonate contents at the base and top of the cores would support this interpretation. In such settings, Th-230 reveals useful data to constrain the stratigraphy of the late Pleistocene sediments, and may compensate for the absence of a viable oxygen isotope stratigraphy.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFMOS53B1106N
- Keywords:
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- 0473 Paleoclimatology and paleoceanography (3344;
- 4900);
- 1040 Radiogenic isotope geochemistry;
- 1051 Sedimentary geochemistry