HR: 0800h
AN: PP51F-1372    [Abstracts]
TI: High-Resolution Accumulation Rate and Authigenic U Variations On The Nazca Rise , Peru Margin, Over the last 25ka
AU: * Higgins, S M
EM: sean@ldeo.columbia.edu
AF: MIT, Dept. of Earth, Atmospheric, & Planetary Sciences, Cambridge, MA 02139 United States
AU: Altabet, M A
AF: SMAST, U. Mass-Dartmouth, 706 S.Rodney Blvd., New Bedford, MA 02744 United States
AU: Higginson, M J
AF: Forensic Alliance LTD., F5 Culham Science Center Abingdon, Oxfordshire, New OX14 3ED United Kingdom
AU: Boyle, E A
AF: MIT, Dept. of Earth, Atmospheric, & Planetary Sciences, Cambridge, MA 02139 United States
AB: As part of a larger regional study into ocean circulation changes along the Peru Margin over the last 25 ka, we have a developed an excess Th-230 (xsTh-230) record of sediment mass accumulation rates (MAR) and authigenic U (Uauth) at an offshore site (140km off margin) on the Nazca Rise. Over an interval between 7 and 25 ka, these profiles consists of 50 xsTh-230 and U measurements for an average resolution of ~350 yr/sample. This has allowed us to evaluate concerns about sediment redistribution affecting interpretations of other proxies, like $\delta$N-15, as well as make more direct comparisons with shallower marginal sites. The Site RR9702A-69TC (3.2 km water depth) is located at $16\deg$S $76\deg$W and has an average sedimentation rate of 5.4 cm/ka based on AMS C-14 dates and correlation to nearby locations. The xsTh-230 profile combined with estimates of dry bulk density and sedimentation rates indicates that sediment focusing (a surplus of Th-bearing sediments) is about 1.5 to 2x the expected throughout the record. Th-230 data and %CaCO$_{3}$ analyses enabled us to produce detailed records of carbonate and non-carbonate MAR. The Th-normalized detrital MAR (primarily eolian) decreases by a factor of two (1.2 to 0.6 g/cm$^{2}$/ka) across the deglacial (15 to 12 ka). In contrast, Th-normalized CaCO$_{3}$ MAR shows values increasing slowly through the LGM (0.1 to 0.15 g/cm$^{2}$/ka) then decreasing from maximum at 15ka to no Holocene accumulation by 11 ka. In addition to xsTh-230, measurements of Th-232 and Uauth (often interpreted as a total organic carbon (TOC) proxy provide us independent measures of detrital flux and productivity variations, respectively. The Th-232 derived detrital flux is consistent with estimates of eolian accumulation from nearby site ODP Site 1237 and is relatively constant through the Last Glacial Maximum (LGM). Uauth varies from 4 to 10 ppm during the LGM, decreasing through the deglacial to Holocene values of 1 to 2 ppm. However, the Th-232 detrital flux does not co-vary with Uauth concentrations (R$^{2}$ =0.2) . Instead, there is a much better correlation between CaCO$_{3}$ flux and Uauth (R$^{2}$=0.5). This indicates that the variability (factor of 4-5x) observed in Uauth was not caused primarily by dilution by terrigenous inputs.
DE: 4860 Radioactivity and radioisotopes
DE: 4267 Paleoceanography
DE: 4558 Sediment transport
DE: 3022 Marine sediments--processes and transport
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2004 AGU Fall Meeting