Sediment redistribution and grainsize effects on Th-230-normalized mass accumulation rates and focusing factors in the Panama Basin

摘要

Here, we examine how redistribution of differing grain sizes by sediment focusing processes in Panama Basin sediments affects the use of Th-230 as a constant-flux proxy. We study representative sediments of Holocene and Last Glacial Maximum (LGM) time slices from four sediment cores from two different localities close to the ridges that bound the Panama Basin. Each locality contains paired sites that are seismically interpreted to have undergone extremes in sediment redistribution, i.e., focused versus winnowed sites. Both Holocene and LGM samples from sites where winnowing has occurred contain significant amounts (up. to 50%) of the (230T)h within the >63 gm grain size fraction, which makes up 40-70% of the bulk sediment analyzed. For sites where focusing has occurred, Holocene and LGM samples contain the greatest amounts of Th-230 (up to 49%) in the finest grain-sized fraction (<4 pm), which makes up 26-40% of the bulk sediment analyzed. There are slight underestimations of Th-230-derived mass accumulation rates (MARS) and overestimations of Th-230-derived focusing factors at focused sites, while the opposite is true for winnowed sites. Corrections made using a model by Kretschmer et al. (2010) suggest a maximum change of about 30% in Th-230-derived MARs and focusing factors at focused sites, except for our most focused site which requires an approximate 70% correction in one sample. Our Th-230-corrected Th-232 flux results suggest that the boundary between hemipelagically- and pelagically-derived sediments falls between 350 and 600 km from the continental margin.(C) 2017 Elsevier B.V. All rights reserved.