26Al, 10Be and U–Th isotopes in Blake Outer Ridge sediments: implications for past changes in boundary scavenging

Abstract

We have investigated the distributions of 26 Al, 10 Be, U–Th isotopes, and stable 27 Al and 9 Be in core CH88-11P (30°40′N, 74°41′W; 3337 m) from the Blake Outer Ridge (BOR) in the western North Atlantic, an area that is characterized by high terrigenous input. Results show authigenic 26 Al/ 27 Al of <2–8×10 −14 (atom/atom), authigenic 10 Be/ 9 Be of 0.5–2.6×10 −8 (atom/atom), <0.2–1.8×10 6 atoms/g of 26 Al, and 5.3–15.1×10 8 atoms/g of 10 Be in the core over the last glacial/interglacial cycle. These values, as well as total 230 Th ex / 232 Th and 10 Be/ 9 Be ratios, were all at their minima during the last glacial maximum (LGM), reflecting intensified terrestrial influx to the area. Aluminosilicate material (clays) in this influx preferentially scavenged 230 Th over the two other particle-reactive nuclides, 26 Al and 10 Be, such that their boundary scavenging during LGM followed the order 230 Th> 10 Be> 26 Al, as opposed to 10 Be≥ 230 Th> 26 Al in the Holocene. They appear at variance with the order 10 Be> 26 Al≈ 230 Th as would be predicted from the relative particle reactivities of the three species. These scavenging characteristics point to 10 Be/ 26 Al as a potentially more suitable proxy for paleoproductivity than 10 Be/ 230 Th ex . Mass-balance considerations for 26 Al and 10 Be show a five-fold LGM-to-Holocene increase in deep-water circulation in the study area, with little change in ocean productivity except during the deglaciation when it increased noticeably.