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

We have investigated the distributions of 26Al, 10Be, U–Th isotopes, and stable 27Al and 9Be 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 26Al/27Al of <2–8×10−14 (atom/atom), authigenic 10Be/9Be of 0.5–2.6×10−8 (atom/atom), <0.2–1.8×106 atoms/g of 26Al, and 5.3–15.1×108 atoms/g of 10Be in the core over the last glacial/interglacial cycle. These values, as well as total 230Thex/232Th and 10Be/9Be 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 230Th over the two other particle-reactive nuclides, 26Al and 10Be, such that their boundary scavenging during LGM followed the order 230Th>10Be>26Al, as opposed to 10Be≥230Th>26Al in the Holocene. They appear at variance with the order 10Be>26Al≈230Th as would be predicted from the relative particle reactivities of the three species. These scavenging characteristics point to 10Be/26Al as a potentially more suitable proxy for paleoproductivity than 10Be/230Thex. Mass-balance considerations for 26Al and 10Be 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.