Neodymium isotopes in benthic foraminifera: Core-top systematics and a down-core record from the Neogene south Atlantic

Records of the past neodymium (Nd) isotope composition of the deep ocean can resolve ambiguities in the interpretation of other tracers. We present the first Nd isotope data for sedimentary benthic foraminifera. Comparison of the ɛNd of core-top foraminifera from a depth transect on the Cape Basin side of the Walvis Ridge to published seawater data, and to the modern dissolved SiO2–ɛNd trend of the deep Atlantic, suggests that benthic foraminifera represent a reliable archive of the deep water Nd isotope composition. Neodymium isotope values of benthic foraminifera from ODP Site 1264A (Angola Basin side of the Walvis Ridge) from the last 8 Ma agree with Fe–Mn oxide coatings from the same samples and are also broadly consistent with existing fish teeth data for the deep South Atlantic, yielding confidence in the preservation of the marine Nd isotope signal in all these archives. The marine origin of the Nd in the coatings is confirmed by their marine Sr isotope values. These important results allow application of the technique to down-core samples. w ɛNd datasets, along with ancillary Cd/Ca and Nd/Ca ratios from the same foraminiferal samples, are interpreted in the context of debates on the Neogene history of North Atlantic Deep Water (NADW) export to the South Atlantic. In general, the ɛNd and δ13C records are closely correlated over the past 4.5 Ma. The Nd isotope data suggest strong NADW export from 8 to 5 Ma, consistent with one interpretation of published δ13C gradients. Where the ɛNd record differs from the nutrient-based records, changes in the pre-formed δ13C or Cd/Ca of southern-derived deep water might account for the difference. Maximum NADW-export for the entire record is suggested by all proxies at 3.5–4 Ma. Chemical conditions from 3 to 1 Ma are totally different, showing, on average, the lowest NADW export of the record. Modern-day values again imply NADW export that is about as strong as at any stage over the past 8 Ma.