Recording changes in ENADW composition over the last 340 ka using high-precision lead isotopes in a Fe–Mn crust

Abstract

We report high-precision Pb triple spike data and U–Th isotopic ages on a high-resolution depth profile from a Fe–Mn crust (121DK, northeastern Atlantic 24°N, 21°W, 2000 m). Our results bring important information particularly on the origin of the Pb isotopic composition of the northeastern Atlantic deep water and more generally on the use of Pb isotopes as a paleo-proxy to reconstruct weathering inputs to the north Atlantic Ocean. Growth rates determined using ( 230Th) ex and ( 230Th/ 232Th) ex (2.9±0.2 and 3±0.3 mm/Ma respectively) are found to be more reliable than ( 234U ex), sensitive to sea water contamination, and are similar to the 10Be results (3±0.3 mm/Ma). Very small variations in 206Pb/ 204Pb (18.942–18.964), 208Pb/ 204Pb (39.057–39.083) and 207Pb/ 204Pb (15.696–15.701) are resolved, defining linear trends in Pb isotope diagrams. High-precision Pb triple spike analysis of samples from crust BM1969.05, northwestern Atlantic (39°N, 61°W, 1829 m), are found to entirely reproduce the multi-collector plasma source mass spectrometry data recently published. This interlaboratory cross-calibration establishes that the west north Atlantic deep water (WNADW) does not represent a suitable Pb contributor to the northeastern Atlantic and rather suggests that the Pb isotopic composition of WNADW is significantly altered before or during transport toward the northeastern Atlantic basin. The Pb isotopic compositions of both suspended loads from the Amazon mouth and offshore sediment cores are consistent on average with crust 121DK. We therefore propose that the change in Pb isotopic composition of WNADW may be caused by input of suspended particle loads from the Amazon and possibly Orinoco rivers or by interaction with sediments deposited by those rivers. We speculate further that the short-term compositional variations observed in this study may be caused by fluctuations in the relative contributions of Amazon tributaries to the bulk composition supplied to the ocean because these tributaries sample terrains of drastically different ages (Phanerozoic Andes and Precambrian Amazonian shield).