A lead isotopic study of circum-antarctic manganese nodules

Abstract

Lead isotope ratios in manganese nodules from the Circum-Antarctic ocean display systematic geographical variations. Eastward, in the direction of current movement, from south of Australia, they progressively decrease in the Pacific sector and into the western Atlantic sector until ∼30°W. From this longitude, lead isotope ratios increase in the eastern Atlantic sector and reach their highest values in the Indian sector near 110°E. South of western Australia they abruptly decrease. These two lead isotope domains correspond closely to the Circum-Antarctic neodymium isotope domains previously recognized by Albarède and Goldstein (1992). However, the intraocean isotopic variations are much clearer for Pb than Nd, reflecting the shorter oceanic residence time of Pb (∼10 2 y) compared with Nd (∼10 3 y), and confirming that Pb is a much more sensitive marine tracer. Throughout the Circum-Antarctic ocean, the lead isotopic changes follow the pattern of present-day oceanic circulation. The Pb shows an overall continental signature, which is strongest in the Indian sector. The most probable candidate as the source of the continental signal is North Atlantic Deep Water (NADW ) which enters the Circum-Antarctic in the south Atlantic. As NADW mixes with Circumpolar water, lead isotope ratios increase eastward in the Indian sector, reflecting progressive loss of the Pacific-derived Pb signal. The abrupt decrease of lead isotope ratios between the Indian and Pacific sectors appears to be a consequence of addition of Pacific seawater. Through the Pacific, lead isotope ratios gradually decrease eastward, approaching values of East Pacific Rise metalliferous sediments, ocean ridge basalts, and south Pacific Mn nodules. This trend results from an increasing influence of Pacific ocean water containing a component derived from Pacific volcanism. The Pacific signature persists into the Argentine basin in the southwest Atlantic Ocean, reflecting significant input of water from the Drake Passage northward into the Atlantic. The Pacific Pb component is seen in the Atlantic and Indian oceans near southern Africa as well as in the Atlantic and Indian sectors of the Circum-Antarctic. Lead and neodymium isotopes correlate in the Atlantic-Indian sectors but not in the Pacific sector, reflecting differences in residence times and modes of water mass mixing. The renewal of Circum-Antarctic water occurs on a timescale which is short compared to the residence time of Nd but long compared to Pb. As a result, dissolved Pb is subject to considerable changes over the timescale of Circumpolar current movement, while Nd is less easily affected. NADW is added locally in the south Atlantic and moves eastward as a water mass. As NADW mixes with Circumpolar water (CPW) through the Indian sector, changes affecting Nd/Pb in NADW and CPW occur in tandem, and neodymium and lead isotopes are correlated. Through the Pacific sector, gradual addition of Pacific seawater occurs over a large region and Nd/Pb relationships are not coupled. Because simple isotopic mixing systematics are not conserved there, neodymium and lead isotopes vary independently. The consistency of lead isotope variations with present-day ocean bottom water circulation is remarkable, considering that the Mn nodules as sampled reflect the integrated flux of Pb over ∼ 10 5-10 6 y, the oceanic mixing time is ∼103 y, the residence time for Pb is ∼10 2 y, and the circulation time of the CircumAntarctic ocean is ∼30 y. Processes occurring over such different timescales can show such a relationship only if the system is stable over the longest time interval. Thus, on average, element sources and ocean circulation through the Pleistocene have been similar to the present-day, despite significant perturbations during glacial periods.