Secular variation of Nd and Pb isotopes in ferromanganese crusts from the Atlantic, Indian and Pacific Oceans

Abstract

Two ferromanganese crusts from the Indian Ocean and one from the Atlantic Ocean have been analysed for 10Be/ 9Be, 143Nd/ 144Nd and 208,207,206Pb/ 204Pb ratios as a function of depth beneath their growth surfaces. 10Be/ 9Be ratios provide growth rate estimates for these crusts between 1.55 and 2.82 mm Ma −1 and further suggest that 87Sr/ 86Sr in crusts do not in any case examined so far provide reliable estimates for growth rates. A crust ALV-539 from 35°N in the western N. Atlantic has ϵ Nd and Pb-isotope variations indistinguishable from crust BM-1969.05 from 39°N in the N. Atlantic [K.W. Burton, H.-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] when 10Be/ 10Be ratios are used to estimate growth rates. Both crusts provide evidence for a marked change in deepwater composition in the western N. Atlantic with a reduction in ϵ Nd and an increase in 206Pb/ 204Pb from ∼8 Ma ago towards the present day. The two crusts from the Indian Ocean show comparatively small variations in ϵ Nd between −8.0 and −7.0 over the last 20 Ma and do not show the large shift in ϵ Nd seen in the Atlantic crusts. Comparison of ϵ Nd in the crusts analysed here with those published previously [H.-F. Ling, K.W. Burton, R.K. O'Nions, B.S. Kamber, F. von Blanckenburg, A.J. Gibb, J.R. Hein, Evolution of Nd and Pb isotopes in central Pacific seawater from ferromanganese crusts, Earth Planet. Sci. Lett. 146 (1997) 1–12; K.W. Burton, H-F. Ling, R.K. O'Nions, Closure of the central American isthmus and its impact on North Atlantic deepwater circulation, Nature (London) 386 (1997) 382–385] shows that provinciality in the present-day ϵ Nd structure of the Pacific, Atlantic and Indian Oceans has been maintained over ∼20 Ma or more despite the palaeogeographic changes that have occurred within this period. These include the closure of the Panama gateway and the uplift of the Himalayas. Superimposed on this broad inter-ocean structure are changes in ϵ Nd of the western N. Atlantic which may relate to the Panama gateway closure and shifts in the ϵ Nd of equatorial Pacific deepwater from 3–5 Ma ago. The absence of any such structure in ϵ Nd of the southwest and central Indian Ocean suggests that Himalayan erosion products such as preserved in the Bengal Fan sediments have not contributed significantly to Indian Ocean deepwater over the last 20 Ma. There is no straightforward relationship between records of 87Sr/ 86Sr in the global ocean and ϵ Nd in ocean deepwater as would be expected if inputs of radiogenic Sr and unradiogenic Nd were coupled.