SECULAR VARIATION OF ND AND PB ISOTOPES IN FERROMANGANESE CRUSTS FROMTHE ATLANTIC, INDIAN AND PACIFIC OCEANS

Two ferromanganese crusts from the Indian Ocean and one from the Atlan tic Ocean have been analysed for Be-10/Be-9, Nd-143/Nd-144 and Pb-208, Pb-207,Pb-206/Pb-204 ratios as a function of depth beneath their growt h surfaces. Be-10/Be-9 ratios provide growth rate estimates for these crusts between 1.55 and 2.82 mm Ma(-1) and further suggest that Sr-87/ Sr-86 in crusts do not in any case examined so far provide reliable es timates for growth rates. A crust ALV-539 from 35 degrees N in the wes tern N. Atlantic has epsilon(Nd) and Pb-isotope variations indistingui shable from crust BM-1969.05 from 39 degrees N in the N. Atlantic [K.W . Burton, H.-F. Ling, R.K. O'Nions, Closure of the central American is thmus and its impact on North Atlantic deepwater circulation, Nature ( London) 386 (1997) 382-385] when Be-10/Be-10 ratios are used to estima te growth rates. Both crusts provide evidence for a marked change in d eepwater composition in the western N. Atlantic with a reduction in ep silon(Nd) and an increase in Pb-206/Pb-204 from similar to 8 Ma ago to wards the present day, The two crusts from the Indian Ocean show compa ratively small variations in epsilon(Nd) between -8.0 and -7.0 over th e last 20 Ma and do not show the large shift in epsilon(Nd) seen in th e Atlantic crusts, Comparison of epsilon(Nd) in the crusts analysed he re with those published previously [H.-F. Ling, K.W. Burton, R.K. O'Ni ons, 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. L ing, R.K. O'Nions, Closure of the central American isthmus and its imp act on North Atlantic deepwater circulation, Nature (London) 386 (1997 ) 382-385] shows that provinciality in the present-day epsilon(Nd) str ucture of the Pacific, Atlantic and Indian Oceans has been maintained over similar to 20 Ma or more despite the palaeogeographic changes tha t have occurred within this period. These include the closure of the P anama gateway and the uplift of the Himalayas. Superimposed on this br oad inter-ocean structure are changes in epsilon(Nd) of the western N. Atlantic which may relate to the Panama gateway closure and shifts in the epsilon(Nd) of equatorial Pacific deepwater from 3-5 Ma ago. The absence of any such structure in epsilon(Nd) of the southwest and cent ral Indian Ocean suggests that Himalayan erosion products such as pres erved in the Bengal Fan sediments have not contributed significantly t o Indian Ocean deepwater over the last 20 Ma. There is no straightforw ard relationship between records of Sr-87/Sr-86 in the global ocean an d epsilon(Nd) in ocean deepwater as would be expected if inputs of rad iogenic Sr and unradiogenic Nd were coupled. (C) 1998 Elsevier Science B.V.