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ELSEVIER Earth and Planetary Science Letters 158 (1998) 121–130 Sources of Pb for Indian Ocean ferromanganese crusts: a record of Himalayan erosion? M. Frank Ł , R.K. O’Nions Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK Received 4 December 1997; accepted 17 March 1998 Abstract A high resolution Pb isotope time-series for the last 26 Ma, dated by 10 Be=9 Be chronology, is reported for a north Indian Ocean ferromanganese crust. This record is compared with available Pb isotope time-series of six other crusts from the Atlantic, Indian and Pacific Oceans, each of which is based on 10 Be=9 Be chronology. The seven Pb isotope records reveal some remarkable features. In contrast to the Nd isotope time-series of these crusts which show a long-term (¾60 Ma) provinciality between the three main ocean basins, the Pb isotopes only show comparable provinciality over the last ¾5 Ma. Prior to about 15 Ma ago no distinct Indian Ocean Pb isotope signal existed. Within this established framework of Pb isotope distribution in the oceans the 208 Pb=206 Pb data for the north Indian Ocean crust reported here are anomalous. The 208 Pb=206 Pb ratio is particularly high and exceeds a value of 2.08 during the time interval from 20 to 8 Ma ago. Consideration of potential sources of Pb in the Indian Ocean which might provide such high 208 Pb=206 Pb ratios suggests that this crust most probably has recorded a time-varying erosional input of Pb from the Himalayas. The timing of the isotopic shift is in good agreement with maximum Himalayan exhumation rates deduced from crystallisation and cooling ages of synorogenic granites (20–14 Ma) and the sedimentation history of the Bengal Fan. 1998 Elsevier Science B.V. All rights reserved. Keywords: Himalayan Orogeny; erosion; Pb-208=Pb-206; ferromanganese composition; crust; Indian Ocean 1. Introduction Ferromanganese crusts (hereafter called crusts) scavenge and incorporate trace metals from ambient seawater and thereby record its isotopic composition with respect to Nd and Pb [1–10]. Nd and Pb isotope profiles of crusts, dated using 10 Be=9 Be ratios, make it possible to relate isotopic variations in sea water at a particular location to changes in the supply and distribution of Nd and Pb in the oceans. It Ł Corresponding author. Tel.: C44 1865 272055; Fax: C44 1865 272072; E-mail: [email protected] is now evident that the present-day provinciality of eNd values in the three main ocean basins has been a persistent feature over the last ¾60 Ma [10] and Nd isotope stratification in the Pacific Ocean has existed for ¾30 Ma [6] despite the major paleogeographic changes that have occurred over this period of time. These include, for example, the closing of the Panama Gateway to deep-water circulation starting from about 12 to 8 Ma ago [11] to its final closure about 3 Ma ago [12], and the opening of the Drake Passage about 23 Ma ago [13]. Any variations arising from changes in ocean circulation are superimposed on the long-term eNd signature typical 0012-821X/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 1 2 - 8 2 1 X ( 9 8 ) 0 0 0 5 5 - 7 122 M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 for each ocean basin. Notable amongst these changes are the decrease in eNd during the last about 5 Ma in the western North Atlantic Ocean and the difference in eNd between the Atlantic and the Pacific Oceans which has increased continuously from about 6 eNd prior to 20 Ma ago to about 9–10 eNd in the present-day ocean [10]. On a global scale, changes in rates of erosion such as those accompanying the main period of Himalayan exhumation, which started about 24 Ma ago [14], have been considered a possible cause for the long-term increase in the marine 87 Sr=86 Sr ratio (cf. Ref. [15]). However, in marked contrast, it has been shown recently that Himalayan erosion products have not had a significant impact on either the global nor local Indian Ocean deep-water budget of Nd [10]. There is much less information available on Pb isotope input into the oceans, but the pre-anthropogenic distribution of Pb isotopes (of the last about 300 ka) is well-known and also shows a clear interocean provinciality [3,4]. The distinction between the three ocean basins with respect to Pb isotopes might be expected to have been even greater than for Nd over the last 60 Ma because of the shorter average oceanic residence time of Pb (about 80–100 years [16,17]), compared to Nd (about 1000 years [1,18,19]), but as shown below this was not the case. In this study the Pb isotope time-series of crust SS-663 (northern Indian Ocean) and six other crusts for which Nd, Pb and Be isotope profiles have been published previously [6–8,10], provide new constraints on the Pb isotopic evolution of the Atlantic, Indian and Pacific Oceans over the last ¾60 Ma. The results of this study suggest that material supplied by erosion of the Himalayas has had a major influence on the Pb isotope composition of northern Indian Ocean deep-water during the last about 26 Ma. Table 1 Results of the 10 Be=9 Be measurements and growth rate determinations for crust SS-663 Depth (mm) 10 Be=9 Be Surface 0.5–1.2 3.5–4.5 7.0–8.5 14.7–16.0 22.0–23.5 31.5–33.0 8:75 š 0:70 7:86 š 0:27 4:35 š 0:14 2:60 š 0:10 0:70 š 0:04 0:28 š 0:03 0:04 š 0:02 (1E-8) Age (Ma) 0.00 0:23 š 0:01 1:52 š 0:05 2:64 š 0:10 5:49 š 0:32 7:52 š 0:75 11:67 š 4:73 the initial 10 Be=9 Be ratio at the growth surface of the crust has remained constant. This is in good agreement with growth rates between 2.2 and 3.4 mm Ma 1 derived from decay profiles of 230 Thexcess and 230 Thexcess =232 Th of the uppermost 2 mm of this crust [22]. A high-resolution (37 sample) Pb isotope profile has been obtained from the outer 56 mm of the crust using TIMS techniques described 2. Results 10 Be=9 Be ratios have been determined on a depth profile comprising seven samples of ferromanganese crust SS-663 from the northern Indian Ocean (12º570 S, 76º060 E, water depth 5250 m) using procedures established on the Oxford ISOLAB [20,21] (Table 1). An average growth rate of 2:8š0:1 mm Ma 1 (Fig. 1) has been estimated assuming that Fig. 1. 10 Be=9 Be ratios in crust SS-663 from the northern Indian Ocean versus depth beneath the growth surface. The ages are calculated with a half life for 10 Be of 1.5 Ma. M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 123 Table 2 Pb isotope results of crust SS-663 Depth (mm) Age (Ma) a 206 Pb=204 Pb b 207 Pb=204 Pb b 208 Pb=204 Pb b 0.0–0.4 1.1–1.5 2.0–2.4 2.6–3.0 3.4–3.8 3.4–3.8 c 4.4–4.8 5.5–5.9 6.7–7.1 6.7–7.1 d 8.2–8.6 10.0–10.4 11.3–11.7 12.7–13.1 12.6–13.0 d 14.8–15.2 15.9–16.3 17.0–17.4 18.0–18.4 19.5–19.9 19.9–20.3 21.0–21.4 22.1–22.5 23.2–23.6 24.5–24.9 25.6–26.0 28.3–28.7 30.0–30.4 32.2–32.8 33.8–34.2 34.9–35.3 36.6–37.0 38.9–39.3 41.5–41.9 47.8–48.2 50.7–51.3 55.0–56.0 0.0 0.5 0.8 1.0 1.3 1.3 1.6 2.0 2.5 2.5 3.0 3.6 4.1 4.6 4.6 5.4 5.8 6.1 6.5 7.0 7.2 7.6 8.0 8.4 8.8 9.2 10.2 10.8 11.6 12.1 12.5 13.1 14.0 14.9 17.1 18.2 19.8 18.881 18.903 18.904 18.879 18.912 18.902 18.887 18.864 18.785 18.859 18.853 18.828 18.831 18.880 18.851 18.837 18.815 18.813 18.784 18.764 18.779 18.749 18.777 18.763 18.733 18.746 18.773 18.769 18.787 18.723 18.756 18.749 18.676 18.638 18.616 18.621 18.655 15.712 15.715 15.716 15.690 15.723 15.714 15.718 15.696 15.679 15.693 15.698 15.673 15.685 15.735 15.699 15.710 15.700 15.707 15.679 15.669 15.691 15.677 15.721 15.712 15.675 15.686 15.701 15.702 15.731 15.702 15.708 15.730 15.675 15.673 15.653 15.663 15.664 39.167 39.160 39.173 39.093 39.192 39.168 39.174 39.096 38.992 39.095 39.117 39.034 39.076 39.240 39.115 39.139 39.101 39.126 39.045 39.000 39.056 39.003 39.148 39.120 39.001 39.025 39.084 39.091 39.182 39.047 39.101 39.135 38.926 38.877 38.776 38.808 38.851 a Ages are calculated by extrapolation of growth rates defined by 10 Be=9 Be ratios in the outer part of the crust. Ages in italics were estimated from extrapolation of the 10 Be=9 Be-derived growth rates beyond 12 Ma. b Repeated measurements of the NIST SRM981 Pb standard during the period of this study yields 206 Pb=204 Pb D 16:902 š 0:019, 207 Pb=204 Pb D 15:451 š 0:023, 208 Pb=204 Pb D 36:576 š 0:070 .n D 14/. Errors represent the external 2σ standard deviation and Pb isotope ratios have been adjusted relative to the accepted value [41]. The 2σ errors of repeat measurements of samples during the period of this study were better than for the repeated standard measurements (š0.012 for 206 Pb=204 Pb, š0.015 for 207 Pb=204 Pb and š0.044 for 208 Pb=204 Pb; n D 18) but nevertheless the external 2σ error was applied. c Repeat measurement of the same sample solution. d Repeat measurements of sample aliquots. previously [23,24]. The high-resolution Pb isotope profile (Table 2, Fig. 2) is in very good agreement with a published low-resolution profile for the same crust [10] and has been extended using the low- ermost three samples from this source. Assuming that crust SS-663 has grown at the rate defined by the 10 Be=9 Be ratios over the entire profile, then the Pb isotope profile in Fig. 2 has recorded the com- 124 M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 position of Indian Ocean deep-water from 26 Ma ago until present. The application of a modified Coconstant flux-based dating technique for this crust [Frank et al., in prep.] shows good agreement with the 10 Be=9 Be-based results for the last 10 Ma, but suggests that the growth rate before 10 Ma ago was somewhat lower, which results in an age of about 30 Ma for the base of the crust. The Pb isotope variations for crust SS-663 are displayed in Fig. 2 in terms of 206 Pb=204 Pb, 207 Pb=206 Pb, and 208 Pb=206 Pb ratios together with the Pb isotope time-series available for other crusts [6–8,10]. A particular and outstanding feature of crust SS-663 is the large increase of the 208 Pb=206 Pb ratio from the base of the crust to a broad peak between 20 and 8 Ma years ago with a maximum ratio of 2.088 at about 13 Ma ago followed by a general decrease until present. 3. Discussion 3.1. Mixing of Nd and Pb in the ocean The Pb isotope time-series available for six crusts from the Atlantic, Indian and Pacific Oceans, each based on their respective 10 Be=9 Be chronologies, are compared with crust SS-663 in Fig. 2. In contrast to the Nd isotope time-series obtained for these crusts [6,7,10], the Pb isotope profiles show much less provinciality over the last 60 Ma. This is, perhaps, contrary to expectations from residence times of Nd and Pb in the oceans. A similar observation was made for crusts from the Pacific Ocean, where the Pb isotopic composition of surface scrapings have a remarkably low variance compared to that of the compositions of the surrounding continental and island arc rocks [4]. If there is no other, and unaccounted for, source of Pb which determines the low variance of Pb isotope composition in the 125 Table 3 Correlation coefficients between eNd and Pb isotopic ratios in crusts over the last 30 Ma Crust BM1969.05 ALV539 109D-C SS663 VA13=2 CD29-2 D11-1 206 Pb=204 Pb 0.90 0.93 0.69 0.81 0.54 0.94 0.87 207 Pb=206 Pb 0.89 0.92 0.64 0.79 0.66 0.70 0.92 208 Pb=206 Pb 0.77 0.87 0.39 0.52 0.27 0.23 0.59 The values of crusts CD29-2 and D11-1 only represent the unphosphatized sections. Bold face values are significant at the 99% level. Pacific, then these data imply that an efficient mixing process must exist for Pb in the Pacific; it may also suggest that the average residence time of Pb in the oceans is longer than previously thought. A longer residence time for Pb would imply a larger advective length scale within the thermohaline circulation. Comparison of Nd and Pb isotope variations in crusts lends some support to this view. From the 30 Ma isotope records of the seven crusts available, the correlation coefficient between eNd and 208,207,206 Pb=204 Pb changes sign when going from the North Atlantic through the Indian Ocean into the Pacific (Table 3, Fig. 3). In the two North Atlantic crusts, eNd and 206 Pb=204 Pb show a strong negative correlation and the opposite is true for eNd and the 208 Pb=206 Pb and 207 Pb=206 Pb ratios. These relationships reflect the variability of the input of radiogenic Pb and unradiogenic Nd from continental North America into the North Atlantic. Crust 109 D-C from the Indian sector of the Southern Ocean shows the same pattern of correlations, albeit at a lower statistical significance. These characteristics may be inherited from the influence of North Atlantic Deep-water (NADW) which is mixed with Fig. 2. Comparison of (a) 206 Pb=204 Pb, (b) 207 Pb=206 Pb, and (c) 208 Pb=206 Pb ratios of ferromanganese crusts with ages based on 10 Be=9 Be chronology. The North Atlantic crusts have diamond symbols [7,10], the southern Indian Ocean crust open squares [10], northern Indian Ocean crust closed squares and bold lines, and deep Pacific crust open circles [6]. In (a) the filled circles mark the data of the Pacific seamount crusts [6] and in (b) and (c) the filled circles mark the low-resolution data of sections older than 32 Ma of the Pacific seamount crusts [6]. Those profiles without any symbols represent the high-resolution records of the two Pacific seamount crusts [8]. The shaded area highlights the last 5 Ma which is the time over which separate Atlantic, Indian and Pacific Ocean signatures of the Pb isotopes in the water column have existed. The lightly shaded area in (c) indicates a period of maximum 208 Pb=206 Pb ratios in the northern Indian and Pacific crusts (20–8 Ma ago). 126 M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 servations support the notion that the Pb isotopic signal produced in the North Atlantic is traceable along the path of the thermohaline circulation through the Atlantic Ocean [4] at least into the Indian sector of the Southern Ocean [3]. In the Pacific Ocean input of material with an unradiogenic Pb and radiogenic Nd isotopic composition has exerted a different control to that observed in the Atlantic Ocean. 3.2. Pb isotopic evolution of the ocean Fig. 3. 206 Pb=204 Pb vs. eNd and 207 Pb=206 Pb vs. eNd for all seven crust profiles. Sources for the Pb isotope data and symbols are the same as in Fig. 2. Circum Polar Deep-water (CDW) on its way to the location of crust 109 D-C. In contrast, at the location of crust VA13=2, which grew from deep-water in the equatorial Pacific, the sign of the correlation coefficients changes. There is a weak positive correlation between eNd and 206 Pb=204 Pb and either a weak negative or no correlation with 207 Pb=206 Pb and 208 Pb=206 Pb ratios, respectively. The two intermediate water depth records in the equatorial Pacific (CD29-2 and D11-1) display a strong positive correlation between eNd and 206 Pb=204 Pb and a significant negative correlation between eNd and 207 Pb=206 Pb. The correlation between eNd and the 208 Pb=206 Pb ratio is not significant. In the northern Indian Ocean crust SS-663 the correlation pattern of the Nd and Pb isotopes is similar to that of the intermediate water depth records from the equatorial Pacific. These ob- In contrast to the distinctive eNd distribution between the Atlantic and Indian Oceans [10], 206 Pb=204 Pb and 207 Pb=206 Pb have been distinct only from ¾5 Ma ago (Fig. 2). Prior to that time no clear provinciality is evident. The cause of the major shift in Atlantic Pb isotopes from 5 Ma ago is not fully understood. It is possible that the closure of the Panama Gateway may have contributed to the development of a strong separate Pb isotopic signature in the western North Atlantic [7,10]. However, the development of the Atlantic signature was probably further amplified by an increase in detrital input into the Atlantic Ocean during the course of the Northern Hemisphere glaciation starting 3.2–2.6 Ma ago [25,26]. This material would have generally high 206 Pb=204 Pb values and low 208 Pb=206 Pb and 207 Pb=206 Pb values derived from continental North America. This influence is reflected in the steep gradient of Pb isotope change in the two North Atlantic crusts and is supported by the correlated Nd isotopic records [7,10]. In further support of this view, recently published Nd and Pb isotope records of authigenic components in sediment cores from the Arctic Ocean [27] are similar to those from the western North Atlantic crusts and emphasize the importance of detrital material supplied by the glaciers from North America in determining the Pb and Nd isotopic composition of the deep North Atlantic Ocean. From 45 to 5 Ma ago the Pb isotopic signatures of the Atlantic and Pacific Ocean crusts were persistently different. There is an indication that before about 45 Ma ago the Pb isotope ratios of the Pacific crusts D11-1 and CD29-2, although clearly distinct for each crust [8], were more similar to Atlantic-type values [6,8]. The similarity of those Pb isotope ratios in the Atlantic and Pacific crusts (Fig. 2) is consistent with an efficient exchange of water masses M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 through an open ocean between North and South America at that time although more work is required to address this issue fully. The Pb isotopic signal of Pacific deep-water crust VA13=2 remained separate from the Atlantic-type values, possibly due to a hydrothermal influence at that time [Frank et al., in prep.] Crust 109 D-C from the southern part of the Indian Ocean, south of the Mid-Indian Ridge, which is at present bathed either in CDW or Antarctic Bottom Water (AABW) [28,29], has 206 Pb=204 Pb and 207 Pb=206 Pb ratios which are indistinguishable from values for Atlantic Ocean crusts prior to 5 Ma ago. They correspond quite well to the present-day signature of the CDW in the Atlantic and Indian sectors of the Southern Ocean [3]. In marked contrast, crust SS663 has recorded 206 Pb=204 Pb and 207 Pb=206 Pb ratios indistinguishable from the Pacific time-series prior to ¾15 Ma ago which may well reflect a relatively unrestricted circulation between the Pacific and Indian Oceans. In addition, the relationship between 208 Pb=206 Pb and 207 Pb=206 Pb in this crust suggests a secular change from a more Pacific-type character 26–16 Ma ago to one located in the field of the Indonesian Arcs after 16 Ma ago [10]. The evolution and weathering of the Indonesian Arcs and the closure of the Indonesian Passage for deep-water flow ¾21 Ma ago [30] were most probably responsible for these changes. 3.3. Pb sources for the northern Indian Ocean: Himalayan erosion? The most striking feature of the Pb isotope timeseries of the seven crusts summarized in Fig. 2 is the high 208 Pb=206 Pb ratios of northern Indian Ocean crust SS-663 with the broad peak in the period from 20 to 8 Ma ago which shows even higher values than those recorded by the Pacific crusts. The 208 Pb=206 Pb ratios and to a lesser extent also the 207 Pb=206 Pb ratios in this crust are anomalous relative to other Indian Ocean Pb isotope data which are intermediate between the Atlantic and Pacific values — they monitor an input of Pb from a source with a distinct Pb isotope composition. An obvious candidate to be considered is the erosional input associated with the Himalayan uplift which occurred at approximately the same time. Although direct information about 127 the Pb isotopic composition of the input into the Indian Ocean originating from Himalayan weathering is unavailable at present, there is some evidence in support of this view. It has been estimated that about 80% of the detritus deposited in the Bengal Fan sediments over the last about 20 Ma has been derived from the high-grade metasedimentary rocks of the High Himalayan Crystalline sequence [31]. Pb isotope analyses of these metasediments and associated leucogranites in Ref. [32] and compiled in Ref. [33] from Refs. [34–36] form arrays in plots of 207 Pb=204 Pb vs. 206 Pb=204 Pb and 208 Pb=204 Pb vs. 206 Pb=204 Pb which are distinct from all ferromanganese crust records, with the array of SS-663 being the closest (Fig. 4). These observations show that the variability of the Pb isotope record of crust SS663 has been controlled by Pb derived either from weathering of the Himalayan rocks or a source with comparable Pb isotope composition. An examination of other potential sources of Pb input into the Indian Ocean has so far failed to reveal any with such unusual Pb isotope compositions. It has long been known that Indian Ocean basalts have in general relatively high 208 Pb=204 Pb and 207 Pb=204 Pb ratios [37] compared with other oceanic basalts. Hart [38] has viewed these as part of a largescale isotope anomaly in the Southern Hemisphere mantle, the so-called Dupal anomaly. A comparison of Pb data from Indian Ocean basalts showing this anomaly with the record of crust SS-663 suggests that these basalts are an unlikely source. Although some basalt samples do have ratios as high as those in crust SS-663, the corresponding 206 Pb=204 Pb ratios are much too low. Similarly volcanics from the Indonesian Arcs have Pb isotope compositions which cannot account for the 208 Pb=204 Pb and 207 Pb=204 Pb variability of SS-663 whilst the Pb data of this crust are in good agreement with the pelagic sediment arrays from the Indian Ocean compiled in [4]. Pb isotope data from eastern Africa are scarce and highly variable. Although it remains possible that a further search may yield other options, a Himalayan source is certainly viable, particularly because the Bengal and Indus Fans are the most important detrital inputs into the northern Indian Ocean. Assuming that the 10 Be=9 Be dating of crust SS663 is correct back to 26 Ma ago, then our data suggest that the maximum erosional input from the 128 M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 Himalayas occurred between about 20 and 8 Ma ago (Fig. 2) whilst the Co-based dating [Frank et al., in prep.] gives an age of about 25 Ma for the beginning of the 208 Pb=206 Pb peak. Both estimates correspond quite well to ages derived from crystallization and cooling ages of various synorogenic granites in the Himalaya, which suggest that a phase of rapid exhumation and concomitant erosion started about 24 Ma ago and reached its maximum between 20 and 14 Ma ago [14]. During this period up to 30 km of crustal thickness are estimated to have been removed from the High Himalaya by erosion [39]. The ferromanganese crust data also concur with results derived from isotopic and clay mineralogical inves- tigations of Bengal Fan sediments deposited during the last 20 Ma, which show that the sedimentation rates of the Bengal Fan decreased significantly at about 7.4 Ma ago, parallel with a change from a physical to a more chemical weathering regime [40]. Pb isotope ratios obtained from the surfaces of Indian Ocean crusts, which represent the pre-anthropogenic Pb isotope distribution in the oceanic deep-water, yield a similar but less pronounced trend as that shown in Fig. 4 when compared to the data from the Pacific and Atlantic [4] and thus corroborate the importance of Himalayan erosion also for today’s Pb isotopic signature of the Indian Ocean. eNd values for the Bengal Fan sediments (18–0 Ma) are 16 š 2 [40], which is much lower than the values of 7 to 8 found for the two Indian Ocean crusts [10]. Thus, Nd derived from Himalayan erosion does not appear to have contributed in a major way to the Nd budget of northern Indian Ocean deep-water although a small negative shift of about 0.6 eNd coincides with the 208 Pb=206 Pb maximum in crust SS-663. Thus, whereas large quantities of Nd and Pb from the Himalayas end up in the sediments of the Bengal Fan, apparently dissolved Nd from this source is too low to have a dominating effect on Indian Ocean deep-water (or for that matter surface water) isotopic composition [10] as obviously is the case for Pb. Thus, there is an apparent decoupling between Nd and Pb. Whether or not Nd is retained more efficiently in the Bengal Fan sediments than is Pb due to different adsorption behaviour depends on the mass balance between Pb and Nd in the erosional input on the one hand and the dissolved Pb and Nd in Indian Ocean deep-water on the other. In addition, more particle reactive Pb is more efficiently scavenged from the water column and therefore a local isotopic signal such as from the Himalayas may only be recorded close to the source of input whereas the Nd isotopic signal might be mixed and diluted. 4. Conclusions 207 Pb=204 Pb 206 Pb=204 Pb 208 Pb=204 Pb Fig. 4. vs. and vs. 206 Pb=204 Pb ratios for all seven crust profiles. Sources for the Pb isotope data are the same as in Fig. 2. The arrays for the Himalayan gneisses and leucogranites are taken from Ref. [32] and the compilation in Ref. [33]. Time-series of seven ferromanganese crusts demonstrate that there was no distinctive Indian Ocean Pb isotope signal prior to about 15 Ma ago. The presently observed separation into Pacific, In- M. Frank, R.K. O’Nions / Earth and Planetary Science Letters 158 (1998) 121–130 dian and Atlantic-type values has only existed for the last about 5 Ma. This is contrary to the expectations from the pronounced long-term (¾60 Ma) provinciality of Nd isotope time-series when comparing published estimates of the respective oceanic residence times of both elements. It is suggested that Pb isotopes have been more sensitive to paleoceanographic changes such as the development of the Indonesian Arcs and the stepwise evolution from an equator-dominated global circulation system during the Tertiary towards the present mode. Particularly high 208 Pb=206 Pb ratios in a crust from the northern Indian Ocean are suggested to represent a record of varying erosional input from the uplift of the Himalyas. The start of the peak ratios at about 25–20 Ma ago is in good agreement with maximum exhumation rates deduced from crystallisation and cooling ages of synorogenic Himalayan granites (20–14 Ma) and the termination of the peak ratios at about 7.4 Ma agrees well with a marked decrease of the sedimentation rates in the Bengal Fan. The fact that the Nd isotopes in the same crust do not show a strong influence of a Himalayan contribution can possibly be explained by a more efficient lateral mixing of Nd in the Indian Ocean compared with Pb which shows a strong impact close to the source of the input signal. On the other hand the mass balance between Nd and Pb in the eroded material and the Indian Ocean deep-water may be responsible for the apparent decoupling of Pb and Nd isotopes. Acknowledgements This work was funded by a HCM grant of the EU and a contract within the TMR network program ‘The Marine Geochemical Record of Continental Tectonics and Erosion’ of the EU to M.F. We acknowledge J.R. Hein, US Geological Survey, Menlo Park, CA and V. Banakar, National Institute of Oceanography, Goa, India for kindly providing material of crust SS-663. We wish to thank Balz Kamber and Friedhelm von Blanckenburg for discussions and their help with the mass spectrometer and Mike Searle for discussions. 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