D RESEARCH ARTICLES tized and cut by carbonate fluorapatite Climate and Ocean Dynamics (CFA) veins. Cut Dll-l (11°39'N, 161°41'E) is from the Marshall Islands at a and the Lead Isotopic Records in locality 3250 km to the west of that of CD29-2. Dl 1-1 was recovered from a depth Paci f ic Ferromangan ese Crusts of 1.8 km, is -15 cm thick, and was depos- ited on an altered hvaloclastite substrate. John N. Christensen, Alex N. Halliday, Linda V. Godfrey, James R. Hein, David K. Rea As hydrogenous iron-manganese crusts grow, at rates of millimeters per million years, they record changes in the lead isotopic composition of ambient seawater. Time-re- solved lead isotopic data for cut slabs of two central Pacific iron-manganese crusts that have been growing since about 50 million years ago were measured in situ by laser ablation, multiple-collector, inductively coupled plasma mass spectrometry. The lead isotopic compositions have remained remarkably uniform over the past 30 million years, but the record of small variations corresponds with other paleoceanographic indicators of climatechange, including weathering and glaciation. This implies that despitethe short residence time of lead in the oceans, global mechanisms may influence lead isotopic compositions in the central Pacific, far from continental inputs, because of changes in weathering, ocean circulation, and degree of mixing. Thus lead isotopic data could be used to probe climate-driven changes in ocean circulation through time. The dynamics of Earth's atmosphere-ocean system are controlled in part by the pole- ward energy transfer mediated by major ocean currents. Furthermore, changes in continental weathering and riverine contri- butions to the oceans may also be linked with climate, and hydrothermal inputs vary with tectonism. A record of these past changes should be retrievable from the vari- ations in isotopic compositions of elements in pristine seawater precipitates (chemical sediments) such as carbonates and hydrog- enous Fe-Mn crusts (1-7). However, inter- pretations of these records have often been controversial. Until recently, little use has been made of records of Pb isotopic varia- tions in the oceans, because the recent record is strongly affected by anthropogenic contributions and because a well-preserved record has been difficult to acauire. Lead and Nd isotopic records from Fe-Mn crusts have been obtained bv thermal ionization mass spectrometry (4, 6). The new tech- niaue of laser ablation. multivle-collector. inductively coupled plasma mass spectrom- etry (LA-MC-ICPMS) has the advantage over conventional Pb isotopic techniques in that rapid and reproducible high-preci- sion Pb isotopic data can be acquired direct- ly from a cut slab of Fe-Mn crust (8). Using this techniaue. we show here that the Pb in . , central Pacific Ocean deep water, well away from the continents, fluctuated in its isoto- pic composition over the past 50 million years and that these fluctuations are similar to records of major paleoceanographic and paleoclimatic changes. Ferromanganese crusts. The Fe-Mn crusts we studied were dredged from central Pacific seamounts. Both of these crusts have been analyzed for Sr, Nd, Pb, and Be isoto- pic compositions (4, 6, 9). Crust CD29-2 (16"42.4'N, 168'14.2'W) was recovered from a depth of 2.3 km from Karin Ridge in the Johnston Island Economic Exclusion Zone (Line Islands) south of the Hawaiian Ridge (Fig. 1). It is -10 cm thick, well preserved, and formed on a substrate of altered hyaloclastite that is partly phospha- Layers in CD29-2 coAtain from 89 to 96 volume % 6-Mn02 (vernadite); 1 to 4% quartz plus plagioclase; and, below 50 mm from the crust surface, 7 to 10% CFA. Layers in D11-1 vary from 71 to 100% 6-Mn02; 1 to 3% quartz plus plagioclase; and, below 40 mm from the crust surface, 14 to 29% CFA (10). In both crusts, iron content varies between 9 and 15 weight %, Mn varies from 18 to 20 weight %, and Pb varies from 670 to 1500 parts per million (ppm). Growth rates of Fe-Mn crusts can be measured with the use of 1°BePBe data, and reliable ages extend back to approximately 10 million years ago (Ma). Measured growth rates (4) are 2.1 mm per million years (My) for crust CD29-2 (since 10 Ma) and 1.4 mm/My (since 6 Ma) and 2.7 mm/ My (from 7 to 10 Ma) for D11-1. L' lnear extrapolation of these growth rates would indicate that the crusts started growing about 55 to 60 Ma (4, 11). At that time, CD29-2 and D11-1 were in a larger ocean than now and were located close to the equator, on the basis of reconstructed Pa- cific plate motions (1 2). Lead isotope stratigraphy. The results of LA-MC-ICPMS isotopic analyses (1 3) (Fig. 2) agree very well with conventional Pb isotopic data for corresponding positions, as demonstrated for CD29-2, and can be con- verted into well-defined changes in compo- sition with time (Fig. 3) with the use of the respective growth rates (4). With a growth rate of 2.1 mm/My for crust CD29-2, each analysis raster represents the Pb isotopic 70's J. N. Christensen, A. N. Halliday, and D. K. Rea are at the Deoartment of Geoloaical Sciences. Universitv of Michi- aan. 2534 C. C. ~itti;e Buildina. Ann Arbor. MI 48109. 4CE 8rJE '2OE '60E 160'W 120W 83bV 40'W ~SA. L. V. Godfrey is at the &partme; of ~eol&kg sciences, ,-omell University, Ithaca, NY ,4853, USA, J, Fig. 1. Map of the Pacific Ocean showing the sample locations of the two Fe-Mn crusts. Schematic R. Hein is with the U.S. Geological Survey, 345 Middle- representations of surfacecurrents are shown as grey arrows, and deep water flow is shown as black field Road, MS999, Menlo Park, CA 94025, USA. arrows (38). www.sciencemag.org SCIENCE VOL. 277 15 AUGUST 1997 91 3