CLIMATE AND OCEAN DYNAMICS AND THE LEAD ISOTOPIC RECORDS IN PACIFIC FERROMANGANESE CRUSTS

As hydrogenous iron-manganese crusts grow, at rates of millimeters per million years, they record changes in the lead isotopic composition o f ambient seawater. Time-resolved 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, m ultiple-collector, inductively coupled plasma mass spectrometry. The l ead isotopic compositions have remained remarkably uniform over the pa st 30 million years, but the record of small variations corresponds wi th other paleoceanographic indicators of climate change, including wea thering and glaciation. This implies that despite the short residence time of lead in the oceans, global mechanisms may influence lead isoto pic compositions in the central Pacific, far from continental inputs, because of changes in weathering, ocean circulation, and degree of mix ing. Thus lead isotopic data could be used to probe climate-driven cha nges in ocean circulation through time.