SEAWATER STRONTIUM ISOTOPIC VARIATIONS FROM 2.5 MILLION YEARS AGO TO THE PRESENT

Measurements of marine carbonate samples indicate that during the past 2.5 million years the 87Sr/86Sr ratio of seawater has increased by 14 × 10-5. The high average rate of increase of 87Sr/86Sr indicates that continental weathering rates were exceptionally high. Nonuniformity in the rate of increase suggests that weathering rates fluctuated by as much as ±30 percent of present-day values. Some of the observed shifts in weathering rates are contemporaneous with climatic changes inferred from records of oxygen isotopes and carbonate preservation in deep sea sediments.