SEAWATER SR ISOTOPE VARIATION OVER THE PAST 300 KYR AND INFLUENCE OF GLOBAL CLIMATE CYCLES

THE past 300,000 years have been characterized by glacial/interglacial fluctuations accompanied by glacio-eustatic changes in sea level and changes in continental erosion arising from varying low-latitude rainfall and river drainage. Here we use measurements of strontium isotopes in foraminifera and corals to place limits on variations in the Sr isotope composition of sea water in response to these changing inputs. We find small variations of about 20 p.p.m. in the Sr-87/Sr-86 ratio, which for the foraminiferal record seem to follow a cycle close to the 100-kyr periodicity seen for a number of other climate-related phenomena. These are superimposed on a general increase in Sr-87/Sr-86 through the Cenozoic to the present. On short timescales these changes are likely to be controlled by variations in the global riverine Sr flux, and thus by weathering rates, rather than in the hydrothermal Sr flux at mid-ocean ridges. We show that transient variations over a 50-kyr timescale can be explained on the basis of the present-day global drainage system in conjunction with the perturbations expected to accompany climate changes over the past 300 kyr.