Time-scales of passive tracers in the ocean with paleoapplications

Abstract

Quantifying time-responses of the ocean to passive and active tracers is critical when interpreting paleodata from sediment cores. Surface-injected tracers are not spreading instantaneously or uniformly throughout the ocean. To obtain insights into these time-scales, a transition matrix is used to compute successive states of a passive tracer concentration in the global ocean. Times to equilibrium are longer than 1500 years for any one region in the global ocean and, near the equilibrium, concentration gradients give time-lags from hundreds to thousands of years between the Atlantic and Pacific bottom, depending on the injection region and the nature of the boundary condition. Ice volume (glacio-eustatic) corrections to deep-sea and surface values cannot be assumed instantaneous, challenging previous paleodata interpretations from sediment cores. Preliminary results on time-responses of the ocean to active tracers, using a global circulation model limited to the North Atlantic box, show no major differences between a freshwater and a dye input, at least during the first 50 years of the experiment and for a small amount of freshwater.