AGE-DEPENDENT MIXING OF DEEP-SEA SEDIMENTS

Rates of bioturbation measured in deep-sea sediments commonly are tracer dependent; in particular, shorter lived radiotracers (such as Th-234) often yield markedly higher diffusive mixing coefficients than their longer-lived counterparts (e.g., Pb-210). At a single station in the 1240-m deep Santa Catalina Basin, we document a strong negative correlation between bioturbation rate and tracer half-life. Sediment profiles of Th-234 (half-life = 24 days) yield an average mixing coefficient (60 cm2 y-1) two orders of magnitude greater than that for Pb-210 (half-life = 22 y, mean mixing coefficient = 0.4 cm2 y-1). A similar negative relationship between mixing rate and tracer time scale is observed at thirteen other deep-sea sites in which multiple radiotracers have been used to assess diffusive mixing rates. This relationship holds across a variety of radiotracer types and time scales. We hypothesize that this negative relationship results from ''age-dependent mixing,'' a process in which recently sedimented, food-rich particles are ingested and mixed at higher rates by deposit feeders than are older, food-poor particles. Results from an age-dependent mixing model demonstrate that this process indeed can yield the bioturbation-rate vs. tracer-time-scale correlations observed in deep-sea sediments. Field data on mixing rates of recently sedimented particles, as well the radiotracer activity of deep-sea deposit feeders, provide strong support for the age-dependent mixing model. The presence of age-dependent mixing in deep-sea sediments may have major implications for diagenetic modeling, requiring a match between the characteristic time scales of mixing tracers and modeled reactants.