Comparing POC export from 234Th/ 238U and 210Po/ 210Pb disequilibria with estimates from sediment traps in the northwest Mediterranean

Abstract

We compare POC fluxes estimated using 234Th/ 238U and 210Po/ 210Pb disequilibria at the DYFAMED site, northwestern Mediterranean Sea. We also compare the POC fluxes estimated from these two isotope pairs with fluxes below the euphotic zone measured in moored sediment traps. The POC flux at 200 m estimated from 234Th and 210Po deficits and the POC/ 210Po or POC/ 234Th on >70 μm filterable particles measured through three seasons (early spring, late spring, summer) ranged from 3.8 to 17.5 mmol C/m 2/d based on 234Th/ 238U and from 4.4 to 7.0 mmol C/m 2/d based on 210Po/ 210Pb disequilibrium. In comparison, sediment trap fluxes of POC at approximately 200 m ranged from 0.2 to 6.0 mmol C/m 2/d over the same interval. Values of POC/ 210Po and POC/ 234Th ratios in sediment trap material collected in time series or separated according to settling velocity (SV) were generally lower than values in the >70 μm filterable particles at the same depth. The variation in POC/ 210Po and POC/ 234Th in material separated according to SV showed no clear relationship with SV and was controlled more by particle composition and degree of degradation. Both 234Th and 210Po showed sustained deficits in late spring and summer, despite low fluxes recorded in the trap. Lateral processes (transport of particles along isopycnals or intrusion of shelf waters to the site) and violations of temporal assumptions (steady-state vs. non-steady-state) may be responsible for this disparity. Based on the results of this study, we conclude that 210Po/ 210Pb disequilibrium is as valid as 234Th/ 238U as a proxy for estimating POC flux in the ocean.