Radioactive (Pu, 210Pb, 234Th and 226Ra) and chemical (Mn, Fe, Al) tracers to evaluate sediment accumulation and biodiffusion in the southern Gulf of Mexico

In this paper, we present results that have important implications for understanding the sedimentation process and for evaluating the carbon cycle in deep-sea sediment of the southern Gulf of Mexico (sGM) in the current context of global climate change. Here we show results about short- and long-term biodiffusion rates obtained from natural ( 234 Th, 210 Pb, 214 Pb, 226 Ra) and anthropogenic ( 239+240 Pu) radionuclides in eleven sediment cores collected in the sGM: i) the outer continental shelf (2 cores), ii) the continental slope (5 cores), and iii) the abyssal plain (4 cores). The short- and long-term biodiffusion rates (D b-Th and D b-Pb ) ranged from 95 to 4135 cm 2 kyr −1 and 54 to 549 cm 2 kyr −1 , respectively. The D b-Th (100 d time scale) values were less than one order of magnitude higher than the D b-Pb (100 yrs time scale), possibly because of the oligotrophic condition of the surface ocean in the sGM to limit the biological activity in sediments. Changes in redox conditions were confirmed by the enrichment of Fe and Mn in the sediment profiles. These redox changes are likely responsible for increased Ra mobilization in the sediments and, consequently, for changes in the accumulation of unsupported 210 Pb. This process acts as an autochthonous source of 210 Pb ex within the sediment layers and explains the disequilibrium observed between 214 Pb and 210 Pb in subsurface sections of some sediment cores. Numerical simulation of 239+240 Pu profiles using an advection-diffusion equation in non-steady-state conditions fit well (R 2 > 0.86) with measured 239+240 Pu profiles in four of six cores. The biodiffusion rates obtained from the Pu model were comparable to those obtained from 210 Pb, but different than the short-term biodiffusion rates elucidated with 234 Th. The accumulation rates from Pu deconvolutions were similar to data reported previously by 14 C dating.