Sediment accumulation rates and vertical mixing of deep-sea sediments derived from~(14)C and ~(210)Pb in the southern Gulf of Mexico

摘要

Here we present new results on the sedimentation rates and vertical mixing in the deep-water region (966-3741 m) of the southern Gulf of Mexico, from a collection of 42 cores retrieved from the continental slopes and the abyssal plain. These cores were dated with the aid of accelerator mass spectrometry (AMS) ~(14)C determinations on planktic foraminifera. We applied the Modified Mixing Box Model (MMBM) to evaluate the mean sedimentation rates (S_m), to estimate the age of surface sediments (T_0) that result from mixing processes. S_m range between 5 and 9 cm kyr~(-1) in cores from the abyssal plain and the continental slopes, highest rates are consistently observed in specific locations on the slopes (17-24 cm kyr~(-1)). T_0 values range between 1000 and 2000 years, in cores from the abyssal plain and the continental slope sites, most probably as a result from the relatively low sedimentation rates and consequent mixing of older sediments. However, some cores from the continental slopes show consistently higher sedimentation rates, and T_0 values that are close to zero age. Here we further show ~(210)Pb profiles from 15 cores to evaluate the non-homogeneous mixing processes in surface sediments (0-10 cm). Biodiffusion coefficients (D_b) derived from the solution of advection-diffusion model range between 14 and 51 cm~2 kyr~(-1) on the abyssal plain and between 42 and 158 cm~2 kyr~(-1) on the continental slopes. Fluxes of ~(210)Pb_(ex) range between 60 and 111 Bq m~(-2) yr~(-1) on the abyssal plain, and higher values are consistently found on the continental slopes (158-247 Bq m~(-2) yr~(-1) ). We find the age of surface sediments (T_0; derived from the MMBM of ~(14)C) are linked with the transit time in the mixed layer through biodiffusion processes, derived from ~(210)Pb profiles. One of the implications of these results is the relative low importance of benthic macrofauna in respect to the meio- and microfauna bioturbation processes to explain sediment mixing patterns in the deep-sea sediments of the Gulf of Mexico, most probably reflecting a low export of organic carbon to the sediments controlled by the oligotrophic conditions of its surface waters.