Sediment accumulation rates and vertical mixing of deep-sea sediments derived from14C and 210Pb in the southern Gulf of Mexico

Abstract

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) 14C determinations on planktic foraminifera. We applied the Modified Mixing Box Model (MMBM) to evaluate the mean sedimentation rates (Sm), to estimate the age of surface sediments (T0) that result from mixing processes. Sm 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). T0 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 T0 values that are close to zero age. Here we further show 210Pb profiles from 15 cores to evaluate the non-homogeneous mixing processes in surface sediments (0–10 cm). Biodiffusion coefficients (Db) derived from the solution of advection-diffusion model range between 14 and 51 cm2 kyr−1 on the abyssal plain and between 42 and 158 cm2 kyr−1 on the continental slopes.Fluxes of 210Pbex 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 (T0; derived from the MMBM of 14C) are linked with the transit time in the mixed layer through biodiffusion processes, derived from 210Pb 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.