Biogenic sedimentation and geochemical properties of deep-sea sediments of the Demerara Slope/Abyssal Plain: Influence of the Amazon River Plume

Abstract

The Amazon River Plume has a major impact on sedimentation at abyssal depths throughout a broad area (450,000km2) and far (500–1000km) from the river mouth. As the plume mixes into the North Atlantic Ocean, it may support ecological niches favorable for N2 fixation driven by the symbiosis between diatoms and diazotrophic cyanobacteria. This proposed diatom-diazotroph association (DDA) should result in increased carbon export from a limited region of plume influence. Here we used 32 multicores collected throughout the Demerara Abyssal Slope and Plain at depths ranging from 3000 to 5000m to assess patterns and rates of export and accumulation of organic carbon (Corg or POC) and biogenic SiO2 (bSi). Sediment accumulation rates were determined from 14C ages of foraminifera obtained at 2–5 depths from 9 cores and were further constrained by 230Th measurements. The sedimentation rate (1–5cmky−1), representing the average accumulation over the past 8000years, was combined with Corg and bSi sediment concentrations to establish biogenic matter burial fluxes. Burial of POC represents 4–16% of the Corg raining to the deep sea floor, which is high for water depths>4000m. Burial of bSi represents 1–9% of the total bSi rain. Clastic sedimentation dominates regions underlying the plume, while carbonate sedimentation dominated otherwise. Sediment mixing (bioturbation) was studied in 13 cores using 210Pb as a tracer and showed no spatial patterns. Diffusional mixing coefficients ranged from 0.03 to 3.2cmyear−1, values consistent with other deep sea mixing rates, however, sites with the highest mixing rates did not have the highest Corg content. Sediment focusing was studied in at 7 sites using 230Th as a tracer; surficial sediments had focusing factors that ranged from 1 to 2, indicating little sediment redistribution. Biogenic sedimentation in general and bSi deposition in particular is not concentrated in the region of DDA abundance. The NW axis of primary plume trajectory is coincident with the maximum deposition and burial of POC and bSi but not CaCO3.