Long-term Observation of Particle Fluxes in the Eastern Atlantic: Seasonality, Changes of Flux with Depth and Comparison with the Sediment Record

Abstract

Long-term particle fluxes have been measured with time-series sediment traps off Cape Blanc (CB), in the southern Guinea Basin (GBS), in the northern one (GBN) and off Namibia (Walvis Ridge, WR). Seasonality was most strongly expressed at the GBS and WR sites. Production half-time (length of time to generate one half of the annual productivity; Berger and Wefer 1990) ranged from 4.6 to 1.8 months suggesting almost constant (e.g at Cape Blanc) to highly-peaked (e.g. at GBS) production systems. Off Cape Blanc, mean annual total flux was 45.3 g m-2 to a water depth of 3204 m, exhibiting substantial variation of 62% (deviation from mean value). Holocene sediment accumulation rates amounted to 17 g m-2 yr-1. At GBN, mean total flux was 36.1 g m-2 at 3939 m of water and the interannual variation reached only 11%. South of the equator at GBS, a total flux of 36.4 g m-2 to a water depth of 3382 m was determined. Bulk sediment accumulation rates in the Gulf of Guinea ranged between 17 and 23 g m-2, but revealed higher values at GBS. At the Walvis Ridge, we obtained a mean total deep-water flux of 31.1 g m-2 with 42% interannual variation; sediment accumulation rates amounted to 15 g m-2. Average annual organic carbon flux to the seafloor was 1.6 (Cape Blanc), 2-2.1 (Guinea Basin) and 3.2 g m-2 (Walvis Ridge); these values are typical for open-ocean-(coastal) upwelling transition systems. They revealed no relationship to literature-derived annual primary production values. Organic carbon preservation was generally poor and estimated as 7.5% (Cape Blanc), 2.9%-3.0% (Guinea Basin) and 1.9% (Walvis Ridge) of the carbon fluxes into the traps, respectively. Carbon accumulation was not related to the deep-water fluxes nor to primary production estimates taken from literature. Mean annual total, lithogenic and biogenic opal fluxes generally increased with depth in the water column (lithogenic fluxes about two-fold), obviously due to the contribution of a substantial fraction of fine-grained, resuspensed material originating at topographic elevations. In contrast, organic carbon fluxes decreased with water depth at all sites following an exponential decline. Surprisingly, the accumulation of refractory lithogenic material in the underlying sediments was only 22–56% of the deep-water fluxes and resembled the subsurface fluxes more closely. We assume that the additional resuspended fraction originated at topographic elevations and was not incorporated yet into the sedimentary record but is being transported in suspension and dispersed in the ocean.