Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ15N-signals of suspended matter and surface sediments

Abstract

The study deals with the modern situation of the northern Benguela Upwelling, directing particular attention to the shelf region off central Namibia (21 to 24° S). At the centre of the investigation is the comparison of δ15N‐records in surface sediments (δ15Nsediment) with suspended particulate matter (δ15NSPM) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ15NSPM with depth and elucidate potential offsets between δ15NSPM and δ15Nsediment. The parallel spatial trend of δ15Nsediment and surface ocean δ15NSPM shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ15N‐signatures are found right off the coast where water temperatures are lowest. Concomitantly high productivity rates and low bottom oxygen suggest the upwelling of denitrified source waters. With increasing distance offshore, δ15N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to “normal” with δ15N‐signals continuously increasing towards the mesopelagic ocean. The decrease in δ15Nsediment and surface ocean δ15NSPM with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive 15N‐enrichment of the nitrate pool as it is gradually used up by phytoplankton growth. On the basis of the available evidence, the downward trend of δ15N results from decreased relative nitrate consumption, resting on a combination of reduced primary production and the existence of an ulterior source of nutrients. Nutrient replenishment seems to occur via an additional upwelling front at the edge of the shelf as well as tapping of subsurface nitrate through sufficiently deep penetration of wind‐ and wave‐induced mixing over large areas of the shelf. Both mechanisms are considered capable of working against the expected nutrient drawdown (i.e. 15N‐enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ15N‐variations in sediment cores retrieved from this area. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Environmental factors influencing cold-water coral ecosystems in the oxygen minimum zones on the Angolan and Namibian margins

“…a δ 15 N values between 1.4 to 3.7‰. This range resembles a more terrestrial 485 signal (-1 to 3‰; Montoya, 2007) and is well below the average isotopic ratio of the marine waters of 486 5.5‰ (Meisel et al, 2011). Also δ 13 C values of -23.5‰ are in line with the δ 13 C values of terrestrial 487 matter which is on average -27 ‰ in this area (Boutton, 1991;Mariotti et al, 1991).…”

Warming and Cooling: The Medieval Climate Anomaly in Africa and Arabia

“…The northern Benguela segment of the upwelling system in central Namibia appears to have cooled during the MCA (NAM‐1, ODP 1084B; Sites 14 and 15), while the southern Benguela segment in western South Africa has warmed (GeoB 8331, GeoB 8323; Sites 16 and 17). The two segments are quasi‐independent subsystems, separated by the “Lüderitz Gate,” a discontinuity barrier associated with the Lüderitz upwelling cell (Duncombe Rae, ; Meisel et al, ; Mohrholz et al, ). The Lüderitz Gate acts as an effective barrier to the movement and exchange of some marine species between the southern and northern parts of the Benguela ecosystem.…”

Environmental factors influencing benthic communities in the oxygen minimum zones on the Angolan and Namibian margins

“…3). Tidal waves will be amplified due to a critical match between the characteristic slope of the internal M2 tide and the bottom slope of the Angolan margin, as is known from other continental slope regions (Dickson and McCave, 1986;Mienis et al, 2007). As argued above, this turbid layer is likely important for the nutrition of the slightly deeper situated CWC mounds, since vertical mixing is otherwise hindered by the strong stratification.…”

“…This stratification is restricting vertical mixing and thereby limits ventilation of the oxygen-depleted subsurface water masses. In addition rivers transport terrestrial organic matter to the margin, which is reflected by the isotopic signals of the SPOM (−1 ‰ to 3 ‰; Montoya, 2007) which is well below the average isotopic ratio of the marine waters of 5.5 ‰ (Meisel et al, 2011). Also δ 13 C values are in line Figure 8.…”