Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments
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 δ<sup>15</sup>N-records in surface sediments (δ&...
| Main Authors: | , , |
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| Format: | Article |
| Language: | English |
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Pensoft Publishers
2011-08-01
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| Series: | Fossil Record |
| Online Access: | http://www.foss-rec.net/14/153/2011/fr-14-153-2011.pdf |
| _version_ | 1797330810042318848 |
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| author | S. Meisel U. Struck K.-C. Emeis |
| author_facet | S. Meisel U. Struck K.-C. Emeis |
| author_sort | S. Meisel |
| collection | DOAJ |
| description | 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 δ<sup>15</sup>N-records in surface sediments (δ<sup>15</sup>N<sub>sediment</sub>) with suspended particulate matter (δ<sup>15</sup>N<sub>SPM</sub>) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ<sup>15</sup>N<sub>SPM</sub> with depth and elucidate potential offsets between δ<sup>15</sup>N<sub>SPM</sub> and δ<sup>15</sup>N<sub>sediment</sub>. The parallel spatial trend of δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ<sup>15</sup>N-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, δ<sup>15</sup>N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to "normal" with δ<sup>15</sup>N-signals continuously increasing towards the mesopelagic ocean. The decrease in δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive <sup>15</sup>N-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 δ<sup>15</sup>N 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. <sup>15</sup>N-enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ<sup>15</sup>N-variations in sediment cores retrieved from this area.
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doi:<a href="http://dx.doi.org/10.1002/mmng.201100005" target="_blank">10.1002/mmng.201100005</a> |
| first_indexed | 2024-03-08T07:24:49Z |
| format | Article |
| id | doaj.art-00d76b0096644f1ab6cd92d9e6649fd2 |
| institution | Directory Open Access Journal |
| issn | 2193-0066 2193-0074 |
| language | English |
| last_indexed | 2024-03-08T07:24:49Z |
| publishDate | 2011-08-01 |
| publisher | Pensoft Publishers |
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| series | Fossil Record |
| spelling | doaj.art-00d76b0096644f1ab6cd92d9e6649fd22024-02-02T22:02:28ZengPensoft PublishersFossil Record2193-00662193-00742011-08-0114215316910.5194/fr-14-153-2011Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sedimentsS. Meisel0U. Struck1K.-C. Emeis2Museum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung an der Humboldt-Universität zu Berlin, 10115 Berlin, GermanyMuseum für Naturkunde, Leibniz-Institut für Evolutions- und Biodiversitätsforschung an der Humboldt-Universität zu Berlin, 10115 Berlin, GermanyInstitut für Biogeochemie und Meereschemie, Universität Hamburg, Bundesstr. 55, 20146 Hamburg, GermanyThe 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 δ<sup>15</sup>N-records in surface sediments (δ<sup>15</sup>N<sub>sediment</sub>) with suspended particulate matter (δ<sup>15</sup>N<sub>SPM</sub>) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ<sup>15</sup>N<sub>SPM</sub> with depth and elucidate potential offsets between δ<sup>15</sup>N<sub>SPM</sub> and δ<sup>15</sup>N<sub>sediment</sub>. The parallel spatial trend of δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ<sup>15</sup>N-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, δ<sup>15</sup>N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to "normal" with δ<sup>15</sup>N-signals continuously increasing towards the mesopelagic ocean. The decrease in δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive <sup>15</sup>N-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 δ<sup>15</sup>N 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. <sup>15</sup>N-enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ<sup>15</sup>N-variations in sediment cores retrieved from this area. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201100005" target="_blank">10.1002/mmng.201100005</a>http://www.foss-rec.net/14/153/2011/fr-14-153-2011.pdf |
| spellingShingle | S. Meisel U. Struck K.-C. Emeis Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments Fossil Record |
| title | Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments |
| title_full | Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments |
| title_fullStr | Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments |
| title_full_unstemmed | Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments |
| title_short | Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments |
| title_sort | nutrient dynamics and oceanographic features in the central namibian upwelling region as reflected in delta sup 15 sup n signals of suspended matter and surface sediments |
| url | http://www.foss-rec.net/14/153/2011/fr-14-153-2011.pdf |
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