Post-depositional stability of long-chain alkenones under contrasting redox conditions

Prahl, Fredrick G.; Lange, G. J. de; Lyle, M.; Sparrow, Margaret A.

doi:10.1038/341434a0

Cited by 213 publications

(99 citation statements)

References 14 publications

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“…That the changes recorded at the different sites are of different sign suggest that the alkenone unsaturation ratio is unaffected by diagenesis (Prahl et al 1989) and that the records reflect SST variability in the past. The data reflect both proximity of the sampling location to upwelling cells, and changes in upwelling intensity of the upwelling cells through time: pristine upwelling water has a baseline SST of 12°C, which is also seen in the present-day surface sediments as two discrete upwelling cells between 23 and 26°S.…”

Section: Resultsmentioning

confidence: 98%

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Emeis

Struck

Leipe

et al. 2007

Int J Earth Sci (Geol Rundsch)

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The coastal upwelling system offshore Namibia is ideally suited to address a focal question of the Integrated Marine Biogeochemistry and Ecosystem Research Programme: what are the mechanisms that drive long-term changes in ecosystems? Considerable interannual variability in climatic forcing is indicated by long time series of meteorological and remote sensing observation; these accompany considerable interannual to interdecadal changes in the upwelling intensity over the last 100 years, as well as a centennial trend. On longer time scales, the only archives available are sediment records spanning the late Holocene. To decipher the sediment record, we mapped surface-sediment patterns of proxies for physical (sea surface temperature/SST from alkenone unsaturation indexes) and nutrient (d 15 N on bulk sedimentary N) variables. Their present-day surface-sediment patterns outline the coastal upwelling cells and filaments and associated high productivity area. Analysed in an array of dated sediment cores, the spatial patterns of SST suggest long-term ([100 years) variability in the location and intensity of individual upwelling cells. The patterns of d 15 N outline an area of intense denitrification near the coast, and advection of water with low-oxygen concentrations in the undercurrent from the North. d 15 N exhibits considerable downcore variability, in particular over the last 50 years. The variability appears to be governed by differences in extent of denitrification and thus of the shelf oxygen balance, which appears to have deteriorated in the last 50 years. Together, the data suggest that SST and denitrification conditions have remained in the narrow bounds outlined by today's patterns in surface sediments, but that spatially small variability in upwelling intensity and make-up of upwelling feed waters induced considerable changes in the lower trophic levels of the coastal upwelling ecosystem over the last 6,000 years. Attempts to correlate proxy records from sediments with observational time series and regional climate reconstructions were not successful, possibly because annual to interannual environmental signals are erased in the process of sediment formation.

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Section: Resultsmentioning

confidence: 98%

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Emeis

Struck

Leipe

et al. 2007

Int J Earth Sci (Geol Rundsch)

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“…The O 2 penetration stopped after the deposition of a new turbidite and the diagenetic conditions changed to sub-oxic. Due to the development of the progressive oxidation front in the f-turbidite, the organic matter content above the colour boundary is only 0.14%, whereas below the boundary the normal value of 1 % Cora is found (PRAHL et al, 1989). The samples were collected at a depth of 720-722 cm f;om a thin laminar diffusive black-purple coloured front located above the paleo-oxidation front (front 1) and about 6 cm below the paleo-oxidation front at 746-748 cm (front 2) (Fig.…”

Section: Methodsmentioning

confidence: 99%

“…By their nature, these remarkably homogenous turbidites are extremely suited to study the effects of oxidation and the subsequent diagenetic alterations (e.g. WILSON et al, 1986;PRAHL et al, 1989;THOMSON et al, 1989). Oxidation of the top of these turbidites caused drastic colour changes of the sediment, mostly from brown-pale green (oxidized) to dark green (reduced) (MIDDELBURG and DE LANGE, 1988).…”

Section: Introductionmentioning

confidence: 99%

Occurrence of thin, metal-rich layers in deep-sea sediments: a geochemical characterization of copper remobilization

Visser

Middelburg

et al. 1993

Deep Sea Research Part I: Oceanographic Research Papers

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Abstract-Distinctly coloured trace metal enriched fronts with a six-fold Cu-enrichment occur in the interbedded pelagic-turbiditic sediments from the Madeira Abyssal Plain. To decipher their mode of formation. two sequential extraction techniques, an acid-gradient and a "Tessier-type" scheme, were applied. Two different front types could be distinguished: Type-l fronts are mostly black and enriched in Co, Zn, Ni and Cu, and associated with the oxidation fronts that developed after deposition of turbidites. Type-2 fronts are purple, extremely enriched in Cu, and associated with sulphur. These fronts are sometimes found in relation with a relict oxidation front, but often show no clear relation with lithology. Two different modes of formation are postulated. (1) The transition-metal enrichment in the type-l fronts is typical for a manganese-oxide association often found in marine sediments. After the dissolution of manganese-oxide during sub-oxic diagenesis, the transition metals were liberated and subsequently relocated and precipitated in response to a redox-gradient. (2) The Cu-S association in type-2 fronts is also formed during sub-oxic diagenesis. The main process leading to the formation of this Cu-S enrichment is the mobilization of inherited Cu-S compounds from organic-rich turbidites. The accumulation of these compounds is possibly related to redox changes at relict oxidation fronts and to gradual. probably slowly evolving. redox changes that occur at present.

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“…The distal, organic matter-rich turbidites, derived from the northwest African margin, are almost perfectly mixed and laid down as a homogeneous deposit (De Lange et al, 1987;Morris, 1987;Prahl et al, 1989). All of these turbidites contain a sharp oxidation front down to which bottom-water oxygen has penetrated the sediment.…”

Section: Introductionmentioning

confidence: 99%

Changes in kerogen composition across an oxidation front in Madeira Abyssal Plain turbidites as revealed by pyrolysis GC-MS

Damsté¹,

Hoefs²,

Leeuw³

et al. 1998

Proceedings of the Ocean Drilling Program, 157 Scientific Results

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Isolated kerogens from Madeira Abyssal Plain turbidite samples were qualitatively and semiquantitatively studied by means of analytical pyrolysis for the effect of oxygen exposure on organic matter preservation. The initial organic matter in the turbidite is very homogeneous and relatively high and offers an ideal case for the study of oxygen effects on organic matter preservation without the usual complications of varying sources or sedimentation rates and bioturbation. From all four studied turbidites, one sample from the oxidized part of the turbidite was compared with two samples from the unoxidized part. The aliphatic character of the kerogens after postdepositional oxidation has increased significantly, as revealed by the abundance of the n-alkanes and n-alk-1-enes in the oxidized turbidite. At the same time, the contribution of the isoprenoid alkanes and alkenes and the alkylthiophenes to the pyrolysate has decreased significantly. The relative abundances of alkylbenzenes, alkylnaphthalenes, alkylphenols, alkylindenes, and alkan-2-ones in the pyrolysates have remained relatively constant upon oxidation. The alkylbenzene, alkylphenol, alkylnaphthalene, n-alkane, and n-alk-1-ene, isoprenoid alkane, and alkene internal distributions are substantially altered in the oxidized samples. This study shows that postdepositional oxidation of sedimentary organic matter by oxygen is far more severe than by sulfate alone. Furthermore, it is clear that oxidation of the organic matter is, to a significant degree, selective.

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Post-depositional stability of long-chain alkenones under contrasting redox conditions

Cited by 213 publications

References 14 publications

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Variability in upwelling intensity and nutrient regime in the coastal upwelling system offshore Namibia: results from sediment archives

Occurrence of thin, metal-rich layers in deep-sea sediments: a geochemical characterization of copper remobilization

Changes in kerogen composition across an oxidation front in Madeira Abyssal Plain turbidites as revealed by pyrolysis GC-MS

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