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| Titel |
Records of past mid-depth ventilation: Cretaceous ocean anoxic event 2 vs. Recent oxygen minimum zones |
| VerfasserIn |
J. Schönfeld, W. Kuhnt, Z. Erdem, S. Flögel, N. Glock, M. Aquit, M. Frank, A. Holbourn |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 12, no. 4 ; Nr. 12, no. 4 (2015-02-24), S.1169-1189 |
| Datensatznummer |
250117829
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| Publikation (Nr.) |
 copernicus.org/bg-12-1169-2015.pdf |
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| Zusammenfassung |
| Present day oceans are well ventilated, with the exception of mid-depth oxygen minimum zones (OMZs)
under high surface water productivity, regions of sluggish
circulation, and restricted marginal basins. In the Mesozoic, however,
entire oceanic basins transiently became dysoxic or anoxic. The Cretaceous
ocean anoxic events (OAEs) were characterised by laminated organic-carbon
rich shales and low-oxygen indicating trace fossils preserved in the
sedimentary record. Yet assessments of the intensity and extent of
Cretaceous near-bottom water oxygenation have been hampered by deep or
long-term diagenesis and the evolution of marine biota serving as oxygen
indicators in today's ocean. Sedimentary features similar to those found in
Cretaceous strata were observed in deposits underlying Recent OMZs, where
bottom-water oxygen levels, the flux of organic matter, and benthic life have been studied thoroughly. Their implications for constraining past bottom-water oxygenation are addressed in this review. We compared OMZ sediments from the
Peruvian upwelling with deposits of the late Cenomanian OAE 2 from the north-west
African shelf. Holocene laminated sediments are encountered at bottom-water
oxygen levels of < 7 μmol kg−1 under the Peruvian
upwelling and < 5 μmol kg−1 in California Borderland
basins and the Pakistan Margin. Seasonal to decadal changes of sediment
input are necessary to create laminae of different composition. However,
bottom currents may shape similar textures that are difficult to discern
from primary seasonal laminae. The millimetre-sized trace fossil
Chondrites was commonly found in Cretaceous strata and Recent oxygen-depleted
environments where its diameter increased with oxygen levels from 5 to 45 μmol kg−1. Chondrites has not been reported in Peruvian sediments but
centimetre-sized crab burrows appeared around 10 μmol kg−1, which may
indicate a minimum oxygen value for bioturbated Cretaceous strata. Organic
carbon accumulation rates ranged from 0.7 and 2.8 g C cm−2 kyr−1 in laminated OAE 2 sections in Tarfaya Basin, Morocco, matching late
Holocene accumulation rates of laminated Peruvian sediments under Recent
oxygen levels below 5 μmol kg−1. Sediments deposited at
> 10 μmol kg−1 showed an inverse exponential
relationship of bottom-water oxygen levels and organic carbon accumulation
depicting enhanced bioirrigation and decomposition of organic matter with
increased oxygen supply. In the absence of seasonal laminations and under
conditions of low burial diagenesis, this relationship may facilitate
quantitative estimates of palaeo-oxygenation. Similarities and differences
between Cretaceous OAEs and late Quaternary OMZs have to be further explored
to improve our understanding of sedimentary systems under hypoxic
conditions. |
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