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Journal of the Geological Society; 2000; v. 157; issue.3; p. 667-677
© 2000 Geological Society of London

Regular Article

Enhanced productivity on the Iberian margin during glacial/interglacial transitions revealed by barium and diatoms

J. THOMSON1, S. NIXON1, C. P. SUMMERHAYES1, E. J. ROHLING1, J. SCHÖNFELD2, R. ZAHN2, P. GROOTES3, F. ABRANTES4, L. GASPAR4 & S. VAQUEIRO4

1 Southampton Oceanography Centre, Empress Dock, Southampton SO14 3EZ, UK
2 GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1–3, D-24148 Kiel, Germany
3 Leibniz-Labor für Altersbestimmung und Isotopenforschung, Christian-Albrechts Universität, Kiel, Germany
4 Instituto Geologico e Mineiro, Departamento de Geologia Marinha, Estrada da Portela, Zambujal—Alfragide, Portugal

Scientific editing by Mike Hambrey.

The Portuguese margin is at a critical location for studies of the ocean’s behaviour during glacial/interglacial climatic changes, and the rapid accumulation rates of the sediments enable high-resolution palaeoclimatic investigation. The sedimentary record of the past 350 ka has been investigated in a 35 m long core from 3.5 km water depth on the slope at 40°N by geochemical, isotopic and micropalaeontological techniques. The CaCO3 content of this core as a function of time contains significant Milankovitch orbital frequencies of 18.8, 23.7, 38.0 and 100.6 ka, but these are driven primarily by dilution by clay-flux variations rather than by CaCO3 productivity variations. The largest signals in the productivity indicators Corg, Ba/Al and diatom abundance are all observed as simultaneous peaks at the oxygen isotope stage boundaries 10/9 and 6/5, with the signal magnitude in the order 10/9>6/5 for all three indicators. Smaller coincident signals in Corg, Ba/Al but not diatoms are also observed at the oxygen isotope stage 2/1 boundary. Other less prominent peaks in the Corg and Ba/Al profiles occur elsewhere, including Heinrich Event horizons, but these are not always simultaneous and none contain evidence of the dissolution-prone diatom microfossils. The 10/9, 6/5 and 2/1 oxygen isotope stage transitions represent the three most extreme glacial/interglacial sea level rises in the past 350 ky, possibly in the same sequence of magnitude, when sea level rose rapidly by 120+m from glacial low stands to interglacial low stands to interglacial high stands. The productivity signals at these transitions are contained within <5 ka (including bioturbation).


Keywords: Pleistocene, Iberian margin, sedimentation, oxygen isotopes, barium.




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