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Geological Society, London, Special Publications; 1992; v. 64; p. 285-297;
DOI: 10.1144/GSL.SP.1992.064.01.19
© 1992 Geological Society of London

Diagenetic Conditions

Strong deglacial minimum in the {delta}13C record from planktonic foraminifera in the Benguela upwelling region: palaeoceanographic signal or early diagenetic imprint?

R. Schneider, A. Dahmke, A. Kölling, P. J. Müller, H. D. Schulz & G. Wefer

Fachbereich Geowissenschaften, Universität Bremen, Klagenfurter Straße, 2800 Bremen 33, Germany

A strong deglacial decrease in the {delta}13C values of the planktonic foraminifera G. inflata and G. bulloides, spanning a 1000-year period, was measured in a core from the Benguela upwelling region. The deglacial {delta}13C minimum exceeds background values by more than –1{per thousand}, and is between –1.6 to –2.0{per thousand} below Holocene values. The unusual low {delta}13C values could reflect a change in the isotopic composition of the upwelled South Atlantic Central Water (SACW) and/or changes in the stratification of the surface waters in the Benguela coastal upwelling region. The observed {delta}13C decrease correlates with a proposed zone of authigenic CaCO3 precipitation at 6 m depth in the anaerobic sediments. This raises the question whether the {delta}13C minimum was caused by diagenetic alteration of the {delta}13C record or not. Calculations based on estimated interstitial water {delta}13CTCO2 values and flux rates of Ca predict a 5% authigenic CaCO3 precipitation additional to the primary CaCO3 content in the sediment, which could account for an average diagenetic imprint of –1.0{per thousand} {delta}13CCaCO3 if precipitated on foraminiferal tests. However, missing visual evidence (SEM) for authigenic CaCO3 phases greatly weakens the case for diagenetic causes for the negative {delta}13C spike. The deglacial negative {delta}13C spike is therefore assumed to be a primary palaeoceanographic signal.