Strong deglacial minimum in the δ13C record from planktonic foraminifera in the Benguela upwelling region: palaeoceanographic signal or early diagenetic imprint?

Abstract A strong deglacial decrease in the δ13C 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 δ13C minimum exceeds background values by more than −1‰, and is between −1.6 to −2.0‰ below Holocene values. The unusual low δ13C 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 δ13C decrease correlates with a proposed zone of authigenic CaCO3 precipitation at 6 m depth in the anaerobic sediments. This raises the question whether the δ13C minimum was caused by diagenetic alteration of the δ13C record or not. Calculations based on estimated interstitial water δ13CTCO2 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‰ δ13CCaCO3 if precipitated on foraminiferal tests. However, missing visual evidence (SEM) for authigenic CaCO3 phases greatly weakens the case for diagenetic causes for the negative δ13C spike. The deglacial negative δ13C spike is therefore assumed to be a primary palaeoceanographic signal.

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