Ehlert, Claudia, Grasse, Patricia and Frank, Martin (2013) Changes in silicate utilisation and upwelling intensity off Peru since the Last Glacial Maximum – insights from silicon and neodymium isotopes. Quaternary Science Reviews, 72 . pp. 18-35. DOI 10.1016/j.quascirev.2013.04.013.

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Abstract

We combine the stable silicon isotope composition (δ30Si) of diatoms and the radiogenic neodymium isotope compositions (εNd) of past seawater extracted from the authigenic fraction of the sediments (Mn–Fe coatings of particles and benthic foraminifers), as well as the radiogenic isotope compositions (Nd, Sr) of the detrital material itself to reconstruct silicic acid utilisation, water mass mixing, and upwelling intensity from the same marine sediments in the Peruvian upwelling region during the past 20,000 years. Additionally, the sedimentary signals were compared to the water column isotope compositions.

Along the Peruvian shelf, the dissolved εNd in the water column ranges from −5.7 to +0.6. The corresponding εNd signatures of the coatings and the benthic foraminifers of the surface sediments range from −4.5 to +1.8 and from −2.5 to +2.2, respectively. The detrital εNd (87Sr/86Sr) signatures range from −6.3 to 0 (0.70508–0.71049). All phases show a trend from more radiogenic εNd values in the north towards less radiogenic values in the south broadly reflecting local weathering inputs and hinterland geology.

The εNd signatures of the coatings extracted from sediment core SO147-106KL located in the present day centre of coastal upwelling near 12°S have been essentially constant (−1.5) throughout the past 20,000 years, while the detrital εNd (87Sr/86Sr) varied between values of −0.7 (0.70620) during the Last Glacial and −4.5 (0.70849) during the late Holocene reflecting changes in the origin of the sediment and current transport from a more northerly towards a more southerly source and from overall stronger to weaker upwelling. The δ30Si signature of both total biogenic opal (δ30Siopal) and of hand-picked diatoms (δ30Sidiatom) ranged from +0.3‰ (Last Glacial) to +1.4‰ (late Holocene) confirming large variations in upwelling intensity driving silicic acid utilisation by diatoms. Our combined bSiO2 MAR, δ30Siopal and detrital radiogenic isotope results demonstrate that the strongest El Niño-Southern Oscillation conditions off Peru of the past 20 ka have prevailed during the past 5 ka.

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