Changes in diatom productivity and upwelling intensity off Peru since the Last Glacial Maximum: Response to basin-scale atmospheric and oceanic forcing

New records of stable silicon isotope signatures (δ30Si) together with concentrations of biogenic opal and organic carbon from the central (9° S) and northern (5° S) Peruvian margin reveal changes in diatom productivity and nutrient utilization during the past 20,000 years. The findings are based on a new approach using the difference between the δ30Si signatures of small (11-32μm) and large (>150μm) diatom fractions (Δ30Sicoscino-bSi) in combination with the variance in diatom assemblages for reconstruction of past upwelling intensity. Combination of our records with two previously published records from the southern upwelling area off Peru (12-15° S) shows a general decoupling of the environmental conditions at the central and southern shelf mainly caused by a northward shift of the main upwelling cell from its modern position (12-15° S) towards 9° S during Termination 1. At this time only moderate upwelling intensity and productivity levels prevailed between 9° S and 12° S interpreted by a more northerly position of Southern Westerly Winds and the South Pacific Subtropical High. Furthermore, a marked decrease in productivity at 12-15° S during Heinrich Stadial 1 coincided with enhanced biogenic opal production in the Eastern Equatorial Pacific, which was induced by a southward shift of the Intertropical Convergence zone and enhanced northeasterly trade winds. Modern conditions were only established at the onset of the Holocene. Past changes in preformed δ30Si signatures of subsurface waters reaching the Peruvian Upwelling System did not significantly affect the preserved δ30Si signatures.