A sediment-nutrient-oxygen feedback responsible for productivity variations in Late Miocene sapropel sequences of the western Mediterranean

Cyclic sediments observed throughout the Mediterranean during the Late Miocene have been related to precessional forcing of ocean stratification. Individual couplets, typically 2-m-thick sequences of sapropels and diatom-rich marls, can be reliably traced from western Spain to Crete, and were formed in restricted marginal basins. Micropaleontological evidence indicates paradoxically that the organic carbon-rich sapropels were formed under low productivity conditions marked by surface water stratification and deep anoxia, whereas the diatom-rich marls were formed under high productivity conditions marked by upwelling. Here we present geochemical evidence, mainly from detailed phosphorus determinations and paleo-redox proxies, indicating that a sediment-nutrient-oxygen feedback (herein dubbed the SNO Effect) is in part responsible for driving the observed productivity variations. During stratification, anoxic conditions in these basins cause the release of the limiting nutrient phosphorus from reducible oxide phases in the sediments. Basin stagnation causes the buildup of phosphorus below the photic zone. Subsequent overturn driven by precessionally-driven winds injects phosphorus-rich bottom waters into the photic zone. The biotic response to this overturn is high productivity in marls directly overlying the sapropels culminating in diatom mat formation. Exhaustion of the stored excess phosphorus results in lower productivity marls, which grade back into sapropels due to lower wind stresses and the return of stagnant basin conditions. These findings indicate that the SNO Effect may in part be responsible for precessional-scale productivity variations observed in parts of the Mediterranean, and perhaps present in other restricted ocean basins, like the Miocene Monterey Formation and the modern Santa Barbara Basin. (C) 2002 Elsevier Science B.V. All rights reserved.