Long-term, sustained reduction in ocean productivity initiated at 4.6-4.4 Ma

Abstract

<p>The late Miocene to early Pliocene was a time of global cooling, albeit in a warmer-than present climate state. Increased marine primary productivity characterizes this interval, often referred to as the late Miocene-early Pliocene biogenic bloom (~9-3.5 Ma). To explain its manifestation, paleoceanographers often involve ocean gateway or monsoon-related mechanisms, formulating hypotheses of increased or redistributed nutrients in the ocean. However, the exact cause-and-effect chains remain obscure, since important diachronicity is observed across ocean basins for the main phase and the termination of this event. Here, we compile proxy data for late Miocene to Pliocene paleoproductivity from all major ocean basins, including calcareous and siliceous plankton groups. By systematically evaluating the age-depth model accuracies of previously published records we demonstrate that a globally synchronous and long-sustained reduction in primary productivity was initiated with a sharp decline between 4.6 and 4.4 Ma. Our compilation supports that relatively rapid processes (~200 kyr) influenced nutrient availability towards the end of the biogenic bloom. By evaluating different mechanisms influencing the ocean nutrient budget on such time scales, we propose orbital forcing as an important candidate to have tipped the balance towards a less productive ocean. We show that this decline in productivity coincided with a prolonged period of low orbital eccentricity and a shift towards lower-amplitude obliquity. This specific astronomical configuration prevents the development of extreme seasonal contrasts which could lead to reduced nutrient supply to the ocean due to decreased riverine influx.</p>