Organic carbon burial in Mediterranean sapropels intensified during Green Sahara Periods since 3.2 Myr ago

Katharine M Grant,Jessica D Amies,Diederik Liebrand,Tiah Penny,Xiang Zhao,Andrew P Roberts,Rick Hennekam,Lucas J Lourens,Udara Amarathunga,Pengxiang Hu,Thomas Westerhold,David Heslop,Laura Rodriguez-Sanz,Eelco J Rohling,Yao Qian,Stewart Gilmore

doi:10.1038/s43247-021-00339-9

Abstract

Dark organic-rich layers (sapropels) have accumulated in Mediterranean sediments since the Miocene due to deep-sea dysoxia and enhanced carbon burial at times of intensified North African run-off during Green Sahara Periods (GSPs). The existence of orbital precession-dominated Saharan aridity/humidity cycles is well known, but lack of long-term, high-resolution records hinders understanding of their relationship with environmental evolution. Here we present continuous, high-resolution geochemical and environmental magnetic records for the Eastern Mediterranean spanning the past 5.2 million years, which reveal that organic burial intensified 3.2 Myr ago. We deduce that fluvial terrigenous sediment inputs during GSPs doubled abruptly at this time, whereas monsoon run-off intensity remained relatively constant. We hypothesize that increased sediment mobilization resulted from an abrupt non-linear North African landscape response associated with a major increase in arid:humid contrasts between GSPs and intervening dry periods. The timing strongly suggests a link to the onset of intensified northern hemisphere glaciation.

Highlights

Dark organic-rich layers have accumulated in Mediterranean sediments since the Miocene due to deep-sea dysoxia and enhanced carbon burial at times of intensified North African run-off during Green Sahara Periods (GSPs)
The δ18O record reflects long-term global sea-level/ice volume changes[16], with superimposed negative δ18O excursions that relate to African run-off into the Eastern Mediterranean and warming within fresher surface-water layers[12,17,18]
While changes in local Eastern Mediterranean precipitation over the entire Plio-Pleistocene are not well-constrained, their impacts on ODP967 δ18O are likely to be minimal compared to African run-off because Eastern Mediterranean surface waters are the evaporative source for local precipitation[8,12]

Summary

Introduction

Dark organic-rich layers (sapropels) have accumulated in Mediterranean sediments since the Miocene due to deep-sea dysoxia and enhanced carbon burial at times of intensified North African run-off during Green Sahara Periods (GSPs). Form in response to deep-sea anoxia and nutrient inputs when significantly increased run-off enters the basin via the Nile and wider North African margin[8]. These periods correspond to enhanced boreal summer insolation maxima and minima in Earth’s orbital precession cycle, resulting in a more northerly and intensified African rain belt[8,9].

Methods

Results

Conclusion