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Orbital forcing of Cretaceous river discharge in tropical Africa and ocean response


The tropics have been suggested as the drivers of global ocean and atmosphere circulation and biogeochemical cycling during the extreme warmth of the Cretaceous period1,2; but the links between orbital forcing, freshwater runoff and the biogeochemistry of continental margins in extreme greenhouse conditions are not fully understood. Here we present Cretaceous records of geochemical tracers for freshwater runoff obtained from a sediment core off the Ivory Coast that indicate that alternating periods of arid and humid African climate were driven by orbital precession. Our simulations of the precession-driven patterns of river discharge with a global climate model suggest that ocean anoxia and black shale sedimentation were directly caused by high river discharge, and occurred specifically when the northern equinox coincided with perihelion (the minimum distance between the Sun and the Earth). We conclude that, in a warm climate, the oceans off tropical continental margins respond rapidly and sensitively to even modest changes in river discharge.

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Figure 1: Location of OAE 3 sites and investigated region for simulated river discharge.
Figure 2: Upper Cretaceous (Coniacian to Campanian) records from ODP Site 959.
Figure 3: Simulated total continental freshwater discharge over the course of one precession cycle (orbital cases A to D) compared to control run conditions.


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We thank the Ocean Drilling Program for providing the sample material and technical assistance. H. Heilmann, R. Henning, B. Kockisch, M. Gölden, S. Berg, A. Katzemich, S. Reiss, M. Aiyswaryan, R. Kunze and D. Pollard are thanked for analytical assistance.

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Correspondence to Thomas Wagner.

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Supplementary Notes

This contains information on time-frequency analysis as well as specifications on experimental set-up of GENESIS 2.0. This file also contains Supplementary Figure S1. (DOC 543 kb)

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Beckmann, B., Flögel, S., Hofmann, P. et al. Orbital forcing of Cretaceous river discharge in tropical Africa and ocean response. Nature 437, 241–244 (2005).

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