No abstract

The Holocene Thermal Maximum with peak temperatures prior to 7 ka BP is mostly accentuated in the Northern Hemisphere, still visible in the Southern Hemisphere and possibly did not exist in the tropics. Between this period and the modern warming a remarkable negative temperature trend occurred in the Northern Hemisphere, which was probably the effect of a decreasing orbital-induced insolation during the boreal summer. On average the Northern Hemisphere humidityprecipitation records do not show any significant trend. A mechanistic explanation for the multi-decadal-to century-scale Holocene cold relapses, which mainly occurred in the Atlantic-European regions, exists for only the early Holocene cooling events, which are probably the result of a collapse of the meridional overturning circulation owing to freshwater outbursts from the Laurentide ice sheet. Possibly, the late Holocene cold events after c. 4 ka BP are influenced by the covarying influence of major tropical volcanic eruptions and Grand Solar Minima.

Abstract. Changes in the marine remineralisation of particulate organic matter (POM) and calcium carbonate potentially provide a positive feedback with atmospheric CO 2 and climate change. The responses to changes in remineralisation length scales are systematically mapped with the Bern3D ocean-sediment model for atmospheric CO 2 and tracer fields for which observations and palaeoproxies exist. Results show that the "sediment burial-nutrient feedback" amplifies the response in atmospheric CO 2 by a factor of four to seven. A transient imbalance between the weathering flux and the burial of organic matter and calcium carbonate lead to sustained changes in the ocean's phosphate and alkalinity inventory and in turn in surface nutrient availability, marine productivity, and atmospheric CO 2 . It takes decades to centuries to reorganise tracers and fluxes within the ocean, many millennia to approach equilibrium for burial fluxes, while δ 13 C signatures are still changing 200 000 years after the perturbation. At 1.7 ppm m −1 , atmospheric CO 2 sensitivity is about fifty times larger for a unit change in the remineralisation depth of POM than of calcium carbonate. The results highlight the role of organic matter burial in atmospheric CO 2 and the substantial impacts of seemingly small changes in POM remineralisation.