Climate and marine biogeochemistry during the Holocene from transient model simulations

Abstract. Climate and marine biogeochemistry changes over the Holocene are investigated based on transient global climate and biogeochemistry model simulations over the last 9,500 yr. The simulations are forced by accelerated and non-accelerated orbital parameters, respectively, and atmospheric pCO2. The analysis focusses on key climatic parameters of relevance to the marine biogeochemistry, on the processes that determine the strength of the carbon pumps that drive the ocean–atmosphere carbon flux, and on the oxygen minimum zones (OMZs) in the ocean. The most pronounced changes occur in the eastern equatorial Pacific (EEP) OMZ, and in the North Atlantic. Changes in global mean values of biological production and export of detritus remain modest, with generally lower values in the mid-Holocene. The simulated ocean–atmosphere CO2-flux is of the right order of magnitude to explain the observed atmospheric pCO2 evolution, but with different timing. As the most significant result, we find a substantial increase in volume of the OMZ in the EEP continuing into the late Holocene in the non-accelerated simulation. The concurrent increase of age of the water mass within the EEP OMZ suggests that this growth is driven by a slow down of the circulation in the interior of the deep Pacific. This results in large scale deoxygenation in the deeper Pacific and hence the source regions of the EEP OMZ waters from mid-to-late Holocene. The simulated expansion of the OMZ raises the question whether the currently observed deoxygenation is a continuation of the orbitally driven decline in oxygen, or if it is already a result of the occuring climate change from anthropogenic forcing as widely assumed. An additional explanation would be that the anthropogenic forcing amplifies the natural forcing. The increase in water mass age and EEP OMZ volume can only be simulated with the non-accelerated model simulation. The simulations thus demonstrate that non-accelerated experiments are required for an analysis of the marine biogeochemistry in the Holocene.