THE ABIOTICALLY DRIVEN BIOLOGICAL PUMP IN THE OCEAN AND SHORT-TERM FLUCTUATIONS IN ATMOSPHERIC CO2 CONTENTS

Current debates on the significance of the oceanic ''biological pump'' in the removal of atmospheric CO2 pay more attention to the act of biological carbon-dioxide fixation (primary productivity) in the sea, but pay less or no attention to the equally relevant aspect of the transfer of the fixed carbon to a sink before its oxidation back to CO2. The upper ocean obviously disqualifies as a sink for biologically fixed CO2 because of gas-exchange with the atmosphere. The deep ocean, on the other hand, can be a sink at least at time scales of the ocean turnover. Transfer of newly-fixed CO2 to the deep sea can be accelerated by abiogenic matter introduced to the sea surface from terrestrial sources. This matter acts as ballast and increases the density and settling rates of aggregates of freshly synthesized organic matter thereby facilitating their rapid removal from the upper ocean. Higher supply of abiogenic matter enhances the sequestering of fresh organic matter and in effect shifts the zone of organic matter remineralization from the upper ocean to the deep sea. Consistent with this abiogenic forcing, the rate of organic matter remineralization and the subsequent storage of the remineralized carbon in the deep sea are linked to bulk fluxes (mass accumulation rates) in the deep sea. This mechanism acts as an ''abiotic boost'' in the workings of the oceanic ''biological pump'' and results in an increase in deep sea carbon storage; the magnitude of carbon thus stored could have caused the observed short term fluctuations in atmospheric CO2-contents during the glacial-interglacial cycles.