Ocean Carbon Pump in Past Limited by Phosphorus
Filed under: Discovery,News & Resources,Scientific Ocean Drilling
Biological carbon pump as described by Broecker, 1982. There is a continous exchange of carbondioxide between surface water and atmosphere. Organic material is produced from primary production in the light-flooded surface water. Remains of organic material is remineralized and transfered back to surface water e.g. by upwelling. Other remains become part of the sedimentation to the ocean floor, where it is buried in the sediment and thus removed from the carbon cycle. The thermocline is the boundary between surface water and the lower water masses.
From environmentalresearchweb.org
by Liz Kalaugher
Today the Eastern Equatorial Pacific is the most important marine source of atmospheric carbon dioxide. That’s because the low biological productivity in the region limits the amount of carbon transported to the ocean floor in dead organic matter so that it isn’t enough to cancel out the carbon dioxide brought to the surface by upwelling.
However, in past glacial periods when the region received more dust rich in iron – a mineral often essential for plankton growth – it’s likely that iron limitation wasn’t a problem and that the biological carbon pump was stronger. Now a team from the UK, Switzerland and Spain has come up with clear evidence for this.
“Existing paleoproductivity records from the Eastern Equatorial Pacific have provided highly equivocal estimations of the biological carbon pump during glacial times,” Laetitia Pichevin of the University of Edinburgh told environmentalresearchweb. “The main paradox stemmed from the observation of a concomitant increase in organic carbon export – pointing towards an invigorated biological carbon pump – and a decrease in opal accumulation, suggesting an inefficient biological pump of carbon during glacial periods.”
Pichevin and colleagues from ETH Zürich, Switzerland, the University of Barcelona, Spain, and the Scottish Universities Environment Research Centre, UK, examined a sediment core taken by the Ocean Drilling Program from the Cocos Ridge.
They took the first stable silicon-isotope ratio measured on diatom frustules in a sediment core collected from a low-latitude ocean. Together with other isotopic and geochemical measurements, these data indicated that biological productivity was not limited by iron and silicon availability during glacial periods, and that the biological carbon pump was much more efficient.
It appears that during glacial periods phosphorus availability was the limiting factor on plankton growth in the region, in contrast to today when silicic acid shortage appears to cause an upper limit.
“Importantly, the timing of change in nutrient availability is synchronous with the observed atmospheric carbon dioxide increase at the end of the last glacial period,” said Pichevin. “Our work therefore demonstrates that change in the carbon budget in the low-latitude Pacific in response to iron supply acts as a primary feedback on global climate at glacial-interglacial timescales through greenhouse gas emission.”
According to Pichevin, the alleviation of iron limitation during glacial periods caused a decrease in the silicon:carbon uptake ratio by diatoms, leading to a drop in silicic acid requirements and opal production, without implying a simultaneous decrease in the carbon export.
“This also casts doubts on the use of sedimentary opal records as a productivity indicator in general and to assess changes in the C rain rate ratio,” she added. “Coupling between the opal and carbon pumps is in fact complex and strongly conditioned by iron availability.”
The amount of carbon dioxide released through upwelling of deep water in the region today depends on the El Nino-La Nina oscillation, which appears to be speeding up as climate changes.
“Our study brings new understanding and insights into the oceanic carbon budget in the tropical Pacific and hence could improve predictions of oceanic carbon emissions in a warming planet,” said Pichevin. “Estimates of future atmospheric dust suggest that dust emissions may either increase or decrease with future climate change, depending on the scenario or the numerical model used. Recent work however implies that industrial dust contains particularly reactive iron that is readily usable by marine producers. However it is difficult to predict which way this will go.”
Now the team plans to reconstruct a high-resolution record of dust/iron supply in nearby sediment cores to further assess the link between iron supply and an invigorated biological carbon pump. Ultimately the researchers, who reported their work in Nature, hope to use these records to constrain marine biogeochemical models for climate prediction.
