Servizio Comunitario di Informazione in materia di Ricerca e Sviluppo - CORDIS

Development & integration of ocean carbonate chemistry proxies to better understand the interrelationship between global carbon cycle &climate change

Among the most important challenges remaining to be addressed by Quaternary paleoceanographers is the mechanism responsible for lowering pCO2 during the Last Glacial Maximum (LGM) and possible feedback mechanisms with climate change (e.g. Sigman and Boyle, 2000). During 6C we used a multi-proxy approach to reconstruct the oceanic carbonate chemistry and understand the natural relationship with the global carbon cycle. One important component was the development of new tools for reconstructing environmental parameters (see TIP results 1-6). The other was to combine analytical records of the sedimentary archive (combining several proxies) in combination with numerical models in order to 1) distinguish the mechanisms that control the operation of the oceanic carbon cycle, 2) identify water masses as sinks or sources of atmospheric CO2 and hence, 3) better constrain the role and the impact of the carbon cycle on climate oscillations. Knowledge of the nature and amplitude of natural fluctuations in the past are a precondition to assess the stability of modern subsystems and their potential range of variations in the future.

With regard to the second objective we have combined multiple proxies (boron isotopes, B/Ca, Mg/Ca, Cd/Ca and alkenones) to study sediments from the northern Arabian Sea. We could demonstrate that the magnitude of CO2 degassing from this area increased significantly at ~18 ka, and may thus have played an important role in initiating the rise in atmospheric CO2 levels at the start of the last deglaciation.

It is generally accepted that the oceans were instrumental in regulating glacial-interglacial changes in atmospheric CO2, but there is uncertainty over past changes in the location and magnitude of oceanic sources and sinks of CO2. Our reconstructions indicate that the northern Arabian Sea has been a source of CO2 to the atmosphere since 30 ka. The delta11B and B/Ca proxies further suggest that this source (and the intensity of upwelling) increased in intensity from the last glacial maximum to the Holocene. This hypothesis accords with findings from most other studies of the region that suggest the summer monsoon was less intense in the LGM as the Tibetan plateau was heated less strongly at this time. Around ~18 ka the change from relatively low pCO2 values to higher pCO2 values is coincident with the start of the rise in atmospheric CO2 during the last deglaciation. In this context it is noteworthy that it has been observed that the mean effective moisture levels from the Asian monsoon margin started to increase between 18.5 and 17 ka, suggesting that this may represent onset of the summer monsoon after the LGM. Hence, intensification of upwelling in the Arabian Sea and degassing of CO2-rich surface waters may well have played a role in the increase in atmospheric CO2 that was further enhanced by increased La Nina-type conditions in the equatorial Pacific between ~14-16 ka.

Although much progress has been achieved with regard to all 3 objectives stated above, we cannot answer objective 1 and 3 as detailed as we had hoped for at the beginning of the project. To address these objectives requires to fully reconstructing the carbonate chemistry of the global ocean. Hereto, planktonic foraminiferal proxies would constrain the chemistry of the surface water and benthic foraminifera would be used to determine the bottom water chemistry. However, several proxies turned out to be influenced by more environmental parameters than just the target parameter. For instance, we proposed to use "size normalized weight" (SNW) of planktonic foraminifera as an estimator for bottom water carbonate ion concentrations. Unfortunately, we had to conclude that "cryptic speciation" in planktonic foraminifera masks the true bottom water values and that SNW is therefore not a reliable proxie. In addition, we didn't reach the desired precision and accuracy for single shell boron isotope analysis required to reliably approximate bottom water pH. These drawbacks made it impossible to reconstruct the bottom water carbonate chemistry.

Since "down core" bottom water carbonate chemistry estimates along depth transects of critical ocean areas are the only means to distinguish between the processes that gave rise to the observed changes (physico-chemical carbon uptake or release versus the organic carbon pump or the alkalinity pump) and since, it is also the only way to reconstruct lysocline dynamics, we are currently unable to fully address two of the main objectives that remain the "Holy Grail" for future climate change predictions (but it should be stressed that we have contributed significantly to the objectives).

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