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6C - Carbonate chemistry, carbon cycle and climate change

Summary information

Funding:FP5 – Research project
Total cost:2389796
Ec contribution:1652751
Start date:2002-11-01
End date:2006-10-31
Duration:48 months
Coordinator:Jelle Bijma (jbijma@awi-bremerhaven.de)
Organisation:Alfred Wegener Institute for Polar and Marine Research (AWI) – Germany
Themes:Ocean acidification
Keywords:Carbonate chemistry; Climate change; CO2; Carbon Cycle; Isotopes; Trace elements; Biomarkers
Project name:6C - Carbonate chemistry, carbon cycle and climate change
Project summary:Problems to be solved
The mechanism(s) responsible for lower atmospheric pCO2 during the Last Glacial Maximum (LGM) and possible feedbacks with climate change are still unresolved. Without a properly implemented natural relationship between the global carbon cycle and climate change, model predictions of future climate scenarios due to anthropogenic CO2 release are doomed to fail.

Abstract
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. Our main objective is to use a multi-proxy approach in order to reconstruct the oceanic carbonate chemistry over the past 130,000 years. In combination with numerical models this will allow us 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.

Objectives
The overall objective of 6C is to hind-cast the processes that control the natural inter-relationship between the variation in atmospheric pCO2 and climate change on glacial-interglacial (G-IG) time scales (by reconstructing temporal and spatial changes in the ocean carbonate chemistry) and to quantify and predict changes in atmospheric pCO2 on anthropogenic time scales (by quantifying the negative feed-back of pelagic calcifiers).
Project outputs:Scientific Achievements
During the course of 6C, the project researchers 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. 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.

The development of the envisioned “foram paleobarometer” did not work because the organic matter contained in fossil foraminifera did not include symbiont specific compounds. In addition, the project did not reach the desired precision and accuracy for “single shell boron isotope analysis” required to reliably approximate bottom water pH. However, one should be optimistic that this will be possible in the near future.

The project has combined multiple proxies (boron isotopes, B/Ca, Mg/Ca, Cd/Ca and alkenones) to study sediments from the northern Arabian Sea and was able to 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 11B 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 Niña-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 at the beginning of the project. Addressing these objectives requires to fully reconstruct 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, the project proposed to use “size normalized weight” (SNW) of planktonic foraminifera as an estimator for bottom water carbonate ion concentrations. Unfortunately, the project results lead to the conclusion that “cryptic speciation” in planktonic foraminifera may mask the true bottom water values and that SNW is therefore not a reliable proxie. In addition, the project did not reach the desired precision and accuracy for single shell boron isotope analysis required to reliably approximate bottom water pH. Both of 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.

The project proposed and investigated the possible feedback of decreased calcification potential of planktonic calcifiers to increased anthropogenic CO2 production and hence ongoing ocean acidification. Although, the impact will be relatively small on short time scales, it has to be taken into account on longer timescale (Ridgwell et al., 2006).

Socio-economic relevance and policy implications
The results and conclusions at the end of the project aid us in the understanding of the natural controls and feedbacks between the carbon cycle and global climate change. Eventhough, we were not able to reconstruct the complete carbonate chemistries as initially anticipated, is the increased understanding of the natural inter-relationships (a prerequisite to project possible future climate change scenarios) a relevant issue to society as a whole and for policy makers in particular. It is anticipated that through personal contacts with writers of the IGBP reports our results and conclusions will diffuse to policy makers.

Conclusions
The project has experienced some drawbacks and a one year no-cost extension has been granted. Despite the extra time, however, several proxies turned out to be unreliable (SNW), impossible (paleobarometer) or could not be developed with sufficient accuracy and precision (11B on single benthic foraminiferal shells). Therefore it was impossible to meet all deliverables set out in the original proposal. Yet, significant variations of the carbonate system have been documented (e.g. surface water pH), and as the data sets are completed, this project has contributed to a better knowledge of the natural inter-relationship between the marine carbon cycle and climate change. Finally, it should be noted that several new new programes and many new project have been initiated by the consortium and that 6C keeps on producing new and exiting results.

Project results
A list of publications can be found on the project website (see above) and several abstracts are available from the cordis website at http://cordis.europa.eu/.