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A 23,000-year record of surface water pH and pCO2 in the western equatorial Pacific Ocean

The concentration of carbon dioxide (CO2) in the atmosphere plays a major role in defining the Earth's climate, and studies have shown that atmospheric CO2 levels were lower in glacial times than in inter-glacial periods. Since we also know that the oceans play a key role in controlling the level of CO2 in the atmosphere today, a major goal of scientific research is to understand how this role may have varied in the past. This result shows that the equatorial Pacific Ocean was a major source of CO2 to the atmosphere between 14 and 16 thousand years ago - a time when the concentration of CO2 in the atmosphere was rising rapidly and the Earth's climate was warming.

The planktonic foraminifer, Globigerinoides sacculifer, faithfully records the boron isotope composition (d11B) of dissolved B(OH)4- in the seawater from which the foraminifer grew its shell, and this is directly related to the pH of the seawater. Once the pH of the seawater is known the pCO2 of the waters can be calculated if alkalinity concentrations are also known. We measured the d11B of samples of G. sacculifer from the western equatorial Pacific covering the period 0.4 - 23.2 thousand years ago (ka). The results show that for most of this time the surface waters of this area of the ocean were in equilibrium with atmospheric CO2. However, during the period 13.8 - 15.6 ka the pCO2 values of the surface waters were ~100ppmv higher than atmospheric levels.

In the modern western equatorial Pacific the physical structure of the water column inhibits upwelling of nutrient-rich water into the euphotic zone, leading to the formation of a warm pool that has relatively low levels of biological productivity and surface water pCO2 levels that are in equilibrium with the atmosphere. To the east, upwelling brings nutrient and CO2-rich waters to the surface. The low iron concentration of these waters means that the nutrients are not fully utilized (i.e., this is a high nutrient-low chlorophyll (HNLC) area) which results in this area of the ocean being the largest natural source of CO2 to the atmosphere.

The longitudinal divide between the warm pool (from which the studied core was recovered) and HNLC areas moves to the east during El Nino events, such that the extent and intensity of the HNLC is reduced and the thermocline shallows in the western equatorial Pacific. In general, surface water pCO2 values vary from close to atmospheric levels in "normal" and El Nino years, but during La Nina (increased upwelling) periods the warm pool retreats to the west and the surface water pCO2 reaches ~80 ppmv at the sample site. Hence, we hypothesise that the pCO2 record reflects a period of more frequent La Nina conditions between 13.8-15.7 ka.

This conclusion is supported by several other studies that have also suggested that interstadials are characterised by La Nina conditions. For example, increased wind driven upwelling along the Oman margin and higher productivity in the Cariaco Basin have both been linked with La Nina-type conditions.

Finally, the period of high pCO2 observed here is approximately coincident with the deglaciation carbon isotope minimum event (that reached its greatest intensity at 15.9±0.2 ka), and is also apparent in our data. This observation has been ascribed to increased upwelling of CO2-rich sub-Antarctic Mode Water as a consequence of the reestablishment of circum-polar deep water that itself resulted from melt-back of Antarctic sea ice. Our data suggest that a significant portion of this upwelling occurred in the equatorial Pacific.

Current status & use
Data from this and other studies are compatible with the hypothesis that there was an increase in the intensity of upwelling in the eastern equatorial Pacific at a time that is coincident with the steepest rise in atmospheric CO2 levels during the last deglaciation. The extent to which such upwelling played a role in the increase in atmospheric CO2 during this time requires more precise dating of the relative timing of the various paleo-records as well as modelling of the physical controls of ocean-atmosphere CO2 exchange (e.g., wind stress). Nevertheless, our study shows that d11B studies of planktonic foraminifers are a powerful tool with which to investigate Pleistocene variations in ocean-atmosphere CO2 exchange.

Expected benefits:
Together with other records this method will help in
-identifying sources and sinks of atmospheric CO2 in the past and thereby
-improving our understanding of how sensitive the atmosphere is to changes in CO2
- understanding what role the ocean plays in controlling atmospheric CO2.

Dissemination and use potential
It is planned to develop a faster and easier method to measure d11B using multi-collector ICPMS. This would also strongly increase the value of this method to study CO2 fluctuations in the past oceans for other palaeoclimatologists.

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