Abrupt sea surface pH change at the end of the Younger Dryas in the central sub-equatorial Pacific inferred from boron isotope abundance in corals (Porites)

摘要

The 'delta B-11-pH' technique was applied to modern and ancient corals Porites from the sub-equatorial Pacific areas (Tahiti and Marquesas) spanning a time interval from 0 to 20.720 calendar years to determine the amplitude of pH changes between the Last Glacial Period and the Holocene. Boron isotopes were measured by Multi-Collector - Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) with an external reproducibility of 0.25 parts per thousand, allowing a precision of about +/- 0.03 pH-units for pH values between 8 and 8.3. The boron concentration [B] and isotopic composition of modern samples indicate that the temperature strongly controls the partition coefficient K-D for different aragonite species. Modern coral delta B-11 values and the reconstructed sea surface pH values for different Pacific areas match the measured pH expressed on the seawater scale and confirm the calculation parameters that were previously determined by laboratory calibration exercises. Most ancient sea surface pH reconstructions near Marquesas are higher than modern values. These values range between 8.19 and 8.27 for the Holocene and reached 8.30 at the end of the last glacial period (20.7 kyr BP). At the end of the Younger Dryas (11.50 +/- 0.1 kyr BP), the central sub-equatorial Pacific experienced a dramatic drop of up to 0.2 pH-units from the average pH of 8.2 before and after this short event. Using the marine carbonate algorithms, we recalculated the aqueous pCO(2) to be 440 +/- 25 ppmV at around 11.5 kyr BP for corals at Marquesas and similar to 500 ppmV near Tahiti where it was assumed that pCO(2) in the atmosphere was 250 ppmV. Throughout the Holocene, the difference in pCO(2) between the ocean and the atmosphere at Marquesas (Delta pCO(2)) indicates that the surface waters behave as a moderate CO_2 sink or source (-53 to 20 ppmV) during El Nino-like conditions. By contrast, during the last glacial/interglacial transition, this area was a marked source of CO_2 (21 to 92 ppmV) for the atmosphere, highlighting predominant La Nino-like conditions. Such conditions were particularly pronounced at the end of the Younger Dryas with a large amount of CO_2 released with Delta pCO(2) of +185 +/- 25 ppmV. This last finding provDEes further evDEence of the marked changes in the surface water pH and temperature in the equatorial Pacific at the Younger Dryas-Holocene transition and the strong impact of oceanic dynamic on the atmospheric CO_2 content.