ALBERT

Description: The object of the present study is to introduce a means of comparing the Vostok and marine chronologies. Our strategy has been to use the delta18O of atmospheric O2 (denoted delta18Oatm) from the Vostok ice core as a proxy for the delta18O of seawater (denoted delta18Osw). Our underlying premise in using delta18Oatm as a proxy for delta18Osw is that past variations in delta18Osw (an indicator of continental ice volume) have been transmitted to the atmospheric O2 reservoir by photosynthesizing organisms in the surface waters of the world's oceans. We compare our record of delta18Oatm to the delta18Osw record which has been developed from studies of the isotopic composition of biogenic calcite (delta18Oforam) in deep-sea cores. We have tied our delta18Oatm record from Vostok to the SPECMAP timescale throughout the last 135 kyr by correlating delta18Oatm with a delta18Osw record from V19-30. Results of the correlation indicate that 77% of the variance is shared between these two records. We observed differences between the delta18Oatm and the delta18Osw records during the coldest periods, which indicate that there have been subtle changes in the factors which regulate delta18Oatm other than delta18Osw. Our use of delta18Oatm as a proxy for delta18Osw must therefore be considered tentative, especially during these periods. By correlating delta18Oatm with delta18Osw, we provide a common temporal framework for comparing phase relationships between atmospheric records (from ice cores) and oceanographic records constructed from deep-sea cores. Our correlated age-depth relation for the Vostok core should not be considered an absolute Vostok timescale. We consider it to be the preferred timescale for comparing Vostok climate records with marine climate records which have been placed on the SPECMAP timescale. We have examined the fidelity of this common temporal framework by comparing sea surface temperature (SST) records from sediment cores with an Antarctic temperature record from the Vostok ice core. We have demonstrated that when the southern ocean SST and Antarctic temperature records are compared on this common temporal framework, they show a high degree of similarity. We interpret this result as supporting our use of the common temporal framework for comparing other climate records from the Vostok ice core with any climate record that has been correlated into the SPECMAP chronology.

Description: The response of marine phytoplankton to the ongoing increase in atmospheric pCO2 reflects the consequences of both increased CO2 concentration and decreased pH in surface seawater. In the model diatom Thalassiosira weissflogii, we explored the effects of varying pCO2 and pH, independently and in concert, on photosynthesis and respiration by incubating samples in water enriched in H218O. In long-term experiments (6-h) at saturating light intensity, we observed no effects of pH or pCO2 on growth rate, photosynthesis or respiration. This absence of a measurable response reflects the very small change in energy used by the carbon concentrating mechanism (CCM) compared to the energy used in carbon fixation. In short-term experiments (3 min), we also observed no effects of pCO2 or pH, even under limiting light intensity. We surmise that in T. weissflogii, it is the photosynthetic production of NADPH and ATP, rather than the CO2-saturation of Rubisco that controls the rate of photosynthesis at low irradiance. In short-term experiments, we observed a slightly higher respiration rate at low pH at the onset of the dark period, possibly reflecting the energy used for exporting H+ and maintaining pH homeostasis. Based on what is known of the biochemistry of marine phytoplankton, our results are likely generalizable to other diatoms and a number of other eukaryotic species. The direct effects of ocean acidification on growth, photosynthesis and respiration in these organisms should be small over the range of atmospheric pCO2 predicted for the twenty-first century.