Siple Dome ice reveals two modes of millennial CO2 change during the last ice age

Reconstruction of atmospheric CO₂ during times of past abrupt climate change may help us better understand climate-carbon cycle feedbacks. Previous ice core studies reveal simultaneous increases in atmospheric CO₂ and Antarctic temperature during times when Greenland and the northern hemisphere experienced very long, cold stadial conditions during the last ice age. Whether this relationship extends to all of the numerous stadial events in the Greenland ice core record has not been clear. Here we present a high-resolution record of atmospheric CO₂ from the Siple Dome ice core, Antarctica for part of the last ice age. We find that CO₂ does not significantly change during the short Greenlandic stadial events, implying that the climate system perturbation that produced the short stadials was not strong enough to substantially alter the carbon cycle.

Whether all rapid climate events during the last ice age impacted the global carbon cycle is not clearly understood. Ahn and Brook present a high-resolution record of atmospheric CO₂ from Antarctica and suggest that only Greenland stadials associated with massive iceberg discharge influenced atmospheric CO₂.

Characterizing microbial diversity and the potential for metabolic function at -15 °c in the Basal ice of taylor glacier, antarctica

Measurement of gases entrapped in clean ice from basal portions of the Taylor Glacier, Antarctica, revealed that CO₂ ranged from 229 to 328 ppmv and O₂ was near 20% of the gas volume. In contrast, vertically adjacent sections of the sediment laden basal ice contained much higher concentrations of CO₂ (60,000 to 325,000 ppmv), whereas O₂ represented 4 to 18% of the total gas volume. The deviation in gas composition from atmospheric values occurred concurrently with increased microbial cell concentrations in the basal ice profile, suggesting that in situ microbial processes (i.e., aerobic respiration) may have altered the entrapped gas composition. Molecular characterization of 16S rRNA genes amplified from samples of the basal ice indicated a low diversity of bacteria, and most of the sequences characterized (87%) were affiliated with the phylum, Firmicutes. The most abundant phylotypes in libraries from ice horizons with elevated CO₂ and depleted O₂ concentrations were related to the genus Paenisporosarcina, and 28 isolates from this genus were obtained by enrichment culturing. Metabolic experiments with Paenisporosarcina sp. TG14 revealed its capacity to conduct macromolecular synthesis when frozen in water derived from melted basal ice samples and incubated at −15 °C. The results support the hypothesis that the basal ice of glaciers and ice sheets are cryospheric habitats harboring bacteria with the physiological capacity to remain metabolically active and biogeochemically cycle elements within the subglacial environment.

Eastern equatorial pacific productivity and related-CO2 changes since the last glacial period

Understanding oceanic processes, both physical and biological, that control atmospheric CO(2) is vital for predicting their influence during the past and into the future. The Eastern Equatorial Pacific (EEP) is thought to have exerted a strong control over glacial/interglacial CO(2) variations through its link to circulation and nutrient-related changes in the Southern Ocean, the primary region of the world oceans where CO(2)-enriched deep water is upwelled to the surface ocean and comes into contact with the atmosphere. Here we present a multiproxy record of surface ocean productivity, dust inputs, and thermocline conditions for the EEP over the last 40,000 y. This allows us to detect changes in phytoplankton productivity and composition associated with increases in equatorial upwelling intensity and influence of Si-rich waters of sub-Antarctic origin. Our evidence indicates that diatoms outcompeted coccolithophores at times when the influence of Si-rich Southern Ocean intermediate waters was greatest. This shift from calcareous to noncalcareous phytoplankton would cause a lowering in atmospheric CO(2) through a reduced carbonate pump, as hypothesized by the Silicic Acid Leakage Hypothesis. However, this change does not seem to have been crucial in controlling atmospheric CO(2), as it took place during the deglaciation, when atmospheric CO(2) concentrations had already started to rise. Instead, the concomitant intensification of Antarctic upwelling brought large quantities of deep CO(2)-rich waters to the ocean surface. This process very likely dominated any biologically mediated CO(2) sequestration and probably accounts for most of the deglacial rise in atmospheric CO(2).

Macromolecular synthesis by yeasts under frozen conditions

Although viable fungi have been recovered from a wide variety of icy environments, their metabolic capabilities under frozen conditions are still largely unknown. We investigated basidiomycetous yeasts isolated from an Antarctic ice core and showed that after freezing at a relatively slow rate (0.8 degrees C min(-1)), the cells are excluded into veins of liquid at the triple junctions of ice crystals. These strains were capable of reproductive growth at -5 degrees C under liquid conditions. Under frozen conditions, metabolic activity was assessed by measuring rates of [(3)H]leucine incorporation into the acid-insoluble macromolecular fraction, which decreased exponentially at temperatures between 15 degrees C and -15 degrees C and was inhibited by the protein synthesis inhibitor cycloheximide. Experiments at -5 degrees C under frozen and liquid conditions revealed 2-3 orders of magnitude lower rates of endogenous metabolism in ice, likely due to the high salinity in the liquid fraction of the ice (equivalent of approximately 1.4 mol l(-1) of NaCl at -5 degrees C). The mesophile Saccharomyces cerevisae also incorporated [(3)H]leucine at -5 degrees C and -15 degrees C, indicating that this activity is not exclusive to cold-adapted microorganisms. The ability of yeast cells to incorporate amino acid substrates into macromolecules and remain metabolically active under these conditions has implications for understanding the survival of Eukarya in icy environments.

Stable Carbon Cycle-Climate Relationship During the Late Pleistocene

A record of atmospheric carbon dioxide (CO2) concentrations measured on the EPICA (European Project for Ice Coring in Antarctica) Dome Concordia ice core extends the Vostok CO2 record back to 650,000 years before the present (yr B.P.). Before 430,000 yr B.P., partial pressure of atmospheric CO2 lies within the range of 260 and 180 parts per million by volume. This range is almost 30% smaller than that of the last four glacial cycles; however, the apparent sensitivity between deuterium and CO2 remains stable throughout the six glacial cycles, suggesting that the relationship between CO2 and Antarctic climate remained rather constant over this interval.

Ice-sheet and sea-level changes

Future sea-level rise is an important issue related to the continuing buildup of atmospheric greenhouse gas concentrations. The Greenland and Antarctic ice sheets, with the potential to raise sea level approximately 70 meters if completely melted, dominate uncertainties in projected sea-level change. Freshwater fluxes from these ice sheets also may affect oceanic circulation, contributing to climate change. Observational and modeling advances have reduced many uncertainties related to ice-sheet behavior, but recently detected, rapid ice-marginal changes contributing to sea-level rise may indicate greater ice-sheet sensitivity to warming than previously considered.