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Bauska, TK, Brook EJ, Marcott SA, Baggenstos D, Shackleton S, Severinghaus JP, Petrenko VV.  2018.  Controls on millennial-scale atmospheric CO2 variability during the last glacial period. Geophysical Research Letters. 45:7731-7740.   10.1029/2018gl077881   AbstractWebsite

Changes in atmospheric CO2 on millennial-to-centennial timescales are key components of past climate variability during the last glacial and deglacial periods (70-10 ka), yet the sources and mechanisms responsible for the CO2 fluctuations remain largely obscure. Here we report the C-13/C-12 ratio of atmospheric CO2 during a key interval of the last glacial period at submillennial resolution, with coeval histories of atmospheric CO2, CH4, and N2O concentrations. The carbon isotope data suggest that the millennial-scale CO2 variability in Marine Isotope Stage 3 is driven largely by changes in the organic carbon cycle, most likely by sequestration of respired carbon in the deep ocean. Centennial-scale CO2 variations, distinguished by carbon isotope signatures, are associated with both abrupt hydrological change in the tropics (e.g., Heinrich events) and rapid increases in Northern Hemisphere temperature (Dansgaard-Oeschger events). These events can be linked to modes of variability during the last deglaciation, thus suggesting that drivers of millennial and centennial CO2 variability during both periods are intimately linked to abrupt climate variability. Plain Language Summary Ice cores provide unique records of variations in atmospheric CO2 prior to the instrumental era. While it is clear that changes in atmospheric CO2 played a significant role in driving past climate change, it is unclear what in turn drove changes in atmospheric CO2. Here we investigate enigmatic changes in atmospheric CO2 levels during an interval of the last glacial period (similar to 50,000 to 35,000 years ago) that are associated with abrupt changes in polar climate. To determine the sources and sinks for atmospheric CO2, we measured the stable isotopes of carbon in CO2 and found that the primary source of carbon to the atmosphere was an organic carbon reservoir. Most likely, this carbon was sourced from a deep ocean reservoir that waxed and waned following changes in either the productivity of the surface ocean or stratification of the deep ocean. We also found that atmospheric CO2 can change on the centennial timescale during abrupt climate transitions in the Northern Hemisphere. This observation adds to a growing body of evidence that abrupt changes in atmospheric CO2 are an important component of past carbon cycle variability.

Bauska, TK, Baggenstos D, Brook EJ, Mix AC, Marcott SA, Petrenko VV, Schaefer H, Severinghaus JP, Lee JE.  2016.  Carbon isotopes characterize rapid changes in atmospheric carbon dioxide during the last deglaciation. Proceedings of the National Academy of Sciences of the United States of America. 113:3465-3470.   10.1073/pnas.1513868113   AbstractWebsite

An understanding of the mechanisms that control CO2 change during glacial-interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO2 (delta C-13-CO2) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO2 from 17.6 to 15.5 ka, these data demarcate a decrease in delta C-13-CO2, likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO2 rise of 40 ppm is associated with small changes in delta C-13-CO2, consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in delta C-13-CO2 that suggest rapid oxidation of organic land carbon or enhanced air-sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO2 coincident with increases in atmospheric CH4 and Northern Hemisphere temperature at the onset of the Bolling (14.6-14.3 ka) and Holocene (11.6-11.4 ka) intervals are associated with small changes in delta C-13-CO2, suggesting a combination of sources that included rising surface ocean temperature.

Seibt, U, Brand WA, Heimann M, Lloyd J, Severinghaus JP, Wingate L.  2004.  Observations of O-2 : CO2 exchange ratios during ecosystem gas exchange. Global Biogeochemical Cycles. 18   10.1029/2004gb002242   AbstractWebsite

We determined O-2:CO2 exchange ratios of ecosystem fluxes during field campaigns in different forest ecosystems (Harvard Forest/United States, Griffin Forest/United Kingdom, Hainich/Germany). The exchange ratios of net assimilation observed in chamber experiments varied between 0.7 and 1.6, with averages of 1.1 to 1.2. A measurement of soil gas exchange yielded an exchange ratio of 0.94. On the other hand, the observed canopy air O-2:CO2 ratios, derived from the concurrent variations of O-2 and CO2 abundances in canopy air, were virtually indistinguishable from 1.0 over the full diurnal cycle. Simulations with a simple one-box model imply that the combined processes of assimilation, respiration, and turbulent exchange yield canopy air O-2:CO2 ratios that differ from the exchange ratios of the separate fluxes. In particular, the simulated canopy air O-2:CO2 ratios (1.01 to 1.12) were clearly lower than the exchange ratios of net turbulent fluxes between the ecosystem and the atmosphere (1.26 to 1.38). The simulated canopy air ratios were also sensitive to changes in the regional O-2:CO2 ratio of air above the canopy. Offsets between the various exchange ratios could thus arise if the component ecosystem fluxes have different diurnal cycles and distinct exchange ratios. Our results indicate that measurements of O-2 and CO2 abundances in canopy air may not be the appropriate method to determine O-2:CO2 exchange ratios of net ecosystem fluxes.