Ed Brook, Thomas Bauska, and Alan Mix (Corvallis, OR, USA)
With 2 Figures
Trace gas records from traditional ice cores provide vital records of changes in atmospheric composition but deep ice coring is expensive, time consuming, and sample quantities are typically limited. In some locations on the margins of Greenland and Antarctica ancient ice outcrops at the surface and can be sampled in very large quantities (Fig. 1 shows the Taylor Glacier outcrop location in Antarctica). Such samples provide the opportunity to make meas-urements that are otherwise difficult on traditional ice cores, for example the radiocarbon content of CH4 (Petrenko et al. 2009), or high precision measurements of stable isotope ratios of trace gases (for example, recent work on the isotopic composition of N2O by Schilt et al. 2014, see Fig. 2). Ice margin samples also offer the possibility of very high-resolution sampling in key intervals.
We focus here on a complete record of CO2 and δ13C-CO2 for the last deglaciation ob-tained from the Taylor Glacier, in the Dry Valleys region of Antarctica (Fig. 1). Samples were taken by shallow (~ 4 m) coring in an ~360 m cross- glacier transect after extensive field reconnaissance and analysis of atmospheric CH4 to establish the stratigraphy.
Ice ranging in age from early Holocene to Eemian outcrops at various locations on Taylor Glacier. Although the ice is deformed by folding, it is possible to reconstruct time series of gas records by correlating CH4 concentration records and the δ18O-O2 with well dated ice core records. We take advantage of the large sample sizes available to use a high precision dual inlet method to measure δ13C-CO2 (Bauska et al. 2014) from 22–11 ka. The reproducibility of the isotope measurements is ~ 0.02 ‰. The high precision results combined with high-res-olution sampling reveal more detail about carbon cycle changes than previous data sets.
In Figure 2 we plot CO2 and δ13C-CO2 as well as CH4, N2O and δ15N-N2O (the latter three records from Schilt et al. 2014). The age scale is based on correlating CH4 variations with the very well dated WAIS Divide ice core (Marcott et al. 2014) and confirmation of field results with laboratory measurements of CH4, CO2, N2O and δ18O-O2 (Schilt et al. 2014, Baggenstos et al., in prep.) and comparison of those records with established ice core data.
Corrections for gravitational fractionation are based on δ15N-N2 measurements made at the Scripps Institute of Oceanography (Baggenstos and Severinghaus, in prep.).
CH4 and CO2 concentration trends show all of the expected abrupt changes and inflection points known from other records, allowing us to precisely date the Taylor Glacier records and place them in a global stratigraphic framework. Relative to WAIS Divide we expect (and ob-serve) that the Taylor Glacier record is smoothed due to diffusion in the firn column because the original deposition site of Taylor Glacier ice was in a relatively lower accumulation rate region.
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The high precision δ13C-CO2 record is fully consistent with previous reconstructions but al-lows us to see changes on short timescales (Fig. 2). During the Last Glacial Maximum (LGM) CO2 and δ13C-CO2 were not constant; small variations (< 5 ppm and 0.08 ‰), suggest that the carbon cycle was not completely at steady state. From 18 ka to 15.5 ka δ13C-CO2 shows a strong 0.3 ‰ decrease that corresponds to an increase in CO2 of 35 ppm. We show that the decrease happened in two roughly equal steps. An initial decrease of ~0.15 ppm between 18 and 16.5 ka corresponds with the initial increase of CO2 by ~ 20 ppm. At ~ 16.5 ka a rapid additional drop of δ13C-CO2 of ~0.15 ‰ occurred over a period of several centuries, and corresponds with an additional increase of CO2 by ~ 7 ppm. From 15.5 ka to 11 ka, CO2 in-creased by 40 ppm and δ13C-CO2 gradually increased. Superimposed on this trend is a second sharp decline in δ13C-CO2 that started at ~12.9 ka, coincident with the start of the CO2 rise during the Younger Dryas. δ13C-CO2 reached a minimum at ~12.5 ka and recovered over the next ~1000 years.
We combine the data with carbon cycle box model experiments to examine processes that are plausibly responsible for the deglacial rise in atmospheric CO2. We employ a Keeling plot technique where in a classic two-component system the y-axis intercept of a linear regression to the data (y0) is the δ13C signature of the reservoir controlling the atmosphere. In the more complex mixing between the atmosphere, ocean and terrestrial biosphere a carbon cycle mod-el must be used to account for process like air-sea gas exchange and ocean mixing which can lead to a non-linear, time-variant relationship between CO2 and δ13C-CO2.
We use the model results to outline a scenario that couples deglacial climate history and our carbon cycle observations. During the early part of HS1, the collapse of AMOC (McManus et al. 2004) decreased heat transport to the North Atlantic. In response, large areas of the Northern Hemisphere (NH) cooled and Southern Hemisphere (SH) warmed, the ITCZ shifted southward and SH westerlies shifted southward or strengthened (Wang et al.
2004, Cheng et al. 2009, Denton et al. 2010). We hypothesize that a shift of the westerlies off the SH continents and/or increased SH precipitation lead to a precipitous decline in dust delivery over the Subantarctic ocean, driving the bulk of the CO2 rise from about 18 –15.5 ka.
The southward migration of the ITCZ also lead to a drying in parts of the NH, possibly caus-ing a reduction in land carbon, most notably around 16.5 ka when the first abrupt change in
Fig. 1 Left: Location of Taylor Glacier blue ice outcrop, middle: sketch map of sampling region (inset shows location of sample transect (green line). Right: example of high resolution coring.
Isotopic Constraints on Greenhouse Gas Variability during the Last Deglaciation
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δ13C-CO2 occurred. Alternatively, or additionally, the changing SH westerlies around 16.5 ka leading to enhanced air-sea gas exchange and possibly greater upwelling.
During the later half of HS1, dust deposition had effectively reached interglacial levels, and the δ13C-CO2 data are consistent with the CO2 rise being driven mostly by warming ocean temperatures and an additional release of ocean biological carbon. However, the initial rise in CO2 during the YD could have been triggered by either a second loss of land carbon or addi-tional enhancement of SH westerlies, driven ultimately by AMOC reduction and NH cooling.
Our new data provide strong constraints on the mechanisms behind glacial-interglacial CO2 variability. The δ13C-CO2 record shows that most of the 75 ppm increase in atmospheric CO2 could plausible be attributed to a combination of a release of organic carbon from the
Fig. 2 Blue: CH4, CO2, δ13C-CO2, andδ15N-N2O from the Taylor Glacier blue ice archive. δ15N-N2O results were used by Schilt et al. (2014) to partition the N2O change in to oceanic and terrestrial sources. Terrestrial changes dominate on centennial timescales but inferred changes in marine emissions are consistent with current understand-ing of changes in ocean oxygenation.
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ocean and rising ocean temperature, but the CO2 rises occurred in a series of steps, each with a d13C fingerprint that suggests that different mechanisms may have been triggered at various times during the deglacial transition. Primary release of CO2 accumulated in the ocean from respired organic matter occurred relatively early in the transition. Global temperature changes lagged the release of deep ocean carbon, supporting a trigger for the deglaciation in ocean cir-culation or ocean biological processes. At least twice during the deglaciation a rapid release of carbon depleted in 13C to the atmosphere occurred over a few centuries, suggesting that abrupt and significant releases of CO2 to the atmosphere may be common nonlinear features of Earth’s carbon cycle.
References
Bauska, T. K., Brook, E. J., Mix, A. C., and Ross, A.: High-precision dual-inlet IRMS measurements of the stable isotopes of CO2 and the N2O/CO2 ratio from polar ice core samples. Atmospheric Measurement Techniques 7/11, 3825 –3837 (2014)
Cheng, H., Edwards, R. L., Broecker, W. S., Denton, G. H., Kong, X., Wang, Y., Zhang, R. and Wang, X.: Ice age terminations. Science 326/5950, 248 –252 (2009)
Denton, G. H., Anderson, R. F., Toggweiler, J. R., Edwards, R. L., Schaefer, J. M., and Putnam, A. E.: The last glacial termination. Science 328/5986, 1652–1656 (2010)
Marcott, S. A., Bauska, T. K., Buizert, C., Steig, E. J., Rosen, J. L., Cuffey, K. M., Fudge, T. J., Severing-haus, J. P., Ahn, J., Kalk, M. L., Mc Connell, J. P., Sowers, T., Taylor, K. C., White, J. W., and Brook, E.
J.: Centennial-scale changes in the global carbon cycle during the last deglaciation. Nature 514/7524, 616 – 619 (2014)
McManus, J. F., Francois, R., Gherardi, J. M., Keigwin, L. D., and Brown-Leger, S.: Collapse and rapid resumption of Atlantic meridional circulation linked to deglacial climate changes. Nature 428/6985, 834 – 837 (2004)
Petrenko, V. V., Smith, A. M., Severinghaus, J. P., Brook, E. J., Lowe, D., Riedel, K., Brailsford, G., Hua, Q., Schaefer, H., Reeh, N., Weiss, R. F., and Etheridge, D.: 14CH4 measurements in Greenland ice: investigat-ing last glacial termination CH4 sources. Science 324/5926, 506 –508 (2009)
Schilt, A., Brook, E. J., Bauska, T. K., Baggenstos, D., Fischer, H., Joos, F., Petrenko, W., Schaefer, H., Schmidt, J., Severinghaus, J. P., Spahni, R., and Stocker, T. F.: Isotopic constraints on marine and terrestrial N2O emissions during the last deglaciation. Nature 516/7530, 234 –237 (2014)
Wang, X., Auler, A. S., Edwards, R. L., Cheng, H., Cristalli, P. S., Smart, P. L., Richards, D. A., and Shen, C.
C.: Wet periods in northeastern Brazil over the past 210 kyr linked to distant climate anomalies. Nature 432/7018, 740 –743 (2004)
Prof. Ed Brook, Ph.D.
Thomas Bauska, Ph.D.
Prof. Alan Mix, Ph.D.
College of Earth, Ocean, and Atmospheric Sciences Oregon State University
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