The recent completion of drilling at Vostok station in East Antarctica has allowed the extension of the ice record of atmospheric composition and climate to the past four glacial–interglacial cycles. The succession of changes through each climate cycle and termination was similar, and atmospheric and climate properties oscillated between stable bounds. Interglacial periods differed in temporal evolution and duration. Atmospheric concentrations of carbon dioxide and methane correlate well with Antarctic air-temperature throughout the record. Present-day atmospheric burdens of these two important greenhouse gases seem to have been unprecedented during the past 420,000 years.

Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica

/pdf/climate-and-atmospheric-history-of-the-past-420-000-years-2jdtde1xna.pdf

Evidence from North Atlantic deep sea cores reveals that abrupt shifts punctuated what is conventionally thought to have been a relatively stable Holocene climate. During each of these episodes, cool, ice-bearing waters from north of Iceland were advected as far south as the latitude of Britain. At about the same times, the atmospheric circulation above Greenland changed abruptly. Pacings of the Holocene events and of abrupt climate shifts during the last glaciation are statistically the same; together, they make up a series of climate shifts with a cyclicity close to 1470 ± 500 years. The Holocene events, therefore, appear to be the most recent manifestation of a pervasive millennial-scale climate cycle operating independently of the glacial-interglacial climate state. Amplification of the cycle during the last glaciation may have been linked to the North Atlantic's thermohaline circulation.

http://science.sciencemag.org/content/sci/278/5341/1257.full.pdf

A Pervasive Millennial-Scale Cycle in North Atlantic Holocene and Glacial Climates

Surface winds and surface ocean hydrography in the subpolar North Atlantic appear to have been influenced by variations in solar output through the entire Holocene. The evidence comes from a close correlation between inferred changes in production rates of the cosmogenic nuclides carbon-14 and beryllium-10 and centennial to millennial time scale changes in proxies of drift ice measured in deep-sea sediment cores. A solar forcing mechanism therefore may underlie at least the Holocene segment of the North Atlantic's "1500-year" cycle. The surface hydrographic changes may have affected production of North Atlantic Deep Water, potentially providing an additional mechanism for amplifying the solar signals and transmitting them globally.

http://science.sciencemag.org/content/sci/294/5549/2130.full.pdf

Persistent Solar Influence on North Atlantic Climate During the Holocene

Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

https://dspace.stir.ac.uk/bitstream/1893/30981/1/intcal20_northern_hemisphere_radiocarbon_age_calibration_curve_055_cal_kbp.pdf

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 cal kBP)

https://www.whoi.edu/cms/files/sowers10qsr_71583.pdf

Atmospheric methane isotope records covering the Holocene period

Air trapped in glacial ice offers a means of reconstructing variations in the concentrations of atmospheric gases over time scales ranging from anthropogenic (last 200 yr) to glacial/interglacial (hundreds of thousands of years). In this paper, we review the glaciological processes by which air is trapped in the ice and discuss processes that fractionate gases in ice cores relative to the contemporaneous atmosphere. We then summarize concentration-time records for CO2 and CH4 over the last 200 yr. Finally, we summarize concentration-time records for CO2 and CH4 during the last two glacial-interglacial cycles, and their relation to records of global climate change.

Gases in ice cores

[1] A historical record of changes in the N2O isotope composition is important for a better understanding of the biogeochemical cycles involving atmospheric N2O. Here we combine measurements of trapped gases in the Greenland Ice Sheet Project II (GISP II) ice core and the interstitial air from the snowpack at the South Pole to reconstruct atmospheric N2O isotope records back to 1785 A.D. Results from six measurements of ice with gas ages between 1785 and 1819 A.D. indicate that the δ15N and δ18O of atmospheric N2O was 10.1 ± 0.6 and 47.4 ± 1.0‰, respectively. These values are 1.9 and 2.9‰ higher than present-day values as measured in this study. A simple two-box atmospheric model was constructed to simulate the various sources and sinks of atmospheric N2O over the last 250 years. Model results suggest that the total anthropogenic N2O emissions in 1995 were between 4.2 and 5.7 Tg N yr−1. The ice core and box model results were compared to δ15N and δ18O measurements of air trapped in the snowpack at South Pole spanning the 20th century. Our results suggest that δ15N and δ18O of atmospheric N2O have dropped by 1.7 and 0.9‰, respectively, during the last century. These data support previous predictions of decreasing atmospheric isotope ratios that are related to a ∼30% increase in total N2O emissions which are primarily related to agricultural activities.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2002GB001911

Extending records of the isotopic composition of atmospheric N2O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core

High Precision Correlations of Greenland and Antarctic Ice Core Records Over the Last 100 Kyr

During the last glacial cycle, greenhouse gas concentrations fluctuated on decadal and longer timescales. Concentrations of methane, as measured in polar ice cores, show a close connection with Northern Hemisphere temperature variability, but the contribution of the various methane sources and sinks to changes in concentration is still a matter of debate. Here we assess changes in methane cycling over the past 160,000 years by measurements of the carbon isotopic composition 13 C of methane in Antarctic ice cores from Dronning Maud Land and Vostok. We find that variations in the 13 C of methane are not generally correlated with changes in atmospheric methane concentration, but instead more closely correlated to atmospheric CO2 concentrations. We interpret this to reflect a climatic and CO2-related control on the isotopic signature of methane source material, such as ecosystem shifts in the seasonally inundated tropical wetlands that produce methane. In contrast, relatively stable 13 C values occurred during intervals of large changes in the atmospheric loading of methane. We suggest that most methane sources—most notably tropical wetlands—must have responded simultaneously to climate changes across these periods.

/pdf/independent-variations-of-ch-4-emissions-and-isotopic-411zmvteik.pdf

Todd Sowers

Papers

Atmospheric methane isotope records covering the Holocene period

Gases in ice cores

Extending records of the isotopic composition of atmospheric N2O back to 1800 A.D. from air trapped in snow at the South Pole and the Greenland Ice Sheet Project II ice core

High Precision Correlations of Greenland and Antarctic Ice Core Records Over the Last 100 Kyr

Independent variations of CH 4 emissions and isotopic composition over the past 160,000 years