Controls on Millennial‐Scale Atmospheric CO2 Variability During the Last Glacial Period

Changes in atmospheric CO2 on millennial‐to‐centennial timescales are key components of past climate variability during the last glacial and deglacial periods (70‐10ka) yet the sources and mechanisms responsible for the CO2 fluctuations remain largely obscure. Here we report the 13C/12C ratio of atmospheric CO2 during a key interval of the last glacial period at sub‐millennial resolution, with coeval histories of atmospheric CO2, CH4 and N2O concentrations. The carbon isotope data suggest that the millennial‐scale CO2 variability in MIS3 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 (DO 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.National Science Foundatio

Ice stratigraphy at the Pakitsoq ice margin, West Greenland, derived from gas records

Horizontal ice-core sites, where ancient ice is exposed at the glacier surface, offer unique opportunities for paleo-studies of trace components requiring large sample volumes. Following previous work at the Pâkitsoq ice margin in West Greenland, we use a combination of geochemical parameters measured in the ice matrix (δ¹⁸O[subscript ice]) and air occlusions (δ¹⁸O[subscript atm], δ¹⁵N of N₂ and methane concentration) to date ice layers from specific climatic intervals. The data presented here expand our understanding of the stratigraphy and three-dimensional structure of ice layers outcropping at Pâkitsoq. Sections containing ice from every distinct climatic interval during Termination I, including Last Glacial Maximum, Bølling/Allerød, Younger Dryas and the early Holocene, are identified. In the early Holocene, we find evidence for climatic fluctuations similar to signals found in deep ice cores from Greenland. A second glacial-interglacial transition exposed at the extreme margin of the ice is identified as another outcrop of Termination I (rather than the onset of the Eemian interglacial as postulated in earlier work). Consequently, the main structural feature at Pâkitsoq is a large-scale anticline with accordion-type folding in both exposed sequences of the glacial-Holocene transition, leading to multiple layer duplications and age reversals

In situ cosmogenic radiocarbon production and 2-D ice flow line modeling for an Antarctic blue ice area

Radiocarbon measurements at ice margin sites and blue ice areas can potentially be used for ice dating, ablation rate estimates and paleoclimatic reconstructions. Part of the measured signal comes from in situ cosmogenic ¹⁴C production in ice, and this component must be well understood before useful information can be extracted from ¹⁴C data. We combine cosmic ray scaling and production estimates with a two-dimensional ice flow line model to study cosmogenic ¹⁴C production at Taylor Glacier, Antarctica. We find (1) that ¹⁴C production through thermal neutron capture by nitrogen in air bubbles is negligible; (2) that including ice flow patterns caused by basal topography can lead to a surface ¹⁴C activity that differs by up to 25% from the activity calculated using an ablation-only approximation, which is used in all prior work; and (3) that at high ablation margin sites, solar modulation of the cosmic ray flux may change the strength of the dominant spallogenic production by up to 10%. As part of this effort we model two-dimensional ice flow along the central flow line of Taylor Glacier. We present two methods for parameterizing vertical strain rates, and assess which method is more reliable for Taylor Glacier. Finally, we present a sensitivity study from which we conclude that uncertainties in published cosmogenic production rates are the largest source of potential error. The results presented here can inform ongoing and future ¹⁴C and ice flow studies at ice margin sites, including important paleoclimatic applications such as the reconstruction of paleoatmospheric ¹⁴C content of methane

An improved estimate for the delta C-13 and delta O-18 signatures of carbon monoxide produced from atmospheric oxidation of volatile organic compounds

Atmospheric carbon monoxide (CO) is a key player in global atmospheric chemistry and a regulated pollutant in urban areas. Oxidation of volatile organic compounds (VOCs) is an important component of the global CO budget and has also been hypothesized to contribute substantially to the summertime urban CO budget. In principle, stable isotopic analysis of CO could constrain the magnitude of this source. However, the isotopic signature of VOC-produced CO has not been well quantified, especially for the oxygen isotopes. We performed measurements of CO stable isotopes on air samples from two sites around Indianapolis, US, over three summers to investigate the isotopic signature of VOC-produced CO. One of the sites is located upwind of the city, allowing us to quantitatively remove the background air signal and isolate the urban CO enhancements. as well as the isotopic signature of these enhancements. In addition, we use measurements of &Delta;14CO2 in combination with the CO:CO2 emission ratio from fossil fuels to constrain the fossil-fuel-derived CO and thereby isolate the VOC-derived component of the CO enhancement. Combining these measurements and analyses, we are able to determine the carbon and oxygen isotopic signatures of CO derived from VOC oxidation as &minus;32.8&permil;&plusmn;0.5&permil; and 3.6&thinsp;&permil;&plusmn;1.2&thinsp;&permil;, respectively. Additionally, we analyzed CO stable isotopes for 1 year at Beech Island, South Carolina, US, a site thought to have large VOC-derived contributions to the summertime CO budget. The Beech Island results are consistent with isotopic signatures of VOC-derived CO determined from the Indianapolis data. This study represents the first direct determination of the isotopic signatures of VOC-derived CO and will allow for improved use of isotopes in constraining the global and regional CO budgets.</p

Multiple carbon cycle mechanisms associated with the glaciation of Marine Isotope Stage 4

Here we use high-precision carbon isotope data (δ13C-CO2) to show atmospheric CO2 during Marine Isotope Stage 4 (MIS 4, ~70.5-59 ka) was controlled by a succession of millennial-scale processes. Enriched δ13C-CO2 during peak glaciation suggests increased ocean carbon storage. Variations in δ13C-CO2 in early MIS 4 suggest multiple processes were active during CO2 drawdown, potentially including decreased land carbon and decreased Southern Ocean air-sea gas exchange superposed on increased ocean carbon storage. CO2 remained low during MIS 4 while δ13C-CO2 fluctuations suggest changes in Southern Ocean and North Atlantic air-sea gas exchange. A 7 ppm increase in CO2 at the onset of Dansgaard-Oeschger event 19 (72.1 ka) and 27 ppm increase in CO2 during late MIS 4 (Heinrich Stadial 6, ~63.5-60 ka) involved additions of isotopically light carbon to the atmosphere. The terrestrial biosphere and Southern Ocean air-sea gas exchange are possible sources, with the latter event also involving decreased ocean carbon storage

Radiometric ⁸¹Kr dating identifies 120,000-year-old ice at Taylor Glacier, Antarctica

We present the first successful ⁸¹Kr-Kr radiometric dating of ancient polar ice. Krypton was extracted from the air bubbles in four ~350 kg polar ice samples from Taylor Glacier in the McMurdo Dry Valleys, Antarctica, and dated using Atom Trap Trace Analysis (ATTA). The ⁸¹Kr radiometric ages agree with independent age estimates obtained from stratigraphic dating techniques with a mean absolute age offset of 6 ± 2.5 ka. Our experimental methods and sampling strategy are validated by 1) ⁸⁵Kr and ³⁹Ar analyses that show the samples to be free of modern air contamination, and 2) air content measurements that show the ice did not experience gas loss. We estimate the error in the ⁸¹Kr ages due to past geomagnetic variability to be below 3 ka. We show that ice from the previous interglacial period (MIS 5e, 130-115 ka before present) can be found in abundance near the surface of Taylor Glacier. Our study paves the way for reliable radiometric dating of ancient ice in blue ice areas and margin sites where large samples are available, greatly enhancing their scientific value as archives of old ice and meteorites. At present, ATTA ⁸¹Kr analysis requires a 40-80 kg ice sample; as sample requirements continue to decrease ⁸¹Kr dating of ice cores is a future possibility.Keywords: paleoclimatology, glaciology, geochronolog

Spatial pattern of accumulation at Taylor Dome during the last glacial inception: stratigraphic constraints from Taylor Glacier

A new ice core retrieved from the Taylor Glacier blue ice area contains ice and air spanning the Marine Isotope Stage (MIS) 5/4 transition (74 to 65 ka), a period of global cooling and glacial inception. Dating the ice and air bubbles in the new ice core reveals an ice age-gas age difference (Δage) approaching 10 ka during MIS 4, implying very low accumulation at the Taylor Glacier accumulation zone on the northern flank of Taylor Dome. A revised chronology for the Taylor Dome ice core (80 to 55 ka), situated to the south of the Taylor Glacier accumulation zone, shows that Δage did not exceed 2.5 ka at that location. The difference in Δage between the new Taylor Glacier ice core and the Taylor Dome ice core implies a spatial gradient in snow accumulation across Taylor Dome that intensified during the last glacial inception and through MIS 4

A New Method for Analyzing ¹⁴C of Methane in Ancient Air Extracted from Glacial Ice

We present a new method developed for measuring radiocarbon of methane (¹⁴CH₄) in ancient air samples extracted from glacial ice and dating 11,000–15,000 calendar years before present. The small size (~20 μg CH₄ carbon), low CH₄ concentrations ([CH₄], 400–800 parts per billion [ppb]), high carbon monoxide concentrations ([CO]), and low ¹⁴C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air samples was quantitatively removed using the Sofnocat reagent. ¹⁴C procedural blanks were greatly reduced through the construction of a new CH₄ conversion line utilizing platinized quartz wool for CH₄ combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and ¹⁴C activity of extraneous carbon added in the new CH₄ conversion line were determined to be 0.23 ± 0.16 μg and 23.57 ± 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 μg. The overall procedural blank for all stages of sample handling was 0.75 ± 0.38 pMC for ~20-μg, ¹⁴C-free air samples with [CH₄] of 500 ppb. Duration of the graphitization reactions for small (<25 μg C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. ¹⁴C corrections for each step of sample handling have been determined. The resulting overall ¹⁴CH₄ uncertainties for the ancient air samples are ~1.0 pMC.This is the publisher’s final pdf. The published article is copyrighted by the University of Arizona and can be found at: https://journals.uair.arizona.edu/index.php/radiocarbon/index

Spatial pattern of accumulation at Taylor Dome during Marine Isotope Stage 4: stratigraphic constraints from Taylor Glacier

New ice cores retrieved from the Taylor Glacier (Antarctica) blue ice area contain ice and air spanning the Marine Isotope Stage (MIS) 5–4 transition, a period of global cooling and ice sheet expansion. We determine chronologies for the ice and air bubbles in the new ice cores by visually matching variations in gas- and ice-phase tracers to preexisting ice core records. The chronologies reveal an ice age–gas age difference (Δage) approaching 10 ka during MIS 4, implying very low snow accumulation in the Taylor Glacier accumulation zone. A revised chronology for the analogous section of the Taylor Dome ice core (84 to 55 ka), located to the south of the Taylor Glacier accumulation zone, shows that Δage did not exceed 3 ka. The difference in Δage between the two records during MIS 4 is similar in magnitude but opposite in direction to what is observed at the Last Glacial Maximum. This relationship implies that a spatial gradient in snow accumulation existed across the Taylor Dome region during MIS 4 that was oriented in the opposite direction of the accumulation gradient during the Last Glacial Maximum