Publications

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2018
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.

2017
Baggenstos, D, Bauska TK, Severinghaus JP, Lee JE, Schaefer H, Buizert C, Brook EJ, Shackleton S, Petrenko VV.  2017.  Atmospheric gas records from Taylor Glacier, Antarctica, reveal ancient ice with ages spanning the entire last glacial cycle. Climate of the Past. 13:943-958.   10.5194/cp-13-943-2017   AbstractWebsite

Old ice for paleo-environmental studies, traditionally accessed through deep core drilling on domes and ridges on the large ice sheets, can also be retrieved at the surface from ice sheet margins and blue ice areas. The practically unlimited amount of ice available at these sites satisfies a need in the community for studies of trace components requiring large sample volumes. For margin sites to be useful as ancient ice archives, the ice stratigraphy needs to be understood and age models need to be established. We present measurements of trapped gases in ice from Taylor Glacier, Antarctica, to date the ice and assess the completeness of the stratigraphic section. Using delta O-18 of O-2 and methane concentrations, we unambiguously identify ice from the last glacial cycle, covering every climate interval from the early Holocene to the penultimate interglacial. A high-resolution transect reveals the last deglaciation and the Last Glacial Maximum (LGM) in detail. We observe large-scale deformation in the form of folding, but individual stratigraphic layers do not appear to have undergone irregular thinning. Rather, it appears that the entire LGM-deglaciation sequence has been transported from the interior of the ice sheet to the surface of Taylor Glacier relatively undisturbed. We present an age model that builds the foundation for gas studies on Taylor Glacier. A comparison with the Taylor Dome ice core confirms that the section we studied on Taylor Glacier is better suited for paleo-climate reconstructions of the LGM due to higher accumulation rates.

2014
Rosen, JL, Brook EJ, Severinghaus JP, Blunier T, Mitchell LE, Lee JE, Edwards JS, Gkinis V.  2014.  An ice core record of near-synchronous global climate changes at the Bolling transition. Nature Geoscience. 7:459-463.   10.1038/ngeo2147   AbstractWebsite

The abrupt warming that initiated the Bolling-Allerod interstadial was the penultimate warming in a series of climate variations known as Dansgaard-Oeschger events. Despite the clear expression of this transition in numerous palaeoclimate records, the relative timing of climate shifts in different regions of the world and their causes are subject to debate. Here we explore the phasing of global climate change at the onset of the Bolling-Allerod using air preserved in bubbles in the North Greenland Eemian ice core. Specifically, we measured methane concentrations, which act as a proxy for low-latitude climate, and the N-15/N-14 ratio of N-2, which reflects Greenland surface temperature, over the same interval of time. We use an atmospheric box model and a firn air model to account for potential uncertainties in the data, and find that changes in Greenland temperature and atmospheric methane emissions at the Bolling onset occurred essentially synchronously, with temperature leading by 4.5(-24)(+21) years. We cannot exclude the possibility that tropical climate could iag changing methane concentrations by up to several decades, if the initial methane rise came from boreal sources alone. However, because even boreal methane-producing regions lie far from Greenland, we conclude that the mechanism that drove abrupt change at this time must be capable of rapidly transmitting climate changes across the globe.

2009
Schaefer, H, Petrenko VV, Brook EJ, Severinghaus JP, Reeh N, Melton JR, Mitchell L.  2009.  Ice stratigraphy at the Pakitsoq ice margin, West Greenland, derived from gas records. Journal of Glaciology. 55:411-421.   10.3189/002214309788816704   AbstractWebsite

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 Pakitsoq ice margin in West Greenland, we use a combination of geochemical parameters measured in the ice matrix (delta(18)O(ice)) and air occlusions (delta(18)O(atm), delta(15)N of N(2) 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 Pakitsoq. Sections containing ice from every distinct climatic interval during Termination I, including Last Glacial Maximum, Bolling/Allerod, 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 Pakitsoq 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.

2005
Brook, EJ, White JWC, Schilla ASM, Bender ML, Barnett B, Severinghaus JP, Taylor KC, Alley RB, Steig EJ.  2005.  Timing of millennial-scale climate change at Siple Dome, West Antarctica, during the last glacial period. Quaternary Science Reviews. 24:1333-1343.   10.1016/j.quascirev.2005.02.002   AbstractWebsite

Using atmospheric methane and the isotopic composition of O-2 as correlation tools, we place the 6D record of ice from the Siple Dome (West Antarctica) ice core on a precise common chronology with the GISP2 (Greenland) ice core for the period from 9 to 57 ka. The onset of major millennial warming events in Siple Dome preceded major abrupt warmings in Greenland, and the pattern of millennial change at Siple Dome was broadly similar, though not identical, to that previously observed for the Byrd ice core (also in West Antarctica). The addition of Siple Dome to the database of well-dated Antarctic paleoclimate records supports the case for a coherent regional pattern of millennial-scale climate change in Antarctica during much of the last ice age and glacial-interglacial transition.