Active Polar Research

1) Testing the "clathrate gun" hypothesis with a D/H record of atmospheric methane from the GISP II ice core
2) Constructing a 400,000 year climate record from the Moulton blue ice field in W. Antarctica.
3) Constructing a paleoatmospheric record ofthe isotopic composition of methane and nitrous oxide
4) Constructing a 110,000 year record of atmospheric [N2O] from the GISP II ice core.



                    The GISP II Drill Dome

Testing the "clathrate gun" hypothesis with a D/H record of atmospheric methane from the GISP II ice core

NSF ATM- 0117291   (8/01-7/03), 
Atmospheric CH4 levels throughout the last 100,000 years have been shown to vary on timescales ranging from tens of thousands of years to decades (Figure A). The millennial scale CH4 oscillations which are so prevalent during the last glacial period are closely tied to changes in surface temperatures over Greenland. Given that changes in temperature over Greenland cannot have had a substantial impact on global CH4 emissions, other explanations need to be formulated. One explanation for the CH4 oscillations involves rapid changes in CH4 emissions from wetlands. The covariation between CH4 and Greenland temperature point to a strong teleconnection between N. Hemisphere temperature and the global hydrologic cycle. The other leading explanation for the covariation involves the destabililization of shallow water clathrates brought about by temperature increases coincident with those recorded in Greenland ice. This later hypothesis has recently been referred to as the ‘clathrate gun hypothesis’.


Figure A: Isotopic temperature (Grootes et al., 1994) and CH4 records (Brook et al., 1996) from GISP II ice core along with the multispecies benthic d13Cforam record from ODP hole 893A (Santa Barbara Basin, Kennett et al., 2000). The timescales are from the original references. The large amplitude d13C depletions are indicative of clathrate destabilization events in the Santa Barbara Basin and appear to be highly correlated to periods of rapid CH4 increases.
 
 

I propose to test the clathrate gun hypothesis by constructing a high resolution record of the D/H isotopic composition of atmospheric methane throughout the last 40kyr  with special emphasis on the previously documented rapid CH4 events. The basic premise I am working from is the fact that the D/H isotopic composition of marine  clathrates are ~100 ‰ higher than any terrestrially derived CH4. If the rapid CH4 increase observed in the ice cores is the result of clathrate degassing, then I predict the  D/H ratio of atmospheric CH4 should increase by approximately 26 ‰. A signal of this size should be much larger than the analytical uncertainty associated with the  extraction/analytical technique. Thus, by constructing a record of atmospheric CH4 D/H variations across the rapid CH4 events I should be able to determine whether or not
clathrate destablization contributed to the increased atmospheric CH4 concentrations.



 

Constructing a 400,000 year climate record from the Moulton blue ice field in W. Antarctica.  PI's: Todd Sowers (Penn State) and Jim White (INSTAAR)

NSF OPP-9909474  4/1/00-10/02

 During the 99/00 Antarctic field season a group of US scientists retrieved a 600m horizontal ice core from the Mt. Moulton blue ice field.  Previous studies of the tephra layers in this area indicate a strong likelihood that we can construct a 480,000 year climate record from the area. 40Ar/39Ar dating of six tephra layers (in stratigraphic order) from the area range from 15,000 ± 2,000 years  BP (=15±2ka) to 480±10ka.   During the 99/00 field season a group of US scientists from New Mexico Tech and UNH cut a continuous horizontal ice core across the blue ice field (using chainsaws) and returned the core to the US for detailed analyses of the tephra layers and glaciochemical studies of the area.  We propose to add two research initiatives to the funded project; 1) continuous analyses of the D/H ratio of ice (dDice), and 2), detailed analyses of the elemental and isotopic composition of trapped gases all along the Moulton horizontal ice core.  Our initial goal was to confirm the continuous nature of the Moulton ice core by comparing the gas records with the continuous Vostok gas records spanning the last 423 kyr (Petit et. al., 1999). 
We also generatee a  d18Oice record for West Antarctica over the past 480kry.  This record will be extremely important for ground truthing the dDice record from Vostok over the same period.  In addition, we expect to extract intracontinental dDice information which will be important in assessing the regional expression of this variable throughout Antarctica.  Such information will be very important for understanding how the recently published Taylor Dome dDice record fits in with the overall climate history of Antarctica, as well as in interpreting the climate records expected to be recovered from both the Siple Dome and inland WAIS deep coring initiatives.


moulton.pic

To download the raw Moulton gas data, click here.





Constructing a paleoatmospheric record ofthe isotopic composition of methane and nitrous oxide

Active from 3/1/96-2/28/99

Funding agency: NSF Office of Polar Programs

Project description:
        The proposed work involves reconstructing records of the
isotopic composition of paleoatmospheric methane (CH4) and nitrous
oxide (N2O) covering the last 200,000 years.  Fossil air samples are
presently available from the interstitial spaces surrounding the snow
near the surface of the Antarctic ice sheet as well as from trapped
gases in Antarctic ice cores.  The primary objective of the work is to
further our understanding of the biogeochemical cycles of these two
greenhouse gases throughout the anthropogenic period as well as
over glacial/interglacial timescales.   The justification for this
isotope work exists because current records of the concentration
variations do not provide sufficient information to determine the
cause of the observed fluctuations.  Due to the fact that the various
sources and sinks of these trace gases carry different isotope
signatures, records of the isotopic composition variations will provide
additional information which will help to decipher the nature of the
concentration fluctuations.  For the anthropogenic period (= last 200
years), high resolution measurements of the d13C of atmospheric
CH4 from a shallow ice core will help to determine the relative
contributions of biogenic (wetlands, rice fields and ruminants) and
abiogenic (biomass burning and natural gas) CH4 emissions which
have caused the CH4 concentration to increase at an exponential rate
during this period.     Isotopic data on CH4 and N2O over
glacial/interglacial timescales will help determine the underlying
cause of the large concentration variations which have already been
published.  For CH4, the isotope data should help to sort out the
latidudinal distribution of CH4 emissions which caused the rapid
concentration variations which follow the Dansgaard-Oeschger events
recorded in the Greenland ice core temperature record.  For N2O, the
isotope data are sensitive to changes in the ratio of marine/terrestrial
N2O production which caused lower glacial N2O concentrations.
        The proposed work has not been attempted in the past because
of the large amounts of ice which are needed for standard isotope
ratio analyses.  This project makes use of a new generation mass
spectrometer which is capable of generating precise isotopic
information on nanomolar (10-9) quantities of CH4 and N2O.  This
new instrument is capable of measuring the isotopic composition of
samples which are 1000 times smaller than those needed for a
standard isotope ratio instrument.  In addition, the new instrument
measures the isotopic composition of both CH4 and N2O from a single
air sample (100cc STP) with a precision of ±0.16ä for all isotope
ratios. This level of precision is sufficient to gain a substantial
amount of new information on the biogeochemical cycles involving
CH4 and
N2O


 

Constructing a 110,000 year record of atmospheric [N2O] from the GISP II ice core

Active from 7/1/94-7/1/97

Funding agency: NSF Office of Polar Programs

Project description:
  We propose to construct a 200,000 year record of the 
concentration of N2O in  the atmosphere from the occluded air parcels 
in the GISP II ice core.  This record  will provide information on 
nitrogen biogeochemical cycles over  glacial/interglacial timescales.  
        The concentration of N2O in the atmosphere today is close to
 310 ppbv.  Due  to its ability to absorb long wave radiation, N2O is 
considered to be a  "greenhouse" gas.   Thus, variations in the 
paleoatmospheric levels will have  impacted the latitudinal 
distribution of outgoing radiation, in turn impacting  global climate.  
        The concentration of N2O in air is determined by the sources 
and sinks of N2O  on a global scale.  The only sources of N2O are 
terrestrial and marine biospheres  and the major sink of N2O is 
photodissociation in the stratosphere.  Estimates of  the magnitude of 
the sources and sinks are not presently well constrained but it 
 appears as though the terrestrial biosphere contributes about 80% of 
the global  N2O emissions with the other 20% being made up from an
oceanic contribution.   Comparing an atmospheric N2O record with 
other bioactive gases, such as  CO2 and CH4, will help in understanding 
the nature of changes in global carbon  cycling throughout the past 
200,000 years.  In addition,  N2O will be a useful  stratigraphic tool 
for correlating Greenland and Antarctic ice cores since the N2O 
 records versus time must be the same for each ice core when the 
records are  compared on a common timescale.   
______________________________________________________________
________
Bibliography Brook, E., T. Sowers, and J. Orchardo, Rapid variations in 
atmospheric methane concentration during the past 110,000 years, 
Science, submitted.

Sowers, T.A., E. Brook, D. Etheridge, A. Fuchs, M. Leuenberger, T. 
Blunier, J. Chappellaz, J.M. BArnola, M. Wahlen, B. Deck, C. 
Weyhenmeyer., An inter-laboratory comparison of techniques for 
extracting and analyzing trapped gases in ice cores, JGR 
atmospheres, in press.