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.
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.
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.
To download the raw Moulton gas data, click
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
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,
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.