Science Highlights: Ice Core Science 21

Dating ice cores

JAKOB SCHWANDER Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland; [email protected] Introduction 200 [ppb An accurate chronology is the ba- NO

100 3 ] sis for a meaningful interpretation - 80 ] of any climate archive, including ice 2 0 O

2 40 [ppb cores. Until now, the oldest ice re- H 0 200 Dust covered from a continuous core is [ppb 100 that from Dome Concordia, Antarc- 2 ] ] tica, with an estimated age of over -1 1.6 0

Sm 1.2 µ

800,000 years (EPICA Community [ Conduct. 0.8 160 [ppb Members, 2004). But the recently SO

80 4 2 recovered cores from near bedrock ]

60 - ] at Kohnen Station (Dronning Maud + 40 0 Na Land, Antarctica) and Dome Fuji [ppb 20

40 [ 0 Ca (Antarctica) compete for the longest ppb 2

20 + climatic record. With the recovery of ] 80 0 + more and more ice cores, the task of ] 4 establishing a good common chro- 40 [ppb NH nology has become increasingly im- 0 portant for linking the fi ndings from 1425 1425 .5 1426 1426 .5 1427 the different records. Moreover, in Depth [m] order to create a comprehensive Fig. 1: Seasonal variations of impurities in early Holocene ice from the North GRIP ice core. picture of past climate dynamics, it Summer layers are indicated by grey lines. is crucial to aim at a common chro- al., 1993, Rasmussen et al., 2006). is assessed with such a model, then nology for all paleo-records. Here Ideally the counting uncertainty is one can construct a chronology of the different methods for dating ice on the order of 1%. the ice if past accumulation rates cores are summarized. a(z) are known. Ice fl ow modeling H dz′ Layer counting Ice is a fl uid with an empirical rela- age(z) = ∫ Due to the seasonal variation of tem- tion between stress and strain rate z ε z′ ⋅a(z') perature, sunlight or atmospheric (power law with an exponent of ap- (H is surface elevation of ice sheet) circulation, concentrations of many prox. 3, known as Glen’s law (Glen, trace substances or isotopes show 1955)). With the knowledge of all The estimate of past accumulation cyclic variations in the ice cores. In material properties and boundary rates is the Achilles’ heel of dating the ideal case, these annual cycles conditions, the plastic deforma- by fl ow models. Such estimates are can be counted many thousand tion of a glacier or ice sheet could mostly based on a functional depen- years back in time. Depending on be modeled in detail (Fig. 2). How- dence of precipitation and local tem- the conditions in the ice (tempera- ever, these boundary conditions perature (estimated from the stable ture, thinning of layers, etc.), varia- and properties are generally poorly isotope records). There are, how- tions are smoothed by diffusion or known. The bedrock topography is ever, other causes for variations in altered by chemical reactions or not resolved in suffi cient detail and past accumulation rates, like chang- clustering. Diffusion can be reversed past surface conditions (tempera- es in circulation or changes in local mathematically to a certain degree ture, precipitation) can only be es- surface topography. In order to im- by a deconvolution algorithm (John- timated indirectly. The same holds prove the accuracy of the age mod- sen, 1977), which extends the depth for the parameters that determine el, parameters can be constrained range for layer counting. Yet, there the fl ow properties, like ice temper- by assigning well-dated time hori- is always a lower limit below which ature, crystal orientation, impurities zons to corresponding depths. Such the annual variations are lost. One and basal sliding. Flow profi les at events are, e.g., glacial terminations reason for this is that wind is mixing a certain site can often be approxi- or minima of Earth’s magnetic fi eld the snow on the surface by scour- mated by simple fl ow models that that have been dated with best ing and relocation. Another reason are determined by local parame- possible accuracy on other paleo- is that the annual variations are al- ters, like ice sheet thickness, basal archives (Parrenin et al., 2001). The ready smoothed out when they are sliding, basal melting, and few pa- age uncertainty obtained by fl ow compacted to solid ice at the fi rn-ice rameters to describe the horizontal models depends on the accuracy of transition. Best accuracy for layer fl ow. These parameters are either accumulation rate estimates (in the counting is obtained by combining measured or constrained by age fi x- order of 20%) and on the existence different tracers (Fig. 1) (Fuhrer et points. If thinning εz at any depth z of local fl ow disturbances.

PAGES NEWS, VOL.14, N°1, APRIL 2006 22 Science Highlights: Ice Core Science

historic volcanic signals) to several thousand years.

Age of the gaseous records The upper 50-120 m of an ice sheet is made of consolidated snow (fi rn), permeable to air. Air occluded at the firn-ice transition is therefore substantially younger than the ice. Fig. 2: Cross-section through an idealized ice sheet. Layers are stretched and thinned by plastic This age difference can be estimated fl ow. The two highlighted layers (blue) comprise the same number of annual layers. with densifi cation/diffusion models Radiometric dating gaard-Oeschger events), variations (Schwander et al., 1997). The air at Because both layer counting and in the Earth’s magnetic fi eld or solar the fi rn-ice transition is on the order fl ow models may be subject to sys- activity, etc. Global records of meth- of one to a few decades. The age 18 tematic errors, absolute radiometric ane and δ Oatm, for example, have of the ice at the fi rn-ice transition clocks are needed. A number of ra- been used to synchronize ice core re- spans a large range of a few tens to dioactive isotopes have been used cords from Greenland and Antarctica several thousand years under pres- to estimate the age of natural ice (Blunier and Brook, 2001). ent conditions, depending mainly samples or the occluded air. These In the second half of the 19th on temperature and accumulation include fi ssion products, 3H (Tritium), century, James Croll proposed that rates. Locations with high accumu- 210Pb, 32Si, 39Ar, 85Kr, 14C, 81Kr, 36Cl, 10Be, the Pleistocene ice ages are caused lation rates and relatively high an- and Uranium series. Fission prod- by periodic changes in the Earth’s nual temperatures show the smallest ucts resulting from nuclear weapon orbit around the Sun. Milutin Mila- ages, while sites in central Antarctica testing in the atmosphere during the nkovitch improved the astronomical with accumulation rates of a few cm 1950s and 60s, can easily be detected theory, leading to a presently gen- per year show the largest ages. The by total beta-activity measurements. eral acceptance of the causality be- difference between the age of the Other radiometric dating methods tween Earth’s orbital parameters and ice and the age of the air has not re- have failed to be competitive with climate variations. There are three mained constant over time. Under current dating tools because the ini- astronomical parameters that de- glacial conditions this age difference tial concentrations of the radioactive termine the latitudinal and seasonal was generally several times larger isotopes are too variable. However, variation of insolation on Earth: Pre- than at present. Due to uncertain- these methods remain interesting, cession (periods: 22, 24 kyr), Obliq- ties in estimating the accumulation e.g., to estimate the age of very old uity (41, 40 kyr), and Eccentricity (404 rate and the close-off depth in the ice near bedrock. kyr and several around 100 kyr). past, the error in the age difference Tuning to the Milankovitch cy- is around 10% Synchronisation with other cles is accomplished by changing paleo-records the chronology of a record such that REFERENCES Bender, M. L., 2002: Orbital tuning chronology for the Ice core chronologies often rely on its correlation with the target curve Vostok climate record supported by trapped gas indirect dating, i.e. by comparison (e.g. summer insolation at a certain composition. Earth and Planetary Science Letters, with other, better-dated paleo-ar- latitude) is increased. A shortcom- 204: 275-289. Blunier, T. and Brook, E. J., 2001: Timing of millennial- chives, including other ice cores. In ing of the method is the problem scale climate change in Antarctica and Greenland the ideal case, the well-dated record of overtuning. Recent orbital tuning during the last glacial period. Science, 291: 109-112. is annually laminated (Hughen et work has therefore been as con- Johnsen, S. J., 1977: In Proc. of Symp. on Isotopes and al., 1998, Zolitschka, 1998). Yet, these servative as possible (Karner et al., Impurities in Snow and Ice, Int. Ass. of Hydrol. Sci., Commission of Snow and Ice, I.U.G.G. XVI, General varved sediments are often affected 2002, Lisiecki and Raymo, 2005). The Assembly, Grenoble Aug.Sept., 1975. IAHS-AISH by hiatuses or do not extend to the uncertainty in the phasing between publication: 210-219. present but they can provide valu- paleo-records and the astronomical Johnsen, S. J., Clausen, H. B., Dansgaard, W., Fuhrer, K., Gundestrup, N., Hammer, C. U., Iversen, P., able information on the duration of cycles is on the order of 5 kyr. Ice Jouzel, J., Stauffer, B. and Steffensen, J. P., 1992: climatic events. Other archives are core records can be correlated to the Irregular glacial interstadials recorded in a new well suited for absolute dating meth- Milankovitch cycles either directly or Greenland ice core. Nature, 359: 311-313. Schwander, J., Sowers, T., Barnola, J.-M., Blunier, T., ods, like U/Th-dated speleothems or indirectly via synchronization, e.g., Malaizé, B. and Fuchs, A., 1997: Age scale of the corals. with marine sediment records (EPICA air in the summit ice: Implication for glacial-inter- The prerequisite for a correlation Community Members, 2004). Direct glacial temperature change. Journal of Geophysical with ice core records is that events tuning includes the temperature-sen- Research, 102: 19483-19494. exist in the ice core record that are sitive stable isotope ratios of the ice, 18 For full references please consult: unequivocally assignable to a syn- the δ O of oxygen in the trapped air www.pages-igbp.org/products/newsletters/ref2006_1.html chronous event in the other record. (Parrenin et al., 2001), and the O2/N2 Such events can be volcanic erup- ratio (Bender, 2002). Age uncertain- tions, Milankovitch-type variations, ties obtained by synchronization abrupt climate variations (Dans- techniques vary from one year (e.g.

PAGES NEWS, VOL.14, N°1, APRIL 2006