The University of Maine DigitalCommons@UMaine Earth Science Faculty Scholarship Earth Sciences 2004 Dating the Siple Dome (Antarctica) Ice Core By Manual and Computer Interpretation of Annual Layering Kendrick C. Taylor Richard B. Alley Debra A. Meese Matthew K. Spencer Ed J. Brook See next page for additional authors Follow this and additional works at: https://digitalcommons.library.umaine.edu/ers_facpub Part of the Earth Sciences Commons Repository Citation Taylor, Kendrick C.; Alley, Richard B.; Meese, Debra A.; Spencer, Matthew K.; Brook, Ed J.; Dunbar, Nelia W.; Finkel, Robert C.; Gow, Anthony J.; Kurbatov, Andrei V.; Lamorey, Gregg W.; Mayewski, Paul Andrew; Meyerson, Eric A.; Nishiizumi, Kunihiko; and Zielinski, Gregory A., "Dating the Siple Dome (Antarctica) Ice Core By Manual and Computer Interpretation of Annual Layering" (2004). Earth Science Faculty Scholarship. 26. https://digitalcommons.library.umaine.edu/ers_facpub/26 This Article is brought to you for free and open access by DigitalCommons@UMaine. It has been accepted for inclusion in Earth Science Faculty Scholarship by an authorized administrator of DigitalCommons@UMaine. For more information, please contact [email protected]. Authors Kendrick C. Taylor, Richard B. Alley, Debra A. Meese, Matthew K. Spencer, Ed J. Brook, Nelia W. Dunbar, Robert C. Finkel, Anthony J. Gow, Andrei V. Kurbatov, Gregg W. Lamorey, Paul Andrew Mayewski, Eric A. Meyerson, Kunihiko Nishiizumi, and Gregory A. Zielinski This article is available at DigitalCommons@UMaine: https://digitalcommons.library.umaine.edu/ers_facpub/26 Journal of Glaciology, Vo l. 50, No.170, 2004 Dating the Siple Dome (Antarctica) ice core by manual and computer interpretation of annual layering 1 2 3 2 KENDRICK C.TAYLOR, RICHARD B. ALLEY, DEBRA A. MEESE, MATTHEW K. SPENCER, 4 5 6 3 ED J. BROOK, NELIA W. DUNBAR, ROBERT C. FINKEL, ANTHONY J. GOW, 7 1 7 7 ANDREI V. KURBATOV, GREGG W. L A MOR EY, PAUL A. MAYEWSKI, ERIC A. MEYERSON, 8 7 KUNIHIKO NISHIIZUMI, GREGORY A. ZIELINSKI 1Desert Research Institute, University and Community College System of Nevada, Reno, Nevada 89512, U.S.A. E-mail: [email protected] 2Department of Geosciences and EMS Environment Institute,The Pennsylvania State University, University Park, Pennsylvania 16802-7501, U. S. A. 3U.S. Army Cold Regions Research and Engineering Laboratory, 72 Lyme Road, Hanover,New Hampshire 03755-1290, U.S.A. 4Geology and Environmental Science,Washington State University, 14204 NE Salmon Creek Ave.,Vancouver,Washington 98686, U.S.A. 5New Mexico Bureau of Mines and Mineral Resources, Earth and Environmental Sciences Department, New MexicoTech, 801Leroy Place, Socorro, New Mexico 87801,U.S.A. 6Center forAccelerator Mass Spectrometry, Lawrence Livermore National Laboratory, Box 808, Livermore, California 94550, U.S.A. 7Climate Change Institute, University of Maine, 303 Bryand Global Sciences Center,Orono, Maine 04469, U.S.A. 8Space Sciences Laboratory, University of California, Berkeley, Berkeley, California 94720, U.S.A. ABSTRACT. The Holocene portion of the Siple Dome (Antarctica) ice core was dated by interpreting the electrical, visual and chemical properties of the core. The data were interpreted manually and with a computer algorithm.The algorithm interpretation was adjusted to be consistent with atmospheric methane stratigraphic ties to the GISP2 (Greenland Ice Sheet Project 2) ice core, 10Be stratigraphic ties to the dendrochronology 14C record and the dated volcanic stratigraphy.The algorithm interpretation is more con- sistent and better quantified than the tedious and subjective manual interpretation. INTRODUCTION drifting and melting, and changes in annual ice accumu- lation. At some locations and depths, the seasonal signal The deep Siple Dome ice core (SDM-A) was drilled in 1999 dominates other influences, making it easy to interpret the to investigate paleoclimate and glaciological topics. It is records (e.g. the Summit Greenland ice cores have out- located on the Siple Coast,West Antarctica, between Kamb standing annual layers between 1600 and 2000 m). At other and Bindschadler Ice Streams (former Ice Streams C and locations, the seasonal signal is not recorded because surfi- D) (81.65‡ S,148.81‡ W; 621m elevation) and extends 1004 m cial processes mix snow deposited over several years before to the bed. A variety of chemical, physical and isotopic it can metamorphose to ice (e.g. Taylor Dome,Vostok, Ant- measurements were made on the core. To interpret these arctica). Siple Dome is between these extremes, and measurements it is necessary to know the age of the ice as a interpretation of annual layers is possible but not trivial. function of depth. At Siple Dome, the current annual ice ac- The datasets must be interpreted to identify annual cumulation rate is13.2 cm aÀ1 (Hamilton, 2002).The annual layers, which are then counted to determine the age of the layers can be identified by seasonal variations in the chem- ice. The interpretation can be done manually, examining istry, electrical conductivity and visual appearance of the and interpreting the data associated with each annual layer, ice.The depth to which the annual layers can be recognized or by using a computer algorithm.Weapplied both methods is limited by diffusion of ions and isotopes in the ice that to the Siple Dome ice core and developed a time-scale that averages out annual variations, and by ice flow that reduces extends to 8610years BP, where an age of 0 BP is assigned to the thickness of annual layers as depth increases. AD1950. Annual layers in ice cores are associated with approxi- mately regularly spaced local maxima in a seasonal indica- AVAILABLE DATASETS tor, such as the acidity of the ice or hoar-frost layers. It would be easy to interpret the records if each year was ex- Electrical conductivity measurement uses a direct current actly like the previous year and was associated with a layer to measure the electrical impedance of ice as a function of thick enough that the seasonal indicator could be conveni- depth (Hammer, 1983; Taylor and others, 1992), and is ently measured. The situation is complicated by other fac- controlled by the acidity of the ice. Seasonal variations in tors such as variations in the amplitude of the seasonal electrical conductivity occur because of seasonal variations maximum, surficial post-depositional processes such as in the acidity of the ice. The equipment used in this 453 Taylor and others: Dating the Siple Dome ice core Table 1.Methane age^depth control points Depth at Siple Dome Age of corresponding Age uncertainty2 features in GISP21 m kyr BP kyr 514.78 8.61 0.55 621.73 11.91 0.54 646.71 13.20 0.65 674.88 15.46 0.73 680.38 15.73 0.68 708.08 18.30 1.00 729.15 23.81 1.13 739.72 24.78 1.09 1 The GISP2 age is the years before AD1950 and considers the gas-age^ ice-age difference in the GISP2 and Siple Dome cores, following the tech- nique used by Blunier and Brook (2001). 2The uncertainty includes the uncertainty in the dating of the GISP2 core, matching of the Siple Dome and GISP2 methane records, and in the gas-age^ice-age differences. Many different light sources including diffuse-side, diffuse- underneath and fiber-optic-point sources were adjusted to produce the best conditions for viewing the core in a dar- kened room. At least two observers examined every1m long section of core for variations in transparency and scattering Fig. 1. Comparison of different records with seasonal vari- associated with annual layering. Visible annual layers are ations. Measurements of the nitrate, chloride, electrical typically formed by warm summer air temperatures and conductivity (ECM) and complex conductivity are shown near-surface solar heating that cause the formation of from 100 m depth. Also shown on the electrical records are coarse-grained, low-density hoar layers, that contrast with interpretations ofannual layers by the computeralgorithm dis- winter higher-density, finer-grained deposits (Alley and cussed in this paper.The visual stratigraphy (bottom panel) others, 1997b). The grain-size difference is typically pre- was interpreted by two groups. One group (Pennsylvania served during metamorphism (e.g. Alley and Woods,1996), State University; square symbol) ranked their confidence in producing ice in which summer layers have fewer, larger the interpretation of each layer on a scale of one to three, with bubbles per unit volume than nearby winter layers. Differ- greater values indicating a greater level of confidence. The ences in the appearance of the ice caused by variations in other group (Cold Regions Research and Engineering La- density and the size of grains and bubbles were manually boratory; triangle symbol on bottom axis) did not record their noted in logbooks. confidence in their interpretation of each layer. The concentrations of the major ions (Naþ,Mg2þ,Kþ, 2þ þ À À 2À Ca ,NH4 ,Cl ,NO3 ,SO4 ) and methanesulfonic acid investigation measures the current flowing between two were measured by ion chromatography in the uppermost electrodes that are in contact with the ice and have a poten- 24 m of the core with a sample interval of 2 cm, correspond- tial difference of 1000 V.The electrodes are 1cm apart, with ing to eight to ten samples per year. Continuous measure- 2 À À a surface area of 4 mm . The spatial resolution of the meas- ments of Cl ,NO3 and liquid conductivity were also urement is about 3 mm. made to identify annual layers (Kreutz and others,1997) in The complex conductivity measurement uses an alter- the uppermost 154m. nating current to measure the electrical impedance of the The visual, electrical and chemical measurements all ice as a function of depth.There is a direct electrical connec- respond to seasonal variations (Fig.