HR: 08:30h
AN: U31A-02    [Abstracts]
TI: The last glacial retreat on the Mauna Kea summit (Hawaii, 20°N) constrained by cosmogenic 3He dating of moraines
AU: Lave, J
EM: jerome.lave@ujf-grenoble.fr
AF: LGCA CNRS Univ. Joseph Fourier, Maison des Geosciences, Grenoble, 38041 France
AU: * Blard, P
EM: blard@cerege.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 54501 France
AU: * Blard, P
EM: blard@cerege.fr
AF: CEREGE CNRS Univ Paul Cezanne, Europole de l Arbois, Aix en Provence, 13545 France
AU: Pik, R
EM: rpik@crpg.cnrs-nancy.fr
AF: CRPG CNRS, Rue Notre Dame des Pauvres, Vandoeuvre les Nancy, 54501 France
AU: Bourles, D
EM: bourles@cerege.fr
AF: CEREGE CNRS Univ Paul Cezanne, Europole de l Arbois, Aix en Provence, 13545 France
AB: The top of Mauna Kea volcano (Hawaii island) is the only highland area of the central Pacific Ocean that has preserved direct morphological evidences of the last glaciation. In order to constrain the age of the last glacial retreat, 9 cobbles and boulders have been sampled at ca. 3600 m on 3 small recessional moraines posterior to the abandonment of the major lateral moraine of the Makanaka Glacial Stage (MGS). At ca. 3200 m, 3 boulders have also been sampled on an outwash formation anterior to the MGS. All the samples were selected for their rich mafic phenocrysts content (olivines and clinopyroxenes), making them well suited for the precise and accurate method of cosmogenic 3He (3Hec) exposure dating. At ca. 3200 m the low outwash boulders yield concordant 3Hec exposure ages of 18±1 ka BP. On the other hand, the ca. 3600 m lateral moraines are characterized by younger and largely distributed ages between 15 and 9 ka BP. The block sizes are clearly correlated with their exposure ages and suggest that the morainic surfaces have suffered an erosion of several centimetres by soil deflation. Discarding the youngest ages associated to the smallest cobbles (less than 50 cm), the three morainic cordons ages can be interpreted as the proof of a rapid retreat between 15 and 13 ka BP. The major morainic formation of the MGS ranges therefore between 18 and 15 ka BP. These 3Hec ages are compatible but slightly younger than the preliminary exposure 36Clc ages (from 15 to 20 ka BP) measured on boulders of the Makanaka Glacial Members on Mauna Kea [Dorn et al., 1990]. However this glacial retreat timing seems to be in relative agreement with the persistence of low local temperature until 17 ka, and then with a progressive Late Glacial warming, as recorded by the Central Pacific SST [Lee et al, 2001; Kiefer and Kienast, 2005]. These 3Hec data are also concordant with the timing of the final glacial retreat observed in the tropical Andes [Smith et al., 2005], and could indicate a synchronous climatic evolution at the scale of the inter and sub-tropical Pacific regions. In addition, the persistence of an ice cap on the Mauna Kea summit beyond 14 ka implies that the ELA was still depressed by ca. 750 m relative to present time: considering a difference of 2 C of the SST between 14 ka BP and today [Lee et al, 2001], such ELA lowering required either a higher local lapse rate in the Central Pacific (0.5 C/km), or higher snow precipitations on the Mauna Kea summit. Dorn et al., Nat. Geogr. Res. & Explor., 7, 1991 Kiefer and Kienast, QSR, 24, 2005 Lee et al., Paleoceanography, 16, 2001 Smith et al., Science, 308, 2005
DE: 0776 Glaciology (1621, 1827, 1863)
DE: 1150 Cosmogenic-nuclide exposure dating (4918)
DE: 1827 Glaciology (0736, 0776, 1863)
DE: 3309 Climatology (1616, 1620, 3305, 4215, 8408)
DE: 4918 Cosmogenic isotopes (1150)
SC: Union [U]
MN: Fall Meeting 2005