TY - JOUR
T1 - Evidence for large century time-scale changes in solar activity in the past 32 Kyr, based on in-situ cosmogenic 14C in ice at Summit, Greenland
AU - Lal, Devendra
AU - Jull, A. J.T.
AU - Pollard, David
AU - Vacher, Loic
N1 - Funding Information:
We are grateful to J. W. Bieber for providing us with data from the South Pole neutron monitor, which is operated by the University of Delaware with support from NSF grant ATM-0000315. We owe our gratitude to Mark Twickler for his help and guidance in selection and procurement of the valuable GISP2 ice samples. We thank Jeff Severinghaus for valuable discussions and D. Desiletts for valuable suggestions on analyzing the neutron monitor records. We record our grateful thanks to Richard Alley for valuable comments on the manuscript. This work was supported in part from grants, NSF OPP-9909484 and NSF ATM-9905299 (to Lal), and in part by NASA grant NAG5-4832 (to Jull). One of us (D.L.) thanks Joan Eichen and late Myron Eichen for their support and encouragement.
PY - 2005/6/15
Y1 - 2005/6/15
N2 - We present results of estimates of cosmic ray flux in Greenland at the Summit (3200 m.a.s.l, 72.6°N, 38.5°W), during the past 32 Kyr. We derive these estimates based on concentrations of in-situ cosmogenic 14C produced in ice crystals. Based on the secular equilibrium concentration of in-situ produced 14C in quartz in terrestrial rocks, we find that on century time-scales, the cosmic ray production rate of 14C at the Summit was close to its estimated long-term average production rate, except during 3 periods: (i) during 8500-9500 yr B.P. and 27,000-32,000 yr B.P, when the production rate was higher by about a factor of 2, and (ii) during 12,000-16,000 yr B.P, when the production rate was lower by a factor of 1.5. The observed variation in cosmic ray flux at the polar site is best attributed to changes in solar activity resulting in variable modulation of terrestrial cosmic ray flux. Changes in the geomagnetic field in the past do not affect the cosmic ray flux at polar latitudes. Likewise, climate changes do not affect the in-situ 14C record in ice. During the first two epochs, the solar activity must have been very low, as during Maunder Minimum (virtually no sunspots), resulting in essentially no modulation of the cosmic ray flux by the solar plasma. During the low cosmic ray flux epoch, 12,000-16,000 yr B.P., the observed decrease in cosmic ray flux corresponds to high solar activity as seen in 1958 (sun-spot number ∼ 190). We discuss the proxy evidence from tree ring and sediment based records of atmospheric 14C/12C ratios during the three epochs. These records have been used as a measure of changes in cosmic ray flux, and solar activity in the past. However, since they are also appreciably affected by climatic changes, a comparison of the two records is potentially valuable for delineating the nature of past changes in solar activity, and large-scale ocean circulation and air-sea exchange.
AB - We present results of estimates of cosmic ray flux in Greenland at the Summit (3200 m.a.s.l, 72.6°N, 38.5°W), during the past 32 Kyr. We derive these estimates based on concentrations of in-situ cosmogenic 14C produced in ice crystals. Based on the secular equilibrium concentration of in-situ produced 14C in quartz in terrestrial rocks, we find that on century time-scales, the cosmic ray production rate of 14C at the Summit was close to its estimated long-term average production rate, except during 3 periods: (i) during 8500-9500 yr B.P. and 27,000-32,000 yr B.P, when the production rate was higher by about a factor of 2, and (ii) during 12,000-16,000 yr B.P, when the production rate was lower by a factor of 1.5. The observed variation in cosmic ray flux at the polar site is best attributed to changes in solar activity resulting in variable modulation of terrestrial cosmic ray flux. Changes in the geomagnetic field in the past do not affect the cosmic ray flux at polar latitudes. Likewise, climate changes do not affect the in-situ 14C record in ice. During the first two epochs, the solar activity must have been very low, as during Maunder Minimum (virtually no sunspots), resulting in essentially no modulation of the cosmic ray flux by the solar plasma. During the low cosmic ray flux epoch, 12,000-16,000 yr B.P., the observed decrease in cosmic ray flux corresponds to high solar activity as seen in 1958 (sun-spot number ∼ 190). We discuss the proxy evidence from tree ring and sediment based records of atmospheric 14C/12C ratios during the three epochs. These records have been used as a measure of changes in cosmic ray flux, and solar activity in the past. However, since they are also appreciably affected by climatic changes, a comparison of the two records is potentially valuable for delineating the nature of past changes in solar activity, and large-scale ocean circulation and air-sea exchange.
KW - Cosmic ray fluxes
KW - Cosmogenic C in polar ice
KW - Geomagnetic indices
KW - Solar activity
KW - Sun spot numbers
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U2 - 10.1016/j.epsl.2005.02.011
DO - 10.1016/j.epsl.2005.02.011
M3 - Article
AN - SCOPUS:20344379167
SN - 0012-821X
VL - 234
SP - 335
EP - 349
JO - Earth and Planetary Science Letters
JF - Earth and Planetary Science Letters
IS - 3-4
ER -