14 C Activity and Global Carbon Cycle Changes over the Past 50,000 Years
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“Active” refers here to those reservoirs that exchange carbon on time scales of 10 1 to 10 4 years.
The Δ 14 C expressed in ‰ is calculated as ( Fm × e λ t – 1) × 1000 where Fm is the fraction of modern 14 C after marine reservoir correction λ is the true 14 C decay constant and t is the calendar age ( 37 ).
Materials and methods are available as supporting material on Science Online.
P. B. Ganset al., Geol. Soc. Am. Abstr. Programs31, 56 (1999).
D. Lal, in Solar-Terrestrial Relationships and the Earth Environment in the Last Millenia, vol. 95 of Proceedings of the International School of Physics, Enrico Fermi (North Holland, Bologna, Italy, 1988), p. 216.
J. Masarik, J. Beer, J. Geophys. Res.104, 12,099 (1999).
The same experiment was performed with the use of a zonally averaged global ocean circulation model with atmospheric and biospheric carbon exchange as described in ( 38 ). The more detailed model produced atmospheric Δ 14 C for the past 50 ka B.P. within ±10‰ of the simple box model. Subsequent glacial perturbation experiments were carried out in the box model as a computational expedient.
Measurements of air bubbles in ice cores show that glacial atmospheric CO 2 concentration averaged ∼200 parts per million by volume (ppmv) compared to a preindustrial value of ∼280 ppmv ( 39 ). A reduction of terrestrial biomass to 65% of its preindustrial size is within the bounds of previous estimates ( 40 ) and is necessary to balance a lowering of glacial δ 13 C of oceanic ΣCO 2 by an average of –0.3‰ ( 41 ).
Reduced deep-ocean ventilation is consistent with potentially restricted NADW formation and penetration in the glacial ocean. Global transects of radiocarbon distribution in the oceans suggest that NADW formation is responsible for sequestering up to 75% of the radiocarbon in the deep ocean ( 42 ).
This change represents a 120-m lowering of sea level and the resultant shift of carbonate sedimentation off the continental shelves into deep water where dissolution is much more extensive. Discussion and details of the glacial carbon cycle boundary conditions are summarized in SOM Text fig. S3 and table S2.
J. F. McManus R. Francois J.-M. Gherardi L. D. Keigwin S. Brown-Leger in preparation.
K. D. Alverson R. S. Bradley T. F. Pederson Eds. Paleoclimate Global Change and the Future (Springer Verlag New York 2003).
This work was supported by NSF (OCE-0117356) Lawrence Livermore National Laboratory (LDRD-97-ERI-009) and the U.S. Department of Energy (W-7405-Eng-48). We thank the ODP Core Repository in Bremen for core sampling; B. Frantz and P. Zermeno for sample preparation assistance; C. Laj M. Frank J.-P. Valet and J. Stoner for providing their data of past geomagentic field intensities; and S. Johnsen for making available the revised GRIPss09sea chronology. This is WHOI contribution no. 11061.