High-precision thorium-230/uranium-243/uranium-238 dating of the surface ocean radiocarbon record and its geochemical and geophysical implications

Reconstruction of atmospheric and surface ocean 14C content during the last deglaciation is crucial to understanding changes in 14C production and carbon cycle mixing rates. Tropical and cold-water fossil corals provided excellent archives of atmospheric and ocean 14C content, because they can be dated by 14C and 230Th/234U/238U dating methods with high precision and high accuracy.;Chapter two of this dissertation presents a radiocarbon reservoir age record of high latitude western North Atlantic Ocean during the last deglaciation via the use of fossil cold-water corals growing in waters that are rapidly exchanged with nearby surface waters. The high latitude North Atlantic Ocean reservoir age data combined with recalculated reservoir ages based on published coexisting terrestrial and marine material and Vedde ash radiocarbon dates from central and eastern North Atlantic show modern values (380+/-140 year, n=14) during the Bolling/Allerod (BA) warm period and a 200 year increase in reservoir age (590+/-130 year, n=10) during the entire Younger Dryas (YD) cold episode. The reservoir age then decreased to 270+/-20 year (n=2) at the Preboreal/YD transition, although the dates are too sparse to place confidence in this estimate. Intrusion of 14C depleted Antarctic Intermediate Water (AAIW) to the high latitude North Atlantic and reduction of NADW formation are possible causes for the coincident shift to high reservoir ages in the North Atlantic surface ocean and increased atmospheric Delta 14C during the beginning of the YD event.;Chapter three presents a reconstruction of tropical ocean surface radiocarbon reservoir ages for the Holocene deglaciation. Coral data from Barbados, Kiritimati, and Vanuatu combined with previously published coral data from Tahiti, Papua New Guinea, and Marquesas provide coverage of the tropical Atlantic and Pacific Oceans (Bard et al., 1998; Cutler et al., 2004; Edwards et al., 1993; Fairbanks et al., 2005; Paterne et al., 2004). Reservoir age calculated during last glaciation (Barbados, 335 +/- 110 (n=26); Kiritimati, 335 +/- 50 (n=23); Vanuatu, 385 +/- 140 (n=27); Tahiti, 260 +/- 120 (n=14); Papua New Guinea, 490 +/- 190 (n=16); Marquesas, 510 +/- 160 (n=5)) are similar to modern values. Based on these results, it is reasonable to apply a constant reservoir age to correct the marine radiocarbon age during the last deglaciation. We also re-anchored a floating tree ring chronology using Barbados and Kiritimati coral radiocarbon record (Kromer et al., 2004). The agreement between the re-anchored floating tree ring and reservoir age corrected Vanuatu and Tahiti coral record confirmed the absence of significant reservoir age variation during the BA warm interval (Bard et al., 1998, Cutler et al., 2004). Finally, we explored parameters that might influence the reservoir age of tropical surface water. The variation of trade wind stress rather than variation of deep-ocean circulation is the most important factor of tropical surface reservoir age. These results should be helpful in constraining global ocean circulation models and simulations.;Chapter four displays an atmospheric Delta14C record from paired 230Th/234U/238U and 14C dates of tropical surface fossil corals from Kiritimati and Barbados during the last deglaciation. The high resolution Delta 14C record reveals a series centennial to millennial scale fluctuations during the last deglaciation. We illustrate the sensitivity of Delta 14C changes to the uncertainty in possible 14C half-lives, and further evaluate the millennial and centennial time scale variability by also considering the effect of the geomagnetic field change. Our half-life and geomagnetic field corrected Delta14C curve retains the long term trend but results in more reasonable amplitude and smaller variations. Our new Delta14C record decreased from 190 per mil to 50 per mil from 19000 to 14000 yr BP, with a Delta14C plateau at around 70 per mil between 16000 and 14500 yr BP, a 40 per mil increase from 13000 to 12500 yr BP, and a 60 per mil decrease during 12500 to11600 yr BP. The comparison between our atmospheric Delta14C and deep-Pacific benthic-planktonic 14C age differences suggests that the reported changes in deep ocean circulation were not the major factor in atmospheric Delta14C variations during the last deglaciation. The correlation between the estimated 14C production rates computed from ice core 10Be records and the atmospheric Delta14C record suggests that 14C production rate changes played a more important role in the last deglaciation atmospheric Delta14C variations than generally believed. The coincidence between the prominent Delta 14C fluctuations and several dominant climate boundaries begs the question whether solar fluctuations may initiate some of the well documented climate shifts or oscillations.