Ocean circulation and geomagnetic influences on the atmospheric 14C record spanning the past 50,000 years from u-th and 14C dated fossil corals

• Main Author: Mortlock, Richard Andrew
• Format: Text
• Language: unknown
• Published: No Publisher Supplied 2017
• Subjects: Pacific; NADW; North Atlantic Deep Water; North Atlantic
• Online Access: https://dx.doi.org/10.7282/t3qz2dtr; https://rucore.libraries.rutgers.edu/rutgers-lib/53745/

The radiocarbon content (14C) of the atmosphere is primarily controlled by changes in production, regulated by solar output and shielding in the earth’s magnetic field, further influenced by changes in ocean circulation. Fossil corals provide an excellent archive for reconstructing past changes in the 14C/12C atmospheric ratio (Δ14C) because both the 14C age and the calendar age, determined by U-series dating methods, can be obtained from the same sample. This is a requirement for computing accurate estimates of Δ14C. Accuracy in coral Δ14C records, however, requires that corrections be made for the age offset between the surface water in which the coral grew and the contemporaneous atmosphere, called the marine reservoir age correction. In this study the marine reservoir correction is calculated for tropical Atlantic (Barbados) and Pacific (Kiritimati Atoll and Araki Island) corals to generate a record of Δ14C from 50 to 7 thousand years before present (kyr BP). Atlantic and Pacific coral Δ14C data precisely match the timing and amplitude changes predicted in ocean-atmosphere coupled models demonstrating shutdown of North Atlantic Deep Water (NADW) at the start of the Younger Dryas cold interval (12.9 kyr BP) lowers the 14C age of Atlantic surface water. A sharp decline in coral Δ14C at 14.7 kyr BP coincides with proxy evidence for switch on of NADW. Taken together, these records provide credible evidence linking changes in NADW production to changes in the 14C content of the atmosphere. In contrast to 15 to 7 kyr BP when changes in Δ14C largely reflect production changes in North Atlantic Deep Water, prominent features in Δ14C from 50 to 15 kyr BP are due to the long-term effects of increased production of atmospheric 14C during a weakened geomagnetic field at the time of the Laschamp (41 kyr BP) and Mono Lake (31.5 kyr BP) excursions. Removing the excess 14C produced in the atmosphere during excursion events from the Δ14C record effectively removes the rapid Δ14C decline during the “Mystery Interval” (17.5 to 14.5 kyr BP) that has been widely attributed to release of 14C-depleted CO2 during ventilation of the deep ocean.