A Large Drop in Atmospheric ¹⁴ C/ ¹² C and Reduced Melting in the Younger Dryas, Documented with ²³⁰ Th Ages of Corals

Abstract

Paired carbon-14 ( 14 C) and thorium-230( 230 Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14 C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14 C and 230 Th ages suggests that the atmospheric 14 C/ 12 C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14 C/ 12 C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.

U-Th dating of deep-sea corals

“…The ␦ 234 U value for modern hermatypic corals measured in the Minnesota lab is 145.8 Ϯ 1.9‰ (2 ) (Cheng et al, 1999;Edwards et al, 1993). This ratio is about 3‰ lower than earlier reports because we have used the new 234 value determined by Cheng et al (1999).…”

“…Later, more precise and accurate thermal ionization mass spectrometry (TIMS) confirmed this result in a variety of locations (Edwards et al, 1987a,b;Gallup et al, 1994;Muhs et al, 1994;Stirling et al, 1995;Szabo et al, 1994). Drill cores of submerged corals that have been precisely dated by 230 Th methods contain our most detailed and continuous record of sea-level since the last glacial maximum (Bard et al, 1993;Bard et al, 1996;Edwards et al, 1993). Coupled high precision radiocarbon and uranium series dates from surface corals constrain the history of atmospheric ⌬ 14 C beyond the tree ring calibration (Bard et al, 1993;Bard et al, 1990;Edwards et al, 1993).…”

“…As the deep-sea samples were collected from nine different depths across the Pacific, Atlantic, Indian and Southern Oceans, these data indicate that the sea water ␦ 234 U value is conservative in the upper 2000 meters of the world's oceans within about Ϯ2‰. In addition, surface coral data indicate that since the last glacial maximum (Bard et al, 1993;Edwards et al, 1993) and for sea level high stands over the past 200,000 years (Gallup et al, 1994;Gallup and Edwards, 1997) the ␦ 234 U of surface seawater has been the same as the modern value within several per mil (Henderson et al, 1993). Since seawater ␦ 234 U appears to be temporally and spatially constant, the ␦ 234 U values in fossil deep-sea corals may be used as a check of diagenetic alteration.…”

“…Procedures for chemical separation and instrumental analysis of uranium and thorium are modifications of those previously described for surface corals (Chen et al, 1986;Edwards et al, 1993;Edwards, 1988;Edwards et al, 1987b). Ultrasonically or chemically cleaned samples were slowly dissolved in nitric acid.…”

“…Drill cores of submerged corals that have been precisely dated by 230 Th methods contain our most detailed and continuous record of sea-level since the last glacial maximum (Bard et al, 1993;Bard et al, 1996;Edwards et al, 1993). Coupled high precision radiocarbon and uranium series dates from surface corals constrain the history of atmospheric ⌬ 14 C beyond the tree ring calibration (Bard et al, 1993;Bard et al, 1990;Edwards et al, 1993). In addition, TIMS dates provide precise ages for coral tracer-based studies of past oceanographic conditions (Beck et al, 1997;Beck et al, 1992;Gagan et al, 1998;Guilderson et al, 1994;McCulloch et al, 1996).…”