Marine-derived C calibration and activity record for the past 50,000 years updated from the Cariaco Basin

An expanded Cariaco Basin ¹⁴C chronology is tied to ²³⁰Th-dated Hulu Cave speleothem records in order to provide detailed marine-based ¹⁴C calibration for the past 50,000 years. The revised, high-resolution Cariaco ¹⁴C calibration record agrees well with data from ²³⁰Th-dated fossil corals back to 33 ka, with continued agreement despite increased scatter back to 50 ka, suggesting that the record provides accurate calibration back to the limits of radiocarbon dating. The calibration data document highly elevated Δ¹⁴C during the Glacial period. Carbon cycle box model simulations show that the majority of observed Δ¹⁴C change can be explained by increased ¹⁴C production. However, from 45 to 15 ka, Δ¹⁴C remains anomalously high, indicating that the distribution of radiocarbon between surface and deep ocean reservoirs was different than it is today. Additional observations of the magnitude, spatial extent and timing of deep ocean Δ¹⁴C shifts are critical for a complete understanding of observed Glacial Δ¹⁴C variability.     

Porosity of the melting zone and variations in the solid mantle upwelling rate beneath Hawaii: inferences from 238U-230Th-226Ra and 235U-231Pa disequilibria

Measurements of ²³⁸U-²³⁰Th-²²⁶Ra and ²³⁵U-²³¹Pa disequilibria in a suite of tholeiitic-to-basanitic lavas provide estimates of porosity, solid mantle upwelling rate and melt transport times beneath Hawaii. The observation that (²³⁰Th/²³⁸U) > 1 indicates that garnet is required as a residual phase in the magma sources for all of the lavas. Both chromatographic porous flow and dynamic melting of a garnet peridotite source can adequately explain the combined U-Th-Ra and U-Pa data for these Hawaiian basalts. For chromatographic porous flow, the calculated maximum porosity in the melting zone ranges from 0.3–3% for tholeiites and 0.1–1% for alkali basalts and basanites, and solid mantle upwelling rates range from 40 to 100 cm yr⁻¹ for tholeiites and from 1 to 3 cm yr⁻¹ for basanites. For dynamic melting, the escape or threshold porosity is 0.5–2% for tholeiites and 0.1–0.8% for alkali basalts and basanites, and solid mantle upwelling rates range from 10 to 30 cm yr⁻¹ for tholeiites and from 0.1 to 1 cm yr⁻¹ for basanites. Assuming a constant melt productivity, calculated total melt fractions range from 15% for the tholeiitic basalts to 3% for alkali basalts and basanites.     

Scavenging and particle residence times determined from 234Th/238U disequilibria in the coastal waters of Mecklenburg Bay

Activities of the naturally occurring radionuclide ²³⁴Th were determined in water samples of Mecklenburg Bay (SW Baltic Sea) using a new Th-specific diatomite adsorption technique followed by liquid scintillation spectrometry. Activities of “dissolved” (operationally defined as Th in the centrifugate) and particulate ²³⁴Th varied in the range of 1.4–6.9 and 0.9–9.3 mBq l⁻¹, respectively. A significant correlation between K_d and SPM concentration was found. From this particle-concentration effect, the “colloidal pumping” model predicts that 98% of the “dissolved” Th is associated with colloids rather than being truly dissolved. Relative to calculated activities of the parent nuclide ²³⁸U in the Bay, the ²³⁴Th data yielded mean ²³⁴Th scavenging residence times in the range of 1.2–9.7 days. Particulate ²³⁴Th activities are inversely correlated to SPM concentrations. Particle residence times ranged from a few days in winter up to 20 days in spring characterized by less intense bottom currents. The hydrodynamic regime is the master variable controlling scavenging of Th and other similarly particle-reactive elements in Mecklenburg Bay.