Prosper K. Zigah

Assessing the blank carbon contribution, isotope mass balance, and kinetic isotope fractionation of the Ramped Pyrolysis/Oxidation instrument at NOSAMS

We estimate the blank carbon mass over the course of a typical Ramped PyrOx (RPO) analysis (150–1000°C; 5°C×min –1 ) to be (3.7±0.6) μg C with an Fm value of 0.555±0.042 and a δ 13 C value of (–29.0±0.1) ‰ VPDB. Additionally, we provide equations for RPO Fm and δ 13 C blank corrections, including associated error propagation. By comparing RPO mass-weighted mean and independently measured bulk δ 13 C values for a compilation of environmental samples and standard reference materials (SRMs), we observe a small yet consistent 13 C depletion within the RPO instrument (mean–bulk: μ=–0.8‰; ±1σ=0.9‰; n =66). In contrast, because they are fractionation-corrected by definition, mass-weighted mean Fm values accurately match bulk measurements (mean–bulk: μ=0.005; ±1σ=0.014; n =36). Lastly, we show there exists no significant intra-sample δ 13 C variability across carbonate SRM peaks, indicating minimal mass-dependent kinetic isotope fractionation during RPO analysis. These data are best explained by a difference in activation energy between 13 C- and 12 C-containing compounds ( 13–12 ∆E ) of 0.3–1.8 J×mol –1 , indicating that blank and mass-balance corrected RPO δ 13 C values accurately retain carbon source isotope signals to within 1–2‰.

Allochthonous sources and dynamic cycling of ocean dissolved organic carbon revealed by carbon isotopes

We present concentration and isotopic profiles of total, size, and polarity fractionated dissolved organic carbon (DOC) from Station ALOHA, an oligotrophic site in the North Pacific Ocean. The data show that between the surface and 3500 m, low molecular weight (LMW) hydrophilic DOC, LMW hydrophobic DOC and high molecular weight (HMW) DOC constitute 22-33%, 45-52% and 23-35% of DOC respectively. LMW hydrophilic DOC is more isotopically depleted (δ13C of -23.9‰ to -31.5‰ and Δ14C of -304‰ to -795‰; mean age of 2850 to 15000 14C years) than the LMW hydrophobic DOC (δ13C of -22‰ to -23‰ and Δ14C of -270‰ to -568‰; 2470 to 6680 years) and HMW DOC (δ13C of ~ -21‰ and Δ14C of -24‰ to -294‰; 135-2700 years). Our analyses suggest a large fraction of DOC may be derived from allochthonous sources such as terrestrial and hydrothermal DOC, and cycle on much longer timescales of >10,000 yr, or enter the ocean as pre-aged carbon.