Isotopic evidence for quasi-equilibrium chemistry in thermally mature natural gases.

Natural gas is a key energy resource, and understanding how it forms is important for predicting where it forms in economically important volumes. However, the origin of dry thermogenic natural gas is one of the most controversial topics in petroleum geochemistry, with several differing hypotheses proposed, including kinetic processes (such as thermal cleavage, phase partitioning during migration, and demethylation of aromatic rings) and equilibrium processes (such as transition metal catalysis). The dominant paradigm is that it is a product of kinetically controlled cracking of long-chain hydrocarbons. Here we show that C2+n-alkane gases (ethane, propane, butane, and pentane) are initially produced by irreversible cracking chemistry, but, as thermal maturity increases, the isotopic distribution of these species approaches thermodynamic equilibrium, either at the conditions of gas formation or during reservoir storage, becoming indistinguishable from equilibrium in the most thermally mature gases. We also find that the pair of CO2 and C1 (methane) exhibit a separate pattern of mutual isotopic equilibrium (generally at reservoir conditions), suggesting that they form a second, quasi-equilibrated population, separate from the C2 to C5 compounds. This conclusion implies that new approaches should be taken to predicting the compositions of natural gases as functions of time, temperature, and source substrate. Additionally, an isotopically equilibrated state can serve as a reference frame for recognizing many secondary processes that may modify natural gases after their formation, such as biodegradation.

Stable Isotope Analysis of Intact Oxyanions Using Electrospray Quadrupole-Orbitrap Mass Spectrometry.

The stable isotopes of sulfate, nitrate, and phosphate are frequently used to study geobiological processes of the atmosphere, ocean, as well as land. Conventionally, the isotopes of these and other oxyanions are measured by isotope-ratio sector mass spectrometers after conversion into gases. Such methods are prone to various limitations on sensitivity, sample throughput, or precision. In addition, there is no general tool that can analyze several oxyanions or all the chemical elements they contain. Here, we describe a new approach that can potentially overcome some of these limitations based on electrospray hyphenated with Quadrupole Orbitrap mass spectrometry. This technique yields an average accuracy of 1-2‰ for sulfate δ34S and δ18O and nitrate δ15N and δ18O, based on in-house and international standards. Less abundant variants such as δ17O, δ33S, and δ36S, and the 34S-18O "clumped" sulfate can be quantified simultaneously. The observed precision of isotope ratios is limited by the number of ions counted. The counting of rare ions can be accelerated by removing abundant ions with the quadrupole mass filter. Electrospray mass spectrometry (ESMS) exhibits high-throughput and sufficient sensitivity. For example, less than 1 nmol sulfate is required to determine 18O/34S ratios with 0.2‰ precision within minutes. A purification step is recommended for environmental samples as our proposed technique is susceptible to matrix effects. Building upon these initial provisions, new features of the isotopic anatomy of mineral ions can now be explored with ESMS instruments that are increasingly available to bioanalytical laboratories.

Abrupt pre-Bølling-Allerød warming and circulation changes in the deep ocean.

Several large and rapid changes in atmospheric temperature and the partial pressure of carbon dioxide in the atmosphere--probably linked to changes in deep ocean circulation--occurred during the last deglaciation. The abrupt temperature rise in the Northern Hemisphere and the restart of the Atlantic meridional overturning circulation at the start of the Bølling-Allerød interstadial, 14,700 years ago, are among the most dramatic deglacial events, but their underlying physical causes are not known. Here we show that the release of heat from warm waters in the deep North Atlantic Ocean probably triggered the Bølling-Allerød warming and reinvigoration of the Atlantic meridional overturning circulation. Our results are based on coupled radiocarbon and uranium-series dates, along with clumped isotope temperature estimates, from water column profiles of fossil deep-sea corals in a limited area of the western North Atlantic. We find that during Heinrich stadial 1 (the cool period immediately before the Bølling-Allerød interstadial), the deep ocean was about three degrees Celsius warmer than shallower waters above. This reversal of the ocean's usual thermal stratification pre-dates the Bølling-Allerød warming and must have been associated with increased salinity at depth to preserve the static stability of the water column. The depleted radiocarbon content of the warm and salty water mass implies a long-term disconnect from rapid surface exchanges, and, although uncertainties remain, is most consistent with a Southern Ocean source. The Heinrich stadial 1 ocean profile is distinct from the modern water column, that for the Last Glacial Maximum and that for the Younger Dryas, suggesting that the patterns we observe are a unique feature of the deglacial climate system. Our observations indicate that the deep ocean influenced dramatic Northern Hemisphere warming by storing heat at depth that preconditioned the system for a subsequent abrupt overturning event during the Bølling-Allerød interstadial.