Insights into oil cracking based on laboratory experiments

The objectives of this pyrolysis investigation were to determine changes in (1) oil composition, (2) gas composition and (3) gas carbon isotope ratios and to compare these results with hydrocarbons in reservoirs. Laboratory cracking of a saturate-rich Devonian oil by confined, dry pyrolysis was performed at T=350–450 °C, P=650 bars and times ranging from 24 h to 33 days. Increasing thermal stress results in the C₁₅₊ hydrocarbon fraction cracking to form C_6–14 and C_1–5 hydrocarbons and pyrobitumen. The C_6–14 fraction continues to crack to C_1–5 gases plus pyrobitumen at higher temperatures and prolonged heating time and the δ ¹³C_ethane–δ¹³C_propane difference becomes greater as oil cracking progresses. There is considerable overlap in product generation and product cracking. Oil cracking products accumulate either because the rate of generation of any product is greater than the rate of removal by cracking of that product or because the product is a stable end member under the experimental conditions. Oil cracking products decrease when the amount of product generated from a reactant is less than the amount of product cracked. If pyrolysis gas compositions are representative of gases generated from oil cracking in nature, then understanding the processes that alter natural gas composition is critical.     

Magnetic hydrogen atmosphere models and the neutron star RX J1856.5–3754

RX J1856.5−3754 is one of the brightest nearby isolated neutron stars (INSs), and considerable observational resources have been devoted to it. However, current models are unable to satisfactorily explain the data. We show that our latest models of a thin, magnetic, partially ionized hydrogen atmosphere on top of a condensed surface can fit the entire spectrum, from X-rays to optical, of RX J1856.5−3754, within the uncertainties. In our simplest model, the best-fitting parameters are an interstellar column density   N _H≈ 1 × 10²⁰ cm⁻²  and an emitting area with   R ^∞≈ 17 km  (assuming a distance to RX J1856.5−3754 of 140 pc), temperature   T ^∞≈ 4.3 × 10⁵ K  , gravitational redshift   z _g ∼ 0.22  , atmospheric hydrogen column   y _H≈ 1 g cm⁻²  , and magnetic field   B ≈ (3–4) × 10¹² G  ; the values for the temperature and magnetic field indicate an effective average over the surface. We also calculate a more realistic model, which accounts for magnetic field and temperature variations over the NS surface as well as general relativistic effects, to determine pulsations; we find that there exist viewing geometries that produce pulsations near the currently observed limits. The origin of the thin atmospheres required to fit the data is an important question, and we briefly discuss mechanisms for producing these atmospheres. Our model thus represents the most self-consistent picture to date for explaining all the observations of RX J1856.5−3754.     

Major ice sheet response in eastern New England to a cold North Atlantic region, ca. 16-15 cal ka BP

A large ice sheet still covered almost all of Maine and eastern New England until ca. 15 cal ka BP, reaching south of 45 °S, despite rising summer insolation intensity and major ice recession elsewhere outside the North Atlantic region. Furthermore, the well-studied moraine belt along eastern coastal Maine, including the prominent Pineo Ridge delta/moraine complex and Pond Ridge moraine, indicates repeated readvances and stillstands between ca. 16 and 15 cal ka BP. This moraine belt reflects a considerable ice sheet response over eastern North America during this time period, coeval with the latter half of the European Oldest Dryas period. Moraine deposition was concurrent with reduction or elimination of North Atlantic meridional overturning, starting with the earlier onset of peak IRD and Heinrich Event 1 (HE-1). The existing ¹⁴C chronology suggests that the coastal moraine belt and the persistence of the ice sheet until ∼ 15 cal ka BP was a response to the severe cooling of the North Atlantic region after ∼ 17 cal ka BP.     

Heavy metals behavior in coastal sediments of southern California: a critical review and synthesis

The present paper reviews and discusses the studies on heavy metals behavior in coastal sediments of southern California which have been published since 1970.The available data allow for the estimation of pollution-free ‘base-line’ concentrations of some heavy metals in southern California sediments (Ag, 0.4 ppm; Cd, 0.4 ppm; Cr, 25 ppm; Cu, 9 ppm; Ni, 15 ppm; Pb, 10 ppm; and Zn, 44 ppm) which may be used as references for monitoring future metal pollution.The transport of most, if not all, heavy metals through the near-shore and shelf environments is controlled by their association with particles, a significant fraction of which is sewage-derived and rich in organic carbon. Changes in the relative abundances of the heavy metals are brought about mainly by settling differentiation of their host particles, and much less so by diagenetic solubilization. The bulk of the heavy metals are transported, within fine-grained particles, out from the shelf into deep ocean regions.Simple linear mixing of polluted outfall materials with clean natural sediments cannot explain the distribution of heavy metals in near-shore and basinal sediments in southern California, unless significant protective effects of organic coatings (near outfalls) and of grain-size distributions are taken into account. Heavy metal pollution from oil seepages are significant for only a very limited number of metals (Ni, Ba, etc.) and only on a local scale.The urgent need for standardization of heavy metal leaching techniques is demonstrated and emphasized.     

Stable isotope composition of dissolved sulfate and hydrogen sulfide in the Black Sea

The stable isotope ratio of sulfur (³⁴S/³²S) in dissolved sulfate and hydrogen sulfide was measured for 20 water samples from two deep hydrocasts from the south-central Black Sea. The isotope ratio of total reduced sulfur was also measured for surface sediment collected below each hydrocast. The range in the δ³⁴S measurements for sulfate was +18.20 to $\text{+20.17‰}$ and for hydrogen sulfide ?38.71 to $\text{?4.85‰}$. The distribution pattern for δ ³⁴S in both sulfate and sulfide appears to be the result of in situ sulfate reduction.