Geochemistry of low-molecular weight hydrocarbons in hydrothermal fluids from Middle Valley, northern Juan de Fuca Ridge

Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 2073-2092, doi:10.1016/j.gca.2006.01.015.Hydrothermal vent fluids from Middle Valley, a sediment-covered mid-ocean ridge on the northern Juan de Fuca Ridge, were sampled in July, 2000. Eight different vents with exit temperatures of 186 to 281°C were sampled from two areas of venting: the Dead Dog and ODP Mound fields. Fluids from the Dead Dog field are characterized by higher concentrations of ΣNH3 and organic compounds (C1-C4 alkanes, ethene, propene, benzene and toluene) compared with fluids from the ODP Mound field. The ODP Mound fluids, however, are characterized by higher C1/(C2+C3) and benzene:toluene ratios than those from the Dead Dog field. The aqueous organic compounds in these fluids have been derived from both bacterial processes (methanogenesis in low-temperature regions during recharge) as well as from thermogenic processes in higher-temperature portions of the subsurface reaction zone. As the sediments undergo hydrothermal alteration, carbon dioxide and hydrocarbons are released to solution as organic matter degrades via a stepwise oxidation process. Compositional and isotopic differences in the aqueous hydrocarbons indicate that maximum subsurface temperatures at the ODP Mound are greater than those at the Dead Dog field. Maximum subsurface temperatures were calculated assuming that thermodynamic equilibrium is attained between alkenes and alkanes, benzene and toluene, and carbon dioxide and methane. The calculated temperatures for alkene-alkane equilibrium are consistent with differences in the dissolved Cl concentrations in fluids from the two fields, and indicate that subsurface temperatures at the ODP Mound are hotter than those at the Dead Dog field. Temperatures calculated assuming benzene-toluene equilibrium and carbon dioxide-methane equilibrium are similar to observed exit temperatures, and do not record the hottest subsurface conditions. The difference in subsurface temperatures estimated using organic geochemical thermometers reflects subsurface cooling processes via mixing of a hot, low-salinity vapor with a cooler, seawater salinity fluid. Because of the disparate temperature dependence of alkene-alkane and benzene-toluene equilibria, the mixed fluid records both the high and low temperature equilibrium conditions. These calculations indicate that vapor-rich fluids are presently being formed in the crust beneath the ODP Mound, yet do not reach the surface due to mixing with the lower-temperature fluids.This work was funded by NSF OCE-9906752

Methane Clumped Isotopes: Progress and Potential for a New Isotopic Tracer

The isotopic composition of methane is of longstanding geochemical interest, with important implications for understanding petroleum systems, atmospheric greenhouse gas concentrations, the global carbon cycle, and life in extreme environments. Recent analytical developments focusing on multiply substituted isotopologues (‘clumped isotopes’) are opening a valuable new window into methane geochemistry. When methane forms in internal isotopic equilibrium, clumped isotopes can provide a direct record of formation temperature, making this property particularly valuable for identifying different methane origins. However, it has also become clear that in certain settings methane clumped isotope measurements record kinetic rather than equilibrium isotope effects. Here we present a substantially expanded dataset of methane clumped isotope analyses, and provide a synthesis of the current interpretive framework for this parameter. In general, clumped isotope measurements indicate plausible formation temperatures for abiotic, thermogenic, and microbial methane in many geological environments, which is encouraging for the further development of this measurement as a geothermometer, and as a tracer for the source of natural gas reservoirs and emissions. We also highlight, however, instances where clumped isotope derived temperatures are higher than expected, and discuss possible factors that could distort equilibrium formation temperature signals. In microbial methane from freshwater ecosystems, in particular, clumped isotope values appear to be controlled by kinetic effects, and may ultimately be useful to study methanogen metabolism

Potencijalna upotreba izotopa važnih za okoliš u ispitivanju migracije onečišćujućih tvari

This article presents the use of natural abundance stable isotope (hydrogen, carbon, nitrogen, oxygen, chlorine) analysis data as a tool for providing important information about the origin of contaminants, the contribution of different sources to a multi-source plume, characterisation of their complex transport (rate and mechanisms) and for evaluating the success of contaminated site remediation. Isotopic signatures of contaminants are useful tracers of their sources, while isotopic fractionation can be used to quantitatively assess the progress of an environmental process such as biodegradation. This new isotopic approach is reliable and can offer more information than traditional techniques in pollutant migration studies, particularly after waste disposal. During biological degradation of any organic compound, molecules containing lighter isotopes are degraded, and the portion of heavier isotopes in the substrate is increased, identifying specific microbial roles in biogeochemical cycling. Since isotopic fractionation is proportional to degradation, depending on the type of contamination, a microbial degradation of 50 % to 99 % of the initial concentration can be quantified using isotope ratio measurements.Cilj ovog rada je da se prikaže korištenje podataka analize prirodne obilnosti stabilnih izotopa (vodika, ugljika, dušika, kisika i klora) kao alata za dobivanje važnih informacija o porijeklu onečišćujućih tvari, doprinosu različitih multikomponentnih onečišćivača, karakterizaciji njihova kompleksnog transporta (brzine i mehanizma) i praćenja uspjeha remedijacije onečišćenih mjesta. Izotopski sadržaji onečišćujućih tvari koriste se kao traseri za određivanje njihovih izvora, dok se izotopsko frakcioniranje može iskoristiti za kvantitativnu procjenu toka procesa kao što je biodegradacija. Takav nov izotopski pristup je pouzdan i nudi više informacija od tradicionalnih tehnika kontrole putovanja onečišćivala, napose nakon odlaganja opasnog otpada na zemljištu. Za vrijeme biodegradacije nekog organskog spoje molekule koje sadržavaju lake izotope lakše se degradiraju, a dio težih izotopa u supstratu se povećava, što upućuje na mikrobiološku ulogu u biokemijskom ciklusu. Kako je izotopsko frakcioniranje proporcionalno degradaciji zavisno od tipa onečišćenja, korištenjem podataka mjerenja izotopskih odnosa može se procijeniti mikrobiološka degradacija od 50 % do 99 % od početne koncentracije

Electronic Tickets, Smart Cards, and Online Prepayments: When and How to Advance Sell

Advance selling occurs when sellers allow buyers to purchase at a time preceding consumption (Shugan and Xie 2000). Electronic tickets, smart cards, online prepayments, and other technological advances make advance selling possible for many, if not all, service providers. These technologies lower the cost of making complex transactions at a greater distance from the seller's site. They also give sellers more control over advance selling by decreasing arbitrage. As technology enhances the capability to advance sell, more academic attention is vital. This paper strives to exploit these technologies by developing advance-selling strategies. Until recently, advance-selling research focused on the airline industry and specific characteristics of that industry. These characteristics included the price insensitivity of late arrivals (e.g., business travelers) compared with early arrivals (e.g., leisure travelers), demand uncertainty across flights on the same day, and capacity constraints. Recent findings by Shugan and Xie (2000) show that advance selling is a far more general marketing tool than previously thought. It does not require these industry-specific characteristics. It only requires the existence of buyer uncertainty about future valuations. Moreover, sellers without the ability to price discriminate can use advance selling to improve profits to the level of first-degree price discrimination. This finding is important because buyers are nearly always uncertain about their future valuations for most services (e.g., the utility of next year's vacation or a future college education). In this paper, we take the next step from Shugan and Xie (2000). We show that advance-selling profits do not come from buyer surplus, but from more buyers being able to purchase. We determine when and how to advance sell in a variety of situations, including situations with limited capacity, second-period arrivals, refunds, buyer risk aversion, exogenous credibility, continuous preference distributions, and premium pricing. We determine when advance selling improves profits and, when it does, how to set advance prices. We ask and answer seven questions. First, when should sellers advance sell? Second, how much can advance selling improve profits compared with only spot selling? Third, what factors impact the profitability of advance selling and how? Fourth, should advance prices be higher or lower or the same as spot prices? Fifth, how do capacity constraints impact advance-selling strategies? Sixth, should sellers limit the number of advance sales? Finally, what is the possible impact of buyer risk aversion? First, we provide precise conditions when sellers should advance sell. For example, without capacity constraints, we show that sellers should advance sell when marginal costs are sufficiently low to make it profitable to sell to buyers with low valuations and sufficiently high to convince buyers that the spot price will be higher than the advance price. Second, we find that advance selling can almost double the profits from optimal spot selling to early arrivals. We also show that advance selling has no impact on consumer surplus in markets with homogenous consumers and no capacity constraints. Therefore, advance selling can increase social welfare because seller profits increase. Third, we find that two very important factors impacting the profitability of advance selling are seller credibility and marginal costs. Buyers only advance buy when they expect an advantage from advance buying over spot buying. Without capacity constraints, sellers must credibly convince buyers that the advance price is at a discount to the spot price. We show that this condition is met under different circumstances. For example, large marginal costs can create credibility because buyers believe that these costs will lead to high spot prices. Fourth, we find (although optimal advance prices can be at a discount to the spot price) that sometimes a premium is optimal. Premiums are optimal when capacity is large (but limited) and marginal costs are not too large. Buyers advance purchase at a premium to spot prices when capacity is limited and spot prices are low. (Note that this is not a risk premium, and risk aversion is not required.) No prior research has suggested this strategy because that research relies on the assumption that early arrivals are more price sensitive than later ones. Without that assumption, premium advance pricing is sometimes optimal. Fifth, we find that binding capacity constraints can impact the profitability of advance selling in opposite ways. On one hand, capacity constraints create seller credibility. Buyers believe that spot prices will be high when they know spot capacity is limited (and, perhaps, more limited by advance sales). On the other hand, when capacity is limited, the need to increase sales from discounted advance prices diminishes. Sixth, consistent with Desiraju and Shugan (1999) we find that limiting advance sales can be profitable, but only under restrictive conditions. These conditions are: (1) selling to all early arrivals would leave insufficient capacity in the spot period to sell to all second-period arrivals with high valuations, (2) the optimal spot price is high, and (3) marginal costs are sufficiently small to make advance selling profitable. Finally, we find that buyer risk aversion can sometimes increase the profitability of advance selling. Our findings provide precise guidelines for a large number of service providers that will have the technical capability to advance sell. For those service providers, advance selling provides a creative pricing strategy that can potentially provide substantial improvements in profits.Pricing, Advance Selling, Advance Pricing, Tickets, State-Dependent Utility, Services Marketing, Dynamic Pricing