Evidence of polycyclic aromatic hydrocarbon (PAH) exposure in fish from the Antarctic peninsula

PAH exposure was assessed in fish from Arthur Harbor, Antarctica and sites remote from human activities by measuring the concentrations of polycyclic aromatic hydrocarbon (PAH) metabolites in bile and PAH in tissues and stomach contents. The highest concentrations of biliary metabolites and tissue PAH were present in fish captured near an Argentine supply ship (Bahia Paraiso) that ran aground in Arthur Harbor in 1989. The presence of biliary PAH metabolites in fish collected near Palmer Station, a small US scientific station in Arthur Harbor, suggests low level exposure to contaminants. However, PAH tissue concentrations in fish from Palmer Station were similar to those from remote sites. Both tissue PAH and metabolite concentrations were low in fish captured at sites remote from human activities. The confirmation of PAH metabolites in the bile of fish from Arthur Harbor is direct evidence that the presence of humans has resulted in Antarctic fish being exposed to petroleum-derived PAH.     

The added value of joint PP–PS inversion for reservoir characterization: A case study using Jubarte PRM seismic data (offshore Brazil)

The added value of the joint pre-stack inversion of PP (incident P-wave and reflected P-wave) and PS (incident P-wave and reflected S-wave) seismic data for the time-lapse application is shown. We focus on the application of this technique to the time-lapse (four-dimensional) multicomponent Jubarte field permanent reservoir monitoring seismic data. The joint inversion results are less sensitive to noise in the input data and show a better match with the rock physics models calibrated for the field. Further, joint inversion improves S-impedance estimates and provides a more robust quantitative interpretation, allowing enhanced differentiation between pore pressure and fluid saturation changes, which will be extremely useful for reservoir management. Small changes in reservoir properties are expected in the short time between the time-lapse seismic acquisitions used in the Jubarte project (only 1 year apart). The attempt to recover subtle fourth-dimensional effects via elastic inversion is recurrent in reservoir characterization projects, either due to the small sensitivity of the reservoirs to fluid and pressure changes or the short interval between the acquisitions. Therefore, looking for methodologies that minimize the uncertainty of fourth-dimensional inversion outputs is of fundamental importance. Here, we also show the differences between PP only and joint PP–PS inversion workflows and parameterizations that can be applied in other projects. We show the impact of using multicomponent data as input for elastic seismic inversions in the analysis of the time-lapse differences of the elastic properties. The larger investment in the acquisition and processing of multicomponent seismic data is shown to be justified by the improved results from the fourth-dimensional joint inversion.     

Constitutive relations for fault slip and earthquake instabilities

Constitutive relations for fault slip are described and adopted as a basis for analyzing slip motion and its instability in the form of earthquakes on crustal faults. The constitutive relations discussed include simple rate-independent slip-weakening models, in which shear strength degrades with ongoing slip to a residual frictional strength, and also more realistic but as yet less extensively applied slip-rate and surface-state-dependent relations. For the latter the state of the surface is characterized by one or more variables that evolve with ongoing slip, seeking values consistent with the current slip rate. Models of crustal faults range from simple, single-degree-of-freedom spring-slider systems to more complex continuous systems that incorporate nonuniform slip and locked patches on faults of depth-dependent constitutive properties within elastic lithospheric plates that may be coupled to a viscoelastic asthenosphere. Most progress for the rate and state-dependent constitutive relations is at present limited to single-degree-of-freedom systems. Results for stable and unstable slip with the various constitutive models are summarized. Instability conditions are compared for spatially uniform versus nonuniform slip, including the elastic — brittle crack limit of the nonuniform mode. Inferences of constitutive and fracture parameters are discussed, based on earthquake data for large ruptures that begin with slip at depth, concentrating stress on locked regions within a brittle upper crust. Results of nonlinear stability theory, including regimes of complex sustained stress and slip rate oscillations, are outlined for rate and state-dependent constitutive relations, and the manner in which these allow phenomena like time-dependent failure, restrengthening in nearly stationary contact, and weakening in rapidly accelerated slip, is discussed.     

Temporal gravity and elevation changes at Pacaya volcano, Guatemala

Repetitive gravity surveys at Pacaya Volcano from 1975 to 1979 revealed time-dependent changes in the gravity field, which although related to volcanic activity, could not be uniquely attributed to elevation changes or mass redistributions because elevation control was lacking. Elevation control was established in July 1979 using precision leveling. Relative elevation and gravity measurements in June and July of 1979, January 1980 and June 1980 indicate concurrent gravity and elevation changes contemporaneous with variations in eruptive activity. From June 1979 to January 1980, while fumarolic activity was dominant, relative to the most remote station, the volcano deflated by at least 195 mm and the gravity field increased by up to 221 μgal. From January 1980 to June 1980, preceding a Strombolian eruption beginning about June 1980, the volcano inflated by at least 19 mm and the gravity field decreased by up to 231 μgal. Gravity change maps for the intervals of January 1978 to June 1979, June 1979 to January 1980, and January 1980 to June 1980 show areas subject to repeated positive and negative gravity change. Some of those areas coincide with areas of maximum elevation change observed in the June 1979–January 1980 and January 1980–June 1980 intervals; however, gravity changes were observed in areas lacking elevation changes. Adjusting observed gravity changes for elevation changes using a free-air value of −3.086 μgal/cm does not substantially alter the pattern, position, or amplitude of the gravity changes. The relationship between gravity changes, elevation changes, and volcanic activity requires a mechanism producing gravity decreases with little inflation during times of increased eruptive activity, and producing gravity increases with subsidence during times of declining eruptive activity. Such a pattern of changes could be produced by a near-surface magma body in which high-density degassed magma is displaced volume for volume by low-density vesiculated magma during time of increased eruptive activity, and in which loss of gasses by fumarolic activity produces a density increase and a reduction in volume of the magma body during periods of declining eruptive activity. Such a pattern of changes could also be induced by a low-density, vesiculated magma body moving upward in the volcanic pile by piecemeal stoping where the high-density rocks of the volcano are replaced on a volume for volume basis by low-density magma during periods of increasing eruptive activity; and by later density increases and magma body volume reductions accompanying devolatilization and devesiculation during periods of declining eruptive activity. Simple density change and density contrast models involving shallow magma bodies at depths of 100 to 200 m indicate density changes or contrasts of about 0.4 g/cm³ could produce the gravity changes.     

Effects of finite rifting times on the development of sedimentary basins

Most thermo-mechanical models for the development of sedimentary basins have assumed that the rifting responsible for the formation of the basin occurred instantaneously and have examined the post-rift development of the basin. This assumption greatly simplifies the mathematical treatment, but is not in accord with what is found in nature, where 10-to 50-m.y. rifting events commonly accompany the formation of sedimentary basins and continental margins. The effects of a finite rifting time on the development of sedimentary basins are examined using an analytic technique which allows an arbitrary rifting history in both time and space and which considers the effects of both vertical and horizontal heat transfer. This technique allows the thermal structure of the lithosphere to be calculated throughout the rifting event and thus permits the subsidence history and surface heat flow of the developing basin to be traced.The effect of a finite-duration extension event is that heat is lost during rifting increasing the syn-rift subsidence at the expense of the post-rift. Lateral heat flow, which was not included in previous studies of the effect of finite rifting times, has a significant effect on the subsidence history, distribution of sediments and thermal history. In particular, the post-rift subsidence is decreased by more than 25% for a 20-m.y. rifting event and by more than 10–15% for a rifting event as short as 10 m.y. This will significantly decrease the subsidence rates in the post-rift stage and implies that inferences concerning the structure, development and thermal history of the basin derived from using “β-curves” to interpret backstripped subsidence can be greatly in error.Variations in syn-rift sediment accumulation and lithospheric thermal structure at the end of rifting resulting from different rifting histories can interact with other factors, such as the flexural response of the lithosphere to sediment loading, to affect the final width of the basin, the total amount of sediments that accumulate and the basin stratigraphy.     

Petrology of basalts from Loihi Seamount,Hawaii

Loihi Seamount is the southeasternmost active volcano of the Emperor-Hawaii linear volcanic chain. It comprises a spectrum of basalt compositional varieties including basanite, alkali basalt, transitional basalt and tholeiite. Samples from four dredge collections made on Scripps Institution of Oceanography Benthic Expedition in October 1982 are tholeiite. The samples include highly vesicular, olivine-rich basalt and dense glass-rich pillow fragments containing olivine and augite phenocrysts. Both quartz-normative and olivine-normative tholeiites are present. Minor and trace element data indicate relatively high abundances of low partition coefficient elements (e.g., Ti, K, P. Rb, Ba, Zr) and suggest that the samples were derived by relatively small to moderate extent of partial melting, of an undepleted mantle source. Olivine composition, MgO, Cr and Ni abundances, and Mg/(Mg+Fe), are typical of moderately fractionated to relatively unfractionated “primary” magmas. The variations in chemistry between samples cannot be adequately explained by low-pressure fractional crystallization but can be satisfied by minor variations in extent of melting if a homogeneous source is postulated. Alternatively, a heterogeneous source with variable abundances of certain trace elements, or mixing of liquids, may have been involved. Data for ³He/⁴He, presented in a separate paper, implies a mantle plume origin for the helium composition of the Loihi samples. There is little variation in the helium isotope ratio for samples having different compositions and textures. The helium data are not distinctive enough to unequivocally separate the magma sources for the tholeiitic rocks from the other rock types such as Loihi alkalic basalts and the whole source region for Loihi may have a nearly uniform helium compositions even though other element abundances may be variable. Complex petrologic processes including variable melting, fractional crystallization and magma mixing may have blurred original helium isotopic signatures.     

Two-dimensional kinematic and rheological modeling of the 1912 pyroclastic flow,Katmai, Alaska

Pyroclastic flow emplacement is strongly influenced by eruption column height. A surface along which kinetic energy is zero theoretically connects the loci of eruption column collapse with all coeval ignimbrite termini. This surface is reconstructed as a two-dimensional energy line for the 1912 Katmai pyroclastic flow in the Valley of Ten Thousand Smokes from mapped flow termini and the runup of the ignimbrite onto obstructions and through passes. Extrapolation of the energy line to the vicinity of the source vent at Novarupta suggests the eruption column which generated the ignimbrite eruption was approximately 425 m high. The 1912 pyroclastic flow travelled about 25 km downvalley. Empirical velocity data calculated from runup elevations and surveyed centrifugal superelevations indicate initial velocities near Novarupta were greater than 79–88 m s^–1. The flow progressively decelerated and was travelling only 2–8 m s^–1 when it crossed a moraine 16 km downvalley. The constant slope of the energy line away from Novarupta suggests the flow was systematically slowed by internal and basal friction. Using a simple physical model to calculate flow velocities and a constant kinetic friction coefficient (Heim coefficient) of 0.04 derived from the reconstructed energy line, the flow is estimated to have decelerated at an average rate of –0.16 m s^–2 and to have taken approximately 9.5 minutes to travel 25 km down the Valley of Ten Thousand Smokes. The shear strength of the flowing ignimbrite at the moraine was approximately 0.5 kPa, and its Bingham viscosity when it crossed the moraine was 3.5 × 10³ P. If the flow was Newtonian, its viscosity was 4.2 × 10³ P. Reynolds and Froude numbers at the moraine were only 41–62 and 0.84–1.04, respectively, indicating laminar, subcritical flow.     

Seismic wave attenuation in fluid-saturated porous media

Contrary to the traditional view, seismic attenuation in Biot's theory of fluid-saturated porous media is due to viscous damping of local (not global) pore-fluid motion. Since substantial inhomogeneities in fluid permeability of porous geological materials are to be expected, the regions of highest local permeability contribute most to the wave energy dissipation while those of lowest permeability dominate the fluid flow rate if they are uniformly distributed. This dichotomy can explain some of the observed discrepancies between computed and measured attenuation of compressional and shear waves in porous earth. One unfortunate consequence of this result is the fact that measured seismic wave attenuation in fluid-filled geological materials cannot be used directly as a diagnostic of the global fluid-flow permeability.     

Probability of earthquake recurrence with nonuniform stress rates and time-dependent failure

Current methods for calculation of long-term probabilities for the recurrence of large earthquakes on specific fault segments are based upon models of the faulting process that implicitly assume constant stress rates during the interval separating earthquakes and instantaneous failure at a critical stress threshold. However, observations indicate that the process of stress recovery following an earthquake involves rate variations at all time scales in addition to stress steps caused by nearby earthquakes. Additionally, the existence of foreshocks, aftershocks and possible precursory processes suggest that there may be significant time dependence of the earthquake nucleation process. A method for determining the conditional probabilities for earthquake occurrence under conditions of irregular stressing is developed that could be useful at all time scales including those pertinent to short-and intermediate-term prediction. Used with models for earthquake occurrence at a stress threshold, the addition of variable stressing introduces a simple scaling of the conditional probabilities by stress level and stress rate. A model for the time-dependent nucleation of earthquake slip has been proposed recently that is based upon laboratory observations of fault strength. This failure criterion results in large but relatively short duration changes in the probability of earthquake recurrence particularly following stress steps. Applied to populations of earthquakes the models predicts a 1/t decay of seismicity following stress steps as observed for aftershocks and for frequency of foreshock-mainshock pairs. The model suggests that variations of seismicity rates of small earthquakes in the nucleation zone of the expected earthquake directly indicate variations in probability of recurrence of the large earthquake.