Fluid flow and the Heart Mountain fault: a stable isotopic, fluid inclusion, and geochronologic study

Numerous studies have proposed that movement along the 3400‐km² low‐angle (<3°) Heart Mountain detachment fault of north‐western Wyoming and south‐western Montana was facilitated by the presence of lubricating fluids. A recent stable isotope study suggested that the fluids along the Heart Mountain fault originated from large hydrothermal systems associated with Eocene intrusive centers. Herein, we present results from a combined stable isotope, fluid inclusion, and ⁴⁰Ar/³⁹Ar geochronology study of the relationship between shallow crustal fluids from Eocene intrusive centers in the New World Mining district and fluids associated with the Heart Mountain fault. Our results suggest that: (i) Eocene intrusive bodies within the New World Mining District focused hydrothermal fluids along specific units and structures from the ore deposit centers along the Heart Mountain fault detachment surface; (ii) hydrothermal waters were focused along the Heart Mountain fault from the breakaway region near Silvergate, Montana toward Heart Mountain in Cody, Wyoming; and (iii) hydrothermal activity along the Heart Mountain fault occurred at the same time as emplacement of the Eocene intrusives, between 50.1 and 48.1 Ma. These data, taken together, suggest that the hydrothermal activity generated by intrusion of Eocene rhyodacites and dacites provided the source of fluids found along the Heart Mountain fault plane.     

Barite–pyrite mineralization of the Wiesbaden thermal spring system, Germany: a 500-kyr record of geochemical evolution

Barite–(pyrite) mineralizations from the thermal springs of Wiesbaden, Rhenish Massif, Germany, have been studied to place constraints on the geochemical evolution of the hydrothermal system in space and time. The thermal springs, characterized by high total dissolved solids (TDS) contents and predominance of NaCl, ascend from aquifers at 3–4 km depth and discharge at a temperature of 65–70°C. The barite–(pyrite) mineralization is found in upflow and discharge zones of the present‐day thermal springs as well as at elevations up to 50 m above the current water table. Hence, this mineralization style constitutes a continuous record of the hydrothermal activity, linking the past evolution with the present state of this geothermal system. The sulphur isotope signatures of the mineralization indicate a continuous decrease of the δ³⁴S of sulphate from +16.9‰ in the oldest barite to +10.1‰ in the present‐day thermal water. The δ³⁴S values of barite closely resemble various recently active thermal springs along the southern margin of the Rhenish Massif and contrast strongly with different regional ground and mineral waters. The mineralogical and isotopic signatures, combined with calculations based on uplift rates and the regional geological history, indicate a minimum activity of the thermal spring system at Wiesbaden of about 500 000 years. This timeframe is considerably larger than conservative models, which estimate the duration of thermal spring systems in continental intraplate settings to last for several 10 000 years. The calculated equilibrium sulphur isotope temperatures of coexisting barite and pyrite range between 65 and 80°C, close to the discharge temperature of the springs, which would indicate apparent equilibrium precipitation. Kinetic modelling of the re‐equilibration of the sulphate–sulphide pair during water ascent shows that this process would require 220 Myr. Therefore, we conclude that pyrite is formed from precursor Fe monosulphide phases, which rapidly precipitate in the near‐surface environment, preserving the isotope fractionation between dissolved sulphate and sulphide established in the deep aquifer. Equilibrium modelling of water–mineral reactions shows slight supersaturation of barite at the discharge temperature. Pyrite is already strongly supersaturated at the temperatures estimated for the aquifer (110°C) and processes in the near‐surface environment are most probably related to contact of the thermal water with atmospheric oxygen, resulting in formation of oxidized intermediate sulphur species and precipitation of Fe monosulphide phases, which subsequently recrystallize to pyrite.     

Strangers in the grave? Investigating local provenance in a Greek Bronze Age mass burial using δS analysis

The present study employs δ³⁴S analysis to investigate the potential of this method in the investigation of local/geographic origin for the 12 individuals buried collectively over the ruins of a Bronze Age building, under a tumulus, in Thebes, Greece, and to help in the understanding of this unique case in Greek archaeology. Sulphur isotope analysis of bone collagen is applied with increasing frequency to elucidate aspects of paleodiet and movement in archaeology. Sulphur is linked to local geology and can also be used to indicate proximity of people and animals to the sea. The osteoarchaeological analysis of the assemblage revealed that it included individuals of both sexes and all age groups, a configuration which could traditionally be interpreted as an extended family. The absence of synchronous burials in Thebes amplifies the importance of this assemblage. At the same time, traditional archaeological practice has been unable to elucidate the conditions that led to this event, proposing a biological affinity for the individuals and personal reasons for their extinction. However, sulphur isotope analysis clearly demonstrated that at least one of the individuals had spend the last years of life in an environment different from the rest, a fact likely to indicate a rather random composition of this assemblage.     

Response of Alum Rock springs to the October 30, 2007 Alum Rock earthquake and implications for the origin of increased discharge after earthquakes

The origins of increased stream flow and spring discharge following earthquakes have been the subject of controversy, in large part because there are many models to explain observations and few measurements suitable for distinguishing between hypotheses. On October 30, 2007 a magnitude 5.5 earthquake occurred near the Alum Rock springs, California, USA. Within a day we documented a several‐fold increase in discharge. Over the following year, we have monitored a gradual return towards pre‐earthquake properties, but for the largest springs there appears to be a permanent increase in discharge. The Alum Rock springs discharge waters that are a mixture between modern (shallow) meteoric water and old (deep) connate waters expelled by regional transpression. After the earthquake, there was a small and temporary decrease in the fraction of connate water in the largest springs. Accompanying this geochemical change was a small (1–2°C) temperature decrease. Combined with the rapid response, this implies that the increased discharge has a shallow origin. Increased discharge at these springs occurs both for earthquakes that cause static volumetric expansion and for those that cause contraction, supporting models in which dynamic strains are responsible for the subsurface changes that cause flow to increase. We make a quantitative comparison between the observed changes and model predictions for three types of models: (i) a permanent increase in permeability; (ii) an increase in permeability followed by a gradual decrease to its pre‐earthquake value; and (iii) an increase of hydraulic head in the groundwater system discharging at the springs. We show that models in which the permeability of the fracture system feeding the springs increases after the earthquake are in general consistent with the changes in discharge. The postseismic decrease in discharge could either reflect the groundwater system adjusting to the new, higher permeability or a gradual return of permeability to pre‐earthquake values; the available data do not allow us to distinguish between these two scenarios. However, the response of these springs to another earthquake will provide critical constraints on the changes that occur in the subsurface and should permit a test of all three types of models.     

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone,south Hungary

This study reconstructs the palaeohydrogeologic evolution of the shallow‐to‐moderate Mesozoic subsidence history for the Mecsekalja Zone (MZ), a narrow metamorphic belt in the eastern Mecsek Mountains, Hungary. Brittle deformation of the MZ produced a vein system with a cement history consisting of five sequential carbonate generations and one quartz phase. Vein textures suggest different fluid‐flow mechanisms for the parent fluids of subsequent cement generations. Combined microthermometric and stable‐isotope measurements permit reconstruction of the character of subsequent fluid generations with different flow types, as defined by vein textures, yielding new information regarding the hydraulic behaviour of a metamorphic crystalline complex. Textural observations and geochemical data suggest that fracture‐controlled flow pathways and externally derived fluids were typical of some flow events, while percolation through the rock matrix and the relationship to the Cretaceous volcanism and dyke emplacement were typical of others. The difference in the mode of calcite deposition from pervasive fluids (i.e. pervasive carbonatisation along grain boundaries versus deposition in antitaxial veins) between two calcite generations related to the volcanism inspired a stress‐dependent model of antitaxial vein growth. Textural and isotope variations in a vein generation produced by the same parent fluid indicate rock‐dependent hydraulic behaviour for different rock types, distinct action of the contemporaneous fracture systems and different extents of fluid–rock interaction. Cathodoluminescence microscopy and fluid‐inclusion microthermometry shed light on the possible role of hydraulic fracturing in the formation of massive calcite. The time of formation was estimated from the isotope composition of the oldest calcite generation and its presumptive relationship with the sedimentary sequences to the north, whereas microthermometry permitted conciliation of the reconstructed flow sequence with the Mesozoic subsidence history of the Mórágy Block (including the MZ).     

Origin of deep saline groundwaters in the Vienne granitic rocks (France): constraints inferred from boron and strontium isotopes

As part of a preliminary geological characterization programme to assess the feasibility of an underground laboratory in granitic rock, a series of 17 deep boreholes (maximum depth, 900 m) was drilled by ANDRA in the Vienne district, France. A salinity gradient was demonstrated in the granitic waters with concentrations varying from approximately 1 g L^?1 at 150 m depth at the top of the basement (beneath the sedimentary cover) to 10 g L^?1 in the deeper part (from 400 to 600 m depth). Sr and B isotope ratios were measured in order to better understand the origin of the salinity and to evaluate the degree of water–rock interaction in the system. The results obtained were compared to those of mineral spring waters emerging from the granitic basement in the Massif Central. Evidence in support of a significant marine contribution include: (i) the Cl–Br investigations agree with a marine origin for the saline groundwaters without evolution from seawater; (ii) the ⁸⁷Sr/⁸⁶Sr ratio of the Vienne deep groundwaters (0.7078–0.7084) is in agreement with a palaeo‐seawater isotopic signature; (iii) measured δ¹¹B values for the deepest brine samples are enriched in ¹¹B (up to 36.1‰) relative to the granitic springs. The combined use of δ¹¹B, Cl, B, Br, Sr contents and ⁸⁷Sr/⁸⁶Sr ratios makes it possible to define and quantify a mixing model between marine and crustal end‐members in order to explain the origin of the deep saline groundwaters in the Vienne granitic rocks.     

Human biology in the Classic Maya collapse: Evidence from paleopathology and paleodiet

We review evidence from human biology—paleopathological and isotopic paleodietary studies on ancient Maya skeletons—to assess the validity of ecological models of the Classic Maya collapse, in which elevated disease and deteriorating diet are commonly assumed. To be upheld, the health arguments of ecological models require that the Maya disease burden (1) was greater than that for many other societies and (2) increased over the span of occupation. The dietary argument requires (1) consistent change in diet from Preclassic and Early Classic Periods to the Terminal Classic and (2) increasing social divergence in diet. A correlation between diet and disease is necessary to link these arguments. Neither pathology nor isotopic data consistently support these criteria. Instead, it appears that local environmental and political factors created diversity in both disease burden and diet. In view of the human biological data, we are skeptical of ecological models as generalized explanations for the abandonment of Classic Maya sites in the southern lowlands.     

Isotopic and zooarchaeological investigation of later medieval and post-medieval cattle husbandry at Dudley Castle,West Midlands

Abstract

Zooarchaeological analysis of a substantial assemblage of animal bones excavated from Dudley Castle, West Midlands, revealed two significant changes in cattle management in the late medieval and post-medieval periods: a statistically significant increase in size in the later 14th century; and a gradual reduction in slaughter age. To explore whether these altered husbandry practices were accompanied by modifications to the diet and environment of these animals, a targeted study of carbon and nitrogen stable isotope values from 135 cattle bones was conducted. This analysis revealed that δ¹³C and δ¹⁵N values were broadly consistent between the mid-13th and mid-18th centuries. Only two statistically significant diachronic variations were identified: a temporary decrease in δ¹³C values in phase 7 (1397–1533); and lower variation in δ¹³C values in phase 9 (1647–1750). A number of explanations for these changes are offered. While the zooarchaeologically attested shifts in cattle husbandry were not accompanied by isotopically detectable changes to diet and environment, the analysis of stable isotopes highlighted other changes that were not detected using traditional zooarchaeological analysis. The complimentary benefits of zooarchaeological and isotopic approaches are extolled.     

What's in a bone? Recent advances in archaeological bone chemistry

This paper focuses on advances in archaeological bone chemistry since 1989. At that time studies generally shifted from development and application of techniques in bone chemistry to more experimental approaches. In stable isotope studies, controlled feeding experiments have been carried out to determine the routing of various dietary components into bone tissues. Biological apatite has been added to collagen as a tissue of study for stable carbon isotope analysis, providing the ability to study much older remains. Other elements have been added such as hydrogen for the study of paleoclimate and oxygen and strontium isotopes to study life history. Trace element studies have focused on methods of detecting and controlling for diagenesis, and barium has been added to the list of useful dietary indicators. Attempts to extract DNA from archaeological bone have been successful, however, this field is still in its early stages in terms of applications to understanding prehistoric biological relationships.     

δ18O Analyses of the Humpbacked Conch (Gibberulus gibberulus): Evaluating a Proxy for Reconstructing Sea-Surface Temperature at Chelechol ra Orrak,Palau

ABSTRACT

In Palau, Micronesia, marine resources, particularly shellfish, played a vital role in human subsistence for millennia. Despite the vast array of molluscan species in archaeological assemblages, there is a dearth of data on nearshore palaeoecology or prehistoric shellfish foraging practices. In this study, we analysed stable oxygen isotopes (δ¹⁸O) values present in the calcium carbonate shell of Gibberulus gibberulus from the Chelechol ra Orrak archaeological site to reconstruct average nearshore sea-surface temperatures (SST) from approximately 1500–1100 cal BP. Modern shellfish samples and environmental data were collected from intertidal zones near the site and x-ray diffraction (XRD) was employed to identify the biomineralogical composition of G. gibberulus. These steps provide necessary information for the selection of the proper oxygen isotope-to-SST conversion formula. The selected formula was applied to δ¹⁸O samples from archaeological shells to reconstruct prehistoric SST averages. The results of this proxy validation study verify that G. gibberulus accurately records ambient SST and can be used to reconstruct ancient nearshore conditions. These findings also contribute to the establishment of an environmental baseline, which can be used to examine how environmental changes may have influenced the availability of molluscan taxa that in turn influenced human subsistence practices through time.