Petrographic Fabrics Research Articles

Preliminary approach to pottery technology of the Late Bronze Age and Early Iron Age settlement El Morredon (Ebro valley, Spain) (C.1200–500 BCE) through archaeometric methods

This study presents a novel approach to pottery technology of the Urnfield culture in the Middle Ebro Valley (Spain) through petrography and geochemical analyses. In the archaeological site of ‘El Morredón’, a large settlement occupied during the Late Bronze and Early Iron Age, two petrographic fabrics and two main chemical groups have been identified, with the addition of grog as a temper in all samples as the main characteristic of the assemblage. Raw material procurement and paste recipes have been defined, revealing the specific technological choices of this community, which suggest the existence of a technological group in the Middle Ebro valley. Nonetheless, further studies with this methodology in other Urnfield’s sites in the northeast of the Iberian Peninsula are still needed to validate this hypothesis.

Exploring Roman and Late Roman Common Buff Wares from the rural site of Sa Mesquida (Mallorca, Balearic Islands): Continuity of local production?

In previous studies, the Reference Group (RG) for common wares produced in the 1st and 2nd centuries AD at the kiln found at the Roman rural site of Sa Mesquida (western Mallorca) was established. Recent excavations have revealed that the Roman villa was founded at the beginning of the Augustan period or slightly earlier and that some ceramics found in these foundational layers resemble the ceramic products of the kiln. We also know now that the kiln itself and a large cistern were reused in the Late Roman period (4th and 5th centuries AD) as rubbish dumps, also containing regional (mostly produced in the island of Eivissa, but also others that are macroscopically like the products of the kiln) and other imported common wares. The macroscopic similarities between some of the earlier and later materials generated a series of important questions related to the possibility of a long-lasting local pottery-making tradition at this site or more broadly in western Mallorca. To explore this possibility, 74 common wares from different well-stratified layers were analytically characterized using a combination of Wavelength Dispersive X-Ray Fluorescence and Optical Microscopy by thin-section and both typologically and compositionally compared to the Early Imperial common buff wares produced at the site. The results provided a better understanding of the common wares at the site from a diachronic perspective. The comparison of the new ceramics analyzed from several contexts found at Sa Mesquida with the already established reference group for the common wares showed, in some cases, strong similarities in terms of chemical composition and petrographic fabrics both for early Roman and some Late Roman ceramics. On the other hand, the study also enabled the identification of other non-local groups (primarily products from the neighboring island of Eivissa), providing new information on the provenance and regional distribution of buff wares, a ceramic class whose importance has been often denied.

Eocene animal trace fossils in 1.7-billion-year-old metaquartzites

The Paleoproterozoic (1.7 Ga [billion years ago]) metasedimentary rocks of the Mount Barren Group in southwestern Australia contain burrows indistinguishable from ichnogenera Thalassinoides, Ophiomorpha, Teichichnus, and Taenidium, known from firmgrounds and softgrounds. The metamorphic fabric in the host rock is largely retained, and because the most resilient rocks in the sequence, the metaquartzites, are too hard for animal burrowing, the trace fossils have been interpreted as predating the last metamorphic event in the region. Since this event is dated at 1.2 Ga, this would bestow advanced animals an anomalously early age. We have studied the field relationships, petrographic fabric, and geochronology of the rocks and demonstrate that the burrowing took place during an Eocene transgression over a weathered regolith. At this time, the metaquartzites of the inundated surface had been weathered to friable sandstones or loose sands (arenized), allowing for animal burrowing. Subsequent to this event, there was a resilicification of the quartzites, filling the pore space with syntaxial quartz cement forming silcretes. Where the sand grains had not been dislocated during weathering, the metamorphic fabric was seemingly restored, and the rocks again assumed the appearance of hard metaquartzites impenetrable to animal burrowing.

Reconstructing paleoceanographic conditions during the middle Ediacaran: Evidence from giant ooids in South China

The Ediacaran was a period of intense environmental change in Earth history. Transient pulses of oxygenation and shifts in ocean chemistry have been interpreted from a range of geochemical proxies, including rare earth elements and yttrium (REY), but it is still a challenge to reconstruct paleoceanographic conditions due to the scarcity of reliable recorder. In this study, we explore the possibility that a newly-discovered deposit of giant ooids in the Nanshanping outcrop in South China contains a relatively intact record of the REE composition of Ediacaran seawater, providing an important window into the redox state of the surface ocean at this time. These ooids are relatively large in size (~1–6 mm), and exhibit densely banded laminae. A regional chemostratigraphic correlation shows that the giant-ooid-bearing succession in South China is in the upper part of the second Ediacaran positive carbon isotopic shift (EP2), and thus its occurrence was not associated with the Shuram Excursion. Additionally, the occurrences of giant-ooid-bearing oolites in different blocks were not synchronous during the middle Ediacaran. In-situ laser ablation analysis of giant-ooid cortices reveals that the shale-normalized REY patterns are similar to those of modern seawater, including depleted light REEs, superchondritic Y/Ho ratios, and positive La anomalies. Yet, no Ce anomalies (1.0 ± 0.15) in numerous ooid cortices seems to indicate a suboxidized condition of surface ocean in South China at that time. The petrographic and geochemical evidence support the weak diagenetic alteration of the pure giant ooids during burial. They formed in open-ocean settings with energetic hydrodynamic environments and were affected by early dolomitization that might preserve mostly original petrographic fabrics and REY signatures from further diagenetic overprinting. The characteristics of Ce anomalies and REY compositions acquired from the giant ooids indicate that the surface ocean was suboxidized in South China during the late stage of EP2, implying the very low atmospheric O2 level at that time. Our results suggest that giant ooids may be a reliable analytical target that provide a snapshot into palaeoceanographic conditions during this crucial period of dynamic changes in early Earth history.

Cooking and common wares in the Late Antique rural site of Plaça Major de Castellar del Vallès (Catalonia, Spain): archaeometric characterization

This paper presents an archaeometric analysis of utilitarian ceramics from a Late Antique rural site in the area of Valles (Catalonia, Spain), with the aim of investigating their provenance and shedding light on some aspects of their production technology. A total of 55 samples of cooking and common wares were analyzed using a combination of instrumental analytical techniques, including thin-section optical microscopy, X-ray diffraction, and WD-X-ray fluorescence, in order to carry out a petrographic, mineralogical, and chemical characterization of the materials. A variety of petrographic fabrics were identified, generally composed of inclusions derived from granitic rocks, along with metamorphic inclusions in many cases. This variability is not related either to ceramic typologies or to chronological phases but to slight variations in the raw materials and/or paste recipes. Compositional similarities point to a broadly common origin for a large part of the ceramic assemblage. All fabrics identified may be compatible with the hypothesis of a local provenance, although a wider regional provenance cannot be excluded on geological grounds. This study is a step forward into the understanding of cooking and common ware production in northeastern Iberia during Late Antiquity.

Islamic Pottery from Ancient Termez (Uzbekistan): New Archaeological and Archaeometric Data

The study deals with the Islamic pottery from Termez (southern Uzbekistan), with a special focus on the wares dated to the 9th-12th centuries. The city was a major urban settlement, along one of the routes of the Silk Road. Ceramics, both glazed and unglazed, were produced in several workshops located in the lower city (shahristan) and its suburbs (rabad). Glazed and unglazed wares, including cooking pots, two pottery moulds and two sphero-conical vessels from two excavated areas at Termez are examined by X-ray Fluorescence, X-ray Diffraction, and petrographic thin section analysis. Clayey sediments from different areas of the site are incorporated as a local reference for comparison. The aim is to examine the provenance of the vessels and to determine the production techniques. The results reveal that all the vessels were produced by using calcareous clays and most of them exhibit similar geochemical composition; nevertheless, several chemical groups and petrographic fabrics were identified. XRD points that firing temperatures were generally between 800 and 1000-1100°C, being the latter prevalent. The results of this study are remarkable since archaeometric researches on Central Asian pottery are still few.

Amphora production in the Guadalquivir valley (Spain) during the Late Roman period: petrographic, mineralogical, and chemical characterization of reference groups

This paper presents a scientific analysis of Late Roman amphorae from four kiln sites located in the Guadalquivir river basin: Azanaque-Castillejo (AZ), Isla de la Barqueta (IB), Las Monjas-Soto del Rey (SR), and Picachos (PIC). This region was a significant producer and exporter of oil in the Roman Empire and also during the Late Roman period. The amphorae analyzed belong to type Dressel 23 and were used for trading oil to other Mediterranean regions. A total of 36 amphora samples were analyzed by using a combination of instrumental analytical techniques, including thin-section optical microscopy, X-ray diffraction, and X-ray fluorescence, in order to establish reference groups based on the petrographic, mineralogical, and chemical characterization of the materials. The amphorae from the four kiln sites showed approximately similar petrographic fabrics and chemical compositions, as well as strong technological similarities. However, there are slight petrographic and/or chemical differences that allowed for the differentiation of three reference groups (AZ, SR, and IB-PIC). These groups were also compared with existing reference groups for Early Roman amphorae in the same area, and compositional differences were also observed. This is the first time that compositional reference groups for Late Roman amphora kiln sites in the Guadalquivir valley are characterized. Hence, the results of this research not only contribute new evidence on the study of oil amphora production in this region, but they will also serve as a basis for the identification and sourcing of these amphorae in consumption sites and, consequently, for a better understanding of trade networks during the Late Roman period.

Magnetic fabric development in the Lower Seve thrust from the COSC-1 drilling, Swedish Caledonides

The COSC-1 project drilled the several hundred meters thick basal shear zone of the Lower Seve nappe with mylonites in mica schists, amphibole schists and gneisses. In zones of high magnetic susceptibility from 1910 to 2450 m we studied magnetic and petrographic fabrics, and magnetic mineralogy. Borehole imaging allowed for geographic reorientation of the samples and offered the opportunity to study anisotropy of magnetic susceptibility (AMS) in relation to tectonic evolution of the Seve thrust. We measured AMS at room temperature, added low-temperature and field-dependent AMS for a subset of samples, and compared magnetic with petrographic fabrics. Triaxial and prolate magnetic fabrics with degree of anisotropy (P′) up to 3.2 together with abundant S-C fabrics and strain partitioning around porphyroclasts indicate dominant simple shear until 2300 m. Magnetite and ilmenohematite mimic the rock fabric due to fabric parallel alignment and/or magnetic interaction and either contribute to increase or decrease of P′, depending on the dominating rock fabric elements. Field-dependency of pyrrhotite and magnetite in kmax-direction further increases P′. Homogeneous and oblate petrographic and magnetic fabrics in the greenschist-grade overprinted rocks below 2300 m with subhorizontal kmax-kint-girdle distributions indicate dominant flattening. AMS depicts shear fabrics including magnetite and ilmenohematite, and is additionally increased by retrograde magnetite-rutile intergrowth in ilmenohematites. We interpret that shape and degree of AMS are controlled by (a) tectonic deformation and strain, (b) alteration and magnetic grain interaction, and (c) field-dependency of deformed pyrrhotite and/or magnetite. We observed that all petrographic and magnetic subfabrics are coaxial, and lineations are mainly E-W to SE-NW directed confirming the transport direction of the Caledonian allochthonous. From our microstructural and AMS results we suggest that thrusting of the Lower Seve unit commenced under simple shear conditions at higher metamorphic grades and subsequently switched to more pure shear under greenschist-grade conditions.

Active Ooid Growth Driven By Sediment Transport in a High-Energy Shoal, Little Ambergris Cay, Turks and Caicos Islands

Ooids are a common component of carbonate successions of all ages and present significant potential as paleoenvironmental proxies, if the mechanisms that control their formation and growth can be understood quantitatively. There are a number of hypotheses about the controls on ooid growth, each offering different ideas on where and how ooids accrete and what role, if any, sediment transport and abrasion might play. These hypotheses have not been well tested in the field, largely due to the inherent challenges of tracking individual grains over long timescales. This study presents a detailed field test of ooid-growth hypotheses on Little Ambergris Cay in the Turks and Caicos Islands, British Overseas Territories. This field site is characterized by westward net sediment transport from waves driven by persistent easterly trade winds. This configuration makes it possible to track changes in ooid properties along their transport path as a proxy for changes in time. Ooid size, shape, and radiocarbon age were compared along this path to determine in which environments ooids are growing or abrading. Ooid surface textures, petrographic fabrics, stable-isotope compositions (δ^(13)C, δ^(18)O, and δ^(34)S), lipid geochemistry, and genetic data were compared to characterize mechanisms of precipitation and degradation and to determine the relative contributions of abiotic (e.g., abiotic precipitation, physical abrasion) and biologically influenced processes (e.g., biologically mediated precipitation, fabric destruction through microbial microboring and micritization) to grain size and character. A convergence of evidence shows that active ooid growth occurs along the transport path in a high-energy shoal environment characterized by frequent suspended-load transport: median ooid size increases by more than 100 μm and bulk radiocarbon ages decrease by 360 yr westward along the ∼ 20 km length of the shoal crest. Lipid and 16S rRNA data highlight a spatial disconnect between the environments with the most extensive biofilm colonization and environments with active ooid growth. Stable-isotope compositions are indistinguishable among samples, and are consistent with abiotic precipitation of aragonite from seawater. Westward increases in ooid sphericity and the abundance of well-polished ooids illustrate that ooids experience subequal amounts of growth and abrasion—in favor of net growth—as they are transported along the shoal crest. Overall, these results demonstrate that, in the Ambergris system, the mechanism of ooid growth is dominantly abiotic and the loci of ooid growth is determined by both carbonate saturation and sediment transport mode. Microbes play a largely destructive, rather than constructive, role in ooid size and fabric.

Proterozoic microbial mats and their constraints on environments of silicification.

The occurrence of microfossiliferous, early diagenetic chert in Proterozoic successions is broadly restricted to peritidal marine environments. Such coastal environments are amongst the most environmentally variable of marine environments, experiencing both enhanced evaporation and potential influx of terrestrial freshwaters. To better understand potential conditions under which silicification occurs, we focus on microfossiliferous early diagenetic chert from the Mesoproterozoic Bylot Supergroup, northern Baffin Island. Spectacular preservation of silicified microbial mats, their associated mineral phases, and the petrographic fabrics of the chert itself require that silicification occurred at the sediment-water interface, penecontemporaneously with mat growth. In some cases, silica is the primary precipitated mineral phase and is not associated with replacement of precursor mineral phases. In other cases, silica deposition includes the mimetic replacement of carbonate, gypsum, and halite mineral phases. These petrographic constraints suggest that silicification potentially occurred under a range of fluid chemistries associated with environmental variability in nearshore peritidal environments. Here we provide the first direct thermodynamic modeling of hypothetical Proterozoic seawater solutions, seawater-derived brines, and mixed seawater-freshwater solutions, and demonstrate that peritidal environments are capable of providing a wide range of fluid chemistries under which early diagenetic silica can both precipitate and replace primary mineralogical phases. Despite the thermodynamic potential for silica deposition under a wide range of fluid compositions, chert is not ubiquitous in Proterozoic nearshore environments, suggesting that the kinetics of silica polymerization exert a primary control over deposition.

New insights on Early Bronze Age IV pottery production and consumption in the southern Levant: The case of Khirbat Iskandar, Jordan

This work reports the results of a multi-analytical approach, including optical microscopy (OM), X-ray powder diffraction (XRPD), scanning electron microscopy and energy dispersive X-ray analysis (SEM-EDX), aiming to define the mineralogical and petrographic composition of 35 ceramic samples dated in the Early Bronze Age IV (EB IV) from the archaeological site of Khirbat Iskandar (Jordan). OM analyses distinguished six petrographic fabrics and identified numerous inclusions, such as limestone, angular calcite, silica-rich and volcanic rock fragments as well as elongated fragments of bivalve shells. XRPD and SEM-EDX analyses revealed morphological and compositional features, which led to the identification of calcite, quartz, dolomite, hematite, and gehlenite. The concurrent presence of calcite and traces of gehlenite suggested a firing temperature within the range of 750–850°C.Although numerous studies have been published on EB IV ceramic repertoires in the southern Levant, few of them concern different phases within the period. Khirbat Iskandar is a key-site for EB IV as it provides a multi-phase occupational sequence, possibly stretching the entire length of the period, that makes the site a regional stratigraphic benchmark for the study of a still enigmatic period in southern Levantine archaeology. Therefore, a comparative study of two-phases of ceramics from Area B at the site can further provide a unique contribution to the study of pottery production, consumption, and exchange during the late Early Bronze Age in the southern Levant.

A multi-proxy stalagmite record from northwestern Namibia of regional drying with increasing global-scale warmth over the last 47 kyr: The interplay of a globally shifting ITCZ with regional currents, winds, and rainfall

Stalagmite Orum-1 from a cave near Orumana in northwestern Namibia provides a multi-proxy record of regional drying with increasing global-scale warmth over the last 47kyr, in a region with few long well-dated location-specific paleoclimate records. Data from Stalagmite Orum-1 include carbon and oxygen stable isotope ratios, proportions of aragonite and calcite, pronouncedly differing petrographic fabrics, positions of layer-bounding surfaces, variation in layer-specific width, and changes in layer thickness, all of which combine to support change from wetter to drier conditions. Combined with fourteen U-Th ages, they suggest that climate was wetter in northwestern Namibia during globally cold MIS 3 than it is today, and with more grass than is present today. The climate at Orumana became drier during the deglacial transition after the Last Glacial Maximum, but carbon isotope data indicate that C4 grasses persisted. In the Holocene, even greater aridity led to a reduction in grass cover and to the present C3-dominated vegetation. Hiatuses in Stalagmite Orum-1 suggest even drier conditions during the Bølling-Allerød and during the early Holocene thermal maximum.Wetter conditions at Orumana during glacial times may have resulted from movement of the Intertropical Convergence Zone southward, in a shift that was significant west of longitude 13°E but perhaps less significant east of that line. It may have been accompanied by a lesser southward shift of the Angola-Benguela Front at sea and/or the Inter-Ocean Convergence Zone on land, leading to increased rainfall in northern Namibia (but perhaps not farther south). Extrapolation from the present to warmer conditions in the next century would suggest that further drying in northern Namibia and southern Angola may occur.

Petrographic and chemical characterization of Bronze Age pottery from the settlement of Mount San Paolillo (Catania, Italy)

The excavations at Mount San Paolillo (Catania, Italy) led to the discovery of a Prehistoric site that still represents the most important evidence of Middle and Late Bronze Age settlement in this area. During the excavations, archeologists located a hut, a store for ceramic storage vessels, and a pottery workshop, all of which provided a large quantity of heterogeneous ceramics with apparent typological parallels in other areas of Sicily, such as Syracuse, Augusta, and Messina. A large number of specimens were selected in order to cover all the macroscopic types and the main classes. The results identified four petrographic fabrics. Most of the ceramics are characterized by abundant tempers consisting of volcanic rock fragments and occasionally of grog. Only a few samples contained common fine-grained quartz. The groundmass ranges from non-micaceous to very micaceous. In some cases, there is evidence of mixed clays. Analysis of the chemical composition of the ceramics revealed the existence of two groups with low and high CaO contents. The high Fe2O3 content (more than 8.7 wt%) is probably due to the use of temper from altered pyroclastic rocks which are of local provenance as SEM-EDX data suggest, even if petrographic and chemical results suggest that different raw materials may have been used. The data provided by these archeometric analyses have made a significant contribution to the study of Middle and Late Bronze Age pottery from the Catania area, by offering insights into the methods, production processes, and high technical level of the prehistoric craftsmen.

The Meaning of Stromatolites

Stromatolites document microbial interactions with sediments and flowing water throughout recorded Earth history and have the potential to illuminate the long-term history of life and environments. Modern stromatolites, however, provide analogs to only a small subset of the structures preserved in Archean and Proterozoic carbonates. Thus, interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian, scale-dependent models of microbial responses to nutrient availability, seawater chemistry, influx of sediment grains, shear, and burial. Models that integrate stromatolite scales, macroscopic organization, and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity. An improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms that contributed to stromatolite growth in early oceans.

Provenance of marble sculptures from the National Museum of Carthage (Tunisia)

The results of a combined minero-petrographic and isotopic study carried out on representative marble sculptures from the Museum of Carthage are presented. Pentelikon source has been identified for nine of the analyzed items, Carrara and Prokonnesos sources have been also identified for three and one items respectively, confirming in some cases previously-made hypothesis. Local Chemtou and Djebel Ichkeul stones present very different petrographic fabrics compared with the most recurrent marbles. The undertaken approach adds value to the descriptions of the studied artifacts and could be extended to other art pieces of this and other museums.

Cretaceous methane-seep deposits from New Zealand and their fauna

Six Cretaceous methane-seep deposits are reported from the Raukumara Peninsula, eastern North Island, New Zealand. Dinoflagellate dating indicates a Late Albian to mid-Cenomanian age for three deposits from Port Awanui, and a mid-Campanian age for two deposits from Waipiro Bay and for one from Owhena Stream. The dominant petrographic fabric of the carbonates is detrital micrite, and the numerous fenestrae and vugs are filled with various cements including two types of non-detrital micrite, and botryoidal and banded fibrous calcites. These petrographic features, in combination with negative δ13C values of early diagenetic micrites (as low as −29.2‰ vs. V-PDB) and 13C-depleted molecular fossils such as the archaeal biomarkers pentamethylicosane (−97‰ vs. V-PDB) and biphytane (−99‰), and the bacterial biomarker anteiso-C15 fatty acid (−47‰), reveal that sulfate-dependent anaerobic oxidation of methane (AOM) induced the formation of these deposits. The carbonates preserve a typical mollusk dominated, Late Mesozoic, deep-water seep fauna including the large modiomorphid bivalve Caspiconcha, the lucinid bivalve Ezolucina, and limpets, hokkaidoconchids and a large abyssochrysoid among the gastropods, with close biogeographic relationships to North Pacific seep faunas of Cretaceous age. In contrast to the general marine mollusk fauna of New Zealand, which shows a high degree of endemism in both the Cenozoic and Recent, New Zealand's Cretaceous to present-day seep faunas consist largely of taxa known from coeval seeps around the world. Thus while deep-water seeps in New Zealand were repeatedly or continuously colonized by members of the global seep fauna, the general mollusk fauna of the New Zealand region developed a considerable degree of endemism during this time.

Crystallization in flow - I. Palaeocirculation track by texture analysis and magnetic fabrics

SUMMARY In order to better define the dependence of the mineral texture on flow, anisotropy of magnetic susceptibility (AMS) and microscopic observations have been performed on calcite-rich precipitations occurring in a horizontal pipe where the flow direction is clearly defined. A total of 65 cubes were cut from five slices, magnetic studies identified-pseudo-single domain magnetite as the major AMS carrier. Horizontal foliation characterizes the magnetic fabrics and the lineation is parallel to the pipe axis, that is, the flow direction. The origin of this lineation is discussed and is interpreted to be mainly the consequence of elongated pseudo-single domain magnetite. To complete the magnetic fabric studies, shape preferred orientation statistics were performed on 563 calcite sections in the (0001) calcite plane. It shows elongated shapes with a general orientation parallel to the pipe axis. The mean shape orientation is the average of two distinct subpopulations that deviated slightly from the pipe axis. Observation on calcite shapes and the direction of the magnetic lineation are coherent, suggesting that it is possible to track hydrothermal palaeocirculation using magnetic lineation and petrographic fabrics.

Recognition of Fossil Prokaryotes in Cretaceous Methane Seep Carbonates: Relevance to Astrobiology

Recovery of prokaryotic body fossils from methane seep carbonates such as those of the Cretaceous Tepee Buttes of Colorado serves as a model for sampling in future astrobiological missions. The fossils, found primarily at the interface between paragenetic fabrics, suggest a sharp physicochemical gradient. Evidence of these microbial fossils occurs at a variety of scales. In the field, microbialite is found as meter-scale thrombolitic zones and centimeterscale stromatolitic crusts lining voids inferred to be the sites of ancient methane seepage. Petrographic fabrics suggestive of microbialite include indistinct peloids (0.1-1 mm in diameter) and crusts of authigenic micrite. Primary evidence obtained from scanning electron microscopy coupled with energy-dispersive x-ray spectroscopy analysis comprises pinnate bacteria (0.3 microm in diameter and 1-1.5 microm long), sheaths (2-4 microm in diameter), coccoids (0.5-1 microm in diameter, up to 40 per cluster), and the presence of framboidal pyrite (6-8 microm in diameter). These results are in agreement with studies of other ancient and modern seeps and suggest a morphological conservatism of microbial form that can be incorporated into studies of extraterrestrial environments where it is presumed that reduced gases drive the metabolic activity of prokaryote-like organisms. Target areas that could serve as conduits for reduced gas seeps include tectonic or impact-driven faulting, zones of cryosphere melting, or other disruptions in crustal coherence. Ancient seeps, preserved as localized anomalous evaporite deposits in the sedimentary cover, could be detected by remote sensing.

Mixing-Zone Diagenesis in the Subsurface of Florida and the Bahamas

Abstract Three cores from Great Bahama Bank and southern Florida contain examples of mixing-zone diagenesis where typical shallow-water, aragonite-rich sediments were altered to low-Mg calcite. The 40–50 m thick mixing zones are identified on the basis of downcore covariant trends from depleted meteoric (δ18O = -3.2 ± 0.7‰; δ13C = -1.6 ± 1.8‰) to enriched marine isotopic values (δ18O = +1.0 ± 0.3‰; δ13C = +1.9 ± 0.7‰). The age of the mixing zones varies from late Pliocene to Pleistocene, so these cores provide three independent examples of mixing-zone diagenesis. Mixing-zone diagenesis produced a limestone composed of low-Mg calcite with petrographic fabrics including moldic porosity, blocky to dogtooth calcite cementation, and recrystallization of lime mud to micrite and microspar. The mixing-zone diagenetic model predicts alteration in only part of the mixing zone. Because the alteration recorded in these cores shows continuous alteration through the entire mixing zone, we consider the mixing of waters itself to be largely irrelevant. Rather, diagenesis was probably driven by the difference in solubility between aragonite and calcite with some additional dissolution of aragonite driven by the microbial oxidation of buried organic matter. The mixing zone has been proposed as a likely location for dolomitization. With the exception of some clearly sea-floor dolomite in one core (Unda), the mixing zones in these cores lack dolomite despite the sediment clearly having been altered in the mixing zone. The lack of dolomite in these mixing zones, combined with the extensive literature questioning the chemical reasoning for mixing-zone dolomite, supports rejection of the mixing-zone dolomite model.

Preparation of Oriented Block Samples of Lignite for Petrographic Analysis

ABSTRACT The manufacture of particulate samples for coal petrographic analysis destroys the original petrographic fabric of the coal, so that fine-scale (millimeter to centimeter) variations in petrographic composition through the seam cannot be identified. The use of oriented block samples prepared from pillar samples allows the physical integrity of the coal seam to be preserved and thus studied. While the preparation of polished block samples does not generally present any problems for coals of sub-bituminous rank or higher, the high moisture content of lignites means that the polished surface is prone to desiccation. This leads to the formation of shrinkage cracks, which cause distortion of the surface and render microscopic analysis under high-power magnification (500) virtually impossible. In order to overcome this problem, samples of lignite were dehydrated in acetone and then impregnated with a low-viscosity epoxy resin. A thin film of immersion oil during analysis further inhibits the affects of any desiccation on the polished surface. Long-term storage under nitrogen has been effective in preserving the high quality of the polished surface for more than four years.