Secular variations in seawater chemistry as a control on the chemistry of basinal brines: test of the hypothesis

The hypothesis that basinal brines inherited their major ion chemistries and elevated salinities from evaporated paleoseawaters is tested by comparing the compositions of basinal brines in Silurian (Michigan basin, Illinois basin, Appalachian basin in eastern Ohio) and Jurassic/Cretaceous (Central Mississippi Salt Dome basin, Arkansas shelf, and south‐central Texas) host rocks, when the world oceans were ‘CaCl₂ seas’, with those from Permian and Pennsylvanian rocks (Palo Duro basin, Central Basin Platform, and Delaware basin, Texas and New Mexico) when the world oceans were ‘MgSO₄ seas’. Basinal brines examined are assumed to have originally formed from evaporation of the same seawaters that produced major evaporites. Sulfate, Mg and K levels in basinal brines are below the concentrations expected from evaporation of seawater of any type, which emphasizes the importance of diagenetic mineral–brine interactions in controlling basinal brine chemistry. There are no major differences in SO₄, Mg and K concentrations between basinal brines hosted by rocks originally formed during ages when the world oceans were MgSO₄ seas versus CaCl₂ seas. Basinal brines in Pennsylvanian–Permian rocks are compositionally distinct (relatively high Na and low Ca) from basinal brines in Silurian, Jurassic and Cretaceous host rocks, which may reflect original differences in seawater chemistry. Basinal brines enriched in Ca and depleted in Na relative to evaporated seawater of any type have traditionally been interpreted to form by albitization of plagioclase feldspar. A new explanation for Ca enrichment and Na depletion of basinal brines is the mixing of evaporated CaCl₂‐type seawater with more dilute water. Some basinal brines are similar in major ion composition to evaporated seawater of a particular age, for example basinal brines in the Cretaceous Edwards Group carbonates, Texas, where dolomitization is the only reaction required to convert evaporated Mesozoic CaCl₂ seawater into Edwards Group brine.     

Basin‐scale fluid movement patterns revealed by veins: Wessex Basin,UK

Calcite veins at outcrop in the Mesozoic, oil‐bearing Wessex Basin, UK, have been studied using field characterization, petrography, fluid inclusions and stable isotopes to help address the extent, timing and spatial and stratigraphic variability of basin‐scale fluid flow. The absence of quartz shows that veins formed at low temperature without an influence of hydrothermal fluids. Carbon isotopes suggest that the majority of vein calcite was derived locally from the host rock but up to one quarter of the carbon in the vein calcite came from CO₂ from petroleum source rocks. Veins become progressively enriched in source‐rock‐derived CO₂ from the outer margin towards the middle, indicating a growing influence of external CO₂. The carbon isotope data suggest large‐scale migration of substantial amounts of CO₂ around the whole basin. Fluid inclusion salinity data and interpreted water‐δ¹⁸O data show that meteoric water penetrated deep into the western part of the basin after interacting with halite‐rich evaporites in the Triassic section before entering fractured Lower and Middle Jurassic rocks. This large‐scale meteoric invasion of the basin probably happened during early Cenozoic uplift. A similar approach was used to reveal that, in the eastern part of the basin close to the area that underwent most uplift, uppermost Jurassic and Cretaceous rocks underwent vein formation in the presence of marine connate water suggesting a closed system. Stratigraphically underlying Upper Jurassic mudstone and Lower Cretaceous sandstone, in the most uplifted part of the basin, contain veins that resulted from intermediate behaviour with input from saline meteoric water and marine connate waters. Thus, while source‐rock‐derived CO₂ seems to have permeated the entire section, water movement has been more restricted. Oil‐filled inclusions in vein calcite have been found within dominant E‐W trending normal faults, suggesting that these may have facilitated oil migration.     

Chemical changes in buried animal bone: Data from a postmedieval assemblage

Macroscopic chemical analysis of animal bone recovered from dated excavation contexts of known pH from Castle Bromwich Hall, West Midlands, UK, allows an assessment of the rate and effects of bone decomposition, and the evaluation of current models of chemical decay. The results show great variation, and it is suggested that factors such as mechanical disturbance have a more significant effect on the differential destruction of excavated bone assemblages than chemical decay. The implications of this conclusion for the attempted reconstruction of past faunas, diet and behaviour are summarized, recommending caution in the acceptance of assemblages as unbiased samples.     

Can trace elements in fossils provide information about palaeodiet?

The main aim of this study is to try to see if, despite the diagenetic changes undergone by the fossil bones buried in Venta Micena (Orce, Spain), the concentration of trace elements permits the differentiation of particular groups. It is possible that some chemical elements allow us to identify different dietary groups in accord with their archaeological context. Different multivariant methods—correlation, principal component analysis and cluster analysis—were applied to the data, and in all cases the results show that two elements (Ba and Zn) seem able to discriminate between groups with different diets. In this sense, diagenesis cannot explain all the variability found in the concentrations of trace elements in fossils from the Orce region. © 1998 John Wiley & Sons, Ltd.     

An X-ray Diffraction (XRD) and X-ray Fluorescence (XRF) investigation in human and animal fossil bones from Holocene to Middle Triassic

We examined by Rietveld refinement of X-ray Diffraction patterns a series of 61 human and animal fossil bones in an age range from present time to Middle Triassic (around 245 Ma). This approach, supplemented by elemental analysis according to X-ray Fluorescence, has permitted to obtain a quantitative evaluation of the mineralogical phases in the specimens, thus allowing to reconstruct the mineralization process involved. Concerning the apatite phase, after adopting a monoclinic geometry for the unit cell, the method permits to determine with fair degree of precision the unit cell volume, which is found to decrease in relatively short geological times as a function of the fluorine substitution process for the hydroxyl group –OH. After excluding the role of a high-temperature fire treatment to the bones, it is found that a certain linear correlation may exist over the geological time scale involved between the age of the specimen and the average crystallite size. This observation permits to use the XRD pattern as an evaluation of the age of the fossil, although the stratigraphic observations, where available, remain as the main reliable source for dating such biomaterials. The uncertainty related to this age estimation may be hardly better than 15–20% in absolute, because of various assumptions involved in the XRD methodology used and in the variability of the burial environment that may be subjected to discontinuous changes over the very long time of deposition.     

Rapid loss of endogenous DNA in pig bone buried in five different environments

The rate at which endogenous DNA from differently prepared (butchered, boiled and baked) compact pig bones degrades in five different Danish terrestrial and marine environments over 12 months was investigated. Although > 70% of the estimated endogenous mtDNA is lost after just four weeks of exposure, no cytosine deamination of DNA was recognised. A correlation between the presence of oxygen and the amount of preserved DNA was observed. The results provide valuable information on the interaction between the endogenous DNA and the depositional environment in the early stages of bone diagenesis, which can be a support in the interpretation of the initial diagenetic pathways of archaeological bone.     

Permeability of the Mercia Mudstone: suitability as caprock to carbon capture and storage sites

The Upper Triassic Mercia Mudstone is the caprock to potential carbon capture and storage (CCS) sites in porous and permeable Lower Triassic Sherwood Sandstone reservoirs and aquifers in the UK (primarily offshore). This study presents direct measurements of vertical (k_v) and horizontal (k_h) permeability of core samples from the Mercia Mudstone across a range of effective stress conditions to test their caprock quality and to assess how they will respond to changing effective stress conditions that may occur during CO₂ injection and storage. The Mercia samples analysed were either clay‐rich (muddy) siltstones or relatively clean siltstones cemented by carbonate and gypsum. Porosity is fairly uniform (between 7.4 and 10.7%). Porosity is low either due to abundant depositional illite or abundant diagenetic carbonate and gypsum cements. Permeability values are as low as 10^?20 m² (10nD), and therefore, the Mercia has high sealing capacity. These rocks have similar horizontal and vertical permeabilities with the highest k_h/k_v ratio of 2.03 but an upscaled k_h/k_v ratio is 39, using the arithmetic mean of k_h and the harmonic mean of k_v. Permeability is inversely related to the illite clay content; the most clay‐rich (illite‐rich) samples represent very good caprock quality; the cleaner Mercia Mudstone samples, with pore‐filling carbonate and gypsum cements, represent fair to good caprock quality. Pressure sensitivity of permeability increases with increasing clay mineral content. As pore pressure increases during CO₂ injection, the permeability of the most clay‐rich rocks will increase more than carbonate‐ and gypsum‐rich rocks, thus decreasing permeability heterogeneity. The best quality Mercia Mudstone caprock is probably not geochemically sensitive to CO₂ injection as illite, the cause of the lowest permeability, is relatively stable in the presence of CO₂–water mixtures.     

Assessing bone transformation in late Miocene and Plio‐Pleistocene deposits of Kenya and South Africa

Bone reactivity offers a potential way to record local physical–chemical conditions prevailing in fossilization environments and archaeological sites. In the present study, a series of fossil bone samples from the karstic environments of the Bolt's Farm cave system (Cradle of Humankind, South Africa) and from fluvio‐lacustrine environments of the Tugen Hills (Gregory Rift, Kenya) is analysed. The chemical composition and infrared and nuclear magnetic resonance (NMR) spectroscopic properties of fossil samples point to a transformation of the biogenic apatite and formation of secondary apatite. Depending on the sample, the secondary apatite corresponds to a carbonate‐bearing hydroxy‐ or fluor‐apatite. The maximum fraction of secondary apatite is close to 60%, coinciding with previous observations in experimental alteration of bone in aqueous solutions and suggesting that a fraction of pristine biological apatite is likely to be preserved. The present results also suggest that the acetic acid treatment of fossil samples moderately increases their average crystallinity but may dissolve carbonate‐rich domains of secondary apatite.     

‘Not All That Is White Is Lime’—White Substances from Archaeological Burial Contexts: Analyses and Interpretations

Archaeological burial contexts may include a variety of white substances, but few analyses have been published. This study reports on the physico‐chemical characterization of such residues from seven archaeological sites. It is often assumed that white materials from burial contexts are lime. Our findings demonstrate that they can be gypsum, calcite (chalk), aragonite, brushite, degraded metal, natural (gum) resins or synthetic polymer–based products. These may be present as the result of diagenetic processes, funerary practices or modern contamination. This paper provides an analytical approach for the holistic investigation of white materials encountered in burial contexts.     

Effects of pore fluid flow and chemistry on compaction creep of calcite by pressure solution at 150°C

Uni‐axial compaction creep experiments were performed on crushed limestone and analytical grade calcite powders at 150°C, a pore fluid pressure of 20 MPa, and effective axial stresses of 30 and 40 MPa. Previous experiments have shown that compaction under these conditions is dominated by intergranular pressure solution (IPS). The aim of the present tests was to determine the inter‐relationship between pore fluid chemistry, compaction rate and the rate‐controlling process of IPS. Intermittent flow‐through runs conducted using CaCO₃ solution showed no effect on creep rate at low strains (<4–5%) but a major acceleration at high strains (5–10%). Measurements of the Ca concentration present in fluid samples revealed the build‐up of a high super‐saturation of CaCO₃ during compaction under zero flow conditions, especially at high strains. Active flow‐through led to a drop in Ca concentration, which corresponded with creep acceleration. Addition of NaCl to the pore fluid, at a concentration of 0.5 m , increased the creep rate of the analytical grade calcite samples roughly in proportion to the enhancement of calcite solubility. Addition of Mg²⁺ and to the pore fluid, in concentrations of 0.05 and 0.001 m, respectively, caused major retardation of compaction creep. Integrating our mechanical, flow‐through and chemical data points strongly to diffusion‐controlled IPS being the dominant deformation mechanism in the calcite‐water system under closed‐system (zero flow) conditions at low strains (<4–5%), giving way to precipitation control at higher strains. Our fluid composition data suggest that this transition is because of accumulation of impurities in the pore fluid. As Mg²⁺ and phosphate ions are common in natural pore fluids, it is likely that retarded precipitation will be the rate‐limiting step of IPS in carbonates in nature. To quantify diagenetic compaction and porosity‐permeability reduction rates by IPS in carbonates needs to account for this.