Volcanogenic Source Research Articles

Mercury evidence of Deccan volcanism driving the Latest Maastrichtian warming event

Abstract The timing and ecological impacts of the Deccan Traps large igneous province eruption are vigorously debated. Pre–Cretaceous-Paleogene (KPg) boundary impacts of Deccan volcanism have been widely identified in marine sediments, but direct evidence of terrestrial impacts remains rare. We used mercury concentrations and isotopic compositions, a proxy for volcanic activity, to assess impacts on terrestrial environments. We studied two drill cores across the KPg boundary in eastern China that represent two different depositional environments: clastic deposits in the Jiaolai Basin and carbonate deposits in the Pingyi Basin. Both drill cores exhibit strong Hg enrichment prior to the KPg boundary. Near consistent mass-independent fractionation (MIF) of odd-Hg isotopes (odd-MIF) in the Jiaolai Basin likely indicates a volcanogenic source of Hg spikes below the KPg boundary. Odd-MIF isotopes in the Pingyi Basin likewise suggest a volcanogenic Hg source but with a terrestrial Hg signature of lower Δ199Hg values before and after the Hg spike interval. The Hg enrichment level can be stratigraphically correlated to the beginning of the Latest Maastrichtian warming event (LMWE) and is consistent with a strong, negative carbon-isotope excursion (CIE) in both δ13Corg (organic matter) and δ13Ccarb (carbonate), suggesting a disturbance of the global carbon cycle induced by a major pulse of Deccan Traps volcanism. Our discovery of a ter-restrial record of pre-KPg boundary Deccan volcanism provides robust evidence of global influence of the Deccan Traps large igneous province during the LMWE.

Seeking Earth’s oldest geological record: an unexpected discovery of well-preserved 3834 Ma metatonalite

One of Greenland’s largest bodies of Archean (meta)sedimentary rocks occurs on the ∼5.5 by 2.0 km nunatak Isortup Nunataa (∼65°31′N 49°54′W) and is ∼35 km north of the Eoarchean Isua supracrustal belt with its world’s best record of early sedimentary and volcanic systems. The nunatak was visited to ascertain if its metasedimentary rocks are also Eoarchean and thereby provide extra insight into the early Earth. The metasedimentary rocks are derived from a ca 3060 Ma arc-like volcanogenic source and can be assigned to the Akia terrane that crops out immediately to the west. However, glacial erratics scattered over the nunatak indicate there are well-preserved Eoarchean rocks including 3834 Ma metatonalite hidden to the east under the Inland Ice. This demonstrates there are still occurrences of Eoarchean rocks out there to be found—by a mix of logic and luck. These findings will all enhance our knowledge of the early Earth, to help answer the big questions about the important early events that shaped our planet. KEY POINTS The nunatak Isortup Nunataa (West Greenland) is dominated by ca 3060 Ma arc-like volcanogenic metasedimentary rocks that are assigned to the Akia terrane, which crops out immediately to the west. Glacial erratics, including 3834 Ma metatonalite scattered over the nunatak, indicate there are well-preserved Eoarchean rocks hidden to the east under the Inland Ice. Any new findings of Eoarchean rocks will enhance our understanding of the early Earth.

Volcanism and Volcanogenic Submarine Sedimentation in the Paleogene Foreland Basins of the Alps: Reassessing the Source-to-Sink Systems with an Actualist View

This work critically reviews the Eocene–Oligocene source-to-sink systems accumulating volcanogenic sequences in the basins around the Alps. Through the years, these volcanogenic sequences have been correlated to the plutonic bodies along the Periadriatic Fault System, the main tectonic lineament running from West to East within the axis of the belt. Starting from the large amounts of data present in literature, for the first time we present an integrated 4D model on the evolution of the sediment pathways that once connected the magmatic sources to the basins. The magmatic systems started to develop during the Eocene in the Alps, supplying detritus to the Adriatic Foredeep. The progradation of volcanogenic sequences in the Northern Alpine Foreland Basin is subsequent and probably was favoured by the migration of the magmatic systems to the North and to the West. At around 30 Ma, the Northern Apennine Foredeep also was fed by large volcanogenic inputs, but the palinspastic reconstruction of the Adriatic Foredeep, together with stratigraphic and petrographic data, allows us to safely exclude the Alps as volcanogenic sources. Beyond the regional case, this review underlines the importance of a solid stratigraphic approach in the reconstruction of the source-to-sink system evolution of any basin.

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

AbstractAtmospheric dust is a primary source of iron (Fe) to the open ocean, and its flux is particularly important in the high nutrient, low chlorophyll (HNLC) Southern Ocean where Fe currently limits productivity. Alleviation of this Fe limitation in the Subantarctic Zone of the Atlantic by increased dust‐borne Fe supply during glacial periods has been shown to increase primary productivity. However, previous work has found no such increase in productivity in the Pacific sector. In order to constrain the relative importance of Southern Ocean Fe fertilization on glacial‐interglacial carbon cycles, records of dust fluxes outside of the Atlantic sector of the Southern Ocean at the Last Glacial Maximum (LGM) are required. Here we use grain size and U‐series analyses to reconstruct lithogenic and CaCO3 fluxes and Nd, Sr, and Pb isotopes to ascertain the provenance of terrigenous material delivered to four deep water cores in the SW Pacific Ocean over the last ~30 kyr. We find evidence for an increase in the relative proportion of fine‐grained (0.5–12 μm) terrigenous sediment and higher detrital fluxes during the LGM compared to the Holocene. The provenance of the LGM dust varied spatially, with an older, more “continental” signature (low εNd, high 87Sr/86Sr) sourced from Australia in the northern cores, and a younger, more volcanogenic source in the southern cores (high εNd, low 87Sr/86Sr), likely sourced locally from New Zealand. Given this increase in lithogenic flux to the HNLC subantarctic Pacific Southern Ocean during the LGM, factors besides Fe supply must have regulated the biological productivity here.

Geochemical characteristics of the Late Cretaceous radiolarian cherts from North Andaman Island, Bay of Bengal, India

The radiolarian cherts of Late Cretaceous age are reported in the Andaman-Nicobar ophiolites of Bay of Bengal, India. They are of chocolate-coloured (Type-I) and light pink-coloured (Type-II) varieties, juxtaposed and tectonically associated with phyllites, claystones and basalts and are exposed in eastern part of North Andaman Island. These cherts are composed of radiolarian tests in a fine-grained ferruginous matrix. Type-I radiolarian cherts have low SiO2, and high Al2O3, Fe2O 3 (T) , MgO, Na2O and TiO2 and trace elements as compared to Type-II cherts. Al–Fe–(∑REE–Ce) diagram of the studied cherts indicates a mixed terrigenous and volcanogenic source. LaN/CeN ratios (0.76–0.89 for Type-I and 0.71–0.88 for Type-II) and Ce-anomalies (Ce/Ce* = 1.15–1.33 for Type-I and 1.07–1.38 for Type-II) and other elemental ratios in these cherts suggest that they were deposited in continental margin environments. It has been suggested that the studied samples of cherts were deposited at different places, were scrapped off the subducting plate, became tectonically juxtaposed. They were obducted onto the leading edge of the Eurasian continent during the Late Cretaceous prior to the currently active Andaman–Java subduction, that was probably initiated during the Late Miocene.

Organic carbon isotopes in terrestrial Permian-Triassic boundary sections of North China: Implications for global carbon cycle perturbations

Abstract The end-Permian mass extinction (ca. 252 Ma) represents the most severe biotic crisis of the Phanerozoic, and it was accompanied by profound environmental perturbations, especially to the global carbon cycle, as indicated by sharp negative carbon isotope excursions (CIE) in both carbonates (δ13Ccarb) and organic matter (δ13Corg). To date, carbon isotope records are mostly from marine Permian-Triassic transitional sequences with relatively few high-resolution carbon isotope profiles having been generated for terrestrial facies. Terrestrial Permian-Triassic sequences suitable for high-resolution carbon isotope study are rare globally and are difficult to correlate with better-studied marine sequences. However, carbon isotope records from continental facies are essential to a full understanding of global carbon cycle changes during the Permian-Triassic transition. Here, we present bulk δ13Corg profiles for three terrestrial sections in North China representing Permian-Triassic transitional beds. These profiles exhibit similar patterns of secular variation defining three stages: (1) a pre-CIE interval, (2) a CIE interval, characterized by a rapid negative shift of 1.7‰–2.2‰ within the middle part of the Sunjiagou Formation, and (3) a post-CIE interval. The similarity of the CIE in all three study sections facilitates correlations among them, and its presence in the Permian-Triassic transitional beds suggests that it is equivalent to the negative CIE at the Permian-Triassic boundary in the Meishan global stratotype section and point (GSSP) and in coeval marine and terrestrial sections globally. The end-Permian CIE was probably triggered by a massive release of 13C-depleted carbon from volcanogenic sources leading to elevated atmospheric pCO2, although oceanic sources of CO2 cannot be ruled out at present.

Bulk-carbonate and belemnite carbon-isotope records across the Pliensbachian-Toarcian boundary on the northern margin of Gondwana (Issouka, Middle Atlas, Morocco)

The data presented here provide the first high-resolution investigation of carbon isotope and geochemical analyses derived from the Pliensbachian-Toarcian boundary, of Issouka, Middle Atlas, Morocco. The isotope data recorded in micrite reveal a stepwise negative carbon isotope excursion with values dropping to −1.8‰ within the Polymorphum Zone. This excursion coincides with major marine biological changes and extinctions and corresponds with European records, supporting the assertion that the excursion was global in extent. The Issouka section is relatively expanded compared to other well–studied sections in Europe. The excursion at the Pliensbachian–Toarcian boundary also shows several similarities with the negative Early Toarcian Event. In contrast, carbon isotope values derived from coeval belemnites show positive values. The belemnite δ13C data presented here suggest spatial heterogeneity in the carbon isotopic composition of dissolved inorganic carbon (DIC) in the Early Jurassic ocean. Overturning or upwelling of a stratified water mass is inconsistent with our data, as it requires the belemnites to have lived elsewhere and only later to have migrated into the Middle Atlas area where they became fossilized. The oxygen isotope values from belemnite calcite show no distinct trend across the Pliensbachian-Toarcian boundary event, indicative of either no significant change in temperatures or change in seawater δ18O. We suggest that the introduction of any light carbon (e.g. from a volcanogenic source) must have resulted in spatial variability in the δ13C of the dissolved inorganic carbon of seawater. Alternatively, a regional change in the source of the carbonate carrying the isotope signal, could lead to a negative shift in the δ13Cmicrite signature without any relation to variations in the global carbon isotope trend.

Marine 187Os/188Os isotope stratigraphy reveals the interaction of volcanism and ocean circulation during Oceanic Anoxic Event 2

High-resolution osmium (Os) isotope stratigraphy across the Cenomanian–Turonian Boundary Interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs, revealing the impact of palaeoenvironmental processes. In every section the 187Os/188Os profiles show a comparable trend; radiogenic values in the lead up to Oceanic Anoxic Event 2 (OAE 2); an abrupt unradiogenic trend at the onset of OAE 2; an unradiogenic interval during the first part of OAE 2; and a return to radiogenic values towards the end of the event, above the Cenomanian–Turonian boundary. The unradiogenic trend in 187Os/188Os is synchronous in all sections. Previous work suggests that activity of the Caribbean LIP (Large Igneous Province) was the source of unradiogenic Os across the OAE 2 and possibly an instigator of anoxia in the oceans. Here we assess this hypothesis and consider the influence of activity from other LIPs; such as the High Arctic LIP.A brief shift to high radiogenic 187Os/188Os values occurred in the Western Interior Seaway before the onset of OAE 2. We evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of OAE 2; differential input of nutrients from continental and volcanogenic sources, coupled with efficient palaeocirculation of the global ocean and epeiric seas, enhanced productivity due to higher nutrient availability, which permitted penecontemporaneous transport of continental and LIP-derived nutrients to trans-equatorial basins.

Mineralogical and geochemical studies on the Gowd-e-Morad (Ni, Co, As–Cu) mineral deposit, Anarak (central Iran)

The Ni, Co, As, and Cu deposit of Gowd-e-Morad is located 20 km northwest of Anarak in Central Iran. In this hydrothermal deposit, mineralization occurs as veins in a fault breccia zone hosted by the Chahgorbeh (schist and metabasite) complex. The main ores are made up of Ni, Co, and Cu arsenides. Petrologic studies and results obtained from geochemical analyses have indicated that the Ni, Co, As, and Cu are derived from ultramafic rocks while Pb and Zn are likely to be derived from schist. Based on the geochemical evidence, particularly the high correlation between Ni, Co, and As, it is proposed that this deposit be categorized as a “five elements” mineral deposit. Fluid inclusion studies have shown homogenization temperatures (TH) in the range 113−206 ˚C and salinity 3−13.5 % wt eq. NaCl. Therefore this “five elements” mineral deposit has been determined as a low temperature, epithermal deposit type. It is proposed that the low fluid temperatures are a result of an environment of formation which was distal to a volcanogenic source systems and the major influence of meteoric waters in the hydrothermal system.

Thorium-derived dust fluxes to the tropical Pacific Ocean, 58 Ma

Eolian dust in pelagic deep sea sediments can be used to reconstruct ancient wind patterns and paleoenvironmental response to climate change. Traditional methods to determine dust accumulation involve isolating the non-dissolvable aluminosilicate minerals from deep sea sediments through a series of chemical leaches, but cannot differentiate between minerals from eolian, authigenic and volcanogenic sources. Other geochemical proxies, such as sedimentary 232Th and crustal 4He content, have been used to construct high-resolution records of atmospheric dust fluxes to the deep sea during the Quaternary. Here we use sedimentary Th content as a proxy for terrigenous material (eolian dust) in ∼58Myr-old sediments from the Shatsky Rise (ODP Site 1209) and compare our results with previous dust estimates generated using the traditional chemical extraction method and sedimentary 4Hecrustal concentrations. We find excellent agreement between Th-based dust estimates and those generated using the traditional method. In addition our results show a correlation between sedimentary Th and 4Hecrustal content, which suggests a source older than present day Asian loess supplied dust to the central subtropical Pacific Ocean during the early Paleogene.

Selenium as paleo-oceanographic proxy: A first assessment

Selenium (Se) is an essential trace element, which, with multiple oxidation states and six stable isotopes, has been suggested as a potentially powerful paleoenvironmental proxy. In this study, bulk Se concentrations and isotopic compositions were analyzed in a suite of about 120 samples of fine-grained marine sedimentary rocks and sediments spanning the entire Phanerozoic. While the Se concentrations vary greatly (0.22–72ppm), the δ82/76Se values fall in a fairly narrow range from −1 to +1‰ (relative to NIST SRM3149), with the exception of laminated black shales from the New Albany Shale formation (Late Devonian), which have δ82/76Se values of up to +2.20‰. Black Sea sediments (Holocene) and sedimentary rocks from the Alum Shale formation (Late Cambrian) have Se to total organic carbon ratios (Se/TOC) and δ82/76Se values close to those found in modern marine plankton (1.72±0.15×10−6mol/mol and 0.42±0.22‰). For the other sedimentary sequences and sediments, the Se/TOC ratios show Se enrichment relative to modern marine plankton. Additional input of isotopically light terrigenous Se may explain the Se/TOC and δ82/76Se data measured in recent Arabian Sea sediments (Pleistocene). The very high Se concentrations in sedimentary sequences that include the Cenomanian–Turonian Oceanic Anoxic Event (OAE) 2 may reflect an enhanced input of volcanogenic Se to the oceans. As the latter has an isotopic composition not greatly different from marine plankton, the volcanogenic source does not impart a distinct signature to the sedimentary Se isotope record. The lowest average δ82/76Se values are observed in the OAE2 samples from Demerara Rise and Cape Verde Basin cores (δ82/76Se=−0.14±0.45‰) and could reflect fractionation associated with microbial or chemical reduction of Se oxyanions in the euxinic water column. In contrast, a limiting availability of seawater Se during periods of increased organic matter production and burial may be responsible for the elevated δ82/76Se values and low Se/TOC ratios in the black shales of the New Albany Shale formation. Overall, our results indicate that to unlock the full proxy potential of marine sedimentary Se records, we need to gain a much more detailed understanding of the sources, chemical speciation, isotopic fractionations and cycling of Se in the marine environment.

Magma to mud to magma: Rapid crustal recycling by Permian granite magmatism near the eastern Gondwana margin

Early Permian (Cisuralian) S-type monzogranites of the Bundarra Supersuite, New England Orogen (NEO), eastern Australia, have chemical and isotopic compositions indicative of their being derived from young, weathered, volcanogenic sedimentary source rocks. High δ18Ozrn values of zircon precipitated from the melt phase of the magma (ca. 11.5‰) confirm that the previously measured high granite whole-rock values (δ18OWR) are a primary magmatic feature. Abundant inherited detrital zircon cores with a range of dates (360–300Ma) and distinctly lower δ18Ozrn values (ca. 5–10‰) were derived from mostly young (Carboniferous) volcanogenic source rocks that became less primitive and more isotopically diverse with time. The age difference between the youngest inherited zircon population and the earliest zircon growth during magma genesis is about 15Ma. An arc-related volcanogenic sedimentary pile built up over about 50Ma was weathered, buried, and partially melted, producing voluminous peraluminous magmas within no more than ca. 15Ma of the youngest source sediment being deposited. The time interval between the initiation of melting and post-emplacement magma crystallization was about 5Ma. The S-type Bundarra Supersuite in the NEO is a rare case in which the entire geologic cycle has been captured by zircon. Recognizing this is uniquely possible because of the relatively simple populations of inherited and newly-grown zircon, and their distinctive oxygen isotopic compositions.

Application and evaluation of biomagnetic and biochemical monitoring of the dispersion and deposition of volcanically-derived particles at Mt. Etna, Italy

article i nfo Article history: Received 6 August 2009 Accepted 4 January 2010 Available online xxxx Biomagnetic monitoring, using tree leaves as passive surfaces for particle collection, has been shown to be a promising technique for assessing the dispersion and deposition of particles in the context of anthropogenic pollution. By comparing leaves' magnetic properties with trace metal levels measured in the leaves, we here assess the utility of the biomagnetic technique as a sensitive, fast and inexpensive method for assessment of volcanic plume deposition. Samples of sweet chestnut leaves (Castanea sativa) were collected from the area surrounding Mt. Etna volcano in Sicily during the 2008 growing season when the volcano was displaying mild eruptive activity. Previous work has shown that the trace metal concentrations of these leaves show promise as a bio-indicator of volcanic gas, aerosol and ash deposition on the flanks of Mt. Etna. For 2008, ICP- MS analysis of the elemental abundances within the leaves showed that As, Cd, Cu, Mo, Tl, K, B, Al and Co displayed elevated concentrations downwind of the volcanic source, to the E-ESE, but with overall reduced concentrations relative to 2007. Less explosive activity than 2007 and a broader, more easterly wind field may have distributed the volcanic plume over a wider area, both of which would account for reduced trace element concentrations in 2008. Correspondence of elevated concentrations in both years (2007 and 2008) with their respective wind fields suggests that plume deposition is the controlling factor rather than variability in the soils and that these leaves do indeed have potential as bio-indicators of the plume's dispersion. Magnetic analysis of the leaves shows that the spatial distribution of saturation isothermal remanent magnetisation (SIRM) and magnetic susceptibility (χlf) values display a strong correlation with the wind-influenced plume transport direction for 2008, with elevated concentrations of magnetic minerals on the eastern flanks, in broad agreement with the ICP-MS data. This spatial distribution provides further evidence of plume variability as the main control on the concentrations of magnetic particles on the leaf surfaces and on elemental uptake by the tree and suggests that biomagnetic monitoring may also hold promise as a method of assessing the dispersion and impacts of volcanic plumes. The dominant magnetic mineral on the leaf surfaces is a magnetite-like mineral (contributing >90% of the SIRM), of coarse, multi- domain (MD) grain size (∼ 5t o 15 μm). A volcanogenic source is most likely, as magnetites from anthropogenic sources are typically an order of magnitude smaller in grain size (∼0.1-1 μm). These new results from the use of environmental magnetism on leaves, in a volcanically-affected region, suggest that biomagnetic monitoring offers a new, sensitive and rapid means for the assessment of volcanic plume deposition over wide areas.

The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate

The lower Lesser Himalayan sequence marks the northern extremity of the exposed Indian plate, and is generally interpreted as a passive margin. Five lines of evidence, however, collectively suggest a continental arc setting: (1) igneous intrusions and volcanic rocks occur at this stratigraphic level across the length of the Himalaya, (2) ages of intrusive and metavolcanic (?) rocks cluster at 1780–1880 Ma but also indicate a long-lived igneous process, (3) detrital zircon ages in clastic rocks cluster at 1800–1900 Ma, with a unimodal age distribution in some rocks, (4) the mineralogy and chemistry of metasedimentary rocks differ from typical shales and suggest a volcanogenic source, (5) trace-element chemistries of orthogneisses and metabasalts are more consistent with either an arc or a collisional setting. Intercalation of volcanic rocks with clastic sediments and a general absence of Proterozoic metamorphic ages do not support a collisional origin. An arc model further underscores the profound unconformity separating lower-upper Lesser Himalayan rocks, indicating that a Paleoproterozoic arc may have formed the stratigraphic base of the northern Indian margin. This, in turn, may indicate disposition of the Indian plate adjacent to North America in the ca. 1800 Ma supercontinent Columbia. Felsic orthogneisses (“Ulleri”) likely represent shallow intrusions, not Indian basement.

A review of research on Late Cretaceous volcanic-sedimentary sequences of the Mandla Lobe: implications for Deccan volcanism and the Cretaceous/Palaeogene boundary

Regional mapping and detailed petrographic studies of the volcanic-sedimentary sequences of the Mandla Lobe, coupled with lateral tracing has enabled the recognition of 37 physically distinct lava flows, and major oxide chemistry grouped them into eight chemical types. The lava pile becomes structurally complex to the east of Jabalpur where the juxtaposition of distinct groups of lava flows is observed near the Deori (flows 1–4 abutting flows 5–14) and Dindori areas. At Dindori, distinct lava packages (flows 15–27 and 28–37) are juxtaposed along the course of Narmada River, indicating the presence of four post-Deccan faults in the Nagapahar, Kundam, Deori and Dindori areas, and arguing against the idea of a small regional dip. Major geochemical breaks, when traced from section to section, exhibit shifts in height of approximately 150 m near Nagapahar and 300 m near Deori and Dindori. These findings, when considered in conjunction with magnetic chron reversal heights, support the presence of four NE–SW-trending faults. Major and trace element abundances in 15 lava flows in the Jabalpur area are similar to those of the south-western Deccan flows. The Ambenali Formation and a few flows of the Khandala and Bushe formations are present in north-eastern Deccan. Improved stratigraphic correlation is needed to define the lateral continuity and spatial distribution of the western-defined formations in regions to the east. The dominance of smectites in the detrital assemblage of the Lameta Formation suggests their derivation from a volcanogenic source rock. IR spectra support the commonality of the mineralogical attributes of the Lameta Formation and the Deccan basalt in their mutual resemblance of absorption bands. Structural formulae indicate that smectites of the Lameta deposits are rich in octahedral Mg and Fe. The abundance of Fe together with Mg further favours derivation from the Deccan basalt as do similarities in the concentrations of immobile trace elements, REE patterns, and the negative Ce anomalies observed. These findings correlate the smectite-dominant detrital clays with the Deccan basalt, implying the availability of the latter during deposition of the Lameta Formation in the Maastrichtian. These findings do not match models suggesting an extremely short period of Deccan volcanism (<0.5 myr) at the Cretaceous/Palaeogene transition; it is congruent with the models advocating a more prolonged volcanism.

Geochemistry and petrology of a Middle Tithonian limestone-marl rhythmite in the Neuquén Basin, Argentina: depositional and burial history

Abstract The Middle Tithonian Los Catutos Member (Vaca Muerta Formation, Neuquén Basin), is lithologically and geochemically similar to limestone-marl alternations from the Late Jurassic of the northern hemisphere. Both marls and limestones are pelbiomicrites and biopelmicrites principally composed of pellets, radiolaria, forams, ostracods, equinoids, spicules of sponges and gastropods, cemented by several generations of calcite cement. Smectite and interlayers are the major epiclastic components of the fraction below 2 μm, reflecting pedogenic processes developed on volcanogenic source rocks. More abundant kaolinite in some marls reflects stronger humid conditions in the source area and enhanced terrigenous supply. Al 2 O 3 content is demonstrated to be a reliable indicator of clastic input. The same is not true for silica, often related to high biogenic productivity of siliceous organisms. Rocks show total organic carbon (TOC) contents up to 1.95% and constitute regular to good sources for hydrocarbons, although thermally immature. Rhythmites formed gently sloping mounds accumulated in a distal submarine ramp under low-energy and poorly oxygenated open-sea conditions. Sedimentation rates were high due to high productivity on the sea surface, and supply of terrigenous and carbonate sediments transported by suspension plumes originated in shallow, photic areas. δ 13 C values correspond well with the published curves for the Tithonian sea water and with other records from Tethyan limestones. A preliminary analysis of negative excursions of δ 13 C point to a productivity crisis or a mixture of water layers in a stratified sea with a periodicity of 400 ka, which could be a result of changes in the orbital eccentricity of the Earth. Light isotopic composition of O in bulk rocks is the result of diagenetic neomorphism and cement precipitation. Calculated palaeotemperatures from δ 18 C are consistent with those derived from measured vitrinite reflectance ( Ro% ) and burial history reconstruction. Data indicate initial burial during the Tithonian extending up to the Lower Cretaceous, a short period of uplift (Intravalaginian tectonic phase), and renewed uplift during the Cenomanian followed by significant Late Cretaceous sedimentation and Pliocene thrusting.

Distribution, salinity and pH dependence of elements in surface waters of the catchment areas of the Salars of Coipasa and Uyuni, Bolivian Altiplano

The variation in concentrations of a wide variety of hydrochemical parameters in surface watercourses of the Bolivian Altiplano is statistically characterised. A variation of four to five orders of magnitude is characteristic for several parameters (Al, As, B, Cd, Cl −, Cu, Fe, Li, Mn, Na, S, and SO 4 2−), reflecting (i) evapoconcentration between mountainous watersheds and saline desert evaporative sinks, and (ii) geologically varied sources such as volcanic/thermal springs, fumarolic sulphur deposits and widespread sulphide mineralization. A tendency to increasing salinity from catchment headwaters to the salars is observed. pH values are generally rather high (median 8.3), and exhibit a similar, but weaker increasing trend. Arsenic concentrations are high (median=34 μg/l) and are probably largely derived from volcanogenic sources (fumarolic native sulphur deposits and thermal springs) on the Andean Cordillera Occidental. Arsenic exhibits positive correlations with pH and salinity, indicating its mobility as an oxy-anion under the generally high pH conditions and its susceptibility to evapoconcentration. Arsenic appears to be only weakly attenuated relative to chloride, probably by adsorption onto iron hydroxide precipitates or flocs.

Large shifts in the isotopic composition of seawater sulphate across the Permo–Triassic boundary in northern Italy

Carbonate-associated sulphate (CAS) extracted from a Permo–Triassic succession at Siusi in northern Italy is shown to preserve a true seawater-sulphate isotope record. Two periods of increasing δ 34S and δ 18O in CAS provide evidence for increased oceanic anoxia in the Late Permian and the Early Triassic. These two anoxic episodes are separated by an event characterised by the addition of isotopically light sulphur and oxygen to the oceanic sulphate pool. Simple mass balance calculations suggest that this sulphate originates from the reoxidation of bacterially derived H 2S during oceanic mixing, rather than a volcanogenic source. A dramatic fall in CAS-δ 18O directly above the P– T boundary, not accompanied by a large change in CAS-δ 34S, records an oceanic deoxygenation event probably caused by the release of methane from gas hydrates, subsequently recorded in the carbonate-carbon isotope record. The decline of Early Triassic oceanic anoxia is not recorded by a fall in CAS-δ 34S, but is preserved by declining CAS-δ 18O. This is because of an increase in the flux of reactive iron to the oceans during the Early Triassic anoxic episode, triggered by the demise of land plants. This permanently removes a greater proportion of light sulphur from the oceanic sulphate reservoir as pyrite, and means that the heavy residual sulphate-sulphur isotope signature of Griesbachian anoxic seawater is preserved as a geochemical ‘fossil’ until the beginning of the Middle Triassic.

Reappraisal of the age and origin of the Casterton Formation, western Otway Basin, Victoria

The Casterton Formation is widely considered to be the basal sedimentary unit of the Otway Basin, deposited during the latest Jurassic to Early Cretaceous in half‐graben structures related to rifting between Australia and Antarctica. The unit has been intersected in a handful of onshore wells as a relatively thin (13–535 m) basal unit of the Otway Supergroup overlying Palaeozoic basement, but is interpreted from seismic data to be more widespread. Although not formally defined, the unit is currently identified on the basis of the association of a sequence of lithic sandstone with minor mudstone, conglomerate and intercalated basaltic flows in the informal type section in Casterton‐1 well. K‐Ar radiometric age determinations of a basalt flow from Casterton‐1 yield an age range of 153 ± 5 to 120 ± 10 Ma (Late Jurassic to Early Cretaceous). However, the sequence is generally taken to be more restricted in age from latest Jurassic to earliest Neocomian. Some stratigraphers separate the Casterton Formation from the overlying Otway Supergroup with a basin‐wide unconformity at the Jurassic/Cretaceous boundary. Fission track and palynological data from the Moyne Falls‐1 and Hawkesdale‐1 wells, in the Tyrendarra Embayment presented in this paper suggest that the unit, as identified, is as young as Aptian and was deposited synchronously with Otway Supergroup sediments. Apatite chlorine composition, fission track and petrographic analyses suggest provenance from a volcanogenic source similar to that inferred in the overlying Otway Supergroup and no evidence was found to support a significant erosional time break. Therefore, it is apparent that the lithological association defining the Casterton Formation was not unique in the evolution of the Otway Basin fill, and the definition of the Casterton Formation requires revision if the unit is to be a useful stratigraphic marker.

Pyrite composition and ore genesis in the Prince Lyell copper deposit, Mt Lyell mineral field, western Tasmania, Australia

The Prince Lyell copper-gold-silver deposit occurs in the late Cambrian Mt Read Volcanics, at Queenstown, Tasmania. Steeply plunging, broadly conformable lenses of disseminated and stringer pyrite-chalcopyrite mineralisation occur in quartz-sericite-chlorite rocks derived from intense alteration of predominantly felsic lavas and volcaniclastic rocks. Middle Devonian deformation has substantially modified primary sulphide textures. Although extensively fractured, pyrite grains in the ore have retained their original pre-deformation internal structure and chemistry which are revealed by etching and electron microprobe analysis. Earliest sulphide mineralisation produced oscillatory zoned, cobalt-rich pyrite (Pyrite I), coeval with chalcopyrite mineralisation. Cobalt-rich pyrite is commonly associated with Cambrian volcanic rocks in western Tasmania and suggests a volcanogenic origin for the ore fluids at Prince Lyell. Pyrite I was corroded by later hydrothermal fluids and reprecipitated as unzoned, trace element-poor pyrite (Pyrite II), commonly as overgrowths on Pyrite I cores. Minor amounts of a second cobalt-rich pyrite (Pyrite III) occurs with Pyrite II in composite pyrite overgrowths. Sulphur isotope ratios from all pyrite generations fall within a small range (3 to 11‰). In situ isotopic analyses showed no consistent δ 34S variation between the various pyrite generations, suggesting recycling of sulphur derived from a single Cambrian volcanogenic source. Hematite alteration, derived from oxidised fluids possibly from the adjacent hematitic Owen Conglomerate, occurs in the structural footwall volcanics and the Great Lyell fault zone. Hematite inclusions in Pyrite II and III indicate that these pyrite generations occurred after or during deposition of the conglomerate. It is postulated that Pyrite II and III were deposited during waning volcanism, contemporaneous with Owen Conglomerate sedimentation in the late Cambrian or early Ordovician. The Great Lyell fault would have acted as a growth fault margin between a terrestrial basin, filling rapidly from the east, and the volcanic terrane to the west. The scenario raises the possibility that the concentration of mineral deposits and hematitic alteration along the Great Lyell fault resulted from the subsurface interaction of reduced volcanogenic fluids and oxidised basin waters along the growth fault contact.