Ian Jarvis

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

Carbon stable-isotope variation through the Cenomanian–Santonian stages is characterized using data for 1769 bulk pelagic carbonate samples collected from seven Chalk successions in England. The sections show consistent stratigraphic trends and δ13C values that provide a basis for highresolution correlation. Positive and negative δ13Cexcursions and inflection points on the isotope profiles are used to define 72 isotope events. Key markers are provided by positive δ13C excursions of up to + 2‰: the Albian/CenomanianBoundary Event; Mid-Cenomanian Event I; theCenomanian/Turonian Boundary Event; the Bridgewick, Hitch Wood and Navigation events of Late Turonian age; and the Santonian/Campanian Boundary Event. Isotope events are isochronous within a framework provided by macrofossil datum levels and bentonite horizons. An age-calibrated composite δ13C reference curve and an isotope event stratigraphy are constructed using data from the English Chalk. The isotope stratigraphy is applied to successions in Germany, France, Spain and Italy. Correlation with pelagic sections at Gubbio, central Italy, demonstrates general agreement between biostratigraphic and chemostratigraphic criteria in the Cenomanian–Turonian stages, confirming established relationships between Tethyan planktonic foraminiferal and Boreal macrofossil biozonations. Correlation of the Coniacian–Santonian stages is less clear cut: magnetostratigraphic evidence for placing the base of Chron 33r near the base of the Upper Santonian is in good agreement with the carbon-isotope correlation, but generates significant anomalies regarding the placement of the Santonian and Campanian stage boundaries with respect to Tethyan planktonic foraminiferal and nannofossil zones. Isotope stratigraphy offers a more reliable criterion for detailed correlation of Cenomanian–Santonian strata than biostratigraphy.With the addition of Campanian δ13C data from one of the English sections, a composite Cenomanian–Campanian age-calibrated reference curve is presented that can be utilized in future chemostratigraphic studies. The Cenomanian–Campanian carbon-isotope curve is remarkably similar in shape to supposedly eustatic sea-level curves: increasing δ13C values accompanying sea-level rise associated with transgression, and falling δ13C values characterizing sea-level fall and regression. The correlation between carbon isotopes and sea-level is explained by variations in epicontinental sea area affecting organic-matter burial fluxes: increasing shallow sea-floor area and increased accommodation space accompanying sea-level rise allowedmore efficient burial ofmarine organic matter, with the preferential removal of 12C from the marine carbon reservoir. During sea-level fall, reduced seafloor area, marine erosion of previously deposited sediments, and exposure of basin margins led to reduced organiccarbon burial fluxes and oxidation of previously deposited organic matter, causing falling δ13C values. Additionally, drowning of carbonate platforms during periods of rapid sea-level rise may have reduced the global inorganic relative to the organic carbon flux, further enhancing δ13C values, while renewed platform growth during late transgressions and highstands prompted increased carbonate deposition. Variations in nutrient supply, changing rates of oceanic turnover, and the sequestration or liberation of methane from gas hydrates may also have played a role in controlling carbon-isotope ratios.

Microfossil Assemblages and the Cenomanian-Turonian (late Cretaceous) Oceanic Anoxic Event

The effects of the Cenomanian-Turonian Oceanic Anoxic Event (OAE) in the Chalk Sea of NW Europe have been investigated using published macrofossil records combined with new detailed sedimentological, foraminiferal, ostracod, calcareous nannofossil, dinoflagellate cyst and stable-isotope data from Dover, England. The ranges of individual fossil species are displayed against lithostratigraphic logs, and their relation to the Cenomanian-Turonian boundary (defined using macrofaunal data) is discussed. A positive carbon stable-isotope excursion, indicating the stratigraphic extent of the OAE, spans the stage boundary. Correlation with successions elsewhere in NW Europe suggests that the OAE was isochronous, and that major biostratigraphic marker horizons are characterised by distinctive δ13C values. All microfossil groups display uppermost Cenomanian abundance and diversity minima which correspond closely to the peak of the carbon stable-isotope excursion. We propose that the OAE was a phase of increased upwelling which led to a widespread expansion and intensification of the oxygen-minimum zone in the oceans. As a result, increasingly dysaerobic bottom waters developed within the Chalk Sea, and were responsible for progressive disappearances in the benthonic microfauna, including the extinctions of many typical Cenomanian taxa. At the same time the oxygen-minimum zone rose in the water column, causing the extinction of deeper water planktonic foraminifera and then the gradual loss of intermediate-water groups. A temporary disappearance of dinoflagellate cysts and a proliferation of calcispheres were associated with these events. As the OAE waned, new species gradually evolved to fill niches left vacant following the extinctions of Cenomanian taxa. The appearance of these new species defines the base of the Turonian, the stage division being a direct consequence of the OAE. It is concluded the OAEs provide a major mechanism for controlling rates of evolution and extinction throughout the Phanerozoic.

An assessment of dissolution techniques for the analysis of geological samples by plasma spectrometry

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) are being used increasingly for the analysis of a wide range of geological materials. The rapidity of these multi-element techniques results in sample dissolution being the limiting step in sample through-put. This study critically evaluates results obtained from two routinely used sample preparation methods: LiBO2 fusion and open-vessel HF-HClO4 digestion, and compares them with data obtained from microwave-heated, sealed-vessel, acid digestion. Detailed procedures are given for each method. Nine standard reference materials (SRM) were used in the comparison. Selected major and trace elements were determined by ICP-AES to assess the general effectiveness of each dissolution procedure, and 24 trace elements plus the 14 rare-earth elements (REE) were determined by ICP-MS. Trace elements determined by both methods show good agreement. The accuracy and precision of the results were dependent on the dissolution technique and the mineralogy/composition of the material. LiBO2 fusion resulted in complete recovery of major and many minor elements including Cr, Hf and Zr, but the volatile elements Pb, Sb, Sn and Zn were lost. The open acid and the microwave digestions produced similar results and proved suitable for the determination of most elements in most materials. Microwave digestion has the advantage of shorter digestion times and uses smaller volumes of reagents. The accuracy of Zr and Hf data was dependent on the sample type. Cr showed a low recovery by open acid attack and this was only partially improved by microwave digestion. Results indicate that a combination of sample preparation techniques is required if quantitative data are sought for the full range of elements studied here.

Late Cretaceous (Campanian) carbon isotope events, sea-level change and correlation of the Tethyan and Boreal realms

Abstract Carbon stable-isotope stratigraphy provides unique insights into environmental change during the Campanian, the longest (83.5–71.3 Ma), but least well-understood stage of the Late Cretaceous. A new carbon isotope (δ13C) profile for a 500-m-thick Campanian–basal Maastrichtian Tethyan pelagic–hemipelagic section near El Kef, northern Tunisia, is calibrated using data from a biostratigraphically well-constrained succession at Kalaat Senan. The general shapes of the Tunisian δ13C reference curve and published Tethyan δ13C profiles from Elles (Tunisia) and Bidart (SW France), and a Boreal curve for the Trunch borehole (eastern England), are remarkably similar in all three areas. A positive carbon isotope event of +0.2‰ δ13C in the mid-Campanian dated at 78.7 Ma and a negative excursion of −0.4‰ in the upper Campanian at 74.8 Ma can be correlated between Tunisia and England. A positive excursion of +0.3‰ at 83.7 Ma spans the Santonian–Campanian boundary. These isotope events enable precise inter-regional correlations that are consistent with published nannofossil data. Review of Campanian sea-level data from North Africa, the Middle East and northern Europe indicates that major shifts in δ13C profiles coincide with changes in eustatic sea-level. Relatively stable δ13C values in the lower Campanian and their long-term fall through the upper Campanian reflect high and then falling eustatic sea-levels, and increased carbonate production. Short-term (∼600 kyr) positive excursions record greater organic productivity and/or organic matter preservation, and decreased carbonate fluxes during periods of rapid sea-level rise and the drowning of carbonate platforms. Excursions were terminated by falling nutrient supply and increased carbonate deposition associated with epicontinental sea expansion and renewed carbonate platform growth during the late transgression and highstand. Negative excursions are linked principally to reworking of marine and terrestrial organic matter during rapid sea-level fall. Carbon isotope stratigraphy is a powerful tool for correlation which can be used to test the validity of Campanian global biostratigraphic frameworks, and improve our understanding of the nature and timing of Late Cretaceous sea-level change.

Geochemistry of pelagic and hemipelagic carbonates: criteria for identifying systems tracts and sea-level change

The elemental (Si, Ti, Al, Mn, Ca, Zr) and carbon stable-isotope (δ13C) geochemistry of a biostratigraphically well-constrained Cenomanian–Turonian (Upper Cretaceous) Chalk succession on the Isle of Wight, southern England, shows systematic variation that corresponds closely to a published sequence stratigraphic model for the Cenomanian. Six sequences and their constituent systems tracts, defined elsewhere using sedimentological criteria, are clearly distinguishable from bulk-sediment elemental profiles, and an additional Upper Cenomanian sequence previously identified in Spain is recognized in England from these geochemical data. The manganese curve is particularly instructive, exhibiting minima around sequence boundaries and through lowstands, rising values from the transgressive surfaces through transgressive systems tracts, maxima around maximum flooding surfaces, and declining values through highstands. Silica and trace-element (Ti, Zr) aluminium ratios peak around transgressive surfaces and maximum flooding surfaces, indicating pulses of increased siliciclastic input. Positive δ13C excursions are confirmed at the base of the Middle Cenomanian and spanning the Cenomanian–Turonian boundary but are not evident in other sequences. Variation in Mn is related to bulk sedimentation rate and detrital versus biogenic supply, which control the Mn flux and the efficiency of the diagenetic Mn ‘pump’ that leads to elevated Mn contents in sediments. Manganese peaks do not generally correlate with positive δ13C excursions, and although near-coincident Mn and δ13C peaks occur around the Cenomanian–Turonian boundary, the former is not necessarily linked to the oceanic anoxic event occurring at that time. The global oceanic Mn flux may have been enhanced during the Cenomanian as a result of hydrothermal activity during rapid sea-floor spreading and oceanic plateau formation. Elemental chemostratigraphy provides a new tool for developing sequence stratigraphic models in pelagic and hemipelagic carbonate successions.

Assessment of Dowex 1-X8-based Anion-exchange Procedures for the Separation and Determination of Ruthenium, Rhodium, Palladium, Iridium, Platinum and Gold in Geological Samples by Inductively Coupled Plasma Mass Spectrometry

Synthetic multielement solutions of the platinum group metals (PGE: Ru; Rh; Pd; Ir; Pt) and gold, with analysis by ICP-AES and ICP-MS, have been used to study the behaviour of the precious metals on Dowex 1-X8 resin. Simple solutions of precious-metal chlorocomplexes showed near-complete adsorption (>99%) of most elements, and only minor breakthrough of Ru and Ru (≈5%). Solutions pre-treated with acid mixtures typically used to decompose geological samples, demonstrated that perchloric acid adversely affects the adsorption of the PGEs on the resin. Solutions treated with HF–HNO 3 –HCl maintained good retention of Ir, Pt, Au (>99%), Pd (>94%) and Ru (>90%), but displayed significant loss (up to 40%) of Rh. A two-step procedure was necessary to elute the precious metals from the resin: 0.3 mol l - 1 thiourea prepared in 0.1 mol - 1 HCl removed Ru, Pd, Pt, Au, and some Rh: 12 mol l - 1 HCl eluted remaining Rh and all Ir. Recoveries ranged from 50 to 100%. At low levels, the determination of PGE and Au in the thiourea fraction by ICP-MS was compromised by high levels of total dissolved solids (TDS), which necessitated dilution of the eluate prior to analysis. The TDS was reduced by decomposing thiourea with HNO 3 and removing SO 4 2 - by precipitation of BaSO 4 , but this led to lower and more erratic results, and increased contamination. An assessment of the optimised procedure employing geological reference materials PTM-1, PTC-1 and SARM7, indicated that acceptable results should be attainable for ICP-MS determination of most elements in geological samples containing high concentrations (>1 µg g - 1 ) of the PGE, for which decomposition of thiourea is unneccessary. The addition of a decomposition step led to low recovery of all elements except Ir, which was present entirely in the HCl eluate. The method is viable for the determination of Ir in a range of geological materials, but modifications will be required if it is to be extended to the other precious metals.

Plasma spectrometry in the earth sciences: techniques, applications and future trends☆

Abstract Plasma spectrometry is one of the most popular and versatile techniques for the analysis of geological and environmental samples, including rocks and minerals, waters, dust, vegetation, soils, sewage sludges and sediments. Inductively coupled or direct current argon plasmas are used as emission sources in ICP-and DCP-atomic emission spectrometry (ICP-AES, DCP-AES); an ICP provides an ion sources in ICP-mass spectrometry (ICP-MS). Reviews of the two plasma sources, sample introduction systems, and the instrumental and analytical performances of emission and mass spectrometers, demonstrates the superiority of higher-temperature, ICP-based systems. ICP-AES and ICP-MS are characterised by wide linear responses of more than five orders of magnitude. They are rapid and highly cost-effective multi-element techniques which can theoretically determine over 70 elements in

Determination of the platinum-group elements in geological materials by ICP-MS using microwave digestion, alkali fusion and cation-exchange chromatography

The available instrumental methods for platinum group element (PGE: Ru, Rh, Pd, Os, Ir, Pt) and gold (Au) determinations are reviewed. Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) and ICP-mass spectrometry (ICP-MS) enable rapid, multi-elemental analysis, their instrumental and analytical characteristics being discussed here. The suitability of ICP techniques to quantitatively determine the PGEs + Au is demonstrated. The detection limits by ICP-AES range from 6 to 29 ng.mL[sup]-1, while those for ICP-MS range from 0.3 to 0.22 ng.mL[sup]-1, for the individual PGEs + Au. A digestion step is generally required prior to the analysis of geological materials by ICP-AES and/or ICP-MS. Digestion procedures are reviewed, with particular attention to the new method of microwave digestion. A comparative study of three digestion methods was undertaken, a range of well-characterised rock reference materials being used to evaluate open acid digestion, microwave acid digestion and alkali fusion procedures. The precision and accuracy of results obtained by ICP-AES and ICP-MS for 46 elements demonstrates that no single digestion method is universally applicable. It is concluded that the best digestion procedure for PGE-bearing materials is a combination of microwave acid digestion followed by a fusion of the residue. Such a method was developed and its suitability is demonstrated using reference materials containing high levels of the PGEs + Au, which enable their direct determination by solution ICP-MS. Even with the superior sensitivity of ICP-MS, low concentrations of the PGEs + Au in most geological materials preclude the quantitation of unseparated samples. Separation methods which have been used (fire assay, coprecipitation, ion exchange, solvent extraction, distillation) are reviewed. Two ion-exchange methods were developed to separate the PGEs + Au from their associated matrix elements allowing their preconcentration prior to analysis. An anion-exchange method can be used in conjunction with ICP-MS for the separation and determination of ≥1 ng.g[sup]-1 Ir and> 1 [mu]g.g[sup]-1 Ru, Rh, Pd, Pt or Au. A cation-exchange ICP-MS procedure can be used to determine the PGEs at a wider range of concentrations. An evaluation using all the available PGE reference materials showed good agreement with reference values in most instances. Stable isotopes were used to evaluate this method and the results confirm that quantitative results may be obtained. The cation-exchange procedure can be scaled-up to larger samples thus enabling the determination of < 1 ng.mL[sup]-1 of individual PGEs. This is demonstrated using 5 g sub-samples and guidelines are given for further increases in sample size. Slurry nebulisation ICP-MS was developed for the determination of the PGEs + Au in solid samples without a prior digestion stage. An assessment of the method using reference materials demonstrated that quantitative results may be obtained for all seven PGEs + Au at levels above 50 to 200 ng.g[sup]-1 (depending on the element). This method is ideally suited to the routine analysis of mineralised samples or where only small sample sizes are available.

Mobility and immobility of redox-sensitive elements in deep-sea turbidites during shallow burial

During the first few tens of thousands of years after an organic-rich turbidite is emplaced on an abyssal plain, surficial oxidation of the unit occurs by downwards diffusion of bottom water O2. From work on piston cores, it is known that a suite of redox-sensitive elements form characteristic relocation peaks in such units around the deepest reaches (upper few dm) achieved by oxidation (fossil oxic/post-oxic boundaries). Four individual turbidites now buried at depths of 130-230 m below seafloor (mbsf), obtained by ODP Leg 157 drilling on the Madeira Abyssal Plain (MAP), have been investigated to determine the effects of burial diagenesis on these geochemical signals over 3-7 My. Porewater data indicate that post-oxic conditions persist to around 130 mbsf on the MAP, below which sulphate reduction predominates. All four units have retained high total organic carbon (Corg) contents (∼1.4%) in their lower reaches, despite the fact that bottom waters have previously oxidised this same organic matter in the turbidite tops to ∼0.2%. The apparent inertness of the turbidite Corg contrasts with the reactivity of organic matter observed in hemipelagic sediment from other ODP investigations. In all four cases, low initial CaCO3 contents have been quantitatively removed along with Corg oxidation, with additional dissolution of carbonate by undersaturated bottom waters. Diagenetic fractions of the elements Se, Cd, Sb, Tl, and V are present in highly-localised peaks and show no evidence of migration. In different combinations, the elements Cu, Co, Ni, and Zn appear to have migrated over short (<5 cm) distances in thin (mm) dark-coloured sulphide bands, probably sourced by reduction of a Mn-oxyhydroxide host phase during early (70 ky) burial. Two examples of local diagenetic pyritisation (up to 20 wt% FeS2) are observed in the bodies of the two turbidites buried to ∼130 mbsf, confirming that sulphate reduction is active at this level. This diagenetic pyrite is associated with high concentrations of As, and lesser enrichments of Ni, Zn, Co, Se, Sb, and possibly Mo.

Rare earth element geochemistry of massive sulfides-sulfates and gossans on the Southern Explorer Ridge

Massive sulfide-sulfate deposits on the Southern Explorer Ridge were analyzed for 14 rare earth elements (REE) by a modified inductively coupled plasma-mass spectrometric technique that included a correction for high Ba content. Bulk samples of finely intermixed sulfides, sulfate, and amorphous silica contain ΣREE concentrations of ≤6 ppm. REE patterns range from (1) strongly enriched in light REE with positive Eu anomalies, to (2) relatively flat with positive Eu anomalies and slightly negative Ce anomalies, to (3) slightly enriched in light REE with moderately negative Ce anomalies. Pattern 1 is similar to that of 300-350 °C solutions discharging at vents on the East Pacific Rise and the Mid-Atlantic Ridge, whereas pattern 3 resembles REE distributions in normal oceanic bottom waters. The sulfide-sulfate patterns are interpreted to result from variable mixtures of hydrothermal and normal seawater. Barite in gossans capping the mounds has an REE pattern almost identical to patterns of high-temperature vent solutions. Hydrothermal barite has lower REE contents and a different REE pattern relative to hydrogenous barite formed slowly on the sea floor.