Stephen F. Crowley

High-Resolution carbon isotope stratigraphy of the basal Silurian Stratotype (Dob's Linn, Scotland) and its global correlation

Since its designation as the Global Stratotype Section and Point (GSSP) for the base of the Silurian System, the choice of Dobs Linn, Southern Scotland, has received criticism due to the difficulties of relating its well-constrained graptolite biostratigraphy to shallow-water sequences elsewhere. Kerogen samples from across the Ordovician-Silurian boundary interval at Dobs Linn have yielded carbon stable-isotope signatures consistent with those recorded elsewhere, in particular showing a clear positive δ13C excursion in the terminal Ordovician. The architecture of the δ 13C curve from Dobs Linn enables very high-resolution stratigraphic subdivision and direct correlation between the deep water Dobs Linn section and time-equivalent carbonate shelf deposits. An integrated stratigraphic scheme using isotope stratigraphy and biostratigraphy of graptolites, conodonts and shelly faunas has been constructed. This direct correlation shows that the shallow water successions, including the former stratotype candidate at Anticosti Island, are generally incomplete, with hiatuses related to the rapid sea-level changes during the Hirnantian stage. This confirms and greatly increases the global utility of Dobs Linn as a boundary stratotype.

Terrestrial impact of abrupt changes in the North Atlantic thermohaline circulation: Early Holocene, UK

Abrupt cooling events are features of Holocene climate and may recur in the future. We use lake records from Hawes Water, NW England, to quantify the impact of two prominent early Holocene climatic events. Subdecadal oxygen isotope records from sedimentary carbonate (18δOc), dated using thermal ionization mass spectrometry (TIMS) U-series analyses, provide evidence for abrupt cold events, lasting ∼50 and ∼150 yr at 9350 and 8380 yr ago, which correlate with the 9.3 ka and 8.2 ka events recognized in Greenland ice cores. At Hawes Water, mean July air temperatures, inferred from chirono-mid assemblages, decreased by ∼1.6 °C during each event. Calculations show that the isotopic excursions were dominantly caused by decreases in the isotopic composition of meteoric precipitation (18δOp) by ∼1.3‰; this is interpreted as a direct downstream response to cooling and freshening of northeast Atlantic surface water by melting ice sheets. Intermediate in magnitude between events observed in Greenland and central Europe, the effects are consistent with a partial shutdown of the North Atlantic thermohaline circulation.

A high resolution Late-Glacial isotopic record from Hawes Water, Northwest England Climatic oscillations: calibration and comparison of palaeotemperature proxies

Understanding of the impact of past climate change on the terrestrial environment requires the development of quantitative palaeotemperature indicators that can be readily measured at closely spaced stratigraphic intervals. A high resolution oxygen isotopic record from Hawes Water, Lancashire, UK, reveals a degree of climatic instability that is greater than that recognised in previous isotopic studies of sites from the Atlantic margin of Northwest Europe. Four oscillations in the Late-Glacial interstadial before the onset of the Younger Dryas stadial are marked by sharp negative excursions in the δ18O record of the carbonate sediments and accompanied by marked changes in chironomid populations and the pollen record. The oscillations are interpreted as cold events: they correlate with events recognised in marine and lacustrine temperature proxy records as well as the isotopic record from the Greenland ice-cores. Results suggest that there has been a significant change in lake water composition since the Late-Glacial but it is argued that the Hawes Water isotopic record, constrained by the results of a study of carbonate precipitation in the modern lake, provides a quantitative record of Late-Glacial temperature change. The 7°C maximum temperature difference inferred from the isotopic data, from the Interstadial optimum to the Younger Dryas stadial, and the magnitude of individual oscillations, compare well with estimates from terrestrial (coleopterid) and marine (diatom sea surface temperature) records: all are significantly greater than the, independently calibrated, 4.2°C change documented from a chironomid record from South Scotland. Such differences need to be resolved if the impacts of climatic events are to be fully understood.

ISOTOPE DATA FROM CRETACEOUS CHALKS AND FORAMINIFERA: ENVIRONMENTAL OR DIAGENETIC SIGNALS?

Scanning electron microscopy of sediments, microfossils, and polished thin sections from the Cenomanian-Turonian boundary section at Dover, England, demonstrates that cement is concentrated within large pores such as foraminifera and calcispheres, whereas the surrounding coccolith-rich sediments are largely lacking in cement. Isotope analysis of single microfossils and cement samples indicates the presence of mixing lines between primary (paleoceanographic) compositions and cements. Consequently, paleoceanographic and stratigraphic interpretations of these sediments, and perhaps other similar sediments based on similar data, may be suspect.

Fish as major carbonate mud producers and missing components of the tropical carbonate factory

Carbonate mud is a major constituent of recent marine carbonate sediments and of ancient limestones, which contain unique records of changes in ocean chemistry and climate shifts in the geological past. However, the origin of carbonate mud is controversial and often problematic to resolve. Here we show that tropical marine fish produce and excrete various forms of precipitated (nonskeletal) calcium carbonate from their guts (“low” and “high” Mg-calcite and aragonite), but that very fine-grained (mostly < 2 μm) high Mg-calcite crystallites (i.e., MgCO3) are their dominant excretory product. Crystallites from fish are morphologically diverse and species-specific, but all are unique relative to previously known biogenic and abiotic sources of carbonate within open marine systems. Using site specific fish biomass and carbonate excretion rate data we estimate that fish produce ∼6.1 × 106 kg CaCO3/year across the Bahamian archipelago, all as mud-grade (the < 63 μm fraction) carbonate and thus as a potential sediment constituent. Estimated contributions from fish to total carbonate mud production average ∼14% overall, and exceed 70% in specific habitats. Critically, we also document the widespread presence of these distinctive fish-derived carbonates in the finest sediment fractions from all habitat types in the Bahamas, demonstrating that these carbonates have direct relevance to contemporary carbonate sediment budgets. Fish thus represent a hitherto unrecognized but significant source of fine-grained carbonate sediment, the discovery of which has direct application to the conceptual ideas of how marine carbonate factories function both today and in the past.

Model for the evolution of oxygen isotope ratios in the pore fluids of mudrocks during burial

Abstract The determination of oxygen isotopic ratios in authigenic silicates and pore fluids provides vital information on the nature of water-rock interactions in the subsurface and the origin of porosity-occluding phases in reservoir rocks. The δ 18 O values of pore fluids generally increase during burial through water-rock interaction. The transformation of detrital smectite to illite is thought to have a major influence on the isotopic evolution of pore fluids, but previous attempts to model this effect have failed to accurately reproduce the observed water-rock isotopic trends with depth. A revised model has been developed which incorporates an empirical rate equation for the illitization reaction. The input variables include the initial mineral and pore water isotopic compositions, time-porosity-depth functions and geothermal histories, and the assumption is made that the isotopic equilibration of the solid phases with pore waters only takes place during reaction. The model is tested against the most complete data set available from the Texas Gulf Coast (USA) and produces a good fit with the available pore fluid and mineral data. The sensitivity of the model is tested for variations in the following parameters: the illite-water fractionation factor; the fractionation of oxygen isotopes between the smectitic and illitic components of the detrital illite-smectite; the oxygen isotope fractionation between pore waters and the interlayer water within the smectite; and the porosity-depth function. The results of the model are shown to be strongly dependent on the porosity-depth function. The best fit between the model and real data is provided by the Dickinson equation for undercompacted shale modified to have a constant porosity below 2600 m to simulate the effects of overpressuring. The model is also sensitive to the degree of oxygen isotopic fractionation between the illite and smectite within the sediment at the time of burial, but not to the extent of fractionation between interlayer water within smectite and the adjacent pore waters. The uncertainty within the illite-water fractionation factor has only a minor effect on the results of the model. Although only incorporating a single mechanism for water-rock interaction, the model provides a tool for testing the interpretations of subsurface pore fluid evolution trends during sediment diagenesis.

Stable isotope composition of modern bryozoan skeletal carbonate from the Otago Shelf, New Zealand

Abstract The oxygen (δ18O) and carbon (δ13C) isotope ratios of 10 species of living Bryozoa collected from the Otago Shelf, New Zealand were analysed to assess the extent to which isotopic equilibrium (relative to inorganic equilibrium isotope fractionation) is attained during the precipitation of skeletal calcium carbonate. The data reveal that whereas eight species of Bryozoa synthesise skeletal carbonate in apparent oxygen isotope equilibrium with respect to environmental conditions, two species (Celleporina grandis and Hippomonavella flexuosa) yield δ18Ocalcite values which indicate significant disequilibrium oxygen isotope fractionation during calcification. Sufficient data are available from one species (C. grandis) to demonstrate that disequilibrium is probably related to kinetic factors associated with diffusion‐controlled transport of HCO3‐ to the site of calcite precipitation. Carbon isotope signatures indicate significant departures from inorganic isotope equilibrium in all but one bryozoan species (Hippomenella vellicata). Although greater uncertainties are associated with estimates of the isotopic composition of total dissolved inorganic carbon (δ13CSDIC), the data suggest that two factors—kinetic fractionation and incorporation of respiratory CO2—are important in controlling carbon isotope disequilibrium. Where bryozoan species exhibit evidence for disequilibrium in both oxygen and carbon isotope systems (C. grandis, H. flexuosa), it is likely that kinetic factors are primarily responsible for observed departures from carbon isotope equilibrium. In contrast, the probable explanation for those species which display evidence for carbon isotope disequilibrium only, is that skeletal carbonate is precipitated from a DIC pool modified by the incorporation of respiratory CO2. Differences between the carbon isotope composition of skeletal elements from the same species and co‐existing species living in the same community suggests that significant variations may occur in the extent to which marine DIC and respiratory CO2 are utilised during calcification. Additional studies of carbon pathways associated with calcification are required to assess the relative effects of kinetic, metabolic, and environmental factors on the carbon isotopic composition of bryozoan skeletal carbonate.

Wave-dominated nearshore sedimentation and ‘forced’ regression: post-abandonment facies, Great Limestone Cyclothem, Stainmore, UK

Investigation of sedimentary successions and depositional structures from the post-abandonment phase of the Great Limestone Cyclothem (Upper Carboniferous), Stainmore, UK suggests that deposition occurred in a sand-dominated coastal setting. Recognition of sedimentary structures produced by storms and interpretation of palaeocurrent and palaeowave data indicate that a W—E oriented, southwardly prograding, high-energy shoreline system existed in the Stainmore region. Two equally plausible interpretations suggest that the trough cross-stratified facies lying towards the top of the shoreface succession may either represent: (1) upper shoreface deposits formed by high energy fairweather waves, or (2) the deposits of a tidally influenced deltaic distributary or incised valley fill estuarine system. These interpretations contrast significantly with the N-S oriented, transgressive barrier island model proposed in an earlier study. The sharp contact between lower shoreface and underlying offshore facies suggests that progradation of the shoreline occurred during a relative sea-level fall (‘forced’ regression). Palaeocurrent evidence indicates that deposition occurred within a system in which sediment was periodically flushed from a contemporary delta lobe and subsequently transported to the Stainmore shoreline by longshore currents.

δ34S of Lower Carboniferous anhydrite, Cumbria and its implications for barite mineralization in the northern Pennines

Recent hydrocarbon exploration in the Solway Basin has revealed the existence of Lower Carboniferous paralic sediments which contain numerous discrete anhydrite beds. Anhydrite S isotope ratios from the Easton-1 Well are compared with new isotopic data from barite mineralization that occurs along the faulted southern margin of the Northumberland-Solway Basin and published data from the Northern Pennine orefield. Similarities between the δ34S of anhydrite and barites from major mines at Haydon Bridge and the Alston district of the Northern Pennine orefield supports a Lower Carboniferous marine sulphate source for mineralization at these sites. Identification of extensive evaporite deposits hi the Solway Basin resolves several longstanding problems associated with mineralization hi the Northern Pennines including the source of barite sulphate and the origin of hypersaline mineralizing fluids. In an earlier investigation of the S isotope geochemistry of Carboniferous-hosted barite mineralization in and around the Alston district of theNothern Pennine orefield (NPO, Fig. 1a), Solomon et al. 1971 drew attention to the similarity between the δ34S of Lower Carboniferous marine sulphate and the sulphur isotopic composition of barite deposits. A model for barite mineralization was proposed in which deep, circulating, BaCl2 brines mixed with shallow (< 1 km depth), sulphate-rich, connate fluids composed of Lower Carboniferous sea water. Although the model proposed by Solomon et al. 1971 is consistent with aspects of barite mineralization, more recent studies of the evolution of pore fluid geochemistry in sedimentary basins (Land 1987; Hanor 1994a, b) argue against a marine connate origin for the sulphate component

Mineralogy and geochemistry of Bay of Bengal deep-sea fan sediments, ODP leg 116: evidence for an Indian subcontinent contribution to distal fan sedimentation

Abstract Sediments recovered during Ocean Drilling Program Leg 116 (Bay of Bengal deep-sea fan) fall into three mineralogically and geochemically distinct groups. The first of these groups (Group I), characterized by quartz-mica-rich turbidites, constitutes the largest proportion of distal fan sediments. Trace element patterns are similar to modern River Ganges suspended sediment and are consistent with a Himalayan (meta)sedimentary/granitic source. The second group of sediments (Group II) is represented by organic carbon-rich, smectite-kaolinite turbidites. Trace element data reveal significant enrichment in compatible and ferromagnesian elements consistent with a significant contribution from a basaltic crustal source. Although mixing of a basaltic source with granitic crust can account for specific geochemical relationships, mixing of these components cannot account for observed rare earth element (REE) patterns. REE data are best explained by mixing of basaltic detritus (e.g. Deccan Trap basalts of central India) with Precambrian tonalitic crust of the Indian subcontinent. A third group of carbonate-rich sediments (Group III), containing a low-latitude marine fauna, is characterized by a clastic component similar in composition to smectite-kaolinite turbidites. Although carbonate-rich sediments are superficially similar to Group II turbidites, geochemical data indicate a reduced contribution from basaltic crustal sources compared with smectite-kaolinite turbidites. A possible southern India/Sri Lankan provenance has been assigned to Group III turbidites on the basis of faunal content and geochemical composition, although insufficient information exists to substantiate this using geochemical data alone. The distribution of lithofacies and provenance-sensitive geochemical signatures suggests that the relative contribution of Himalayan and Indian subcontinent sources to distal fan sedimentation varied with time. Controls on sediment supply include variations in uplift, weathering and erosion rates, eustatic sea-level changes, and switching of major distributary fan channels. Although a degree of correlation is observed between the occurrence of coarser-grained Himalayan-derived turbidites and relative low-stands, the relationship is patchy. Assuming that the rate of sediment supply from the Himalayas to the Bay of Bengal remained relatively constant, the most likely controls on distal fan sedimentation are thought to be related to an interplay between sea-level change and channel switching. As a consequence attempts to reconstruct major Himalayan tectonic and climatic events based on data obtained from a record of distal fan sedimentation may be unreliable due to the discontinuous nature of Himalayan sediment supply to distal fan sites.