Jim D. Marshall

Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation

Stable isotopic data from marine limestones and their constituent fossils and marine cements can provide quantitative evidence for changes in global climate and ocean circulation. Oxygen isotopic data can indicate changes in temperature and ocean composition whereas stratigraphic variation in carbon isotope ratios may reflect changes in the carbon cycle that can be linked to changes in oceanic productivity and atmospheric greenhouse gases. Terrestrial carbonates–meteoric cements, calcretes and speleothems–similarly offer significant potential for understanding the evolution of terrestrial climates by providing evidence for the composition of rainwater and the nature of vegetative cover.Primary environmental isotopic signals may be obscured by the effects of post-depositional diagenetic alteration. Cementation and replacement reactions can take place in a wide range of diagenetic environments; the diagenetic history of an individual limestone is determined by a combination of its mineralogical diagenetic potential and depositional setting, together with subsequent changes in relative sea-level and burial history. Carbon isotopic values are less prone to alteration during diagenesis than oxygen values but shifts can be significant where organogenic carbon is incorporated. Linear covariation of carbon and oxygen values is not a reliable indicator of diagenetic alteration: water-rock interaction and fluid mixing may produce non-linear distributions.Attempts to determine long-term changes in climatic and oceanographie conditions through isotope stratigraphy of shallow-water limestones must include an assessment of the diagenetic history of the materials analysed. Petrographic examination using conventional microscopy backed up, where appropriate, by cathodoluminescence and scanning electron microscopy together with elemental and strontium isotopic analysis can help to identify the effects of diagenetic alteration. Where material with a range of different degrees of alteration is preserved in the same sediment it may be possible to compare patterns of isotopic and elemental variation and to attempt to unravel the effects of diagenesis in order to determine primary, environmental, isotopic signals. Recent research has shown that these techniques can be successfully employed in both Phanerozoic and Precambrian sediments.

Bathymetric and isotopic evidence for a short-lived Late Ordovician glaciation in a greenhouse period

The end Ordovician glaciation is distinct among Phanerozoic glaciations in that CO 2 , levels were generally high, yet major continental ice sheets accumulated on the Gondwana supercontinent. New oxygen isotopic data indicate substantial changes in sea-water temperatures and ice volume coinciding with glacio-eustatic changes in sea level reflecting the growth and decay of the Gondwana ice cap. Major glaciation was apparently confined to the Hirnantian and was 0.5-1 m.y. long, rather than the 35 m.y. of earlier estimates. Carbon isotope values indicate significant changes in carbon cycling as the oceans changed from a state with warm saline bottom waters to a state with cold deep-water circulation and then back again. We believe that the changes in the carbon cycle effected a reduction in P CO 2 levels in the oceans and atmosphere and thus promoted glaciation but were unable to sustain icehouse conditions in a greenhouse world.

The Cenomanian-Turonian boundary at Eastbourne (Sussex, UK) : a proposed European reference section

The Cenomanian‐Turonian boundary (CTB) section at Eastbourne, Sussex, England is the thickest at outcrop in the Anglo‐Paris Basin, is rhythmic throughout, rich in macro-, micro- and nanno-fossils, and shows less diagenetic alteration than other sections in NW Europe. Geochemical and biostratigraphic data are all related to detailed (rhythmic) lithostratigraphy, allowing correlation with the global boundary stratotype section to within 20,000 years and accurate estimates of rates of changes. Very detailed stable isotope curves for oxygen and carbon are presented and show that the build-up phase of the CTB carbon excursion had two portions separated by a trough. Patterns of occurrence of coarse-fraction benthic foraminifera (>250 mm) show two peaks of extinction corresponding to the two build-up phases of the carbon excursion. Benthic and planktic foraminifera, ostracods and calcareous nannofossils all show declines in diversity and abundance across the CTB. Foraminifera also decline significantly in size. Key biomarker events for the CTB include: first occurrences of the ammonites Fagesia catinus and Mammites nodosoides, the bivalve Mytiloides, the planktic foraminifera Helvetoglobotruncana praehelveticaand Marginotruncana renzi, and the calcareous nannofossils Quadrum intermedium , Q. gartneri and Eprolithus octopetalus; and last occurrences of the ammonite Sciponoceras, the planktic foraminifera Rotalipora greenhornensis and R. cushmani, the calcareous nannofossils Corolithion kennedyi, Axopodorhabdus albianus , Lithraphidites acutus, Rhagodiscus asperand Microstaurus chiastius. Cyclostratigraphy allows very precise correlation of isotopic and bioevents with other sections in Europe and North America. Eastbourne is recommended as a NW European standard section.

Global carbon isotopic events associated with mass extinction and glaciation in the late Ordovician

Mass extinctions and glacioeustatic sea-level changes in the lower part of the Hirnantian (final stage of the Ashgill) are accompanied by shifts in marine stable-isotope compositions. Previously published stable-isotope changes have been used to identify the onset and demise of the Gondwana glaciation and to suggest relationships between biotic changes and carbon cycling within the oceans. However, the existing isotopic data set had limitations because it was derived from Ordovician low-latitude settings and from carbonates or organic carbon in separate areas. We report new data from Ordovician high-latitude carbonates and demonstrate parallel shifts in organic and carbonate δ13C from Baltica. Brachiopod shells from a high-palaeolatitude, periglacial setting in Argentina have elevated δ13C values similar to those described previously from low-latitude sites. The new data demonstrate that the positive Hirnantian δ13C excursion, previously only recognised from low-palaeolatitude areas, was widespread and probably global in extent. The poor preservation state of the brachiopods unfortunately prevented the determination of a reliable oxygen isotopic value from the same material. Preliminary carbon isotopic data from thermally immature organic matter from Estonia provide the first indication of a synchronous shift in organic and inorganic δ13C in sediments from the same basin. This work provides new data of critical importance for constraining models of end-Ordovician palaeoceanography and climate change.

High latitude palaeotemperature variation: New data from the Thithonian to Eocene of James Ross Island, Antarctica

An oxygen stable isotope study of molluscan macrofossils from the Tithonian to the Eocene of the James Ross Island and Alexander Island areas, Antarctica, was carried out in conjunction with careful petrographic, minerological and geochemical analyses to assess the state of fossil preservation. The Alexander Island samples all showed evidence of alteration whilst samples from James Ross Island were variably preserved. The isotopic composition of those samples which met the textural and chemical criteria for well preserved primary skeletal carbonate material were then used to construct a record of high latitude marine water temperature variation. This record shows a marked cooling of palaeotemperatures from the late Jurassic to the Albian, a warming in recorded palaeotemperatures during the mid Cretaceous and a gradual cooling from the mid Cretaceous to the Eocene. The isotopic pattern parallels that from low latitude sites and suggests that climatic change was global and that relatively uniform latitudinal palaeotemperature gradients may have been maintained during a time of green-house climate. Using the James Ross Island data to calculate probable polar temperatures over the adjacent Antarctic continent shows that cold temperate or sub-polar conditions would have been established during the Albian, late Maastrichtian and Eocene.

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.

High-paleolatitude Late Cretaceous paleotemperatures: New data from James Ross Island, Antarctica

Oxygen-isotope analysis of well-preserved macrofossils from the Santonian-Campanian of James Ross Island and the Maastrichtian of Vega Island, Antarctica, indicates that cool high-paleolatitude temperatures prevailed during the Late Cretaceous and suggests that cooling occurred between the Santonian-Campanian and the Maastrichtian. Although more than 50% of the material showed diagenetic alteration, 52 unaltered aragonite and calcite samples were analyzed. Mean {delta}{sup 18}O and calculated paleotemperature values were {minus}0.23{per thousand} and 13.6 C, respectively, for the Santonian-Campanian, and 0.66{per thousand} and 11.7 C, respectively, for the Masstrichtian. In conjunction with recent Late Cretaceous paleoclimatic data from high northern paleolatitudes, these data indicate the presence of cool polar regions with broad climatic zonation during the late Cretaceous. This may have partly controlled faunal distributions.

Changes in marine isotopic composition and the late Ordovician glaciation

Well-preserved brachiopod shells and marine cements from limestone coquinas which cap carbonate mudmounds in the Siljan area of central Sweden have the heaviest stable oxygen and carbon isotope values yet reported for Lower Palaeozoic marine sediments. The coquinas formed during the eustatic regression at the onset of the late Ordovician glaciation: the isotopic compositions reflect simultaneous shifts in both carbon and oxygen (more than 5‰ δ13C and up to 2º‰ δ18O) away from more normal Lower Palaeozoic values in similar carbonates from within the mounds. Oxygen isotope data are consistent with a change in the isotopic composition of the sea water probably accompanied by a decrease in temperature. The changes in carbon values suggest enhanced deposition of organic carbon, a process which would have decreased pCO2 in the ocean and the atmosphere and thus contributed to rapid global cooling.

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.