Alastair Ruffell

Climatic and biotic change in the late Triassic

Generally arid conditions that pervaded much of Europe and North America during the late Triassic were interrupted by a wet monsoonal climatic phase during Middle and Late Carnian times. Extensive fluviatile sandstones deposited at this time throughout the region, occur within a thick sequence of playa-lake mudstones. The sandstones occasionally contain kaolinite, suggesting a humid climate. An extreme δ13C depletion in a shallow marine sequence of this age in Israel has been interpreted as evidence for an influx of freshwater. A widespread change from carbonates to clastics in marine sequences at this time may also be climate-related. Water-course cave systems in limestone areas exposed during the late Triassic indicate high levels of runoff during the Middle and Late Carnian. The marine invertebrate fauna shows a significant turnover at the end of the Early Carnian. The terrestrial fauna and flora were relatively unaffected at this time but subsequently diversified prior to a major biotic turnover at the Carnian-Norian boundary. These periods of biotic change appear to be synchronous with the onset and cessation of a Carnian humid phase. The change to a monsoonal climate during this interval has been documented over more than 90° of longitude between 5° and 50° north of the Triassic equator. It may have been caused by rising atmospheric CO2 levels due to volcanism associated with the incipient dispersal of Pangaea.

Palaeoclimatic significance of changes in clay mineralogy across the Jurassic-Cretaceous boundary in England and France

Abstract Clay mineralogical changes across the Jurassic-Cretaceous boundary from the northern to the southern part of western Europe show similar patterns in: the mudrocks exposed on the Yorkshire Coast, the paralic sediments of the Dorset Wessex Basin, and the Jura and Vocontian Trough of S.E. France, This comprises a change from smectite-poor sediments in the Jurassic (with correspondingly abundant illite and kaolinite) to sediments with abundant smectite in the Jurassic-Cretaceous boundary beds, depleted in kaolinite. This change occurs in the Oxfordian in the south, to? latest Volgian in the north of the study area. Kaolinite reappears in abundance in the late Ryazanian or stratigraphic equivalent in all the locations studied. Comparison with other climatic indicators and other areas suggest that these variations can be ascribed to a phase of latest Jurassic-earliest Cretaceous aridity, the onset of which appears to be diachronous across W. Europe. Possible causes for the climatic change towards aridity are examined: the orographic effect of mountains or plateaus to the east of the study area is considered most likely.

Purbeck–Wealden (early Cretaceous) climates

A multidisciplinary colligation including new data and analysis of the evidence for the climates of southern Britain during c. 140 Ma. to c. 120 Ma BP (Berriasian-Barremian — ? earliest Aptian). The climate was at first hot, semi-arid and ‘Mediterranean’ (rather than ‘monsoonal’) in type, probably with seasonally opposed winds (E/W). An irregular long-term trend of increasing rainfall in the moister seasons is evident. This was probably associated with establishment of predominant westerlies during the Jurassic-Cretaceous transition and slightly lower average annual temperatures thereafter until Barremian times. Causes proposed are frequent changes in the regional climatic system due to technically induced adjustments of relief under the special conditions of the semi-enclosed Purbeck–Wealden archipelago and increasing proximity of the widening Protoatlantic sea.

Isotopic evidence for Late Jurassic^Early Cretaceous climate change

Abstract Strontium-, oxygen- and carbon-isotope ratios have been determined from Late Jurassic–Early Cretaceous belemnites from the Volga Basin, Russia, and Kawhia Harbour, New Zealand. 87Sr/86Sr ratios derived from well-preserved belemnites from the Volga Basin support a Middle Tithonian age derived from the analysis of the endemic ammonite fauna. The Kawhia Harbour section records a gradual rise in 87Sr/86Sr values and in comparison with the published 87Sr/86Sr curve suggests that the lower part of the section is latest Oxfordian in age, whilst the upper part of the section correlates well with the biostratigraphic correlation suggestion of an Early–Middle Tithonian age. Although the published strontium calibration curve shows a degree of scatter, our new data confirm the uniform rise in 87Sr/86Sr values from the Late Jurassic into the Early Cretaceous. Such an increase may result from either a decrease in mid-oceanic ridge spreading and/or an increase in weathering rates and flux of radiogenic strontium, although a eustatic drop in sea level and concurrent Western Cordillera uplift suggests that weathering may have been the controlling factor of Late Jurassic seawater strontium-isotope composition. Palaeotemperatures derived from the well-preserved belemnite δ18Ocarb values from the Volga Basin indicate that the Middle Volgian (Late Kimmeridgian) was warm (∼14–20°C), followed by a slight cooling and a subsequent gradual increase to the Jurassic–Cretaceous boundary. The δ18Ocarb values from New Zealand (located at a palaeolatitude of ∼80°S), if interpreted in terms of palaeotemperature, indicate a high degree of variability. Such variability may not be related to palaeotemperature, but to changes in oceanic chemistry resulting from the formation and dissolution of an ice-sheet and/or snow during the Oxfordian–Kimmeridgian. Carbon-isotope trends for the Late Jurassic show a fall in values from the Oxfordian with lowest values occurring in the Early–Middle Tithonian, before rising but without reaching values obtained in the Oxfordian. The overall low δ13Ccarb may be related to a global increase in continental weathering and/or upwelling of cooler oceanic water enriched in oxidised organic carbon (12C-enriched).

Belemnites of Valanginian, Hauterivian and Barremian age: Sr-isotope stratigraphy, composition (87Sr/86Sr, δ13C, δ18O, Na, Sr, Mg), and palaeo-oceanography

We present new data on 87Sr/86Sr, δ13C and δ18O, and elemental compositions of belemnites from 85 m of Valanginian, Hauterivian and Barremian strata at Speeton, Yorkshire, eastern England. The 87Sr/86Sr data provide a global standard for 87Sr/86Sr isotopic dating, and correlation to the biostratigraphic schemes of NW Europe. Values of 87Sr/86Sr increase from 0.707380±0.000003, at the base of the Hauterivian, to 0.707493±0.000004 in the earliest Late Barremian Paracrioceras elegans ammonite Zone before decreasing thereafter towards an Aptian minimum. The downturn in the elegans Zone coincided with the onset of volcanism on the present Ontong Java Plateau. A linear interpretation of the 87Sr/86Sr profile shows that the relative durations of ammonite zones differ by a factor ≤18. The basal Hauterivian unconformably overlies Valanginian strata; the discontinuity in 87Sr/86Sr across this surface represents a gap in sedimentation of 2.0 myr. In our belemnites (mostly of the genera Hibolites, Acroteuthis, and Aulacoteuthis) the absence of a correlation between δ18O and δ13C suggests that strong non-equilibrium fractionation has not affected the isotopic composition of the calcite. Our δ18O values therefore approximate to a valid record of marine palaeo-temperatures. Specimens of the genus Hibolites have δ18O values that are 0.4‰ more positive than those of co-occurring specimens of the genus Acroteuthis. This offset may be explained as resulting from small (0.4‰) departures from equilibrium during precipitation of calcite, different depth habitats, or changing temperature in the Speeton sea in the time that elapsed between deposition of our individual belemnites. The averaged belemnite record of δ18O through the section shows that seawater warmed from around 11°C at the base of the Hauterivian to a maximum around 15°C in the middle of the Hauterivian regale Zone, and returned to a cooler temperature of around 11°C by the middle of the overlying inversum Zone, a temperature that persisted to the basal Barremian. Through the Barremian, temperature increased to a peak of 20°C in the early Late Barremian elegans Zone then, in the same zone, precipitately and temporarily decreased to around 14°C at about the time of onset of volcanism on the Ontong Java Plateau, before they returned to around 16°C in the uppermost part of the section. In specimens of Aulacoteuthis and Acroteuthis, a good correlation between δ18O and the content of Na, Sr, and Mg suggests that incorporation of these trace elements in these genera is largely controlled by temperature. The dependency of concentration on temperature ranges from 7 to 20% per degree Celsius, if equilibrium fractionation of oxygen isotopic composition is assumed, so the Mg, Na and Sr content of these genera may be used as palaeo-temperature proxies. The trace element content of Hibolites shows no relation to stable oxygen isotopic composition and so does not record palaeo-temperature.

Isotopic evidence for temperature variation during the early Cretaceous (late Ryazanian–mid‐Hauterivian)

Oxygen and carbon isotopic compositions have been determined from the belemnite genera Acroteuthis and Hibolites sampled from the early Cretaceous (Ryazanian–Hauterivian) interval of the Speeton Clay Formation, Filey Bay, England. The Speeton Clay Formation consists of a series of claystones and calcareous mudrocks deposited in an epicontinental sea. δ18O values from belemnites, which met petrographic and chemical criteria for well preserved skeletal carbonate, indicate warm marine palaeotemperatures (c. 12–15°C) for much of the early Valanginian whilst cool temperatures (<9°C) are inferred for the earliest Hauterivian. During the remainder of the Hauterivian, temperatures fluctuated considerably and rose to a maximum of 15.5°C. Changes in kaolinite and smectite abundances, considered to reflect humid and arid phases of climate, correlate with warm and cool episodes. The palaeotemperature record, appears to contradict evidence from cephalopod faunas, which show a Tethyan influx during the inferred early Hauterivian cool period. However, this was a transgressive phase and thus the cephalopods could have been less sensitive to temperature than to water column stability and to land barriers. A positive shift in the carbon isotope profile obtained from the Speeton belemnites appears correlatable with carbon isotope profiles recorded from pelagic Tethyan successions, albeit with somewhat differing absolute values. The data support earlier models of carbon isotope variation, in that positive excursions are associated with an inferred global rise in sea level.

Comparison of clay mineral stratigraphy to other proxy palaeoclimate indicators in the Mesozoic of NW Europe

This paper reviews the opportunities and pitfalls associated with using clay mineralogical analysis in palaeoclimatic reconstructions. Following this, conjunctive methods of improving the reliability of clay mineralogical analysis are reviewed. The Mesozoic succession of NW Europe is employed as a case study. This demonstrates the relationship between clay mineralogy and palaeoclimate. Proxy analyses may be integrated with clay mineralogical analysis to provide an assessment of aridity–humidity contrasts in the hinterland climate. As an example, the abundance of kaolinite through the Mesozoic shows that, while interpretations may be difficult, the Mesozoic climate of NW Europe was subject to great changes in rates of continental precipitation. We may compare sedimentological (facies, mineralogy, geochemistry) indicators of palaeoprecipitation with palaeotemperature estimates. The integration of clay mineralogical analyses with other sedimentological proxy indicators of palaeoclimate allows differentiation of palaeoclimatic effects from those of sea–level and tectonic change. We may also observe how widespread palaeoclimate changes were; whether they were diachronous or synchronous; how climate, sea level and tectonics interact to control sedimentary facies and what palaeoclimate indicators are reliable.

Palaeoclimate analysis using spectral gamma-ray data from the Aptian (Cretaceous) of southern England and southern France

Abstract The abundance and half-lives of potassium (K), uranium (U) and thorium (Th) make these three elements the dominant sources of gamma-rays detected in rocks both at outcrop and in the subsurface. With the advent of the spectral gamma-ray tool (SGR), the variation shown by these three elements in rocks has been considered to reflect sorting at flooding surfaces and changes in the weathering regime of sediment source-lands. K and U are considered to be more soluble than Th, thus elevated Th to K or U ratios in soils and eroded clays can reflect hot, humid climates and palaeoclimates. This study makes an assessment of the usefulness of SGR data to palaeoclimate studies by comparing changing Th/K and Th/U ratios from a known period of palaeoclimate change to extensive published and new data from other analytical methods. New SGR data from the Cretaceous of southern England and southern France compare favourably to outcrop sedimentological facies interpretations and laboratory geochemical data. Palaeoclimate data from two other sections (northern Germany, northern Italy) are compared as controls to the study. Together these analyses suggest that there was a gradual change from semi-arid climate in the Barremian and early Aptian, to a semi-humid climate in the mid-Aptian. Superimposed on this long-term rise in humidity were a number of short-term (ammonite zone-length) changes in Th/K and Th/U ratios and the abundance of climate-sensitive materials. This study shows that SGR data are more useful than originally thought. When problems of overprinting from heavy minerals and diagenesis are overcome, outcrop and borehole spectral gamma-ray logs from mudstones or the clay-rich beds of mixed successions may be used to interpret past patterns of hinterland weathering.

Searching for the IRA "Disappeared": Ground-penetrating Radar Investigation of a Churchyard Burial Site, Northern Ireland

A search for the body of a victim of terrorist abduction and murder was made in a graveyard on the periphery of a major conurbation in Northern Ireland. The area is politically sensitive and the case of high profile. This required non-invasive, completely non-destructive and rapid assessment of the scene. A MALA RAMAC ground-penetrating radar system was used to achieve these objectives. Unprocessed and processed 400 MHz data show the presence of a collapse feature above and around a known 1970s burial with no similar collapse above the suspect location. In the saturated, clay-rich sediments of the site, 200 MHz data offered no advantage over 400 MHz data. Unprocessed 100 MHz data shows a series of multiples in the known burial with no similar features in the suspect location. Processed 100 MHz lines defined the shape of the collapse around the known burial to 2 m depth, together with the geometry of the platform (1 m depth) the gravedigger used in the 1970s to construct the site. In addition, processed 100 MHz data showed both the dielectric contrast in and internal reflection geometry of the soil imported above the known grave. Thus the sequence, geometry, difference in infill and infill direction of the grave was reconstructed 30 years after burial. The suspect site showed no evidence of shallow or deep inhumation. Subsequently, the missing persons body was found some distance from this site, vindicating the results and interpretation from ground-penetrating radar. The acquisition, processing, collapse feature and sequence stratigraphic interpretation of the known burial and empty (suspect) burial site may be useful proxies for other, similar investigations. GPR was used to evaluate this site within 3 h of the survey commencing, using unprocessed data. An additional day of processing established that the suspect body did not reside here, which was counter to police and community intelligence.

Lagoonal sedimentation and fluctuating salinities in the Vectis Formation (Wealden Group, Lower Cretaceous) of the Isle of Wight, southern England

Abstract Sedimentation in the Shepherds Chine Member of the Vectis Formation is characterised by a cyclicity of four principal facies on which a strong asymmetry has been imprinted by erosional events. The four lithofacies are: (1) very fine to fine sandstones; (2) heterolithic sand/silt and mudstones; (3) parallel-laminated (pinstripe) mudstones; and (4) black mudstones. The biota, principally associated with lithofacies 2 and 3 (as shelly partings and coquinas), can be grouped into five molluscan associations which range from freshwater to quasi-marine. These associations are poorly correlated with the lithofacies, but fluctuate within and between cycles. Salinity and storm frequency increase towards the top of the formation, heralding the main marine Aptian transgression. Lithofacies and biotas indicate deposition in a lagoon that was shallow and temporarily emergent. The cyclicity is thought to represent the more distal phases of the advance and retreat of deltaic sand bodies, derived from a westerly direction, into the lagoon. Major storm events broke the symmetry of the cycles. A deltaic facies, represented by the Barnes High Sandstone Member, is thought to be laterally linked, reworked deltaic sandstone lobes.