Gregory D. Price

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.

Isotopic signals from late Jurassic–early Cretaceous (Volgian–Valanginian) sub-Arctic belemnites, Yatria River, Western Siberia

This contribution presents the first detailed oxygen and carbon isotope record from the latest Jurassic–early Cretaceous interval of the Yatria River, subpolar Urals, Siberia. Isotopic compositions have been determined on well-preserved belemnite samples from the genera Lagonibelus, Cylindroteuthis and Acroteuthis. These new data indicate a shift to lower temperatures from the late Volgian into the late Valanginian, with some warmer phases recognized within the Ryazanian and earliest Valanginian. The lowest temperatures of the late Valanginian, consistent with subfreezing polar temperatures, are coincident with an inferred eustatic sea-level fall. A late Valanginian positive shift in carbon isotopes correlates with the carbon isotope excursion recorded from Tethyan successions. The most positive carbon isotope values correspond to the most positive oxygen isotope values (and hence lowest palaeotemperatures). In the absence of widespread Valanginian organic-rich black shale deposition, the carbon isotope excursion may point to increased storage of organic carbon in coastal areas and/or enhanced preservation within stratified waters in high-latitude basins. At these higher latitudes, where rates of weathering were presumably much lower because of the prevalent cold climate, the isotopic data may point to pulses of productivity being brought about by increased riverine nutrient transfer and also by nutrients being released by the melting of ice. The correlation between positive carbon isotopes and cool climates may indicate the effectiveness of these high-latitude carbon sinks and their ability to draw down atmospheric CO2, resulting in an ‘inverse greenhouse’ effect.

Sedimentological evaluation of general circulation model simulations for the “greenhouse” Earth: Cretaceous and Jurassic case studies

Abstract Conceptual climate models, based on the workings of the present-day climate system, provided a first-order approach to ancient climate systems. They are potentially very subjective in character. Their main drawback was that they involved the relocation of continents beneath a stable atmospheric circulation modelled upon that of the present. General circulation models (GCMs) use the laws of physics and an understanding of past geography to simulate climatic responses. They are objective in character. However, they require super computers to handle vast numbers of calculations. Nonetheless it is now possible to compare results from different GCMs for a range of times and over a wide range of parameterisations. GCMs are currently producing simulated climate predictions which compare favourably with the distributions of climatically sensitive facies (e.g. coals, evaporites and palaeosols). They have been used effectively in the prediction of oceanic upwelling sites and the distribution of petroleum source-rocks and phosphorites. Parameterisation is the main weakness in GCMs (e.g. sea-surface temperature, orography, cloud behaviour). Sensitivity experiments can be run on GCMs which simulate the effects of Milankovitch forcing and thus provide insights into possible patterns of climate change both globally and locally (i.e. provide predictions that can be evaluated against the rock record). Future use of GCMs could be in the forward modelling of sequence stratigraphic evolution and in the prediction of the diagenetic characteristics of reservoir units in frontier exploration areas. The sedimentary record provides the only way that GCMs may themselves be evaluated and this is important because these same GCMs are being used currently to predict possible changes in future climate.

“Warm” palaeotemperatures from high Late Jurassic palaeolatitudes (Falkland Plateau): Ecological, environmental or diagenetic controls?

Abstract Oxygen and carbon isotopes have been determined from Late Jurassic (Oxfordian-Tithonian) belemnites and inoceramid bivalves from two Deep Sea Drilling Project (DSDP) sites located on the Falkland Plateau. Mean belemnite ∂18O values, derived from well preserved skeletal material, were −1.29‰ from DSDP site 330 and −1.45‰ from DSDP site 511. Assuming a seawater SMOW value of −1.0‰, mean palaeotemperatures calculated from the oxygen isotopic composition are 17.2°C and 17.9°C, respectively. The inoceramid bivalves yielded much lighter ∂18O values (mean −3.58‰). Petrographic and geochemical evidence points to the inoceramid bivalves being altered by diagenesis which accordingly accounts for the observed differences in isotopic values. “Vital effects” or the importation of belemnites or inoceramids from another area, are considered not to account for the observed isotopic trends. The palaeotemperatures interpreted from the belemnites are significantly warmer than other recent estimates of Late Jurassic temperature (from oxygen isotope studies and climate model predictions) from similar southern palaeolatitudes. We suspect our apparent warmer temperatures are because of a combination of increased freshwater runoff depleting surface waters with respect to ∂18O and related to the semi-enclosed nature of the depositional basin retaining warmth, relative to the open ocean of similar latitudes.

Valanginian isotope variation in glendonites and belemnites from Arctic Svalbard: Transient glacial temperatures during the Cretaceous greenhouse

Oxygen and carbon isotope data from Cretaceous (Valanginian) glendonites and belemnites from Arctic Svalbard are presented. Oxygen isotope data from well-preserved glendonites, in conjunction with the ikaite to glendonite pseudomorph transition temperature, are used to provide an estimate of the oxygen isotope composition of ambient seawater. Calculation of such a factor is essential for robust paleotemperature estimates. Using this methodology, our paleotemperatures calculated from the oxygen isotope compositions of coexisting belemnites yield cool temperatures (4–7 °C) consistent with transient glacial polar conditions during the Cretaceous greenhouse. Cool polar temperatures during the Cretaceous help reconcile geologic data with the simulations of general circulation models. Nevertheless, beyond this postulated and transient cool event within the Valanginian, the remainder of the isotope data are interpretable in terms of warm polar conditions during the Cretaceous greenhouse.

New constraints upon isotope variation during the early Cretaceous (Barremian–Cenomanian) from the Pacific Ocean

Carbon and oxygen isotope data from a succession of Cretaceous (Barremian-Cenomanian) age recovered from the Pacific Ocean (DSDP site 463) are presented. The carbon isotope curve reveals a large isotope excursion within the early Aptian where δ 1 3 C values reach ∼4.8 ‰ in the L. cabri-G. ferreolensis foraminifera zone. A decrease in δ 1 3 C values is observed at the base G. algerianus zone, before a return to more positive values at the top of the G. algerianus-T. bejaouaensis zone. The pronounced early Aptian positive event is preceded by a large negative isotope excursion, confined to the G. blowi zone. Synchronous with this excursion are increased total organic carbon values and increases in Mn and Fe concentrations. Integrated biostratigraphic and magnetostratigraphic data, together with the carbon isotope profile, suggest that the organic-rich units of site 463 are correlatable with Oceanic Anoxic Event la. The input of isotopically light volcanic CO 2 in concert with the intensification and upwelling of intermediate water enriched in 1 2 C could account for the observed trends. A potential trigger may have been the destabilization of the water column and the prodigious CO 2 emissions associated with hydrothermal activity and the emplacement of the Ontong Java Plateau. Coupled with faunal evidence, the subsequent positive carbon isotope excursion is interpreted to be resulting from high, but decreasing, productivity and possibly increasing ocean stratification resulting in strong carbon isotopic gradients and 1 3 C-enriched surface waters. The decrease in δ 1 3 C within the G. algerianus zone is coincident with more positive δ 1 8 O values. If these are interpreted in terms of temperature this interval may be characterized by a period of cooling and possibly a waning of C o r g cycling. A return to lower δ 1 3 C values during the middle Albian is considered to be related to the increased influence of upwelling, as opposed to a waning of C o r g cycling. Upwelling introduced isotopically light carbon to the surface, arresting the stratified oceanic conditions.

Isotopic evidence for palaeotemperatures and depth stratification of Middle Cretaceous planktonic foraminifera from the Pacific Ocean

Stable isotopic measurements have been made on both planktonic foraminifera and coccolithic matrix of Middle Cretaceous (Late Albian–Cenomanian) age from two Pacific low latitude sites. The degree of alteration of the foraminifera has been assessed through the application of chemical analyses, cathodoluminescence and Scanning Electron Microscopy (SEM). The rotaliporid foraminifera display an interspecies range of δ18O values from –2.29 to –3.01 ‰ at Deep Sea Drilling Project (DSDP) Site 463 and from –2.74 to –3.55 ‰ at DSDP Site 305. Hedbergellid foraminifera exhibit a δ18O interspecies variation of –2.52 to –3.02 ‰ at Site 305. Isotopic analysis of individual Hedbergella delrioensis and Rotalipora appenninica foraminifera from single samples shows H. delrioensis to have a surprisingly large spread of δ18O values (–2.492 to –3.097 ‰ from Site 463, –2.454 to –3.344 ‰ from Site 305), whilst δ13C values remain confined to a narrower range. Such a spread of oxygen values may be related to a number of factors, including subtle diagenetic alteration, a wide range of temperature-related depth habitats or growth related changes of primary skeletal calcite. The hedbergellids have consistently lighter oxygen and heavier carbon isotopic values than do the rotaliporid foraminifera and hence provide isotopically derived palaeotemperatures consistent with a thermally stratified ocean. At both sites the oxygen isotopic data are consistent with a gradual warming through Albian–Cenomanian time. However, the results suggest that Middle Cretaceous equatorial oceans were possibly only as warm as those of the present day (or slightly warmer), but did not reach the high temperatures claimed in older literature. * Author for correspondence: g.price@qub.ac.uk been the susceptibility of the carbonate shell material to solution and the identification of subtle diagenetic alteration resulting in a masking of the primary palaeoecological and palaeoceanographic signal (see Price, Sellwood & Pirrie, 1996; Huber & Hodell, 1996). Scanning Electron Microscopy (SEM), cathodoluminescence and trace element analysis of the foraminifera, coupled with the stable isotopic analyses have been employed to help eliminate any trends originating from diagenetic overprinting. 2. Geological setting and sampling Samples of Late Albian to Early Cenomanian age have been obtained from DSDP Site 305 (Shatsky Rise, 32.00.13°N; 157.51.00°E). Additionally, Middle–Late Cenomanian samples were obtained from DSDP Site 463 which is located southwest of the Hawaiian Ridge on the Western Mid-Pacific Mountains (21.21.01°N; 174.40.07 °E) (Fig. 1). During Middle Cretaceous times, Sites 305 and 463 were located close to the palaeoequator at ~5°S and 17°S, respectively. They thus represent two sites of great importance in any reconstruction of ocean temperature for the Middle Cretaceous. The lithology of the sampled section at Site 305 consists of a laminated chalk brecciated by drilling. In parts, the only sediment preserved is chalk forming a crust on chert fragments. The lithology of Site 463 consists of uniform white soft chalk/ooze, partly disturbed by drilling. In the lower parts of the section the sediment is slightly more lithified (see Theide et al. 1981). Black and dark grey chert fragments are recorded throughout (Fig. 2). Mineralogically the sediments, from both sites, are composed largely of calcite, with traces of quartz and clay. At both sites planktonic foraminifera are the dominant fossil component. Rotaliporid foraminifera are most abundant in the 211–425 μm size fraction in all samples. Hedbergellids, together with a few benthic foraminifera were also identified. Boersma (1981) has recorded a diverse planktonic fauna from Site 463 including Rotalipora appenninica, R. gandolfi and R. greenhornensis from the sampled section. From this suite of fossils, rotaliporid and hedbergellid planktonic foraminifera, together with individual H. delrioensis and R. appenninica from the 211–425 μm size fraction and matrix samples, were isotopically analysed from each of the sites. 3. Analytical procedures The generally unconsolidated nature of the sediments allowed relatively easy sample disaggregation using an ultrasonic bath. Two-milligram foraminifera and matrix samples were isotopically analysed using standard methods on a VG Sira series II mass spectrometer at the University of Reading. Further isotopic analyses of single specimens of planktonic foraminifera were carried out on a VG PRISM Series II mass spectrometer, with on-line common acid bath system, at the University of Oxford. The δ18O and δ13C compositions are reported in per mil (‰) notation with respect to the PDB international standard. Reproducibility for both δ18O and δ13C was generally better than ± 0.1 ‰, based upon multiple sample analysis. Elemental concentrations (Mn, Sr, Mg and Fe) were determined by inductively coupled plasma (ICP) spectrometry analysis on 2–10 mg subsamples. Reproducibility, based upon replicate analysis, was estimated generally to be less than ± 17 % of the measured concentration for Sr, Mg and Fe and ± 10 % for Mn.