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The Carnian Pluvial Episode was a phase of global climatic change and biotic turnover that occurred during the early Late Triassic. In marine sedimentary basins, the arrival of huge amounts of siliciclastic sediments, the establishment of anoxic conditions, and a sudden change of the carbonate factory on platforms marked the Carnian Pluvial Episode. The sedimentary changes are closely associated with abrupt biological turnover among marine and terrestrial groups as, for example, an extinction among ammonoids and conodonts in the ocean, and a turnover of the vertebrate fauna and the flora on land. Multiple negative carbon-isotope excursions were recorded during the Carnian Pluvial Episode in both organic matter and marine carbonates, suggesting repeated injection of 13C-depleted CO2 into the ocean–atmosphere system, but their temporal and causal links with the sedimentological and palaeontological changes are poorly understood. We here review the existing carbon-isotope records and present new data on the carbon-isotope composition of organic carbon in selected sections of the western Tethys realm that record the entire Carnian Pluvial Episode. New ammonoid, conodont and sporomorph biostratigraphic data were collected and coupled to an extensive review of the existing biostratigraphy to constrain the age of the sampled sections. The results provide biostratigraphically constrained composite organic carbon-isotope curves for the Carnian, which sheds light on the temporal and causal links between the main carbon-isotope perturbations, and the distinct environmental and biotic changes that mark the Carnian Pluvial Episode. The carbon-isotope records suggest that a series of carbon-cycle perturbations, possibly recording multiple phases of volcanic activity during the emplacement of the Wrangellia Large Igneous Province, disrupted Carnian environments and ecosystems repeatedly over a remarkably long time interval of about 1 million years.

The two major oceanic anoxic events of the Cretaceous, those of the Early Aptian (OAE 1a) and the Cenomanian–Turonian boundary (OAE 2), registered some of the highest temperatures reconstructed for the Cretaceous Period, and are thought to be related to the input of volcanically derived carbon dioxide from one or more Large Igneous Provinces. Widely distributed deposition of marine organic matter, the hallmark of OAEs, and intensified silicate weathering in response to a globally accelerated hydrological cycle and/or reaction of seawater with freshly extruded basalt, are both potential mechanisms whereby the content of atmospheric carbon dioxide could have been drawn down to promote cooling, on the assumption that this potential effect was not offset by increased addition of this volcanically derived greenhouse gas. Reduction in the supply of such carbon dioxide, with deposition of organic matter and silicate weathering continuing at the same rate, could also have produced cooling. A transient fall in temperature and increase in marine dissolved oxygen levels is well documented for OAE 2, in the form of the so-called Plenus Cold Event or Benthic Oxic Event, associated with southward invasion of certain boreal faunas and an increase in many redox-sensitive and/or chalcophilic elements in seawater caused by temporary loss of anoxic–euxinic sinks as basalt–seawater interaction continued apace. High-resolution studies of OAE 1a show at least three cooling episodes of probable global distribution, one of which (recorded in the stratigraphy of the so-called C4 Segment) is documented at high enough resolution to show correlation with an increase in carbon-isotope values that was probably due to a rise in the quantities of organic matter being buried globally, with consequent potential drawdown of atmospheric CO2and/or reduction in volcanic input of this greenhouse gas. Both calcium- and lithium-isotope records suggest an increase in silicate weathering over the OAE 1a interval but current relatively low-resolution records cannot at present be tied to any one cooling episode, although the lowest Li-isotope values do derive from the C4 Segment. Evidence for reoxygenation of the ocean during the transient cooling episodes of OAE 1a is meagre, due to the lack of suitable sedimentary archives, although a negative sulfur-isotope excursion in a Pacific shallow-water carbonate section, which can be interpreted as due to oxidation of pyrite and/or sulfur-rich organic matter in the global ocean, suggests that this phenomenon may also have been a feature of the C4 Segment. Further work is required to elucidate the similarities and differences between OAE 2 and OAE 1a, but both phenomena are demonstrably global in reach and represent major disturbances to the carbon cycle with attendant effects on marine temperatures.This article is part of a discussion meeting issue ‘Hyperthermals: rapid and extreme global warming in our geological past’.

The Lower Jurassic Toarcian Stage (c. 183–174 Ma) is marked by one of the largest global exogenic carbon-cycle perturbations of the Phanerozoic, which is associated with the early Toarcian Oceanic Anoxic Event (T-OAE; c. 183 Ma). Climatic and environmental change at the T-OAE is reasonably well constrained in the marine realm, with marine anoxic or euxinic conditions developing locally across both hemispheres, at the same time as the T-OAE negative carbon-isotope excursion. However, high-resolution stratigraphic comparison between different palaeo-ocean basins and with the continental realm can be complicated. Palaeomagnetic reversals can provide a precise and accurate stratigraphic correlation tool between marine and continental sedimentary archives, and even between sedimentary and igneous successions. Here, we present a high-resolution magnetostratigraphic record for the Toarcian Stage in the biostratigraphically complete and expanded Llanbedr (Mochras Farm) Borehole, Cardigan Bay Basin, Wales. This study provides the first geomagnetic polarity reversal scale that is integrated with high-resolution biostratigraphy and carbon-isotope stratigraphy for the entire Toarcian Stage. This stratigraphic framework also provides a new, precise correlation with the basalt lava sequence of the Karoo–Ferrar Large Igneous Province, linking the Pliensbachian–Toarcian boundary and T-OAE climatic and environmental perturbations directly to this episode of major volcanic activity. Supplementary material: Details of the palaeomagnetic data and dip direction are available at https://doi.org/10.6084/m9.figshare.c.4052720

The Central Atlantic Magmatic Province (CAMP) has long been proposed as having a causal relationship with the end-Triassic extinction event (~201.5 Ma). In North America and northern Africa, CAMP is preserved as multiple basaltic units interbedded with uppermost Triassic to lowermost Jurassic sediments. However, it has been unclear whether this apparent pulsing was a local feature, or if pulses in the intensity of CAMP volcanism characterized the emplacement of the province as a whole. Here, six geographically widespread Triassic–Jurassic records, representing varied paleoenvironments, are analyzed for mercury concentrations and mercury/ total organic carbon (Hg/TOC) ratios. Volcanism is a major source of mercury to the modern environment. Clear increases in Hg and Hg/TOC are observed at the end-Triassic extinction horizon, confirming that a volcanically induced global mercury-cycle perturbation occurred at that time. The established correlation between the extinction horizon and lowest CAMP basalts allows this sedimentary mercury excursion to be stratigraphically tied to a specific flood basalt unit for the first time, strengthening the case for volcanic mercury as the driver of sedimentary Hg/TOC spikes. Additional Hg/TOC peaks are also documented between the extinction horizon and the Triassic–Jurassic boundary (separated by ~200 kyr), supporting pulsatory intensity of CAMP volcanism across the entire province and providing the first direct evidence for episodic volatile release during the initial stages of CAMP emplacement. Pulsatory volcanism, and associated perturbations in the ocean–atmosphere system, likely had profound implications for the rate and magnitude of the end-Triassic mass extinction and subsequent biotic recovery.

Geochemical records of biogenic carbonates provide some of the most valuable records of the geological past, but are often difficult to interpret without a mechanistic understanding of growth processes. In this experimental study, Halimeda algae are used as a test organism to untangle some of the specific factors that influence their skeletal composition, in particular their Ca-isotope composition. Algae were stimulated to precipitate both calcite and aragonite by growth in artificial Cretaceous seawater, resulting in experimental samples with somewhat malformed skeletons. The Ca-isotope fractionation of the algal calcite (−0.6‰) appears to be much smaller than that for the algal aragonite (−1.4‰), similar to the behaviour observed in inorganic precipitates. However, the carbonate from Halimeda has higher Ca-isotope ratios than inorganic forms by approximately 0.25‰, likely because of Rayleigh distillation within the algal intercellular space. In identifying specific vital effects and the magnitude of their influence on Ca-isotope ratios, this study suggests that mineralogy has a first-order control on the marine Ca-isotope cycle.

Early Jurassic marine palaeotemperatures have been typically quantified by oxygen-isotope palaeothermometry of benthic and nektonic carbonate and phosphatic macrofossils. However, records of Early Jurassic sea-surface temperatures that can be directly compared with general circulation model simulations of past climates are currently unavailable. The TEX86 sea-surface temperature proxy is based upon the relative abundance of glycerol dialkyl glycerol tetraethers preserved in organic-carbon-bearing sediments. This proxy has been used extensively on Cretaceous and Cenozoic materials and, in one study, on Middle and Upper Jurassic sediments. Here, TEX86 is applied, for the first time, to Lower Jurassic (Sinemurian–Pliensbachian) sediments cored at Deep Sea Drilling Project Site 547 in the North Atlantic. The abundance of glycerol dialkyl glycerol tetraethers in these sediments is very low, despite biomarker and Rock-Eval data suggesting that thermal maturity is, generally, low. Sea floor oxygenation and a high input of reworked terrestrially sourced organic matter may explain the low concentrations. For samples from which it was possible to quantify the relative abundance of glycerol dialkyl glycerol tetraethers, TEX86 values range from 0·78 to 0·88, equating to sea-surface temperatures in excess of >28°C. These temperatures are broadly comparable with new general circulation model simulations of the Sinemurian and Pliensbachian stages and support the general view of a predominantly warm climate. The new proxy data suggest that, under favourable geological conditions, it is possible to extend the record of TEX86-based sea-surface temperatures back into the Early Jurassic.

Despite its assumed global nature, there are very few detailed stratigraphic records of the late Cenomanian to the early Turonian Oceanic Anoxic Event (OAE) 2 from the Southern Hemisphere. A highly resolved record of environmental changes across the Cenomanian–Turonian boundary interval is presented from Ocean Drilling Program Site 1138 on the central Kerguelen Plateau (southern Indian Ocean). The new data lead to three key observations. Firstly, detailed bio- and chemostratigraphy indicate that the record of OAE-2 is not complete, with a hiatus spanning the onset of the event. A decrease in glauconite and highly weathered clays after the onset of OAE-2 marks the end of the hiatus interval, which can be explained by a relative sea-level rise that increased sediment accommodation space on the Kerguelen Plateau margin. This change in depositional environment controlled the timing of the delayed peak in organic-matter burial during OAE-2 at Site 1138 compared with other OAE-2 locations worldwide. A second key observation is the presence of cyclic fluctuations in the quantity and composition of organic matter being buried on the central Kerguelen Plateau throughout the latter stages of OAE-2 and the early Turonian. A close correspondence between organic matter, sedimentary elemental compositions and sediments recording sea-floor oxygenation suggests that the cycles were mainly productivity-driven phenomena. Available age-control points constrain the periodicity of the coupled changes in sedimentary parameters to ~15–50 ka, suggesting a link between carbon burial and astronomically forced climatic variations (precession or obliquity) in the Southern Hemisphere mid-latitudes both during, and after, OAE-2: fluctuations that were superimposed on the impact of global-scale processes. Finally, trace-metal data from the non-sulphidic black shale unit at Site 1138 provide the first evidence from outside of the proto-North Atlantic region for a global drawdown of seawater trace-metal (Mo) inventories during OAE-2.

Large Igneous Provinces (LIPs) are proposed to have caused a number of episodes of abrupt environmental change by increasing atmospheric CO2 levels, which were subsequently alleviated by drawdown of CO2 via enhanced continental weathering and burial of organic matter. Here, the sedimentary records of two such episodes of environmental change, the Toarcian Oceanic Anoxic Event (T-OAE) and preceding Pliensbachian–Toarcian (Pl–To) event (both potentially linked to the Karoo–Ferrar LIP), are investigated using a new suite of geochemical proxies that have not been previously compared. Stratigraphic variations in osmium-isotope ( 187Os/188Os) records are compared with those of mercury (Hg) and carbon isotopes (δ13C) in samples from the Mochras Core, Llanbedr Farm, Cardigan Bay Basin, Wales. These sedimentary rocks are confirmed as recording an open-marine setting by analysis of molybdenum/uranium enrichment trends, indicating that the Os-isotope record in these samples reflects the isotopic composition of the global ocean. The Os-isotope data include the first results across the Pl–To boundary, when seawater 187Os/188Os increased from ∼ 0.40 to ∼ 0.53, in addition to new data that show elevated 187Os/188Os (from ∼ 0.42 to ∼ 0.68) during the T-OAE. Both increases in 187Os/188Os correlate with negative carbon-isotope excursions and increased mercury concentrations, supporting an interplay between terrestrial volcanism, weathering, and climate that was instrumental in driving these distinct episodes of global environmental change. These observations also indicate that the environmental impact of the Karoo–Ferrar LIP was not limited solely to the T-OAE

The Early Jurassic epoch was a time of extreme environmental change: there are well-documented examples of rapid transitions from cold, or even glacial, climates to super greenhouse events, the latter characterized worldwide by hugely enhanced organic carbon burial, multiple large isotopic anomalies, global sea-level change, and mass extinction (Price, 1999; Hesselbo et al., 2000; Jenkyns, 2010; Korte and Hesselbo, 2011). These icehouse–greenhouse events not only reflect changes in the global climate system but are also thought to have had significant influence on the evolution of Jurassic marine biota (e.g. van de Schootbrugge et al., 2005; Fraguas et al., 2012). Furthermore, the events may serve as analogues for present-day and future environmental transitions. Although our knowledge of specific global change events within the Early Jurassic is rapidly improving, such as the Toarcian oceanic anoxic event (or T-OAE), we still do not have a comprehensive understanding of the timing, pacing, or triggers for these environmental perturbations, principally because of the temporally fragmentary nature of the existing data sets. The major goal for this proposed ICDP project is therefore to produce a new global standard for these key 25 million years of earth history by re-drilling and doublecoring a 45 yr old borehole at Mochras Farm on the edge of Cardigan Bay, Wales, and developing an integrated stratigraphy for the cored material. The new data sets will be applied to understand fundamental questions about the longand short-term evolution of the earth system. Cycles that occur regionally and that provisionally appear in the Mochras logs will allow evaluation of the extent to which major environmental change events are astronomically forced, resulting from internal system dynamics, or are triggered by deepearth processes. Fil: Hesselbo, S. P.. University Of Exeter; Reino Unido Fil: Bjerrum, C. J.. Universidad de Copenhagen; Dinamarca Fil: Hinnov, L. A.. University Johns Hopkins; Estados Unidos Fil: Mac Niocaill, C.. University of Oxford; Reino Unido Fil: Miller, K. G.. State University of New Jersey; Estados Unidos Fil: Riding, J. B.. British Geological Survey; Reino Unido Fil: Van de Schootbrugge, B.. University of Utrecht; Países Bajos Fil: Abels, H.. The Mochras Revisited Science Team; Reino Unido Fil: Belcher, C.. The Mochras Revisited Science Team; Reino Unido Fil: Blau, J.. The Mochras Revisited Science Team; Reino Unido Fil: Browning, J.. The Mochras Revisited Science Team; Reino Unido Fil: Cartwright, J.. The Mochras Revisited Science Team; Reino Unido Fil: Condon, D.. The Mochras Revisited Science Team; Reino Unido Fil: Daines, S.. The Mochras Revisited Science Team; Reino Unido Fil: Damborenea, S.. The Mochras Revisited Science Team; Reino Unido Fil: Dickson, A.. The Mochras Revisited Science Team; Reino Unido Fil: Fraguas, A.. The Mochras Revisited Science Team; Reino Unido Fil: Hilgen, F.. The Mochras Revisited Science Team; Reino Unido Fil: Hooker, J.. The Mochras Revisited Science Team; Reino Unido Fil: Huang, C.. The Mochras Revisited Science Team; Reino Unido Fil: Huesing, S.. The Mochras Revisited Science Team; Reino Unido Fil: Jenkyns, H.. The Mochras Revisited Science Team; Reino Unido Fil: Korte, C.. The Mochras Revisited Science Team; Reino Unido Fil: Krijgsman, W.. The Mochras Revisited Science Team; Reino Unido Fil: Lenton, T.. The Mochras Revisited Science Team; Reino Unido Fil: Little, C.. The Mochras Revisited Science Team; Reino Unido Fil: Manceñido, Miguel Oscar. The Mochras Revisited Science Team; Reino Unido. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Mattioli, E.. The Mochras Revisited Science Team; Reino Unido Fil: Meister, C.. The Mochras Revisited Science Team; Reino Unido Fil: Morgan, R.. The Mochras Revisited Science Team; Reino Unido Fil: Newton, R.. The Mochras Revisited Science Team; Reino Unido Fil: Pálfy, J.. The Mochras Revisited Science Team; Reino Unido Fil: Pienkowski, G.. The Mochras Revisited Science Team; Reino Unido Fil: Poulton, S.. The Mochras Revisited Science Team; Reino Unido Fil: Riccardi, A.. The Mochras Revisited Science Team; Reino Unido Fil: Robinson, A.. The Mochras Revisited Science Team; Reino Unido Fil: Ruhl, M.. The Mochras Revisited Science Team; Reino Unido Fil: Suan, G.. The Mochras Revisited Science Team; Reino Unido Fil: Smith, N.. The Mochras Revisited Science Team; Reino Unido Fil: Thibault, N.. The Mochras Revisited Science Team; Reino Unido Fil: Ullmann, C.. The Mochras Revisited Science Team; Reino Unido Fil: Wignall, P.. The Mochras Revisited Science Team; Reino Unido Fil: Williford, K.. The Mochras Revisited Science Team; Reino Unido Fil: Wonik, T.. The Mochras Revisited Science Team; Reino Unido Fil: Xu, W.. The Mochras Revisited Science Team; Reino Unido

Amber from a Lower Cretaceous outcrop at San Just, located in the Eastern Iberian Peninsula (Escucha Formation, Maestrat Basin), was investigated to evaluate its physico-chemical properties. Thermogravimetric (TG) and Differential Thermogravimetric (DTG) analyses, infra-red spectroscopy, elemental and C-isotope analyses were performed. Physico-chemical differences between the internal light nuclei and the peripheral darker portions of San Just amber can be attributed to processes of diagenetic alteration that preferentially took place in the external amber border colonized by microorganisms (fungi or bacteria) when the resin was still liquid or slightly polymerized. δ13Camber values of different pieces of the same sample, from the nucleus to the external part, are remarkably homogeneous, as are δ13Camber values of the darker peripheral portions and lighter inner parts of the same samples. Hence, neither invasive microorganisms, nor diagenetic alteration, changed the bulk isotopic composition of the amber. δ13C values of different amber samples range from -21.1‰ to -24‰, as expected for C3 plant-derived material. C-isotope analysis, coupled to palaeobotanical, TG and DTG data and infra-red spectra, suggests that San Just amber was exuded by only one conifer species, belonging to either the Cheirolepidiaceae or Aracauriaceae, coniferous families probably living under stable palaeoenvironmental and palaeoecological conditions.

Although a division of the Phanerozoic climatic modes of the Earth into "greenhouse" and "icehouse" phases is widely accepted, whether or not polar ice developed during the relatively warm Jurassic and Cretaceous Periods is still under debate. In particular, there is a range of isotopic and biotic evidence that favours the concept of discrete "cold snaps", marked particularly by migration of certain biota towards lower latitudes. Extension of the use of the palaeotemperature proxy TEX86 back to the middle Jurassic indicates that relatively warm sea-surface conditions (26-30°C) existed from this interval (~160 Ma) to the Early Cretaceous (~115 Ma) in the Southern Ocean. The Jurassic and Cretaceous "cold snaps" represent falls of only a few degrees. Belemnite δ18O data gives palaeotemperatures that are consistently lower by ~14°C than does TEX86 and these molluscs likely record conditions below the thermocline. Such long-term warm climatic conditions would only be compatible with the existence of continental ice and if appreciable areas of high altitude existed on Antarctica, and/or in other polar regions, during the Mesozoic Era.

Oceanic anoxic events (OAEs) record profound changes in the climatic and paleoceanographic state of the planet and represent major disturbances in the global carbon cycle. OAEs that manifestly caused major chemical change in the Mesozoic Ocean include those of the early Toarcian (Posidonienschiefer event, T-OAE, ∼183 Ma), early Aptian (Selli event, OAE 1a, ∼120 Ma), early Albian (Paquier event, OAE 1b, ∼111 Ma), and Cenomanian-Turonian (Bonarelli event, C/T OAE, OAE 2, ∼93 Ma). Currently available data suggest that the major forcing function behind OAEs was an abrupt rise in temperature, induced by rapid influx of CO2 into the atmosphere from volcanogenic and/or methanogenic sources. Global warming was accompanied by an accelerated hydrological cycle, increased continental weathering, enhanced nutrient discharge to oceans and lakes, intensified upwelling, and an increase in organic productivity. An increase in continental weathering is typically recorded by transient increases in the seawater values of 87Sr/86Sr and 187Os/188Os ratios acting against, in the case of the Cenomanian-Turonian and early Aptian OAEs, a longer-term trend to less radiogenic values. This latter trend indicates that hydrothermally and volcanically sourced nutrients may also have stimulated local increases in organic productivity. Increased flux of organic matter favored intense oxygen demand in the water column, as well as increased rates of marine and lacustrine carbon burial. Particularly in those restricted oceans and seaways where density stratification was favored by paleogeography and significant fluvial input, conditions could readily evolve from poorly oxygenated to anoxic and ultimately euxinic (i.e., sulfidic), this latter state being geochemically the most significant. The progressive evolution in redox conditions through phases of denitrification/ anammox, through to sulfate reduction accompanied by water column precipitation of pyrite framboids, resulted in fractionation of many isotope systems (e.g., N, S, Fe, Mo, and U) and mobilization and incorporation of certain trace elements into carbonates (Mn), sulfides, and organic matter. Sequestration of CO2 in organic-rich black shales and by reaction with silicate rocks exposed on continents would ultimately restore climatic equilibrium but at the expense of massive chemical change in the oceans and over time scales of tens to hundreds of thousands of years. Copyright 2010 by the American Geophysical Union.

The mid-Cretaceous (∼120-90 Ma) was a time of excess atmospheric CO2, greenhouse climate, and widespread O2 deficiency in the ocean. The Albian is punctuated by brief, intermittent episodes of anoxia/dysoxia, recorded as cyclic black shales in the western Tethys and Atlantic oceans. The Albian section of the Piobbico core (central Italy) contains 30 m (∼10 Ma) of rhythmic black shales that were sampled at a high resolution and examined for calcareous nannofossil assemblages and C and O stable isotopes. Unlike oceanic anoxic events, productivity was not the primary factor controlling the deposition of Albian rhythmic black shales. It is suggested that during warm humid climatic cycles, higher temperatures and/or increased precipitation and runoff produced density stratification at a regional scale. Recurrent lowered salinity in the early Albian and wanning in the late Albian are credited with causing development of a pycnocline, resulting in slower rates of deep-water renewal and consequent anoxia. Copyright 2009 by the American Geophysical Union.

Organic carbon-rich shales from localities in England, Italy, and Morocco, which formed during the Cenomanian-Turonian oceanic anoxic event (OAE), have been examined for their total organic carbon (TOC) values together with their carbon, nitrogen, and iron isotope ratios. Carbon isotope stratigraphy (δ13 Corg and δ13 Ccarb) allows accurate recognition of the strata that record the oceanic anoxic event, in some cases allowing characterization of isotopic species before, during, and after the OAE. Within the black shales formed during the OAE, relatively heavy nitrogen isotope ratios, which correlate positively with TOC, suggest nitrate reduction (leading ultimately to denitrification and/or anaerobic ammonium oxidation). Black shales deposited before the onset of the OAE in Italy have unusually low bulk δ57 Fe values, unlike those found in the black shale (Livello Bonarelli) deposited during the oceanic anoxic event itself: These latter conform to the Phanerozoic norm for organic-rich sediments. Pyrite formation in the pre-OAE black shales has apparently taken place via dissimilatory iron reduction (DIR), within the sediment, a suboxic process that causes an approximately -2‰ fractionation between a lithogenic Fe(III)oxide source and Fe(II)aq. In contrast, bacterial sulfate reduction (BSR), at least partly in the water column, characterized the OAE itself and was accompanied by only minor, iron isotope fractionation. This change in the manner of pyrite formation is reflected in a decrease in the average pyrite framboid diameter from ∼10 to ∼7 μm. The gradual, albeit irregular increase in Fe isotope values during the OAE, as recorded in the Italian section, is taken to demonstrate limited isotopic evolution of the dissolved iron pool, consequent upon ongoing water column precipitation of pyrite under euxinic conditions. Given that evidence exists for both nitrate and sulfate reduction during the OAE, it is evident that redox conditions in the water column were highly variable, in both time and space. Copyright 2007 by the American Geophysical Union.

To understand the climate dynamics of the warm, equable greenhouse world of the Late Cretaceous period, it is important to determine polar palaeotemperatures. The early palaeoceanographic history of the Arctic Ocean has, however, remained largely unknown, because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice. A shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated Upper Cretaceous organic-rich sediment from Alpha ridge, a submarine elevated feature of probable oceanic origin. A lack of carbonate in the sediments from this core has prevented the use of traditional oxygen-isotope palaeothermometry. Here we determine Arctic palaeotemperatures from these Upper Cretaceous deposits using TEX86, a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton, Crenarchaeota. From these analyses we infer an average sea surface temperature of 15 °C for the Arctic Ocean about 70 million years ago. This calibration point implies an Equator-to-pole gradient in sea surface temperatures of 15 °C during this interval and, by extrapolation, we suggest that polar waters were generally warmer than 20 °C during the middle Cretaceous ( 90 million years ago).