Low Palaeolatitudes Research Articles

Orbitally forced ice sheet fluctuations during the Marinoan Snowball Earth glaciation

The presence of dynamic ice sheets during Snowball Earth glaciations is controversial. Geological evidence and ice sheet modelling suggest that ice sheets may have responded to orbital forcing when reached a certain threshold. Two global glaciations occurred during the Neoproterozoic. Snowball Earth theory posits that these were terminated after millions of years of frigidity when initial warming from rising atmospheric CO2 concentrations was amplified by the reduction of ice cover and hence a reduction in planetary albedo1,2. This scenario implies that most of the geological record of ice cover was deposited in a brief period of melt-back3. However, deposits in low palaeo-latitudes show evidence of glacial–interglacial cycles4,5,6. Here we analyse the sedimentology and oxygen and sulphur isotopic signatures of Marinoan Snowball glaciation deposits from Svalbard, in the Norwegian High Arctic. The deposits preserve a record of oscillations in glacier extent and hydrologic conditions under uniformly high atmospheric CO2 concentrations. We use simulations from a coupled three-dimensional ice sheet and atmospheric general circulation model to show that such oscillations can be explained by orbital forcing in the late stages of a Snowball glaciation. The simulations suggest that while atmospheric CO2 concentrations were rising, but not yet at the threshold required for complete melt-back, the ice sheets would have been sensitive to orbital forcing. We conclude that a similar dynamic can potentially explain the complex successions observed at other localities.

Youngest occurrences of rhomaleosaurid plesiosaurs indicate survival of an archaic marine reptile clade at high palaeolatitudes

Rhomaleosaurid plesiosaurians were a common and ecologically significant component of Early Jurassic marine faunas, primarily as large-bodied predators. They declined in abundance and made their last fossil appearance in the Middle Jurassic. However, the geographic pattern of rhomaleosaurid extinction has thus far been obscured by spatial bias in the Middle Jurassic marine reptile fossil record, which is strongly focussed on low-latitude European assemblages. We report two rhomaleosaurid specimens from the Callovian (late Middle Jurassic) of the UK and Russia. Along with Borealonectes from Arctic Canada, these are the youngest-known occurrences of rhomaleosaurids. The UK specimen is the first identified from the Callovian of Europe, despite intensive fossil sampling over almost 200 years and the recovery of hundreds of other plesiosaurian specimens. Its discovery indicates that rhomaleosaurids were present, but extremely rare, at low palaeolatitudes of the Callovian. The Russian specimen is one of relativ...

Further contribution to the low latitude leaf assemblage from the late Oligocene sediments of Assam and its phytogeographical significance

An assemblage of fossil leaves is described from the late Oligocene (Chattian 28.1–23 Ma) sediments of Assam, which was located in a low palaeolatitude (~10–15°N) during the period of sedimentation. It includes four new fossil leaves resembling Firmiana and Pterygota of the Malvaceae s.l. and Paranephelium and Sapindus of the Sapindaceae. The present study suggests that the floral migration between India and southeast Asia had occurred after the late Oligocene. Our study is in congruence with the earlier published data suggesting a floral migration had occurred after the complete suturing of two landmasses by the Neogene.

A new Late Cretaceous to Present APWP for Asia and its implications for paleomagnetic shallow inclinations in Central Asia and Cenozoic Eurasian plate deformation

Based on a compilation of 533 Cretaceous to present-day palaeomagnetic poles obtained from both sedimentary and igneous rocks, we present a new analysis of the so-called 'Asian inclination anomaly' and demonstrated the anomaly to be twofold: a 2nd-order anomaly, characterized by high palaeolatitudes in Indochina and low palaeolatitudes over Tibet and Central Asia, is superimposed on a 1st-order anomaly, characterized by Cenozoic low palaeolatitudes found all over northeastern Asian stable blocks. The analysis herein convincingly shows that the Europe Apparent Polar Wandering Path (APWP) can no longer be used to interpret palaeomagnetic data East of the Urals, including interpretation of Asian Tertiary deformation related to the India-Asia Collision. We, thus, construct a new APWP for East Asia, based on palaeopoles from blocks assumed to be stable. This new APWP is consistent with and reinforces previous analyses of Asian tectonics, such as the age (∼55 Ma) and locus (∼5-10°N) of the Indo-Asian collision, the lateral extrusion of SE Asian continental blocks and the intracontinental shortening in Central Asia. Possible origins of the 1st-order palaeolatitude anomaly are: (1) a geomagnetic origin, due to long-lasting non-dipolar contribution to the magnetic field and (2) a tectonic hypothesis, in which a newly defined East Asia Plate was located ∼10° farther south than expected from the current Europe APWP. Based on a set of six new reconstructions from 90 Ma to present, we show that our tectonic model reconciles geophysical, geological and tectonic observations throughout Eurasia, from Siberia to Europe, including kinematics in the Arctic Ocean, up to northwestern Arctic Alaska. Beyond possible occurrences of non-dipolar field contribution and/or local inclination flattening in the sedimentary data, our model leads us to conclude that Cenozoic tectonics is the dominant contributor to the observed 1st-order ∼10° low palaeolatitude anomaly over Asia during the Tertiary.

Chapter 26 Graptolite palaeobiogeography

Abstract Graptolite faunas exhibited strong biogeographical differentiation during the Early Palaeozoic, particularly in the Ordovician. Skevington recognized two major faunal provinces, the high to mid palaeolatitude ‘Atlantic Province’ and the low-palaeolatitude ‘Pacific Province’. Subsequent workers have generally accepted this pattern of graptolite distribution, but the controls on this pattern have been the subject of considerable debate. Two competing models have emerged: a surface water temperature model and a depth stratification model. It is likely that the some of the physical and chemical oceanic factors that vary with latitude may also vary in a similar way along an onshore to offshore transect. Hence, it may be that both depth and surface temperature play an important role in biogeographical differentiation. Biogeography also played a critical role in the evolutionary history of graptoloids. Important examples include the origination of axonophorans in deep, offshore environments from isograptid and pseudisograptid ancestors and their subsequent migration into shallow water regions; the replacement of the Diplograptina by Neograptina in the low palaeolatitudes during the Late Ordovician extinction event; and the origination of expansograptids in the ‘Atlantic’ Province as shallow water endemics followed by their worldwide dispersal into the oceanic biofacies.

Upper Carboniferous–Lower Permian Palaeoaplysina build-ups on Svalbard: the influence of climate, salinity and sea-level

Abstract Upper Carboniferous–Lower Permian Palaeoaplysina build-ups on Svalbard developed on a partly restricted marine warm-water carbonate platform at low palaeo-latitudes. The Palaeoaplysina build-ups mostly have a flat-topped tabular geometry, representing the mature stage of build-up growth. With decreasing accommodation space, continued vertical growth was hampered and lateral growth became dominant. The tabular growth form of the build-ups is probably a result of lateral amalgamation of several minor mounds. In central Spitsbergen many of the tabular build-ups form parts of continuous ridges or ring-shaped to polygonal build-up complexes encircling a lagoon. Lateral interfingering between bedded fusulinid dolostones along the build-up margins and barren dolostones further out in the lagoon may indicate increasing salinity from the lagoon margin to the central part. These lateral facies changes are often associated with evaporites (gypsum) and low biodiversity, especially abundant euryhaline fusulinids or even barren beds. On the sea side of the Palaeoaplysina build-ups, the frequent renewal of normal saline seawater allowed the highest biodiversity (e.g. stenohaline crinoids). The uppermost part of some build-ups contain Microcodium , indicating intermittent subaerial exposure and vegetation cover during lowstand, probably caused by eustatic sea-level changes and/or local tectonics. The biota of the build-ups and their lateral sediments is typical for a shallow marine chloroforam association, indicating a tropical climate during deposition.

A new Late OrdovicianHirnantiabrachiopod Fauna from NW Turkey, its biostratigraphical relationships and palaeogeographical setting

AbstractLate Ordovician fossils, including the distinctiveHirnantiabrachiopod Fauna, have been found in Lower Palaeozoic successions in Istanbul and Bolu (Yığılca), western Pontides, NW Turkey. The Middle Ordovician (Sandbian) faunas belong to the cooler-water Mediterranean Province, and they are followed by Katian brachiopods includingSulevorthis,Nicolella,Hesperorthis,Glyptorthis,SaukrodictyaandKullervoand ostracods such asPiretella,EochilinaandKlimphores, which represent deposition in warmer waters; however, the Mediterranean Province usually cooler-water brachiopodsDraboviaandLeptestiinaalso occur. The Pendik Formation includes thin bryozoan-rich limestones which probably represent the Boda Global Warming Event. The overlying turbidites contain aHirnantiaFauna, developed within a brachiopod–diplograptid association. Above them there are characteristic Llandovery (Rhuddanian–Aeronian) brachiopods, such asLeangella,Eoplectodonta,StricklandiaandHindellawith the coralsHalysites,PaleofavositesandStreptelasma. In the Bolu area, Katian brachiopods such asMcewanella,Dalmanella,Glyptorthis,Christiania,Oligorhynchia,Nicolella,HowellitesandDrabovinellaalso occur, but there the overlyingHirnantiaFauna is developed within aHirnantia–Mucronaspisassociation. The fauna and sediments indicate that the western Pontides were not very cold during the latest Ordovician. Despite Turkey being placed in higher latitudes by previous authors, it seems more probable that the Pontides were at somewhat lower palaeolatitudes, perhaps at about 40°S in those times; however, the precise palaeogeographical position of the terrane remains uncertain: there are no Hirnantian glaciogenic rocks there, such as are found in the Taurides of southern Turkey.

Anisotropy of thermoremanent magnetisation of Cryogenian glaciogenic and Ediacaran red beds, South Australia: Neoproterozoic apparent or true polar wander?

Determining the effects of compaction-related inclination shallowing of remanence directions is crucial for ascertaining the validity of low palaeolatitudes for Neoproterozoic red beds in South Australia that are central to the debate concerning low-latitude Proterozoic glaciation. The inclination correction (or flattening) factor, f, is defined as tan(ID)/tan(IF), where ID and IF are the inclinations of the measured detrital remanence and the ancient inducing field, respectively. The anisotropy can be estimated using anisotropy of magnetic susceptibility and the anisotropy of high-field isothermal remanence (hf-AIR). The elongation–inclination (E–I) method has also been used to infer inclination shallowing. We add the anisotropy of thermoremanent magnetisation (ATR) to these methods. For the late Cryogenian Elatina Formation arenites, which constitute the bulk of the Elatina data set, the inclination correction using f=0.738 derived from ATR increases the palaeolatitude of the Elatina Formation from 6.5±2.2° to 8.8±3.2°, which confirms that the Elatina glaciation occurred near the palaeoequator. Inclination corrections for the Ediacaran argillaceous Brachina and Wonoka formations, using f=0.35–0.38 derived from ATR, are significantly greater than for the more arenaceous Elatina Formation, which increases their palaeolatitudes from ~12° to ~30°. Carbonates from the basal Ediacaran Nuccaleena Formation yielded f=0.8 from ATR, which represents only a small palaeolatitude correction from 19° to 23°. The anisotropy results imply that the characteristic remanent magnetisations carried by all these units were acquired early as depositional remanent magnetisations, essentially at the time of deposition. The shift of the palaeopoles from argillaceous units indicating significantly higher palaeolatitudes introduces a distinctive loop into the late Cryogenian–Ediacaran–Cambrian pole path for Australia. This loop shows similarities with the North American pole path for this period, for which true polar wander (TPW) has been inferred. However, until ages of Neoproterozoic strata in South Australia are better constrained uncertainty persists on whether the similarities of the Australian and North American pole paths reflect TPW.

Magnetostratigraphy and mid-palaeolatitude VGP dispersion during the Permo-Carboniferous Superchron: results from Paraná Basin (Southern Brazil) rhythmites

SUMMARY A detailed magnetostratigraphic and rock-magnetism study of two Late Palaeozoic rhythmite exposures (Itu and Rio do Sul) from the Itarare Group (Parana Basin, Brazil) is presented in this paper. After stepwise alterning-field procedures and thermal cleaning were performed, samples from both collections show reversed characteristic magnetization components, which is expected for Late Palaeozoic rocks. However, the Itu rocks presented an odd, flat inclination pattern that could not be corrected with mathematical methods based on the virtual geomagnetic pole (VGP) distributions. Correlation tests between the maximum anisotropy of the magnetic susceptibility axis (K1) and the magnetic declination indicated a possible mechanical influence on the remanence acquisition. The Rio do Sul sequence displayed medium to high inclinations and provided a high-quality palaeomagnetic pole (after shallowing corrections of f = 0.8) of 347.5°E 63.2°S (N = 119; A95 = 3.3; K = 31), which is in accordance with the Palaeozoic apparent wander pole path of South America. The angular dispersion (Sb) for the distribution of the VGPs calculated on the basis of both the 45° cut-off angle and Vandamme method was compared to the best-fit Model G for mid-latitudes. Both of the Sb results are in reasonable agreement with the predicted (palaeo) latitudinal S-λ relationship during the Cretaceous Normal Superchron (CNS), although the Sb value after the Vandamme cut-off has been applied is a little lower than expected. This result, in addition to those for low palaeolatitudes during the Permo-Carboniferous Reversed Superchron (PCRS) previously reported, indicates that the low secular variation regime for the geodynamo that has already been discovered in the CNS might have also been predominant during the PCRS.

Biogeography and Mass Extinction: Extirpation and re-invasion of Normalograptus species (Graptolithina) in the Late Ordovician Palaeotropics

SUMMARY Normalograptids constitute a distinctive but not easily identified group of derived axonophorans. A new study of Normalograptus morphology indicates that many Late Ordovician taxa previously identified as Normalograptus are actually referable to Styracograptus and related climacograptoids. These re-identifications have led to the recognition of a complex biogeographic history among normalograptids and their descendants, including an interesting pattern that had previously been overlooked – a late Katian disappearance of Normalograptus from the palaeotropics. Normalograptid palaeogeographic distribution in the Ordovician appears to comprise 5 main phases: (1) evolutionary origin probably in relatively high palaeolatitudes in the early to mid Darriwilian; (2) spread to global distribution in later Darriwilian time; (3) retreat from most low palaeolatitude regions during the early Katian, with perhaps a slightly delayed retreat from Laurentia; (4) a complete extirpation from the low palaeolatitudes by the mid Katian (Ea4); (5) reinvasion of the low palaeolatitudes that accompanies the remarkable ecological and evolutionary replacement of the Diplograptina by species of Normalograptus and their descendants during the latest Katian–early Hirnantian. We propose two sets of hypotheses to explain Late Ordovician Normalograptus distribution. First, given the low species diversity of Normalograptus , we suggest as a null hypothesis that the disappearance of normalograptids from low palaeolatitudes during the late Sandbian and early Katian as simply a stochastic outcome – a small clade simply went extinct by chance alone. The alternative is that the loss of Normalograptus species reflects some form of extinction for cause, either: (1) competitive exclusion; or (2) an inability to adapt to changing environmental conditions. In the case of the competitive exclusion hypothesis, the most likely competitors are species of the Diplograptina.

A critical graptolite correlation into the Lower Ordovician of Gondwana

SUMMARY The early Ordovician was a time of maximum faunal provinciality. Correlation between strata deposited at low palaeolatitudes and those deposited close to the Ordovician pole in Gondwana is accordingly particularly difficult. The graptolite used to define the base of the internationally accepted Floian Stage, Tetragraptus approximatus , is essentially a lower latitude species. The discovery of a closely related and coeval species, T. akzharensis , in the early Ordovician of the Montagne Noire is reported. It occurs at the same level as the acme of the trilobite Taihungshania miqueli . The new record permits a direct tie with the earliest part of the Floian Stage and the shelly faunas typical of much of western Gondwana.

Phytoplankton dynamics across the Ordovician/Silurian boundary at low palaeolatitudes: Correlations with carbon isotopic and glacial events

The Late Ordovician culminated in a major glacial period that has been related to one of the strongest mass extinctions recorded during the Phanerozoic. During this interval, Anticosti Island (Québec, eastern Canada) was located at low to intermediate palaeolatitudes (15–30° S) on the eastern margin of Laurentia. It displays a relatively complete section across the Ordovician–Silurian (O/S) boundary. Upper Ordovician to Lower Silurian strata of Anticosti studied here comprise the Vauréal (Katian), Ellis Bay (Hirnantian) and Becscie (latest Hirnantian–Rhuddanian) formations. Phytoplankton dynamics inferred from the acritarch content of these strata are locally compared with new geochemical environmental proxies (δ13C), with recent palynological data from Baltica and also with phytoplankton dynamics as described from near-polar, high latitude localities of the Gondwana region. Two positive excursions of the carbon isotope record have been identified in the lower and upper Ellis Bay Formation.Overall, phytoplankton trends are as follows: (1) as with chitinozoans, conodonts, brachiopods and corals, acritarchs from Anticosti display a major turn-over during the early Hirnantian (uppermost Vauréal Formation–lower Ellis Bay Formation), with the appearance of taxa that exhibit Silurian affinities (e.g., Tylotopalla sp., Ammonidium sp., Oppilatala sp., Evittia sp., Dilatisphaera spp.). High polymorphism for some common taxa is observed in the interval corresponding to the first δ13C positive excursion. Using sequence stratigraphy, this period may be related to a first glacially driven sea-level drawdown corresponding to terrestrial ice-sheet growth on Gondwana, such as recorded in Morocco. (2) Subsequently, during the late Hirnantian of Anticosti (uppermost Ellis Bay Formation), phytoplanktic communities suffered a crisis, exemplified by the disappearance of several typical, large Late Ordovician taxa (i.e., Orthosphaeridium spp., Peteinosphaeridium spp., large Baltisphaeridum spp., Sacculidium sp.). The lower Becscie strata, corresponding to the time of deglaciation at the O/S boundary interval, only contain a low diversity assemblage dominated by large acritarchs of the genus Hoegklintia and other morphologically related forms, and a few tolerant and long-ranging taxa (e.g., Evittia remota, Micrhystridium spp., leiospheres). Despite a possible taphonomic bias in the uppermost Ellis Bay Formation, the major crisis in the late Hirnantian coincides with the second δ13C excursion and a second, stronger glacially-driven sea-level fall corresponding to the time of the maximum extend of the ice sheets on Gondwana. (3) Such phytoplankton events have also been detected in Gondwanan ice-center area, but with “typical Ordovician taxa crises” occurring apparently later when plotted against positive carbon isotope excursions, as also observed in Baltica. This time-lag may be linked to a basin configuration of the studied areas, where environmental stress consequently differed or linked to the presence of omission surfaces coupled with preservation bias of acritarch assemblages in the uppermost Ordovician strata of Anticosti. In addition to their biostratigraphic and palaeobiogeographic utility, acritarchs turn out to be more specific in highlighting palaeoenvironmental conditions.

Early Silurian carbonate platform ostracods from Iran: A peri-Gondwanan fauna with strong Laurentian affinities

Early Silurian (Aeronian) carbonate platform deposits of the lower Niur Formation of east central Iran yield an ostracod fauna of fourteen species dominated by podocopes with rare palaeocopes and binodicopes. The podocope-rich fauna is unusual in that Early Palaeozoic carbonate platforms are typically dominated by palaeocopes. Despite its mid-palaeolatitude peri-Gondwanan setting and isolation from low palaeolatitudes, the ostracod assemblage of the lower Niur Formation shows striking affinities to species-level with Late Ordovician and Early Silurian ostracod faunas of palaeo-equatorial Laurentia. Biogeographically the lower Niur Formation ostracod fauna also shows species- and genus-links with Late Ordovician and Early Silurian faunas of low-palaeolatitude (0°–30°S) Baltica and mid palaeolatitude peri-Gondwanan Uzbekistan, dispelling the notion of strict endemism for Early Silurian ostracods. Two new species Bulbosclerites jamshidis sp. nov. and Steusloffina fravashi sp. nov. are described.

Chapter 2 A history of Neoproterozoic glacial geology, 1871–1997

Abstract Neoproterozoic glacial records have been discovered on 23 palaeocontinents, their rate of discovery changing little since 1871. Yet, half of all the resulting publications appeared since 2000. The history of research before 1998 is described in five stages defined by publication spikes; subsequent work is not covered because historical perspective is lacking. In stage 1 (1871–1907), ‘Cambrian’ (now Neoproterozoic) glaciation was recognized successively in Scotland, Australia, India, Norway, Svalbard and China. Criteria for recognition included faceted and striated pebbles in matrix-supported conglomerates resting on ice-worn bedrock pavements. In stage 2 (1908–1940), Neoproterozoic glaciation was shown to have been widespread in Africa, Asia and the Americas. Major textbooks summarized these findings, but the rejection of continental drift (to account for late Palaeozoic glacial dynamics) put a chill on research. In stage 3 (1942–1964), the occurrence of glacial deposits within carbonate successions, as well as nascent palaeomagnetic observations, suggested that Neoproterozoic glaciers reached sea-level at low palaeolatitudes, but the belated recognition of sediment gravity flowage caused glacial interpretations to be prematurely abandoned in key areas. In stage 4 (1965–1981), the extent of Neoproterozoic glaciation was rethought in light of plate tectonics. Distinctive chemical sediments (iron±manganese formations and cap carbonates) were identified. In basic climate models, modest lowering of solar luminosity resulted in global glaciation due to ice-albedo feedback, and deglaciation due to greenhouse forcing resulted from silicate-weathering feedback in the carbon cycle. Neoproterozoic glacial geologists were blind to these ideas. In stage 5 (1982–1997), reliable palaeomagnetic data combined with glacial marine sedimentation models confirmed that Neoproterozoic ice sheets reached sea level close to the palaeoequator.

Ediacaran palaeomagnetism and apparent polar wander path for Australia: no large true polar wander

SUMMARY We report new palaeomagnetic data for red beds from the Ediacaran Brachina and Wonoka formations in the Adelaide Geosyncline, South Australia, and discuss their place with previously determined poles in the Ediacaran apparent polar wander path for Australia. Both formations behave similarly on thermal demagnetization, displaying high-temperature components that decay to the origin at 680 ◦ C, consistent with haematite being the only magnetic mineral present. Restoring the strata to the palaeohorizontal yielded positive fold tests for both units at 99 per cent confidence, indicating that acquisition of magnetization occurred before the early Palaeozoic Delamerian Orogeny. For the Brachina Formation (N = 91 specimens) the mean direction after unfolding is declination D = 178.2 ◦ , inclination I = –22.6 ◦ (α95 = 4.4 ◦ ), indicating a palaeolatitude λ = 11.8 ± 2.5 ◦ and a pole position at latitude λp = 46.0 ◦ S, longitude ϕp = 315.4 ◦ E, with confidence semi-axes dp = 2.4 ◦ and dm = 4.6 ◦ . The mean direction for the Wonoka Formation after unfolding (N = 70) is D = 255.9 ◦ , I = –23.7 ◦ (α95 = 6.4 ◦ ), indicating λ = 12.3 +3.8/–3.4 ◦ and a pole position at latitude λp = 5.2 ◦ S, longitude ϕp = 30.5 ◦ E( dp = 3.6 ◦ and dm = 6.8 ◦ ). The mean directions for these units and other Ediacaran units in the Adelaide Geosyncline are significantly different from each other, which excludes blanket remagnetization of the units before Delamerian folding and therefore gives strong preference to their magnetization dating from close to the time of deposition. The late Cryogenian–Ediacaran–Cambrian apparent polar wander path for South Australia spans 150 Myr from ∼635 to 490 Ma and places Australia in low palaeolatitudes throughout the interval studied. The poles differ significantly from each other, suggesting Australia underwent continual drift during that time. Whereas the directional difference between the late Cryogenian Elatina Formation and early Ediacaran Nuccaleena Formation is mainly in inclination, for most other contiguous stratigraphic units the differences are mainly in declination with minor inclination differences, indicating Australia was rotating about a nearby Euler pole in low palaeolatitudes. The large and perhaps rapid polar shifts at 615–590 and 575–565 Ma in the Laurentian apparent polar wander path are not evident in the Ediacaran apparent polar wander path for Australia. Because true polar wander should be recorded globally, in the absence of evidence for any major stratigraphic break in the South Australian succession we conclude that large true polar wander did not occur during the Ediacaran.

The evolution of Iberia during the Jurassic from palaeomagnetic data

A revision of Jurassic palaeomagnetic data from Iberia has been carried out in order to investigate the consistency between the palaeomagnetic information and the tectonic models proposed for the western Mediterranean. Due to the presence of a widespread (but partial) remagnetization which affected most Jurassic sediments in Iberia, selection criteria have been designed to avoid completely remagnetized sites. A total of 72 sites have been considered for the palaeomagnetic discussion (35 sites from the Messejana–Plasencia dolerite dyke, 14 sedimentary sites from the Iberian Range and 23 sites from the Betic Cordillera). Three palaeopoles for Iberia have been selected (from the Iberian Massif and from the Iberian Range) for the period around 200 Ma, the Toarcian–Aalenian and the Oxfordian. Data from the Subbetic Zone (Betic Cordillera) are used to constrain the palaeolatitude of the Iberian microplate. Iberian data are in general agreement with the BC02 master curve and the reconstruction parameters used to transfer the Iberian data to Europe, but lower palaeolatitudes than predicted by BC02 master curve are observed in Iberia for the Late Jurassic. Iberia reached a maximum in palaeolatitude during the Toarcian–Aalenian (the reference point of Madrid was at about 37°), and since then, the palaeolatitude decreased (Madrid was at 22° by the Kimmeridgian). Tectonic reference models for the Western Mediterranean (Stampfli and Borel, 2002, 2004) do not fit the Iberian declinations and palaeolatitudes. New palaeogeographic reconstructions are proposed for the Hettangian–Sinemurian, the Toarcian–Aalenian and the Oxfordian.

Biogeography of Triassic tetrapods: evidence for provincialism and driven sympatric cladogenesis in the early evolution of modern tetrapod lineages.

Triassic tetrapods are of key importance in understanding their evolutionary history, because several tetrapod clades, including most of their modern lineages, first appeared or experienced their initial evolutionary radiation during this Period. In order to test previous palaeobiogeographical hypotheses of Triassic tetrapod faunas, tree reconciliation analyses (TRA) were performed with the aim of recovering biogeographical patterns based on phylogenetic signals provided by a composite tree of Middle and Late Triassic tetrapods. The TRA found significant evidence for the presence of different palaeobiogeographical patterns during the analysed time spans. First, a Pangaean distribution is observed during the Middle Triassic, in which several cosmopolitan tetrapod groups are found. During the early Late Triassic a strongly palaeolatitudinally influenced pattern is recovered, with some tetrapod lineages restricted to palaeolatitudinal belts. During the latest Triassic, Gondwanan territories were more closely related to each other than to Laurasian ones, with a distinct tetrapod fauna at low palaeolatitudes. Finally, more than 75 per cent of the cladogenetic events recorded in the tetrapod phylogeny occurred as sympatric splits or within-area vicariance, indicating that evolutionary processes at the regional level were the main drivers in the radiation of Middle and Late Triassic tetrapods and the early evolution of several modern tetrapod lineages.

The Late Cretaceous environment of the Arctic: A quantitative reassessment based on plant fossils

Late Cretaceous megafossil floras from the palaeo-Arctic of northeastern Russia and northern Alaska are reviewed in respect of their age, composition, structure and floral dynamics. Palaeofloral correlations and comparisons are made between the two regions. Nine angiosperm-rich, predominantly Cenomanian to Coniacian, floras from the palaeo-Arctic are re-evaluated using Climate Leaf Analysis Multivariate Program (CLAMP) calibrated using a global gridded (0.5° × 0.5°) climate data set derived from that used in climate modelling. Additional floras from lower palaeolatitudes were used to derive latitudinal temperature gradients: seven from N. America, five from around 30 °N palaeolatitude in Europe and one from Kazakhstan. The Arctic climatic determinations, similar to previous estimates, support the existence of a northern Pacific Ocean cold gyre and a warm Arctic Ocean. At palaeolatitudes greater than 80 °N floras are insufficiently diverse in woody dicot taxa to use CLAMP, but using CLAMP-derived latitudinal temperature gradients Arctic Ocean coastal environments at 70 Ma and 82 °N, and which supported a diverse dinosaur magafauna, are predicted to have experienced a mean annual temperature of 6.3 ± 2.2 °C, a warm month mean of 14.5 ± 3.1 °C and a cold month mean no colder than − 2.0 ± 3.9 °C. All uncertainties are 2σ. The new estimates are in good agreement with a wide range of non-palaeobotanical climate proxies and render as an outlier warmer temperature estimates for the Arctic Ocean derived from the TEX 86 proxy. Modelling, however, shows that land to ocean temperature gradients could have been steep. The CLAMP estimates also suggest high values for humidity and precipitation consistent with sedimentological indicators and, coupled with warm temperatures, support the existence of a persistent polar cloud cap that helped maintain high terrestrial air temperatures throughout prolonged periods (up to 5 months) of winter darkness.

How cold were the Early Permian glacial tropics? Testing sea-surface temperature using the oxygen isotope composition of rigorously screened brachiopod shells

Abstract: Brachiopod carbonate from Early Permian brachiopod shells from low palaeolatitude north Iran and higher palaeolatitude Pakistan Karakorum were screened for diagenesis and analysed for oxygen isotope ratios to derive seawater palaeotemperatures. Screening techniques employed included SEM ultrastructural analysis, cathodoluminescence (CL), image analysis of CL images, trace-element (Sr, Mn, Fe) determinations, and carbon and oxygen stable-isotope determinations. The Karakorum shells were found to be diagenetically altered, but those from north Iran were judged to be pristine. Using data from pristine material, two distinct time slices were analysed: the early and middle Asselian. Two contrasting δ 18 O values for seawater (0‰ and +1.0‰ V-SMOW) were used to account for different extensions of the Gondwanan ice caps. The δ 18 O data from north Iran indicate a range of seawater temperatures from +24.3 to +30.3 °C (for δ 18 O seawater = 0‰ V-SMOW) or from +30.3 to +35.4 °C (for δ 18 O seawater = +1.0‰ V-SMOW) for the early Asselian. Results for δ 18 O from the middle Asselian indicate seawater temperatures of +24.4 to +28.0 °C (for δ 18 O seawater = 0‰) or +29.2 to +32.8 °C (for δ 18 O seawater = +1.0‰). The maximum calculated temperatures in the middle Asselian are about 2 °C lower than those for the early Asselian. The average temperature for both time slices is similar to modern tropical sea-surface temperatures, indicating that low-latitude Early Permian ocean waters in Iran did not undergo significant cooling during the final Glacial III episode of Gondwanan glaciation. This confirms other evidence based on biotic provinces, which suggests that during the Permo-Carboniferous glaciation, the low-latitude warm belt became narrower and confined to the western Tethys and Cathaysian provinces, and was not subject to a reduction in temperature, but rather a reduction in size.

A palaeogeographic context for Neoproterozoic glaciation

The distributions of 77 Neoproterozoic glacigenic formations are shown on global palaeogeographic maps for 715 Ma (Sturtian), 635 Ma (Marinoan) and 580 Ma (Ediacaran), constructed on grounds independent of palaeoclimatic indicators. The meridional distribution of Sturtian and Marinoan deposits is biased in favour of low palaeolatitudes, whereas Ediacaran deposits are biased in favour of high palaeolatitudes. All carbonate-hosted glacigenic formations ( n = 22) fall within 35° of the palaeoequator. Most (6 of 8) examples of periglacial polygonal sand-wedges occur at palaeolatitudes greater than 30°, whereas most (8 of 9) occurrences of large syn-glacial Fe and Fe–Mn deposits lie within 30° of the palaeoequator. Marinoan syn-deglacial cap dolostones ( n = 24) decline in maximum thickness with palaeolatitude, consistent with poleward ice retreat, normal meridional temperature gradients and a small-obliquity orbit. Meridional (N–S) mean orientations of giant wave ripples in Marinoan cap dolostones from different regions ( n = 10) and absence of zonal (W–E) orientations are consistent with zonal wind-driven waves and not with hurricanes. In general, the results support the validity of the palaeogeographic reconstructions and the pan-glacial character of Sturtian and Marinoan ice ages.