Icehouse Conditions Research Articles

Isotopic tracers of chemical weathering and consequences for marine geochemical budgets

The mechanical breakdown of rock by physical weathering exerts a significant control on chemical weathering rates because it produces surface area. During periods of icehouse conditions on Earth, the grinding of rock by glacial processes should lead to faster chemical weathering of the continents, perhaps particularly during periods of pronounced climatic variability, like the Quaternary. Evidence is reviewed here for both high and cyclical chemical weathering rates during the Quaternary, and the implications for both marine geochemical budgets and climate-chemical weathering feedbacks are discussed.

Integration of sedimentology and ground‐penetrating radar for high‐resolution imaging of a carbonate platform

AbstractGround‐penetrating radar has not been applied widely to the recognition of ancient carbonate platform geometries. This article reports the results of an integrated study performed on an Upper Jurassic outcrop from the south‐east Paris basin, where coral bioherms laterally change into prograding depositional sequences. Ground‐penetrating radar profiles illustrate the different bedding planes and major erosional unconformities visible at outcrop. A ground‐penetrating radar profile conducted at the base of the cliff displays a palaeotopographic surface on which the outcropping bioherms settled. The excellent penetration depths of the ground‐penetrating radar (20 m with a monostatic 200 MHz antenna) images the carbonate platform geometries, ranging between outcrop workscale (a few metres) and seismic scale (several hundreds of metres). This study supports recent evidence of icehouse conditions and induced sea‐level fluctuations controlling the Upper Jurassic carbonate production.

Step-wise change of Asian interior climate preceding the Eocene–Oligocene Transition (EOT)

Understanding the global climate change from greenhouse to icehouse conditions at the Eocene–Oligocene Transition (EOT) 34millionyears ago requires climatic records from oceanic as well as continental realms of the key Late Eocene “doubthouse” period preceding this switch. Here, we report integrated stratigraphic results from well-dated Late Eocene continental mudflat to saline lake paleoenvironments of the Xining Basin (northeastern Tibetan Plateau, western China) recording regional and global change. Cyclostratigraphic analysis strongly suggests continuous dominance of the 41-kyr obliquity cycle in the whole late Eocene interval down to the base of polarity chron C18n.2n at 39Ma with additional input of the ~100-kyr eccentricity cycle up to the base of chron C13r at ~34.7Ma. This might imply that high-latitude climates dominated the area long before the EOT, probably related to incipient ice-volume fluctuations. Furthermore, our results reveal two paleoenvironmental deterioration steps preceding the Eocene–Oligocene Transition. The first step occurs in the top of chron C17n.1n at ~36.6Ma. This age closely corresponds to (1) the high-altitude pollen appearance in chron C16.2r at ~36.4Ma in the same section, (2) the recently dated final retreat of the Tarim Sea in western China, and (3) a shift from precession to obliquity dominance in the Atlantic Ocean. This near co-occurrence suggests global change at this time. We hypothesize this change is related to an increase in incipient ice sheet volume leading to passing threshold conditions for the high-altitude pollen appearance and Tarim Sea retreat, finally leading to decreased moisture availability in the Xining Basin. At the second step, in the base of chron C13r at ~34.7Ma, a substantial increase in clastic sedimentation rates is observed. This might relate to increased climate variability preceding the greenhouse to icehouse transition at the EOT that prevented landscapes to attain equilibrium configurations.

Northern England Serpukhovian (early Namurian) farfield responses to southern hemisphere glaciation

Abstract: During the Serpukhovian (early Namurian) icehouse conditions were initiated in the southern hemisphere. However, nearfield evidence is inconsistent: glaciation appears to have started in limited areas of eastern Australia in the earliest Serpukhovian, followed by a long interglacial, whereas data from South America and Tibet suggest glaciation throughout the Serpukhovian. New farfield data from the Woodland, Throckley and Rowlands Gill boreholes in northern England allow this inconsistency to be addressed. δ 18 O from well-preserved late Serpukhovian (late Pendleian to early Arnsbergian) Woodland brachiopods vary between −3.4 and −6.3‰, and δ 13 C varies between −2.0 and +3.2‰, suggesting a δ 18 O seawater ( w ) value of around −1.8‰ VSMOW, and therefore an absence of widespread ice-caps. The organic carbon δ 13 C upward increasing trend in the Throckley borehole (Serpukhovian to Bashkirian; c . −24 to c . −22‰) and the Rowlands Gill borehole (Serpukhovian; c . −24 to c . −23‰) suggests large-scale burial of organic material, probably in burgeoning lycophyte-dominated coal forest, implying a fall in p CO 2 . p CO 2 reduction appears not to have caused large-scale glaciation until the early Bashkirian, but a scenario of coalescing upland ice-caps through the Serpukhovian with a background of decreasing p CO 2 appears to be similar to the process that initiated Cenozoic Antarctic glaciation.

Polyphase speleogenesis in Lick Creek Cave, Little Belt Mountains, Montana, USA

Lick Creek Cave in northern Montana (USA) is hosted in limestones of the Lower Carboniferous Madison Group near Tiger Butte, an Eocene quartz–syenite porphyry intrusive dome. The cave is located within the zone of contact metamorphism of the dome, which crops out ∼ 300 m from the cave entrance. The cave consists of two genetically distinct cave systems separated by a fracture zone: (1) a ∼ 80 × 50 m dome-shaped cavern in breccias of a Carboniferous paleocave, and (2) anastomosing conduits ∼ 2–10 m across, parallel to the bedding of the Madison Group and extending 100 m up dip to the present cave entrance. The conduits are further subdivided into a tectonised and a maze zone and are variably decorated in several combinations by phreatic isopachous calcite spar cements, with crystals up to several cm long, and with vadose speleothems, including stalactite–stalagmite pairs, flowstone, corallite (cave popcorn), and moonmilk. Our database is comprised of field survey, thin section, XRD, and SEM observations along with 118 δ 18O/ δ 13C analyses and 27 87Sr/ 86Sr measurements from samples of county rock and speleothems. The limestone matrix samples with the heaviest δ 18O/ δ 13C ratios are interpreted as the least recrystallised proxy to Tournaisian seawater. Stable isotope data from other Carboniferous limestones, including paleocave breccias, follow a regional meteoric pathway established elsewhere in the Madison for the Late Carboniferous transition from greenhouse to icehouse conditions. Isopachous calcite spar cements from the conduit zone are interpreted as the result of late-stage, Eocene hydrothermal fluid circulation. Stalactite–stalagmite pairs, flowstone, corallite, and moonmilk carry a signature similar to modern or Quaternary high-alpine meteoric water. Previous workers have determined separate hydrothermal and meteoric δ 18O/ δ 13C stable isotope fields for speleothems in caves in Carboniferous limestones from the Black Hills, South Dakota. We re-define the stable isotope ranges for meteoric and magmatic–hydrothermal calcites based on a comparison of stable isotope data from the Little Belt Mountains with those from the Black Hills. We further propose that the hydrothermal calcite end-member δ 18O composition is around −20‰ PDB, represented by the lowest oxygen isotope values from all data sets, with a corresponding δ 13C of about −7‰ PDB. Sr-isotope data from speleothems, Carboniferous limestone wall rocks, and from the igneous intrusion itself support the interpretation of an Eocene hydrothermal speleogenic event. The integration of petrographic and geochemical data shows that Lick Creek Cave is the result of polyphase speleogenesis in three major episodes: (1) Middle to Late Carboniferous, (2) Eocene, and (3) (sub-)Recent to Recent. The Carboniferous and (sub-)Recent to Recent speleogenesis appear epigenic, i.e., driven by surface-derived waters, whereas the Eocene event was hypogenic, i.e., driven by ascending hydrothermal waters. Each of the three major speleogenic events probably consisted of two or more distinct “phases”, but our database does not permit these phases to be resolved with certainty.

Palynological analyses of Eocene to Oligocene sediments from DSDP Site 338, Outer Vøring Plateau

Against the background of the profound global climatic shift from greenhouse to icehouse conditions during the Eocene–Oligocene transition, major geographic and oceanographic changes were taking place in the Norwegian–Greenland Sea region. The Vøring Plateau was a prominent structural feature which influenced the evolution of water mass circulation in the Nordic seas, and we present detailed palaeoenvironmental reconstructions of this structure. New palynological results suggest that shallow water inner-neritic environments were developed across parts of the Vøring Plateau during early Eocene times, with terrestrial and brackish water palynomorphs indicating that both basement highs to the north, and the crestal part of the Vøring Escarpment, may have been emergent. A transition from marginal-marine to open marine conditions occurred around 44 Ma ago, with the complete subsidence of the Vøring Plateau below sea level, facilitating inter-basinal surface water circulation and promoted a significant increase in photic zone fertility. Carbon sequestration associated with such enhanced productivity in the late Eocene Nordic seas may have contributed to declining Cenozoic atmospheric carbon dioxide levels, thence to declining global temperatures and the development of limited Northern Hemisphere continental ice on Greenland in the latest Eocene.

Distinguishing Milankovitch‐Driven Processes in the Rock Record from Stochasticity Using Computer‐Simulated Stratigraphy

Repetitive patterns of facies recurrence are frequently reported from the shallow‐water sedimentary rock record and are postulated to have been driven by orbital forcing on eustatic sea level. Consequently, multiple statistical techniques have been developed to evaluate whether patterns of stratigraphic succession are more consistent with a periodic sea level signal or are stochastic. Previous studies focused on development and/or application of such methods to test empirical geological records. However, the character of such records cannot be known a priori, as deposition, erosion, and preservation influence the resultant composition, nor can those records be readily manipulated to explore the sensitivity, robustness, and overall validity of statistical methods. Here we simulate carbonate layers using computer‐modeled successions generated by periodic sea level changes. The resulting stratigraphic records were then evaluated statistically. Thickness distributions of simulated lithofacies were compared to distributions predicted for Poisson processes, which by definition are not driven by cyclical sea level changes. Our results suggest that periodic processes produce stratigraphic thickness frequencies that are difficult to distinguish from random frequencies except under high‐magnitude sea level fluctuations. Similarly, autocorrelation fails to correctly recognize cyclic patterns in such simulated records. Models with high‐magnitude sea level fluctuations (icehouse conditions) had thickness frequencies that are suggestive of orbital forcing, whereas low‐magnitude sea level fluctuations (greenhouse conditions) appeared independent even though they were modeled using Milankovitch orbital forcing. The increasing evidence based on spectral data from real rock successions suggests that Milankovitch drivers are common in both icehouse and greenhouse periods. Because statistical approaches are unsuccessful in recognizing the cyclic driver of these simulated records, we infer that it is difficult to disprove independence from real stratigraphy even when orbital forcing is controlling the rock composition. Even in the necessarily simplified world of computer simulations, the numerous factors involved in depositing stratigraphic successions work to complicate or mask any periodic signal, thus generating the appearance of stochasticity in some successions.

Climatic plant power

Levels of atmospheric carbon dioxide constrain vegetation types and thus also non-biological uptake during rock weathering. That's the reasoning used to explain why CO2 levels did not fall below a certain point in the Miocene. Despite the occurrence of high rates of global silicate rock weathering likely to increase the drawdown of atmospheric CO2 — and hence temperatures — CO2 concentrations are thought to have stayed above about 180 parts per million throughout the past 24 million years (compared to about 385 p.p.m. today, and a 'pre-industrial' 280 p.p.m.). Based on terrestrial and geochemical carbon-cycle modelling simulations, Pagani et al. suggest that it may have been the actions of land plants that strongly attenuated long-term CO2 variability and helped prevent the onset of icehouse conditions. As CO2 falls to critically low levels due to increased chemical weathering, they find that vegetation activity is substantially reduced, compromising the capacity to further enhance silicate chemical weathering.

Increased seasonality through the Eocene to Oligocene transition in northern high latitudes

A profound global climate shift took place at the Eocene-Oligocene transition ( approximately 33.5 million years ago) when Cretaceous/early Palaeogene greenhouse conditions gave way to icehouse conditions. During this interval, changes in the Earth's orbit and a long-term drop in atmospheric carbon dioxide concentrations resulted in both the growth of Antarctic ice sheets to approximately their modern size and the appearance of Northern Hemisphere glacial ice. However, palaeoclimatic studies of this interval are contradictory: although some analyses indicate no major climatic changes, others imply cooler temperatures, increased seasonality and/or aridity. Climatic conditions in high northern latitudes over this interval are particularly poorly known. Here we present northern high-latitude terrestrial climate estimates for the Eocene to Oligocene interval, based on bioclimatic analysis of terrestrially derived spore and pollen assemblages preserved in marine sediments from the Norwegian-Greenland Sea. Our data indicate a cooling of approximately 5 degrees C in cold-month (winter) mean temperatures to 0-2 degrees C, and a concomitant increased seasonality before the Oi-1 glaciation event. These data indicate that a cooling component is indeed incorporated in the delta(18)O isotope shift across the Eocene-Oligocene transition. However, the relatively warm summer temperatures at that time mean that continental ice on East Greenland was probably restricted to alpine outlet glaciers.

Facies and Stratigraphic Anatomy of a Temperate Carbonate Sequence (Capo Colonna Terrace, Late Pleistocene, Southern Italy)

Abstract Accurate interpretation of ancient non-tropical carbonate systems requires the development of appropriate facies and sequence stratigraphic models. These models should account for the marked differences in sedimentation style among non-tropical carbonates formed under different energy settings. This paper presents data and a conceptual model of the sequence architecture of non-tropical carbonates of moderate energy environments based on the late Pleistocene deposits of Capo Colonna marine terrace (Calabria, southern Italy). These deposits have the advantage of being continuously exposed along a proximal–distal transect across the paleo-shelf and their geometry has not been disturbed by later tectonic activity. They consist of a transgressive–regressive (T–R) cycle whose genesis can confidently be related to one of the interstadial glacio-eustatic fluctuations of marine isotope stage (MIS) 5. Eight different mixed carbonate–siliciclastic and carbonate facies are present in the studied sequence. Of these, biogenic-dominated facies are invariably made up of skeletal remains of light-independent organisms plus or minus red calcareous algae (the heterozoan association). The relatively young age of the deposits allows calibration of the paleoecological significance of observed facies by direct comparison with modern analogues from the Mediterranean Sea. Four laterally persistent discontinuity surfaces were identified along the transect, and interpreted, from bottom to top, as (1) a basal ravinement surface, (2) a submarine hardground locally acting as a substrate for algal boundstones, (3) a regressive surface of marine erosion, and (4) a surface of subaerial exposure. Sequence stratigraphic analysis indicates that the preserved T–R cycle is asymmetric, with a thicker regressive portion. Highstand deposits can be subdivided into a shore-connected sandy prism and more distal autochthonous and parautochthonous skeletal accumulations and algal boundstones. During forced regression, a thin, sharp-based, strongly progradational unit of calcarenites was deposited above the regressive surface of marine erosion. The deposits of Capo Colonna terrace document the response of non-tropical, moderate-energy carbonate ramps to sea-level fluctuations during icehouse conditions. Their architecture can facilitate the interpretation of other successions from similar settings and provide an end member to be compared and contrasted with high-wave-energy cool-water carbonates.

Late Proterozoic plate tectonics and palaeogeography: a tale of two supercontinents, Rodinia and Pannotia

Abstract The plate tectonic and palaeogeographic history of the late Proterozoic is a tale of two supercontinents: Rodinia and Pannotia. Rodinia formed during the Grenville Event ( c . 1100 Ma) and remained intact until its collision with the Congo continent (800–750 Ma). This collision closed the southern part of the Mozambique Seaway, and triggered the break-up of Rodinia. The Panthalassic Ocean opened as the supercontinent of Rodinia split into a northern half (East Gondwana, Cathyasia and Cimmeria) and a southern half (Laurentia, Amazonia–NW Africa, Baltica, and Siberia). Over the next 150 Ma, North Rodinia rotated counter-clockwise over the North Pole, while South Rodinia rotated clockwise across the South Pole. In the latest Precambrian (650–550 Ma), the three Neoproterozoic continents – North Rodinia, South Rodinia and the Congo continents – collided during the Pan-Africa Event forming the second Neoproterozoic supercontinent, Pannotia (Greater Gondwanaland). Pan-African mountain building and the fall in sea level associated with the assembly of Pannotia may have triggered the extreme Ice House conditions that characterize the middle and late Neoproterozoic. Although the palaeogeographic maps presented here do not prohibit a Snowball Earth, the mapped extent of Neoproterozoic ice sheets favour a bipolar Ice House World with a broad expanse of ocean at the equator. Soon after it was assembled ( c . 560 Ma), Pannotia broke apart into the four principal Palaeozoic continents: Laurentia (North America), Baltica (northern Europe), Siberia and Gondwana. The amalgamation and subsequent break-up of Pannotia may have triggered the ‘Cambrian Explosion’. The first economically important accumulations of hydrocarbons are from Neoproterozoic sources. The two major source rocks of this age (Nepa of Siberia and Huqf of Oman) occur in association with massive Neoproterozoic evaporite deposits and in the warm equatorial–subtropical belt, within 30° of the equator.

Stability of regional brachiopod diversity structure across the Mississippian/Pennsylvanian boundary

The middle Carboniferous was an interval of global change when the climate was transitioning from greenhouse to icehouse conditions. Field collections of paleotropical brachiopod assemblages across the Mississippian/Pennsylvanian boundary reveal a taxonomic turnover event in which the overall diversity structure is conserved, despite an apparent regional extinction of 63% of latest Mississippian genera and an apparent regional origination of 50% of earliest Pennsylvanian. An analysis of the global ranges of the brachiopods encountered in the field reveals that turnover was driven primarily by extirpation and immigration rather than true extinctions and originations. Taxonomic richness and evenness are indistinguishable between the latest Mississippian and earliest Pennsylvanian stages. Additive diversity partitioning shows that the within-collection, between-collections (i.e., within-bed), and between-bed diversity components do not change across the Mississippian/Pennsylvanian boundary for richness or evenness. Rank-abundance plots of genera show the same distribution for both stages, but with no correlation between the Mississippian abundances of range-through genera and their abundance in the Pennsylvanian. Detrended correspondence analysis shows a major change in taxonomic composition across that Mississippian/Pennsylvanian boundary and consistency in the general gradient along which genera were distributed. An estimation of spatio-temporal heterogeneity of taxonomic composition within each stage reveals that the earliest Pennsylvanian was significantly more homogeneous. These results suggest that middle Carboniferous brachiopod assemblages from tropical shallow-water carbonate platform settings were organized by some factor that was independent of the specific taxa present. Furthermore, the increased homogeneity in taxonomic composition in the Morrowan did not affect the overall diversity structure. Strong competitive interactions among taxa do not appear to be important in determining the taxonomic compositions and abundances of brachiopod stage-level assemblages.

About this title - Devonian Change: Case Studies in Palaeogeography and Palaeoecology

The Devonian was a critical period with respect to the diversification of early terrestrial ecosystems. The geotectonic setting was characterized by the switch from the post-Caledonian to the pre-Variscan situation. Plant life on land evolved from tiny tracheophytes to trees of considerable size in combination with a global increase in terrestrial biomass, and vertebrates started to conquer the land. Extensive shallow-marine areas and continental lowlands with a wide range of different habitats existed. These are preserved in a large number of basins all around the world. Climate change finally led from greenhouse to icehouse conditions towards the end of the Devonian. Rapid evolution of terrestrial ecosystems and climate change had a pronounced influence on sedimentation and biodiversity, not only in the terrestrial, but also in the marine realm. This volume contains case studies from Australia, China, Europe, South America and North America, and individual palaeoecosystems and their components have been investigated in different palaeogeographic settings that contribute to a much better understanding of the Devonian Period. This is a contribution to the IGCP 499 project on “Devonian land-sea interaction: evolution of ecosystems and climate”.

Facies-dependent δ13C variation from a Cryogenian platform margin, South Australia: Evidence for stratified Neoproterozoic oceans?

Analysis of a Cryogenian interglacial platform margin in the Adelaide Geosyncline reveals a strong carbonate δ 13C-facies relationship. Detailed chronostratigraphic correlation between sections ranging from shallow platform facies to deep basinal facies indicates the presence of a carbon isotopic gradient of between 8 and 11‰ in time-equivalent strata. Shallow-water back-reef facies have δ 13C values up to 8.2‰, while equivalent basinal sediments have δ 13C values between − 3 and 0‰. Allochthonous blocks that have been transported from the platform margin into basinal environments retain their heavy δ 13C values (4–9‰) and are surrounded by basinal calcareous shales with light δ 13C values (ave. 0.8‰). The regional and stratigraphic consistency of these δ 13C trends suggests a primary marine origin. We interpret this δ 13C-facies correlation to be the result of ocean stratification/stagnation that significantly reduced the rate of deep-ocean ventilation and produced a large deep-water, organically derived carbon reservoir. We suggest that stratification may have been a persistent feature of the global ocean throughout much of Neoproterozoic time. Ocean stratification may explain many of the unusual features that characterise the sedimentary record of this era, including large-scale δ 13C variation, extreme climatic fluctuations, and the presence of cap carbonates. A highly variable climatic regime would be expected with the development of a large deep-water carbon reservoir. Small changes in ocean circulation could rapidly transfer or remove large volumes of carbon to and from the surface-ocean and atmospheric reservoirs, leading to intense greenhouse or icehouse conditions respectively.

Three-dimensional sequence analysis of a subsurface carbonate ramp, Mississippian Appalachian foreland basin, West Virginia, USA

Well-cuttings, wireline logs and limited core and outcrop data were used to generate a regional, three-dimensional sequence framework for Upper Mississippian (Chesterian), Greenbrier Group carbonates in the Appalachian foreland basin, West Virginia, USA. The resulting maps were used to document the stratigraphic response of the basin to tectonics and to glacio-eustasy during the transition into ice-house conditions. The ramp facies include inner ramp red beds and aeolianites, lagoonal muddy carbonates, mid-ramp ooid and skeletal grainstone shoal complexes, and outer ramp wackestone–mudstone, that grades downslope into laminated silty lime mudstone. The facies make up fourth-order sequences, a few metres to over 90 m (300 ft) thick. The sequences are bounded along the ramp margin by lowstand sandstones and calcareous siltstones. On the ramp, sequence boundaries are overlain by thin transgressive siliciclastics and aeolianites, and only a few are calichified. Maximum flooding surfaces on the outer ramp lie beneath deeper water facies that overlie lowstand to transgressive siliciclastic or carbonate units. On the shallow ramp, maximum flooding surfaces overlie siliciclastic-prone transgressive systems tracts, that are overlain by highstand carbonates with significant grainstone units interlayered with lagoonal lime mudstones. The fourth-order sequences are the major mappable subsurface units; they are bundled into weak composite sequences which are bounded by red beds. In spite of differential subsidence rates across the foreland basin (1 to 3 cm/k.y. up to 25 cm/k.y.), eustatic sea-level changes controlled regional sequence development. Thrust-load induced differential subsidence of fault-blocks, coupled with in-plane stress, controlled the rapid basinward thickening of the depositional wedge, whose thickness and facies were influenced by subtle structures such as arches trending at high angles as well as parallel to the margin.

Neoproterozoic glaciation in the Earth System

The Neoproterozoic contains severe glacial intervals (750–580 Ma) including two extending to low palaeomagnetic latitudes. Paucity of radiometric dates indicates the need for chronostratigraphic tools. Whereas the marine 87 Sr/ 86 Sr signatures show a steady rise, δ 13 C fluctuates, the most reproducible variations being negative signatures in carbonate caps to glacial units, but more diagenetic work is needed. Four conceptual models for the icehouse conditions are contrasted: Zipper-Rift Earth (diachronous glaciation related to continental rift margins), High-tilt Earth (high-obliquity and preferential low-latitude glaciation), Snowball Earth (extreme glaciation related to runaway ice–albedo feedback) and Slushball Earth (coexistence of unfrozen oceans and sea-level glaciers in the tropics). Climate models readily simulate runaway glaciation, but the Earth may not be able to recover from it. The Slushball state requires more extensive modelling. Biogeochemical models highlight the lack of CO 2 buffering in the Neoproterozoic and the likely transition from a methane- to a CO 2 -dominated climate system. Relevant processes include tropical weathering of volcanic provinces, and new land biotas stimulating both clay mineral formation and P delivery to the oceans, facilitating organic C burial. Hence a step change in the Earth System was probably both facilitated by organisms and responsible for moderating Phanerozoic climate.

The major-ion composition of Permian seawater

The major-ion (Mg 2+, Ca 2+, Na +, K +, SO 4 2−, and Cl −) composition of Permian seawater was determined from chemical analyses of fluid inclusions in marine halites. New data from the Upper Permian San Andres Formation of Texas (274–272 Ma) and Salado Formation of New Mexico (251 Ma), analyzed by the environmental scanning electron microscopy (ESEM) X-ray energy-dispersive spectrometry (EDS) method, along with published chemical compositions of fluid inclusions in Permian marine halites from North America (two formations of different ages) and the Central and Eastern European basins (eight formations of four different ages) show that Permian seawater shares chemical characteristics with modern seawater, including SO 4 2− > Ca 2+ at the point of gypsum precipitation, evolution into Mg 2+-Na +-K +-SO 4 2−-Cl − brines, and Mg 2+/K + ratios ∼5. Permian seawater, however, is slightly depleted in SO 4 2− and enriched in Ca 2+, although modeling results do not rule out Ca 2+ concentrations close to those in present-day seawater. Na + and Mg 2+ in Permian seawater are close to (slightly below) their concentrations in modern seawater. Permian and modern seawater are both classified as aragonite seas, with Mg 2+/Ca 2+ ratios >2, conditions favorable for precipitation of aragonite and magnesian calcite as ooids and cements. The chemistry of Permian seawater was modeled using the chemical composition of brine inclusions for three periods: Lower Permian Asselian-Sakmarian (296–283 Ma), Lower Permian Artinskian-Kungurian (283–274 Ma), and Upper Permian Tatarian (258–251 Ma). Parallel changes in the chemistry of brine inclusions from equivalent age evaporites in North America, Central Europe, and Eastern Europe show that seawater underwent secular variations in chemistry over the 50 million years of the Permian. Modeled SO 4 2− concentrations are 20 mmol per kg H 2O (mmolal) and 19 mmolal in the Asselian-Sakmarian and Artinskian-Kungurian, with higher concentrations in the Upper Permian Tatarian (23 mmolal). Modeled Ca 2+ is at or above its concentration in modern seawater throughout the Permian. Mg 2+ is close to (slightly below) its concentration in modern seawater (55 mmolal) in the Asselian-Sakmarian (52 mmolal), and Tatarian (52 mmolal), but slightly higher than modern seawater in the Artinskian-Kungurian (60 mmolal). Mg 2+/Ca 2+ ratios are 3.5 (total range = 2.7 to 5.5) in the Lower Permian and rose slightly to 3.7 (total range = 3.1 to 5.8) in the Upper Permian, primarily due to decreases in Ca 2+. These results are consistent with models that predict oscillations in the major-ion composition of Phanerozoic seawater on the basis of changes in the midocean ridge/river water flux ratio driven by changes in the rate of midocean ridge crust production. The Permian was characterized by low sea levels, icehouse conditions, and southern hemisphere glaciation. Such conditions, analogous to the present ice age, and the similarities between Permian seawater and modern seawater, all suggest that general Phanerozoic supercycles, driven by mantle convection and global volcanicity, also control the major-ion chemistry of seawater.

Paleoclimatic inferences from paleopedology and magnetism of the Permian Maroon Formation loessite, Colorado, USA

The Maroon Formation in the eastern Eagle basin (Colorado) consists of >700 m of lithified loess with >200 paleosols interpreted as Protosols and Argillisols on the basis of field, petrographic and geochemical data. Additionally, magnetic susceptibility aids assessment of the intensity of pedogenesis. Bulk magnetic susceptibility (χ b ) through the section repeatedly fluctuates between low values (average 3.51 × 10 −8 ± 1.59 × 10 −8 m 3 /kg) in parent loessite and higher values (average 5.70 × 10 −8 ± 2.70 × 10 −8 m 3 /kg) in paleosols. Moreover, magnetic susceptibility positively correlates with abundance of clay-sized material as well as Al 2 O 3 and K 2 O and effectively distinguishes Protosols and Argillisols. Low- temperature demagnetization indicates the presence of ultra-fine-grained magnetite. The integration of geochemical, petrographic, and rock magnetic data suggest that changes in magnetic susceptibility reflect pedogenesis and relate primarily to climate- and time-dependent pedogenic production and concentration of ultra-fine- grained magnetite. The Maroon Formation loess and associated soils accumulated in an overall arid system, documented in part by formation of incipiently formed paleosols developed into Argillisols by eolian clay and carbonate additions rather than by in situ clay formation. However, the paleosols showing bulk magnetic susceptibility values of greater than 200 χ b document a high- frequency (10 4 −10 5 yr) fluctuation between arid times of loess accumulation and slightly wetter times of reduced silt influx and resultant pedogenesis. This fluctuation likely reflects glacial-interglacial climate shifts that operated in low-latitude Pangea during icehouse conditions. These results suggest that climate-related magnetic susceptibility variations within loess successions can be preserved and useful in very ancient (pre– Pliocene–Pleistocene) sequences.

Paleozoic stromatactis and zebra carbonate mud-mounds: Global abundance and paleogeographic distribution

Carbonate mud-mounds with zebra and stromatactis structures are present in every Paleozoic system and series, but are more common in Devonian and Carboniferous deposits, reaching their acme in Mississippian System (lower Carboniferous) rocks. Global distributions illustrate that mud-mounds spanned the planet ranging from tropical to polar circles. Such a wide latitudinal span signifies that they not only grew in and occupied warm depositional environments, but also in settings where oceanic waters were cold and seasonally light limited. Moreover, their proliferation during the Devonian and Carboniferous was at a time when planet-wide climatic ice-house conditions are thought to have prevailed. Mud-mounds, therefore, may also be products of cool and cold-water carbonate sedimentation.

Microbial boundstone dominated carbonate slope (Upper Carboniferous, N Spain): Microfacies, lithofacies distribution and stratal geometry

The Carboniferous, particularly during the Serpukhovian and Bashkirian time, was a period of scarce shallow-water calcimicrobial-microbialite reef growth. Organic frameworks developed on high-rising platforms are, however, recorded in the Precaspian Basin subsurface, Kazakhstan, Russia, Japan and Spain and represent uncommon occurrences within the general trend of low accumulation rates and scarcity of shallow-water reefs. Sierra del Cuera (Cantabrian Mountains, N Spain) is a well-exposed high-rising carbonate platform of Late Carboniferous (Bashkirian-Moscovian) age with a microbial boundstone-dominated slope dipping from 20° up to 45°. Kilometer-scale continuous exposures allow the detailed documentation of slope geometry and lithofacies spatial distribution. This study aims to develop a depositional model of steep-margined Late Paleozoic platforms built by microbial carbonates and to contribute to the understanding of the controlling factors on lithofacies characteristics, stacking patterns, accumulation rates and evolution of the depositional architecture of systems, which differ from light-dependent coralgal platform margins. From the platform break to depths of nearly 300 m, the slope is dominated by massive cement-rich boundstone, which accumulated through the biologically induced precipitation of micrite. Boundstone facies (type A) with peloidal carbonate mud, fenestellid and fistuliporid bryozoans, sponge-like molds and primary cavities filled by radiaxial fibrous cement occurs all over the slope but dominates the deeper settings. Type B boundstone consists of globose centimeter-scale laminated accretionary structures, which commonly host botryoidal cement in growth cavities. The laminae nucleate around fenestellid bryozoans, sponges, Renalcis and Girvanella-like filaments. Type B boundstone typically occurs at depths between 20–150 m to locally more than 300 m and forms the bulk of the Bashkirian prograding slope. The uppermost slope boundstone (type C; between 0 and 20–100 m depth) includes peloidal micrite, radiaxial fibrous cement, bryozoans, sponge molds, Donezella, Renalcis, Girvanella, Ortonella, calcareous algae and calcitornellid foraminifers. From depths of 80–200 m to 450 m, 1–30 m thick lenses of crinoidal packstone, spiculitic wackestone, and bryozoan biocementstone with red-stained micrite matrix are episodically intercalated with boundstone and breccias. These layers increase in number from the uppermost Bashkirian to the Moscovian in parallel with the change from a rapidly prograding to an aggrading architecture. The red-stained strata share comparable features with Lower Carboniferous deeper-water mud-mound facies and were deposited during relative rises of sea level and pauses in boundstone production. Rapid relative sea-level rises might have been associated with changes in oceanographic conditions not favourable for thecalcimicrobial boundstone growth, such as upwelling of colder, nutrient-rich waters lifting the thermocline to depths of 80–200 m. Downslope of 150–300 m, boundstones interfinger with layers of matrix-free breccias, lenses of matrix-rich breccias, platform- and slope-derived grainstone and crinoidal packstone. Clast-supported breccias bound by radiaxial cement are produced by rock falls and avalanches coeval to boundstone growth. Matrix-rich breccias are debris flow deposits triggered by the accumulation of red-stained layers. Debris flows develop following the relative sea-level rises, which favour the deposition of micrite-rich lithofacies on the slope rather than being related to relative sea-level falls and subaerial exposures. The steep slope angles are the result of in situ growth and rapid stabilization by marine cement in the uppermost part, passing into a detrital talus, which rests at the angle of repose of noncohesive material. In the Moscovian, the aggradational architecture and steeper clinoforms are the result of increased accommodation space due to tectonic subsidence and due to a reduction of slope accumulation rates (from 240±45−605±35 m/My to 130±5 m/My). The increasing number of red-stained layers and the decrease of boundstone productivity are attributed to environmental changes in the adjacent basin, in particular during relative rises of sea level and to possible cooling due to icehouse conditions. The geometry of the depositional system appears to be controlled by boundstone growth rates. During the Bashkirian, the boundstone growth potential is at least 10 times greater than average values for ancient carbonate systems. The slope progradation rates (nearly 400–1000 m/My) are similar to the highest values deduced for the Holocene Bahamian prograding platform margin. The fundamental differences with modern systems are that progradation of the microbial-boundstone dominated steep slope is primarily controlled by boundstone growth rates rather than by highstand shedding from the platform top and that boundstone growth is largely independent from light and controlled by the physicochemical characteristics of seawater.