Biostratigraphic Boundaries Research Articles

Major shifts in biogeographic regions of freshwater fishes as evidence of the Anthropocene epoch.

Animals and plants worldwide are structured in global biogeographic regions, which were shaped by major geologic forces during Earth history. Recently, humans have changed the course of events by multiplying global pathways of introduction for nonindigenous species and propagating local species extirpations. Here, we report on how introductions and extirpations have changed the distributions of freshwater fishes worldwide and how it affected their natural biogeographic regions. We found major shifts in natural regions, with the emergence of an intercontinental region arising from the fusion of multiple faunas, which we named Pan-Anthropocenian Global North and East Asia (PAGNEA). The PAGNEA region is evocative of the Pangea supercontinent, as flows of introductions show that dispersal has become possible again across multiple continents, suggesting that human activities have superseded natural geological forces. Our results constitute evidence on the expected modification of biostratigraphic boundaries based on freshwater fish, which are abundant in the fossil record, thereby supporting the concept of the Anthropocene epoch.

Pennsylvanian-age plant macrofossil biostratigraphy in tropical Pangaea: uniformitarianism, catastrophes and the ‘Cantabrian’ problem

Abstract Upper Carboniferous stages in terrestrial strata are consistently recognizable throughout the Euramerican Realm based on the ranges of plant macrofossils rather than named biozones. Uniformitarianism is an invaluable principle used to understand much of Earth's history. However, it has been recognized that relatively short time intervals exist when major changes occurred in the biota and sedimentation style at a non-uniformitarian time scale. Many well-defined Upper Carboniferous stage boundaries are located at such events. The Cantabrian Stage was proposed in 1969 based on the assumption that the ‘Florensprung’ (‘floral jump’) of Gothan, a dramatic change in terrestrial floras at the traditional Westphalian–Stephanian boundary, indicated that strata were missing, and the postulated ‘gap’ had to be closed by finding strata representing this ‘missing’ time elsewhere. The suggestion of a ‘gap’ or hiatus at this level reflects uniformitarian thinking, but is incorrect. As recent work has shown that a drastic climatic change reached a threshold in the tropical palaeoequatorial parts of Pangaea at the Westphalian–Stephanian (approximately the Moscovian–Kasimovian) boundary as a consequence of a drying trend, floras changed rapidly, and the traditional Westphalian–Stephanian boundary is thus one of the most easily recognizable biostratigraphic boundaries in Pennsylvanian terrestrial beds. The Cantabrian Stage, proposed to fill the non-existent gap, does not exist and cannot be recognized, either in its type area or elsewhere.

Ages of the Ediacaran Volyn-Brest trap volcanism, glaciations, paleosols, Podillya Ediacaran soft-bodied organisms, and the Redkino-Kotlin boundary (East European Craton) constrained by zircon single grain U-Pb dating

Combined single zircon crystal CA-ID-TIMS U-Pb dating, micropaleontological study, and quantitative XRD analysis were used to correlate sections of the Podillya-Moldova Ediacaran succession with sections of the Volyn Large Igneous Province (Volyn LIP). Five U-Pb zircon ages were obtained from Volyn LIP rocks, and four from three distinct Podillya bentonite horizons. Biostratigraphic determinations were made for one section in Volyn, and two sections in Podillya. It was concluded that Volyn LIP activity had two phases. A first phase of mafic and felsic magmatism was characterized by eruption of numerous basalt lava flows, and lasted from c. 580 to 567 Ma; a second phase of pyroclastic felsic magmatism, with no lavas, lasted from 567 to c. 545 Ma. The two phases can be distinguished in the quantitative mineral composition of rocks from the Kobryn 1K borehole in western Belarus. In the central area of the Volyn LIP, near Brest, rocks of the felsic phase were removed by erosion before deposition of the Kotlin subgroup. The Podillya Ediacaran succession, from the top of the Hrushka formation to the top of the Kalyus beds, (i.e. the Mohyliv-Podilsky group), hosts soft-bodied organisms. This succession corresponds to the felsic phase of the Volyn LIP. The age of the Redkino-Kotlin subgroup biostratigraphic boundary, at the base of the Nagoryany formation, is constrained at 555–545 Ma, close to the end of the Volyn LIP activity. The newly discovered glaciation in Volyn is dated as c. 567 Ma. The period of development of kaolinitic paleosols on top of the Volyn LIP effusive rocks is dated at c. 570–550 Ma.

Foraminifers in upper Viséan–lower Serpukhovian limestones (Mississippian) from South Wales: regional correlation and implications for British foraminiferal zonal schemes

Foraminiferal assemblages have been recorded in sections from the Gower Peninsula and North Crop in South Wales, of Holkerian (Stormy Limestone Formation), late Asbian to lower late Brigantian (Oxwich Head Limestone Formation) and late Brigantian (Oystermouth Formation) age. The absence of lower Asbian foraminifers in the studied successions is confirmed. These formations and biostratigraphic boundaries are correlated with those in northern England, where similar foraminiferal assemblages are recorded. It is considered that Loeblichia paraammonoides does not occur in Wales. Its record is based on a misidentification of L . ammonoides and its presence in Britain is questioned. The distribution of Bibradya in Britain seems to be restricted to the late Asbian. Archaediscus at angulatus stage and ancestral forms of Palaeotextularia are confirmed in the uppermost Holkerian. Ancestral forms of Koskinobigenerina and Climacammina recorded below the upper Asbian and lower Brigantian, respectively, are considered to be misidentifications. Archaediscus at tenuis stage recorded from the Oystermouth Formation can form a plausible marker for the equivalent Three Yard Limestone and Four Fathom Limestone members in northern England. The acme of discoidal archaediscids is equivalent to the Four Fathom Limestone Member. In contrast to a ready comparison with foraminiferal microfaunas from northern England, comparison with North Wales is problematic. Nevertheless, it would appear that most assemblages previously cited as early Asbian, would now likely be reassigned to the late Asbian, similar to the observations in South Wales.

Cretaceous/Palaeogene boundary transition induced lattice defects in illite and kaolinite associated with the Um-Sohryngkew river section, Meghalaya, India

Cetaceous-Palaeogene Boundary (KPB) transition records restricted to a unique, continuous, late Maastrichtian-early Danian, shallow marine Um Sohryngkew river succession (bearing planktonic foraminifers' CF4 to P1a biozones) have not been studied in detail. Earlier studies sub-divided it into lower, middle and upper parts, defined by the dominance of illite> illite/smectite; illite>kaolinite> montmorillonite and kaolinite > montmorillonite assemblages, respectively. However, limited studies were carried out on the KPB transition events, particularly for those reproduced in the illite (C2/m) and kaolinite (P1) lattice structures. Thus, illite and kaolinite cell parameters and interatomic distances have been studied to understand KPB transition events. For this purpose, layer-wise clay (0.2–2 μm) separates were analyzed by X-ray diffraction method and subjected to Rietveld refinement. Obtained data show substantial expansion in the c-axis of the kaolinite grains associated with the shaly marlite (P0 biozone) layer. Also, b- and c- axes of the kaolinite associated with the yellowish brown (CF3 biozone) layer indicate expansion and contraction, respectively. Moreover, Al–O bond in the kaolinite reflects contraction. Obtained cell parametric data and interatomic distances when plotted across the stratigraphic succession show notable contraction along b- and c- axes of the illite associated with the yellowish brown and shaly marlite layers of the CF3 and P0 biozones, respectively. The illite containing 26 atoms, linked to 38 and 34 bonds present in the intensely weathered lower and middle parts of the CF4 biozone, respectively. The illite in the moderately weathered upper part of the CF4 – CF1 biozones bearing 32 atoms and 44 bonds, represent Si–K and Al–K bond distortions together with the loss of Si4+ and K+. The coprolite-bearing shaly marlite layer (between the CF1 and P0 biozones) shows drastic changes in the illite structure, containing 48 atoms and 44 bonds, characterized by K–O, Si–K and Al–O bond contractions along with the retention of K+ and Al3+. Moreover, Al–O, Si–K, Al–K and Al–Al bonds connected strongly due to reinforcement of Al ions. A very low degree of alteration is noticed in case of preceding yellowish brown (CF3 biozone) layer. Owing to the weakening of the Al–O and Si–O bonds in the intensely weathered illite lattices of this layer show distortion as well as Al3+ and Si4+ losses, suggest incidence of thermal high. Sudden degeneration of illite lattices noticed in the shaly marlite layer is also marked with the planktonic foraminifers’ extinction, signifying biostratigraphic boundary. Thus, contraction in the c- axis as well as notable decrease in the bond lengths of illite (in this layer) is linked to KPB transition which includes major tectonic events, sea level changes and global-climatic changes caused environmental stress in the sea shelf. The substantial distortion in the illite lattices of the biozone CF2 corresponds to the most vigorous second phase of the Deccan volcanic eruption that occurred during 65.4–65.2 Ma.

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Возрастное скольжение слоев: факты и геологические следствия (к 150-летию фундаментальной работы Н.А. Головкинского)

The fundamental facial law, determining the relationship between facies of sedimentary rocks in the sedimentary basin in lateral and vertical extensions, was formulated by the Russian geologist N.A. Golovkinsky a century and a half ago. Theoretical statements and views proposed by Golovkinsky have not lost their importance and relevance nowadays. In the article considered an important aspect of diachroneity (heterochroneity) of layer associations and their litho- and biostratigraphic boundaries. The methodological approach of measuring its degree (the window of age moving) is proposed. Golovkinskiy’s conceptions are developing fruitfully within the framework of Seismostratigraphy and Sequence Stratigraphy, and their main content remains in demand in the light of new realities of cognitive process (nonlinear science, NBICS convergence, endovision).

Microbiostratigrapy of the Paleocene-Lower Eocene Sequences in the Bibi Hakimeh 2 Subsurface Section Located in the SW of Iran

This paper investigates the biostratigraphy of the Palaeocene-Lower Eocene sequences in the Bibi Hakimeh 2 subsurface/subterranean sections located in the Bibi Hakimeh Oil Field. The Palaeocene-Lower Eocene sequences are induced as part of the Pabdeh Formation. They are 49 m in thickness and are made up of clayey limestone with chert intercalations. Biostratigraphic investigations identified 10 genera and 51 species of Planktonic Foraminifera which with respect to the specific planktonic species and according to the newest Wade Biozonation led to the determination of 9 biozones and 4 subzones. The Palaeocene-Early Eocene biostratigraphic boundary determined previously in accordance with the Wynd Biozonation was reviewed with respect to the new biozonation. Moreover, the impression of the PETM on the Planktonic Foraminifera found in the Bibi Hakimeh subsurface section, has been occurred at P/E boundary on global scale was studied. Based on this research we can conclude the P/E biostratigraphic boundary border is occurred ranges between 2125 - 2124 meters in Pabdeh formation.

Chemostratigraphy and biostratigraphy of Devonian, Carboniferous and Permian sediments encountered in eastern Saudi Arabia: An integrated approach to reservoir correlation

The following study was completed on inorganic geochemical data acquired from 850 core and cuttings samples taken from Devonian, Carboniferous and Permian sediments encountered in six wells in the arabian gulf, eastern saudi arabia. The principal objective of the study was to employ chemostratigraphy to produce a ‘robust’ high resolution correlation scheme. A secondary aim was to integrate this with existing biostratigraphic data as part of a multidisciplinary approach to reservoir correlation.Data were derived for 50 elements using ICP-OES (Inductively Coupled Plasma – Optical Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma – Mass Spectrometry), but definition of chemostratigraphic boundaries are based on changes in the following key element ratios: Cr/P, Th/Nb, Th/Ta, Zr/Th, Zr/P, Nb/Ti and Nb/P. Variations in these parameters are largely dependent on changes in source/provenance, which affects the relative concentrations of detrital heavy minerals.To avoid geochemical variations relating to changes in grain size/lithology, separate frameworks were constructed for data derived from sandstone and mudrock lithologies. Both comprise a hierarchical order of zones, subzones and divisions, with subdivisions also being identified in the sandstone dataset. The schemes are considered to be ‘robust’ as chemozones (general term used to refer to any zone, subzone, division or subdivision) are clearly defined using geochemical profiles and binary diagrams and most are correlative between three or more wells. Given the similarities in definitions of zones, subzones and divisions in the two schemes, and the fact that these chemozones are considered to be roughly time equivalent, both were combined to produce a ‘final’ chemostratigraphy scheme. In a further stage of interpretation, the correlation was refined by integrating the interpretations of this chemostratigraphy study with lithostratigraphic markers (identified by significant trends in GR that exist in two or more wells) and an existing correlation based on biostratigraphy.In the Tawil and Jauf formations there are obvious inconsistencies between the placement of chemostratigraphic and biostratigraphic boundaries, with some subzones occurring within different palynozones in adjacent wells. Such disparities may be explained by the existence of separate fluvio-deltaic/shallow marine sandstones having a similar source/provenance but being deposited in different parts of the study area at different times. This interpretation is supported by the available seismic data showing that the Jauf, and to a lesser extent the Tawil, formations take the form of laterally discontinuous sandstones. By contrast, the chemozones associated with the overlying Jubah Formation, Berwath Formation, Unayzah Group (Juwayl and Nuayyim formations) and Base Khuff Clastics Member show a very close match with the placement of palynozones in each well and were probably deposited at a similar time on both field and subregional scales.

Conodonts from the Sesong Slate and Hwajeol Formation (Guzhangian to Furongian) in the Taebaeksan Basin, Korea

AbstractAbundant conodont elements have been recovered from the Hwajeol Formation, to allow five zones to be erected: Proconodontus, Eoconodontus notchpeakensis, Cambrooistodus minutus, Cordylodus proavus, and Fryxellodontus inornatus‐Monocostodus sevierensis‐Semiacontiodus lavadamensis zones, in ascending order. More confident biozones are recognized in the Sesong Slate and lower Hwajeol Formation in the Makgol section, a part of the southern limb of the Baekunsan syncline, Taebaeksan Basin, Korea, especially focusing on the conodont biostratigraphic boundary of two units, and the subdivision potential of the previous “Proconodontus Zone”, lowermost biozone of the Hwajeol Formation. Similarly, only a few conodont elements recovered from upper 14.5 m interval, namely the Furongian portion of the Sesong Slate, in the Makgol section did not allow erection of a biozone. Nevertheless, this part of the unit plus the basal 2.5 m interval of the Hwajeol Formation is characterized by the occurrence of Prooneotodus rotundatus (Druce and Jones), Teridontus nakamurai (Nogami), Phakelodus elongatus (An) and Phakelodus tenuis Müller, This interval marks the early Furongian “Prooneotodus rotundatus Zone”. The rest of the measured section yielded relatively abundant conodonts, so three conodont biozones are proposed, based on the successive appearance of key species: Proconodontus tenuiserratus, Proconodontus posterocostatus, and Proconodontus muelleri zones, in ascending order, and thus allowing subdivision of the previous “Proconodontus Zone”. The four conodont biozones are correlated with the relevant biozones of North and South China, and North America.

浅析P/T界线层中牙形刺Hindeodus parvus的标准性

本文讨论了中国(江西修水东岭、四川广元上寺和浙江长兴煤山)、印度(克什米尔Guryul Ravine)、前苏联(阿塞拜疆Achura)等地的海相P/T界线层中牙形刺Hindeodus parvus，及其首次出现的点位，阐述了其标准性对海相P/T界线划分的重要意义。建议以产于单相且沉积连续剖面，主齿和细齿均保存完好的Hindeodus parvus的首次出现的点位，作为P/T生物地层界线划分的标准。The article discusses the Conodont Hindeodus parvus in marine P/T boundary layer in China (Dongling Village of Xiushui County, Jiangxi Province; Shangsi Village of Guangyuan City, Sichuan Province; Meishan Town of Changxing County, Zhejiang Province), India (Kashmir Guryul Ravine), and Union of Soviet Socialist Republics (Azerbaijan Achura), and the point location where they appear for the first time. It also explains the important significance of setting criterion for demar-cating marine P/T boundary layer, suggesting that the partition criterion for P/T biostratigraphic boundary would be set on the point locations where they firstly appear in single-phase sediment- successive section, with well-preserved cardinal tooth and serration of Hindeodus parvus.

Boundary Between Upper Cretaceous and Lower Paleocene in the Zagros Mountain Ranges of Southwestern Iran

Abstract:In the present study, over 3000 m of Upper Cretaceous sediments (Tarbur Formation) in seven stratigraphic, columnar sections were studied. The area is located in the Zagros mountain ranges of southwestern Iran, attributed to the converging continental Arabian Shield, and is interpreted as the result of subduction and collision. Based on foraminiferal studies of the Tarbur Formation in the sections, we have established many new biozones in the stratigraphic sections. However, investigations of the biozones indicate that there is no lithostratigraphic variation between the Upper Cretaceous and Lower Paleocene sediments in some of the studied sections. The concept of widespread formations and lithologic correlations is not applicable in this area, probably as a result of the tectonic unrest at that time. The biostratigraphic boundary between the Cretaceous and the Lower Paleocene is nevertheless well defined by the intraformational boundary in the sections.

Lutetian magnetostratigraphic calibration of larger foraminifera zonation (SBZ) in the Southern Pyrenees: The Isuela section

In this paper, we present a chronostratigraphic calibration of the shallow benthic zones (SBZs) of larger foraminifera during Lutetian times based on new magnetostratigraphic data (Isuela section) and two new densely sampled biostratigraphic sections (Isuela and Gabardiella). These sections are located in the External Sierras (Southern Pyrenees) and represent an ideal location to avoid previous chronostratigraphic problems. 1) They belong to a shallow carbonate shelf environment with abundant larger foraminifera, which ensure the reliability of SBZ data. 2) The time span covers almost the entire Lutetian interval. 3) The profiles are sufficiently long to establish trustworthy local scales, both the SBZs (520m and 760m in Isuela and Gabardiella respectively) and the paleomagnetic local polarity sequence (LPS) (520m). 4) The excellent outcropping conditions also permit a detailed paleomagnetic and biostratigraphic sampling to be performed.The LPS at the Isuela section was built after sampling every 2.6m. In total, more than 359 samples were thermal (244) and alternating field (115) demagnetized. Characteristic remanent magnetization (ChRM) usually dropped at 560°C. Only 43% the samples (359 demagnetizations) gave reliable directions (high quality) for the LPS. A further short section (Gas line) was also cored (38 samples in 90m) to obtain more detailed data for the upper part of the section. At the Isuela section, the results allow three pairs of trustable magnetozones to be assembled within the Lutetian. A further 255 samples were taken for biostratigraphic purposes, 55 sites at the Isuela and 52 at the Gabardiella sections being determinant. This density enabled a thorough study of changes in fossil associations to be performed. Assuming a constant accumulation rate within the chron boundaries, absolute ages have been estimated for the biostratigraphic boundaries. Combining all these data allows the previous calibration of SBZ during Lutetian times in the geological time scale to be refined. In particular, with this new data, we identify the SBZ 13/SBZ 14 boundary at 44.46Ma reaching C20r; a boundary interval spanning from SBZ 14 to SBZ 15 (41.36 and 42.45Ma) is found within C20n and C19n; and, finally, the top of SBZ 15 can be accurately located at 41.12Ma (C19r).

Late Paleozoic macrofloral assemblages from Weibei Coalfield, with reference to vegetational change through the Late Paleozoic Ice-age in the North China Block

Abstract Based on investigations of seven stratigraphic sections in the Weibei Coalfield, a typical coal basin in the North China Block, the succession of Late Paleozoic plant macrofossil assemblages from the Benxi (or Penchi in old spelling), Taiyuan, Shanxi, Lower Shihhotse and Upper Shihhotse Formations were redefined. These zones are the: Paripteris spp . – Linopteris spp . – Conchophyllum richthofenii Assemblage (Benxi Fm.), Neuropteris pseudovata – Lepidodendron posthumii Assemblage (Taiyuan and lower Shanxi Fms.), Emplectopteridium alatum – Emplectopteris triangularis – Cathaysiopteris whitei Assemblage (upper Shanxi and Lower Shihhotse Fms.), Gigantonoclea lagrelii – Fascipteris hallei – Lobatannularia heianensis Assemblage (Upper Shihhotse Fm.). As a result of this biostratigraphic reanalysis, it has become clear that biostratigraphic boundaries do not exactly correlate to lithostratigraphic boundaries as has been thought. The biostratigraphy of the terrestrial deposits in the North China Block were correlated to the IUGS Global Chronostratigraphy. The more precise chronostratigraphic constraints that were obtained as a result of this study make it possible to more precisely correlate the vegetational successions to the concurrent waxing and waning of the Late Paleozoic ice sheets.

Plio-Pleistocene transition in the continental record from Lake Baikal: Diatom biostratigraphy and age model

Lake Baikal basin in the continental interior Asia preserved a unique continuous sedimentary archive of the late Cenozoic; large-scale regional environmental changes of the past were recorded in the composition of fossil diatom flora. Here, the Lake Baikal planktonic diatom biostratigraphy is extended over the Plio-Pleistocene transition, it includes 11 new local diatom assemblage zones (LDAZ) for the time interval 1.25–3.6 Ma. The comparison of the parallel BDP-96-1 and BDP-98 drill core records allows constructing the complete diatom biostratigraphic record at the time of the inception of the Northern Hemisphere glaciations; diatom biostratigraphic zones around Matuyama/Gauss paleomagnetic reversal are correlated with individual stages of marine oxygen isotope stratigraphy. Contrary to previous assumptions, the period 2.8–2.6 Ma in Lake Baikal record was characterized not by a prolonged minimum in diatom abundance but by a distinct maximum in the abundance of small-celled diatom taxa Tertiarius baicalensis and Stephanodiscus binderanus et var. hyalinus, which proliferated in the lake during the interval of relatively warm and humid climate. The Lake Baikal diatom biostratigraphic change at around 1.8 Ma occurs only at a species level of the genus Cyclotella. By contrast, around 2.8–2.5 Ma, spectacular appearances and extinctions occurred at a high taxonomic level of genera (including the genera of Tertiarius, Thalassiobeckia and Stepahanopsis) making the biostratigraphic changes around Matuyama/Gauss reversal more prominent. At least three distinct biostratigraphic boundaries are observed: at 2.69 Ma, 2.65 Ma and at 2.46 Ma. None of these boundaries, however, coincides with the paleomagnetic reversal at 2.58 Ma. Thus the new well-resolved Lake Baikal biostratigraphic record points to the difficulty of assigning a single ‘golden spike’ biostratigraphic boundary around Matuyama/Gauss paleomagnetic reversal in continental Asia.

Updated biostratigraphy of Jurassic (lower Kimmeridgian) deposits containing the ammonite Ataxioceras from the eastern Iberian Range, northeastern Spain

Ammonite biostratigraphy and its correlation potential are evaluated for Ataxioceras-bearing deposits in the eastern Iberian Range, E–NE Spain. The Chronostratigraphic Standard Hypselocyclum Zone of sub-Mediterranean areas is analysed. Correlation of the new Ataxioceras lothari Biozone is proposed utilising first appearance datum (FAD)-based biostratigraphy, resulting from a review of ammonite (Ataxioceratinae) systematics. The lower boundary of the A. lothari Biozone and its chronostratigraphic equivalent Lothari Chronozone are defined by the FAD of Ataxioceras Fontannes s.s., and their upper boundaries by the FAD of Crussoliceras Enay. Distinction of the sub-biozones (subzones) A. hippolytense and A. lothari is confirmed, but these units are slightly reinterpreted. No ammonite subdivisions are defined for the A. hippolytense subzone, although two informal “intra-subzone biodivisions” (A. lothari and Geyericeras aragoniense) are recognised within the A. lothari subzone, the latter defined for the first time. The correlation potential of the proposed biostratigraphic scheme (zone and intra-zone levels using Ataxioceratinae) is high for marine successions from both epicontinental and oceanic-fringe environments under Tethyan influence (southern Europe and north-western Africa). The potential for correlation at the intra-zone level is higher for epicontinental than for oceanic-fringe areas; yet the precise correlation of some biostratigraphic boundaries is difficult at the “intra-subzone biodivision” level between the eastern Iberian Range and SE France. Updated identification and subdivision of the equivalent Metahaploceras strombecki Biozone in oceanic-fringe sections are presented, together with this unit's correlation potential with respect to the proposed A. lothari Biozone.

Evidence for glacioeustatic control of large, rapid sea-level fluctuations during the Albian-Cenomanian: Dakota formation, eastern margin of western interior seaway, USA

Sequence stratigraphic analysis of the Albian-Cenomanian marginal marine succession (Dakota Formation) along the eastern margin of the Western Interior Seaway in eastern Nebraska reveals large-scale fluvial incision (13–28 m) that coincides with sequence boundaries. Construction of a relative sea-level curve constrained by high-resolution palynological correlations indicates rapid incision rates (>10 m/m.y.). Lithologic and palynologic analyses of the strata above and below the D 2 sequence boundary show that this hiatus includes the Albian-Cenomanian biostratigraphic boundary. Comparison with the eustatic sea-level curves published by Haq and Al-Qahtani (2005) and Sahagian et al. (1996) places the D 2 sequence boundary in eastern Nebraska at a global sea-level minimum at 99.5 m.y.a. The interpreted sea-level changes from this succession are comparable to other Albian-Cenomanian successions from around the world indicating a eustatic mechanism. We attribute these large-scale and rapid sea-level falls during the mid-Cretaceous greenhouse world to glacioeustasy. The implication that a glacioeustatic mechanism operated during the mid-Cretaceous suggests that the definition of a “greenhouse world” as being ice-free needs to be reevaluated. Perhaps many, if not all, of the “greenhouse” climatic settings included some ice, either in the form of alpine glaciers, small “continental” ice sheets, or both. Understanding past climate changes and their ramifications may be the key to predicting future climatic changes.

Holocene diatom stratigraphy in the Archipelago Sea, northern Baltic Sea

Two sediment cores from the Archipelago Sea in the northern Baltic Sea were examined for their siliceous microfossils in order to study the Holocene palaeoenvironmental history of the area. The diatom record was divided into local diatom assemblage zones (LDAZ). An age model was constructed using independent palaeomagnetic and AMS-14C methods. The early history of the Archipelago Sea was freshwater. Initial brackish-water influence is observed at 7,950 ± 80 cal. BP (LDAZ4), but fully brackish conditions were established at 7,700 ± 80 cal. BP (LDAZ5). Diatom assemblages indicate increasing salinity, warming climate, and possible increasing trophic state during the transition from lacustrine to brackish-water conditions. The decreasing abundance of Pseudosolenia calcar-avis (Schultze) Sundstrom and the increasing abundance of the ice-cover indicator species Pauliella taeniata (Grunow) Round and Basson indicate reduced salinity and climatic cooling after ∼5,000 cal. BP. LDAZ boundaries do not always correlate with changes in the sediment appearance, which underlines the importance of defining biostratigraphic boundaries independently to the sediment visual character, in contrast with the conventional practice for classifying the Baltic Sea sediments.

Synoptic reconstruction of a major ancient lake system: Eocene Green River Formation, western United States

Numerous 40Ar/39Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ∼8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, 40Ar loss, and 39Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven 40Ar/39Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries. Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfilled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled Tipton and Laney Members. Sediment accumulation patterns thus reflect basin-center–focused accumulation rates when the basin was underfilled, and supply-limited accumulation when the basin was balanced filled to overfilled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ∼150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fluctuating profundal facies, which likely reflects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fields entered the Green River Formation lakes from the north. Our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfilled) lakes were initially dominant (53.5–52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fill lakes first occurred in all Green River Formation basins at 52.0–51.3 Ma and again between 49.6 and 48.5 Ma. Evaporative (underfilled) lakes occurred in various basins between 51.3 and 45.1 Ma, coincident with the end of the early Eocene climatic optima and subsequent onset of global cooling defined from marine record. However, evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate. Evaporative terminal sinks were initially located in the Greater Green River and Piceance Creek Basins (51.3–48.9 Ma), then gradually migrated southward to the Uinta Basin (47.1–45.2 Ma). This history is likely related to progressive southward construction of the Absaroka Volcanic Province, which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient. The Greater Green River and Piceance Creek Basins were eventually filled from north to south with Absarokaderived detritus at sedimentation rates 1–2 orders of magnitude greater than the underlying lake deposits.

The Early to Middle Pleistocene boundary in the Baza Basin (Spain)

The continental vertebrate fauna of Cúllar Baza-1 (Granada, Spain) occur immediately above the Matuyama/Brunhes polarity boundary, and therefore represent an initial record for the Middle Pleistocene. All polarity zones for the Late Pliocene through Middle Pleistocene are found in an 80 m section, which includes this fossil quarry. The existing collection of abundant remains of micro-mammals, macro-mammals and lithic artifacts (indicating human activity) can now be assigned an earliest Middle Pleistocene age of 0.75 Ma. Another section, 25 km across the Neogene Baza Basin, also has the Matuyama/Brunhes polarity boundary. However, in this case the Huéscar-1 fossil quarry and the Puerto Lobo sites are both below the polarity boundary, thus representing a record of the late Early Pleistocene at ∼0.9 Ma. Differences in micro-mammal species across these Matuyama/Brunhes boundaries are significant and justify an approximately coincident biostratigraphic boundary between the Biharian and Toringian stages.