Late Quaternary Sequences Research Articles

Análisis Fitolítico de la Formación Tezanos Pinto (Pleistoceno Tardío—Holoceno Temprano) en el Sector Sudoeste de la Provincia de Entre Ríos (Argentina)

Abstract. PHYTOLITH ANALYSIS OF TEZANOS PINTO FORMATION (LATE PLEISTOCENE-HOLOCENE) IN THE SOUTHWESTERN REGION OF ENTRE RIOS PROVINCE (ARGENTINA). Loess deposits are the main component of the Late Quaternary sequences of the Pampa plains (South America). The Last Glacial Maximum loess represents a depositional unit of the Pampean Aeolian System that evidences the expansion of semiarid conditions to the northeast of the region. The Tezanos Pinto Formation is the typical loessic unit of the late Pleistocene—early Holocene of the northeastern Pampa region. Advances in the knowledge of the micropaleobotanical content of this geologic formation are presented in this work, especially the obtained results of the Southweastern area of Entre Rios Province. The analyzed phytolith assemblages show a high homogeneity with some variations that allowed their quantitative differentiation across the analyzed sedimentary profiles. The presence of grass phytoliths jointly with palm, podostemoid and ciperoid elements allowe...

Non-pollen palynomorph studies in the Neotropics: The case of Venezuela

This paper summarises the NPP studies developed so far in a wide range of environments from Venezuelan lowlands, midlands and highlands, as an example of a growing discipline in a Neotropical area. The studies discussed include both modern analogues from surface sediments and Late Quaternary sequences combining pollen and spores with NPP analyses. Emphasis is placed on the utility of NPP as palaeoecological indicators both individually and collectively (NPP assemblages), as well as in combination with other proxies. The main advantages of using NPP instead of only pollen and spores are highlighted using case studies as examples. Among them, the occurrence of NPP in samples barren for pollen and spores, the independent indicator nature of NPP, and their capacity to reveal previously unnoticed environmental shifts (notably those related to local conditions), are emphasised. The main results obtained in all these analyses are discussed in the frame of several areas of the NPP study that are considered crucial (methods, taxonomy, indicator value and statistical treatment). Some ideas for future developments are proposed in each of these areas, and the need for closer collaboration among NPP specialists is strengthened, in a way towards unification and standardisation.

Formation of a stratified subglacial ‘till’ assemblage by ice-marginal thrusting and glacier overriding

O Cofaigh, C., Evans, D. J. A. & Hiemstra, J. F. 2010: Formation of a stratified subglacial ‘till’ assemblage by ice-marginal thrusting and glacier overriding. Boreas, 10.1111/j.1502-3885.2010.00177.x. ISSN 0300-9483. A thick sequence of glaciotectonically stacked till and outwash is preserved in a coastal embayment at Feohanagh, southwest Ireland. The sequence contains a variety of diamicton lithofacies, including laminated, stratified and massive components, but stratified diamictons dominate. Stratification/lamination is imparted by the presence of numerous closely spaced subhorizontal and anastomosing partings, which give a fissile appearance to the diamictons. Many partings are the result of sandy or thin gravelly layers within the diamictons. Some diamictons contain interbeds and lenses of sand, mud and gravel, which still preserve the original stratification. The sequence at Feohanagh is the product of a two-stage depositional process in which initial glaciolacustrine sedimentation in an ice-dammed lake was followed by glaciotectonic thrusting and overriding, during which the lake sediments were reworked and variably deformed. Similar late Quaternary sequences of glaciotectonically stacked stratified sediments and till have been described from around the coastal margins of Ireland and Britain, where they constitute glaciotectonite–subglacial traction till continuums rather than true lodgement tills as traditionally implied. Thick stratified diamicton assemblages are likely to occur in areas where steep topography provides pinning points for the glacier margin to stabilize and deliver large volumes of sediment into a glaciolacustrine or glaciomarine setting before proglacial and subglacial reworking of the sediment pile. The resulting geological–climatic unit, often defined as ‘till’, will contain a large amount of stratified and variably deformed material (laminated and stratified diamictons will be common), including intact sediment rafts, reflecting low strain rates and short sediment transport distances.

Longitudinal correlation of Late Quaternary terrace sequences of Widden Brook, southeastern Australia

Terrace remnants on Widden Brook, southeastern Australia, were examined and correlated longitudinally to establish their evolutionary history. Three discontinuous terrace sequences, the Baramul, Widden and Kewarra, were identified in a 26 km reach using sedimentology, topography and chronology. Each terrace sequence occurred within a geomorphically distinct valley setting: an upstream constriction, a valley expansion and a highly constricted downstream section. Radiocarbon and optically stimulated luminescence ages indicated that each terrace sequence was formed during the Late Pleistocene and Holocene (16.7–0.5 ka cal BP). However, their sedimentology and topography were shown to differ significantly. We present evidence that both climate and the exceedance of intrinsic geomorphic thresholds were major contributing factors responsible for the formation of these terrace sequences.

Late Quaternary Seismic Sequence Stratigraphy of the Gulf of Kachchh, Northwest of India

Abstract High-resolution seismic reflection and bathymetric data in 10 to 50 m water depth in the Gulf of Kachchh, northwest of India, have been analysed together with earlier results from the area. They have revealed geomorphic features and Late Quaternary seismic sequence stratigraphy of 25-m-thick sediments. Seabed topography is uneven except in the east, carpeted by thick acoustically transparent to semitransparent sediment clays and stratified limestone beds. The sediments consist of (i) the transparent unit “A” up to 12 m thick, (ii) thick 8 to 10 m diffuse seismic reflection free unit “D,” and (iii) 4 to 5 m thick hummocky reflections unit “K.” The geomorphic/subsurface features are pinnacles (1 to 2 m high) and mounds (4 to 5 m high) on the seafloor and subsurface in the south and west, valleys (1 to 3 m) on the seafloor in the south and subsurface in the north and onlap of reflectors in the north. They mark corals, seafloor incisions, and sediment influx. Spatial shifts of valleys that mark a hig...

Late Quaternary changes in intermediate water oxygenation and oxygen minimum zone, northern Japan: A benthic foraminiferal perspective

A strong oxygen minimum zone (OMZ) currently exists at upper intermediate water depths on the northern Japanese margin, NW Pacific. The OMZ results largely from a combination of high surface water productivity and poor ventilation of upper intermediate waters. We investigated late Quaternary history (last 34 kyr) of ocean floor oxygenation and the OMZ using quantitative changes in benthic foraminiferal assemblages in three sediment cores taken from the continental slope off Shimokita Peninsula and Tokachi, northern Japan, at water depths between 975 and 1363 m. These cores are well located within the present‐day OMZ, a region of high surface water productivity, and in close proximity to the source region of North Pacific Intermediate Water. Late Quaternary benthic foraminiferal assemblages experienced major changes in response to changes in dissolved oxygen concentration in ocean floor sediments. Foraminiferal assemblages are interpreted to represent three main groups representing oxic, suboxic, and dysoxic conditions. Assemblage changes in all three cores and hence in bottom water oxygenation coincided with late Quaternary climatic episodes, similar to that known for the southern California margin. These episodes, in turn, are correlated with orbital and millennial climate episodes in the Greenland ice core including the last glacial episode, Bølling‐Ållerød (B/A), Younger Dryas, Preboreal (earliest Holocene), early Holocene, and late Holocene. The lowest oxygen conditions, marked by dysoxic taxa and laminated sediments in one core, occurred during the B/A and the Preboreal intervals. Suboxic taxa dominated mainly during the last glacial, the Younger Dryas, and most of the Holocene. Dysoxic conditions during the B/A and Preboreal intervals in this region were possibly caused by high surface water productivity at times of reduced intermediate ventilation in the northwestern Pacific. Remarkable similarities are evident in the late Quaternary sequence of benthic foraminiferal assemblage change between the two very distant continental margins of northern Japan and southern California. The oscillations in OMZ strength, reflected by these faunal changes, were widespread and apparently synchronous over wide areas of the North Pacific, reflecting broad changes in intermediate water ventilation and surface ocean productivity closely linked with late Quaternary climate change on millennial and orbital timescales.

Utility of coal petrology for sequence-stratigraphic analysis

Abstract Sequence stratigraphy seeks to predict the depositional architecture of sediments based on the principle that their superposition and lateral arrangement are largely determined by the rate at which accommodation is made below depositional base level relative to the supply rate of sediments. Depositional base level is generally correlated with sea level which in paralic peat/coal deposits controls the position of the groundwater table above which oxidation, ablation and erosion would occur. The current Late Quaternary sequence is used to discuss some of the constraints on coal formation that stem from the interaction of short- and long-period modulations on sequence-forming eustatic cycles. The results show that the mid-transgressive systems tract is unlikely to produce significant coal deposits, which is supported by a survey of the relevant literature from eight different countries. It reveals a bimodal distribution of coal deposits with the strongest mode being situated in the early transgressive systems tract, while a broader, secondary mode occurs in the early and mid-highstand position. This concentration contrasts with the complete absence of any significant coal deposits in the early and mid-lowstand systems tract, and only a low occupancy of the late highstand systems tracts. The reasons for this distribution are briefly discussed, and attention is drawn to the possible over-stating of the coal-forming potential of the late lowstand systems tract. Following a brief discussion of the sequence-stratigraphic interpretation of coal measures, emphasis is put on the application of sequence stratigraphy to the study of coal seams and their composition at various levels of petrographic detail. While in low-rank coals, botanical affinities contribute much to the lithotype-based analysis, a genetic assessment of bituminous coal seams relies primarily on physical and chemical palaeo-environmental signatures. Simple properties, such as maximum coal thickness, low detrital mineral content and maximum preservation of vitrinite, particularly of telovitrinite, define a good balance between the former rates of accommodation versus peat accumulation. Iso-metamorphic variations in telovitrinite reflectance and its dispersion measured by the coefficient of variation have also contributed to the palaeo-environmental interpretation. This paper confirms the identification within coal seams of most of the sequence-stratigraphically significant surfaces, e.g. sequence and parasequence boundaries, transgressive, flooding, and maximum flooding surfaces. It also gives examples of some new ones, referred to as accommodation reversal, non-marine flooding, give-up transgressive, terrestrialisation, and paludification surfaces. Some of these are specific to coal measures, but the give-up transgressive and accommodation reversal surfaces may have wider significance.

Late Quaternary (marine isotope stages 6-1) seismic sequence stratigraphic evolution of the Otago continental shelf, New Zealand

A proposed chronostratigraphic model for the Late Quaternary evolution of the Otago continental shelf, New Zealand, includes fluvially incised sequence boundaries eroded during marine isotope stages 2, 4 and 6, bounding three sequences subdivided into lowstand, transgressive and highstand/regressive systems tracts. Datable material is limited to the uppermost 1 m of the sequence, and consequently the model is based on correlation of new high-resolution seismic reflection data (Geopulse™) to a compilation of independent global sea-level curves. A second chronostratigraphic model that lacks a stage 4 sequence boundary and distinct stage 3 highstand/forced regressive deposit is discounted because its subsidence-corrected paleoshoreline depths do not match lowstands in the eustatic curves or temporally constrained sequences at other far-field locations. In the preferred chronostratigraphic model, sea-level lowstands during stages 2, 4 and 6 reached maximum depths of 126–131 m, 101–111 m and 116–135 m, respectively, producing sequence boundaries, landward-pinching deltaic wedges on the outer Otago shelf, and submarine canyon incision. Lowstand paleoshoreline depths have been corrected for subsidence from thermal cooling, sediment compaction and loading, but may be erroneously deep owing to erosion of paleoshoreline indicators. Clastic, shallow marine wedges and back-barrier valley-fill deposits accumulated during the stage 6–5, 4–3 and 2–1 marine transgressions. Contrary to the traditional sequence stratigraphic model, deltaic and strandline units deposited during highstand through falling sea level (stages 5–4, 3–2, 1) volumetrically dominate Otago Late Quaternary sequences. Discrete units and minor marine erosion surfaces within the transgressive and regressive systems tract deposits are interpreted as strong evidence for seventh- (∼ 20 ka) or higher-order sea-level fluctuations influencing sedimentation on the Otago shelf.

Stratigraphical and palynological appraisal of the Late Quaternary mangrove deposits of the west coast of India

The organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the Late Quaternary sequence of Kerala–Konkan Basin. Three generations of such deposits have been identified. The older one is dated to around 43,000–40,000 14C yr B.P., with a few dates beyond the range of radiocarbon. The younger ones date from the Middle Holocene to latest Pleistocene (10,760–4540 14C yr B.P.) and the Late Holocene (<4000 14C yr B.P.). Pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation. Peat accumulation during the period 40,000–28,000 14C yr B.P. can be correlated with the excess rainfall, 40–100% greater than modern values, of the Asian summer monsoon. The low occurrence of mangrove between 22,000 and 18,000 14C yr B.P. can be attributed to the prevailing aridity and/or reduced precipitation associated worldwide with Last Glacial Maximum, because exposure surfaces and ferruginous layers are commonly found in intervals representing this period. The high rainfall of 11,000–4000 14C yr B.P. is found to be the most significant as the mangrove reached an optimum growth around 11,000 14C yr B.P. but with periods of punctuated weaker monsoons. From the present and previous studies, it has been observed that after about 5000 or 4000 14C yr B.P., the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems. A case study of Hadi profile provided an insight to the relevance of magnetic susceptibility (χ) to record the ecological shift in Late Holocene.

Internal structure and evolution of the Late Quaternary sequence in a shallow embayment: The Amvrakikos Gulf, NW Greece

The Late Quaternary stratigraphic pattern of the Amvrakikos Gulf in the eastern Mediterranean has been studied on the basis of the analysis of high-resolution seismic profiles and short sediment cores. Lowstand, transgressive and highstand systems tracts have been identified that are configured in a major depositional sequence of a fifth-order sea level cycle. During the last glaciation, when the sea level was lowered by ∼55 m relative to its present position (ca. 50 ka BP), the Amvrakikos Gulf was probably emerged, giving rise to: (a) subaerial erosion and fluvial incision in the western shallow part of the Gulf; and (b) development of a paleo-lake in the eastern deepest domain. Until the early phase of the post-glacial transgression (ca. 11–50 ka BP), a paleo-river, characterized by a dense network of V-shaped valleys, was draining the western part of the Gulf and flowing into the Ionian Sea. In the isolated eastern part, lowstand lacustrine deposits (deltaic and prodeltaic units) accumulated within the paleo-lake. At the latter stage of the last post-glacial transgression (ca. 11 ka BP), seawater entered the Gulf and the transgressive systems tract was deposited, consisting of incised-channel fills, beach–shoreface deposits and delta/prodelta wedges. Buried terraces have been identified also within this transgressive tract that reveal a step-like sea level rise. Since ca. 6 ka BP, when the sea level reached its present-day position, a distal prodeltaic unit has been deposited and overlain by prograding deltaic wedges. The spatial distribution of these wedges provides evidence of a continuous shifting of the river mouths during Holocene. The recent sedimentation patterns are related to the terrigenous inputs primarily from the Arachthos River, located in the northeastern part of the Gulf, and secondarily from the Louros River, located in the northwestern part.

Discontinuity-Bounded Alluvial Sequences of the Southern Gangetic Plains, India: Aggradation and Degradation in Response to Monsoonal Strength

Discontinuity-bounded late Quaternary sequences in the southern Gangetic Plains (Himalayan Foreland Basin) reflect floodplain aggradation and degradation in response to forcing by the powerful Southwest Indian Monsoon. The major Himalayan and cratonic rivers in this area occupy narrow, incised valleys and do not inundate the adjoining broad interfluves, which display soil development, small plains-fed rivers, lakes, eolian deposits, and badland ravines. However, these areas formerly experienced active alluviation because thick floodplain muds underlie the interfluve. An age model suggests that interfluve areas near the major rivers aggraded periodically between about 27 ka and 90 ka (Marine Oxygen Isotope Stages 3–5). They subsequently degraded or accumulated sediment only locally, probably reflecting decreased monsoonal precipitation around the Last Glacial Maximum (Marine Isotope Stage 2), when major river valleys moved to an underfit condition. Increased precipitation during the 15 to 5 ka period of monsoon recovery probably increased discharge and promoted incision and widespread badland formation. Discontinuities at deeper levels reflect earlier degradation phases, and are marked by reworked gravel lenses, thin channel bodies, and carbonate-cemented surfaces. Carbonate veins (infilled joints) that cut strata below but not above one discontinuity suggest some local tectonic influence. There is no evidence that sea-level fluctuations influenced stratal patterns in this elevated, inland region. Discontinuity-bounded sequences linked to climate change may characterize alluvial successions in many continental settings, especially along the cratonic margins of foreland basins, where subsidence rates are modest.

Past Human Activity and Geomorphological Change in a Guano-Rich Tropical Cave Mouth: Initial Interpretations of the Late Quaternary Succession in the Great Cave of Niah, Sarawak

This paper presents initial interpretations of the processes and events responsible for the late Quaternary sequence in the West Mouth of the Great Cave of Niah, in the hot and humid lowland rainforest and swamp forest of Sarawak in Malaysian Borneo. It evaluates the geomorphological context of the site within the known pattern of rapid late Quaternary climate change. Attention is given to the proximity to the sea and the likelihood of humid tropical or cooler drier conditions. The stratigraphic succession is described and four units or lithofacies (2C, 2, 3 and 4) are recognized as being of particular geomorphological and archaeological importance. The key processes operating within the site are the accumulation and subsequent failure and flow of bat and bird guano, hillslope colluviation, and ephemeral stream flow and pond development. Units 2C and 2 contain the critical archaeology, including the Deep Skull from an anatomically modern human, discovered by Tom Harrisson. These were formed by colluviation from a complex cave-mouth rampart and stream flow from within the cave. The stream transported fine-grained sediment to a shallow pond, and both the stream and pond deposits show evidence for prolonged desiccation. Human activity is associated with these surfaces. The human remains and related archaeology are preserved because a mudflow (Unit 3) plowed into and overrode the land surface upon which the humans had lived, resulting in the deformation and burial of the surface and the preservation of the archaeological material. Provisional radiocarbon dates indicate that Units 2C and 2 accumulated from before ca. 45,000 B.P. until ca. 38,000 B.P. Dates bracketing the Deep Skull give this an age of ca. 45,000 B.P. to ca. 43,000 B.P. Overlying the mudflow, Unit 4, a silty diamicton with a relatively high carbonate and organic content, appears to have formed by a mix of natural colluvial and human transport processes, and is associated with human cultural material. Unpublished radiocarbon dates indicate that this deposit formed from before ca. 19,500 B.P. to ca. 8500 B.P. (uncalibrated).This interpretation of the site and its finds has required detailed reconstruction of the changing palaeogeography within and beyond the cave entrance and the nature and rate of geomorphological processes operating within the region, which have been placed within models for rapid Quaternary environmental change. The results suggest that during the earlier period of human presence in the Great Cave of Niah(earlier than ca. 45,000 B.P. until ca. 38,000 B.P.), the climate was episodically wet with much longer periods of relative dryness. During the later period of human occupancy (ca. 19,500 B.P. to ca. 8500 B.P. [uncalibrated]), the evidence is less secure and a slightly moister climate is suggested.

A late quaternary paleoenvironmental sequence from Dor, Carmel coastal plain, Israel

The 10.5 m deep “D‐Dor”; core was taken at Dor (Tantura Lagoon), on the Carmel coastal plain, Israel. The established chrono‐stratigraphic sequence (based on x‐ray radiographs, and both luminescence and radiocarbon dating) covers the last about 26,000 years. It provides the paleoenvironmental framework for the transition from hunter‐gathering to agriculture in the Levant. Three clay units were identified, overlying kurkar (calcareous sandstone) and covered by 6.3 m of sand. The bottom clay unit is a paleosol. Pollen was not preserved in this unit. Gray clay (the top of which was dated to about 12,000 cal. YBP) was deposited, overlying the paleosol, in a wetland environment. Pollen was preserved only in the upper part of this unit. It indicates a slightly drier climate than today's, probably correlative with the Younger Dry as. At the beginning of the Holocene, between 10,300 and 9,550 cal. YBP, a new marsh originated, depositing dark clay. High concentrations of well‐preserved pollen allowed the reconstruction of several fluctuations in humidity. When the marsh was first formed, precipitation was higher than today, and oak maquis was more extensive in the area. The date of the earliest submerged Pre‐Pottery Neolithic settlement embedded in its upper part indicates that the marsh dried out no later than 9,400–8,550 cal. YBP. Around 5,000 years ago, long after the Early Holocene marsh had dried up, sand began to accumulate in the region as a consequence of the Holocene sea level rise, covering several submerged Neolithic settlements off the Carmel coast.

Biostratigraphical correlation between the late Quaternary sequence of the Thames and key fluvial localities in central Germany

The shared characteristics of limestone bedrock geology and resultant calcareous groundwater have allowed excellent preservation of mammalian and molluscan faunas within the terrace sequences of the Lower Thames and the rivers of the Muschelkalk region of Thuringia, central Germany. The mammalian and molluscan assemblages from the Lower Thames have underpinned the dating of one of the most important late Middle Pleistocene sequences in Britain and probably also Europe; one that is the repository of a highly significant Lower and Middle Palaeolithic archive. The most complete terrace records in Thuringia are those from the River Wipper, in the region of Bilzingsleben, and the Ilm, around Weimar. Both here and in the Lower Thames, interglacial deposits representing the four major post-Elsterian temperate-climate complexes (=oxygen isotope stages (OIS) 11, 9, 7 and 5 of the oceanic record) have been identified. In the Thames the interglacials are represented by fluvially deposited sediments, whereas in Thuringia they are frequently represented by travertines that formed around calcareous springs, often containing exquisitely preserved fossils. Evidence from Lower Thames interglacial deposits within four different terrace formations (Boyn Hill/Orsett Heath, Lynch Hill/Corbets Tey, Taplow/Mucking and Kempton Park/East Tilbury Marshes) is reviewed, in addition to which new evidence from a site at Hackney Downs, East London, is summarized. The deposits at the last-mentioned site are part of the Lynch Hill/Corbets Tey Formation and include interglacial sediments attributed to OIS 9. As well as the record of travertine complexes from each terrace level within the Bilzingsleben staircase, the celebrated travertine sequence at Weimar-Ehringsdorf, on Terrace 4 of the Ilm, is described. The biostratigraphical and palaeoenvironmental evidence from the Ehringsdorf travertines compares closely with that from interglacial deposits at Aveley, in the Mucking Formation of Lower Thames; both are attributed to OIS 7, with comparison possible at the oxygen isotope substage level.

Late quaternary seismic sequences and sedimentation along the coast off Taishan, Guangdong, as revealed by high resolution seismic data

Seismic stratigraphic analysis of 150 km of high-resolution seismic profiles (GEOPULSE and 3.5 kHz) obtained along the muddy coast off Taishan, Guangdong provided valuable information on the offshore Quaternary stratigraphy, tectonics, sedimentary evolution, and environmental and engineering geology of this area. The following late Quaternary stratigraphic sequences were mapped. (1) Pleistocene alluvial sitty sand and shallow marine clay; (2) coarse fluvial channel sand; (3) nearshore and bay clayey sediments. The incision of fluvial channels occurred in two stages: during the Late Pleistocene and at the beginning of the Holocene. The Pleistocene fluvial channels have been filled by coarse sand overlain by more than 10 m of late Pleistocene clay and 20–30 m of Holocene clay. The coastal area is dissected by two major fault systems: the first group strikes ENE-WSW and controls the evolution of the adjacent Quaternary basin, while the second one trends NNE-SSW. These fault systems must be taken into account in any coastal engineering considerations. In addition, both granitic and metamorphic basements were recognized.

Chemical and isotopic composition of modern water bodies in the Lake Kopais Basin, central Greece: analogues for the interpretation of the lacustrine sedimentary sequence

Stable-isotope measurements and modern aquatic chemistry are used to aid the reconstruction of past hydrological changes recorded in a 126 ka-long sediment core from Lake Kopais, Greece. This is the most southeasterly European site with a long Late Quaternary sequence. Although the lake has been artificially drained, calcareous sediments from the centre of the basin, 38°26′16ʺN, 23°03′01ʺE, provide a continuous record from the early part of the last interglacial to the Early Holocene. Because the physiography and hydrology are complex, modern analogues for past environmental conditions are essential. The modern waters are of Ca–Mg–HCO3 or Mg–Ca–HCO3 type, reflecting the prevalence of limestones and metamorphic rocks in the catchment. They show a clear evolutionary trend in δ18O and δD values from the source areas to the modern lakes. However, the modern water bodies are less evolved than the palaeolake. A larger evaporating surface, higher values of δ18O in precipitation, decreased precipitation/evaporation ratios and hydrological closure increased the δ18O values of endogenic carbonates at different times in the past. However, Palaeolake Kopais never desiccated completely or became saline. Analyses of the organic fraction from the core show that: (i) it is mainly of algal origin; (ii) productivity and carbon cycling in the lake waters were relatively stable through time; and (iii) the bulk δ13C values vary between −29‰ and −25‰, indicating that the lake waters were never appreciably carbon-limited and that C4 plants did not invade the catchment. Although significant shifts in the δ18O and δ13C values of endogenic low-Mg calcite are attributable mainly to past variations in water balance, weak δ18O–δ13C covariance during marine isotope stages (MIS) 5e–5c suggests that the palaeolake leaked significantly through karstic sinkholes. Since it was effectively operating as an open system, δ18O may have retained the isotopic imprint of increased summer precipitation. Between MIS 5a and MIS 4, strong isotopic covariance suggests minimal outflow and drier conditions than today. After an increase in winter snowmelt during MIS 3, the climate again became cold and dry in the full-glacial (MIS 2). A fluctuating increase in moisture occurred during the late-glacial to early Holocene transition (MIS 2–1).

Dinoflagellate cyst assemblages as tracers of sea‐surface conditions in the northern North Atlantic, Arctic and sub‐Arctic seas: the new ‘n = 677’ data base and its application for quantitative palaeoceanographic reconstruction

AbstractThe distribution of dinoflagellate cyst (dinocyst) assemblages in surface sediment samples from 677 sites of the northern North Atlantic, Arctic and sub‐Arctic seas is discussed with emphasis on the relationships with sea‐surface parameters, including sea‐ice cover, salinity and temperature of the coldest and warmest months. Difficulties in developing a circum‐Arctic data base include the morphological variation within taxa (e.g. Operculodinium centrocarpum, Islandinium? cezare and Polykrikos sp.), which probably relate to phenotypic adaptations to cold and/or low salinity environments. Sparse hydrographical data, together with large interannual variations of temperature and salinity in surface waters of Arctic seas constitute additional limitations. Nevertheless, the use of the best‐analogue technique with this new dinocyst data base including 677 samples permits quantitative reconstruction of sea‐surface conditions at the scale of the northern North Atlantic and the Arctic domain. The error of prediction calculated from modern assemblages is ±1.3 °C and ±1.8 °C for the temperature of February and August, respectively, ±1.8 for the salinity, and ±1.5 months yr−1 for the sea‐ice cover. Application to late Quaternary sequences from the western and eastern subpolar North Atlantic (Labrador Sea and Barents Sea) provide reconstructions compatible with those obtained using the previous dinocyst data base (n = 371), which mainly included modern data from the northern North Atlantic. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Micromorphological evidence supporting Late Weichselian glaciation of the Northern North Sea

Thirteen samples from three cores and boreholes are examined using micromorphology to test existing interpretations of Late Quaternary sedimentary sequences from the Norwegian Channel, North Sea Fan and the North Sea Plateau. Previous studies have interpreted these sediments using arbitrary parameters as reflecting Late Weichselian subglacial and glacimarine conditions associated with the Scandinavian Ice Sheet and Norwegian Channel ice stream. This study develops existing micromorphological criteria to interpret the samples as reflecting specific processes of subglacial deformation and proximal and distal glacimarine sedimentation during and subsequent to the Last Glacial Maximum. The study concludes by outlining diagnostic criteria for the identification of these sediment types from core and borehole samples of other Quaternary sediments.

Abstract: Late Quaternary sequence development on a slowly subsiding, ramp margin: offshore Alabama/northwest Florida

Abstract: Late Quaternary sequence development on a slowly subsiding, ramp margin: offshore Alabama/northwest Florida

Sequence Stratigraphy and Composition of Late Quaternary Shelf-Margin Deltas, Northern Gulf of Mexico

High-resolution seismic profiles and foundation borings from the northwestern Gulf of Mexico record the physical attributes and depositional histories of several late Quaternary sequences that were deposited by wave-modified, river-dominated shelf margin deltas during successive periods of lowered sea level. Each progressively younger deltaic sequence is thinner and exhibits a systematic decrease in the abundance and concentration of sand, which is attributed to a shift in the axis of trunk streams and greater structural influence through time. Our study shows that (1) contemporaneous structural deformation controlled the thickness of each sequence, the oblique directions of delta progradation, the axes of major fluvial channels, and the geometries of delta lobes at the shelf margin, (2) sedimentation was rapid in response to rapid eustatic fluctuations and structural influence, (3) boundaries of these high-frequency sequences are the correlative conformities of updip fluvial incision, and coincide with downlap surfaces at the shelf margin, (4) the downlap surfaces are not true surfaces, but zones of parallel reflections that become progressively higher and younger in the direction of progradation, (5) the downlap zones are composed of marine muds that do not contain high concentrations of shell debris that would be expected in condensed sections, (6) possible paleosols capping the two oldest sequences are regressive surfaces of subaerial exposure that were preserved during transgressions, and (7) no incised valleys or submarine canyons breach the paleoshelf margin, even though incised drainages were present updip and sea-level curves indicate several periods of rapid fall. (Published in American Association of Petroleum Geologists Bulletin 80: 505 530.) R. A. Morton and J. R. Suter AAPG Bulletin, Vol. 80, No. 4, AAPG 1996, reprinted by permission of the American Association of Petroleum Geologists whose permission is required for future use.