Glaciomarine Environments Research Articles

Retreat patterns and dynamics of the former Norske Trough ice stream (NE Greenland): An integrated geomorphological and sedimentological approach

A better understanding of past extent and dynamics of the Greenland Ice Sheet (GrIS) is required to provide context for present-day observations, to constrain numerical climate models and to predict future scenarios of ice-sheet response to recent climatic change. The presence of a grounded GrIS on the NE Greenland shelf during the Last Glacial Maximum (LGM) is supported by high-resolution seismic data combined with multi-proxy analyses of sediment gravity cores from Norske Trough. Our results indicate that an ice stream advanced to the outer shelf during the LGM. Recessional moraines and grounding zone wedges on the seafloor of the outer to middle shelf show that initial retreat was episodic, punctuated by two major stillstands. Pinning points for grounding-zone stabilization were controlled by pre-LGM trough topography. In contrast, preserved large-scale glacial lineations on the seafloor of the inner shelf, formed during the advance, indicate that ice retreat from the inner trough to the coast-proximal position was rapid in order to maintain the former carved morphology. Initial ice retreat was underway before 16.6 cal Kyr BP, earlier than previously known for this sector of the NE Greenland shelf. The inner shelf was free from grounded ice at least by 12.5 cal Kyr BP, and likely before. Retreat occurred in a glaciomarine setting and the ice sheet was fringed by a floating ice shelf. There is no evidence of grounded-ice readvances during the Younger Dryas, although a floating ice shelf with vigorous sea-ice and iceberg rafting production occupied the inner shelf at 11.3 cal Kyr BP. 14C dates reveal initial stepwise ice retreat followed by a fast retreat that coincides with Heinrich Stadial 1 and the Bølling-Allerød interstadial, respectively. During both periods there was increased inflow of warm Atlantic Water, indicating a strong ocean/climate control on the late deglaciation of Norske Trough. Following the late deglaciation, postglacial (Holocene) sedimentation records a change from an ice-distal glaciomarine environment, to an open marine environment and complete ice shelf disintegration. On the innermost shelf of the Norske Trough, the ice shelf disintegrated and reformed once before the final break-up occurred at 9.6 cal Kyr BP.

Sedimentary evolution from greenhouse to icehouse of Neoproterozoic and age constraints in the northern Yangtze Craton

The late Tonian is a critical period of the global climate transition from greenhouse to icehouse. Systematic analysis of its sedimentary evolution is critical to properly understand the initiation of snowball Earth and the nature of Cryogenian glaciers. However, it is difficult to infer this continuous response to climate change from the perspective of studying the continuous sedimentary sequence and sedimentary evolution, because of the depositional hiatus between the Tonian and Snowball Earth strata in almost all cratons. A unique and continuous Neoproterozoic clastic rock succession developed well in the northern Yangtze Craton. This study analyzes the sedimentology, paleocurrent, major and trace elements, volcanic tuff geochronology, sedimentary sequences, and depositional environment of the Tonian Liantuo Formation to the Cryogenian Gucheng Formation in the Shennongjia area of the northern Yangtze Craton with a focus on sedimentary evolution. UPb zircon radiometric ages from interbedded tuffs obtained by LA-ICP–MS method allow constraining the age of the sedimentary evolution in the northern Yangtze Craton to the late Tonian-early Crygenian. The northern Yangtze Craton began deposition at 763.1 ± 6.2 Ma and the sedimentary environment was dominated by terrestrial fluvial action with a provenance from the continental magmatic arc on the South Qinling Belt. At 752.1 ± 6.5 Ma, a transition from the fluvial-dominated terrestrial system to the transgressive tidal-influenced marine system was coeval with the peak rifting related to the Rodinia breakup. After 722.5 ± 2.1 Ma, the northern Yangtze Craton entered the glaciomarine environment under the influence of a global cooling trend. Our analysis of the sedimentary evolution from the late Tonian to the Sturtian glaciation in the northern Yangtze Craton forced by tectonic- climate deepens our understanding of the paleoenvironment at the beginning of snowball Earth.

Environmental changes in Krossfjorden, Svalbard, since 1950: Benthic foraminiferal and stable isotope evidence

ABSTRACT Environmental changes for the past ca. 50 years were studied in a short sediment core from inner Krossfjorden, Svalbard, investigating benthic foraminifera and stable isotopes (δ18O, δ13C). A depth–age model based on anthropogenic 137Cs time markers indicates that record covers the period from 1955 to 2007 and has sediment accumulation rates of ca. 0.3 to 1 cm/year. The benthic foraminifera are arctic and/or common in glaciomarine environments. Six fauna assemblages were identified using stratigraphically constrained cluster analysis. Benthic foraminiferal fauna assemblages are mainly dominated by Cassidulina reniforme. Elphidium clavatum is dominant from 1973 to 1986 and 2002 to 2007, likely due to greater turbidity in the water column. We interpret the increased percentages of Spiroplectammina biformis over the same intervals to reflect a slightly lower salinity probably caused by meltwater. During a short time period, 1970 to 1973, Stainforthia concava dominates the benthic foraminiferal fauna interpreted to reflect increased productivity within a marginal ice zone. Other species as Islandiella norcrossi, Nonionellina labradorica, Islandiella helenae, and Melonis barleanus also indicate more nutrient-rich waters are present but not very abundant throughout the record probably due to the glacier proximal position of the study site. The stable isotope record (δ18O) shows lighter values from 2001 to 2007, which seem to correlate well with oceanographic monitoring data showing increasing core temperatures of West Spitsbergen Current.

A comparison of different acoustic methods for sedimentary classification of King George Bay, Antarctica

Multibeam sonar and high-resolution seismic mapping are marine geophysical techniques frequently employed to map the distribution and type of marine sediments. Both techniques involve the transmission and reception of acoustic pulses reflected by the seafloor and typically generate consistent and satisfactory results at a relatively low-cost relative to direct sampling. However, mainly due to their operational peculiarities (frequency range, beam width, etc.), each method generates some discrepancies regarding sedimentary distribution for the same area. Here, we compare three well-established marine geophysical techniques (Multibeam Echosounder (MBES) bathymetry and backscatter intensity, Sub-Bottom Profiler (SBP) echo-characterization, defined as the acoustic response of marine sediments as a function of its sedimentological properties, and seismic amplitude) applied to characterize seabed geology, analyzing comparing the results observed from each method. The study area is the glaciomarine environment of King George Bay, South Shetland Islands, Antarctica. We demonstrate that each method responds to specific characteristics of the seafloor, mainly because of divergent interactions between the acoustic signal and morphological features at different scales and sedimentological interfaces. Despite some consistencies, considerable mismatches in terms of classification approaches are apparent. Nevertheless, we show how these different classification approaches can be interpreted together to enable more precise geological understandings.

New data on the Late Paleozoic Ice Age glaciomarine successions from Tasmania (SE Australia)

The Late Paleozoic Ice Age (LPIA) is one of the coldest periods in Earth history, and led to the diachronous development of widespread ice centers across Gondwana in the Carboniferous and Permian. In Tasmanian Basin, located between northern Victoria Land (Antarctica) and Australia, the lowermost part of the Parmeener Supergroup (Late Carboniferous to Triassic), consisting of the Wynyard Tillite and its correlatives (Truro and Stockers tillites), recorded LPIA glacial sedimentation linked with ice covers that developed in the region. We carried out a detailed facies analysis of two drillcores which recovered glaciogenic sequences deposited in the Tasmanian Basin. Facies associations vary from possibly sub-glacial or ice-contact to ice-distal. Diamictite is the most common facies and its deposition is driven both by gravity and sediment remobilization processes and suspension settling with ice rafted debris accumulation. Mudstone layers, with and without dropstones, are interposed between diamictite intervals, recording ice-distal to non-glacial conditions respectively. Facies associations are indicative of subaqueous deposition in glaciomarine environment. The glacial sequence stratigraphy approach was applied and glacial system tracts and bounding surfaces, which define glacial sequences, were identified. The stacking pattern of the facies associations allow us to demonstrate that the glacial sequences in the Tasmanian Basin recorded different phases of advance and retreat of the glacial front into the basin, at about the end of the main glacial phase.

Geophysical investigation in sediment cores and its relationship with the governing sedimentary processes at Bransfield Basin, Antarctica.

This research aims to investigate sediment cores from a glaciomarine environment based on their petrophysical parameters to elucidate lithological and sedimentary issues, as well as to identify a geophysical signature based on their parameter's response. To achieve this objective, six marine sediment cores were collected in the Central Bransfield Basin - Antarctica, the lengths ranging from 1.5 to 5.2 meters in water depths ranging from 304 to 1463 meters. The cores were submitted to different analyses and values from density, magnetic susceptibility, electrical resistivity, p-wave velocity, total gamma radiation, silt, clay and sand contents, and mean grain size for each core location were considered. Four large lithologies were identified according to their geological and geophysical characteristics. The first group is subglacial deformation till (GC16); the second is massive diamicton (GC13 and GC12), the third group is composed of diamictons from the basin (AM10) and lower slope (GC09); the fourth lithological group is composed of siliceous mud from an upper slope location (GC06A). The characteristics recorded across Central Bransfield Basin (from South Shetlands Islands to Antarctic Peninsula) highlighted the relationship between the lithological content and associated depositional processes with the geophysical proprieties as density and magnetic susceptibility.

Neoproterozoic syn-glacial carbonate precipitation and implications for a snowball Earth.

The Neoproterozoic 'snowball Earth' hypothesis suggests that a runaway ice-albedo feedback led to two intense glaciations around 717-635million years ago, and this global ice cover would have drastically impacted biogeochemical cycles. Testing the predictions of this hypothesis against the rock record is key to understanding Earth's surface evolution in the Neoproterozoic. A central tenet of the snowball Earth hypothesis is that extremely high atmospheric CO2 levels-supplied by volcanic degassing over millions of years-would be required to overcome a strong ice-albedo feedback and trigger deglaciation. This requires severely diminished continental weathering (and associated CO2 drawdown) during glaciation, and implies that carbonate minerals would not precipitate from syn-glacial seawater due to a lack of alkalinity influxes into ice-covered oceans. In this scenario, syn-glacial seawater chemistry should instead be dominated by chemical exchange with the oceanic crust and volcanic systems, developing low pH and low Mg/Ca ratios. However, sedimentary rocks deposited during the Sturtian glaciation from the Adelaide Fold Belt-and contemporaneous successions globally-show evidence for syn-sedimentary dolomite precipitation in glaciomarine environments. The dolomitic composition of these syn-glacial sediments and post-glacial 'cap carbonates' implies that carbonate precipitation and Mg cycling must have remained active during the ~50million-year Sturtian glaciation. These syn-glacial carbonates highlight a gap in our understanding of continental weathering-and therefore, the carbon cycle-during snowball Earth. In light of these observations, a Precambrian global biogeochemical model (PreCOSCIOUS) was modified to explore scenarios of syn-glacial chemical weathering, ocean chemistry and Sturtian carbonate mineralogy. Modelling results suggest that a small degree of chemical weathering during glaciation would have been capable of maintaining high seawater Mg/Ca ratios and carbonate precipitation throughout the Sturtian glaciation. This is consistent with a severe ice age during the Sturtian, but challenges predictions of biogeochemical cycling during the endmember 'hard snowball' models. A small degree of continental weathering might also help explain the extreme duration of the Sturtian glaciation, which appears to have been the longest ice age in Earth history.

Subglacial sediment deformation in the Ross Sea, Antarctica

The Ross Sea embayment is the outlet for one-third of all Antarctic ice flow, receiving input from both East and West Antarctica. Marine-based ice streams, whose stability is susceptible to global sea-level changes, expand and contract across the Ross Sea continental shelf, eroding and deforming the substrate and transporting sediment subglacially. Deposits from the Last Glacial Maximum in the western Ross Sea provide an opportunity to investigate paleo-ice stream processes using sediment cores associated with glacial landforms. This study combines micro-to macro-sedimentological analyses to characterize lithofacies that represent a glacial-interglacial succession. We observe subtle variations in sediment properties within the subglacial unit that suggest till heterogeneity. Physical characteristics that vary both laterally and vertically indicates different sediment response to glacial stress, resulting in non-pervasive deformation within the till. These observations are consistent with the interpretation that a dynamic ice sheet with variable flow regimes existed across the Ross Sea during the Last Glacial Maximum. Further microscopic evidence of dynamic subglacial conditions includes sediment aggregates preserved as discrete grains in the form of soft sediment clasts or till pellets. Till pellet-rich sediment is commonly associated with the transition from a subglacial to proximal glaciomarine environment, although here we demonstrate they are widespread in both facies. Their widespread occurrence downcore and distribution across the continental shelf provides sedimentological evidence of a deformable bed beneath an ice stream during the Last Glacial Maximum. Ultimately, better tools for discriminating between glacial deposits will allow for more detailed reconstructions of ice-sheet history, and thus determine the forcing mechanisms to which the ice is responding.

Sedimentary provenance and role of tectonic inheritance on the control of the Macaúbas group, eastern margin of São Francisco Craton (SE Brazil)

The Neoproterozoic Macaúbas Group represents the precursor basin system of the Araçuaí–West Congo orogen, located in the southern margin of São Francisco Craton, and records a Tonian volcano-sedimentary succession, at the base, overlain by a Cryogenian sequence related to the Neoproterozoic global glacial event. Detailed stratigraphic surveys along the Tonian units allowed the identification of two lithofacies associations related to alluvial fan and braided river environments, while the three Cryogenian lithofacies associations indicate a glaciomarine environment associated with extensive tectonics that has evolved into a marine environment. The results show that the age spectra of the Tonian and Cryogenian units are similar, differing mainly for an increased peak in the Cryogenian sample in ca. 1.0 Ga, where the primary source is probably related to those magmatic rocks associated with the Stenian-Tonian extensional breakup of the São Francisco-Congo paleocontinent. The KS-Test shows that not all the Tonian samples are similar to each other, as in all the Cryogenian samples. The detailed stratigraphic and structural surveys associated with the geochronological U–Pb data allow us to interpret that the Tonian and the basal Cryogenian sequences were deposited in two episodes of an active intracontinental tectonic context related to fault reactivations along the WNW-ESE Pirapora aulacogen structures in superimposed basin-cycles, the middle-to late-Cryogenian sequences extrapolated the WNW-ESE boundaries while the basin local depocenter and its surroundings expanded to a large depocenter during the basin evolution.

Permo-Carboniferous Climate Change: Geochemical Evidences from Lower Gondwana Glacial Sediments, Rangit Valley, Sikkim Lesser Himalaya, India

The Rangit Gondwana Basin of Sikkim in the lesser Himalaya witnessed a wide span of climate change during the Permo-Carboniferous period. The principal objective of the present study is to document this Permo-Carboniferous climate change in the form of a geochemical signature preserved in the siliciclastic facies of the Rangit Pebble Slate Formation. The stratigraphic sequences of the Rangit Gondwana Basin are categorically well defined and subdivided into upper and lower sequences on the basis of their depositional environment. The lower sequences of the Rangit Gondwana basin are comprised of massive diamictite with large stromatolitic dolomite boulders and dropstone embedded in the coarser sandstone which indicate the cold glaciomarine environment of deposition, whereas upper sequences consist of repeated alternate beds of sandstone, black shale and coal seam with a regular interval depicting the fluvial and deltaic environment of deposition. The enrichment of SiO2, Al2O3, TiO2, MnO, MgO, and K2O indicates that these sediments were mostly derived from felsic rock source areas. Chemical Index of Alteration (CIA) and Index of Compositional Variability (ICV) CaO + Na2O + K2O/Al2O3, and SiO2 vs. (Al2O3 + K2O + Na2O) values suggest that the sediments maturity and paleoclimatic environment deposition of the sediments of lower sequences was cold and semi-humid whereas the deposition of sediments of upper sequences was warm and humid. The A-CN-K ternary plot and CIA vs ICV binary plot also indicate and verify that the source areas were subjected to prolonged intense chemical weathering from low to high grade due to shifting of cold to warm humid paleo-climatic condition.

Constraining late paleozoic ice extent in the Paganzo basin of western Argentina: Provenance of the lower Paganzo group strata

The western margin of Gondwana records evidence of mid-Carboniferous glaciation (Visean- Bashkirian) in the strata of the Paganzo Basin and adjacent areas. Previous studies focused on constraining the orientations of ice flow and generalizing the extent and occurrence of glacial ice in the basin. However, there is uncertainty occurs concerning the location and extent of glaciation and the locations of glacial centers during deposition of the Guandacol Formation located in the western portion and the time correlative Malanzán Formation in the eastern parts of the basin respectively. Understanding these strata and the conditions that led to the deposition of these strata has important implications for understanding glaciation extent along the western margin of Gondwana during the late Paleozoic ice age. To refine the glaciation history, we present new paleocurrent, facies, and a comparative analysis of previously published and one new detrital zircon geochronology data set. Together, these data provide new insight into sediment dispersal patterns and glacial centers within the basin. Our data indicate that both units were deposited under cold climatic conditions that occurred across the entire basin, but that glaciers were restricted to the western portion of the basin. The facies analysis for both formations indicates very different depositional environments for the two units: glacial and glaciomarine environments in the west and non-glacial alluvial, alluvial fan and lacustrine and/or marine environments in the east. Detrital zircon geochronology indicates separate localized provenance signatures for the two formations. Furthermore, the detrital zircon populations allude to disconnected depositional centers. At Huaco, glacial flow in the Guandacol is oriented toward the northwest (i.e. 313°) and appears to be draining from an uplift that contains similar zircon age populations as the Sierra de Valle Fértil and Sierra de Pie de Palo ranges. In the Olta-Malanzán paleovalley system, drainage was off valley walls and down the valley axis (i.e. south and southwest) within the Sierras de Chepes region with limited sediment sourced from the east just beyond the paleovalley system. Paleoflow measurements reported from other known glacial localities along the western portion of the basin reflect a radial flow pattern within and away from deeply incised valleys that clearly point to upland glacial centers within the Cuyania and Precordilleran terranes.

STRUCTURAL, STRATIGRAPHIC AND METALLOGENETIC ASPECTS OF THE PARAGUAY FOLD AND THRUST BELT: IMPLICATIONS FOR GOLD MINERALIZATION AND COLLAGE OF THE GONDWANA

ABSTRACT - Paraguay Belt occupies the western portion of the Tocantins Province, surrounding the Southeast of the Amazonian Craton and the eastern border of the Rio Apa Block, suggesting continuity with Tucavaca Belt in Bolivia. The rocks of the Paraguay belt were initially deposited in a glaciomarine environment in sites proximal to the cratonic area and deeper marine under the influence of turbidite flows in distal sites (Cuiabá Group, Bauxi and Puga Formation). The cap carbonates, thick limestone and dolostone succession of the Araras Group and siltstones and diamictites of the Serra Azul Formation related to Glaskiers glaciation overlay these diamictites (related to Marinoan glaciation). On the top there are terrigenous sediments of the Alto Paraguay Group, represented by sandstones of Raizama and claystones of Diamantino formations, respectively. The belt can be divided into three distinct structural zones: The Internal Domain is comprised of turbidite and glaciogenic sequences. Glaciogenic rocks on the base and carbonaceous and terrigenous sediments on the top occur in the External Domain. Horizontal platformal cover on the Amazonian Craton rocks are characterized by open folds. Structural studies allowed characterization of continuous deformational phases: the main deformational phase generated regional inverse folds with a NE-SW trend and fan geometry. Several regionally widespread lode-type gold deposits related to four types of the quartz veins were identified: type 1 is in concordance to bedding, type 2 is parallel to Sn, type 3 is parallel to Sn+2, and vertical Type 4 (Au-rich) is orthogonal to Sn. Late deformation developed in the Cuiabá region, recorded the closure of the ocean and the invertion where the hydrothermal fluids are the responsible for the orebodies formation.
 Keywords: Paraguay Belt, Structural, Stratigraphy, Metalogenesis.

Reservoir heterogeneity of Ordovician sandstone reservoir (Bedinan Formation, SE Turkey): Diagenetic and sedimentological approachs

The sandstone-bearing reservoir levels (Bedinan Formation) in the Diyarbakır Basin, SE Turkey show strong heterogeneity in porosity, permeability and capillary pressure, which influences oil production rates of the studied 6 wells and beclouds the prediction of hydrocarbon reserves. Therefore, comprehensive understanding of the sandstone reservoir characteristics has been necessitated to optimize its ultimate hydrocarbon recovery, maximize its lifetime and to make more accurate performance predictions for the use in economic risk assessment. In this study, integrated analysis of paleogeography, depositional characteristics, petrographic features, provenance, pore structure, reservoir heterogeneity rating, diagenetic stage and its possible impacts on reservoir quality were examined by employing thin section analysis, sedimentary structure analysis, petrophysical interpretations, geostatistic methods, X-ray diffraction (XRD) and non-destructive X-ray computed tomography. It is concluded that the Upper Ordovician sands were laid down under shallow marine conditions (lower shoreface to upper shoreface) in an ice-distal glaciomarine environment and have been subjected to a complex history of diagenesis. The sandstone succession was sourced from plutonic rocks and derived from a stable craton. The presence of detrital and diagenetic rooted clay-coating minerals (illite, kaolinite, illite/smectite mixed clay layer) together with cementation by quartz overgrowth and carbonate concretions in the matrix of the sandstone layers are the main factors deteriorating the reservoir quality by plugging pore throats and decrease production rates and oil recovery. However, the type of clay-coating minerals and the amount of those are the key factors modifying reservoir architecture in comparison with the cementing materials. Besides that the integration of illitization of kaolinite and smectite in mesodiagenetic stage, the presence of minerals such as quartz, feldspar and illite altering the pore spaces from oil-wet to water-wet and compaction induced micropores are culprits in causing high water saturation in the sandstones even in the zone above oil-water contact.

Subglacial meltwater supported aerobic marine habitats during Snowball Earth

The Earth's most severe ice ages interrupted a crucial interval in eukaryotic evolution with widespread ice coverage during the Cryogenian Period (720 to 635 Ma). Aerobic eukaryotes must have survived the "Snowball Earth" glaciations, requiring the persistence of oxygenated marine habitats, yet evidence for these environments is lacking. We examine iron formations within globally distributed Cryogenian glacial successions to reconstruct the redox state of the synglacial oceans. Iron isotope ratios and cerium anomalies from a range of glaciomarine environments reveal pervasive anoxia in the ice-covered oceans but increasing oxidation with proximity to the ice shelf grounding line. We propose that the outwash of subglacial meltwater supplied oxygen to the synglacial oceans, creating glaciomarine oxygen oases. The confluence of oxygen-rich meltwater and iron-rich seawater may have provided sufficient energy to sustain chemosynthetic communities. These processes could have supplied the requisite oxygen and organic carbon source for the survival of early animals and other eukaryotic heterotrophs through these extreme glaciations.

Lithofacies and glacio-tectonic deformation structures of the Tiesi'ao/Dongshanfeng Formation on the Yangtze Block, South China: Implications for Sturtian Glaciation dynamics

The Cryogenian Period (~720–635 Ma) witnessed the most extensive and prolonged glaciation events in Earth history. Vast ice sheets of at least two global glaciations, the Sturtian (~720–660 Ma) and Marinoan (<654–635 Ma), advanced to sea level at low latitudes, a scenario that is at the heart of the general Snowball Earth hypothesis. This hypothesis is supported by results of recent work in geochronology and geochemistry, but certain parts of the glacial sedimentary records remained debated regarding ice sheet dynamics. Here we report findings from four sections of the Sturtian-age Tiesi'ao/Dongshanfeng Formation and related strata on the Yangtze Block in South China. Our work has documented the presence of four lithofacies: massive diamictite, crudely stratified diamictite, stratified to massive pebbly sandstone and dropstone-bearing laminated siltstone and our interpretation is that these rocks were deposited in proximal glacio-marine environments during five successive episodes of glacial advance and retreat. We have also identified an interval of deformation structures near the base of the Tiesi'ao/Dongshanfeng Formation that is characterized by upward increasing strain intensity, indicating that these structures were caused by overriding ice sheets. All the observations collectively suggest that the Sturtian ice sheets in South China were warm-based, rapidly moving, and sensitive to changes in climate. Further, our data suggest that the ice grounding line reached the slope area, implying a major drop in global sea level, enormous ice sheet thickness, or both during the Sturtian Glaciation.

A new palynological assemblage (Carboniferous) from the Chacoparaná Basin, Western Gondwana, and revision of the biostratigraphic scheme

A new palynozone for the upper Serpukhovian–Bashkirian (uppermost Mississippian–lower Pennsylvanian) strata of the Chacoparaná Basin (Argentina) is formally proposed based on palynological assemblages from the Santiago Temple YCF.CO1, YCF.CO2 and YCF.CO3 boreholes, drilled in Córdoba province. Thirty samples were analyzed and 222 palynomorph species identified. The Anapiculatisporites concinnus (Ac) Assemblage Zone is present only in the Santiago Temple boreholes and is divided into two subzones (lower and upper). The Ac Assemblage Zone has been identified in the lower part of the Ordóñez Formation, which is characterized by the presence of diamictites that would have been deposited in glaciolacustrine and glaciomarine environments. The Ac Assemblage Zone is correlated with subzones A and B of the Raistrickia densa–Convolutispora muriornata Zone defined in the western basins of Argentina. The known zones in the Chacoparaná Basin succession are redefined based on a review of borehole information, and the new zone is incorporated. Palynostratigraphically, the basin is divided into the Anapiculatisporites concinnus Assemblage Zone (upper Serpukhovian–lower Bashkirian), Potonieisporites–Lundbladispora Interval Zone (late Bashkirian–Gzhelian), Cristatisporites Interval Zone (late Gzhelian–early Kungurian) and Striatites (Kungurian–early Roadian) Assemblage Zone.

Seasonal changes in foraminiferal assemblages along environmental gradients in Adventfjorden (West Spitsbergen)

Foraminifera are numerically abundant components of the benthic fauna; thus, they are very important compounds in glaciomarine environments. However, ongoing oceanographic changes may influence their distribution, abundances and diversity differently at different times of the year. In the present study, we aimed to assess seasonal environmental changes in an Arctic fjord (Adventfjorden, W Spitsbergen) and the structure and distribution of benthic foraminiferal assemblages during four seasons (January, May, August, November). The study revealed that the benthic foraminiferal assemblages in Adventfjorden are susceptible to environmental changes related to enhanced inflow of shelf-transformed water (STW). We distinguished three zones in the fjord: (1) the deeper outer part influenced by STW, with living specimens staying in the few uppermost layers of sediment, (2) the shallower interior influenced by freshwater outflows and high sediment load, resulting in the dominance of opportunistic glaciomarine taxa constantly buried in the sediment, and (3) the central part, where the seasonal variation in the foraminifera assemblage reflects the interplay between the influence of riverine inputs and STW. Our study also revealed that the presence of STW in Adventfjorden favours an early spring appearance of juvenile foraminifera; however, the majority of the specimens are later withered and do not contribute to the summer pool of mature individuals. On the other hand, the majority of the mature specimens can survive during the winter because of sufficient organic matter stocks and year-long ice-free conditions.

Beryllium isotope signatures of ice shelves and sub-ice shelf circulation

Be isotopes are a useful tracer of sediment source and transport pathways but have not been widely tested in glacio-marine environments. We measured Be isotopes in a range of depositional environments from open marine, sub-ice shelf and subglacial settings throughout Prydz Bay, one of Antarctica's largest ice drainage systems. We find that strong sub-ice shelf and bottom current circulations can advect 10Be-rich open marine sediments into an ice shelf cavity, and 10Be-poor terrestrial sediments onto the continental shelf at the ice shelf outflow, meaning that 10Be concentrations reflect sub-ice shelf circulation patterns rather than depositional environment. However, HCl-extractable 10Be/9Be ratios can provide a more robust discrimination of sediment deposited in open marine and sub-ice shelf settings. Thus, Be isotopes are a useful tracer of both environmental setting and sub-ice shelf circulation strength in both modern and paleo-ice sheet margins.

Cryogenian iron formations in the glaciogenic Kingston Peak Formation, California

The Kingston Peak Formation records glacial sedimentation during the Cryogenian in Death Valley, California, and contains iron formation horizons. These iron formations are part of a thick sedimentary succession containing glaciogenic diamictites, together with mass flow breccias, conglomerates, sandstones and siltstones. The Kingston Peak iron formations are mineralogically and sedimentologically simple, consisting of finely laminated hematitic siltstones that contain up to 50 wt% Fe. These iron formations have been the subject of controversy as they have previously been described as being either absent, or the product of volcanism, diagenesis, hydrothermalism or weathering. Here we present a detailed case study of the sedimentology, stratigraphy, geochemistry and iron isotope composition of the Kingston Peak iron formations. Unequivocal textural evidence indicates a synsedimentary origin for these iron formations. The iron isotopic signature (0.2 < δ56Fe < 1.65) is indicative of partial oxidation of a ferrous iron reservoir. Therefore the Kingston Peak iron formations are interpreted to be primary chemical sediments deposited by the mixing of oxygenated glaciogenic fluids with ferruginous seawater. Pulses of oxidants led to the precipitation of iron oxides, which became enriched under periods of sediment starvation in a basin influenced by episodic mass flows and glacial input. Sedimentological evidence implicates deposition in a range of glaciomarine environments, from ice-distal to relatively ice-proximal, at multiple stages throughout the glacial succession and well before the end of the glacial period. These iron formations have geochemical and sedimentological similarities to other glacially-associated Neoproterozoic iron formations that occur globally and have important implications for our understanding of ocean chemistry and glaciation in the Cryogenian.

Exhumed subglacial landscape in Uruguay: Erosional landforms, depositional environments, and paleo-ice flow in the context of the late Paleozoic Gondwanan glaciation

A well-exposed glacial surface sculpted on Precambrian crystalline basement rocks occurs below the glacial succession of the San Gregorio Formation on the eastern border of the Chaco-Parana Basin in Uruguay and was formed in the context of the late Paleozoic Gondwana Ice Age. On the glacial surface are asymmetric parallel streamlined bedrock landforms interpreted as whalebacks. The downglacier (lee-side) faces of the whalebacks have gentle slopes dipping NNW with striated and sometimes polished surfaces on crystalline rocks. These landforms are covered by 10–100-cm-thick layers of tillites and shear-laminated siltstones, suggesting glacial abrasion produced mainly by subglacial till sliding. The subglacial facies are ice-molded, and exhibit meso-scale glacial lineations such as ridges and grooves up to 30 m long and 30 cm deep. The subglacial association is directly overlain by proglacial fine-grained facies (rhythmites) with dropstones indicating a subaqueous depositional environment following ice-margin retreat. The fine-grained facies are erosively cut by a succession of sandstones with wave-generated stratification resting on a basal conglomerate. Intraformational striated surfaces, NNE-oriented, were found on four distinct bedding planes within the sandstone package and interpreted as ice keel scour marks produced by floating ice. The San Gregorio deposits are partially confined in a wide and shallow subglacial trough and the stratigraphic succession is interpreted as the record of a glacial advance-retreat cycle comparable to deglacial sequences from other late Paleozoic localities. The paleo-ice flow to the NNW indicated by subglacial lineations is parallel to that verified in the southernmost Paraná Basin located north of the study area, suggesting a paleogeographic scenario in which glaciers advanced northward into a glaciomarine environment. The proposed palaeogeography does not confirm the previous hypothesis of an ice center on the Sul-Riograndense Shield but, instead, it corroborates a south-derived Uruguayan Ice Lobe advancing to the north, probably with provenance far afield in terranes of the present-day southern African.