Glacial Influence Research Articles

Tunnel-valley fills in the Paraná Basin and their implications for the extent of late Paleozoic glaciation in SW Gondwana

Tunnel valleys are subglacial incisions formed mainly by meltwater erosion on soft sediment overridden by glaciers. Despite being common in the Pleistocene and Ordovician glacial record, where their fills constitute excellent aquifers and hydrocarbon traps, tunnel valleys are poorly recognized in late Paleozoic strata. Other types of glacial and non-glacial valleys, on the other hand, have been widely reported from Carboniferous and Permian glacial successions, most cut into the basement or older sedimentary rocks and interpreted as glacial valleys eroded by outlet glaciers draining high-relief settings. The degree of glacial influence on the formation of these valleys has strong implications for the volume of ice during late Paleozoic glaciations. In this paper, we report the Lapa sandstone, an exhumed valley-fill succession encased in Pennsylvanian glacial rocks of eastern Paraná Basin, southern Brazil, and its subsurface counterparts. The Lapa sandstone forms a slightly sinuous ribbon several tens of kilometers long, 1500 m wide and 100 m thick composed essentially of glaciofluvial facies. By comparing the external geometry and the sediment fill of the valley with several types of erosive-based, elongated sediment bodies, we conclude that the Lapa formed as the infilling of a tunnel valley cut beneath an ice margin that advanced to the north onto poorly consolidated, mud-rich sediments. Valley fills previously mapped in the subsurface area to the west are considered to have been formed by the same mechanism and during the same glaciation event. The Lapa tunnel valley and its subsurface counterparts are the first example reported from the late Paleozoic glacial record in South America, but similar features are also expected in any formerly glaciated basin, and their recognition may help in estimating the extent of glaciation across the region and guiding exploration for hydrocarbon traps.

Diversity patterns of aquatic macroinvertebrates in a tropical high-Andean catchment

Introduction: Glacierised catchments are remote and hostile environments, in which streams from different water sources (e.g., glacier melt, rain/snowmelt, groundwater) converge, creating a complex mosaic of stream sites with varying levels of glacial influence and environmental conditions. This environmental heterogeneity, in turn, influences the assemblage and composition of aquatic communities and produces complex patterns of species diversity at the catchment scale. Objectives: In this contribution, we assessed biodiversity and community composition of aquatic macroinvertebrate communities from 51 stream site types in a glacierised catchment in the tropical Andes. The aim of our study was to: (1) determine diversity, rarity, commonness and spatial distribution patterns of aquatic macroinvertebrate communities from sites with different water sources, and (2) identify which environmental variables influence the density and presence of macroinvertebrate taxa and, in particular, of the subfamilies of the ubiquitous chironomids. Methods: Our study sites were grouped according to their water source and to their percentage of glacier coverage in the catchment (GCC). At each site we sampled aquatic macroinvertebrates, measured environmental variables and assessed community differences and environmental influence with ordination analyses and generalized linear models. Results: Kryal and mixed sites had an important proportion of rare taxa. Mean richness was highest in the mixed sites and lowest in the sites with the highest glacier cover; while sites with an intermediate percentage of glacier cover, had the highest values of α and β diversity. We found that 13 taxa (15.9%) were common to all stream site types. SIMPER analysis showed that Orthocladiinae, Hyalella sp. and Andesiops sp. contributed the most to the dissimilarity between site types (˃ 45% of cumulative contribution). RDA showed that kryal sites were associated with high turbidity and density of Podonomids, and with low temperature, amount of CPOM and densities of both Anomalocosmoecus sp. and Andesiops sp. Orthocladinae was associated with high current velocity and chlorophyll a concentration, whereas Hyalella sp. had a positive relationship with higher pH and streambed stability. Generalized linear models showed that GCC was the main variable explaining all faunal metrics. Current velocity explained macroinvertebrate abundance, water temperature was related to chironomid density and chlorophyll a influenced Orthocladiinae presence-absence. Conclusions: Our results suggest that by favoring the presence of rare taxa and taxa turnover, glacier influence may increase biodiversity in glacierised catchments. In terms of biodiversity conservation, this study confirms an urgent need to increase knowledge of high-Andean stream biodiversity, especially in highly heterogenous glacierised catchments, to better describe regional biodiversity patterns and community composition of these highly vulnerable freshwater ecosystems. Detailed analyses of benthic communities and development of databases are key for conservation strategies. Water management municipalities and/or enterprises should consider water quality and stream types for more sustainable management of these important ecosystems.

Neoproterozoic non-glaciogenic iron formation: Insights from Fe isotope and elemental geochemistry of the Shalong iron formation from the Central Tianshan block, southern Altaids

The return of widespread banded iron formation (BIF) deposition during the Neoproterozoic Era after a more than one-billion-year hiatus appears to be a hallmark of the Neoproterozoic, and may be indicative of critical changes in the Earth’s environmental and tectonic conditions. However, consensus on the Fe sources and origin of Neoproterozoic iron formations (NIFs) remains elusive. Here we present new petrographic and geochemical data from the newly discovered Shalong NIF from the Central Tianshan block in order to constrain the iron source and elucidate the origin and depositional conditions of the NIF. The major and trace element composition of the Shalong NIF indicates that it is a relatively pure chemical sediment with a low detrital input. Rare earth elements and Y patterns (REY) exhibit seawater-like signature such as depletion in light REEs (LREEs) and enrichment in heavy REEs (HREEs). The weak positive Eu anomalies (Eu/Eu∗ = 1.11–1.31) and consistent chondritic Y/Ho ratios of 25 to 29 (average 26) suggest a predominant influx of low-temperature hydrothermal fluids. The geochemistry of the Shalong NIF also supports a predominant hydrothermal iron source characterized by low-temperature hydrothermal fluids. The lack of negative Ce anomalies, as well as weak positive Eu anomalies, and positive and variable δ56Fe values (−0.09‰ to +1.28‰) in the Shalong NIF indicates partial Fe2+ oxidation in a relatively large dissolved iron marine basin under oxygen-limited conditions. The Shalong NIF deposition likely took place on the slope of an extensional oceanic basin in a low energy environment. The Shalong NIF has a close association with metamorphized- bimodal volcanic rocks and predominant hydrothermal iron source. Combined with no sign for any glacial influence and depositional age prior to the global Sturtian glaciations, we suggest that the Shalong NIF is a non-glaciogenic Neoproterozoic iron formation in association with volcanic and hydrothermal activity. Our results, in conjunction with previous studies, suggest that glacial events are not essential or necessary for the origin of all the NIFs. Instead, hydrothermal activity seems to be a more important controlling factor for the build-up of ferruginous basins and subsequent iron oxidation events in the Neoproterozoic leading to the sudden recurrence of NIFs.

Deepcoral habitats of Glacier Bay National Park and Preserve, Alaska.

Glacier Bay National Park and Preserve (GBNPP) in Southeast Alaska is a system of glaciated fjords with a unique and recent history of deglaciation. As such, it can serve as a natural laboratory for studying patterns of distribution in marine communities with proximity to glacial influence. In order to examine the changes in fjord-based coral communities, underwater photo-quadrats were collected during multipurpose dives with a remotely operated vehicle (ROV) in March of 2016. Ten sites were chosen to represent the geochronological and oceanographic gradients present in GBNPP. Each site was surveyed vertically between 100 and 420 meters depth and photo-quadrats were extracted from the video strip transects for analysis. The ROV was equipped with onboard CTD which recorded environmental data (temperature and salinity), in order to confirm the uniformity of these characteristics at depth across the fjords. The percent cover and diversity of species were lowest near the glaciated heads of the fjords and highest in the Central Channel and at the mouths of the fjords. Diversity is highest where characteristics such as low sedimentation and increased tidal currents are predominant. The diverse communities at the mouths of the fjords and in the Central Channel were dominated by large colonies of the Red Tree Coral, Primnoa pacifica, as well as sponges, brachiopods, multiple species of cnidarians, echinoderms, molluscs and arthropods. The communities at the heads of the fjords were heavily dominated by pioneering species such as brachiopoda, hydrozoan turf, the encrusting stoloniferan coral Sarcodyction incrustans, and smaller colonies of P. pacifica. This research documents a gradient of species dominance from the Central Channel to the heads of the glaciated fjords, which is hypothesized to be driven by a combination of physical and biological factors such as glacial sedimentation, nutrient availability, larval dispersal, and competition.

Glaciomarine sequence stratigraphy in the Mississippian Río Blanco Basin, Argentina, southwestern Gondwana. Basin analysis and palaeoclimatic implications for the Late Paleozoic Ice Age during the Tournaisian.

The Late Paleozoic Ice Age (LPIA) has been well recorded in the uppermost Mississippian–Pennsylvanian of Gondwana. Nevertheless, little is known about the temporal and geographic dynamics, particularly during the early Mississippian. We report on exceptional Tournaisian glaciomarine stratigraphic sections from central Argentina (Río Blanco Basin). Encompassing c . 1400 m, these successions contain conspicuous glacigenic strata with age constraints provided by palaeontological data and U/Pb detrital zircon age spectra. A variety of marine, glaciomarine and fan-deltaic environments indicate relative sea-level variations mainly associated with tectonism and repetitive cycles of glacial activity. Provenance analysis indicates a source from the Sierras Pampeanas basement located to the east. Fifteen sequences were grouped into three depositional models: (1) Transgressive Systems Tracts (TST) to Highstand Systems Tracts (HST) sequences unaffected by glacial ice; (2) Lowstand Systems Tracts (LST) to TST and then to HST with glacial influence; and (3) non-glacial Falling-Stage Systems Tracts (FSST) to TST and HST. The glacial evidence indicates that the oldest Mississippian glacial stage of the LPIA in southwestern Gondwana is constrained to the middle Tournaisian. In contrast with previous descriptions of Gondwanan coeval glacial records, our sequence analysis confirms complex hierarchical climate variability, rather than a single episode of ice advance and retreat. Supplementary material: Detailed stratigraphic sections, palaeocurrents and compositional analysis and U/Pb detrital Zr methodology and data are available at: https://doi.org/10.6084/m9.figshare.c.5011424

Glacial influence on late Pleistocene 10Be-derived paleo-erosion rates in the north-western Himalaya, India

Terrestrial cosmogenic nuclide concentrations in fluvial deposits allow estimation of paleo-erosion rates and reconstruction of the response of landscapes to climatic perturbations. In partly ice-covered landscapes, however, incorporation of subglacially-derived sediments that were shielded by ice from cosmic can lead to erroneous erosion rate calculations. Here, we combine in situ-produced 10Be-derived erosion rates, based on sand and pebbles from a fluvial fill terrace and the modern riverbed in the upper Yamuna catchment, with numerical ice flow modelling to quantify this bias. New luminescence and surface exposure ages suggest that aggradation of the exposed deposits occurred between 29.9 ± 2.5 ka and 14.8 ± 2.8 ka. During most of the deposition, glaciers probably covered ∼19% of the catchment. 10Be concentrations of terrace sand samples differ from those of pebble samples. We obtained the lowest erosion rates from quartzite pebbles, which stem from low elevations, and the highest erosion rates from crystalline pebbles, which stem from high elevations in the Yamuna catchment. We explain these different erosion rates by differences in the steepness of the source areas, an effect that prevails throughout the entire aggradation period despite significant former ice-cover. Sand samples, which are thought to be derived from all elevation parts of the catchment, however show lower 10Be concentrations during the aggradation compared to present-day. We argue that this difference is due to a substantial subglacial origin of the sand during the aggradation period, and not necessarily related to enhanced erosion. We conclude that aggradation of the valley fill in the Yamuna catchment is most likely due to reduced discharge, and only marginally related to higher erosion rates during the late Pleistocene.

Differing limnological responses to late Holocene climate variability in the Cordillera Vilcanota, Peruvian Andes

In recent decades, climate change in Peru’s Cordillera Vilcanota has resulted in massive reductions to its cryosphere and the upslope migration of species and agriculture. Little, however, is known about the response of the region’s many lakes that support local biodiversity and supply water to downstream populations. We analyzed fossil diatom assemblages in dated sediment cores from three lakes with differing morphometry and catchment characteristics to document the limnological response to climate variability over the late Holocene. The study lakes contained similar dominant diatom taxa but recorded markedly different assemblage shifts over time. The two deeper lakes, Laguna Sibinacocha (zmax = 92 m) and Chaca Cocha (zmax = 18 m), contained diatom assemblages that oscillated in dominance between benthic fragilarioids (Staurosirella pinnata, Pseudostaurosira brevistriata) and the planktonic Discostella stelligera. The Chaca Cocha diatom record closely matched the mean state changes in climate inferred from independent proxy records, whereas the record from the glacially influenced Laguna Sibinacocha did not match independent records. In contrast, the shallow study site, Lado del Quelccaya (zmax = 5 m), recorded a complacent diatom profile dominated by benthic fragilarioids with negligible planktonic taxa. The disparate diatom trajectories among the study lakes reflect variations in morphometry and glacial influence and the assemblage shifts appear best explained by climate-driven alterations to thermal stratification. These data offer insight into the primary mechanisms driving limnological change in this region and how different lake types throughout the Andes may respond to future warming.

Runoff Changes from Urumqi Glacier No. 1 over the Past 60 Years, Eastern Tianshan, Central Asia

Glaciers are vital to water resources in the arid land of central Asia. Long-term runoff records in the glacierized area are particularly valuable in terms of evaluating glacier recession and water resource change on both a regional and global scale. The runoff records of streams draining basins with 46% current glacier cover, located at the Urumqi Glacier No. 1 in the source area of the Urumqi River in eastern Tianshan, central Asia, were examined for the purpose of assessing climatic and glacial influences on temporal patterns of streamflow for the period 1959–2018. Results suggest that runoff from the catchment correlates well with temperature and associated precipitation data. During the period 1993–2018, it increased by 114.39 × 104 m3, which was 1.7 times the average runoff during the period 1959–1992. A simple water balance model is introduced to calculate the different components of the runoff, including precipitation runoff from glacier surface and from nonglacial areas, glacier mass balance and glacial runoff. Thus, the long-term change of each component and its response to climate change are revealed. We found that the period 1997–2018 is likely to be the “peak water” (tipping point) of the glacial runoff resulting from shrinkage of glacier area.

Foraminiferal Patterns in Deglacial Sediment in the Western Ross Sea, Antarctica: Life Near Grounding Lines

AbstractImproved multibeam swath bathymetry allows targeted coring of glacial landforms aiming at improving our understanding of sedimentary facies that developed in glacimarine settings during the post‐Last Glacial Maximum (LGM) deglaciation. Coupled with radiocarbon dates, we explore foraminiferal records from 18 sediment cores from the western Ross Sea largely from sites near paleo–grounding lines. We investigate post‐LGM foraminiferal assemblages from glacimarine environments, including those proximal and more distal to paleo–grounding lines, including environments influenced by subglacial meltwater outflow and further removed from direct glacial influence and subject to different oceanographic conditions. Agglutinated benthic foraminiferal assemblages dominate open marine facies deposited under the presence of High Salinity Shelf Water and significant primary production, while calcareous foraminiferal assemblages characterize grounding line‐proximal settings, some of which were potentially influenced by Modified Circumpolar Deep Water. Rapid deposition of meltwater plume deposits inhibited and, in some cases, significantly altered foraminifera abundance and diversity. Broadly in the Ross Sea, it appears that the high bathymetric gradient of grounding zone wedges is a key factor promoting rich benthic foraminiferal communities in habitats proximal to grounding lines. Therefore, we demonstrate that paleo–grounding line settings may archive high quality in situ foraminiferal data, which is imperative for paleoenvironmental and geochemical studies on glaciated continental margins worldwide.

Suspension feeders as natural sentinels of the spatial variability in food sources in an Antarctic fjord: A stable isotope approach

• δ 13 C and δ 15 N signatures of benthic suspension feeders differed across Potter Cove. • Food sources presented obvious small-scale spatial heterogeneity across Potter Cove. • Mixing models showed different contributions of food sources across Potter Cove. • Glacial melt-related environmental conditions govern the spatial heterogeneity of food sources. We examined the effect of glacial melt-related retreat environmental conditions on the spatial heterogeneity of food sources for Antarctic nearshore marine benthic communities of Potter Cove, a representative fjord on King George Island on the West Antarctic Peninsula. Focusing on Antarctic suspension feeders, we surveyed the variation of carbon (δ 13 C) and nitrogen (δ 15 N) stable isotopes in three different stations with contrasting characteristics in terms of glacier-sedimentary regimes and distance from the glacier. Bayesian mixing models were used to estimate the relative contribution of eight potential food sources at the study sites and to the consumers' diet. The isotopically signatures of suspension feeders displayed a broad range of assimilate available food sources with distinct contributions at each site. Overall, for most of the suspension feeders analysed, contributions of suspended particulate matter and macroalgae detritus tended to prevail over the rest of the potential food sources. Additionally, we confirm the assimilation of krill faeces by benthic organisms reinforcing its relevance as food source in nearshore Antarctic ecosystems. Mixing models corroborated the influence of glacier discharge on the spatial distribution of available food sources, since their contribution in the suspension feeders diet was different and more heterogeneous in the internal stations (ice-proximal zone) than in the external one (ice-distal zone). The analysis of isotopic signatures of primary benthic consumers (suspension feeders) indicated the spatial heterogeneity of food sources and its composition in coastal Antarctic nearshore, as consequence of the glacial influence on the local environment and hydrology.

The non-glacial and non-cratonic origin of an early Archean felsic volcaniclastic unit, Barberton Greenstone Belt, South Africa

Recent studies have suggested that ~3.445 Ga felsic volcaniclastic rocks of member H6 of the Hooggeneoeg Formation in the Barberton Greenstone Belt, South Africa, show evidence for deposition above a major erosional unconformity under glacial to periglacial conditions and contain debris eroded from older, granitic continental crust. These conclusions represent fundamental re-interpretations of this sequence, its crustal setting, and processes of deposition. We here present evidence that these felsic strata represent terrestrial to shallow water mass flows deposits, storm-influenced shallow-marine units, and turbidity-current-like, storm-driven submarine flows, all of which lack any evidence of a glacial influence during sedimentation. They accumulated as part of a conformable stratigraphic sequence and show no evidence of mantling an erosional surface. The debris consists entirely of volcanic-derived materials. No clasts of plutonic rock are found in the conglomerates. Low-temperature diagenesis and potash metasomatism have altered primary volcanic feldspar in the volcaniclastic debris to tartan twinned microcline, previously thought to be characteristic of granitic plutonic rocks. This potassium feldspar is quite distinctive and unlike the microcline found in plutons surrounding the greenstone belt. This succession is part of a classic felsic volcaniclastic sequence deposited as an apron flanking a major felsic volcanic center.

Glacial influence on the iron and sulfur cycles in Arctic fjord sediments (Svalbard)

Arctic fjord sediments of Svalbard receive terrestrial material from glacial runoff and organic matter from marine primary productivity. Organic carbon mineralization proceeds primarily through sulfate and iron reduction in the fjord sediment. The ongoing retreat of glaciers in the high Arctic is altering the input of glacial material to the fjords, with unknown consequences for the iron and sulfur cycles in the fjord sediments. We measured sulfate reduction rates in sediment cores and analyzed porewater geochemistry, then compared these results to long-term sediment incubations to determine the rates of iron reduction and sulfide oxidation in three glacially influenced fjords on the west coast of Spitsbergen, Svalbard. Despite an abundance of glacially-sourced Fe(III)-oxide minerals, active sulfate reduction took place throughout the sediment. Analyses of the sulfur and oxygen isotopic composition of porewater sulfate and sulfate concentrations suggest that sulfide produced from biological sulfate reduction is reoxidized to sulfate. Long-term sediment incubations indicated sulfide oxidation at all three stations. The rate of sulfide oxidation was controlled by both the rate of sulfate reduction and the quantity and reactivity of Fe(III)-oxides. In our experimental incubations, we detected a decrease in Fe(III) content of the 0.5 M HCl and ascorbate-extractable fractions, but not in the 6 M HCl fraction, indicating that the highly reactive Fe(III) fraction is utilized by microorganisms and serves as the oxidant for sulfide oxidation. Our results show that sulfide oxidation in glacially-influenced fjord sediments is a wide-spread geochemical process. Further warming will drive glacial retreat onto land, where sediment-laden glacial meltwater will be altered during flow through proglacial streams and lakes before entering the marine environment. Fjord sediments will likely become more sulfidic, as glaciers deliver less particulate, highly-reactive metal oxides to the marine environment.

Strategy for Detection and High-Resolution Characterization of Authigenic Carbonate Cold Seep Habitats Using Ships and Autonomous Underwater Vehicles on Glacially Influenced Terrain

Cold seep habitats with authigenic carbonates and associated chemosynthetic communities in glacially-influenced terrains constitute an important part of the benthic ecosystems, but they are difficult to detect in large-scale seabed surveys. The areas they occupy are normally small, and survey platforms and sensors allowing high-resolution spatial characterization are necessary. We have developed a cold seep habitat mapping strategy that involves both ship and autonomous underwater vehicle (AUV) as platforms for multibeam echosounder, synthetic aperture sonar and a digital photo system. Water column data from the shipborne multibeam echosounder data are initially used to detect gas flares resulting from fluid flow from the seabed. The next phase involves mapping of flare areas by synthetic aperture sonar, mounted on an AUV. This yields an acoustic image with a resolution up to 2 cm over a swath of c. 350 metres, allowing detection of seep-related features on the seabed. The last phase involves digital photographing of the seabed, with the AUV moving close to the seabed, allowing recognition of bubble streams, seep-related features and giving a first order documentation of the fauna. The strategy was applied to a 3775 km2 large area on the continental shelf, northern Norway. This is a passive continental margin, with thick deposits of oil- and gas-bearing sedimentary rocks. Extensive faulting and tilting of layers provide potential conduits for fluid flow. The seabed is glacially influenced with a highly-variable backscatter reflectivity. More than 200 gas flares have been identified, and a similar number of cold seep habitats have been characterized in high spatial detail. Two case studies are shown. In the first area, there is a close spatial relation between active gas seepage and carbonate crust fields. The second case study shows that carbonate crust fields are not necessarily spatially associated with currently active seeps, but represent dormant or formerly active gas expulsion. An important finding is that the bathymetric resolution of shipborne multibeam echosounders will often be too low to detect cold seep habitats. This means that a nested multi-resolution approach involving a multitude of platforms and sensors is required to provide the full picture.

Food Web Complexity of High Mountain Lakes is Largely Affected by Glacial Retreat

High mountain lakes provide essential ecosystem services and have a high conservation value. Therefore, understanding how glacier retreat will affect their ecological functioning and water quality is crucial. Here, we tested how shallow high mountain lakes having different glacial influences differ in their abiotic main features and food web structure using a multiple ecological indicator approach. We identified 13 functional groups within the planktonic and littoral communities, each one representing a biotic indicator and a node in a simplified food web network. The abiotic environment and most functional groups differed significantly as a consequence of the glacial influence. In general, runoff from glacial meltwaters resulted in highly simplified food webs. Considering that many turbid glacially fed lakes are losing their hydrological connection with disappearing/retreating glaciers and shifting to a clear state, our results suggest that this shift could enhance food web complexity, but at the cost of losing specific habitats. Further, retreat of large glaciers will form new glacially fed lakes, but it remains unclear whether this will buffer the expected habitat and biodiversity loss.

Subaqueous debrites of the Grand ConglomÉrat Formation, Democratic Republic of Congo: A model for anomalously thick Neoproterozoic: “Glacial” diamictites

ABSTRACTVery thick (> 1 km) successions of matrix-supported conglomerates (diamictites) are a very distinctive component of many Neoproterozoic basins. Classically interpreted as glacially deposited sedimentary rocks, their thickness has been seen as requiring exceptional depositional conditions such as world-wide “panglacial” climates. The Neoproterozoic Grand Conglomérat Formation (GC) of Katanga Province, southeastern Democratic Republic of Congo, is a 1.8-km-thick diamictite succession hosting one of the world's largest stratiform copper deposits. Examination of more than 300 km of recently acquired large diameter (up to 4 inches, 10.2 cm) core identifies the diamictites of the GC as debrites that accumulated as part of a deep-water “mass-transport complex” in a tectonically active and volcanically influenced anoxic rift basin. Detailed sedimentological descriptions of debrite facies and their lateral and vertical variability permits new insights into processes of debris-flow formation and transport, and their wider paleoenvironmental and paleotectonic significance. A genetic model is herein presented that highlights the importance of slumping and subaqueous downslope mixing of basin-margin fan-delta gravels, fault breccias, volcanic debris from contemporaneous basaltic fissure eruptions with basinal muds, and the downslope ponding of flows in narrow fault-bounded depocenters. Preservation of such an exceptionally thick subaqueously deposited debrite-dominated mass transport complex lacking any evidence of shallow-water deposition, requires rapid subsidence. Any direct sedimentary evidence of a glacial influence on sedimentation in the wider basin hinterland (if present) has been destroyed and homogenized by mass flow. Indirect evidence of a cold-climate setting is possibly expressed as lonestones in laminated turbidite facies dropped by either glacial or seasonal ice, and by exceptionally rare scratched clasts that may have been striated subglacially. Descriptions and interpretations presented here provide clues to the origin of other unusually thick debrite and turbidite successions elsewhere in other Neoproterozoic basins; their primary paleoenvironmental significance is that they appear to record ponding and focusing of mass flows in narrow, rapidly subsiding fault-bounded depocenters, rather than any unique glacial paleoclimate.

Seasonal variability in glacial influence affects macroinvertebrate assemblages in North-Andean Patagonian glacier-fed streams

ABSTRACTMacroinvertebrate communities of glacial streams are affected by melting mountain glaciers, which in turn are influenced by landscape and local and global climate change. We analyzed macroinvertebrate communities in 2 North-Andean Patagonian glacier-fed streams. The significant difference in glacial influence between the 2 streams was due to differences in glacier flow velocity and to the presence of a proglacial lake in one of the streams. Our main aim was to analyze whether temporal variations in physical and chemical parameters of meltwater produced shifts in macroinvertebrate taxon richness and community composition. We applied the glaciality index (GI) to consider the combined effect of suspended solids, water temperature, conductivity, and streambed stability. On each stream we established one sampling station; we sampled during the melting season (spring–summer) of 3 consecutive years on 12 occasions. Macroinvertebrate taxon richness, diversity, and evenness increased as glacial influence decreased. Scrapers were the dominant functional feeding group in the stream with the lower glacial influence while detritivores-gathers dominated under high glacial influence. We further analyzed how temporal changes in the GI affected compositional differences among communities using Bray-Curtis dissimilarities. Under a stable GI, communities showed no substantial compositional changes (low Bray-Curtis), whereas under change in glacial influence (ΔGI ≠ 0) the communities decreased in similarity.

Glaciers, flows, and fans: Origins of a Neoproterozoic diamictite in the Saratoga Hills, Death Valley, California

The Kingston Peak Formation of Death Valley, California, provides an exceptional archive of Cryogenian glaciation and concomitant rifting of Rodinia. In the Saratoga Hills, an 800 m thick succession of diamictite-dominated strata is exposed, allowing lithofacies and clast compositions to be studied in detail, and for the relative influence of glacial versus slope processes on sedimentation to be critically assessed for the first time. Two detailed sections, 400 m apart, allow four facies associations to be established: (1) Thinly Laminated Argillaceous Sandstone (interpreted as low density turbidites), (2) Laminated, Deformed, and Brecciated Limestone (interpreted as carbonate-sourced turbidites), (3) Bedded Massive Diamictite (interpreted as glacially-fed debrite), and (4) Interbedded Mudstone, Sandstone, and Diamictite (interpreted as stratified diamictite deposited via ice rafted debris and passing gradually into turbidites separated by intensely deformed intervals). Bedded massive diamictite is dominant in the succession and shows a gradual up section transition into interbedded mudstone, sandstone, and diamictite. This succession is interpreted to record a waning glacial influence: from glacially sourced debris flows to stratified diamictite strongly influenced by ice rafted debris. Clasts in the diamictite include carbonate, siliciclastic intraclasts, granite, diabase, gneiss, and quartzite. Clast presence profiles suggest that in spite of the comparable along-strike facies profiles between the two logs, clast content is significantly different within the same stratigraphic unit. This finding suggests that clast content is of little use in aiding correlation and lithostratigraphic subdivision of the Kingston Peak Formation. The differences in clast compositions is envisaged to be due to complex drainage network. Examinations of the sedimentary structures and stacking pattern of the lithofacies, complex drainage network, and localized deformation features of FA4 all support deposition as part of a trough mouth fan system.

An integrated chemostratigraphic (δ13C-δ18O-87Sr/86Sr-δ15N) study of the Doushantuo Formation in western Hubei Province, South China

High-resolution isotopic analyses were conducted on core samples from the Ediacaran Doushantuo Formation at the Wangjiapeng section, western Hubei Province in South China, whereby two laterally traceable, negative δ13Ccarb excursions (EN1 and EN3) were recognized. The magnitude and duration of these excursions permit intra-basinal and inter-basinal correlation, which indicates that they probably represent a global change in seawater composition. The occurrence of decoupled δ13Ccar-δ13Corg with almost invariable δ13Corg values at Wangjiapeng, Zhongling, Yangjiaping sections is consistent with remineralization of a dissolved organic carbon (DOC) pool by means of sulfate reduction, as recorded in EN3. The synchronous presence of EN3, a shift to higher 87Sr/86Sr and decrease of Mn and Fe contents and δ15N values together points to a glacial influence whereby oxygenation and remineralization of reduced carbon produced 13C-depleted DIC. Glaciations cause a drop in sea level, which itself leads to increased continental shelf area to be exposed to surface weathering, and ultimately to enhanced delivery of radiogenic 87Sr. The increase of 87Sr/86Sr ratios, sulfate and phosphate are consequences of surface runoff into oceanic environments and such perturbations induce biogeochemical changes.

Basinal setting and origin of thick (1·8 km) mass‐flow dominated Grand Conglomérat diamictites, Kamoa, Democratic Republic of Congo: Resolving climate and tectonic controls during Neoproterozoic glaciations

AbstractThe Kamoa sub‐basin, in the south‐eastern part of the Democratic Republic of Congo, is a rift basin that hosts a world‐class stratiform copper deposit at the base of a very thick (1·8 km) succession of matrix‐supported conglomerates (diamictite) (Grand Conglomérat Formation) that has been interpreted by some as the product of deposition in the aftermath of a planet‐wide glaciation. Newly available subsurface data consisting of more than 300 km of drill core throws new light on the origin of diamictite and associated facies types, and their tectonic, basinal and palaeoclimatic setting. Initiation of rifting is recorded by a lowermost subaqueous succession of fault‐related mass flow conglomerates and breccias (the ‘Poudingue’) with interdigitating coeval and succeeding sandstone turbidites (Mwashya Subgroup). Overlying diamictites of the Grand Conglomérat were deposited as subaqueous debrites produced by mixing and homogenization of antecedent breccias and gravel from the Poudingue and Mwashya sediments with basinal muds. Failure of over‐steepened basin margins and debris flow was likely to be triggered by faulting and seismic activity, and was accompanied by syn‐depositional subaqueous basaltic magmatism recorded by peperites and pillow lavas within diamictites. The thickness of diamictites reflects recurring phases of faulting, volcanism and rapid subsidence allowing continued accommodation of rapidly deposited resedimented facies well below wave base. A distal or indirect, glacial influence in the form of rare dropstones and striated clasts is evident, but tectonically‐driven mass flow destroyed any primary record of glacial climate originally present in basin margin sediments. Such basin margin settings were common during Rodinia rifting and their stratigraphy and facies record a dominant tectonic, rather than climatic, control on sedimentation. Deposition occurred on tectonic timescales inconsistent with a Snowball Earth model for Neoproterozoic diamictites involving a direct glacial contribution to deposition.

Spatial and temporal variation of benthic macroinvertebrate assemblages during the glacial melt season in an Italian glacier-fed stream

The biodiversity of glacier-fed streams is particularly threatened by climate change, emphasising the need of monitoring these sentinel systems. The glacier-fed Saldur stream is an International Long Term Ecological Research (ILTER) site in the Italian Central Eastern Alps. Here, we sampled benthic macroinvertebrates and measured environmental variables (discharge, suspended solids, conductivity, water temperature, and channel stability) five times at six sites (5–11 km from the glacier) during an entire glacial melt season (April–September). Our main objectives were (1) to elucidate relationships between the abiotic variables and the faunal composition, (2) to quantify and compare the spatial and temporal variability of the faunal community, and (3) to assess the composition of the benthic macroinvertebrate community in relation to conceptual models. Hosting a higher number of individuals and more diverse communities at sites with reduced glacial influence, the Saldur stream fitted well in the framework of conceptual models. Nevertheless, the spatial variability of the fauna was higher than the temporal variability. This study presents an initial characterisation of the benthic faunal assemblages in the Saldur stream, constituting a reference point for future analyses dealing with potential disruptive factors introduced by climate change and upcoming hydroelectric power production on this stream.