Glacial Influence Research Articles

An interglacial on snowball Earth? Dynamic ice behaviour revealed in the Chuos Formation, Namibia

AbstractThe Sturtian is the oldest (ca 716 Ma) of three pan‐global glaciations in the Cryogenian. At Omutirapo, in northern Namibia, a 2 km wide, 400 m deep palaeovalley is filled by glaciogenic strata of the Chuos Formation, which represents the Sturtian glacial record. Sedimentary logging of an exceptionally high‐quality exposure permits detailed stratigraphic descriptions and interpretations, allowing two glacial cycles to be identified. At the base of the exposed succession, strong evidence supporting glaciation includes diamictites, ice‐rafted dropstones and intensely sheared zones of interpreted subglacial origin. These facies collectively represent ice‐proximal to ice‐rafted deposits. Upsection, dropstone‐free mudstones in the middle of the succession, and the absence of diamictites, imply sedimentation free from glacial influence. However, the reappearance of glacial deposits above indicates a phase of Sturtian glacial re‐advance. Comparison with age‐equivalent strata in South Australia, where evidence for sea‐ice free sedimentation has been established previously, suggests that a Sturtian interglacial may have been extensive, implying global‐scale waxing and waning of ice sheets during a Cryogenian glacial event.

Recent glacially influenced sedimentary processes on the East Greenland continental slope and deep Greenland Basin

This paper presents the morpho-sedimentary characterization and interpretations of the assemblage of landforms of the East Greenland continental slope and Greenland Basin, based on swath bathymetry and sub-bottom TOPAS profiles. The interpretation of landforms reveals the glacial influence on recent sedimentary processes shaping the seafloor, including mass-wasting and turbidite flows. The timing of landform development points to a predominantly glacial origin of the sediment supplied to the continental margin, supporting the scenario of a Greenland Ice Sheet extending across the continental shelf, or even to the shelf-edge, during the Last Glacial Maximum (LGM). Major sedimentary processes along the central section of the eastern Greenland Continental Slope, the Norske margin, suggest a relatively high glacial sediment input during the LGM that, probably triggered by tectonic activity, led to the development of scarps and channels on the slope and debris flows on the continental rise. The more southerly Kejser Franz Josef margin has small-scale mass-wasting deposits and an extensive turbidite system that developed in relation to both channelised and unconfined turbidity flows which transferred sediments into the deep Greenland Basin.

Loss of small glaciers will diminish beta diversity in Pyrenean streams at two levels of biological organization

ABSTRACTAim Small (< 1 km2) alpine glaciers are likely to disappear in this century, resulting in decreased regional habitat heterogeneity in associated streams. Both heterogeneity within and spatial isolation among glacier‐influenced streams can enhance beta diversity of stream‐dwelling organisms. We measured beta at both community and population‐genetic levels within and among streams currently influenced by small Pyrenean glaciers. We aimed to evaluate whether patterns are analogous between the two levels, to apply various approaches for characterizing beta, and to infer the outcome of future glacier loss on regional biodiversity.Location Four glacier‐fed basins in the Parc National des Pyrénées, France.Methods We classified each of 18 stream reaches across the basins into either high‐, mid‐ or low‐‘glaciality’ (glacial influence) groups according to four physicochemical characteristics. At each reach, we collected macroinvertebrate communities and evaluated mitochondrial DNA haplotypes for 11–13 individuals of Baetis alpinus Pictet. Using taxa/haplotypes as basic units, we evaluated community and population‐genetic beta diversity simultaneously. We measured beta diversity in three major ways: as multivariate (Sørensen's dissimilarity, Jost D) and ‘classical’ (gamma/alpha) variation to compare among glaciality groups, and as turnover along the glaciality gradient within each basin.Results For most approaches at both organizational levels, beta was greatest among high‐glaciality reaches, absolute values of variation of beta in high‐glaciality streams were strikingly similar between levels, and the steepest turnover within basins occurred between high‐ and mid‐glaciality reaches. Therefore, high‐glaciality reaches contained assemblages and populations that were unique both within that stream type (among basins) and compared with other stream types within basins.Main conclusions Parallel beta diversity patterns at population‐genetic and community levels suggested that environmental drivers influence these levels analogously. Extreme conditions (e.g. low temperature, high instability, isolation) in high‐glaciality streams probably enhance beta at both levels. Stream beta diversity is likely to decrease substantially with continued glacial reduction in this system.

Global and continental-scale glaciations on the Precambrian earth

Earth’s climate during the Proterozoic Eon was marked by major glacial events with evidence for large continental ice sheets on many cratons, and with sedimentological data indicating that glaciers had extended to sea-level. This paper emphasizes the sedimentological and sequence stratigraphic responses to glaciations and evaluates the major driving forces of glaciations during the Precambrian. First- and second-order sequences are recognized related to continental-scale fragmentation and formation of marine rift basins wherein sedimentary rocks indicate glacial influences and pronounced tectonic-climatic linkages. Coarse syn-rift deposits are typically characterized by mass flow diamictites and conglomerates. It is important to undertake sedimentological facies and sequence stratigraphic analysis of these syn-rift and capping passive margin sequences, as well as of slope turbidite deposits which formed if enhanced subsidence of the basins was occurring. More generally, latitude and syn-rift tectonic uplift can cause formation of glacial ice and enhance distinctive glacial influences on extensional basin sedimentation, thus supporting a causal relationship between thermal subsidence and the formation of glacier ice on inland areas. During the Precambrian, however, it is suggested that long-lived marine-terminated glaciers also situated at low paleolatitudes, were related to an extensional tectonic setting. In such settings, glacial deposits associated with sedimentary sequences of distinctively different origin, e.g. carbonate and chemically mature siliciclastic sequences, can well be used to detect the prominent sequence boundaries to verify depositional systems tracts. Internal sediment stacking patterns in sequences are indicative of dynamic processes along glaciated continental margins and without always having the need for global synchroneity. In glacially influenced rift basins and continental margins it is important to recognize the sequence boundaries of significant subaerial unconformities and their correlative conformities. A sequence boundary is a chronostratigraphically significant surface always produced as a consequence of a change in relative sea-level. These can then be well related to initiation and decay of glaciations, however on-land glacial deposits in a very few cases are prevented from later erosion. Attenuation of continental crust during rifting and breakup of the continent raises relative sea-level and also many of the shallow intra-cratonic basins subsided below sea-level, in favourable conditions being affected by major continental glaciations.Paleoproterozoic and Neoproterozoic glacial deposits are known in North and South America, South Africa, India, Western Australia and Fennoscandia. Against this background, continental-scale to global glaciations in the Precambrian appear to be possible, however views on the causes and timing of these glaciations, as well as on planetary extent of ice cover are still contradictory. There is a need to continue detailed sedimentological studies of pre-glacial and post-glacial deposits as well as to interpret syn-glacial lithofacies for their inferred transportation and depositional processes. Pre-glacial deposits, especially, should provide a new target to help us understand the processes that initiated these Precambrian glaciations. The sequence stratigraphic approach with understanding of the stacking pattern of depositional systems could prevent oversimplification and use of just single events to explain the complexity of evolution of glacially influenced Precambrian continental margin sediments.

Neogene benthic foraminiferal assemblages and paleoenvironmental record for McMurdo Sound, Antarctica

The Antarctic Geological Drilling (ANDRILL) program recovered a 1,138-m-long core, AND-2A, during the austral spring 2007–2008 from the Victoria Land Basin for the Southern McMurdo Sound (SMS) Project. The main objective of the SMS Project was to establish a Neogene (ca. 24–2.5 Ma) history recording ice volume changes from an ice-proximal site, in particular the environmental response of the Antarctic margin during the middle Miocene climate transition. This study focuses on the foraminifera taxa recovered from AND-2A, providing a paleoenvironmental analysis including paleobathymetry and ice-proximity estimates. Three assemblages, Cibicides-Cassidulinoides , Ammoelphidiella , and Ehrenbergina , as well as two subassemblages within the Ammoelphidiella assemblage, Globocassidulina and Nonionella , were identified in this study using cluster analysis. Calcareous eurytopic taxa dominate every assemblage and display similarities between Fresh Shelf Water assemblages and Ice Edge Biofacies that were previously defined in other studies focused in the Southern Ocean, as well as assemblages recovered from previous drilling projects in the Ross Sea region. Paleoenvironmental interpretations include an early Miocene record of fluctuating periods of glacial influence from ice-distal, abrupt cooling followed by a continued warming and transition into a more ice-proximal setting in the uppermost middle Miocene. The well-preserved middle Miocene record reflects more productive stable environmental conditions with significant freshwater input from outlet glaciers and rivers at the East Antarctic margin, and coincides with previously published palynomorph data indicating a short abrupt warming. Furthermore, the persistent appearance of the planktonic species Neogloboquadrina pachyderma and glacial/sea ice assemblages can be related to the onset of progressive cooling at the Antarctic margin, including periods of fluctuating sea ice extent or glacial proximity. Finally, the Pliocene record contains taxa similar to that of Fresh Shelf Water assemblages and Ice Edge Biofacies recovered in the Southern Ocean today. Paleobathymetric estimates range from inner shelf depths (∼50–150 m) in the early Miocene reaching periods of outer-shelf depths (∼400 m) in the uppermost early–middle Miocene and Pliocene.

Structure of marine predator and prey communities along environmental gradients in a glaciated fjord

Spatial patterns of marine predator communities are influenced to varying degrees by prey distribution and environmental gradients. We examined physical and biological attributes of an estuarine fjord with strong glacier influence to determine the factors that most influence the structure of predator and prey communities. Our results suggest that some species, such as walleye pollock (Theragra chalcogramma), black-legged kittiwake (Rissa tridactyla), and glaucous-winged gull (Larus glaucescens), were widely distributed across environmental gradients, indicating less specialization, whereas species such as capelin (Mallotus villosus), harbor seal (Phoca vitulina), and Kittlitz's murrelet (Brachyramphus brevirostris) appeared to have more specialized habitat requirements related to glacial influence. We found that upper trophic level communities were well correlated with their mid trophic level prey community, but strong physical gradients in photic depth, temperature, and nutrients played an important role in community structure as well. Mid-trophic level forage fish communities were correlated with the physical gradients more closely than upper trophic levels were, and they showed strong affinity to tidewater glaciers. Silica was closely correlated with the distribution of fish communities, the mechanisms of which deserve further study.

Deep-water geomorphology of the glaciated Irish margin from high-resolution marine geophysical data

The continental margin offshore of western Ireland offers an opportunity to study the effects of glacial forcing on the morphology and sediment architecture of a mid-latitude margin. High resolution multibeam bathymetry and backscatter data, combined with shallow seismic and TOBI deep-towed side-scan sonar profiles, provide the basis for this study and allow a detailed geomorphological interpretation of the northwest Irish continental margin. Several features, including submarine mass failures, canyon systems and escarpments, are identified in the Rockall Trough for the first time. A new physiographic classification of the Irish margin is proposed and linked to the impact of glaciations along the margin. Correlation of the position and dimensions of moraines on the continental shelf with the level of canyon evolution suggests that the sediment and meltwater delivered by the British–Irish Ice Sheet played a fundamental role in shaping the margin including the upslope development of some of the canyon systems. The glacial influence is also suggested by the variable extent and backscatter signal of sedimentary lobes associated with the canyons. These lobes provide an indirect measurement of the amount of glaciogenic sediment delivered by the ice sheet into the Rockall Trough during the last glacial maximum. None of the sedimentary lobes demonstrates notable relief, indicating that the amount of glaciogenic sediment delivered by the British–Irish Ice Sheet into the Rockall Trough was limited. Their southward disappearance suggests a more restricted BIIS, which did not reach the shelf edge south of 54°23′ N. The various slope styles observed on the Irish margin represent snapshots of the progressive stages of slope development for a glacially-influenced passive margin and may provide a predictive model for the evolution of other such margins.

Chironomid (Diptera) distribution and diversity in Tibetan streams with different glacial influence

Abstract. 1. We collected chironomid larvae and measured basic environmental variables in eight high mountain streams with different degrees of glacial influence in southern Tibet.2. In total, 32 chironomid taxa were collected. The Orthocladiinae subfamily dominated both in taxon number and in abundance.3. Both average taxon number and abundance were lower in glacier‐fed streams compared with those of non‐glacial origin. The total taxon number collected (γ diversity) was similar regardless of glacial influence. However, spot diversity (α diversity) was higher in non‐glacial streams, while glacier‐fed streams supported higher species turnover (β diversity).4. Detrended correspondence analysis scattered the study sites along the first ordination axis, representing a combination of distance from glacier and channel stability. Two‐way indicator analysis distinguished three groups of sites. Group 1 represented the sites closest to the glacier and were characterised by unstable channel conditions and low temperature with characteristic taxa Diamesa sp. 1, Orthocladius (Eud.) sp. and Rheocricotopus sp. Group 2 was made up of glacier‐fed streams situated further from glaciers, with unstable channels and characterised by Orthocladius (Euo.) sp. Group 3 contained non‐glacial streams as well as a glacier‐fed stream further from the glacier margin. For these sites, stable channels and high conductivity were characteristic and Cricotopus (C.) sp., Pseudosmittia sp, Polypedilum sp., Eukiefferiella gracei group and Pagastia sp. 1 were the dominating taxa.5. We propose a general distribution pattern of chironomids in the streams of glacial and non‐glacial influence in Tibet, which is in accordance with the model proposed by Milner et al. (2001a) Freshwater Biology, 46, 1833–1847.

Glacial influence on the geochemistry of riverine iron fluxes to the Gulf of Alaska and effects of deglaciation

Riverine iron (Fe) derived from glacial weathering is a critical micronutrient source to ecosystems of the Gulf of Alaska (GoA). Here we demonstrate that the source and chemical nature of riverine Fe input to the GoA could change dramatically due to the widespread watershed deglaciation that is underway. We examine Fe size partitioning, speciation, and isotopic composition in tributaries of the Copper River which exemplify a long-term GoA watershed evolution from one strongly influenced by glacial weathering to a boreal-forested watershed. Iron fluxes from glacierized tributaries bear high suspended sediment and colloidal Fe loads of mixed valence silicate species, with low concentrations of dissolved Fe and dissolved organic carbon (DOC). Iron isotopic composition is indicative of mechanical weathering as the Fe source. Conversely, Fe fluxes from boreal-forested systems have higher dissolved Fe concentrations corresponding to higher DOC concentrations. Iron colloids and suspended sediment consist of Fe (hydr)oxides and organic complexes. These watersheds have an iron isotopic composition indicative of an internal chemical processing source. We predict that as the GoA watershed evolves due to deglaciation, so will the source, flux, and chemical nature of riverine Fe loads, which could have significant ramifications for Alaskan marine and freshwater ecosystems.

Environmental harshness and global richness patterns in glacier‐fed streams

ABSTRACTAim To test for a possible effect of environmental harshness on large‐scale latitudinal and elevational patterns in taxon richness of macrofauna in arctic and alpine glacier‐fed streams.Location Svalbard (79° N), Iceland (65° N), Norway (62° N), Switzerland and Italy (46° N), France (43° N), New Zealand (43° S) and Ecuador (0°), covering an elevational gradient from sea level to 4800 m a.s.l.Methods We gathered data from 63 sites along 13 streams and created an index of glacial influence (the glacial index, GI) as an integrative proxy for environmental harshness. The explicative power of the GI, environmental variables, latitude and elevation on taxon richness was tested in generalized linear models. Taxon richness along geographical gradients was analysed at standardized levels of GI in contour plots. Beta diversity and assemblage similarity was calculated at different GI intervals and compared with a null‐model.Results Overall, taxon richness decreased exponentially with increased GI (r2= 0.64), and of all included factors, GI had the highest explicative power. At low values of GI we found that local taxon richness varied along the coupled gradients of latitude and elevation in a hump‐shaped manner. However, this pattern disappeared at high values of GI, i.e. when environmental harshness increased. Beta diversity increased, while similarity among assemblages decreased towards high GI values.Main conclusions In our study system, the number of taxa able to cope with the harshest conditions was largely independent of the regional taxon pool, and environmental harshness constituted a ‘fixed’ constraint for local richness, irrespective of latitude and elevation. Contrary to expectations, we found that beta diversity was highest and similarity lowest among the harshest sites, suggesting that taxon richness was not solely driven by niche selection based on environmental tolerances, but also stochastic ecological drift, leading to dispersal‐limited communities.

Chapter 3 Recognition of glacial influence in Neoproterozoic sedimentary successions

Abstract This chapter provides an overview and key references of glacial processes and resulting sedimentary products in subglacial, terrestrial proglacial and glaciomarine or glaciolacustrine settings. These settings are characterized by a wide variety of processes ranging from subglacial lodgement and deformation, ice-push and sediment remobilization, which in turn result in a wide range of products such as diamictite, conglomerate, sandstone, siltstone and mudstone. The sedimentary record of proglacial settings exhibits the most lateral and vertical variability due to the dynamic nature of ice margins and the most direct record of climatic fluctuations. Many Neoproterozoic successions, however, preserve glaciomarine deposits that can provide a more continuous and high-resolution (though indirect) record of change. This chapter will enable the reader to identify features that may be used to infer a glacial influence on the formation of ancient deposits. The chapter also outlines some of the important issues that require consideration when evaluating palaeoclimatic models for Neoproterozoic sedimentary successions. These include the equivocal significance of most commonly used proxies such as occurrence of diamictite, outsized clasts in laminated sediments, clast characteristics, lithostratigraphic trends and sequence boundaries. Careful analysis of multiple lines of sedimentary evidence, together with other proxies of climatic changes, can yield meaningful reconstructions and provide a basis for testing palaeoclimate models for this time period. A summary table outlining the characteristics of diamictite with different depositional origins is also included in order to assist with the interpretation of the Neoproterozoic sedimentary record.

Salinity and irradiance effects on growth and maximum photosynthetic quantum yield in subarctic Saccharina latissima (Laminariales, Laminariaceae)

In subarctic regions, melting of glaciers creates stressful environmental conditions, such as reduced salinity and irradiance, in coastal benthic habitats such as kelp beds. Our goal was to determine whether these stressors differently affect kelp juveniles of Saccharina latissima originating from two environmentally distinct shores; one under oceanic and one under estuarine glacial influence. Laboratory tests assessed the effects of varying salinities and irradiances on growth and maximum quantum yield (Fv/Fm) of photosystem II of S. latissima. Overall, growth rates decreased with decreased salinity and irradiance. Growth rates of juveniles from the glacially influenced shore were significantly lower than those of the oceanic shore in most salinity and irradiances treatments. Juveniles from both shores grew negligibly at salinities below 13 and an irradiance of 5 μmol photons m-2 s-1. At salinity 10, Fv/Fm was significantly reduced but Fv/Fm was not affected by decreased irradiance. S. latissima proved to be relatively tolerant to reduced salinities and irradiances but we detected limits to its resilience. Our results suggest that S. latissima populations exhibit phenotypic plasticity in their growth responses. This plasticity appears to be constrained within specific seasonal growth patterns in accordance with their environment of origin.

Chapter 43 The Squantum Member of the Boston Basin, Massachusetts, USA

Abstract The Neoproterozoic diamictite-bearing Squantum Member is located in the Boston Basin in eastern Massachusetts, USA. The Boston Basin forms part of the Avalonia island arc terrane ( c. 650 Ma), and appears to have originated as a rift-type basin in an extensional setting along the northern margin of Gondwana, although its exact position is debated. Inferred palaeoenvironmental reconstructions of the Boston Basin have alternated between a fluvial basin where ice played a major role in transporting much of the coarse material and an evolving marine basin dominated by non-glacial subaqueous mass flow, submarine fans and turbidity-current deposition. The age of the Squantum is bracketed between c. 595 and 570 Ma, and is correlated by some to the glaciogenic diamictite succession of the Gaskiers Formation (eastern Newfoundland) as part of the putative global Gaskiers Glaciation c. 582–585 Ma. However, the Squantum Member consists of diamictite, graded sandstone and siltstone units, and fine-grained laminated argillite/mudstone units typical of debris flow and turbidite facies that accumulate in a submarine setting. A glacial influence is not readily identified and revolves around early interpretations of the diamictite as being ‘till-like’, the presence of laminated horizons that resemble glaciolacustrine ‘varvites’ and the disputed recognition of ice-rafted dropstones. There are no associated carbonates and, consequently, no geochemical data are available in connection with the Squantum Member.

Chapter 22 Evidence for Early and Mid-Cryogenian glaciation in the Northern Arabian–Nubian Shield (Egypt, Sudan, and western Arabia)

Abstract Evidence of Early- to Mid-Cryogenian ( c. 780 Ma and c. 740 Ma) glacial activity is summarized for the northern Arabian–Nubian Shield (ANS), including structural framework, stratigraphy, lithological descriptions and relationships with younger and older units, banded iron formation chemostratigraphy, other characteristics, geochronological constraints, and discussion. The ANS is a broad tract of juvenile continental crust, formed from accreted arc-backarc basin terranes developed around the margins of the Mozambique Ocean. As a result, these successions formed in marine environments at some distance from continental margins. Deposits include banded iron formation (BIF) and possibly glacial diamictite scattered over broad regions of the Central Eastern Desert of Egypt, NW Arabia and possible correlative units in NE Sudan. The older ( c. 780 Ma) examples (Meritri group, NE Sudan; basal Mahd group, Arabia) occur in the central ANS, on the southern flank of an important lithospheric boundary, an ophiolite-decorated suture zone. Mahd group diamictite is thin (1–5 m thick) and rests above the earliest (Cryogenian) ANS unconformity. The Meritri group interval near Port Sudan is much thicker and part of a deformed passive margin. Both Mahd and Meritri group deposits need further study before they are accepted as glaciogenic; confirmation of this interpretation would indicate that Neoproterozoic glacial activity began at least as early as 780 Ma ago. The younger ( c. 740 Ma) glacial deposits include diamictite and BIF: the Atud diamictite and BIFs of the Central Eastern Desert of Egypt and the correlative Nuwaybah diamictite and BIF of NW Arabia. Northern ANS-BIF is a well-layered chemical sediment of interlaminated hematite-magnetite and jasper. A glacial origin for the Atud-Nuwaybah diamictites is inferred because large clasts and matrix zircons have ages (Palaeoproterozoic and Neoarchean) and compositions (especially quartzite, arkose, and microdiamictite) that require transport from outside the ANS Cryogenian basin. Northern ANS-BIF may also reveal glacial influence, having been deposited in response to reoxygenation of a suboxic ocean. The 740 Ma diamictite and/or BIF may correlate with Tambien Group diamictites in Ethiopia ( Miller et al. 2011 ). Northern ANS diamictite and BIF were deposited in an oceanic basin of unknown size, as indicated by association with abundant ophiolites; they are strongly deformed, obscuring many primary features. There is no strong evidence for or against Ediacaran glaciation in the ANS, largely because the region was uplifted at this time. The c. 600 Ma ANS peneplain may have been partly cut by Ediacaran glaciation. Some of the post-accretionary basins of Arabia could preserve glaciogenic deposits of Ediacaran age, but assessing this possibility requires further investigation.

Physical and biological changes to a lengthening stream gradient following a decade of rapid glacial recession

AbstractIn mountains, environmental gradients are steep in both terrestrial and aquatic systems, and climate change is causing upward shifts of physical and biological features of these gradients. Glacial streams are an interesting system to evaluate such shifts both because streams have a linear nature (for simplicity of analysis), and because the stream habitat will at least temporarily lengthen as it follows receding glaciers upward. The Tschierva Glacier, Swiss Alps, receded 482 m upstream from 1997 to 2008. We tested the null hypothesis that the physical and biological stream gradient below this glacier maintained the same structure between these time periods, but simply shifted upward following the glacial source. We compared longitudinal patterns of water temperature and zoobenthic community structure in 1997 and 2007–2008 during three seasons (spring, summer, fall) along the uppermost ca. 5 stream km. Upward shifts were evident, including colonization of the newly exposed stream reaches by cold‐adapted taxa, and the appearance in 2007/2008 of four lower‐altitude species that were previously absent. Overall, however, results rejected the null hypothesis, instead revealing significant changes in gradient structures. These included a more steeply increasing temperature profile downstream of the glacier and increased amplitude of seasonal community turnover in 2007/2008 vs. 1997. Long‐term (1955–2007) flow records revealed increasing short‐term and seasonal hydrologic variability, which might have influenced the increased intra‐annual community variability. The steepening of the temperature gradient was likely caused by a warming lake‐outlet tributary upon which glacial influence was diminished between 1997 and 2007/2008. These results suggest that upward‐shifting gradients in glacial streams can involve complex interactions with other landscape elements and that local‐scale climate response can progress even more rapidly than the rate of glacial recession.

Distribution patterns in the biomass of macrozoobenthic communities in Admiralty Bay (King George Island, South Shetlands, Antarctic)

The distribution patterns of macrozoobenthic communities as shown by the biomass of the higher taxa were investigated in Admiralty Bay. Material was collected at depths ranging from 4 to 500 m (102 quantitative sam- ples), representing the full depth range of this basin and including areas characterized by the different levels of glacial influence. Five community groups were distin- guished by multivariate analysis (clustering, MDS) based on the Bray-Curtis similarity index. In the Ezcurra Inlet, the area characterized by intensive glacial processes resulting in a high sedimentation rate, the distribution of biomass was not depth related, but was associated with the intensity of glacial disturbance along the axis of the fjord. The innermost part of this fjord, which receives strong outflows from glaciers, had a low biomass value (2.9 g/ 0.1 m 2 ) and was dominated by polychaetes. The middle part of the fjord which is not so heavily disturbed had higher biomass values (46.1 g/0.1 m 2 ) due to the domi- nance of bivalves. A clear depth gradient of biomass dis- tribution was observed in bottom areas located far from glaciers, in the central basin of the bay, with three zones within the depth ranges 4-30, 40-380, and 400-500 m. The highest mean biomass values (92.5 g/0.1 m 2 ) were found in the middle sublittoral (40-380 m) dominated by ascidians. In the shallow and deep assemblages, the biomass was lower (33.8 g/0.1 and 30.9 g/0.1 m 2 respectively), most probably associated with the ice dis- turbance in the shallows and lower food supply in the deepest part of the shelf.

One, two or no record of late neoproterozoic glaciation in South-East Cameroon?

Severe climate changes culminating in at least three major glacial events have been recognized in the Neoproterozoic sedimentary record from many parts of the world. Supportive to the global nature of these climatic shifts, a considerable amount of data have been acquired from deposits exposed in Pan-African orogenic belts in southwestern and western Africa. By comparison, published data from the Pan-African belts in Central Africa are scarce. We report here evidence of possibly two glacial events recorded in the Mintom Formation that is located on the margin of the Pan-African orogenic Yaoundé belt in South-East Cameroon. In the absence of reliable radiometric data, only maximum and minimum age limits of 640 and 580 Ma, respectively, can at present be applied to the Mintom Formation. The formation consists of two lithostratigraphic ensembles, each subdivided in two members (i.e., in ascending stratigraphic order the Kol, Métou, Momibolé, and Atog Adjap Members). The basal ensemble exhibits a typical glacial to post-glacial succession. It includes diamictites comprising cobbles and boulders in a massive argillaceous siltstone matrix, and laminated siltstones followed by, in sharp contact, a 2 m-thick massive dolostone that yielded negative δ 13C values (<−3‰ V-PDB) similar to those reported for Marinoan cap carbonates elsewhere. However, uncertainty remains regarding the glacial influence on the siliciclastic facies because the diamictite is better explained as a mass-flow deposit, and diagnostic features such as dropstones have not been seen in the overlying siltstones. The Mintom Formation may thus provide an example of an unusual succession of non-glacial diamictite overlain by a truly glacial melt-related cap-carbonate. We also report the recent discovery of ice-striated pavements on the structural surface cut in the Mintom Formation, suggesting that glaciers developed after the latter had been deposited and deformed during the Pan-African orogeny. Striations, which consistently exhibit two principal orientations (N60 and N110), were identified in two different localities, in the west of the study area on siltstones of the Kol Member, and in the east on limestones of the Atog Adjap Member, respectively. N60-oriented striae indicate ice flow towards the WSW. Assigning an age to these features remains problematical because they were not found associated with glaciogenic deposits. Two hypotheses can equally be envisaged, i.e., either the striated surfaces are correlated: (1) to the Gaskiers (or Neoproterozoic post-Gaskiers) glaciation and represent the youngest Ediacaran glacial event documented in the southern Yaoundé belt; or (2) to the Late Ordovician Hirnantian (Saharan) glaciation, thereby providing new data about Hirnantian ice flows in Central Africa.

Mississippian–Early Permian brachiopods from western Argentina: Tools for middle- to high-latitude correlation, paleobiogeographic and paleoclimatic reconstruction

Brachiopod faunas from western Argentina are reviewed to improve the current understanding of their biostratigraphic and paleobiogeographic value for intercontinental correlation and paleoclimatic reconstruction, especially for middle to high paleolatitudes. The oldest faunal assemblage, the late Tournaisian–early Viséan Malimanian fauna, indicates an incipient biogeographic segregation of southwestern South America from contemporaneous peri-Gondwana and northern hemisphere regions, likely due to the enhanced thermal gradient that predated the pronounced latest Viséan global cooling and the major latitudinal biotic differentiation on a global scale. Paleogeographical distribution of key brachiopod taxa provides evidence of faunal interchange along western Gondwana during late Early Carboniferous. The paleoclimate of the region by this time has been interpreted as temperate preglacial conditions, in a higher paleolatitudinal setting. Global cooling and strong differentiation between marine biota of circum-polar and paleoequatorial regions occurred after late Viséan. The Barrealian fauna ( Rugosochonetes-Bulahdelia and Levipustula assemblages), represents the oldest faunal record in western Argentina of this major latitude-related biotic segregation linked with the onset of the mid-Carboniferous glaciation. The Barrealian fauna appeared during a major late Viséan–Serpukhovian glacial episode with short-lived glacial and interglacial stages. Climatic fluctuations were probably related to local faunal changes and marine biotic exchange between higher and middle paleolatitudes of southwestern and eastern Gondwana. Widespread postglacial deposits bearing Bashkirian faunas demonstrate the beginning of a progressive climatic amelioration trend until early Permian times, barely affected by short-lived colder paleoclimatic fluctuations. The incoming of the late Bashkirian–early Moscovian Marginovatia-Maemia fauna was linked to a climatic change in the region following the disappearance of glacial conditions in westernmost Argentina. Ocean currents from the north, that reached the region bringing boreal brachiopods, were warm enough since Bashkirian times to build a thermal barrier for the biotic interchange with peri-Gondwana regions under subpolar settings and a faunal disconnection was established. In addition, asymmetrical climatic record between western Argentina and regions still under strong glacial influence, such as Patagonia and eastern Australia, reflect the clockwise rotation of Gondwana and consequent migration of western Argentina to lower latitudes. After a preceding mainly regressive depositional interval, the marine incursion bearing the Tivertonia- Streptorhynchus fauna accompanied the most extensive transgression that covered western Argentina. It was probably linked with the beginning of a thalassocratic regime by the global major sea level rise of late Asselian–Sakmarian times. The Tivertonia-Streptorhynchus assemblage represents a middle paleolatitude temperate and mixed fauna, which migrated toward southern Gondwana regions, indicating a marine faunal connection during a climatic amelioration trend. The last marine incursion in western Argentina brought the Costatumulus fauna, linked to a brief interruption of the mostly temperate paleoclimatic conditions established since the Pennsylvanian. A short-lived minor glacial pulse associated with the Costatumulus fauna reflected local mitigated climatic fluctuations, by the paleolatitudinal gradient to circumpolar areas where a major glacial pulse occurred during Sakmarian–Artinskian times.

Application of Quartz Sand Microtextural Analysis to Infer Cold-Climate Weathering for the Equatorial Fountain Formation (Pennsylvanian-Permian, Colorado, U.S.A.)

Abstract Climatic interpretations of the upper Paleozoic (Permo-Pennsylvanian) Fountain Formation, a coarse-grained fan-delta system that formed in western equatorial Pangea, are difficult to constrain owing to a general lack of climatic indicators so typical of coarse clastic systems. We applied scanning electron microscopy (SEM) to analyze quartz grains in this system in an attempt to test the hypothesis of a glacial influence on these strata. SEM observations of first-cycle quartz grains from these strata reveal microtextures formed from fracturing during grain transport, even after diagenetic overprinting occurred under moderate burial conditions (up to 3.5 km depth and 100°C). Transport-induced microtextures can be grouped based on inferred fracture process into: (1) high-stress fractures, consisting of fractures created through sustained high shear stress, such as grooves, deep troughs, and gouges, and are inferred to occur predominantly during glacial transport; (2) percussion fractures, consisting of fractures created by grain-to-grain contact during saltation or traction flow, such as randomly oriented v-shaped cracks and edge rounding; and (3) polygenetic fractures, such as conchoidal fractures, arc-shaped steps, linear steps, and linear fractures, that occur under a wide range of transport processes and thus possess no environmental significance. Delineation of high stress, percussion, and polygenetic fracture types demonstrate that the Fountain Formation quartz grains exhibit microtextures similar to both till and glaciofluvial deposits, suggesting that periods of upland glaciation occurred in the source region of the Fountain Formation (Ute Pass uplift). The abundance of high-stress fractures peaks at two stratigraphic intervals. These intervals are inferred to record the presence of ice in the Ute Pass uplift and are correlative with polygonally fractured paleosurfaces in the Fountain Formation that are interpreted to reflect cold-temperature weathering. Moreover, the peak intervals are approximately coeval with inferred episodes of ice maxima from high-latitude localities, as well as other low-latitude localities. Geologically reasonable stream gradients and estimated transport distance suggest a best-estimate elevation of the ice terminus of ~ 1500 m, but possibly ranging to 3000 m. These data suggest that upland glaciers episodically existed within this equatorial setting and that further use of this technique may reveal more evidence of ice in other proximal deposits of the ancestral Rocky Mountains, as well as other systems of various geologic ages.

Stratigraphic signature of the late Palaeozoic Ice Age in the Parmeener Supergroup of Tasmania, SE Australia, and inter-regional comparisons

Recent research in eastern Australia has established that rather than being a single, long-lived epoch, the late Palaeozoic Ice Age comprised a series of glacial intervals each 1–8 million years in duration, separated by non-glacial intervals of comparable duration. In order to test whether the glacial events recognized in New South Wales and Queensland have broader extent, we conducted a reappraisal of the Parmeener Supergroup of Tasmania, southeast Australia. A facies analysis of the Pennsylvanian to Permian section was carried out, allowing rationalization of the succession into four recurrent facies associations: a) glacigenic facies association, restricted to the basal Pennsylvanian/earliest Permian Wynyard Formation and correlatives, b) glacially/cold climate-influenced to open marine shelf facies association, which accounts for large parts of the Permian succession, c) deltaic facies association, which specifically describes the Lower Permian “Lower Freshwater Sequence” interval, and d) fluvial to estuarine facies association, which specifically addresses the Upper Permian Cygnet Coal Measures and correlatives. Indicators of sediment accumulation under glacial influence and cold climate are restricted to four discrete stratigraphic intervals, all of which indicate that glaciation was temperate in nature. The lowermost of these, incorporating the basal Wynyard Formation and its correlatives, and overlying Woody Island Formation, shows evidence of proximal glacial influence (subglacial, grounding-line fan and ?fjordal facies), and is likely a composite of one or more Pennsylvanian glacial event(s) and an earliest Permian (Asselian) glacial. The second, of late Sakmarian to early Artinskian age, comprises an initial more proximal ice-influenced section and an overlying more distal ice-influenced interval. The third (Kungurian to Roadian) and fourth (Capitanian) intervals are both distal glacimarine records. The four intervals are of comparable age to glacials P1–P4, respectively, recognized in New South Wales and Queensland (notwithstanding apparent discrepancies of < 2 million years in age), and display similar facies characteristics and vertical contrasts to those intervals. Accordingly, it is concluded that the late Palaeozoic stratigraphy of Tasmania preserves a glacial/cold climate record correlatable to that of mainland eastern Australia, lending support to the hypothesis that these events were widespread across this portion of Gondwana.