Single Moraine Research Articles

Interpreting exposure ages from ice‐cored moraines: a Neoglacial case study on Baffin Island, Arctic Canada

ABSTRACT10Be dating of moraines has greatly improved our ability to constrain the timing of past glaciations and thus past cold events. However, the spread in ages from a single moraine is often greater than would be expected from measurement uncertainty, making paleoclimatic interpretations equivocal. Here we present 28 new 10Be ages from ice‐cored Neoglacial moraines on Baffin Island, Arctic Canada, and explore the processes at play in moraine formation and evolution through field observations and a numerical debris‐covered glacier model. The insulating effect of debris cover modifies glacier lengths and results in the development of ice‐cored moraines over multiple advances and thousands of years. Although ice cores can persist for several millennia, spatially variable ice core melt‐out contributes to moraine degradation and boulder destabilization, making it likely that the 10Be clock is reset on moraine boulders in these settings. Thus, exposure ages from ice‐cored moraines must be interpreted with caution. The oldest ages, after excluding samples with inheritance, provide the best estimates of initial moraine formation. Three Baffin Island moraines yield 10Be ages suggesting formation at 5.2, 4.6 and 3.5 ka, respectively, adding to a growing body of evidence for significant summer cooling millennia before the Little Ice Age.

Revised palaeoclimatic significance of Mueller Glacier moraines, Southern Alps, New Zealand

AbstractA sequence of Late Holocene moraines on the foreland of the Mueller Glacier, Southern Alps, New Zealand, forms part of a local moraine‐age database used to establish a regional glacier chronology and subsequently to investigate potential intra‐hemispheric and global climate forcing mechanisms. We present new sedimentological and geomorphological evidence that a set of these moraine ridges, previously considered to represent individual advances, constitutes a single moraine complex (the ‘Mueller Memorial Moraine’) formed by supraglacial transport of a large volume of landslide debris to the glacier terminus. Because a moraine formed in this way is not necessarily associated with an advance triggered by a climate event, we question the palaeoclimatic significance of the Mueller Memorial Moraine, as well as that of other moraines in comparable settings. Our findings suggest that the mode of formation and glacio‐dynamical context of moraines whose ages contribute to existing palaeoclimate reconstructions need to be re‐examined in order to assess the reliability of these reconstructions. Copyright © 2015 John Wiley & Sons, Ltd.

Non-stochastic colonization by pioneer plants after deglaciation in a polar oasis of the Canadian High Arctic

Initial plant colonization is critical in determining subsequent ecosystem development. In a High-Arctic oasis showing atypical “directional primary succession”, we quantified the microhabitat characteristics associated with colonization by pioneer vascular plants of a bare moraine. The study moraine, formed during the Little Ice Age, is located within the proglacial area at the southern front of Arklio Glacier, Ellesmere Island, Canada. We established two line-transects on this moraine to quantify microhabitats for vascular species. Microsites favorable for plants were concave depressions, probably increasing the likelihood of colonization. At microsites distant from stable boulders, which probably protect seeds/seedlings from wind desiccation, plant colonization was less likely. Furthermore, favorable microhabitat properties differed depending on topographical location within the moraine, suggesting that, even within a single moraine, microhabitats favorable for plant colonization are heterogeneously-distributed. This moraine was characterized by two major pioneer species, Epilobium latifolium and Salix arctica. Their species-specific microhabitat requirements highlight the importance of biotic factors in colonization processes. Favorable sites for plants are generally distributed at random in harsh environments. However, we showed that initial plant colonization is a deterministic process rather than random, indicating the possibility of non-stochastic processes even during the early phase of ecosystem development in High-Arctic ecosystems.

Glacial advance and stagnation caused by rock avalanches

New model results show that a glacier advance caused by deposition of a rock avalanche on the ice will be followed by stagnation of the advanced ice lobe, producing distributed, hummocky deposits quite different from the single moraine ridges typically dated in paleoclimatic reconstructions. We obtain these results using a numerical flow-line glacier model with superimposed rock debris. In particular, our results imply that the single moraine ridge known as the Waiho Loop of the Franz Josef Glacier, New Zealand, is not the result of an advance triggered by a rock avalanche, else there would be a widespread, hummocky, glacial stagnation deposit associated with it.

Multiple, distinct groundwater flow systems of a single moraine–talus feature in an alpine watershed

Recent studies suggest that talus slopes and moraines likely play an important role in groundwater flow and storage in alpine watersheds, though the subsurface processes of these unconsolidated sediment features are not fully understood. To gain insight into these groundwater systems, we investigated the spatial variability in groundwater properties and hydrological trends of a large spring and several nearby (⩽200 m) springs, discharging from a single moraine–talus feature in an alpine watershed in the Canadian Rockies. A key question was whether groundwater flow in these features is reasonably homogeneous. Hydrograph analyses revealed at least two different groundwater responses to precipitation and melt inputs: a rapid and likely localized response and a slower response indicating a subsurface connection to a nearby lake. There was also a large spread in groundwater composition across the large spring and between springs, including a consistent linear trend in major ion chemistry over a 20-m section of the large spring. The spatial and temporal trends in groundwater chemistry data suggests there are three groundwater components associated with this sediment feature, and that their relative contributions vary temporally, though the component associated with the lake appears dominant. The study findings suggest that unconsolidated sediment features can possess multiple, and possibly disconnected, groundwater flow paths exhibiting unique hydrological and geochemical characteristics, and cannot necessarily be treated as a single, homogeneous groundwater component when modeling the hydrology of alpine watersheds.

Cosmogenic 10be age constraints for the wester ross readvance moraine: insights into british ice‐sheet behaviour

This study presents the first absoluteage constraints from a palaeo‐ice‐sheet margin in western Scotland. Cosmogenic 10Be from four Lewisian gneiss boulders on the Gairloch Moraine in NW Scotland have yielded reliable exposure ages. Three of these dates, taken from a single moraine ridge, cluster around c. 15.5–18 ka BP, with a weighted mean of 16.3 ± 1.6 ka BP. These findings indicate that the last British Ice Sheet had retreated to the present‐day coastline in NW Scotland by this time. It is suggested that the Wester Ross Readvance represents an ice‐sheet oscillation during, or in the immediate aftermath of, Heinrich Event 1 (c. 17–18 ka BP).

Late-Glacial History of Lago Argentino, Argentina, and Age of the Puerto Bandera Moraines

In the west-central part of Lago Argentino, the Puerto Bandera moraines are clearly detached from longer, more prominent moraines of the last glaciation and from shorter and smaller Neoglacial moraines. Scientists have long speculated about the age of the Puerto Bandera moraines. Detailed geomorphologic studies in the western area of Lago Argentino, including stratigraphic profiles at Bahı́a del Quemado in the northern branch (Brazo Norte), indicate that the Puerto Bandera moraines were deposited by three pulses of ice. Each of the three pulses is represented by single moraine ridges and belts of tightly arranged ridges. The timing of the three glacier advances was established by radiocarbon dating, including data published by John Mercer. The oldest moraine system, formed during the Puerto Bandera I substade, was deposited ca. 13,000 14C yr B.P. Moraines of the Puerto Bandera II substade were deposited ca. 11,000 14C yr B.P. The youngest moraine system was deposited during a minor readvance, shortly before 10,390 C14 yr B.P., and thus appears to have occurred some time during the European Younger Dryas interval. After this third substade, the ice tongues retreated into the interior branches of Lago Argentino and have remained there since. Evidence found at Bahı́a del Quemado, together with data provided by other authors, attests to a significant climatic change by the middle Holocene, which we believe occurred during the Herminita advance, the first Holocene glacial readvance recognized within the area.

Examination of surface exposure ages of Antarctic moraines using in situ produced 10Be and 26Al

Concentrations of 10Be (t 1 2 = 1.5 × 10 6 y ) and 26 A1 (t 1 2 = 0.72 × 10 6 y ) have been determined by accelerator mass spectrometry (AMS) in a suite of quartz samples taken from sandstone boulders in several moraines in Arena Valley, a dry valley adjacent to the Taylor Glacier in the Quatermain Mountains, Southern Victoria Land, East Antarctica. These isotopes are produced in surficial quartz by cosmic ray spallation of O and Si. The concentrations in these samples ranged from 6.1 × 10 5 to 3.0 × 10 7 at g −1 for 10Be and from 9.4 × 10 6 to 1.2 × 10 8 at g −1 for 26A1, depending upon the extent of exposure at the surface. Analyses of 10Be in several samples from a single moraine at altitudes ranging from 1300 to 1650 m permitted an estimation of the air attenuation pathlength of cosmic ray neutrons; the value of 142 +96 −41 g cm −2 is consistent with other published values. Production rates of 17 +6 −4 at g −1 y −1 for 10Be and 113 +54 −16 at g −1 y −1 for 26A1 at 1300 m and 87°S and a 26Al: 10Be production ratio of 6.5 +1.3 −1.3 were calculated from the data. These values correspond to sea-level production rates at high geomagnetic latitude of 6.4 at g −1 y −1 and 41.7 at g −1 y −1 for 10Be and 26A1, respectively, consistent with determinations based on ~ 11 Ky glacially polished surfaces in the Sierra Nevada in California. These production rates imply exposure ages for the various moraines ranging from 50 Ky to 2.5 My, in accordance with other geological evidence. The 10Be and 26Al ages of these rocks compare favorably with those found using a similar dating method based on in situ production of 3He. Examination of the 3He concentrations in conjunction with the present data set yields an estimate of 3He production of 230 +85 40 at g −1 y −1, corresponding to 85 +31 −15 at g −1 y −1 at sea level. This is lower than previously reported values of 240 ± 60 at g −1 y −1 and ~190 at g −1 y −1 (scaled to sea-level and high geomagnetic latitude) based, respectively, on volcanic flows less than 2000 years old at low latitude, and samples collected from 14,400-year-old volcanic flows. This discrepancy is likely due to diffusive losses which may become significant (up to ~50%) for surface exposure ages on the order of 2.0 My. Nevertheless, the general correlations among the three isotopes suggest that 3He does not suffer gross losses through diffusion.

Variability in Neoglacial Moraine Morphology and Composition, Storbreen, Jotunheimen, Norway: Within-Moraine Patterns and their Implications

The nature and origin of the outermost 'Little Ice Age' moraine in front of a NE-flowing glacier (Storbreen, Jotunheimen) are investigated by analysing the variability along the length of the moraine of a range of characteristics (crossprofile form, surface composition, clast form and quartz sand grain characteristics). Concentrated application of a range of techniques in relation to a single moraine has enabled inferences to be made about glacial transport paths of debris in the moraine and processes of moraine-building. Moraine form is consistent within lateral and latero-terminal sections. Origin is attributed to a combination of some pushing of valley-side debris and dumping of glacially-transported material, in which the ratio of modified subglacial to unmodified supraglacial/englacial debris increases downglacier. A larger southern than northern side of the outer moraine is attributed to a difference in the quantity of debris being supplied from headwall areas, which are steep and heavily-eroded on the southern side but low-angled and vegetated on the northern side.

Till Fabrics of the Cross-Valley Moraines of North-Central Baffin Island, Northwest Territories, Canada1

Analyses of 103 till fabrics taken from proximal and distal slopes of distinctive cross-valley moraines in the Isortoq Valley of north-central Baffin Island show variations in fabric pattern with respect to slope and location within the valley. Changes in dip patterns and fabric strengths are also associated with variations in moraine morphology. Four distinct morphological moraine types are delimited and are called simple-linear, s-shaped, hooked, and asymmetric. There is no gradation in type along a single moraine. Orientation and dip strength are found to vary with moraine type as well as between distal and proximal slopes. Dip patterns of elongate pebbles from proximal and distal sites are also shown to vary with slope, location within the valley and moraine type. These results indicate that the hooked, asymmetric, and simple-linear moraines are formed by either dissimilar processes or changes in intensity of the same process. S-shaped moraines differ morphologically from simple-linear but have the same fabric characteristics. The possibility of annual deposition is examined and concluded to be possible though in no sense proven. The cross-valley moraines are thought to form at the base of an ice cliff grounded in a glacial lake. The influx of summer meltwater provides a possible trigger mechanism and explains the association between the moraines and water-deposited kame features. The asymmetric moraines were deposited by overriding and pushing. Moraines at right angles to the ice cliff were formed by the squeezing of till into basal crevasses or melt-water tunnels.