Brigitte Van Vliet-Lanoë

Frost and soils: implications for paleosols, paleoclimates and stratigraphy

Abstract Frost is a pedogenetic agent. This paper describes the use of frost features in soil for paleoclimatic and stratigraphic interpretation. It is possible to differentiate periods of frost activity from those of temperate pedogenesis by macroscopic and microscopic criteria and to distinguish between soils with pre-, syn- and epigenetic frost effects. An experimental approach is necessary to determine initial frost effects because they are usually difficult to observe at natural active sites. Work with ecologists also clarifies the evolution of frost-affected soils under climatic and vegetational constraints. Vegetation has important effects on the thermal and hydraulic regimes of soils and on erosion susceptibility. The recognition of fossil cryosoils often allows the reconstruction of sequences of paleoclimatic events in periods or environments with little or no micropaleontological evidence. The interrelations between frost and other pedogenetic processes allow a better understanding of climatic evolution and deterioration throughout the Neogene and especially during the Quaternary and the Holocene. Most of the inherited frost features in European soils are younger than 480 ka BP. The differentiation between frost and tectonic perturbations is very important for calculation of soil erosional budgets.

Pleistocene subglacial volcanism in Iceland: tectonic implications

Abstract At several stages during the last 700 kyr, tectonic and volcanic activity due to lithospheric spreading occurred beneath a 1000–1500 m thick ice cap in Iceland. Magmatic activity has been recorded by the emplacement of subglacial volcanic edifices. Table volcanoes are the subglacial equivalent of aerial shield volcanoes. Hyaloclastite ridges are the subglacial equivalent of aerial eruptive fissures. Some hyaloclastite ridges are located in currently inactive areas, whereas they are nearly absent in some parts of the currently active Neovolcanic Zone. A part of this discrepancy can be attributed to glacial erosion. A manual reconstruction of the flowing pattern of the ice cap, based on glacial landforms, shows that some parts of the Neovolcanic Zone were occupied by fast flowing ice streams. In these areas, most hyaloclastite ridges have been removed as eruptions proceeded: fast ice flow and water/debris flows triggered by volcanic eruptions have transported subglacial volcanic products to the sea. Subglacial volcanic products have been preserved beneath ice divides, where ice motion was slower, and in some table volcanoes, where magma supply was sufficient to counteract removal by ice flow. Once the effect of glacial removal has been subtracted, the arrangement of the subglacial volcanic edifices appears clearly. Similarly to the post-glacial eruptive fissures, the hyaloclastite ridges are gathered in swarms associated with central volcanoes located in the Neovolcanic Zone. However, the area covered by hyaloclastite ridges is wider than the extent of the currently active fissure swarms. This discrepancy suggests either continuous wandering of the volcanic activity from one fissure swarm to another for the last 700 kyr, or narrowing of the active rift zone at the end of the last glaciation.

Dynamics and extent of the Weichselian permafrost in western Europe (Substage 5E to stage 1)

Abstract A paleoclimate sequence and a map of the successive extensions of the permafrost during the different stages of the Weichselian is proposed on the basis of cryopedological and paleopedological data. This work shows the existence of two periglacial cycles within the Weichselian; both are characterized by later permafrost in the oceanic provinces of Europe, compared with northern France and Belgium. Explanations are given for the origin of the Pomeranian and Dryas stadials.

6 – Frost Action

Publisher Summary This chapter discusses frost action. Frost activity is present in many regions of the world, from high latitudes to tropical arid zones, where it may penetrate the soil to a depth of 5–15 cm in winter. Ground freezing is a pedogenetic agent among other soil-forming processes. It results in temperature-driven soil desiccation associated with the main process of ice segregation. The impact of this process depends on various soil characteristics, and on the temperature regime. The segregation of ice lenses results in the development of distinctive soil features, the most typical being platy and lenticular structures at various scales. In conditions of alternating freeze and thaw, the characteristic lenticular and platy microstructures occur together with more or less developed silt and coarse clay cappings on lenticular aggregates and sorting of coarse grains, giving rise to different types of banded fabrics. Other micromorphological features related to ice segregation in soils are silt cappings on coarse grains, vertically oriented coarse grains, microinjection features, grano-and porostriated b-fabrics and granular microstructures. Freezing and thawing and solifluction processes further lead to compaction, displacement, rotation, and deformation of the cryogenic features.

Middle- to late-Holocene storminess in Brittany (NW France): Part II – The chronology of events and climate forcing

This study focuses on the recurring climate conditions required for the largest storms occurring in NW France (Brittany). It is based on the analysed records of storm events along Western Brittany coast (see Part I). In this manuscript (Part II), storm recurrence is explored along with forcing mechanisms. Periods of more frequent storm events over the two last centuries are analysed first in order to link these events with possible forcing mechanisms (North Atlantic Oscillation (NAO) and Atlantic Multidecadal Oscillation (AMO) modes) triggering the most destructive storms. Then, palaeostorm events are discussed at the Holocene scale, from 6000 yr BP to present, to verify the forcing mechanisms. Most recorded events appear to be linked with cooling episodes, mostly in winter, a transition to or from a negative winter NAO mode, a positive AMO mode. Extreme storms occur immediately prior to the ‘Medieval Warm Period’ (MWP). Maximum effects are reached prior to the onset of the MWP and during the Maunder and Dalton solar minima. Low storm activity occurred during the Spörer Minimum linked to an acceleration of the Atlantic Meridional Overturning Circulation (AMOC). Main storm triggers seem to correspond to a positive AMO mode with an unstable jetstream configuration driving a negative NAO. In this study, four specific weather configurations were defined to explain each type of recorded storminess. The strongest storms correspond to low AMO and decennial-negative NAO modes (e.g. ‘Little Ice Age’), or high AMO in association with dominant low NAO modes, as during the early Middle Age and present-day period. Fresh or warm oceans in association with a positive NAO mode are stormy but with very low sting storms frequency. Although in agreement with the orbital forcing and the Holocene glacial history, increasing storm frequency and intensity is most probably partly biased by continuous sea-level rise and resulting erosion.

Chironomids (Diptera) in river floodplains: their status and potential use for palaeoenvironmental reconstruction purposes

The chironomid subfossils from a single sediment core in the floodplain of the Aa river (Saint-Omer basin, Northern France) were analysed. This study was a pio- neering attempt to use sedimentary chironomid assemblages and lithostratigraphic analysis to reconstruct the palaeoenvironment and the past hydrodynamic conditions in a river system during the mid-Holocene period. In this paper, a chironomid subfossil typology is proposed, based on the current scientific knowledge about the present dis- tribution of chironomid species through the cross-section of a floodplain (from lotic to more lentic habitats). A hydrological status is attributed to 27 subfossil chironomid taxa (limnophilous taxa and rheophilous taxa) from the Saint-Omer sediment sample. Based on these chironomid hydrological statuses and in regard with the lithostrati- graphic data, the dynamics of chironomid subfossil assemblages showed the occur- rence of some fluctuations between periods with lower (lentic periods SMch-1, SMch- 3 and SMch-5) and higher (lotic periods SMch-2 and SMch-4) Aa river discharge rates during the Late-Atlantic and Subboreal periods. The lotic periods were correlated with several climatic events known to have occurred during these periods in Europe. Sev- eral hypotheses are put forward and discussed, as to the potential relationships between the levels of precipitation and the hydrological conditions in the Saint-Omer basin. Al- though chironomid analysis in river systems is still in its developmental phase, the conclusions drawn from the present findings on subfossil chironomids show that these can provide a useful tool for palaeoenvironmental reconstruction purposes.

Middle- to late-Holocene storminess in Brittany (NW France): Part I - morphological impact and stratigraphical record

Our study aims to understand the recurring climatic conditions prevailing during the largest storms reaching NW France (Brittany). These storms are responsible for the breaching of coastal barriers and major flooding of lowlands. In a first part of our work, we examine the morphological impact and stratigraphic record of storm events along Western Brittany rocky coasts, with a special focus on the southern coast of the Bay of Audierne, the most exposed coast of the region. In a second paper (‘Middle- to Late-Holocene Storminess in Brittany (NW France): Part II’), we shall focus on the chronology of storm events and their climate forcing conditions. Drilling transects and stratigraphic analyses were first undertaken to constrain chronology, strength and wind direction during the main Holocene storm events. New dates, observations and a relative sea-level (RSL) curve were then used to inform discussion of the necessary climatic and morphologic conditions leading to destructive storm events. Most recorded events appear to be linked with cooling episodes of the Holocene and a RSL close to present. Some storms are clearly responsible for breaching and dune building or remobilisation. We demonstrate that storm frequency and intensity appear to rise in a stepwise manner during the late Holocene. Maximum efficiency is reached during the ‘Little Ice Age’ with clustered events probably lasting several days, but major storms also occurred immediately prior to the ‘Medieval Warm Period’. We suggest that recent coastal dune building from c. ad 1100 until now, despite a sea level close to present and continuously rising, may be a direct consequence of the restoration of beaches after periods of recurrent storminess. This building activity often occurred during dry negative North Atlantic Oscillation (NAO) events, in connection with the available sedimentary supply.

The genesis and age of the argillic horizon in Weichselian loess of northwestern Europe

Abstract The argillic horizon developed in the Weichselian loesses of western Europe can be related to mechanical clay translocation under a cold climate following a period of decarbonatation. In several sites from Brittany to southern Poland where the pedostratigraphical record was preserved, the Bt formation can be mainly attributed to the Bolling Interstadial. Younger events exist also in the Early Holocene. In most of these sites, Holocene pedogenesis is reduced to an acidification or even a podzolization in the former eluvial horizon. In other sites, influenced by anthropogenic activities, it is difficult to analyze the chronostratigraphy of the pedological events. Dynamic conditions for clay translocation and stratigraphical arguments are discussed. This type of argillic horizon is an early phase of pedogenesis, considered to be formed under boreal climatic conditions.

Le Mio-Pliocène du Massif armoricain. Données nouvelles

Abstract The ‘red sands’ of the Armorican Massive were attributed to an upper Pliocene marine environment. A reappraisal of the stratigraphy and ESR datings shows that it covers fluviatile to estuarine retrogressive formations, invading paleovalleys, and covering Tortonian, Messinian, Zanclean and Lower Piacenzian high stands. Formation mapping at the level of the Channel shows that it is associated with successive long wave deformations related to the pyrenean alpine compression and Paleogene saprolite erosion.

Chironomid responses (Insect: Diptera) to Younger Dryas and Holocene environmental changes in a river floodplain from northern France (St-Momelin, St-Omer basin)

Lateglacial to Holocene subfossil chironomids from the St-Omer basin at St-Momelin (northern France) were analysed. At the end of the Lateglacial, stratigraphic and chironomid sequences indicate a shallow, oligotrophic and slow-flowing freshwater stream, with abundant aquatic macrophytic vegetation. Cold-water adapted taxa indicate a cold climate. During the early Holocene, from about 9500 to 7700 conventional radiocarbon yr BP, climate improvement is marked by an increase in warm-water adapted taxa and a decrease in cold-water fauna. However, the cold-water fauna persist, probably because of both the instability and continentality of the climate. During the mid Holocene, from about 7700 to 6000 BP, the marine transgression strongly influenced the rivers hydrological regime and consequently the chironomid assemblages. At that time, eutrophic, warm and shallow stagnant-water conditions occured throughout the basin. From 6000 to 3200 BP, the St-Momelin basin was subjected to a marine regression. Chironomids indicate a hydrological regime marked by an alternance of phases with either a dominance of rheophilous or limnophilous fauna. During this period, equivalent percentages of warm- and cold-adapted taxa may be explained by cold-water input in context of a temperate climate.