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Climatostratigraphy – geological-climate stratigraphy

Introduction

In contrast to the rest of the Phanerozoic, the Quaternary has a long-established tradition of sediment sequences being divided on the basis of represented climatic changes, particularly sequences based on glacial deposits in central Europe and mid-latitude North America.  This approach was adopted by early workers for terrestrial sequences because it seemed logical to divide till (glacial diamicton) sheets and non-glacial deposits or in stratigraphical sequences into glacial (Glaciation) and interglacial periods respectively.  In other words the divisions were fundamentally lithological. The overriding influence of climatic change on sedimentation and erosion in the Quaternary has meant that, despite the enormous advances in knowledge during the last century and a half, climate-based classification has remained central to the subdivision of the succession.  Indeed the subdivision of the modern ocean sediment isotope stage sequence is itself based on the same basic concept.  

For at least the first half of the 20th century the preferred scale was that developed for the Alps at the turn of the century by Penck & Brückner (1909-11), although evidence for multiple glaciation and intervening warmer-climate interglacial events had already been proposed by Geikie (1874).  The Alpine scheme was based upon the identification of glaciofluvial accumulations that could be traced upstream to end moraines that marked maximum glacial extent positions, in the Alpine foothills of southern Germany.  The terrace surfaces developed upon the glaciofluvial sediments were immediately underlain by fossil soils that were related by Penck & Brückner to interglacial weathering intervening between the glaciations.  These observations provided the foundation for alternating glacial-interglacial events, of which four were initially identified.  Although later workers added earlier events to the sequence, the basic structure remained dominant for over half a century.

Comparable schemes were developed in many regions subsequently, especially in Europe, the former USSR, Asia and North America, but also Patagonia, New Zealand and even the mountains of East Africa.

For much of this time, there was an almost universal acceptance that there were only four major glaciations, which led to their identification throughout the glaciated regions.  The recognition that beyond glaciated regions, varied precipitation had given rise to substantial changes in lake levels led to the recognition of pluvial lakes in arid regions of North America, Africa and elsewhere.  This assumed equivalence that glacials and pluvials (periods of increased precipitation relative to the present day) led to the virtually worldwide extension of the four-fold glaciation model (cf. Nilsson 1983). 

Terminology

Before the impact of the ocean-core isotope sequences an attempt was made to formalise the climate-based stratigraphical terminology in the American Code of Stratigraphic Nomenclature (1961) where so-called geologic-climate units were proposed.  Here a geologic-climate unit is based on an inferred widespread climatic episode defined from a subdivision of Quaternary rocks (American Code 1961).  Several synonyms for this category of units have been suggested, the most recent being climatostratigraphical units (Mangerud et al. 1974) in which an hierarchy of terms is proposed.  In subsequent, stratigraphic codes, however (Hedberg, 1976; North American Commission on Stratigraphic Nomenclature, 1983; Salvador, 1994), the climatostratigraphic approach has been discontinued since it was considered that for most of the geological column 'inferences regarding climate are subjective and too tenuous a basis for the definition of formal geologic units' (North American Commission, 1983, p 849).  As Gibbard & van Kolfschoten (2005) observe, this view does not find favour with Quaternary scientists, however, since it is difficult to envisage a scheme of stratigraphical subdivision for recent earth history that does not specifically acknowledge the climate-change factor.  Accordingly, Quaternary stratigraphical sequences continue to be divided into geologic-climatic units based on proxy climatic indicators.  Boundaries between geologic-climate units were to be placed at those of the stratigraphic units on which they were based. 

The American Code (1961) defines the fundamental units of the geologic-climate classification as follows:

A Glaciation is a climatic episode during which extensive glaciers developed, attained a maximum extent, and receded.  A Stadial (‘Stade’) is a climatic episode, representing a subdivision of a glaciation, during which a secondary advance of glaciers took place.  An Interstadial (‘Interstade’) is a climatic episode within a glaciation during which a secondary recession or standstill of glaciers took place.

An Interglacial (‘Interglaciation’) is an episode during which the climate was incompatible with the wide extent of glaciers that characterise a glaciation.

In Europe, following the work of Jessen & Milthers (1928), it is customary to use the terms interglacial and interstadial to define characteristic types of non-glacial climatic conditions indicated by vegetational changes; interglacial to describe a temperate period with a climatic optimum at least as warm as the present interglacial (Holocene, Flandrian: see below) in the same region, and interstadial to describe a period that was either too short or too cold to allow the development of temperate deciduous forest or the equivalent of interglacial-type in the same region (West 1977). 

In North America, mainly in the USA, the term interglaciation is occasionally used for interglacial (cf. American Code 1961).  Likewise, the terms stade and interstade may be used instead of stadial and interstadial, respectively (cf. American Code 1961). 

It will be readily apparent that, although in longstanding usage, the glacially-based terms are difficult to apply outside the glaciated regions for which they are defined.  Moreover, Suggate and West (1969) recognised, the term Glaciation or Glacial is particularly inappropriate since modern knowledge indicates that cold rather than glacial climates have tended to characterise the periods intervening between interglacial events over most of the earth.  They therefore proposed that the chronostratigraphical term ‘cold’ stage be adopted for ‘glacial’ or ‘glaciation’.  Likewise they proposed the use of the term ‘warm’ or ‘temperate’ stage for interglacial, both being based on regional stratotypes.  Lüttig (1965) also recognised this problem and attempted to avoid the glacial connotations by proposing the terms cryomer and thermomer for cold and warm periods respectively.  These terms have found little acceptance, however.  The local nature of these definitions indicates that they cannot necessarily be used across great distances or between different climatic provinces or indeed across the terrestrial / marine boundary (see below). 

Perhaps the biggest problem with climate-based nomenclature is where the boundaries should be drawn.  Ideally they should be placed at the climate change but since the events are only recognised through the responses they initiate in depositional or biological systems a compromise must be agreed.  In practice, boundaries are generally placed at mid-points between temperature maxima and minima, e.g. in ocean-sediment sequences.  This positioning is arbitrary but is necessary because of the complexity of climatic changes.  However, problems may arise when attempts are made to determine the chronological relationship of boundaries drawn in sequences of differing temporal resolution or sediment type, and indeed determined using differing proxies.  By contrast in temperate Northwest Europe the base of an interglacial or interstadial is very precisely defined.  It is placed at the point where herb-dominated (cold-climate) vegetation is replaced by forest.  The top (i.e. the base of the subsequent glaciation or cold stage) is drawn where the reverse occurs.  It is unclear, however, how this relates to the timing of the actual climate change recorded or how this is recorded by other proxies.

Ocean sediment sequences

The principal development in the Pleistocene time scale depends on the regularity of the climatic cycle that was discovered around 1875 by Croll and developed especially by Milankovitch.  The first rigorous treatment using wide-ranging techniques was by Hays, Imbrie & Shackleton (1976). Isotope studies from the bottom sediments of the world’s oceans since then have indicated as many as 52 Late Cenozoic glacial ages and that the continental evidence, established up to that point, was so incomplete by comparison with the oceanic sequences that terrestrial glacial-interglacial stratigraphy must depend on the ocean record for chronological foundation.

'I'he marine oxygen-isotope scale makes use of the fact that, when continental ice builds up as a result of global cooling and sea level is lowered, the ice is depleted in 18O relative to the ocean water, leaving the ocean water enriched in 18O. The oxygen-isotope composition of calcareous foraminifera and coccoliths, and of siliceous diatoms, varies in direct proportion to that of the water (cf. Shackleton & Opdyke (1976) for discussion of the limitations of isotope stratigraphy). The 16 stages of Emiliani (1955, 1966) obtained from Caribbean and Atlantic sediment cores were extended to 22 by Shackleton &  Opdyke (1973) after analysis of an equatorial Pacific core.  Later development has seen additional analyses of cores to extend the sequence backwards into the Early Pleistocene and beyond into the Neogene (Tertiary).  Today the sequence is a combination of measurements from cores V19-30, ODP677 and ODP846 (Crowhurst 2002). The isotope stages recognised in core V28-238, from the eastern Pacific (Shackleton & Opdyke, 1976), are generally regarded as the ‘type’ for the later Quaternary, whilst those defined in core ODP 677 and 846 are those for the Pliocene to Middle Pleistocene (Shackleton , 1989; Shackleton & Hall 1989).

The events differentiated in isotope sequences are termed Marine Isotope Stages (abbreviated as MIS); this term is preferred by palaeoceanographers to the previously widely-used oxygen isotope stages (abbreviated to OIS).  This is because of the need to distinguish the isotope stages recognised from those identified from ice-cores or speleothem sequences (Shackleton, personal communication).  The stages are numbered from the present-day (MIS 1) backwards in time, such that cold-climate or glacial events are assigned even numbers and warm or interglacial (and interstadial) events are given odd numbers.  Individual events or substages in marine isotope stages are indicated either by lower-case letters or in some cases by a decimal system, thus MIS 5 is divided into warm substages 5a, 5c and 5e, and cold substages 5b and 5d, or 5.1, 5.3, 5.5, and 5.2 and 5.4 respectively, named from the top downwards.  This apparently unconventional top-downwards nomenclature originates from Emiliani’s (1955) original terminology and reflects the need to identify oscillations down cores from the ocean floor (Gibbard & van Kolfschoten (2005).

A glance at the oxygen-isotope sequences routinely obtained from ocean sediments reveals the complexity of the signal, the nature of which becomes more complex with closer sampling or resolution in higher sedimentation-rate profiles.  The adoption of the simplistic glacial-interglacial alternation is in fact only a very generalised description for periods characterised by predominantly 'cold' or 'warm'-climates, but which in reality show considerably more structure than can be accommodated in the simple geologic-climate terminology. 

Perhaps the biggest problem with climate-based nomenclature, like the marine isotope stratigraphy, is where the boundaries should be drawn.  Ideally the boundaries should be placed at a major climate change.  However, this is problematic because of the multifactorial nature of climate.  But since the events are only recognised through the responses they initiate in depositional systems and biota, a compromise must be agreed. Although there are many places at which boundaries could be drawn, in principle in ocean-sediment cores they are placed at mid-points between temperature maxima and minima.  The boundary points thus defined in ocean sequences are assumed to be globally isochronous.  This is reasonable because of the extremely slow sedimentation rate of ocean-floor deposits and the relatively rapid mixing rate of oceanic waters.  Attempts to date these MIS boundaries are now well-established by Martinson et al. (1987), for example.

Land – sea correlation

In recent years it has become common to correlate directly terrestrial sequences with those in the oceans.  This arises from a desire to correlate local sequences to a regional or global timescale, occasioned by the fragmentary and highly variable nature of terrestrial sequences.  The realisation that more events are represented in the deep sea, and indeed ice-core sequences, than were recognised on land, together with the growth in geochronology, has often led to the displacement of locally-established terrestrial scales.  Instead, direct correlations of terrestrial sequences to the global isotope scale are advanced, but there are serious practical limitations to this approach (cf. Schlüchter 1992, 2002; Gibbard & van Kolfschoten 2005). 

In reality there are very few means of directly and reliably correlating between the ocean and terrestrial sediment sequences.  Direct correlation can be achieved using markers that are preserved in both rock sequences such as magnetic reversals, radiometric dating or tephra layers and, rarely, fossil assemblages (particularly pollen).  However, normally it is impossible over most of the record and in most geographical areas.  Thus these correlations must rely totally on direct dating or less reliably on comparison using the technique of ‘curve-matching’; a widely-used approach in the Quaternary.   The latter can only reliably be achieved where long, continuous terrestrial sequences are available, but it is not straight-forward because of overprinting by local factors.  Moreover, the possibility of failure to identify ‘leads-and-lags’ in timing by the matching of curves is very real.  In discontinuous sequences, which typify land and shelf environments, correlations with the ocean-basin sequences are potentially unreliable, in the absence of fossil groups distributed across the facies boundaries or potentially useful markers.

Recently the growth of stratigraphy recognised from short-duration, often highly characteristic events has led to attempts to use these features as a basis for correlation.  This event stratigraphy (e.g Lowe et al., 1999), typically includes changes of sea level, climatic oscillations or rhythms and the like.  Such occurrences, often termed ‘sub-Milankovitch events’, may be preserved in a variety of environmental settings and thus offer important potential tools for high- to very high-resolution cross-correlation.  Of particular importance are the so-called ‘Heinrich Layers’ which represent major iceberg-rafting events in the North Atlantic Ocean.  These detritus bands can potentially provide important lithostratigraphical markers for intercore correlation in ocean sediments and the impact of their accompanying sudden coolings (‘Heinrich Events’ ) may be recognisable in certain sensitive terrestrial sequences (summary in Lowe & Walker 1997).  Similarly, the so-called essentially time-parallel periods of abrupt climate change termed ‘Terminations’ (Broecker & van Donk 1970), seen in oxygen isotope profiles, can also be recognised on land as sharp changes in pollen assemblage composition or other parameters, for example, where sufficiently long and detailed sequences are available, such as in long lake cores.  However, their value for correlation may be limited in high sedimentation-rate sequences because these ‘terminations’ are not instantaneous but have durations of several thousand years (Broecker & Henderson 1998).

Of greater concern for the development of a high-resolution terrestrial stratigraphy is the precise recognition and timing of boundaries or events from the marine isotope stages on land, and indeed vice versa.  Until very recently this was not perceived as a problem since it has been generally assumed that boundaries identified using a variety of proxies on land are precisely coeval with those seen in ocean sediments. Yet it is well known that different proxies respond at different rates and in different ways to climate changes and these changes themselves may be time-transgressive.  This has been forcefully demonstrated by work off-Portugal by Sanchez-Goñi et al. (1999) and Shackleton et al. (2003) where the MIS 6/5 boundary has been shown to have not been coeval with the Saalian / Eemian stage boundary on land, as previously assumed (Gibbard 2001).  The same point concerns the MIS 1/2 boundary which pre-dates the Holocene / Pleistocene (Flandrian / Weichselian) boundary by some 2-4000 years.  Thus high-resolution land sequences and low-resolution marine sequences must be correlated with an eye to the detail since it cannot be assumed that the boundaries recognised in different situations are indeed coeval. 

Future potential

Nevertheless, dating through astronomical (and sub-astronomical) cycles is clearly a geochronological tool of considerable future potential, already realised in respect of the ocean and ice-core sequences (e.g Björck et al. 1998), and of singular importance to understanding rates of process operation on land once the problems of cross-facies correlation have been overcome.  Perhaps the way forward should be to date fixed events – probably magnetic reversals or major climatic events – as accurately as possible, then use the astronomical cyclicity to provide a finer-scale chronology.  In future it is important that this scheme be phased-in to run in parallel and perhaps eventually to replace the fundamentally palaeontologically-tuned scheme that has served stratigraphical geology so well in the past.

It is this approach which has brought Quaternary geology so far, but at the same time causes considerable confusion to workers attempting to correlate sequences from enormously differing geographical and thus environmental settings.  This is because of the great complexity of climatic change and the very variable effects of the changes on natural systems.

The recognition of climatic events from sediments is an inferential method and by no means straightforward.  Sediments are not unambiguous indicators of contemporaneous climate, so that other evidence such as fossil assemblages, characteristic sedimentary structures (including periglacial structures) or textures, soil development and so on must be relied upon wherever possible to illuminate the origin and climatic affinities of a particular unit.  Local and regional variability of climate complicates this approach in that sequences are the result of local climatic conditions, yet there remains the need to equate them to a global scale.

In the second half of the 20th century, it was a recognised that Quaternary time should be subdivided as far as possible in keeping with the rest of the geological column using time, or chronostratigraphy, as the basic criterion (e.g. van der Vlerk 1959; Gibbard & West 2000).  Because stages are the fundamental working units in chronostratigraphy they are considered appropriate in scope and rank for practical intraregional classification (Hedberg, 1976).  However, the definition of stage-status chronostratigraphical units, with their time – parallel boundaries placed in continuous successions wherever possible, is a serious challenge especially in terrestrial Quaternary climate-dominated sequences.  In these situations boundaries in a region may be time-parallel but over greater distances problems may arise as a result of diachroneity.  It is probably correct to say that only in continuous sequences which span entire interglacial – glacial – interglacial climatic cycles can an unequivocal basis for the establishment of stage events using climatic criteria be truly successfully achieved. There are the additional problems which accompany such a definition of a stage, including the question of diachroneity of climate changes themselves and the detectable responses to those changes.  For example, it is well known that there are various  ‘lag’-times of geological or biological responses to climatic stimuli.  Thus, in short, climate-based units cannot be the direct equivalents of chronostratigraphical units because of the time-transgressive nature of former.  This distinction of a stage in a terrestrial sequence from that in a marine sequence should be remembered.

In general practise today these climatic subdivisions have been used interchangeably with chronostratigraphical stages by the majority of workers.  Whilst this approach, which gives rise to alternating ‘cold’ and ‘warm’ or temperate’ stages, has been advocated for 40 years, there remains considerable confusion about the precise distinction between the schemes.

In languages other than English the situation is more confused.  For example, in German the terms Glazial  and Interglazial are used as equivalents of the English stage.  Such an approach, on the face of it, seems expedient until one considers certain stages that have been correctly, formally-defined in the Netherlands’ Middle and Early Pleistocene, which are commonly used throughout Europe.  Here the Bavelian Stage includes two interglacials and two glacials, likewise the Tiglian Stage comprises at least three interglacials and two glacials (de Jong 1988; Zagwijn 1992).  Each of these interglacials are comparable in their characteristics to the last interglacial or Eemian which is a discrete stage, also defined in the Netherlands.  In these cases workers have fallen back on the non-commital term complex.  One example is the Saalian of Germany, originally defined as a glaciation, this chronostratigraphical stage includes at least one interglacial, as currently defined (Litt and Turner 1993).  Attempts to circumvent the nomenclatural problem by defining a ‘Saalian Complex’ are a fudge at best but one that is occasioned by linguistic and long-term historical precedent, as much as by geological needs. 

The original intention was that ‘cold’ or ‘warm’ or ‘temperate’ stages should represent the first-rank climate oscillations recognised, although it has since been realised that some, if not all, are internally complex.  Subdivision of these stages into substages or zones was to be based, in the case of temperate stages, on biostratigraphy, and in the case of cold stages principally on lithostratigraphy and or pedostratigraphy.  Within the range of radiocarbon dating (c. 30 ka) the most satisfactory form of sub-division is frequently that based on radiocarbon years (cf. Shotton and West 1969).  However, high-resolution investigations, such as the ice-core investigations, have allowed the recognition of ever more climatic oscillations of decreasing intensity or wavelength within the first-rank time divisions.  These events are stretching the ability of the terminology to cope with the escalating numbers of names they generate.  Terms such as ‘event’, ‘oscillation’ or ‘phase’ are currently in use to refer to short or small-scale climatic events (often referred to as 'sub-Milankovitch oscillations').  Clear hierarchical patterns are becoming blurred but perhaps this should be seen as a positive development since the system must reflect the need to classify events that are recognised.  Moreover, as our ability to resolve smaller and smaller scale oscillations increases, a more detailed nomenclature will inevitably emerge.

Therefore for many Quaternary workers chrono- and climatostratigraphical terminology are interchangeable.  Although realistically this situation is clearly unsatisfactory, because of the imprecision that it may bring to interregional and ultimately to global correlation, it is likely to continue for the foreseeable future.  The long-term goal should be to clarify the situation by continuing to develop a formally-defined, chronostratigraphically-based system that is fully compatible with the rest of the geological column, supported by reliable geochronology.

 

Modified from: Gibbard, P.L. 2007 Climatostratigraphy In: Elias, S.A. (ed.) Encyclopedia of Quaternary Science. Elsevier: Amsterdam. 2819-2825.



Subcommission on Quaternary Stratigraphy,
International Commission on Stratigraphy

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