Review article Journal of the Geological Society https://doi.org/10.1144/jgs2020-134 | Vol. 178 | 2021 | jgs2020-134

Terrestrial stratigraphical division in the Quaternary and its correlation

P.L. Gibbard1* and P.D. Hughes2 1 Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK 2 Department of Geography, School of Environment, Education and Development, University of Manchester, Manchester M13 9PL, UK PLG, 0000-0001-9757-7292; PDH, 0000-0002-8284-0219 * Correspondence: [email protected]

Abstract: Initially, terrestrial evidence formed the foundation for the division of Quaternary time. However, since the 1970s there has been an abandonment of the terrestrial stage chronostratigraphy, which is based on locally dominated successions, in favour of the marine oxygen isotope stratigraphy which largely records global-scale changes in ice volume. However, it is now clear that glacial records around the world are asynchronous, even at the scale of the continental ice sheets which display marked contrasts in extent and timing in different glacial cycles. Consequently, the marine isotope record does not reflect global patterns of glaciation, or other terrestrial processes, on land. This has led to inappropriate correlation of terrestrial records with the marine isotopic record. The low resolution of the latter has led to a preferential shift towards high-resolution ice-core records for global correlation. However, even in the short term, most terrestrial records display spatial variation in response to global climate fluctuations, and changes recorded on land are often diachronous, asynchronous or both, leading to difficulties in global correlation. Thus, whilst the marine and the ice-core records are very useful in providing global frameworks through time, it must be recognized that there exist significant problems and challenges for terrestrial correlation. Received 12 July 2020; revised 30 September 2020; accepted 10 October 2020

From its earliest beginnings, the study of geology has been division of Quaternary time. Today the contrast could not be greater, fundamentally linked to the study of sedimentary strata and their with many abandoning the terrestrial chronostratigraphical classi- relationship to each other. This concept provides the very fabric of fication in favour of the ocean-floor isotope and ice-core the subject and its application in presenting and determining the stratigraphies. The driver of cyclic fluctuations in marine oxygen evolution of the Earth through time. This holds as true for the isotopes has long been attributed to orbital forcing (Hays et al. 1976; Quaternary as it does for any other part of the geological column. Imbrie and Imbrie 1980) and this has underpinned the timeframe to Therefore the division of the successions that record the passage of which both the marine isotope and ice-core records are tuned events and time provides the central pillar of palaeoenvironmental (Imbrie et al. 1984; Dansgaard et al. 1985; Ruddiman et al. 1989; and landscape reconstruction through the period. Bender et al. 1994; Lisiecki and Raymo 2005). Whilst undoubtedly Since the 19th century, when it was recognized that glaciers had the marine isotope and ice-core sequences offer a reliable been far more extensive than today, the basis of Quaternary history framework, it is essential to remember that these sequences, had been the distribution and spread of glacial materials in the especially those in ocean-bottom sediments, record both regional landscape (specifically tills, erratic rocks and associated meltwater and global-scale changes. This contrasts substantially with deposits). By late in the century the recognition that glacial ice had terrestrial or shallow marine sequences which preserve locally extended multiple times, their deposits being separated by fossil- dominated successions, reflecting local and regional events. Such bearing beds yielding evidence that temperate climate episodes events are not necessarily represented in the global patterns because intervened, led James Geikie (1894, 1895), among others, to coin the local responses to changes will inevitably lead to modifications the terms ‘glacial’ and ‘interglacial’ to classify these events. to any all-encompassing pattern. For this reason direct correlation of Subsequent development of the divisions and definition of terrestrial sediment sequences with those in the oceans and those in Quaternary time and events naturally followed. One of the most the ice cores should not be undertaken uncritically. Lessons from striking themes that recurs through the decades has been the desire the past have repeatedly shown that simplistic or mechanistic to equate events identified from local sequences with ‘global’ correlation, when examined in detail, is often inappropriate and records. This has happened against the background of repeated unsustainable. paradigm shifts, starting with the Alpine successions of Penck and The complexity of events shown by high-resolution palaeoenvir- Brückner (1911), then the sea-level chronology of Zeuner (1959) onmental investigations, such as those from the ice cores, have and Evans (1971) and most recently the marine oxygen isotope and allowed the recognition of ever more climatic oscillations of polar ice-core stratigraphies. Each of these paradigm shifts was seen decreasing scale. Consequently ‘event stratigraphy’ is now applied as a ‘revolutionary’ advance in understanding of what is to the subdivision of ice-core records and is widely applied and undoubtedly an extremely complex sequence of climatically indeed advocated for wider terrestrial correlations in some contexts driven changes at a range of scales. (e.g. Hughes and Gibbard 2015; Lowe et al. 2019; see below). The Until recognition of the implications of the marine isotope recognition of these events has substantial implications for our succession in the late 1960s and early 1970s (Shackleton 1967; understanding of the nature of landscape development and Broecker and van Donk 1970; Shackleton and Opdyke 1973; Hays terrestrial evolution. However, the unravelling of this evolution et al. 1976) terrestrial evidence formed the foundation for the requires a rigorous time-division foundation. The reconstruction of

© 2020 The Author(s). This is an Open Access article distributed under the terms of the Creative Commons Attribution 4.0 License (http://creativecommons.org/ licenses/by/4.0/). Published by The Geological Society of London. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics

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events should use all available lines of evidence to ensure a rigorous finally it is deposited. Minor exceptions will be chemically local and regional chronostratigraphical scale. Over reliance on ice- precipitated deposits, biogenic deposits or volcanic dust products. core event stratigraphy, such as Greenland interstadials or stadials, Subsequent activity may result in modification of the deposited for global correlation could lead to the same erroneous correlations material by physical chemical processes, such as weathering, or soil that have arisen from attempted equation with marine isotope stages formation, and by plants or animals. (Hughes and Gibbard 2015). This point is reinforced by the fact that Whilst whole books have been written addressing the concepts of events resulting from the same causative agent are not necessarily sediment-process modelling, facies and architectural analysis and globally or even regionally isochronous (see below). taphonomic process investigations, it is not the purpose of this This review examines the bases for construction of terrestrial review to address a classification of sedimentary environments. sequences and their subdivision and, in turn, assesses the validity of Instead it is to consider the stratigraphical approaches to terrestrial their correlation with extra-regional and ‘global’ stratigraphies. and shallow marine stratigraphical sequences, both their correlation Finally, important recent and potential future developments are also and age determination. considered. Terrestrial environments and materials Terrestrial geological successions Terrestrial evidence is defined here as of rocks, sediments and It is natural that the first observations of sedimentary sequences and landforms found on or near land. Put in the simplest of terms, the landforms should begin with our observation of the landscape we nature of the terrestrial or continental environment is one of erosion inhabit. Therefore in the early development of geology, it was to be where, in general, natural processes break down pre-existing rocks expected that investigations primarily focused on the spread of and sediments (cf. Longwell and Flint 1962). These areas are by glacial sediments, with the realization that glaciers had in the recent their nature those above sea-level. However, the rapidity and scale of past been more extensive than today (see the review in Hansen changing sea-level in the Quaternary means that continental 1970). These studies necessarily began in mountain regions, environments also occur in fossil form below sea-level to depths especially the Alps where glaciers were already present, but later in excess of 100 m in coastal regions. Equally, in areas of isostatic extended to the plains of Europe and beyond when it was realized uplift shallow marine sediments may occur elevated to over 200 m the ice had once extended from an ice centre in Scandinavia, as well in regions subject to this process. For this reason, coastal and as smaller centres such as those in Britain including the Scottish and shallow marine sedimentary sequences and landforms can be Welsh mountains and the Lake District. At much the same time included within the general heading since they are effectively there was a realization that shallow marine and coastal sediments in transitional in nature (cf. Krumbein and Sloss 1963). the North Sea and Paris basins and the raised shorelines of the Baltic On land sediment deposition occurs in a vast range of and Mediterranean regions indicated that sea-level had equally environmental situations including rivers, lakes, estuaries, coasts, changed very recently in geological terms. marshlands (paludal environments), peat bogs, valley sides, From these early observations the development of Quaternary solution hollows, caves, dune fields, volcanoes, beneath and in geology, and the realization that climates of the recent past have front of glaciers, etc. The sediments may be clastic, chemical changed profoundly, has been principally based on the study of precipitates or biological materials. The variety of sediments terrestrial or shallow marine sediment sequences and landforms. It produced in these situations is as diverse as the environments is this evidence that until relatively recently provided the sole source themselves. An important characteristic of terrestrial sediments is of our records and sequence of events through the critical time that at the surface they are expressed as landforms and thus interval. terrestrial sediment sequences are not only recorded in vertical Sedimentary environments reflect the interaction of physical, section as geology but also recorded by landforms at the Earth’s chemical and biological conditions under which the material surface. This characteristic means that terrestrial sequences can also accumulates (Krumbein and Sloss 1963). The interplay determines be subdivided and correlated based on their morphology, i.e. the properties of the sediments deposited in an environment and in morphostratigraphy, which is defined as the subdivision of turn determines the pattern and distribution of the sediments sedimentary or rock units primarily from the surface form that deposited. For stratigraphers, it is essential to understand the nature they display (Frye and Willman 1962). Morphostratigraphy is of such environments, their persistence or otherwise through time widely employed in Quaternary science to subdivide a range of and their geographical distribution if they are to reconstruct sedimentary successions and is a key component of geomorphology accurately the sequence of sediments and landforms and their – the study of Earth surface processes and landforms (Hughes 2010, evolution. The climatic and environmental significance of the 2013). Since morphostratigraphy is inherently very fragmentary, contained fossil assemblages further enhance and in many cases correlation can be made with nearby quasi-continuous records determine the interpretation of these parameters. Moreover they which can act as a parasequence in order to build a chronostratig- provide the basis for constructing time equivalence or correlation in raphy, which ideally provides a record of continuous time. For distant sediment sequences. example, moraine records in Greece (Hughes et al. 2006) were For the stratigrapher to reconstruct past environments accurately, correlated with the pollen stratigraphical record from Lake Ioannina a knowledge of sedimentary processes, based on modern-day (Tzedakis 1994; Tzedakis et al. 2002) in order to formally define a observations, is essential. These observations must not only be cold stage chronostratigraphy for Greece (Hughes et al. 2005). based on physical, chemical and biological process themselves, but The nature of terrestrial sedimentation is governed by its also the nature of the sediments formed in a specific situation and geographical limitations in terms of the distribution, occurrence the processes involved in preservation of forms and fossil and by the climatic determinants. Climatic changes in the assemblages alike. Only in this way can the products be interpreted Quaternary further limit the occurrence of specific sedimentary in a meaningful and accurate manner. sequences since the controlling processes are inevitably determined The occurrence of a terrestrial sediment sequence in any place very strongly by the prevailing climatic conditions operating at any implies that the material was derived from a source area, where one place at any one time. The effects of these conditions on presumably the debris was produced as a product of breakdown or ongoing surface processes mean that morphology alone is often pre-existing rock or sediment. The debris is then transported by insufficient for correlating Quaternary terrestrial sequences, and the some means, during which it may or may not be modified, and most complete and continuous records tend to occur in polar ice

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sheets (ice cores) or at lower latitudes in lake sediment sequences. to this problem. Within various depositional systems, facies Records from extant ice on land, such as ice cores, whilst variation remains a significant problem when attempts are made obviously terrestrial, are usually considered separately to terrestrial to demonstrate equivalence either directly or through geochron- sediment and landform sequences, and we explore the relationship ology. Sedimentation on land therefore represents the products of between land–ice correlation later alongside ocean–land correla- highly discontinuous processes, often operating at highly variable tions. In terms of lake sediments, the longest and best-preserved rates. are found in stable basin settings, such as tectonic basins and It is often stated that the geological record preserved in sediments extinct volcanic maars in areas beyond the influence of major is highly incomplete. According to Ager (1993, p.53), ‘the sediment erosional agents, such as the effects of Pleistocene glaciation. This pile at any one place on the Earth’s surface is nothing more than a means that the best potential for long continuous Quaternary tiny and fragmentary record of vast periods of Earth history – the sequences occurs in tectonically active regions of the middle and “Phenomenon of the Gap Being More Important than the Record”’. low latitudes. For example, some of the longest terrestrial If this statement is correct for the shallow marine environments for sequences are found in southern Europe and the Mediterranean which it was primarily intended, it is even more so for continental region (Tzedakis et al. 1997; Turner 1998), Japan (Tarasov et al. settings where the conditions cause the stratigraphical record to be 2011), the Andean tropics of South America (Hooghiemstra 1989; effectively ‘a lot of holes tied together with sediment’. Given that Torres et al. 2005; Felde et al. 2016) and the rift valleys of Africa erosion is the dominant process operating on land areas in general, (Lamb et al. 2018). Long sediment sequences are also present in and non-deposition might be regarded as a secondary determinant, tectonic basins unaffected by glaciation at higher latitudes but the unavoidable consequence is that a ‘complete’ sequence of these are often restricted to continental interiors where Pleistocene events – whatever the term ‘complete’ means in this context – can glaciations were relatively limited because of limited moisture almost never be preserved on the land at a single locality. This is so supply, such as the Lake Baikal in Siberia, Russia (BDP-99 Baikal even though long records, such as those from lakes or ice cores, may Drilling Project Members 2005). provide sequences spanning several glacial–interglacial cycles, but From a long-term perspective, deposition in such situations can these are exceptional cases. Sadler (1981) has considered this be considered temporary since the sediment stored on the land at any problem and demonstrated that a complete sequence is one in which one given time is in transit to the sea, its ‘final resting place’. The no gap in the succession exists that is of longer duration than the sediment therefore has a low preservation potential on a geological time span at which the sequence is studied; i.e. the finer the scale of timescale. Nevertheless, since it is deposited and we can observe it, measurement, the less complete the succession is likely to be. The there is no reason to consider its stored information to be less useful implication of the episodic nature of sedimentation is that at very than that from marine sediments. The study of terrestrial sequences short timespans all sequences are incomplete; what is important is is clearly of central importance for study where local or specific whether the succession can be judged complete relative to the environments are the focus of the investigations for whatever timespan or scale resolution at which the record is required (Smith purpose. A detailed study of this type should not be considered of et al. 2014). any less importance in the case where the sequence cannot be Whilst only a fragmentary record of geological time is normally correlated to a global scale. recorded in preserved terrestrial and shallow marine sediments, it The correlation of terrestrial sediment sequences relies on robust is also important to consider the hierarchy of sequences, i.e. the stratigraphical frameworks and the organization of terrestrial scale of deposition over time at any one place or area. Clearly, the sediments and landforms into distinctive, useful, mappable units scale of the component sequences and sedimentary hiatuses based on their properties or attributes (Salvador 1994, p. 137). separating individual units can differ greatly, and indeed by Stratigraphy comprises two basic elements, sequence subdivision several orders of magnitude (cf. Miall 1997, 2016). Determining and correlation. Both are important for Quaternary studies, although the timespan of these individual component units is a major the latter is the ultimate objective for scientists interested in regional challenge and one that should not be taken lightly. Accurate and palaeogeographical reconstructions, which in turn can be correlated precise time estimates of sequences and hiatuses cannot be with the major global climatic events that characterize the achieved by taking bracketing ages from stratigraphically or Quaternary (Morrison and Wright 1965). Where the main property geographically distantly spaced marker beds (Sadler 1981; Miall or attribute includes landform morphology, sediment characteristics 1997, 2016). The same problems hold for palaeomagnetic events and sediment architecture, their organization into distinctive, useful, where lacunae may result in misleading ages. Nevertheless, mappable units can be achieved following three approaches: accurate correlation demands that sequences be dated either morphostratigraphy, lithostratigraphy and allostratigraphy (Hughes relatively or numerically as precisely as possible. However, 2010). In the vertical section, terrestrial stratigraphical correlation is shallow marine sedimentation on the shelf may be amenable to also commonly facilitated by biostratigraphy (Tzedakis et al. 2001) seismic stratigraphy. In these cases sequence stratigraphic and advances in sediment core-scanning have led to an increase in interpretations are possible, where sequence boundaries (erosional stratigraphical subdivision using techniques allied to lithostratigra- surfaces near shore) can be traced to continuously deposited and phy, such as geochemistry and tephrochronology. The basis of these hence (potentially) precisely dateable sediments. approaches and their use in Quaternary stratigraphy is outlined These concepts hold true throughout the geological column, but briefly below. for the Quaternary an additional complication arises from the need to reconstruct the numerous, and often rapid, climatic events now Stratigraphical implications known to have occurred during the period. The consequence is that stratigraphical problems are often uppermost in the minds of Whilst tectonics and climate are the principal drivers of terrestrial Quaternary scientists. Unlike geologists working on pre-Quaternary landscape evolution, their impact is in all cases modified by local rocks, we are testing stratigraphical principles to their limit and factors. The products of terrestrial mass-wasting and transport are beyond, often by attempting to subdivide impossibly short periods overwhelmingly of local character and distribution. This great range of time across different environments and then to correlate them of processes operating, the rapidity of climatic changes and the beyond local areas. This is particularly obvious, for example, where geographical settings in which terrestrial sediments accumulate glaciation is identified in one region, yet beyond the maximum combine to make sedimentary sequences difficult to relate to those extension of the glaciers it can be difficult to identify and preserved in external areas. Secondary modification also contributes establish the chronologically equivalent non-glacial sedimentary

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accumulations; the greater the distance, the more problematic the achieved and equivalence is established. Whilst in some instances it correlation becomes. has proved possible to link directly the marine and terrestrial records Attempts to overcome this difficulty have led to workers adopting using either marker events (e.g. volcanic eruptions), lithologies or an event-based stratigraphy, an approach which, instead of defining magnetic reversals, in the majority of cases the linkage has been boundaries fixed in time, understands the boundaries to be based on weak criteria. Indeed there is a strong tendency to over- diachronous, reflecting potential leads or lags in responses across extend correlation from one or possibly more markers by counting the Earth’s surface (e.g. Björck et al. 1998). This approach is backwards or forwards, or adopting assumed equivalents, often essentially the same as that of climatostratigraphy, long applied to based on interpretation of climatic origin or genesis of sedimentary the division and correlation of Quaternary sequences (e.g. Pillans deposits. and Gibbard 2012). Whilst this is useful for climatic reconstruction, A most successful means of extending and achieving greater, it can cause confusion with the formal division of time, with its independent accuracy in land–sea correlation is the technique of fixed boundaries. sequence stratigraphy. This approach relies on the fact that sea-level has undergone rapid relative changes multiple times through Extra-regional correlations geological time, and especially during the Quaternary. Relative sea-level changes arise from transfers of water between oceans and The inherently fragmentary nature of terrestrial records means that continents during expansion and contraction of ice sheets, driven by many Quaternary researchers understandably look to correlate climate change (Shennan 2007). They also record vertical move- events on land to those recorded in quasi-continuous records in the ments in the Earth’s crust over a wide range of timescales, from deep oceans or in the ice cores. Much of the assumptions seismic uplift and subsidence, through centennial- and millennial- surrounding such correlation is the role of orbital forcing on scale movements driven by glacial isostasy. Vertical changes in patterns of Quaternary climate change. Indeed, the marine isotopic relative sea level result in major horizontal shifts in coastal records and associated marine records are tuned to orbital environments. The combination of all these factors means that no parameters (Fig. 1). Fortuitously, ice-core records have a semi- single location on Earth records all the changes. The most independent chronology based on counting of annual accumulation, significant advance in studies of Quaternary sea-level change is ice-flow modelling, and the use of climate-independent age the recognition that reconstructions require the integration of markers, such volcanic eruptions (e.g. Parrenin et al. 2007). multiproxy observations with model predictions for a range of However, as with marine records, the ice-core stratigraphies also palaeoenvironmental variables, not only through observations or rely on orbital tuning and ice-core chronologies have often been predictions of relative sea level, but by oxygen isotope records from tuned using the same orbital models, such as SPECMAP, as the ocean sediments used to reconstruct global eustatic sea-level change marine isotope records (e.g. Dansgaard et al. 1985; Bender et al. through the Quaternary. 1994; Bender 2002). This led Parrenin et al. (2007, p. 486) to note In common with many aspects of Quaternary science, research that for ice-core records ‘the accuracy in terms of absolute ages is into sea-level changes advanced significantly with the development limited by the hypothesis of a constant phasing between the climatic of improved methods for numerically dating sediment, especially record used for the orbital tuning procedure and the insolation 14C. Early debates noted the regionally different patterns of emerged variations’. Thus, marine and ice-core stratigraphies are often built and submerged shorelines and sediments. Glacio-isostasy partially using the same assumptions of global climate forcing and this offered an explanation, but the significance of hydro-isostasy, local inevitably introduces some circularity in the entire basis of tectonics and the effects of gravitational attraction on the ocean Quaternary climatostratigraphy, to which all other records, surface only became accepted with advances in geophysical including those on land, are tied. In addition to this fundamental models. weakness, which undermines the stratigraphical independence of As already discussed, oxygen isotope variations (δ18O), in these records for global correlation, there are other problems and foraminifera preserved in deep-ocean sediments are a major challenges for land–sea and land–ice correlations and these are resource for developing continuous reconstructions of ice volume outlined below. and sea level throughout Quaternary time. Benthic oxygen isotopes record a >120 m glacio-eustatic sea-level lowstand Land–sea correlation during the Last Glacial Maximum (LGM) (Fig. 1; Spratt and Lisiecki 2016). Moreover, a major shift in the frequency and The recognition that the ocean isotope stratigraphy provides a quasi- amplitude of glacio-eustatic sea-level lowstands coincided with continuous record of changing climate through the late Cenozoic the initiation of the c. 100 ka climate cyclicity of the ‘early–middle has driven an inevitable trend towards correlating terrestrial Pleistocene transition’ at c. 900–600 ka (Maslin and Brierley sequences directly with the isotope stages established in the deep- 2015). Prior to this, sea-level changes of c. 70 m or less occurred sea succession (Fig. 1). This arises from the desire to correlate within glacial cycles. fragmentary local sequences to a regional or global timescale, where Sequence stratigraphy is dependent upon the large-scale, three- continuity is the focus. The realization that ocean-floor sediments dimensional arrangement of sedimentary strata, and the factors that recorded considerably more events than were recognized on land influence their large-scale geometry (e.g. reviews by Van Wagoner has led to the replacement of locally established terrestrial et al. 1990; Emery and Myers 1996; Miall 1997, 2016; Catuneanu timescales over the last few decades. Instead, direct correlations et al. 2011, Catuneanu 2019). The sequence is a stratigraphical of terrestrial sequences to the global isotope scale have become the concept based upon the three-dimensional arrangement of strata norm, as advocated, for example, by Kukla (1977). The temptation into units bounded both above and below by successional hiatuses to do this is understandable, but since there are serious practical that can be identified and traced from discontinuous accumulations limitations to this approach (cf. Schlüchter 1992), any attempt to to conformable surfaces in a basinward direction from river mouths cross-correlate should be related to fixed points or marker units or or coastal situations (Fig. 2). These surfaces are termed sequence horizons the age of which is not disputed. The recent development boundaries and the strata that comprise the package represent a of reliable geochronological techniques has further encouraged this depositional sequence. The seismic reflectors from stratal surfaces trend (Gibbard and West 2000, 2014). are chronostratigraphical markers, not lithological horizons as One of the biggest issues in relating terrestrial sequences to the earlier thought, and therefore provide invaluable markers for marine isotope stratigraphy is the means by which correlation is temporal correlation.

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Fig. 1. Graphs showing link between the structure of glacial–interglacial cycles indicated in the global benthic marine oxygen isotope LR04 stack of Lisiecki and Raymo (2005) and global sea-levels extracted from a global of seven sea-level records (from Spratt and Lisiecki 2016). The dust flux graph is from the EPICA Dome C ice-core record in Antarctica and is predominately a terrestrial dust signal and an indicator of the state of the global hydrological cycle with increased dust at times of global glacier maxima (Lambert et al. 2012). The bottom graph shows June insolation data at 60°N (Berger and Loutre 1991; Berger 1992). The Roman numerals (I, II, III, IV etc.) over the global benthic δ18O stack indicate the positions of the respective glacial terminations. During the glacial cycles associated with weaker global glaciations and higher sea levels (MIS 8 and 10 – highlighted in the sea-level and dust graphs), the most severe glaciations recorded by global aridity and the greatest dust flux over Antarctica (indicated by red arrows) were not synchronous with the global glacier maxima indicated by the trough in the marine isotopic record (indicated by the red dotted line). Adapted from Hughes, P.D., Gibbard, P.L., Ehlers, J., 2020. The ‘Missing Glaciations’ of the Middle Pleistocene. Quaternary Research 96, 161–183, reproduced with permission of Cambridge University Press.

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In almost all cases the geometrical relationships are identified by plain sedimentation. A third important surface is the transgressive geophysical seismic techniques, particularly long profiles in marine surface, which is generally taken to be the first significant marine settings, on land in extensive exposures, or they are interpreted from flooding surface within the sequence. individual observations from exposures or boreholes. Generalized Within sequences, more subtle facies and geometrical relation- models of depositional successions, including details of the internal ships have been used to characterize so-called systems tracts (Van geometries, are shown in Figure 2. Temporal representation or Wagoner et al. 1988, 1990; Helland-Hansen and Gjelberg 1994; thicknesses vary since sequences develop hierarchically at a Helland-Hansen et al. 1996). Geometrical arrangements of facies or substantial range of scales (e.g. Van Wagoner et al. 1990). The smaller-scale sedimentary cycles (‘parasequences’) may be suffi- principal factor thought to govern the genesis of a depositional ciently characteristic in certain circumstances that systems tracts can sequence in near-continent situations is relative sea-level change. be recognized in single outcrops or boreholes (Van Wagoner et al. This is the net result of global sea-level change, combined with local 1990). A four-fold systems-tract subdivision of depositional subsidence or uplift of the depositional area, both of which can sequences is commonly recognized (Hunt and Tucker 1992, operate at differing rates (Posamentier and Vail 1988). 1995; Catuneanu et al. 2011). Overlying the sequence boundary Fundamental to the sequence stratigraphical technique is the is the lowstand systems tract, interpreted as rising relative sea-level identification of critical or key surfaces that divide sequences, a and shoreline regression. The transgressive systems tract comprises sequence being defined as ‘a unit bounded by any recurring surface strata, the depositional environments of which migrate overall in a of sequence stratigraphy’ (Catuneanu 2019). In the Exxon sequence landward direction (i.e. they are transgressive) and the component stratigraphical model (e.g. Haq et al. 1988) the unconformity stratal surfaces of which onlap pre-existing deposits. Their base is surface represents the proximal area of a sequence boundary which defined by the transgressive surface and the relative sea-level at the passes into expanded sedimentary successions in a basinward shoreline is also interpreted to have been rising. The transgressive direction; i.e. the unconformity surface forms where sediment systems tract is terminated at its top at the maximum flooding accommodation is unavailable. ‘Accommodation’ refers to the surface, above which strata of the highstand systems tract shift space available for the accumulation of sediment, i.e. a surface basinward again, with successive stratal surfaces terminating in which passes through the shoreline and is dependent on sediment progressively more distal locations, forming a geometrical pattern supply and transport. The proximal region of the sequence boundary known as downlap, comparable to the lowstand systems tract. The is characterized by non-deposition or erosion of underlying strata final, regressive systems tract is represented by deposits, the and may include features such as river valleys or glacial troughs shoreline positions of which migrated progressively downward as incised into the underlying surface of previously deposits. In well as basinward, and so was produced during falling relative sea- contrast, the distal region of the sequence boundary may be level and regression (the surface defining the base had been termed represented by the increased volume of sediment eroded from the the ‘basal surface of forced regression’)(Rawson et al. 2002). adjacent landmass. It has been claimed by some that global sea-level change is the Two other important surfaces occur within a sequence. The more dominant influence on the formation of the majority of marine distal portion of the maximum flooding surface represents, like the depositional sequences (e.g. Haq et al. 1988), but this has been sequence boundary, a hiatus in deposition or very slow sedimen- questioned by those applying the method. Naturally, the extent to tation. However, unlike the sequence boundary it develops at the which global sea-level change may influence the local sequence distal end of the sediment transport path as a result of sediment stratigraphical record will depend on both time and location. In many starvation, and may be characterized by condensed marine deposits depositional settings (for example, active plate margins) more frequently containing an abundant pelagic fauna, e.g. ocean abyssal localized relative sea-level changes commonly dominate the

Fig. 2. Sequence stratigraphical summaries modified and redrawn based on Van Wagoner et al. (1990).(A) A type 1 sequence developed on a shelf margin, in which there is a relative fall in sea-level at the shoreline because the eustatic sea- level fall is faster than the rate of subsidence at the shoreline. This diagram lacks a falling-stage systems tract, which was originally included in the highstand tract, because they can be thin and frequently removed by later subaerial erosion forming the unconformity. (B)A type 2 sequence developed on a shelf margin, in which the rate of eustatic fall is slower than the rate of subsidence at the shoreline. HST, highstand systems tract; TST, transgressive systems tract; LST, lowstand systems tract; SMST, shelf- margin systems tract. From Van Wagoner et al. (1990) American Association of Petroleum Geologists, Methods in Exploration Series, 7, Tulsa, Oklahoma, AAPG ©2020, reprinted by permission of the AAPG whose permission is required for further use.

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stratigraphical record (Posamentier and Weimar 1993; Pedoja et al. stratigraphy for establishing a chronostratigraphy that can be 2014; Catuneanu 2019). On the other hand at certain times, such as compared across wide areas. Boundaries of chronozones should during periods of glaciation and deglaciation, high-magnitude, high- be isochronous, i.e. time-parallel (Björck et al. 1998). However, it rate sea-level fluctuations have undoubtedly occurred. would not be possible to define globally isochronous boundaries for Although originally based on Quaternary sequences (e.g. Frazier a chronozone based on isotopic signals in either Greenland and 1974), the approach of sequence stratigraphy has been relatively Antarctica. In event stratigraphy the boundaries between events are little-used in Quaternary studies. However, it has been applied not not specifically designated and problems of time transgression that only to near-continental margin situations (e.g. Miller and Mountain have arisen in applications of the terrestrial chronostratigraphy are 1994; Campo, Bruno and Amorosi 2020), but attempts have been no longer encountered (Björck et al. 1998, p. 289). In effect, this made to also apply it to fluvial (e.g. Blum and Törnqvist 2000) and recognizes that ‘events’ can be diachronous. indeed glacial depositional successions, the latter most frequently in Hughes and Gibbard (2015) recommended that event stratigraphy situations where glacier tongues or ice shelves terminate in the sea can, and indeed should, reside within a broader longer-term or large lakes (e.g. Boulton 1990; Eyles and Eyles 1992; Brookfield chronostratigraphical framework (such as the Late Pleistocene and Martini 1999). Subseries) and should not replace locally defined terrestrial In older geological successions sequence stratigraphy is now chronostratigraphical frameworks. Indeed, Lowe et al. (2008, widely used as a means of subdividing, correlating and dating p. 7) recommended that ‘all site records should initially be sediment accumulations, especially in the hydrocarbons industry designated using the appropriate local terminology, and that the (e.g. Posamentier and Vail 1988). It should also be noted that timing and duration of local or regional climatic/environmental sequence stratigraphy is often applied uncritically; in particular, age events be established independently of the ice-core record’. Once assignment of strata based solely on correlation with a supposedly this is achieved, then correlations with the ice-core event global sea-level curve has not proved to be a robust method. stratigraphy are acceptable, and even desirable, when comparing Nevertheless, as Posamentier and Weimar (1993) have suggested, records across wide areas. It has been shown that changes in the this method holds considerable potential for increasing accurate Greenland ice-core record are matched within independently dated land–sea correlation during the Quaternary. speleothem records as far away as SE China (Cheng et al. 2006). However, intuitively, correlation with the Greenland ice-core record Land–ice correlation is likely to be most reliable for short time intervals (sub-millennial-, centennial- and decadal-scale changes) at high northern latitudes. There has been a trend in the last few decades towards reliance on ice For example, issues over diachronous environmental changes over cores for terrestrial correlation. This is especially the case for the the Late Pleistocene late-glacial interval have meant that locally Late Pleistocene, since the longest ice cores in the Northern defined terrestrial chronostratigraphies cannot be directly compared Hemisphere (Greenland) span this interval cycle (Björck et al. 1998; directly between regions (Lowe et al. 2019). In fact, most terrestrial Walker et al., 1999; Lowe et al., 2001, 2008; Blockley et al., 2012). records display spatial variation in response to global climate Ice-core records from Antarctica are much longer and span multiple fluctuations, and changes recorded on land are often diachronous, glacial–interglacial cycles over the past 800 ka (EPICA 2006). The asynchronous or both, leading to difficulties in global correlation. advantage of ice cores over ocean records is one of resolution. The This is an evitable challenge to any attempts to link terrestrial ice cores offer an unrivalled annual resolution for reconstructing records to a globally comparable or even hemispherically environmental change and as such offer potential for fine-scale comparable records. In those cases where this has been attempted, land–ice stratigraphical correlations. However, there are limitations such as the correlation of the global LGM with Greenland Stadial 3 to relying solely on event stratigraphy for global correlation. in the Greenland ice-core records (Fig. 3), this represents a type of Ice cores provide geochemical archives related to a variety of informal chronostratigraphy that is common to Quaternary science different environmental indicators. Oxygen and deuterium isotope (e.g. Hughes and Gibbard 2015). In practice this approach is records in ice cores are related to air temperatures over the ice sheet acceptable, but cannot replace formal chronostratigraphy. This has at the time of snow precipitation and subsequent accumulation consequences for correlation since the widespread nature of (Johnsen et al. 2001; Jouzel et al. 2007). Dust particles in ice cores diachronous boundaries in Quaternary climatostratigraphy means provide a record of atmospheric dust flux over the ice sheets that the issue of correlation or comparison is a major and persistant (Lambert et al. 2012). Antarctic dust records are broadly question in Quaternary stratigraphy. representative of the global hydrological cycle with increasing dust, indicating a cooler and drier global atmosphere that is directly Correlation or comparison? associated with the extent of global glaciations (Hughes et al. 2020). A comparison of Antarctic ice-core dust records with loess/ Correlation is fundamental to the construction of regional palaeosol sequences from the Chinese Loess Plateau (Kukla et al. succession and, in turn, its relation to extra-regional or global 1994) appears to support the synchroneity of global changes in scales throughout the geological column. To correlate is ‘to show atmospheric dust load (Rousseau et al. 2007; Lambert et al. 2008). correspondence in character and stratigraphic position between In the Northern Hemisphere, there is evidence that the Greenland such geologic phenomena as formations or fossil faunas of two or ice-core isotopic signal is also mirrored in high-resolution tropical more separated areas’ and correlation is defined as ‘demonstration cave speleothems, such as in Hulu Cave, China (Wang et al. 2001). of the equivalence of two or more geologic phenomena in different However, as with marine isotopes, environmental signals recorded areas; it may be lithologic or chronologic’, according to the by dust or oxygen/deuterium isotopes in ice cores may not always American Geological Institute’s Dictionary of Geological Terms translate as an environmental signal in other types of sequences. (Bates and Jackson 1984, p. 113). Moreover, it is well established that there is asynchrony between In this context, the question of the quality or strength of the ice-core records in the Northern and Southern Hemispheres, correlation requires consideration. Where a terrestrial chronostra- especially when considering shorter timescales. For example, tigraphy has been established, the means by which any particular millennial-scale events preserved in the Antarctic and Greenland deposit, event or similar entity can be related to a global scale, ice cores are to some extent asynchronous, out-of-phase and especially the marine isotope–stratigraphical divisions, arises. sometimes opposite (Blunier et al., 1998; Steig and Alley, 2002; Today, in a wide range of cases where evidence for relating EPICA, 2006). This causes problems in using ice-core event continental sequences or individual depositional units is weak or

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Fig. 3. (A) Defining the Late Pleistocene global Last Glacial Maximum (LGM) using the ice-core record from Greenland (NGRIP). The top graph is the oxygen isotope record (Andersen et al. 2006) and the bottom graph dust concentration (Ruth et al. 2007), both from the NGRIP core. H2, H3 = Heinrich Events 2 and 3. GI-2, 3 and 4 = Greenland Interstadials 2, 3 and 4. (B) Comparison of the global oxygen isotope record based on a stack of 57 records from around the world (top diagram from Lisiecki and Raymo 2005) and the high-resolution coral-derived sea-level record from Thompson and Goldstein (2006). The coral-derived sea-level record is directly dated using U-series techniques. The interval represented by Greenland Stadial 3 is indicated by the dashed lines. The offset between the δ18O and the sea-level records and wider global climate at the global LGM highlights the probems of using the marine isotope record for fine-scale correlation. Both graphs are adapted from Hughes and Gibbard (2015). Reprinted from Quaternary International, Vol. 383, Hughes, P.D. and Gibbard, P.L., A stratigraphical basis for the Last Glacial Maximum (LGM), 174–185, Copyright 2015, with permission from Elsevier.

even lacking, correlation may be attempted by comparison of curves Eemian Stage Interglacial at 110 ka based on pollen in marine cores or plots based on perceived interpretation of climatic changes from off Portugal. This important example highlights that there is a lag sedimentary or fossil assemblages, etc. Such a process may indeed between terrestrial vegetational and the marine isotope records (see show corresponding patterns of change which may be interpreted as Turner 2002 for a discussion). Furthermore, there is also a lag demonstrating time equivalence. However, in reality this process between the timing of the end of the Last Interglacial between cannot be termed ‘correlation’ (see above) unless concrete evidence northern and southern Europe, with the elimination of forest is available to demonstrate the equivalence of the strata concerned. occurring in the north at c. 115 ka whilst in the south tree Instead it should more properly be termed ‘comparison’. populations persisted until c. 110 ka (Tzedakis 2003). This further Comparison is ‘the act or instance of comparing; the quality of emphasizes the caution needed in using the marine isotope record being similar’ (The Concise Oxford Dictionary 1976). Clearly any for correlating with terrestrial archives, and stage boundaries based establishment of potential equivalence based on the latter is on this archive are likely to be transient over millennial timescales. unreliable, since it is open to mis- or re-interpretation, and is likely to be unsafe. Indeed the danger of relying on the alignment of The nature of the marine isotope succession curves or plots to demonstrate the time equivalence of distant strata is that the process itself could potentially mask changes that, A further matter concerns the nature of the marine isotope although following an apparently similar course, are not coeval but succession itself. It is important to realize when comparing or out-of-phase. An excellent example is illustrated when considering attempting to correlate continental sequences with those on the the boundary between the end of the last interglacial and the onset of ocean floor that isotope stratigraphy is not strictly a chronostratig- the last cold stage in Europe. In the marine isotope record the start of raphy despite being referred to as such in some classic papers (e.g. the cold stage is placed at c. 115 ka based on a substantial cooling Martinson et al. 1987). The nature of the marine isotope effect at this time (Shackleton et al. 2002, 2003). This is consistent stratigraphy, its history and development and the consequent with Lisiecki and Raymo (2005) who place the Marine Isotope implications for chronostratigraphy are reviewed in Railsback Stage (MIS) 5d and 5e peaks (MIS 5.4 and 5.5) at 109 and 123 ka, et al. (2015) and explored below. respectively. However, the end of the Eemian Interglacial in Isotope stratigraphy is a method of determining relative ages of Portugal, recognized in the palynological record in an offshore core, sediments based on measurement of isotopic ratios of a particular shows that the boundary is actually well within MIS 5d (Shackleton element. It works on the principle that the proportions of some et al. 2003). The fact that the Eemian Interglacial does not isotopes incorporated in biogenic minerals (calcite, aragonite, correspond to the precise climatostratigraphical relationship sug- phosphate) change through time in response to fluctuating gested by MIS 5e envisaged by Shackleton (1969) was already palaeoenvironmental and geological conditions. Oxygen isotopes, noted by Sánchez-Goñi et al. (1999) who placed the end of the the most regularly applied for stratigraphy in the Quaternary, record

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detailed changes in ocean temperature and ice volume. In 1987) a process that is accepted as valid within the limitations of the sediments, data from calcitic microfossils, notably foraminifera, resolution achievable in these highly condensed sequences. record fluctuating temperatures and the growth and decay of ice However, the extremely slow sedimentation rate of ocean-floor sheets, allowing the recognition of oxygen isotope stages (or more deposits and the relatively rapid mixing rate of oceanic waters make properly marine isotope stages). The isotope contents are controlled high-precision correlation difficult. Nevertheless deep-ocean MIS by temperature, global ice volume and salinity. Earlier papers (e.g. records offer correlation with a resolution of a few thousand years or Shackleton 1967) assumed that benthic foraminiferal signals were less within a particular marine basin. Drift deposits on the deep thought to be largely unaffected by temperature (constant ocean floor may have significantly higher sedimentation rates which temperature of the world’s deep oceans) but this has since been is why they are targeted for coring. Even in relatively low shown to be over-optimistic (see Shackleton 2000; Skinner and sedimentation rate deposits of the central North Atlantic, Ferretti Shackleton 2005). Elderfield et al. (2012) used Mg/Ca ratios in the et al. (2010) were able to extract a detailed isotope record of the benthic foraminifera to remove the temperature effects from the Early to Middle Pleistocene boundary period MIS 19–21. isotope measurements, to reconstruct a signal for global ice volume, A further problem that arises from the definition of boundaries and hence sea-level. Shakun et al. (2015) developed this concept in ocean-sediment sequences is that the marine isotope stratigraphy and used a global compilation of 49 paired sea surface temperature– adopted as a reference standard by the majority of workers is based planktonic δ18O records to extract the mean δ18O of surface ocean not on a single-site, stratotype core but on a synthetic ‘stack’ of 57 seawater over the past 800 kyr, to separate out signals of global ice globally distributed benthic δ18O sequences that have been volume and global sea surface temperatures and then to model sea- ‘aligned using an automated graphic correlation algorithm’ level change. Because both global ice volume and sea-level (Lisiecki and Raymo 2005). This synthetic curve (Fig. 1)is parameters were driven by Milankovitch climatic cycles, it has intended to provide ‘a much needed common timescale and been possible to identify and correlate marine (oxygen) isotope reference for comparison (correlation target)’. However, this single stages in detail across the globe, and δ18O curves provide a relatively ‘ideal global’ marine isotope sequence is therefore not a refined (c.6–20 ka resolution) timescale for Quaternary to Neogene chronostratigraphy since it synthesizes curves from around the time (Rawson et al. 2002). As such this is a form of world, i.e. it does not present a record from any one locality and astrochronology, and does produce ‘absolute’ numerical ages in therefore it infringes the practice and the rules of definition of the same way that top-down layer counting works with other media. chronostratigraphical units (Salvador 1994; Gibbard 2013). This The critical point is that these marine (oxygen) isotope curves problem was recognized by Shackleton et al. (1995) when they provide a continuous record of change and as such have come to effectively observed single-type sequences for the isotope stages represent the reference for correlation around the world (Railsback for the Late Pleistocene in core V28–238, from the eastern Pacific et al. 2015). Whilst there is no doubt that the record is fundamental (Shackleton and Opdyke 1973), while those defined in cores ODP to our understanding of broad ‘global-scale’ changes throughout the 677 and 846 are those for the Middle Pleistocene and Pliocene, later Cenozoic, there are several substantial hurdles to correlation of respectively (Shackleton and Hall 1989; Shackleton et al., 1995). these sequences to the land record. In chronostratigraphy substantial The problem with adopting a single standard marine isotope record reliance is placed on the identification of boundaries, the basal is that it will contain local effects and analytical biases that must be boundary of each unit, be it the largest (erathem) to the smallest recognized for correlation. An obvious worry is that if widely (chronozone) being defined from a specifically defined sediment spaced core sequences are aligned statistically, any geographical sequence. For each unit only the base is defined, the top of a unit variations in responses to climatic changes will be masked by the being defined by the base of that succeeding it. This allows for the alignment process – a situation that is known to occur (cf. Skinner addition of any missing time without the need to redefine units if and Shackleton 2005). Moreover, if, as seems likely, most if not all intervening time is later identified. climatic event boundaries are diachronous at durations of less than However, in a quasi-continuous sequence, such as that a few thousand years, the use of climatic events for worldwide represented by the marine isotope stratigraphy, it is difficult to fine-scale chronostratigraphical subdivision potentially becomes determine where it is best to draw the stage boundaries. Ideally, the unsustainable over large areas (Moreno 2014; Rach et al. 2014; boundaries should be placed at the beginning point of a major Hughes and Gibbard 2015), at least in the Late Pleistocene. The climatic or environmental change. Railsback et al. (2015) discuss solution to this problem lies in the (re-)establishment of regional this issue in detail and noted that Emiliani (1955) explicitly dated sequences in all environments, the inter-regional cross- conceptualized marine isotope stages as intervals of time with correlation of which will highlight any variations in timing of boundaries at changes in temperature. Both Emiliani (1961) and events, the identification of which must be an important goal of Shackleton (1969) implicitly but clearly followed that model with ultra-high-resolution stratigraphy. substages as successive contiguous intervals of time (Railsback Comparison of sequences from the ice cores and the ocean et al. 2015). However, this is problematic because of the isotope profiles serves to emphasize a further issue, the question of multifactorial nature of climate and the difficulty in identifying representation of short-term events within the larger-scale glacial– the point where the climatic change actually occurred on the plots. interglacial climate cycles. Clearly the ice-core records, especially Since climatic events are only recognizable through the responses those where the accumulation rate is sufficiently high, record they initiate in depositional systems and biota, a compromise is considerably more detailed records of climatic events compared to required. those represented in the oceans. This is a consequence of So where precisely are the stage boundaries? In principle in accumulation, or more properly sedimentation rate, a factor which ocean-sediment sequences boundaries are placed on the plots at pervades the entire geological record. midpoints between temperature maxima and minima. The boundary Observations from modern processes indicate that sedimentation points thus defined are assumed to be globally isochronous, but they rates vary enormously depending on the environment of deposition, are really graphic artifacts (Fig. 1), not necessarily reflecting actual potentially by as much as 11 orders of magnitude (e.g. Sadler 1981), changes at that point in time and space. On land temperatures may allowing for breaks in sedimentation. Whilst there is a hierarchy to be very locally influenced and may also show a time lag in this variation, modelling exercises (e.g. Crowley 1984)have comparison to those indicated in the deep sea (see Railsback et al. demonstrated that as sedimentation rate decreases the number of 2015 and their figure 5). Dates for the MIS boundaries have been timelines preserved decreases exponentially, and the completeness established based principally on orbital tuning (e.g. Martinson et al. of the record of depositional events decreases linearly. The

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consequence of this conclusion is that low-magnitude events are of-phase with global environmental changes. The marine isotopic progressively eliminated from the sequence (Miall 1997, 2016), record has long been considered to represent the key proxy for shifts confirming the observation noted above. The implication of this is between glacial and interglacial cycles at the global scale by virtue that the representation of the high-resolution events that occur on of the dominance of continental ice sheets on the isotopic land or in the shallow marine environment are, in general, unlikely composition of the oceans (Shackleton 1967). Whilst it is well to be recorded in the ocean sequences, providing a further hurdle to known amongst palaeoceanographers that the marine isotope record equating sequences in these disparate settings. Clearly exceptions is more complex than just an ice-volume signal (Skinner and will occur, including major influxes of sediment arising from Shackleton 2005; Shakun et al. 2015), this is often overlooked by drainage of major waterbodies or deposition of volcanic tephra, for those working on terrestrial records. Moreover, it is apparent that the example. marine isotope record is misleading for correlations not only at the Further problems that arise in correlating continental and deep- local and regional scales but also at the global scale. This has water ocean sequences are the occurrence of bioturbation and recently been highlighted when examining the nature of global sedimentary hiatuses in the latter. These two issues are intimately glaciations through glacial–interglacial cycles (Hughes et al. 2013, related and therefore can, for the sake of brevity, be considered 2020; Hughes and Gibbard 2018). together. It is frequently believed that the bottom environment of the Marine isotope records have been used to characterize the deep sea is one of quiescence where sedimentary particles, structure of glacial cycles. During the later Quaternary, they display including the remains of organisms, etc, gently rain down in an a clear asymmetry though between interglacials with a slow descent extremely slow but constant stream to accumulate and remain into glacials and a rapid termination at their end (Broecker and van undisturbed. Whilst this may be true of certain regions, the presence Donk 1970)(Fig. 1). This vision of ice-age cycles has driven the of bottom-dwelling and burrowing fauna is well established, their idea of global glacier maxima at the troughs of the marine isotope activities giving rise to turbation or mixing of the surface sediments curve that has persisted for decades. However, as dating techniques which blurs or attenuates the signal of environmental change have improved and become widely applied to date glacial (Manighetti et al. 1995; Anderson 2001). Generally, this disturb- successions around the world, it has become clear that the pattern ance is thought to penetrate to depths of 10–20 cm, a process that is of global glaciation does not match this paradigm. Until the late part regarded by palaeoceanographers as of little significance for the of the 20th centrury it was notoriously difficult to date surface long-term record. Although there is a demonstrably clear record of landforms and thus the legacy of even the most recent Late changes through the vertical sediment sequences, doubt must arise Pleistocene glaciation remained poorly dated and often indirectly as to the precision that can be achieved in sequences where a interpreted from the basal ages of post-glacial sediment sequences. sedimentation rate of a few centimetres or less in 1 ka occurs However, since the advent of cosmogenic exposure dating, which (Leeder 2011). This implies that the bioturbation depth spans the has become widely applied in the past 20 years to date landform deposits of at least 3 ka or more, limiting the resolution to 20 ka or surfaces, it has revolutionized our understanding of Late Pleistocene thereabouts in the majority of situations (McCave 1995). However glaciation. This has also been paralleled by a dramatic increase in Ferretti et al. (2010) have demonstrated that the effects of the dating of subsurface materials often closely related to glaciation bioturbation are generally much less than previously assumed, especially through optically stimulated luminescence dating and to following a programme of 1 cm sediment sampling from the central lesser extent cosmogenic nuclide depth profile dating. The North Atlantic succession. Whilst abyssal plain sedimentation is mounting evidence has led to a shift in our understanding of the certainly characterized by very fine-grained deposits, under certain timing of the maximum extent of glaciations through the last glacial circumstances hiatuses or substantial debris influxes can occur cycle, which equally has implications for understanding global (Baldwin and McCave 1999). In particular, close to continental glacier dynamics in earlier glacial cycles. slopes or mid-ocean ridges, the input of substantial volumes of For the last glacial cycle (Late Pleistocene Subseries) the global material by turbidity currents causes local erosion and recycling of glacial maximum has traditionally been placed at c.1814CkaBP(c. accumulated pre-existing sediment. Equally adjacent to tectonically 21 cal ka BP) based on the orbitally tuned marine oxygen isotope active regions sediment mixing is potentially possible associated curve (e.g. Martinson et al. 1987; Lisiecki and Raymo 2005). with earthquake activity, submarine volcanism, the release of gases, However, as noted above, the marine isotope record has short- etc. Submarine currents can give rise to resuspension and recycling comings for fine-scale stratigraphical correlations. The correlation of sediments in some situations. However, fortunately for general of marine isotope records ideally can be accomplished with a purposes, hiatuses of longer duration than the shortest-term precision of a few hundred years. However, the numerical dating of Milankovitch cycles (i.e. c. 20 ka) are relatively rare in deep-water these records is less precise, having an uncertainty of c. 5 or 6 ka. ocean sediments. For the purposes of astronomical time-scale This is because of the lag in deepwater circulation between the construction, and medium-resolution palaeoclimate and global oceans that causes asynchroniety between the marine isotope stratigraphical studies, such records remain the best available (e.g. record, global ice volume and ocean temperatures at millennial Pälike and Norris 2006; Smith et al. 2014). timescales (Skinner and Shackleton 2005). This means that the trough in the marine isotope record at the end of saw-tooth glacial cycles is imprecise for defining global glacier maxima. For Terrestrial–marine correlation: issues for understanding example, for the Late Pleistocene Hughes and Gibbard (2015) the nature of glacial cycles argued that this is better defined in ice-core records and they correlated the global LGM with Greenland Stadial 3 (27.3–23.4 ka) Whilst the marine isotope record provides a very useful global (Fig. 3). This closely corresponds with sea-level minima dated reference timescale of the Quaternary (Fig. 1) the issues described in independently from the marine isotope sequence using U-series the previous section mean that its use as a record of global techniques (Thompson and Goldstein 2006) and also the maximum environmental change is more limited. Whilst the signal contains a extent of the Laurentide Ice Sheet (Balco et al. 2002; Ullman et al. global ice volume component it is a valuable proxy, so long as the 2015). Other ice sheets around the North Atlantic Ocean, such as the leads and lags are understood. This has resulted in problems when British–Irish ice sheet, also reached their maximum extent between correlating environmental changes recorded in different settings 27 and 24 ka (Clark et al. 2012; Scourse et al. 2019), and ice sheets across the world. This is especially true at the local scale where local rapidly retreated and thinned soon after Heinrich Event 2 (Hughes and regional environmental changes are often opposite or even out- and Gibbard 2015; Hughes et al. 2016). This illustrates that the

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‘global’ LGM was driven by events in this region and, importantly, New directions it is a regional rather than a truly global phenomenon. This further emphasizes the problems of correlation versus comparison when In recent years several new techniques have been developed that applying the marine isotope record to correlate fine-scale offer the opportunity to clarify, re-examine or refine stratigraphical stratigraphical events in the Quaternary. correlation. Here examples of these advances are reviewed. In addition to the problems of fine-scale correlations of global ice One area that has advanced apace is Quaternary geochronology volume with events such as the LGM, the marine isotope record has and associated dating techniques, which are considerably more more serious shortfalls when considering the overall pattern of diverse than for earlier periods of the geological timescale. Some of global glaciations through glacial–interglacial cycles. The late trough the techniques are applicable only to Quaternary deposits, while in the glacial marine isotope stage record was for nearly 50 years others can also be applied to older sequences. The time ranges of the assumed to correlate with the timing of the maximum extent of the major Quaternary dating methods over which each of the methods large continental ice sheets, following the assumptions of Shackleton (after Pillans and Gibbard 2012) are shown in Figure 4. Quaternary (1967). This assumption has dominated our perception of dating methods have been summarized in several publications, and Quaternary glaciations for decades with the result that the concept include Easterbrook (1988), Rutter and Cato (1995), Wagner of a global LGM has overwhelmingly influenced correlation of all (1998), Noller et al. (2000), Walker (2005), Elias (2007) and Elias kinds of terrestrial records for the Late Pleistocene. In effect it has and Mock (2013). essentially operated as a fixed tie point in Quaternary stratigraphy. Continued advances in dating techniques hold great promise for However, whether this tie point is real or imagined has recently been the future but it is unlikely that alone they will overcome the serious opened to question as more and more geochronological and fine- correlation problems that are encountered in Quaternary continental resolution stratigraphical evidence for global glaciations has been sequences. This is especially true when using geochronological acquired. Hughes et al. (2013) examined the evidence for the timing techniques for pre-Late Pleistocene deposits and landforms and of glaciations around the world and noticed that a large number of even for fine-scale stratigraphical correlations in the Late sites did not match the common concept of the global LGM. Many Pleistocene and Holocene. Some of the most significant advances sites saw glacier maxima earlier in the last glacial cycle, sometimes have occurred in radiometric dating which provide numerical ages by many tens of thousands of years. This situation was noted earlier (Fig. 4). Continuous developments and improvements in dating by Gillespie and Molnar (1995) who observed that mountain glaciers techniques mean that calculated ages change as new findings are in the mid-latitudes were asynchronous to the large continental ice made. For example, in cosmogenic exposure dating, production sheets. However, Hughes et al. (2013) found that it was not just the rates have been refined in recent years, such that for 10Be dating the mid-latiude glaciers that were asynchronous but some of the largest production rate has shifted by 20% from c. 5 to <4 10Be atoms/ continental ice sheets reached their maxima at markedly different gramme/annum, which has resulted in similar percentage increases times to the global LGM indicated in the marine isotope record. in calculated exposure ages. For other nuclides, such as 36Cl, there is Moreover, whilst the marine isotope record is known not to be a still significant disparity between different production rates, which perfect mirror of global ice volume (Skinner and Shackleton 2005; means that exposure ages can vary by up to 24% (Allard et al. Shakun et al. 2015), the general relationship was not doubted by 2020). Other radiometric dating techniques are also occasionally Hughes et al. (2013) and Hughes and Gibbard (2018). Rather, it was affected by changes in calibration curves, such as for radiocarbon the spatial and temporal representation of global ice volume that was dating (e.g. Reimer et al. 2013) or changing values for isotope half- questioned. This is because of the hugely dominant effect of changes lives, such as in U/Th dating (e.g. Cheng et al. 2013). In addition to in the volume of the Laurentide Ice Sheet on the marine isotopic these issues all radiometric ages come with uncertainties associated composition of the global oceans. Nevertheless, this effect does limit with laboratory (internal) and systematic (external + internal) error the utility of the marine isotope record as a representative indicator of (Schmitz 2012), although improvements in dating techniques in global glaciation and accompanying environmental changes, recent years mean that this is often just a few percent. However, all especially climatic changes, closely associated with glacier expan- geochronological techniques rely on correct interpretation of the sion and contraction in different parts of the world. geological context and this is probably the biggest obstacle to The problem of the marine isotope record and the pattern of correct correlation. In ideal situations multiple different dating global glaciation identfied for the Late Pleistocene has major techniques should be used to test and confirm geochronological implications for understanding earlier glacial cycles. Hughes et al. frameworks that are subsequently used for correlation. Concerning (2020) took the lessons of the last glacial cycle and applied them to the application of numerical dating, it is also important to remember earlier glacial cycles in the Middle Pleistocene. It soon became that geochronology (geological time independent of the rock apparent that similarities existed between some major glaciations, record), is a separate classification from chronostratigraphy (time which were absent or different in extent from others. For example, in based on representative rock sequences), the two schemes running all glacial cycles there appeared to be the phenomenon of double in parallel. glaciation maxima, early and late in the glacial cycles. In the glacial Until very recently the principal and only practical means of cycles characterized by the largest glacier global extents (such as displaying geology on the ground was the geological map. Whilst MIS 16, 12 and 6 in the Middle Pleistocene, as well as the MIS 5d-2 this remains an excellent means of recording, portraying and in the Late Pleistocene), the second glaciation maximum corre- communicating several sets of two-dimensional information, it sponded closely to the global glacier maximum in the marine requires a significant amount of expert knowledge to interpret isotope record (LGM, Penultimate Glacial Maximum, etc). (Royse et al 2012). However, recent advances in Geographical Conversely, in glacial cycles associated with weaker global Information Systems, and three-dimensional modelling based on glaciations (e.g. MIS 14, 10 and 8) the first glaciation maximum large datasets that have allowed them to be handled on desktop early in the glacial cycle was larger (Fig. 1). This is despite the computers, have initiated a seed-change in the viewing, marine isotope record suggesting the classic second and late glacial manipulation and interpretation of geological information (Jones maximum was the largest. This is because the later peak represents et al. 2009). This is allowing construction of a new generation of the maximum of the Laurentide Ice Sheet but not other ice masses geological models for onshore and offshore areas, including around the world. This highlights again that the saw-tooth complex urban areas, such as London (e.g. Royse et al. 2012; asymmetry of the marine isotope record is misleading for under- Mathers et al. 2014)(Fig. 5). These models provide a means for standing the nature of global glaciations and their extents. integrating and visualizing evidence from many different sub-

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Fig. 4. Dating methods applicable to the Quaternary, showing age ranges over which each method can be applied (after Pillans and Gibbard 2012). Reprinted from The Geologic Time Scale 2012, Gradstein, F., Ogg, J., Schmitz, M., Ogg, G.(eds), Chapter 30 – The Quaternary Period (Pillans, B., Gibbard, P.L.), p. 980–1009, Copyright 2012, with permission from Elsevier.

disciplines, thus allowing a model to display more closely some stratigraphical components on the sequences as a whole can be of the natural heterogeneity of real geological systems. As well as assessed, for example the interdependence of tectonics, sedimen- allowing geoscientists to take account of the third dimension, in tation, stratigraphy, material properties and preservation. the near future models will be able to be manipulated to display Regarding the chronostratigraphical division of Quaternary time changes through time (the fourth dimension). Nevertheless, the (Fig. 6), as Gibbard and West (2000, 2014) have noted, the current limitations of the data on which the assessments are based must situation is unsatisfactory. This is because two partially compatible be understood, as in all geological modelling exercises. As systems are being widely applied, yet neither is completely suitable technological improvements allow workers to introduce greater for today’s knowledge of events. On the one hand, terrestrially levels of realism into their models, it is becoming more critical to based terminology, like that used in NW Europe (e.g. Mitchell et al. describe accurately and to understand the nature of the sequences 1973) is still widely in use, whilst a significant number of articles being investigated. The accuracy of these models depends not have adopted the marine isotope nomenclature. As has been stated only on the density and quality of the original data, but also on repeatedly, both these systems have limitations, but the potential the theoretical understanding of the underlying geology (Royse difficulties of correlating (rather than comparing) with the ocean et al 2012; Westerhoff et al. 2009). sequences in reality make the latter an unreliable enterprise (cf. Such a resource provides a fundamental tool for geological Gibbard and West 2014). Direct correlation with the ocean or ice- research, where the impact of individual geological or core sequences holds a number of pitfalls, the greatest obstacle

Fig. 5. Three-dimensional block geological model of the Middle to Lower Thames Valley area. BGS Image ‘3D Geological Map of London’. Permit Number CP20/065. Image Courtesy of the British Geological Survey ©_UKRI 2020.

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Fig. 6. Global chronostratigraphical correlation table for the Quaternary showing marine isotope stages and relationship to terrestrial chronostratigraphical stages (copied from a figure in Hughes et al. 2020 that is modified from Cohen and Gibbard 2011). From Hughes, P.D., Gibbard, P.L., Ehlers, J., 2020. The ‘Missing Glaciations’ of the Middle Pleistocene. Quaternary Research 96, 161–183, reproduced with permission of Cambridge University Press.

being the comparison of high-resolution discontinuous continental Clearly a system is required that provides a solid foundation for sequences with low-resolution yet quasi-continuous ocean basin stratigraphical division incorporating the modern understanding of sequences. events during the Quaternary, but which equally respects the rules

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Fig. 7. Virtual axial dipole moment (VADM) calibration of the PISO-1500 relative palaeointensity stack. The dots indicate palaeointensity minima that have ages close to those attributed to geomagnetic excursions and reversals (cf. Fig. 7 for excursion/reversals). The dashed line represents the threshold value of 2.5×1022 Am2 that may trigger excursions and reversals (modified from Channell et al. 2009). Reprinted from Earth and Planetary Science Letters, Vol. 283, Channell, J.E.T., Xuan, C., Hodell, D.A., Stacking paleointensity and oxygen isotope data for the last 1.5 Myr (PISO- 1500), Copyright 2009, with permission from Elsevier.

and methodology of rigorous stratigraphical classification. The reference profile spanning the last 1.5 ma, that can be used to identification of additional events demonstrates the need to confront correlate marine, lacustrine, loess and ice-core sequences (Fig. 7). the definition of additional time divisions in the terrestrial sequence. This independent marine chronology could potentially eliminate Conversely, the uncritical adoption of the marine isotope stage difficulties inherent in oxygen isotope stratigraphy such as phase sequence for division on the land begs the question: since the lags, and the assumption that δ18O is purely a glacial ice-volume or boundaries are imprecisely defined, should these divisions be sea-level signal (Imbrie and Imbrie 1980; Skinner and Shackleton adopted in the definition of high-resolution terrestrial sequences (cf. 2005). They speculate that potentially the timing and phase Gibbard and West 2014)? The shorter the duration of the changes relationships of the climate system components may be resolvable the more acute the problem becomes. Land–sea correlation therefore independent of oxygen isotope stratigraphy, providing new insights remains the greatest challenge in chronostratigraphy of the late into the mechanisms and timing of glacial–interglacial climate Cenozoic. Accurate land–ocean correlation ideally requires the change and its correlation. identification of global markers, that can potentially be identified in Reversals and excursions of the Earth’s geomagnetic field occur the widest spectrum of environments. throughout the Quaternary and hold great potential for correlation How then are the equivalents of short-term event subdivisions to across various environments, creating marker horizons that are be reliably determined in ocean sediments? One possible means of readily detected globally in sedimentary and volcanic accumula- addressing this problem is the application of new geomagnetic tions (Singer 2014). However, their application to correlation techniques, such as magnetic polarity stratigraphy that is based on demands an accurate, high-resolution reference chronology for the recognition of polarity zones in sediments. This, an approach several main reasons. These include the potential to provide precise resulting from the changes in the magnetic dipole field, is an inter-correlation of the ocean and terrestrial sediment and ice-core independent phenomenon that can be recorded in the widest range records, which can be used to compare and contrast the timing and of sedimentary records. The reversal horizons provide timelines for intensities of events from spatially distant regions. Of equal precise correlation at the reversal events. However in the last 20 geochronological/stratigraphical importance is that the geomag- years, relative geomagnetic palaeointensity (RPI) has been netic field instabilities associated with both reversals and determined in a large number of investigations and holds potential excursions lead to enhanced production of cosmogenic isotopes. for millennial-scale stratigraphical correlation within the relatively Therefore the application of cosmogenic isotopes for surface long-duration polarity chrons. As Channell et al. (2009) have exposure dating or as chronostratigraphic markers requires a demonstrated, the association of some RPI minima with short complete succession of geodynamo instabilities (e.g. Dreyfus et al. magnetic excursions means that an RPI stratigraphy can now be 2008; Singer et al. 2009). The timing of polarity reversals that established in the palaeomagnetic reversal record. define chronozone boundaries and the Geomagnetic Polarity For the application of palaeointensity records to stratigraphy a Timescale, and the determination of the frequent excursions now calibrated reference profile (or profiles) must be developed. As for known to have taken place during the intervals between the benthic oxygen isotopes (cf. Lisiecki and Raymo 2005), a sequence reversals offers a new potential means of high-resolution has been generated from multiple records which, although stratigraphy for regional, extra-regional and even land–sea increasing the clarity of the signal, may average local variations correlation. This geomagnetic instability timescale for the by the statistical process (see above). The PISO-1500 stack, Quaternary offers the potential for high-resolution chronostratig- compiled by Channell et al. (2009), represents a stratigraphic raphy (Singer 2014)(Fig. 8).

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Fig. 8. The Geomagnetic Intensity Time Scale for the Quaternary period. The PISO-1500 palaeointensity record is taken from Channell et al. (2009) (cf. Fig. 6) (modified from Singer 2014). Reprinted from Quaternary Geochronology, Vol. 21, Singer, B.S., A Quaternary Geomagnetic Instability Time Scale, 29–52, Copyright 2014, with permission from Elsevier.

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Conclusions require boundary definitions and designation of boundary strato- types for all units defined for all environments. (1) Terrestrial stratigraphy comprises highly discontinuous, (13) Recent advances in techniques, especially in geochron- laterally variable, high-sedimentation rate sequences. ology and three-dimensional geological mapping, offer new (2) A distinct characteristic of some terrestrial records is their insights into stratigraphical correlation. In addition, tools that can surface expression, or geomorphology. However, these types of be applied across different sedimentary and volcanic situations to records are inherently fragmentary because of ongoing surface recognize events, such as palaeomagnetic reversals and excursions, processes. hold great potential for refined terrestrial chronostratigraphy and (3) Aside from extant ice sheets, the longest and best-preserved land–sea and land–ice correlations. terrestrial records occur in stable basin settings, such as tectonic (14) These points reinforce the view that it is only through a basins and extinct volcanic maars in areas beyond the influence of carefully and systematically defined sequence of units for each major erosional agents, such as the effects of Pleistocene glaciation. region that a durable Quaternary geological timescale can be Except under these very exceptional circumstances, terrestrial established and developed in the future. sequences will never preserve a ‘complete’ record of events in a region. (4) Nevertheless, as terrestrial organisms we are greatly concerned about terrestrial events and as such the records Acknowledgements We thank Professor R.G.West for his constant support and encouragement, and his critical comments on the manuscript. We reconstructed from ocean sediments or ice cores will not replace also thank Philip Stickler and David Walton (Department of Geography, land-based sequences. University of Cambridge) and Graham Bowden (University of Manchester) for (5) Not all investigations require the correlation of local their cartographic assistance. We are grateful to Professor Martin Head and an sequences with those constructed from other environments, and in anonymous referee for their comments that improved our manuscript and the editor Dr Stephen Lokier for managing the review process. Finally, we thank particular ‘global’ records, in order to be valid. In particular, Patricia Pantos and the JGS editorial team for encouraging this submission. researchers should avoid spurious correlation, such as correlating with marine isotope stages or other records when it is not necessary Author contributions PH: writing – original draft (supporting), writing or appropriate for the investigation in question. – review & editing (equal); PLG: conceptualization (lead), writing – original (6) The subdivision of high-resolution sequences from contin- draft (lead), writing – review & editing (equal) ental situations requires the application of systematic stratigraphical division (i.e. precise boundaries); the marine isotope stratigraphy Funding This research received no specific grant from any funding agency in potentially provides support and clarification with dating of the public, commercial, or not-for-profit sectors. successions, if not classification, especially when coupled with sequence stratigraphy. Data availability - Data sharing is not applicable to this article as no (7) Correlation of land and deep-sea sequences requires the datasets were generated or analysed during the current study. - This paper demonstration of the temporal equivalence of strata (i.e. isochro- represents a review paper and all data sources are from cited reference sources. neity), in terms of lithology, process, fossil content or time; comparison is not correlation. Where appropriate, onshore deposits Scientific editing by Stephen Lokier can be traced seaward by seismic stratigraphy. The package of sediments (the sequence) produces a genetic equivalence, but a References sequence boundary is an erosional surface; it is therefore likely to be Ager, D.V. 1993. The Nature of the Stratigraphical Record. J. Wiley and Sons, diachronous. Chichester. (8) For example, the marine isotope record is not a good Allard, J.L., Hughes, P.D., Woodward, J.C., Fink, D., Simon, K. and Wilcken, K.M. 2020. 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