When did the Last Interglacial end?

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  • QU.4T'ERNARY RESEARCH 4, 246-252 (1974)

    When Did the Last Interglacial End?

    R. P. SUGGATE~

    Received April 1, 1974

    Disagreement on the time of the end of the Last Interglacial stems from lack of an agreed definition of an Interglacial. Although strictly a climatic episode, Interglacial as commonly used is essentially a chronologic unit equivalent to an Age, corresponding in time range to the chronostratigraphic unit Stage.

    The name Last Interglacial has gained a global connotation so that its definition must take into account global rather than local effects of temperature fluctuations. An Interglacial begins with a warming to full interglacial warmth (as warm as the present day). It continues until a cooling of full glacial severity occurs, and includes any lesser toolings within a period of fluctuating climate. Such lesser toolings are recorded, particu- larly in deep-sea cores, following a period of warmth about 125,000 y.a. The cooling that led to the next full glacial cold began about 50,000 yr later. The Last Interglacial lasted from about 123,000 to 73,000 yr BP, equivalent to stage 5 of Shackleton and Opdyke.

    INTRODUCTION

    Amidst the wealth of important papers given to a symposium concerned with the timing of the end of the present intergla- cial, published in Quaternary Research 2, 1972, there lies a total confusion as to when the Last Interglacial can be said to have ended. The guest editorial by Kukla et al. (1972) emphasized the contrasts in previ- ously published assignments of duration of the Last Interglacial and asserted (p, 263) that lL. . . . at least for the purposes of the meeting the concept similar to the practice of European palynologists was tacitly adopted. The term interglacial is under- stood here to mean an uninterrupted warm interval, in which the environment on a global scale reached at least the present level of warmth. The editorial indicated that this symposium adopted the period

    1 New Zealand Geological Survey, P.O. Box 30368, Lower Hutt, New Zealand.

    *The first letters of the words Last Intergla- cial need to be in capitals to indicate the effec- tively formal use of the term.

    about 128,000-116,000 yr BP, for the Last Interglacial. Yet no such generalization is apparent in the published papers. Several authors did not define the Last Interglacial even though they used it as an analogy for the present one ; others did not need to define it. The following contrasts are found in the dating of its end:

    116,000 yr BP-Fairbridge (1972: 294) Kukla and Kukla (1972:

    42) 97,000 yr BP-Morner (1972, Figs. 1, 2;

    pp. 342-343) 73,000 yr BP--McIntyre and Ruddiman

    (1972: 350).

    Wright (1972: 280) clearly stated one rea- son for divergency, The 70,000 yr date [for the end of the Last Interglacial] is now challenged by evidence of a low stand of sea level (-71 m) in Barbados and thus presumably extensive glaciation between 125,000 and 105,000 yr BP, according to thorium dates. This cold period is referred to as the 110,000 YBP event by Sancetta

    246 Copyright @ 1974 by University of Washingcon. All rights of reproduction in any form reserved.

  • WHEN DID THE LAST INTERGLACIAL END? 1i

    et al. (1972: 366), and is correlated with a northern hemisphere winter insolation minimum by Kukla and Kukla (1972, Fig. 3). Sancetta et al. (lot. tit.) stated that The cooling associated with the 110,000 YBP event represents, in the [North Atlan- tic] core studied, two-thirds of a swing towards full glacial conditions, and was considerably more intense than that associ- ated with the younger event at 92,000 YBP. Although both events are recorded in the Camp Century ice core (Dansgaard et al., 1972, Fig. l), the magnitude of the earlier one is not known because of a break in the record. In this ice core, the events appear to have been of exceedingly short. duration.

    The period of 127,000-71,000 yr BP ap- parently had three temperature maxima separated by two minima, although Emi- liani (1972, Fig. 1A) appears to have dated these climatic episodes in the range of lOO,OOO-70,000 yr BP. The first tempera- ture maximum was almost certainly the warmest, probably warmer than the present day, and the later two may have been as warm as or a little less warm than the pres- ent day. The question is-should the whole period be regarded as one interglacial (cf. McIntyre and Ruddiman, 1972: 350), as one interglacial followed by two stadials and two interstadials of the Last Glaciation (cf. Fairbridge, 1972, Fig. 7)) or as three interglacials? The question needs to be answered in the most globally useful man- ner, without undue weight being given to any particular local sequence, especially if that sequence is far from oceans that pro- vide a global means of smoothing the ex- tremes of local fluctuations.

    CURRENT NOMENCLATURE OF SEQUENCES IN THE

    LATER QUATERNARY

    Several schemes of subdivision, based on climatic change inferred from data derived from different sediment types, were used by contributors to the symposium. As well as more or less traditional schemes such as the

    Flandrian-Weichselian-Eemian sequence, three numerical or alphabetical sthemes were used :

    Author of srheme of

    subdivision Sourvc of data Subdivisions Emiliani 1 kcp-sea (ores ;,I . *; 1 < .,I 1

    ( lSkj,i I

    Ericson et ~2. Ikep-sea cores s I ,z (1!)61)

    Kukln (1961) Loess stratigraphy H A

    The 2 = 1 = A subdivision is the post- glacial; prior to that the status of units varies. Beca.use the dating of t.he X/Y boundary is not agreed (89,500 yr BP, Kennett and Huddlestun 1972: 391; 72,000, Kukla et nl., 1972: 264), the correlation implied above between the Ericson and Emiliani units is not quite certain. Further- more Emiliani (197, Fig. 1) dated the be- ginning of his core stage 5 at about 100,000 yr BP whereas the beginning of Ericsons pelagic zone S is at about 130,000 yr; the discrepancy resul t.s almost certainly from difficulties of dating deep sea-cores rather than from difficulties of correlatng between cores. It may be noted that the numerical stages of Shackleton and Opdyke (1973) are those of Emiliani, but their date for the beginning of stage 5 is 128,000 yr BP.

    Subdivisions of Ericsons units were made by Kennett and Huddlestun (1972: 386) who numbered the subunits backward in tirnc progrcssivcly in the snn~e dir&ion as Ericsons main units. Emilianis unit 5 was subdivided alphabetically backward in time by Shackleton (1969)) in the direction opposite to the numbering by Emiliani. Kuklas units were subdivided numerically forumrtl in time by Kukla and KoFi (1972: 3771 in the direction opposite to t,hat of Kuklas alphabetical progression of main units.

    These detailed contrasts in usage are reflected also in the three main systems themselves and have a parallel in the con- trast b&m-een the forward nurnbc~ring of the

  • 248 R. P. SUGGATE

    Mallorca interglacial high-sea-level se- quence (Tyrrhenian-I being older than Tyrrhenian-II and Tyrrhenian-III) by Butser and Cuerda (1962) and the baclc- ward numbering of the Barbados sequence (Barbados-I being younger than Barba- dos-11 and Barbados-III) by Mesolella et al. (1969). Another forward numbering sys- tem is that of pollen zones in northwest Europe and another backward numbering system is that of the Terminations of glacial cycles of Broecker and van Donk (1970).

    Clearly the various conflicting practices make it difficult to comprehend the litera- ture easily. Whereas the problem of the direction of numbering could be resolved by simple agreement-preferably by number- ing forward in time in the direction of the sequence of events, the problem of the status of units is more difficult. Of the nu- merical or alphabetical units, only those of Ericson et al. (1961) are of Interglacial and Glacial status, in the sense that they are intended to be equivalent to major half- cycles within full climatic cycles.3 In order to begin to systematize the various ap- proaches to subdivision on a climatic basis, agreement should be sought for interpreta- tion in terms of major units of this status, with minor half-cycles within full climatic cycles as subunits. It is probable that some pollen sequences, from which comes one concept of an Interglacial (cf. Jessen and Milthers, 1928) record minor half-cycles rather than major ones.

    THE DEFINITION OF INTERGLACIAL [AGE]

    An interglacial episode between two gla- ciations can strictly have taken place only in regions that were glaciated, and its dura-

    The term climatic cycle seems preferable to that of glacial cycle used by Kukla et al. (1972) and Fairbridge (1972) since both the inter- glacial and glacial parts of the cycle need to be included; the use of cycle for a half-cycle, by Wright (1972: 275) and MGrner (1972: 345) is wrong.

    tion will have varied from place to place according to location relative to areas of ice accumulation and ice limits; its time boundaries are accordingly time-transgres- sive. The term Interglacial has, however, been extended widely to nonglaciated regions, to refer to a warm episode between two cold ones that correspond to those with truly glacial effects at higher latitudes or altitudes. To be of value for interregional correlation, it is necessary to define units with boundaries that are not time-trans- gressive, and the regions beyond the ice limits, including the oceans, can provide critical data. The time between the begin- ning and end of such units constitutes an Interglacial Age (a chronologic unit) and the rocks formed during that period of time form an Interglacial Stage (a chrcno- stratigraphic unit).

    The distinction between the time-trans- gressive climatic episode and the chrono- logic and chronostratigraphic units that are not time-transgressive needs to be main- tained (cf. Suggate, 1965). For a term such as Last Interglacial to have any inter- regional or global usefulness, it must be ac- cepted as an abbreviation of Last Intergla- cial Age or Last Interglacial Stage. As it clearly does not refer to rock, it is not a stratigraphic unit and must be considered to be equivalent to Last Interglacial Age.4

    Although in the present discussion the term Last Interglacial is accepted, in order not to direct attention away from the prob- lem of its duration, the unsatisfactory use of Last must be noted. An agreed geo- graphic name should be substituted, which

    ,

    If it is contended that a time unit is dependent on the corresponding time-stratigraphic unit, and accordingly on a section of strata, it suffices to note that the needlessness of such a procedure has already been demonstrated by the Holocene Commission of INQUA. This commission defined the base of the Holocene at 10,000 radiocarbon years, at a temperature level about halfway be- tween the cold of the last glacial and the warmth of the postglacial. It is now seeking a section that would suitably satisfy the supposed require- ment of stratigraphic nomenclature.

  • WHEN DID THE LAST INTERGLACIAL END? 249

    will allow the distinction to be made be- tween the climatic episode, which would use the geographic name unmodified, and the age (and stage), which would add the -an or -ian ending to the geographic name (cf. Eem Interglacial, Eemian Stage).

    It follows from differences in intensity of cold episodes and from the time-transgres- sive nature of such episodes that some may produce glaciation over substantial areas and for substantial lengths of time in high latitudes but none at those lower latitudes that the ice reached during full glacial maxima. Such episodes result from minor cIimatic fluctuations that can be within either a Glacial or an Interglacial [Age]. Their assignment to the one or to the other presents a problem that cannot be resolved without definitions that take into account minor climatic fluctuations.

    The only contributor to the symposium to consider a definition of Interglacial was Fairbridge (1972: 283, 293-296). The closest he came to a definition was (p. 283) Interglacials are defined in their classical stratotype areas of N W Europe, by sedi- mentary sequences characterized by the pollen of deciduous forests, pointing to climates as least as warm as those of the present time. He noted (p. 287) that such climatic peaks are marked by very brief crescendos (less than 10,000 yr) and, at least for midlatitudes, he assigned (Fig. 7) a length of 100,000 yr to the glacial (Weichselian) part of the last climatic cycle. He acknowledged, however, that in- terglacial conditions lasted longer in the tropics than in midlatitudes.

    Because the duration of interglacial con- ditions varied latitudinally and altitudi- nally, it is clear that the term Intergla- cial-if it is to be defined for interregional or worldwide use-must be defined arbi- trarily ; Last Interglacial is indeed used as a worldwide term. Some rules for defini- tions need to be adopted.

    If a temperature level equivalent to that of the present day is adopted as the mini-

    mum to be attained during an Interglacial, correspondingly a temperature level sub- stantially as low as that of the Last Glacial should have been reached during previous Glacials. These temperature levels are those attained during each major climatic cycle of the Quaternary, and these cycles included minor cycles during which tcmper- ature maxima and minima were at int,erme- diate levels.

    Fluctuations of temperature are to he ex- pected during an interglacial age, and cool half cycles following the initial full inter- glacial warmth should be considered to have taken place during the continuing In- terglacial Age provided :

    (a) the temperature did not fall to sub- stantially that of the Last Glacial, and

    (b) the temperature during the following warm half-cycle was greater than half-way between that of the Last Glacial and that of the present day (i.e., greater than the mean tem- perature of the climatic cycle).

    The necessity of assessing the degrees of cold and of warmth prevenk glaciated areas (except at the limits of ice advances) from providing adequate data, as the de- gree of cold will not be assessable. Beyond the glaciated areas, deposits may record the significant degrees of both warmth and cold so that (within the limitations imposed by the time lags of the response of animals and plants and of physical processes, and by the lack of precise synchronism of temperature fluctuations in different areas) regional or worldwide units can be usefully defined. The evidence now available justifies an at- tempt to define the Last Interglacial 1 Age] as a worldwide unit.

    CLIMATIC FLUCTUATIONS BETWEEN 130,000 AND 70,000

    YR BP

    In determining the ends of t,he Last (In- terglacial, the need is to decide which were

  • 250 R. P. SUGGATE

    the major and which the minor half-cycles, as inferred from the best data available, From the papers presented in Q?&ernary Research 2:

    (1) Emiliani (p. 271)) on his tempera- ture curve for Caribbean and Atlantic deep-sea cores, showed three temperature peaks in his core stage 5; the first is as warm as the present day, and the two later ones are almost as warm. The intervening cool oscillations do not approach the cold of the Last Glacial ; indeed they are shown as warmer than the mean temperature of the climatic cycle. If part of core stage 5 represents the Last Interglacial Stage then the whole must do so. Emiliani dated the end of this stage at about 70,000 yr BP.

    (2) Richmond (Fig. 4, p. 321) inter- preted the Rocky Mountains sequence as indicating three peaks of warmth between 130,000 and 80,000 BP, with intervening glacial advances as great as that of the Last Glaciation. The first warm peak was certainly warmer than the present day, and according to Richmond the later two prob- ably were, judged by soils. The three were interpreted as representing interglacials, with two intervening glacials.

    (3) McIntyre and Ruddiman (p. 350) stated The last interglacial between 127,000 and 73,000 yr BP. . . . was not uniform oceanographically. Three times warm Subtropical and Transitional surface water masses advanced northward, then retreated . . . Between these three warm intervals, . . . Polar water extended south of IceIand but not beyond 55*N. The end of this interglacial is seen in the abrupt sweep southward to approximately 45ON lat. of Polar surface waters. This is a clear statement which may be taken as indicat- ing that the cool oscillations within the in- terglacial (as McIntyre and Ruddiman in- terpreted it) were not severe enough to justify an interpretation that places the end of the Last Interglacial prior to about 73,000 yr BP.

    (4) Hayes and Perruza (p. 358) pre- sented carbonate percentages in two eastern

    equatorial Atlantic deep-sea cores, indica- tive of four peaks in what they correlate with Ericsons Zone X. Between these peaks the lowered carbonate percentages are still far higher than the low percentages of zone Y, which is the Last Glacial. There is no reason to exclude the younger peaks from the Last Interglacial.

    (5) Sancetta et al. as a result of the study of a North Atlantic deep-sea core concluded (p. 366) : As elsewhere, the last dominantly warm interval of the Pleisto- cene began abruptly 127,000 YBP. . . . Short intervals of ice-rafted mineral detri- tus occurred at 110,000 YBP . . . and 92,000 YBP. . . . The cooling associated with the 110,000 YBP event represents, in the core studied, two-thirds of a swing to- wards full glacial conditions and was con- siderably more intense than that . . . at about 92,000 YBP. It is clear that the two toolings do not represent full glacial condi- tions and must constitute cool oscillations within the interglacial.

    (6) Matthews (pp. 368-373) inferred a rapid lowering of sea level after the 125,000 yr BP high stand in Barbados (Barbados- III) ; this lowering reached -71 + 11 m. This low sea level is far from being as low as the lowest Last Glacial sea level, and was followed by other high sea levels. It is not adequate to justify a full glacial, so that it does not follow the Last Interglacial but lies within it.

    (7) Kukla and KoFi (p. 374) interpreted sedimentation (mainly eolian) and soil de- veIopment in central Europe as indicating a forested last interglacial ending about 115,000 yr BP, followed by loess deposition and chernozemic soils indicative of a harsh continental climate, followed by an interstadial soil complex prior to the main period of loess deposition. The se- quence indicates a single maximum of warmth comparable with, or warmer than the present day, with a later less warm period. It is not necessary, however, to ac- cept that loess deposition in midcontinent requires worldwide full glacial climate.

  • (8) Kennett and Huddlestun (p. 386) interpreted western Gulf of Mexico cores as indicating two warm peaks between 120,000 and 100,000 yr BP, followed by an abrupt cooling at 90,000 yr BP and a subse- quent minor warm peak prior to the full cold of the Last Glacial. The 90,000 yr BP cooling was not interpreted as being as cold as the Last Glacial. Without a scale to indi- cate the mean temperature of the climatic cycle, the following warm period may or may not have been warm enough to justify retaining it in the Last Interglacial, as the two earlier peaks of warmth must be.

    (9) Dansgaard et al. (p. 397) repro- duced the 180/160 ratio data for the Camp Century ice core, Greenland, for which they inferred an exceptionally severe but excep- tionally short-lived cooling at 89,000 yr BP and a possible earlier one (partly missing at a gap in the core) at 109,000 yr BP. Both these were interpreted as being prior to the Wisconsin Glaciation, the beginning of which they placed at 73,000 yr BP. The second cooling was less severe than those of the Last Glaciation, and the intensity of the first one is not known.

    tams and central Europe-is t.here the evi- dence suggesting the possibility of local full glacial conditions during cool phases in this period of time. Midcontinent areas? however, are not necessarily the most use- ful to provide standard sequences for global use.

    THE END OF THE LAST INTERGLACIAL

    The evidence of deep-sea cores, of the Barbados sea level fluctuations and of the Camp Century ice core clearly indicates that the most probable sequence between about 130,000 yr BP and 70,000 yr BP was of three warm episodes separated by two cool episodes neither of which was as cold as the main cold episode of the Last Glacia- tion. Since the (present interglacial sym- posium papers were published, Shackleton and Opdyke (1973) have presented data from an equatorial Pacific core that does not indicate any cooling of glacial status between about 128,000 and 75,000 yr BP. Nevertheless, short severe toolings seem probable within this interval. From papers presented at the symposium it appears that only in midcontinentin the Rockv Moun-

    For global correlation it is best-and necessary if the requirements for an Inter- glacial discussed above are accepted-to define the Last Interglacial to include the three warm peaks, so that it ended about 73,000 yr BP. Whether the Eemian deposits of northwest Europe are those formed tlur- ing only the first warm period (cf. Shackle- ton, 1969), and if so whether the early in- terstadials in the Last (Weichsel) Glacial should be considered to correspond to the later warm periods of the Last Interglacial, is at present inconclusive. The precise defi- nition of the boundaries of the Intergla- cial-at a midpoint of temperature in the cooling or warming from one major half- cycle to the next (cf. t,he definition of the base of the Holocene by the INQUA Com- mission on the Holocene) or at the begin- ing of the rapid cooling or warming that led directly to full glacial cold or full inter- glacial warmth (cf. Suggate, 1962)-is less important at present than thch problrm of distinguishing between major and minor cycles of climate fluctuation.

    The symposium to which the papers in Quaternary Research 2, were contributed was entitled The present interglacial, how and when will it end? If by interglacial an analogy was intended with the Last In- terglacial, as most contributors accepted, by a cooling that leads to the cold of the next Glacial. The answer to the when de- pends on the definition of an Interglacial, the answer to t,he how is that it will end. If the question had been how soon and how quickly will the world become severely colder, then the problem of definition would not have been so apparent. But it is perhaps for the best that the confusion should have been made so manifest. in a single group of papers.

    WHEN DID THE LAST INTERGLACIAL END? ,51

  • 252 R. P. SUGGATE

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