Imp.Ravazzi AANN OOVVEERRVVIIEEWW OOFF TTHHEE QQUUAATTEERRNNAARRYY CCOONNTTIINNEENNTTAALL SSTTRRAATTIIGGRRAAPPHHIICC UUNNIITTSS BBAASSEEDD OONN BBIIOOLLOOGGIICCAALL AANNDD CCLLIIMMAATTIICC EEVVEENNTTSS IINN IITTAALLYY CCeessaarree RRaavvaazzzzii C.N.R. - Istituto per la Dinamica dei Processi Ambientali, Milano (e-mail: cesare.ravazzi@idpa.cnr.it) ABSTRACT A short review about Quaternary continental stratigraphic units based on biological and climatic events is provided, with several exam- ples taken from Italy. The potential of biochronological and biostratigraphical units is discussed. The suitability of climate events and of composite regional units in continental stratigraphy is also stressed, although a consistent definition and formal status of the latter units are not yet available. RIASSUNTO Le unità stratigrafiche basate su eventi biologici e climatici sono di largo uso nel Quaternario continentale, ma il loro status non è ben definito da accordi internazionali. Nel presente lavoro si evidenzia il significato e l’utilità delle unità biocronologiche, biostratigrafiche, cli- matostratigrafiche e cronostratigrafiche e si auspica che venga mantenuta una rigorosa distinzione nel loro impiego, prendendo spunto da vari esempi in Italia. Viene posto l’accento sull’indipendenza delle unità biocronologiche continentali dalle biozone stratigrafiche. Queste ultime sono applicabili alla distribuzione stratigrafica di microfossili (polline, diatomee) e di piccoli invertebrati, ma non è il caso dei grossi mammiferi. Le unità climatostratigrafiche riguardano sia eventi a scala globale, sia regionale o locale; in ogni caso sono dia- croniche e non danno luogo a correlazioni di significato strettamente cronostratigrafico. La mancanza di una chiara distinzione tra unità biostratigrafiche e climatostratigrafiche ha creato confusione nella terminologia riguardante la parte terminale del Pleistocene superiore e dell’Olocene. Il significato delle unità in uso in questo intervallo viene discusso in un’apposita tabella. Viene infine considerata la natu- ra composita di alcune unità in uso nel Quaternario continentale a scala regionale in Europa occidentale, le quali risultano dalla combi- nazione di tutti i dati disponibili di interesse stratigrafico e cronologico. L’impiego con un significato cronostratigrafico di tali unità non dovrebbe essere confuso con la loro natura e definizione. Key words: Quaternary biostratigraphy, Biochronology, Pollen zonation, Climatostratigraphy. Parole chiave: Biostratigrafia, Quaternario, Biocronologia, Zonazione pollinica, Climatostratigrafia. Il Quaternario Italian Journal of Quaternary Sciences 1166(1Bis), 2003, 11-18 11.. IINNTTRROODDUUCCTTIIOONN Reconstructing the Quaternary stratigraphy in con- tinental environments is a difficult task because of the low degree of succession continuity and of the scattered paleontological documentation. Although biological and climatic events help understanding sedimentary sequen- ces and mapped geological units, the usage of biochro- nology, biostratigraphy, climatostratigraphy and compo- site regional stratigraphy is still limited. This paper provi- des a brief review to the potential of these units, with several examples from Italy. A Late Pleistocene and Holocene chrono- and climatostratigraphic subdivision of continental Italy and part of Europe is also presented. 22.. BBIIOOCCHHRROONNOOLLOOGGYY,, BBIIOOCCHHRROONNOOSSTTRRAATTII-- GGRRAAPPHHYY AANNDD BBIIOOSSTTRRAATTIIGGRRAAPPHHYY Fossiliferous continental deposits formed in a regi- me of virtually continuous sedimentation can be found in deep lacustrine environments, but they record relatively short time periods (103 – 104 years) compared to marine successions. This obstacle may be partially overcome by applying biochronological criteria. Biochronology is the subdivision of geological time by means of biological events, i.e. the evolution of the organisms with respect to time and other paleobiological events, which do not imply consideration of the stratigraphic relationships among the rocks in which the fossils are included (Berggren & Van Couvering, 1978; Raffi & Serpagli, 1993, Tab. 1). Biochrons can be defined thanks to the theoretical base provided by irreversible organic evolu- tion, and by other paleobiological events (e.g. immigra- tion, dispersal, emigration, and extinction). However, often the continental faunal units used in Italy miss a reference to any body of rocks, or, in other terms, body of rocks including faunal assemblages have been not preserved after the fossil collection. In these conditions, it may be impossible to describe biostratigraphic units, and only biochronologic or biochronostratigraphic unit can be conceived1 . The concepts of mammal continen- tal biochronology were developed by Lindsay and Tedford (1990), Fejfar & Heinrich (1990) and Walsh (1998). A Quaternary biochronological scheme of the Italian mammal and mollusc faunal complexes is well established (Gliozzi et al., 1997; Kotzakis, this issue). The interpretational approach provided by biochrono- logy is also inherent the characterization of floristic com- 1 BBiioocchhrroonnoossttrraattiiggrraapphhiicc uunniittss are “the sets of rock formed during biochrons, without reference to any particular strati- graphic section” (Walsh, 1998). BBiioossttrraattiiggrraapphhiicc uunniittss are instead “bodies of rock strata that are defined or characterized on the basis of their contained fossils” (ICS, 1994, p. 53). 12 C. Ravazzi Table 1 - The stratigraphic categories currently used in Quaternary sciences. The table includes units formally defined in the International Code of Stratigraphy (Salvador, 1994) and other categories that, despite not yet formally defined by ICS, are of importance for Quaternary stratigraphy and are of common usage. The relevant references (only for units not included in ICS) are here reported: 1 - Cushing, 1967; Birks & Gordon, 1985; Tzedakis, 1994; 2 - Kerney et al., 1980; 3 - De Giuli et al., 1986; 4 - Fejfar & Heinrich, 1990; Lindsay & Tedford, 1990; Walsh, 1998; 5 - De Giuli et al., 1983; 6 - Mai & Walther, 1978; 7 - Lowe & Walker, 1997; 8 - Walker et al., 1999; 9 - Johnson et al., 1997; 10 - Zagwijn, 1992. The term “composite regional units” is used to refer “regional Stages” of the Dutch stratigraphy. This terminological problem is discussed in the text. SSuummmmaarryy ooff ccaatteeggoorriieess uusseedd iinn tthhee ccoonnttiinneennttaall QQuuaatteerrnnaarryy ssttrraattiiggrraapphhyy (non-referred units are from Salvador, 1994) (*) Mappable units; (+) Stratotype sections applicable 11.. CCHHRROONNOOLLOOGGYY 22.. RROOCCKK PPRROOPPEERRTTIIEESS 33.. FFOOSSSSIILL CCOONNTTEENNTT CChhrroonnoossttrraattiiggrraapphhyy ((**)) ((++)) LLiitthhoossttrraattiiggrraapphhyy ((**)) ((++)) BBiioossttrraattiiggrraapphhyy ((++)) Eonothem Group Pollen superzone1 Erathem Formation Pollen zone System Membre Other biozones and ecozones2 Series Stratum (based on diatoms, Stage cyanobacteria, green algae, Substage PPeeddoossttrraattiiggrraapphhyy ((**)) Ostracods, Molluscs, etc.) Chronozone MMiinneerraallooggyy ((**)) ((++)) BBiioocchhrroonnoossttrraattiiggrraapphhyy EEqquuiivvaalleenntt GGeeoocchhrroonnoollooggiicc UUnniittss Heavy mineral assemblages aanndd BBiioocchhrroonnoollooggyy Eon MMaaggnneettoossttrraattiiggrraapphhyy ((++)) Era (Cenozoic) Polarity zone Biochronological events3 Period (Quaternary) Land Mammal Ages4 Epoch (Pleistocene) IIssoottooppiicc ssttrraattiiggrraapphhyy ((++)) Faunal Units5 Age δ18O stages from sea records Local Faunas5 Subage (MIS = Marine Isotopic Stages) Floral Complexes6 Chron δ18O from polar ice (GS / GI) Local Floras6 Termination Human cultural chronology CChheemmiioossttrraattiiggrraapphhyy 44.. CCOOMMPPOOSSIITTEE AANNDD IINNTTEERRPPRREETTAATTIIOONNAALL SSTTRRAATTIIGGRRAAPPHHIICC CCAATTEEGGOORRIIEESS ((OONNLLYY 44..BB AANNDD 44..EE AARREE DDEETTAAIILLEEDD HHEERREE)) 44..AA.. CCYYCCLLOOSSTTRRAATTIIGGRRAAPPHHYY ((**)) ((++)) 44..BB.. EEVVEENNTT DDIIAACCHHRROONNIICC SSTTRRAATTIIGGRRAAPPHHYY Event (Whittaker et al., 1991) BBaasseedd oonn cclliimmaattiicc eevveennttss iinnffeerrrreedd bbyy ssttrraattiiggrraapphhiicc ppaatttteerrnn [[CClliimmaattoossttrraattiiggrraapphhyy ((++))]] aanndd ootthheerr eevveennttss:: Geologic-climatic (geoclimatic) units7 Global interglacial, Global glaciation7 Stadial, Interstadial Episodes and Sub-episodes (mostly at regional scale) GI/GS (Greenland Stadial / Interstadial Episodes and Subepisodes)8 BBaasseedd oonn bboouunnddaarriieess ooff rreeffeerreenntt ddeeppoossiittss ffrroomm wwhheerree eevveennttss aarree iinnffeerrrreedd ((**)) Rock diachronic Event Units9 Episode Subepisode Phase 44..CC.. UUNNCCOONNFFOORRMMIITTYY BBOOUUNNDDEEDD SSTTRRAATTIIGGRRAAPPHHIICC UUNNIITTSS 44..DD.. SSEEQQUUEENNCCEE SSTTRRAATTIIGGRRAAPPHHYY 44..EE.. RREEGGIIOONNAALL CCOOMMPPOOSSIITTEE SSTTRRAATTIIGGRRAAPPHHYY ((**)) ((++)) CCoommppoossiittee rreeggiioonnaall ssttaaggee1100 A combination of the above mentioned properties (e.g. Paleomagnetic Polarity, Lithostratigraphy, Susceptibility log, Radiometric dating, Pollen stages succession, Climatostratigraphic succession) considered at regional scale and boundary-defined. plexes, first defined in the lowlands of Germany (Mai & Walther, 1978) and recently applied to the continental carpofloral assemblages of the western Po Plain (Martinetto, 1995). Considering the Italian continental paleobiological record, only the distribution of some microfossil, e.g. pollen and diatoms, and of small invertebrates (Ostracods, Chironomids), matches the requirements to establish biostratigraphic units. There are only a few examples of continental biostratigraphy in Italy. Several paleoecological investigations have proposed a local biozonation, based on the succession of biotic assem- blages at a single lacustrine or palustrine small basin. So far there is detailed local biozonation back to 14 kyr BP at the southern Alpine foothill (Schneider, 1978; Wick, 1996), in Central Alps (Tinner, 1998; Pini, 2002), in the central Italian peninsula (Lowe, 1992; Brugiapaglia & de Beaulieu, 1995; Lowe et al., 1996) and in central Sicily (Sadori & Narcisi, 2001). The pollen zonation from the Monticchio maar lake (southern Apennines) and from the Latium extends to the last 100 kyr BP (Magri, 1999; Magri & Sadori, 1999; Allen & Huntley, 2000). One site is zoned back to about 250 kyr BP (Valle di Castiglione: Follieri et al., 1988). Detailed pollen zonation is also available for the Middle Pleistocene lacustrine record of Vallo di Diano (Russo Ermolli, 1994), from part of the Early Pleistocene lacu- strine sequence of Leffe in the Lombardian Pre-Alps (Ravazzi & Rossignol Strick, 1995; Ravazzi & Moscariello, 1998) and from the late Early Pleistocene Colle Curti and Cesi basin sequence (Bertini, 2000). The development of regional pollen zones applies to biogeographically homogeneous regions, which are provided with several pollen records. Although each site is characterized by a peculiar vegetation development (shown by the succession of local pollen zones span- ning an entire warm or cold phase: this is the concept of pollen assemblage superzone, proposed by Tzedakis, 1994), a comparative inspection of pollen curves shows that different sites have in common a similar back- ground pollen, after eliminating local plants from the pol- len sum. This is the case of Latium maar lakes, where the main changes observed in the five long pollen records available have been correlated (Follieri et al., 1989). A quantitative analysis by numerical methods (Sugita 1994) can help defining the biogeographical limits of a regional pollen zone. Regional pollen zones are commonly time-transgressive (Lowe & Walker, 1997); this is partially due to migration time lags (102 kilometers per 103 years). However, once the migration pattern is known and well dated, the time of immigration and / or expansion at a given site by a good pollen pro- ducer may be a precise biostratigraphic marker (e.g. the Holocene migration history of Picea, Ravazzi, 2002). 33.. EEVVEENNTT SSTTRRAATTIIGGRRAAPPHHYY AANNDD CCLLIIMMAATTOOSSTTRRAATTIIGGRRAAPPHHYY Climatostratigraphy (the modern development of geoclimatic stratigraphy) is a type of “event diachronic” stratigraphy based on interpreted climate features from evidence in the rock/sediment record (see Lowe & Walker, 1997). Events are short-term phenomena that leave some trace in the geological record (e.g. volcanic eruptions, sea level changes, etc.). As noted by Walker et al. (1999), "it is primarily the events and not the boun- daries between the events that are specifically designa- ted" (by climatostratigraphic inferences). Furthermore, climatostratigraphy "acknowledges the fact that the stra- tigraphic boundaries marking the onset and ending of a climatic event may well be diachronous (.....)". Climatostratigraphic units are best derived by a combi- 13An overview of the Quaternary ... nation of different stratigraphical categories, such as lithological, biological, isotopic (Walker et al., 1999), and UBSU units used in sequence stratigraphy (Miall, 1997). Whenever emphasis is on globally recognized cli- matic-events, climatostratigraphic subdivisions may con- cern the whole Earth, such as global glaciations, inter- glacials and the interglacial-glacial cycle. Local climato- stratigraphic units may represent events recorded by relevant deposits, for instance at the scale of a basin catchment (e.g. high-intensity rain triggering cata- strophic floods), or of a climatic province (e.g. lake level changes, etc.), or of a whole mountain chain (e.g. local glaciation). Climatic events which occurred at regional scale, together with other derivate types of event strati- graphic units, such as volcanic eruptions, are widely used in Italy (Narcisi & Vezzoli, 1999) and provide a powerful method for marine / continental correlations. For instance, the beginning of the late glacial interstadial (Tab. 2) has been recognized in maar deposits from the Italian peninsula as well as in marine sediments from the Adriatic sea. Inter-regional and marine / continental correlation has been possible thanks to the background arboreal pollen signal, which shows an abrupt shift at 12.4 ka 14C BP (Lowe et al., 1996). This pollen signal has been compared with other paleoclimatic proxies (geochemistry, rock-magnetism, isotopes, diatoms, other algae, invertebrate fossil remains), for a multidisci- plinary evaluation of the climatic event triggering the observed physical and biological changes (Guilizzoni & Oldfield, 1996). The climatic events are commonly very close to the boundaries of the biostratigraphic subdivi- sions from where events are inferred. The International Stratigraphic Guide (Salvador, 1994) misses to consider climatostratigraphy as a basic stratigraphic category of common use. The present author believes that a rigo- rous distinction between biostratigraphic and climato- stratigraphic units needs to be maintained (on this pro- blem see Turner, 2002 discussing the status of the Eemian interglacial). An effort to add precision to the terminology used in different branches of stratigraphy is made in Tab. 2 and 3. 44.. CCOOMMPPOOSSIITTEE RREEGGIIOONNAALL UUNNIITTSS Several Quaternary stratigraphers from Central Europe acknowledges the use of continental “stages” (Gibbard et al., 1991; Zagwijn, 1998). From the paleocli- matological point of view, some of them are “complex stages” (Zagwijn, 1992, p. 585). These units (Tab. 1b) derive from a combination of all available stratigraphic (lithologic, UBSU, magnetic, biologic and climatic) and chronological (geocronometric, biochronological) data. The paleoclimatic complexity of such units is not the only defining characteristics, and therefore I would sug- gest to refer them as composite regional units. Because they are based on diachronic-type stratigraphic units (such as climato- or biostratigraphic units) they are not chronostratigraphic units (not a Stage by definition). However, composite regional units are also intended to provide a chronostratigraphical reference for biogeo- graphically circumstancied regions, hence the term “stage” (which implies a chronostratigraphic unit) has been used with reference to rocks formed during the relevant time interval (Gibbard et al., 1991; Zagwijn, 14 C. Ravazzi Tab. 2 - Framework of Late Pleistocene chronologic and climatostratigraphic reference units used in Western-Central Europe (column climatostratigraphy dealing with the Alps only) and detail of local units applicable to selected regions. ALGM = Alpine Last Glacial Maximum. The age of Northern Germany and Netherlands interstadials (Oerel, Glinde, Moershoofd, Hengelo and Denekamp) is from Behre & van der Plicht (1992), calibrated with the method by Bard et al. (1998). Alpine deglaciation and the late-glacial interstadial may be partially coeval. The term Neoglaciation is from Porter & Denton (1967). 1992). The usage of a composite regional stratigraphy in continental regions is not formalized by the International Stratigraphic Code currently in use (Salvador, 1994), and its chronostratigraphic value is still a matter of debate (Gibbard & West, 2000). Composite regional units are also characterized by a 15An overview of the Quaternary ... Tab. 3 - Late glacial and Holocene chronostratigraphy / geochronology and climatostratigraphic units (from Orombelli & Ravazzi, 1996, modified and updated). Chronozones are framed by conventional 14C ages BP. The late glacial is subdivided by using climatostrati- graphic criteria. This is because the late glacial is characterized by sharp climatic changes which have been recognized either in terre- strial biological records, either in ice and in marine records, but these climatostratigraphic transitions do not fit the boundaries of the relevant chronozones proposed by Mangerud et al. (1974). Consequently, these late glacial chronozones miss any practical interest. Holocene chronozones are provisory maintained here, waiting for further international agreements. Note that the Last Glacial Maximum in the Italian Alps ends about 15 ka 14C BP, well before than in Central and Northern Europe. The earliest interval of the Alpine late gla- cial, or, in other terms, the time interval between the beginning of the Alpine deglaciation and the late glacial interstadial is poorly defi- ned in stratigraphic terms. (* 1) The subdivisions early, middle and late Holocene are informal, with boundaries at ca 7000 and 3000 BP. (* 2) The calendar 14C age BP has been calibrated with the program CALIB 4.0 elaborated by Stuiver and Reimer (1998). The calibrated ages are reported as one Sigma time intervals obtained using a standard deviation of ± 50 yr on the conventional age. (* 3) The framing of the “Holocene thermal optimum” between about 9 and 5 kyr 14C BP is based on: i) the δ18O curve from the Renland ice core (Larsen et al., 1995); ii) the record of Alpine glacier contraction during the early-middle Holocene (Hormes et al., 2001); iii) the paleoecological record of treeline oscillation in the Alps (Wick & Tinner, 1997). This concept of the Holocene thermal optimum is diffe- rent from the hypsithermal (Porter, 1981), used in a previous version of this scheme (Orombelli & Ravazzi, 1996). (* 4) The Younger Dryas is here taken as a climatostratigraphic unit. L.I.A. = Little Ice Age. Dashed lines indicate large-scale diachronic boundaries. (* 5) A substantial increase of climatic humidity in the Alps does not fit this Boreal / Atlantic chronozone boundary but occurred later, about 7.3 Ka 14C BP, i.e. about 8.2 Ka cal BP (Tinner & Lotter, 2001; Pini, 2002). stratotype. For instance, an Eemian stratotype was pro- vided by van der Heide and Zagwijn (1967); see Turner (2002), for a review. Moreover, reference sections and boundaries or boreholes may be designated, such as the Amsterdam borehole for the Eemian (van Leeuwen et al., 2000). The designation of stratotypes and / or reference sections for continental units is mentioned by the International Stratigraphic Code currently in use (ICS, 1994, p. 77). The present author believes referen- ce sections to be necessary for any further study. This procedure is expecially fruitful in case that bioprovinces are partially cohincident with sedimentary basins, a situation which may apply to the Quaternary evolution of the Apenninic intermontane basins and of the Po Plain. 16 C. Ravazzi Unfortunately, so far there are no proposals for compo- site regional units in Italy. 55.. CCOONNCCLLUUSSIIOONN A consistent definition and formal status of conti- nental stratigraphic units based on biological and clima- tic events is not yet available in the official agreements of IUGS and INQUA. This paper has emphasized the importance to maintain rigorous distinction among bio- chronologic, biostratigraphic, climatostratigraphic and chronostratigraphic units. The marked geological, environmental and clima- tic diversity affecting both the Alpine and the Mediterranean regions hampers correlation based on biological and climatic events. 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