Implementación de sistemas CVAA en centros hospitalarios

6.2.1 Connecting the oceanic record with land-based events

The marne sotope stratgraphy reveals eght major cycles of glo- bal glacaton wthn the Brunhes Chron and two more n the upper part of the precedng Matuyama Chron, reflectng an ncrease n the ntensty of glacatons from about 900 ka. The sotopc cycles of approxmately the last 700 ka comprse an average 100 ka pe- rod frequency of whch the duratons range from 88 to 118 ka. Although ampltudes dffer, the cycles generally end wth a _δ18_O

maxmum followed by a strong decrease to an sotopc mnmum ( = deglacaton/termnaton).

Pronounced _δ18_{O sotopc maxma durng the last 700,000 years}

occurred n the fnal parts of MIS 2-4, 6, 12 and 16 (Shackleton 1987) suggestng that only durng these four cold sotope stages have clmatc condtons n Europe been suffcently severe and sustaned to permt the Fennoscandan ce-sheets to expand nto the area south of the Baltc (Boulton et al. 1997). Moreover, these most ntensve _δ18_{O-peaks seem to concde wth the thckest unts}

n the loess-palaeosol record, .e. the cycles B, C, F and H n the Central European successon (Kukla 1975). On the other hand, there are the weakly expressed _δ18_{O maxma of MIS 8 and MIS 14}

whch may ndcate perods of less extensve glacaton and loess accumulaton. Marne transgressonal phases n the North Sea and Baltc Sea margns, mmedately followng major glacatons, can be fxed to the warm sotope substage peaks succeedng a glacal stage _δ18_O-maxmum2_{. An dealsed correlaton scheme based on}

these assumptons then corresponds to Figure 6.1.

However, there are ce-sheet developments other than the Fennos- candan/Brtsh ones to take nto account for the northern hem- sphere. Besdes mnor glacatons that have occurred n Iceland, Greenland and alpne regons such as the Hmalayas, the Alps and the Cordllera, the largest part of the total ce volume durng the glacaton cycles was the Laurentde ce-sheet on the North Amer- can contnent. Stratgraphcal and chronometrc data, manly K/ Chapter 6

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Fgure 6.1: Idealsed correlaton dagram of the Central European terres- tral loess records (Kukla 1975) and Northwest European clmatc ‘stages’ (ncludng the Fennoscandan/Brtsh glacatons and North Sea marne transgressons) wth the marne sotope record ODP-677 and the MIS. The loess record of Chna (Kukla 1987) and the local pollen records from Tenagh Phlppon and Lac du Bouchet (taken from Tzedaks et al. 1997) are gven for comparson.

Ar and Ar/Ar dates from tephras ntercalated n glacal sequences n the Cordlleran regon, demonstrate that the late Quaternary gla- caton events correspond rather well wth the marne sotope record. However, ths drect correlaton should be consdered ten- tatvely (Fullerton & Rchmond 1986) and s no proof for the ex- panson of the Laurentde glacatons. Independent radometrcal dates from a calcte ven at Devl’s Hole, Arzona (Wnograd et al. 1992) largely confrm these palaeoclmatc trends, although there

are dfferences n phases and ampltudes compared to the marne sotope stages whch have to be explaned n more detal. Marne sotope and regonal glacaton maxma may not therefore always be exactly synchronous. Moreover, maxmum glacaton lmts of the dfferent contnental ce-sheets do not necessarly cor- respond to each other nor do they concde wth the most extreme

δ18_{O sotope maxma. Such propertes may explan the remarkable}

dscrepances n the number and dstrbuton of glacal sedmen- tary cycles recorded n the fragmentary Mddle Plestocene re- gonal sequences of md-lattude Europe. In spte of ths ndstnct relatonshp between the ampltudes of the sotope ratos and the regonally dfferent glacaton lmts, the trends n the oxygen so- tope curves can at least be used as a bass for further confnng the ndependent terrestral chronostratgraphcal evdence from the European non-glacal terrestral record, such as regonally dated volcanc ash layers, secondary carbonates and bostratgraphcal markers.

6.2.2 Connecting the terrestial record with marine isotope events

The correspondence of the terrestral large-scale events to the ma- rne sotope stratgraphy s closely related to the followng ques- tons:

- Durng whch parts of the MIS-ntervals were duraton and n- tensty of coolng suffcent to produce ce-sheet expansons nto the southern Baltc and further, nto the southern North Sea basn and the Russan Plan, and how do these relate to the northern Alpne glacatons?

- To whch perods of perglacal loess deposton n Central Eu- rope and Asa do the glacal cycles correspond?

- To what extent does clmatc and neotectonc evdence from (unconformtes n) the fluval terrace systems relate to the gla- cal cycles?

Much of what s known about these ponts comes from the well- documented chronostratgraphy and clmatc hstory of the Late Plestocene. It s generally agreed that the Wechselan Fennos- candan, Devensan Brtsh and Würman Alpne cycle B glaca- tons correspond to MIS 2-4, concdng wth the loess accumula- ton of cycle B (Kukla 1975) n the Central European extraglacal areas. Maxmum glacaton lmts were reached durng MIS 2, a less extensve ce-sheet advance occurred durng MIS 4, but dd not spread outsde Scandnava. Radocarbon dates estmate the age of the Wechselan glacaton maxmum just south of the Bal- tc n MIS 2 at about 20 ka BP3 _{(Boulton et al. 1985). Modellng}

suggests that ce-sheet advancng over lowland Northwest Europe durng the Wechselan glacaton was restrcted to the tme perod between 25 -18 ka (Van Weert et al. 1997). The duratons of ear- ler ce-sheet cover perpheral to the Fennoscandan Sheld prob- ably never exceeded 20,000 years. Deposton of loess n pergla- cal deserts hold a wder tme-range. Indcatve for the duraton n whch deposton may take place, although not contnuous, may be the perod from MIS 4 through MIS 2 comprsng some 60 ka. Because of ther polygenetc character, fossl sols n frst nstance gve overall tme-ranges fllng the gaps n between two succes- sve MIS-fxed subaeral unts n whch they have formed. Snce preservaton potental s hghest for the lowermost B(t)-horzon, representng the frst post-sedmentary sol formaton processes, these may therefore be equated to the warm marne sotope sub- stage followng the loess accumulaton.

based Eeman Stage type forest vegetatons broadly correspond to MIS 5 substage e (cf. Sánchez-Goñ et al. 1999). Stratgraphcal relatonshps and glaco-sostatc nterference n the North Sea ba- sn confrms the datng of the transgressve marne sequences suc- ceedng a major glacaton event, a phenomenon whch s less clear n the marne terraces along the Channel or the Atlantc coast. Duratons of transgressonal phases coverng the southern North Sea may be n the order of some thousands to 10 ka. Tmng of the hghest sea-levels however s regonally determned (Mörn- er 1980, Lambeck 1993).

The correspondence of the Saalan Fennoscandan glacal cycle C to MIS 6 s generally accepted nowadays. MIS 6 also most prob- ably ncludes the youngest Alpne Rssan (III) glacaton and the Central European loess C accumulaton. Some reservatons have to be made for Eastern Europe where correlaton of glacal se- quences appears to be more complcated.

Serous chronostratgraphcal problems arse further back n the Mddle Plestocene. Ths s llustrated n the correlaton scheme of Figure 3.2 compled by Kukla (1977). In partcular the age, or bet- ter tme-range, of the Fennoscandan Mddle Plestocene glaca- ton events s not entrely resolved. Although the Elsteran glaca- ton has produced a dstnct glacal sequence, ts chronostrat- graphcal poston has remaned a matter of debate untl recently. Snce the frst volcanc products of the East Efel regon, K/Ar- and Ar/Ar-dated at about 600 ka, underle the Elsteran glacal sedments n the southern North Sea basn, ths glacaton cannot be as old as MIS 16. Because Mollusca n the subsequent marne Holstenan North Sea sequence are dated to older than 300-350 ka, .e. equvalent or older than MIS 9, the Elsteran glacaton must be assgned ether to MIS 12 or 10. Correspondence wth the latter must be taken nto account because of dfferent stratgraph- cal nterpretatons and nterregonal correlatons. Both optons wll be dscussed. The Elsteran glacaton s assumed to be tme equvalent wth the oldest Alpne Rssan (I) glacaton.

Wde-spread glacal sequences predatng the Elsteran glacaton are, outsde Scandnava, only found n eastern Europe. The south- ernmost glacaton lmt s found n the Don rver basn n the Rus- san Platform type area. Snce ts stratgraphcal poston s below the Mimomys/Arvicola boundary, whch s contemporary n the Mddle Rhne regon wth the frst East Efel volcanc actvty at about 600 ka, and ts deposts are normally magnetsed, the Don- an glacaton most lkely corresponds to MIS 16. On other grounds, ths may also be true for the northern Alpne Mndelan glaca- ton.

The lower boundary stratotype of the Mddle Plestocene at the Brunhes/Matuyama geomagnetc reversal approxmates to the MIS 20/19 transton at termnaton IX. Snce there are no clear 4th_{-order clmatc sgnals pror to the Donan glacaton, no related}

event-stratgraphcal boundary levels can be set for ths perod.

6.3 Global time-stratigraphical settings for the terres-