MODDER MET EEN KLIMAATSVERHAAL
Weer en klimaat, het zijn concepten waar we willens nillens iedere dag mee in aanraking komen. Met de regelmaat van de klok vuren media feiten en cijfers op ons af, gaande van doordeweekse weerberichten tot onheilspellende scenario’s in verband met klimaatsverandering. Niemand ontsnapt nog aan deze overweldigende informatiestroom. Wetenschappers zijn echter al langer gefascineerd door het belang en de complexiteit van het aardse klimaatssysteem. De sleutel tot een beter inzicht in zowel huidige als toekomstige omstandigheden zou wel eens letterlijk begraven kunnen liggen in het verleden, bewaard in de natuurlijke archieven van diepe meren.
Jaarlijkse laagjes of 'warven'
Natuurlijke klimaatsarchieven zijn tijd- en omgevingsgebonden accumulaties van materiaal die bovendien een schat aan informatie bevatten omtrent heersende condities ten tijde van accumulatie. De kracht en pracht van deze archieven schuilt in hun bereik dat dikwijls groter is dan dit van instrumentele data. Iedereen is wel vertrouwd met de afwisseling van donkere en lichte ringen in een doorgezaagde of afgebroken boomstam. Gunstige weersomstandigheden tijdens lente en zomer hebben een positieve invloed op fotosynthese, waardoor bomen veel nieuw weefsel en dus brede groeiringen kunnen aanmaken. Ononderbroken opeenvolgingen van deze jaarlijkse ringen vormen natuurlijke archieven waarin klimaatsinformatie wordt bewaard onder de vorm van variaties in ringdikte.
Een alternatief voor deze boomringen treffen we aan op de bodems van diepe, proglaciale meren. Proglaciale meren worden, per definitie, voornamelijk gevoed door smeltwater van gletsjers of terugtrekkende ijskappen. In en rondom zulke meren herhaalt zich ieder jaar een min of meer identieke cyclus. Tijdens de wintermaanden vormt zich een dikke laag ijs aan het wateroppervlak, bevriezen bodems in het omliggende gebied en wordt alles bedekt onder een verhullende sneeuwmantel. Het meerbekken is gedurende deze periode volledig geïsoleerd van wat zich afspeelt in de buitenwereld. Doorheen de bewegingloze waterkolom bezinken fijne kleideeltjes uit suspensie, waarna ze op de meerbodem belanden en daar een dun laagje vormen. Bij het aanbreken van de lente verdwijnt het meerijs, waardoor de waterkolom opnieuw in verbinding komt te staan met zijn drainagegebied. Bovendien bereiken rivieren een hoger debiet ten gevolge van het vele regenwater en smeltwater van sneeuw en ijs dat in hun loop terecht komt. Deze rivieren zijn in staat om naast water, grote hoeveelheden zand, silt en klei (sediment) te transporteren. In het meer aangekomen, spreidt dit mengsel van water en sediment zich uit als een turbulente wolk, die vervolgens op de bodem neerdaalt in de vorm van een grofkorrelige zomerlaag. Een set bestaande uit een zomer- en een winterlaag vertegenwoordigt dus één jaar aan sedimentatie in een proglaciaal meer en wordt ook wel eens een ‘warve’ genoemd, het equivalent van één groeiring (Figuur 1).
Zuid-Alaska
Deze studie focust op gewarvde sedimenten van drie proglaciale meren in Zuid-Alaska: Eklutna, Kenai en Skilak (Figuur 2). De prominente aanwezigheid van grote gletsjers in de drainagegebieden van Eklutna en Skilak uit zich in een dominante aanvoer van glaciaal smeltwater naar deze meren. Kenai daarentegen, vangt eveneens substantiële hoeveelheden water op uit gletsjervrije valleien. Zowel Eklutna als Kenai liggen in het Zuid-Alaskaanse hooggebergte, terwijl Skilak zich bevindt op de overgang tussen gebergte en laagland. Een goed inzicht in dergelijke landschapselementen is cruciaal bij het interpreteren van meersedimenten. Afhankelijk van vele factoren zal ieder meer immers op een eigen, unieke manier weer en klimaat archiveren en weerspiegelen in de eigenschappen van zijn warven.
Meerbodems werden bemonsterd in de zomer van 2012 door middel van korte boorkernen. Bij zulke procedure worden vanuit een kleine onderzoeksboot lege buizen uit kunststof langs een kabel omlaag gelaten tot aan de bodem van het meer in kwestie, waarna een hamermechanisme deze stapsgewijs dieper in het sediment duwt. Volle buizen komen via dezelfde kabel terug naar boven. Daar worden ze aan beide uiteinden afgesloten en gelabeld. Deze boorkernen kan men nadien openen om hun inhoud te bestuderen.
Signatuur van weer en klimaat
Zoals de theorie ons voorschrijft, bestaat het opgeboorde sediment van de drie meren grotendeels uit warven. De interne klok die bepaald wordt door het jaarlijkse tijdskader van deze laagjes maakt het mogelijk een relatie vast te leggen tussen afzettingsdiepte (diepte in de boorkern) en -ouderdom. Aan iedere warve kan een kalenderjaar toegekend worden, vertrekkende van 2012 bovenaan de boorkernen. Deze methode levert een continue, hoge-resolutie datering op van de verzamelde archieven.
Verschillende eigenschappen, zoals dikte, structuur, korrelgrootte en chemische samenstelling, werden voor iedere warve geanalyseerd en getest op hun lineaire correlatie met weer- en klimaatsomstandigheden (instrumentele data van weerstations) gedurende het door het sediment gerepresenteerde jaar. Zo kunnen ook kenmerken van warven, gevormd vóór het instrumentele tijdperk, vertaald worden in klimatologische termen. Mogelijke relaties werden onderzocht op zowel een jaarlijkse tijdsschaal als op de schaal van meerdere decennia. Hieruit blijkt dat voornamelijk warvendikte interessante resultaten oplevert. Gekende periodes van langdurige koude, o.a. 1945-1976, leidden in Eklutna en Skilak tot de ontwikkeling van dikkere warven (Figuur 3). De oorzaak hiervoor ligt bij een significante groei van grote gletsjers naar lager gelegen terrein. Ten gevolge van de schurende werking van uitbreidend gletsjerijs op het onderliggende gesteente, komen grote hoeveelheden aan geërodeerd materiaal ter beschikking voor verdere afvoer naar de meerbekkens. In Eklutna en Kenai blijkt ook neerslag een niet onbelangrijke invloed te hebben op warvendikte. Hevige regenval is in staat om kalme waterlopen om te toveren tot kolkende rivieren, vertroebeld door hun zware en grove sedimentlading, dewelke uiteindelijk in de meren geloosd wordt om daar een karakteristiek ‘stortvloed’-laagje te vormen. Voorgaande bevindingen leggen echter slechts een fractie bloot van het klimaatsverhaal dat neergeschreven staat in het sedimentaire logboek van deze drie Zuid-Alaskaanse meren…
Toekomstplannen
Vorig jaar werden reeds langere boorkernen verzameld in Eklutna en een naderende winter-expeditie zou eveneens nieuwe monsters uit Skilak en Kenai moeten opleveren. Dit aanvullende studiemateriaal kan bestaande resultaten bevestigen en ons verder helpen bij het achterhalen welke de invloed is van koudere, warmere, nattere en drogere periodes op de omgeving van het subarctische studiegebied. Bovendien laten langere kernen toe om klimaatsvariaties in een verder verleden te identificeren. We spreken hier dan over enkele duizenden jaren, in plaats van de laatste 700 jaar die vertegenwoordigd zijn in de korte kernen.
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