Evi­dence of middle Pleis­to­cene gla­cia­tions in the Swiss fo­re­land is ei­ther la­cking (ve­ry few to no fron­tal or la­te­ral mo­raines) or in­dis­tinct en­ough (scat­te­red quart­zite clasts) to be dif­fi­cult to map and, the­re­fore, to date. The re­cord of such gla­cia­tions is thus main­ly found in se­di­ments of over­dee­pe­ned val­leys and can on­ly be ac­ces­sed through com­plex co­ring cam­pai­gns. Some of these se­di­ments could be da­ted with lu­mi­nes­cence tech­niques back to c. 200 ka but the age of the dee­pest gla­cial se­di­ments and the ti­ming of over­dee­pe­ning as­so­cia­ted with the most ex­ten­sive gla­cia­tions re­mains, ho­we­ver, com­ple­te­ly unk­nown.

In this pro­ject (2024-2028) we use gla­cial se­di­ments trap­ped in caves in the Ju­ra Moun­tains to add spa­tio­tem­po­ral constraints ma­jor Al­pine gla­cia­tions. Clas­tic se­di­ments com­pri­sing of co­arse sands, pebbles and cobbles from the Pen­nine Alps (quart­zites, ser­pen­tines, dio­rites, etc.) en­te­red in­to caves in as­so­cia­tion with gla­cial ru­noff and, thus, re­cord when a gla­cier was close en­ough to pour out­wash in­to the cave sys­tems. Here, we pro­pose a mul­ti-me­thod ap­proach cen­te­red on field in­ves­ti­ga­tions, se­di­ment trans­port ex­pe­ri­ments and bu­rial da­ting with cos­mo­ge­nic nu­clides to constrain how and when these clasts were de­li­ve­red.

De­tai­led geo­mor­pho­lo­gi­cal sur­veying will in­form about the spe­leo­ge­ne­tic evo­lu­tion and se­di­men­ta­tion context to un­ders­tand the trans­port his­to­ry of in­di­vi­dual clasts in­side mul­ti-le­vel cave sys­tems. Our in­ter­pre­ta­tion will be sup­por­ted by in-si­tu mo­ni­to­ring ex­pe­ri­ments to de­ter­mine the cri­ti­cal shear stress nee­ded for clast re­mo­bi­li­za­tion in a com­plex cave set­ting. Wir wer­den 10Be/26Al Bu­rial-Da­tie­run­gen ver­wen­den, um fest­zus­tel­len, wann die Se­di­mente in der Höhle ab­ge­la­gert wur­den. This tech­nique is main­ly re­now­ned through an­thro­po­lo­gi­cal stu­dies and is com­mon­ly used to de­ter­mine flu­vial in­ci­sion rates . Our pro­po­sal is, ho­we­ver, first to ap­ply it for the re­cons­truc­tion of gla­cier ex­tents, which is on­ly pos­sible be­cause of the unique li­tho­lo­gy contrast bet­ween the source of the clasts and their de­po­si­tion site. Bu­rial ages will be com­ple­men­ted by OSL da­ting of youn­ger se­di­ments as­so­cia­ted with gla­cial back­floo­ding and U/Th da­ting will pro­vide mi­ni­mum ages from spe­leo­thems en­com­pas­sing the gla­cial de­po­sits.

The ab­so­lute ages pro­du­ced du­ring this pro­ject will pro­vide fun­da­men­tal and at present com­ple­te­ly la­cking in­for­ma­tion on when gla­ciers from the Alps were large en­ough to in­vade and even co­ver the Ju­ra Moun­tains. Our pre­li­mi­na­ry da­ta sug­gests that this may have been not on­ly du­ring the middle Pleis­to­cene but al­so du­ring the ear­ly Pleis­to­cene for which there are no map­ped out­crops in the Ju­ra. Is this be­cause gla­ciers did not reach that far or be­cause the se­di­ments were sub­se­quent­ly ero­ded du­ring youn­ger gla­cia­tions? Wha­te­ver, our pro­ject will give ac­cess to an un­tap­ped re­cord uni­que­ly pre­ser­ved in the cave se­di­ments and, by com­pa­ring with the known ti­ming of De­cken­schot­ter, contri­bute si­gni­fi­cant­ly to de­ter­mi­ning when the first ma­jor Al­pine gla­cia­tions oc­cur­red. This da­ta can be well in­te­gra­ted in­to on­going re­search ac­ti­vi­ties ai­ming at da­ting se­di­ment cores from over­dee­pe­ned val­leys.

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