The Influence of Debris Flow Activity on the Sediment of Lake Plansee over 3.6 ka (Tyrol, Austria)

Carolin Kiefer Carolin.Kiefer@tum.de

Prof. Dr. Michael Krautblatter (TUM)PD Dr. Christoph Mayr (FAU)M.Sc Patrick Oswald (UIBK)Prof. Dr. Michael Strasser (UIBK)

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Can we see evidence for a change of debris flow volumes in the sedimentary archive analogous to the terrestrial record?

50 km

Enhanced debris flow activity

around lake Plansee

since the 1980s

DIETRICH, A.; KRAUTBLATTER, M. (2017): Evidence for enhanced debris-flow activity in theNorthern Calcareous Alps since the 1980s (Plansee, Austria). – Geomorphology 287: 144-158.https://doi.org/10.1016/j.geomorph.2016.01.013

2-3 times higherthan meanHolocene rates

Terrestrial Record: Aerial Photos, Mapping

Modified after www.tirol.gv.at

S T U D Y S I T E

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S A M P L I N G L O C A T I O N S

Modified after www.tirol.gv.at

Lake area: 2.87 km2

Depth: 77 m

Transect of sediment cores taken in elongation of the Laser-scanned alluvial fan

1 0 . 5 . 2 0 1 9 1 7 . 7 . 2 0 1 9

R E S U LT S L A S E R S C A N N I N G

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DEPOSITION VOLUMEin 2 month period

EROSION VOLUMEin 2 month period

487 m3 of debris were transported in theobserved time period, 139 m3 of whichwere deposited in the lake

SEDIMENT DELIVERY RATIO: 28.5 %

M E T H O D O L O G Y S E D I M E N T A N A LY S I S

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δ13Corg

δ15Ntotal

Piston Corer withHammer ActionØ 63 mm

GEOTEK MSCLDensity & Magnetic Susceptibility

MALVERN Mastersizer 3000Grain Size

Thermo Delta V Plus

Sediment Core

R E S U LT S S E D I M E N T A T I O N R A T E S

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PLAN18-10

Dating method: 137Cs, 14CSedimentation rates indicatedebris flow activity

Strong Increase (2k-3k BP vs post 1930)

R E S U LT S T Y P E S O F D E P O S I T S

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Background Sedimentation

Debris Flow SubaquaticSuspension Flow

Megaevent

Normal grading upwards

White clay layer on top

Homogeneousgrain size

Coarsegrain size

Dark and light laminae

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C O R E - T O - C O R E

C O R R E L A T I O N

PLAN18-10

Transect of sediment coresfrom deep basinto shallow debris fan

52 Event layers were identified:Debris flow(34-55% of total section thickness)SeicheEarthquake induced suspension flow

Megaevent

EarthquakeNamlos 1930

Megaevent

DEEP SHALLOW

R E S U LT S C A R B O N I S O T O P E S S E D I M E N T

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TOC/TN ratios of sediment reveal permanent influenceof terrestrial carbon on the lake sediment

mixed source of aquatic and terrestrial organic matter

R E S U LT S C A R B O N I S O T O P E S S E D I M E N T

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Large debris flow events can be distinguished from background sediments by slightly increased δ13C isotope ratios.

P R E L I M I N A R Y C O N C L U S I O N O N O N G O I N G S T U D I E S

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└ ~ 10-fold increase of sedimentation rates in Lake Plansee after 1930└ potentially linked to increase in heavy precipitation events increased debris flow activity

└ potentially linked to lowered water level (5 m in winter months) from hydroelectric power plant

└ 52 event layers identified: cover 34 % to 55% of total section thickness

└ Three types of event deposits└ Flood-triggered debris flows

└ Earthquake-induced subaquatic suspension flows

└ Megaevents

└ TOC/TN ratios: permanent influence of terrestrial carbon on the lake sediment and mixed source of allochthonous and autochthonous organic matter

└ Large debris flow events can be distinguished from background sediments by increased δ13C isotope ratios

The Influence of Debris Flow Activity on Lake Plansee

R E C O N S T R U C T I O N O F H O L O C E N E D E B R I S F L O W A C T I V I T Y

B A S E D O N C L I M A T E - D R I V E N P R E C I P I T A T I O N D Y N A M I C S

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Figure Sketch modified after VANDAELE et al. (2015)https://doi.org/10.1111/sed.12193