THERMOKARST LAKES IN SUBARCTIC QUEBEC:

A PRELIMINARY VIEW ON PAST AND RECENT DYNAMICS

Frédéric Bouchard (INRS-ETE, CEN-U.Laval) Pierre Francus (INRS-ETE, CEN-U.Laval)

Reinhard Pienitz (CEN-U.Laval) Maggy P. Rousseau (CEN-U.Laval)

Introduction Thermokarst «lakes» (ponds):

-Related to permafrost degradation («collapse scars»)

-Recently been observed in many subarctic sites

-Linked to climate and green-house gases (CO2, CH4), but…

-Evolution cycle?

-Past & recent dynamics?

Payette & Rochefort, 2001

Frozen peat

Unfrozen peat Mineral soil

Disturbed soil

Introduction

Study site:

-15 km NE of Kuujjuarapik-Whapmagoostui (55°20N; 77°30W)

-Southern limit of discontinuous permafrost zone

-Many ponds (different ages?)

1959

2007 100 km

KUUJJUARAPIK- WHAPMAGOOSTUI

N

Tree Line

Methods Once the sediments are sampled…

Two different approaches:

1. Sedimentary processes

-ITRAX Core Scanner -Radiography (density) -XRF (geochemistry)

2. Paleolimnological reconstructions

-Diatoms -L.O.I.

Sampling

ITRAX

Diatoms

Today’s presentation:

3 ponds for sedimentary processes

KWK-6, KWK-12 and KWK-16

1 pond for paleolimnological reconstructions

KWK-6

1959

KWK-12

KWK-16

N

KWK-12

KWK-16 KWK-6

KWK-6

2007 KWK-6 KWK-12 KWK-16

Color Green-blue Black Beige

Depth (m) 3.5 3 2.5

DOC (mg/L) 3.8 6.9 8.7

TSS (mg/L) 3.7 2.5 24.3

Secchi (cm) 86 121 22

Conductivity (mS/cm) 0.614 0.386 0.605

O2 (mg/L) 9.24 9.07 9.25

pH 6.26 6.28 7.24

Salinity 0.33 0.21 0.30

Methods

Preliminary results

Stratigraphy:

A) Laminated light silt-clay (gyttja?)

B) Massive sticky clay (marine? lacustrine?)

Dep

th (m

m)

Dep

th (m

m)

Dep

th (m

m)

Rad

iogr

aphy

R

adio

grap

hy

Rad

iogr

aphy

KW

K-1

6 K

WK

-12

KW

K-6

195

± 15

YB

P

KWK-6

KWK-12

A

A

B

B

Preliminary results

Lost On Ignition (KWK-6)

Water content & organic matter: seems to correlate with ITRAX data

100°C

550°C

ITRAX

Org H2O ρ

A

B

Preliminary results

Diatoms (KWK-6)

-Diatoms appearance (4.75 cm) before sedimentary change (3 cm)

-Productivity MAX (peak) at 2.25 cm

Fragilaria pinnata

Fragilaria capucina

Navicula expecta

Navicula cryptocephala

Achnanthes curtissima

Achnanthes pusilla

Discussion 1. Diatom appearance: 4.75 cm ≠ sedimentary change (3 cm)

Environmental «delay»? Contamination?

2. Productivity MAX: 2.25 cm (195 ± 15 BP) Circumneutral species (N. cryptocephala, N. expecta, A. pusilla, A. curtissima)

3. Flora evolution with pH / alcalinity Alcaliphilous VS circumneutral species

4. No marine species (fresh water only) Pro-glacial lake (Ojibway)? Where are those Tyrell Sea sediments?

ONLY ONE POND… -ANALYSE OTHER PONDS WITH OTHER PROPERTIES -ANALYSE D.O.C. & WATER COLOR → MODELS

Future work -Complete diatoms analysis

(models) -Thin sections of sediment archives

(OM, SEM) -Study ponds morphology by GPR

(winter) -Sediment traps (accumulation rate)

GPR Sediment traps

Thin sections

Diatoms (again)

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