UNIVERSITY OF MINNESOTA
Late-Holocene Moisture and Fire History in the Park Range, Northern Colorado
Bryan Shuman, Anna Henderson, Michael F. Mechenich, and Vania Stefanova
Department of Geography, University of Minnesota, Minneapolis MN 55455
Limnological Research Center, University of Minnesota, Minneapolis MN 55455
Dated sediment cores record fluctuations in the elevation of wave
base in Hidden Lake, CO. Organicrich sediments are limited to elevations >2 m lower than today before
4300 and from 3700-3000 yr BP.
Near-shore cores contain minerogenic sediment and sediment hiatuses during these periods, which
we infer to represent dry episodes.
Few records of millennial-scale climate and ecological variation exist from the Rocky Mountains. Here, we show that
the avalability of water, perhaps the most important resource in the West, increased in the past 4300 years but that
the increase was interupted by a millennium-length drought
beginning about 3700 yrs BP. Fire regimes also differed
from today. Few large fires like the type widely experienced
in the past decade have occurred for about 1000 years.
We collected sediment cores and ground-penetrating radar
profiles from Hidden Lake, a moraine-dammed lake located at the eastern foot of the Park Range in northcentral
Colorado.
400
Wa
ase
b
e
v
500
Core C
H
600
Core A
Zone of Organic
Sediment Accumulation
700
30% LOI
5% LOI
H Hiatus
800
5000
4000
3000
2000
1000
Cal yr BP
Lake-Level Data
Ground-penetrating radar (GPR) profiles show the location of burried paleoshorelines. Bright white reflectors (insets) show a dense layer and unconformity near the western shore (left). Cores were collected along profile B.
Hidden
HiddenLake,
Lake,near
nearWaldon,
Walden,Colorado
Colorado
A. (File 020)
0
1072
1
0
Depths based on 0.33 m/ns in water.
Paleoshoreline
40
80
120
8.5 m
0m
~200 m
B. (022)
Water
20
5.25 m
B. 022
0.6 m
0.3 m
C. 024
0.4 m
0.85 m
0 10 20 0 20 40 60 0
C. (024)
0
250
500
1,000 Meters
The lake is a closed basin fed primarily by groundwater,
and has a small watershed (blue shade on map). These
characteristics are ideal for capturing past hydrologic fluctuations and for pollen and charcoal analysis.
1.85 m
Scales same for all profiles
Dark organic rich sediment extends furthest shoreward
above the bright layer as though the zone of organic sediment accumulation expanded as lake levels rose.
20
Depth (cm)
5.75 m
600
Thanks to Djuna Guliver; Anders Noren and Amy Myrbo of the LRC; Mark Linberg, Director of the Minnesota Cartography Lab. Funding provided by a University of Minnesota Graduate School McKnight Land-Grant Professorship.
0
0
two-way travel time (ns)
View southwest toward
Continental Dividen
and moraine (right).
1.1 1.2 1.3 1.4
1.1 1.2 1.3 1.4 1.5
1
1.1 1.2 1.3 1.4
60 70 80 90 100
60 70 80 90 100
Fossil pollen data confirm
Charcoal and Pollen Data
the long persistence of
lodgepole pine (Pinus contorta; a diploxylon pine) at
Hidden Lake. The current
20 40 0 5 0 5 0 10 0 10 20 0 10 0 5 0 5 0 20 40 60
forest also contains significant subalpine fir (Abies
lasiocarpa), which increased
Post-Fire Meadow
Fuel Build-Up
in abundance in the pollen
data in the past 500 years.
930
An initial peak in fir pollen preceeds the largest fire
episode of the past millennium. The charcoal peak is
2130
cal yr BP
followed by a peak in grass
and other herb pollen. Pollen influx data (not shown) are
consistent with low tree cover, such as found in a meadow.
C
ha
r
(> coa
12 l c
5 on
m ce
ic n
To
ro tr
ta
n at
lP
pi io
ec n
in
e
es
(P
/c
in
m3
us
)
)
D
ip
lo
xy
lo
n
Pi
ne
H
ap
l
Fi oxy
r ( lo
Sp Abi n P
ru es ine
ce )
(P
Sa
ge icea
br
)
us
h
(A
rte
G
m
oo
is
se
ia
)
fo
ot
As
(C
te
he
G rs
ra (A
no
ss st
po
(P era
Is
da
oe o c
ce
a
ea
te
c
ae
e
e
s
ae )
)
)
Core A
GPR Profiles
Bright (dense) layer truncates
a lens of sediment off-shore.
The pattern is consistent with
sediment winnowing during
a period of low lake levels.
1
Ecosystem History Data
1.75 m
Sed thickness based on
0.028 m/ns estimated from
hyperbolas. Velocities
estimates range from 0.023
to 0.032 (near shore).
1.1 1.2 1.3 1.4
60 70 80 90
>1.45 m
A. 0
1
60 70 80 90 100
Wate
r
Till
We chose the lake because of its proximity to the lower
elevational limit of forest, which is often considered to
be moisture limited. Sagebrush (Artemisia spp.) dominates
the broad North Park valley located less than a kilometere
away and less than a hundred meters down slope. Ongoing
work is examining ecotone dynamics near Hidden Lake in
response to moisture variations recorded at the lake.
1.1 1.2 1.3
60 70 80 90 100
0.85 m
Photo from here
A transect of sediment cores collected along GPR
profile B show a dense inorganic layer (grey). The
layer appears thickHidden Lake
Lake Selected
Selected Cores:
Cores: Deep
Deep to
toShallow
Shallow
C
D
E
F
A
est near shore and
Deep Cores
Bulk Density
(g/cm )
marks a transition
I J A
from dense, low water
content
sediment
2026
2020
below
to
less
dense,
930
930
3718
3715
1072
1070
2953
2950
more
floculent
sedi4231
4230
3747
3750
ment above. Cali4370
4395
brated AMS dates
2132
2130
1015
1014
bracketting the layer
Location of calibrated AMS radiocarbon ages (median age shown)
Macrofossil ages in italics; other AMS ages on bulk sediment
Density Core A is to the east (right on GPR profile); core F is to the west (left on GPR).
reveal a slow sediWater depth changes 0.5 m between cores. Core F was from 3.5 m of water.
mentation
rate
(<12
Core I was collected at the deepest point of Profile A; Core J from the deepest
2915
Water Content
point on Profile C.
(Percent wet weight)
cm/kyr) associated
with the layer. The low sedimentation rate and sediment
contrast across the layer is consistent with the interpretation of an unconformity in the GPR profiles. We infer
that the lake level was low 3700-2000 cal yr B.P.
2
3.6 m
View south across
Hidden Lake.
0.8
Fossil charcoal in sediments from
0.6
a deep core (A below) in Hid0.4
0.2
den Lake show repeated episodes
0
of burning during the past 2000
2000
1500
1000
500
0
Cal yr BP
years. Counts of >125 mm charcoal fragments were made from contiguous 0.5 cm intervals in the
core. Synthetic charcoal counts based on repeated random draws of
actual counts were used to set a threshold for significant peaks. A grey
bar shows the range of potential random variation; arrows mark significant peaks. The results show few large events in the past 1000 yrs.
Core E
H
Depth (cm)
Study Site
Zone of Sediment
Winnowing
Fire History
Charcoal Influx
(pieces/cm2/yr)
Moisture History
Depth Below Modern Lake Surface (cm)
Overview
40
60
80
100
Copyright, 2006. Bryan Shuman, University of Minnesota.
Significant
Charcoal Peaks
Range of Values Attributa
to Random Chance