A Quantitative Midge-based Reconstruction of Late Pleistocene-Early Holocene Temperatures in the Sierra Nevada, CA
David F. Porinchu(1), Glen M. MacDonald(2), Katrina A. Moser(3) and Amy M. Bloom(4)
Introduction
Results and Discussion
• the remains of midge flies (see Fig. 1 and Fig. 2), preserved in the
sediment of Sierra Nevada lakes, were used to quantitatively
reconstruct thermal conditions during the Pleistocene-Holocene
transition
• the relative abundance diagram is divided into five zones based on
compositional changes (see Fig. 7)
• the basal chironomid assemblage (14.5 – 13.7 kcal BP) is dominated
by a cold-water taxon, Heterotrissocladius and represents water
temperatures likely < 5oC
• at 13.7 kcal BP the diversity of the midge community begins to
increase with the arrival of additional cold water taxa such as
Sergentia, Hydrobaenus/Oliveridia and Corynocera oliveri-type
• a sharp increase in thermophilous midge taxa occurs at 12.3 kcal BP
with Dicrotendipes, Psectrocladius (Psectrocladius) and Corynocera
nr. ambigua appearing and/or increasing
• the early Holocene midge community stabilizes at ~ 10.7 kcal BP
Greenstone
Lake
Fig. 2 Photomicrograph of a
Microtendipes head capsule (scale
bar is 50 µm). VP = ventromental plate
Fig. 1 Lifecycle of midge
flies.
Transfer Function Development (Modern
Sediment)
• surface sediment was recovered from 57 lakes in the eastern Sierra
Nevada (see Fig. 3) and each lake was characterized by 48
environmental variables (see Table 1)
•
Fig. 7
• redundancy analysis (RDA) identified that surface water temperature
(SWT), elevation, depth, strontium and particulate organic carbon
(POC) accounted for a statistically significant amount of variance in
chironomid community composition (see Fig. 4)
Fig. 3
Study Site and Methods (Late Quaternary
Sediment)
• Weighted Averaging-Partial Least Squares (WA-PLS) was used to
develop a robust chironomid-based surface water temperature
inference model (see Fig. 5)
• a 283 cm sediment core was recovered in July 2001 from Greenstone
Lake (GSL), which is located in the Hoover Wilderness, near Tioga
Pass (see Figs. 3, 6)
• the model was applied to a late- Quaternary chironomid stratigraphy
to reconstruct thermal conditions in the Sierra Nevada following deglaciation
• at present, GSL is a small (8.1 ha), relatively shallow (5.10 m), slightly
alkaline lake (pH = 8.1), surrounded by sub-alpine vegetation, with
surficial talus and slopewash deposits of Quaternary age present in
the catchment
1:1
r2 = 0.73
o
RMSEP=1.16 C
n=44
18
15
Average SWT of closest
modern analogues
14
Chironomid-infered SWT
13
Measured SWT
(09 August 1999)
11
Poorly constrained
temperature estimates
12
10
12
14
16
18
20
22
Fig.6: Map showing location of
GSL and the nearby Conness
Glacier. Coring location
indicated by X
24
o
MeasuredSummer Water Temperature ( C)
8.8
6.7
16
12
14
Fig.5: Relationship between
observed and predicted
summer water temperature
pH
17
• LOI analysis was undertaken at ½ cm intervals (~ 75 year resolution);
midge analysis was undertaken at 1 cm intervals (~ 150 years)
16
10
Elevation Area Depth
(m a.s.l.) (ha)
(m)
mean
2906
10.0
10.5
maximum
3475
121.6 40.0
minimum
2115
0.1
2.0
Greenstone Lake
18
• the chronology for the basal portion of the core is based on two AMS
14C dates (see Table 2); the age-depth function was constructed
assuming a constant rate of deposition
20
10
Fig. 4: Ordination bi-plot illustrating
relationship between calibration lakes
and forward selected environmental
variables
• results from the inference model (see Fig. 8) indicates that the immediate
postglacial period was characterized by SWT < 5oC; likely representing input of
glacial melt-water from the nearby Conness glacier
• between 13.7 and 12.0 kcal BP SWT rose ~ 2oC, reaching 15.5oC at the
onset of the Holocene; fluctuated between 15.5 and 16.5oC for remainder
of the record
• the rate of warming between 13.3 and 12.0 kcal BP is ~ 0.25oC/100 yrs
• if the basal midge assemblage accurately captures SWT during the
immediate post-glacial period the rate of warming between 14.0 and 12.0
kcal BP is ~ 0.5oC/100 years
Inferred July surface
water temperature (oC)
Predicted Summer Water Temperature (oC)
24
22
Calibration dataset lakes
Temp.
(oC)
15.1
21.4
7.7
Salinity LOI
(mg/L)
(%)
19.28 27.36
120.28 57.37
2.20
4.69
Table 1: Selected environmental characteristics of the lakes incorporated
in the calibration set
Lab Number
Depth
(cm)
Material
Age
(14C yr BP)
Calibrated age
(cal yr BP)
Beta-157868
Beta-157867
245-246
229-231
bulk
bulk
10,700 + 80
9270 + 60
12,800
10,470
Table 2: Details of the AMS 14C dates
10000
Conclusions
11000
12000
13000
Age (cal yr BP)
14000
15000
Fig. 8
• provides estimate of the rate of warming associated with post-glacial climatic
amelioration (0.25-0.5oC/100 years)
• the rate of warming during the Pleistocene-Holocene transition is much lower
than that estimated by the IPCC to occur by 2100 AD (1.6-5.8oC)
• expansion of the calibration set will enable more accurate
reconstructions of thermal conditions during the immediate post-glacial
period
Acknowledgements: Amanda Petel (UCLA) participated in developing the multi-proxy
calibration dataset. Funding for this research was provided by the National Science
Foundation (NSF) and the Geological Society of America (GSA).
Affiliations:
1 Department of Geography, The Ohio State University, Columbus, OH 43210
2 Departments of Geography and OBEE, UCLA, Los Angeles, CA 90095
3 Department of Geography, University of Utah, Salt Lake City, UT 84112
4 Department of Geography-Geology, Illinois State University, Normal, IL 61790