The MCR Technique

The MCR technique is based on the assumption that the present climatic tolerance range of a species can be applied to its Quaternary fossil record, so that fossil occurrences of a given species imply a paleoclimate that was within the same tolerance range. MCR studies focus on predators and scavengers, because these groups are assumed to show the most rapid response to climate change. The predators are nearly all generalists that prey on a wide variety of small arthropods. Plant-feeding groups are not considered, because these species cannot become established in new regions until their host plants arrive. In contrast to this, predators and scavengers have been shown to be able to shift distributions at a continental scale in a few tens or hundreds of years. This has been demonstrated in the fossil record of western Europe (Coope 1977) and of North America (Elias 1994), and has been clearly demonstrated for fossil faunas from the Rocky Mountain region (Elias 1991). The fossil assemblages considered in the Rocky Mountain regional study include 74 species in the families Carabidae (ground beetles), Dytiscidae (predaceous diving beetles), Hydrophilidae (water scavenger beetles), Staphylinidae (rove beetles), Scarabaeidae (dung beetles), and Coccinellidae (ladybird beetles).

To determine the climatic tolerances of the beetles in the fossil assemblages, I developed a climate envelope for each species, based on the mean July and mean January temperatures of all the North American locations where the species presently occur. The temperature regimes of these localities were plotted on a diagram of the mean July temperature versus the difference between the mean July and mean January temperature, based on a 25-km-grid North American climate database (Bartlein et al. 1994). This database was used to pair temperature regimes with the modern beetle collection sites, using the geographically nearest grid location to each collecting site. To test MCR accuracy for North American fossil beetle assemblages, I developed a linear regression model (Elias et al. 1996) that tested predicted versus observed modern temperatures at sites with meteorological stations. The regression equations were used to calibrate paleotemperature estimates made by the MCR method. I used the MCR method to predict the modern mean July and mean January temperatures of 35 sites in North America, based on the overlap of climate envelopes of beetle species that live at these sites. The climate envelopes were developed for species found in the Wisconsin-age fossil assemblages (Elias et al. 1996), all of which are extant. A linear regression of observed versus predicted mean July temperatures yielded an r2 value of 0.94. A regression of observed versus predicted mean January temperatures yielded an r2 value of 0.82. The slopes of predicted versus actual temperatures were 0.78+.03 and 0.72+.06, respectively. The linear regressions of predicted on observed TMAX and TMIN values yielded the following equations:

TMAX [calibrated] = [median predicted TMAX x 0.787] + 3.4 TMIN [calibrated] = [median predicted TMIN x 0.716] - 4.9

The standard errors of the regressions were +0.7°C for TMAX and +10°C for TMIN. This indicates that MCR estimates of mean July temperature are probably far more reliable than MCR estimates of mean January temperature. This makes sense from an ecological standpoint, because mountain-dwelling beetles are only active during the summer months.

In this study, I applied MCR analysis to 21 fossil beetle assemblages from eight sites spanning the interval 14,500-400 yr b.p. (table 18.1) and used the linear regression equations (given previously) to calibrate the data. The sites are in the Colorado Front Range region of the Rocky Mountains (figure 18.2). Although there

Table 18.1 Site data and summary of modern and paleoclimatic data

Elevation

Sample Age

Site

(m asl)

04C yr BP)

Late Quaternary

Modern

Change

in Temperature

TMAX

TMIN

TMAX

calibrated

TMIN

calibrated

TMAX

TMIN

(°C)

(°C)

(°C)

(°C)

(°C)

(°C)

July AT

January AT

Lamb Spring

1731

14,500 ±500

10-11

11.7

-31 to -27

-25.8

21.4

-1.3

-9.7

-24.5

Mary Jane

2882

13200

9.8-10.2

11.3

-29.3 to -27.6

-25.4

13.4

-8.6

-2.1

-16.8

Mary Jane

2882

12800

10-10.2

11.4

-29.1 to -27.6

-25.3

13.4

-8.6

-2

-16.7

Sky Pond

3320

10000

8.3-10.0

10.6

-33.2 to -27.2

-26.6

10.0

-10.7

+0.6

-15.9

La Poudre Pass

3100

9850+300

15-18

16.4

-17.5 to -7

-13.7

11.3

-7.7

+5.1

-6.0

Lake Isabelle Delta

3323

9000+285

11.75-14.5

13.8

-31.25 to -15

-21.6

10.8

-8.2

+3.0

-13.3

Sky Pond

3320

8950

9-15.5

13.1

-33 to -17.5

-23.1

10.0

-10.7

+3.1

-12.4

La Poudre Pass

3100

8800+90

13.5-16.5

15.2

-19.5 to -9

-15.2

11.3

-7.7

+3.9

-7.5

Lake Isabelle Delta

3323

8500

10.5-13

12.7

-23.5 to -16

-19.1

10.8

-8.2

+1.9

-10.9

Lake Isabelle Delta

3323

7800+255

11-13

12.9

-14 to -9

-13.2

10.8

-8.2

+2.1

-5

Lake Isabelle Fen

3325

7080+90

10.25-13

12.6

-14.75 to -7.5

-12.9

10.8

-8.2

+1.8

-4.7

Sky Pond

3320

6500

11-15.5

13.9

-27.5 to -17

-20.9

10.0

-10.7

+3.9

-10.2

La Poudre Pass

3100

5360+90

12.5-13.5

13.7

-12.5 to -11.5

-13.5

10.8

-8.2

+2.9

-5.3

Sky Pond

3320

5250

7.5-10.3

10.4

-29.5 to -17

-21.6

10.0

-10.7

+0.4

-10.9

La Poudre Pass

3100

3485+180

11.75-15

14.0

-21.25 to -14.5

-17.8

11.3

-7.7

+2.7

-10.1

Lake Isabelle Fen

3325

3000

10.25-13

12.6

-14.5 to -7.5

-12.8

10.8

-8.2

+1.8

-4.6

Longs Peak Inn

2732

2965+75

12-15.5

14.2

-26.5 to -14

-19.5

14.5

-5

-0.3

-14.5

Longs Peak Inn

2732

2680+80

13.5-15.5

14.9

-24 to -15

-18.9

14.5

-5

+0.4

-13.9

Roaring River

2800

2400+130

14.25-14.75

14.9

-18.25 to -16.5

-17.4

14.3

-5.5

+0.6

-11.9

Mount Ida Bog

3520

900+150

10.25-12

12.2

-14.75 to -9

-13.4

9.3

-11.5

+2.9

-1.9

Longs Peak Inn

2732

395+100

13.5-15.5

14.9

-24.5 to -15

14.5

-5

+0.4

-14.1

Millennial and Century Climate Changes in the Colorado Alpine 375 La Poudre Pass

Mt. Ida Bog

Sky PondJT I Longs Peak Inn —**,Niwot Ridge Mary Jane»,'

Roaring River

Lake Isabelle (delta & fen)

»Lamb Spring

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