Correlating Phytolith and Pollen Data from Wild Rice

Lakes and Designation of First Occurence – A Cautionary Note

Robert Thompson, Amy Myrbo, Shelden Misquadace, Misty Rose Peterson, Matthew Weingart, Emma Locatelli

Abstract Wild rice (Zizania palustris) is central to Ojibwe culture. For example, the Fond du Lac

Band of Lake Superior Chippewa chose the location of their reservation in the 1854

treaty based on the presence of several wild rice lakes in this area. The manoomin

project, a shared effort between the Fond du Lac band and the University of Minnesota,

is studying the history of the wild rice lakes on the reservation using sediment core

samples. The lakes share a basic history, having been formed in depressions left by

melting of ice blocks calving off the retreating glacier. Variations in hydrology impact

this history differently for each lake. Nevertheless, a basic pattern of lakes filling in

gradually through the Holocene is shared by the three lakes cored in the first year of

this study: Dead Fish Lake, Perch Lake and Rice Portage Lake. Each of these lakes was

infilled first with organic-poor silt in the early Holocene, then later with organic rich,

fine-grained sediment (diatomaceous silty sapropel) with interspersed plant

macrofossils. Both phytoliths and pollen were recovered from cores taken from each of

these lakes, and their respective abundance in subsample splits from the uppermost

levels of the lake cores were compared. In each of the lakes, pollen, and in particular

grass pollen, is abundant in the surface sediments of the lakes. While wild rice does not

produce unique pollen, the abundance of grass pollen is thought to reflect the presence

of wild rice. Wild rice phytoliths (in fact any phytoliths) are sparse in the surface levels,

beginning to concentrate 15-20 cm below the surface. Phytoliths have a specific gravity

of 2.3- 2.35, which allows them to sink further than pollen. The disjunct between grass

(presumed wild rice) pollen and wild rice phytoliths demonstrates the need for caution

in interpreting the correlation between phytoliths and pollen recovered from the same

core horizons. In addition, the first appearance of wild rice phytoliths in the

sedimentary record may predate the first appearance of wild rice plants in the lake..

)

Deadfish

Lake Perch Lake

Rice

Portage

Lake

This research is student driven, with the

data collected by interns. Interns were

familiarized with phytolith shapes using

SEM photos, and learning to model the

three dimensional forms as observed in two

dimensions under light microscopy. The

interns were able to identify phytoliths

produced by grasses, and forms unique to

wild rice. Pollen identifications were done

by Matthew Weingart, and Amy Myrbo

provided the Bacon age models for each of

the lakes

0

100

200

300

400

500

600

700

Wild Rice Phytoliths

Grass Phytoliths

0

10

20

30

40

50

60

70

2-3

cm

18-1

9c

m

34-3

5c

m

50-5

1c

m

66-6

7c

m

82-8

3c

m

96-9

8c

m

114

-11

6

2007 1980 1931 1834 1733 1648 1532 1392

Grass Pollen Percentage

0

50

100

150

200

250

300

350

400

450

500

2-3

cm

18-1

9c

m

34-3

5c

m

82-8

3c

m

98.5

-99

.5c

m

114

-11

6c

m

130

-13

2c

m

2006 1980 1929 1648 1524 1395 656

Wild Rice Phytoliths

Grass Phytoliths

0

10

20

30

40

50

60

Grass Pollen Percentages

0

50

100

150

200

250

300

2-3

cm

18

-19

cm

32

-34

cm

98

-99

cm

114

-11

6c

m

130

-13

2c

m

2011 1952 1870 708 432 215

Wild Rice Phytoliths

Grass Phytoliths

0

10

20

30

40

50

60

70

2-3

cm

18-1

9c

m/1

952

32-3

4c

m/1

870

98-9

9c

m/7

08

114

-11

6c

m/4

32

130

-13

2c

m/2

15

2011 1952 1870 708 432 215

Grass Pollen Percentages

More than a meter of sediment has been sampled and analyzed from a single core from each of the lakes, and an important disjunction between phytolith

recovery and pollen recovery is readily apparent for the top layers of all three lakes. Pollen recovery is strong from the surface levels of each of the cores,

whereas phytolith recovery is scant. The core from Deadfish Lake yielded grass phytoliths as 60% of total recovery, which was the highest percentage in the

core. Since Deadfish Lake produces abundant wild rice at the present time this is not surprising. In contrast, grass phytoliths accumulation begins to grow

in the 1930’s, and increases with depth until the level representing 1834, at which time pollen has significantly decreased. It is not yet clear how deep

phytoliths representing the present have sunk in the loosely consolidated surface levels. Lower in the column the phytolith and pollen recovery mirror each

other much more closely. Both show low recovery in the early 1700’s, and another peak in the 1500’s, and fall off again in the late 1300”s. The reasons for

the disjunction are not yet clear. We are investigating several possibilities: Phytoliths recovered from the cores seem to be dominated by forms from wild

rice chaff. The macrostructure of the chaff is designed to help it burrow into bottom sediments, thus accumulating in deeper levels. The decay of this chaff

results in concentrated phytolith deposition. Another problem in interpreting the disjunction is the nature of grass pollen. Pollen from a number of near lake

grasses may be included in the lake. Whatever the ultimate causes, this disjunction requires caution in interpreting the stratigraphy of phytoliths recovered

from lakes. I would like to gratefully acknowledge the support of LacCore,

Fond du Lac Natural

Resources, and NCED.