the catchment, and, to a minor portion, of volcanic rock-clasts. In
addition, loose, coarse sand has been recovered from 21
–25.6 m drill
depth (
Fig. 7C
). The base of these sands can not be exactly de
fined
because of a ca 20 m gap with no core recovery below ca 25.6 m drill
depth. Most likely this failure in core recovery is due to further
extension of unconsolidated coarse material which could not be
obtained by the drilling technique used. Some dm-scale, organic-rich
horizons from the upper part of the pro
file could be related to
reworking of catchment material as they lack any lamination, or may
represent sedimentary gaps with palaeo-soils (
Fig. 7B
). Most out-
standing, however, are three horizons (at 54
–54.65 m, 55.8–56.10 m,
and 74.9
–76.6 m drill depth) of darker, finely laminated sediments
with increased organic content (
Figs. 7F,G
). Beneath the lowermost
organic horizon the amount of pebbles increases, ending with a coarse,
mainly phyllitic breccia in the lowermost 1.5 m of the pro
file (
Fig. 7H
).
This sediment facies resembles the coarse basal sedimentary in
fill of
previously studied maar lakes of similar size (e.g. Eocene Eckfeld Maar
Lake;
Mingram 1998; Bullwinkel 2003
; Lago Grande di Monticchio;
Brauer et al., 2007
).
Thin sections from predominantly minerogenic sediments reveal
the expected clastic input of the local catchment, mostly quartz, feld-
spar, and mica. A faint lamination is caused by grain size variations
only, without changes in sediment composition (
Fig. 8
A). The organic
components of these sediments are mostly allochthonous terrestrial
plant remains, whereas remains of lake biota like diatoms or sponge
spicules are rare. Surprisingly, diatom frustules are rare also in the
laminated organic intervals (
Fig. 8
B
–H) although a few diatom layers
have been found (
Fig. 8
G). Instead, chrysophyte cysts and the colonial
freshwater alga Botryococcus (Chlorophyta) are very abundant
(
Fig. 8
C
–F), clearly indicating open water conditions. Experimental
data have shown a stronger resistance of chrysophyte cysts against
dissolution compared with diatoms (
Miller et al., 1990
). It has to be
proven if this is the case also for the Mýtina sediments. An alternative
explanation could be different ecological requirements since green
algae can be dominant over diatoms under conditions of higher
temperatures with moderate or low ratios of both N/P and Si/P
(
Tilman et al., 1986
). Mass blooms of Botryococcus (
Fig. 8
F) are often
related to nutrient-rich conditions, and may dominate the phyto-
plankton under favourable conditions. Botryococcus is also favoured
under conditions of lake strati
fication and clear epilimnia with high
potential insolation (
Reynolds, 2006
). In any case, the frequent
occurrence of Botryococcus in the laminated, organic-rich horizons
indicates elevated temperatures and stable summer strati
fication of
the lake. Numerous graded layers in the organic-rich horizons, ranging
in thickness from sub-mm to cm scale (
Fig. 8
B), point to a high-
energetic environment with steep slopes of the lake basin.
4.4. Palynology
A
first set of 14 pollen samples has been analysed to underline the
importance of a continuous lake sediment section for palaeoenviron-
mental and palaeoclimate reconstruction.
Palynological investigations reveal suf
ficient (app. 10,000 cm
− 3
)
pollen concentration from predominantly minerogenic sediments and
extremely high concentration (with up to
N1,000,000 cm
− 3
) from the
laminated organic intervals. The overall pollen preservation is good.
Fig. 9. Mýtina Maar. Simpli
fied pollen percentage diagram (selected taxa). Exaggeration (x10) is indicated by line. 9 zones (I–IX) have been distinguished: I = Glacial, II =
Interglacial/ Interstadial (temperate), III = Glacial (cool-moist), IV = Glacial (cool-dry), V = Interglacial/ Interstadial (temperate), VI = no data (no core recovery), VII = Glacial
(cool-moist), VIII = Glacial (cool-dry), IX = beginning terrestrialization. Analyst: M. Stebich.
107
J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112
The most exciting
finding is the presence of several alternating warm
and cold periods in the Mýtina record below the core gap (
Fig. 9
). High
values of non-arboreal pollen in the predominantly minerogenic
sediment intervals point to open landscapes of steppic to woody-
steppic character typical for late Quaternary stadial times of central
Europe. In contrast, the three organic-rich horizons obviously
Table 2
Whole rock chemistry of host rock samples (MY-1/ = samples from borehole MY-1/2007; My, XKZH = samples from Mýtina trench from
Geissler et al., 2005, 2007
).
Sample
Nephelinite
Phyllite
MY-1/2-84
MY-1/3-84
My1
My1-B
My2
My2-B
My-1/1-84
XKZH10
XKZH69
XKZH63
Mass-%
SiO
2
43.30
40.90
40.4
39.8
41.3
41.1
54
55.2
61
76.3
TiO
2
2.74
3.05
2.96
2.93
2.9
2.87
1.05
1.47
1.07
0.84
Al
2
O
3
11.20
12.10
11.5
11.4
11.6
11.6
25.3
24.7
20.2
10.1
Fe
2
O
3(t
⁎)
11.26
⁎
11.86
⁎
5.58
5.63
5.68
5.39
6.58
⁎
7.47
⁎
6.16
⁎
6.51
⁎
FeO
5.26
5.11
5.05
5.27
MnO
0.19
0.19
0.19
0.19
0.19
0.19
0.05
0.08
0.05
0.07
MgO
10.96
11.38
13.64
13.7
12.6
12.68
1.46
1.8
1.4
1.18
CaO
12.19
12.82
12.67
12.54
12.41
12.31
0.13
0.42
0.33
0.29
Na
2
O
3.20
2.83
2.53
2.61
2.3
2.4
0.68
1.53
1.71
0.82
K
2
O
1.89
2.06
1.55
1.53
1.42
1.42
5.98
4.48
4.09
1.42
P
2
O
5
0.75
0.73
0.67
0.72
0.65
0.71
0.09
0.16
0.12
0.07
H
2
O
1.46
1.42
2.21
2.28
2.51
2.75
4.2
2.75
3.98
2.54
CO
2
0.13
0.12
0.11
0.1
0.11
0.1
0.2
0.48
0.12
0.05
Total
99.22
99.38
99.38
98.49
99.33
98.78
99.6
100.54
100.28
100.2
Mg#
0.72
0.73
0.73
0.7
Trace elements (ppm)
Rb
82
69
61
59
208
208
227
136
198
101
Sr
766
776
759
748
738
724
125
129
123
70
Cs
2.89
1.39
0.91
0.89
2.09
2.03
11.2
10.2
10.2
5.8
Ba
896
1143
780
770
752
738
866
790
880
492
Th
9.95
9.78
8.9
9
9.5
9.6
20.4
20
20
18
U
2.53
2.64
2.4
2.4
2.4
2.5
3.55
3.2
3.3
3.6
Pb
5.99
5.46
2.9
3.1
3.7
4
26.7
36
20
7
Y
23.1
23.2
21
21
23
22
33
37
32
30
Zr
274
273
225
220
247
243
238
226
199
210
V
295
329
315
320
316
306
154
109
58
Cr
490
469
726
735
586
582
133
84
41
Ni
178
170
249
225
54
32
25
Zn
96
100
70
72
74
75
122
87
113
La
70.1
72.7
68.4
68.3
70.3
68.2
58.1
55.6
59.9
49
Ce
134
138
131
128
135
129
119
121
120
98.8
Pr
15.3
15.6
14.8
14.8
15.6
14.7
13.6
13.9
14
11.5
Nd
57.8
59.7
56.2
55.7
59
55.6
50.4
50
52.3
42.2
Sm
10.1
10.4
9.61
9.65
10.2
9.66
9.41
9.55
9.67
7.54
Eu
2.93
2.94
2.92
2.83
2.9
2.77
1.88
1.82
1.8
1.46
Gd
7.94
7.95
7.69
7.42
8.17
7.73
7.46
8.2
8.01
6.31
Tb
1.03
1.07
0.97
0.97
1.03
1
1.13
1.19
1.09
0.96
Dy
5.56
5.49
4.94
4.89
5.22
4.89
6.64
6.97
6.27
5.5
Ho
0.97
0.96
0.81
0.83
0.86
0.84
1.36
1.36
1.17
1.06
Er
2.3
2.35
2.06
2.05
2.23
2.1
3.85
3.87
3.5
3.09
Tm
0.3
0.31
0.25
0.26
0.28
0.27
0.56
0.56
0.49
0.43
Yb
1.8
1.78
1.51
1.51
1.66
1.59
3.59
3.95
3.14
3.02
Lu
0.27
0.27
0.21
0.2
0.24
0.23
0.57
0.56
0.5
0.43
Hf
6.03
6.15
5.26
5.35
5.3
5.34
6.21
6.37
5.92
6.17
Fig. 10. Photographs of typical hand specimen of rocks from borehole MY-1 (depth: 84
–85.5 m) probable from uppermost part of debris-flow deposits at the base of the lake
sediment), left: phyllitic sample (pebble), right: nephelinitic host rock (lapilli, volcanic bomb).
108
J. Mrlina et al. / Journal of Volcanology and Geothermal Research 182 (2009) 97
–112