- Email: [email protected]
Diatoms of the McMurdo ice shelf, Antarctica: Implications for sediment and biotic reworking
Palaeogeography, Palaeoclimatology, Palaeoecology, 60 (1987): 77 96
77
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
DIATOMS OF THE McMURDO ICE SHELF, ANTARCTICA: IMPLICATIONS FOR SEDIMENT AND BIOTIC REWORKING 1 DAVIDA E. KELLOGG and THOMAS B. KELLOGG Institute for Quaternary Studies and Department of Geological Sciences, University of Maine, Orono, ME 04469 (U.S.A.) (Received M a r c h 25, 1986; revised and accepted October 29, 1986)
Abstract Kellogg, D. E. and Kellogg, T. B., 1987. Diatoms of the McMurdo Ice Shelf, Antarctica: implications for sediment and biotic reworking. Palaeogeogr., Palaeoclimatol., Palaeoecol., 60: 77-96. In the course of investigations of the origin of the McMUrdo Ice Shelf (MIS), we analyzed diatoms in 59 sediment samples collected from its upper surface. Fragments of centric marine species, most of which occur in the Ross Sea today, were found in all samples, but identifiable marine taxa dominated the total diatom flora in only two samples. In contrast, non-marine diatoms are a b u n d a n t and diverse in most MIS samples. These non-marine species probably live in the numerous melt ponds which are distributed widely over the shelf surface during the warmest few weeks of each austral summer. Non-marine diatoms are clustered in four assemblages, which are probably related in a complex way to geographic position on the ice shelf, sediment age, local geochemical variations, and seasonal blooms. The presence of marine diatoms in MIS sediments is consistent with isotopic evidence t h a t the MIS consists of frozen seawater, and with Debenham's hypothesis t h a t transport of marine macrofossils to its surface is accomplished by freezing on at the base and upward movement t h r o u g h the shelf caused by surface melting. The combined marine and non-marine diatom floras of the MIS suggest a new mechanism to explain mixed diatom floras and sediments, observed in antarctic cores. These composite floras and sediments may be introduced or reintroduced to the marine environment during austral summers by melt water streaming off the front of the MIS or through crevasses. Additionally, icebergs calved from the MIS release their load of mixed biota and sediment when they melt. These observations may pertain to other present and former antarctic ice shelves t h a t are characterized by surface ablation and basal freezing.
Introduction The McMurdo Ice Shelf, located in the southwest corner of the Ross Sea (Fig.l), is bounded by Minna Bluff and the mountains of Southern Victoria Land on the west and south, and by Ross Island and the Ross Ice Shelf on the northeast and east. The waters of McMurdo Sound, which become partly or
~This paper is dedicated to the memory of F r a n k Debenham, who first proposed uplift of marine organisms by basal freezing and surface melting of the Pinnacled Ice. 0031-0182/87/$03.50
completely free of pack ice during the austral summer, lie to the north. Brown Peninsula, snow-covered White Island, and bare volcanic Black Island, are surrounded by the MIS. To the first-time observer, the most notable features of the MIS are the distinctive bands and patches of debris that mantle much of its surface (Fig.l). Scott (1905, p. 154) first described the McMurdo Ice Shelf, and the term "Pinnacled Ice" was used during his last expedition to describe the entire area and the lower Koettlitz Glacier (Scott, 1914). Although Scott's term has priority and conveys a more evocative description of the area, we use "McMurdo Ice Shelf" (MIS) because ice pin-
© 1987 Elsevier Science Publishers B.V.
78
nacles are developed only locally (Kellogg, T. et al., in prep.). Perhaps the most evocative image of the surface was given by Taylor (1914, p. 205):
"Most picturesque in appearance, but as a sledging proposition it can only be described as infernal!" Today this area is approached only by helicopr
i 165 °
166°E
McMurdo
Sound
77o45 '
K78-SM
ROSS
SEA
K76-7
ROSS
.K76-9
K76-12~ K76-13 K76-14
L>.V %. '.\
YY
• LK76-16 \/ K 7 6 - 1 7 K76-1e
7 8 ~-
. K76-19
K78-56
K78-100 KTB-59 --
K78-63 K7S-65 K78-66
K76-24
,
"I,
kjK~s-2~ ~rs-~ Ii,l
K78-1C
Peninsula
White K78-108
Island
K76-60
K76-59
/ K76- 64
Black Island ¢~"~a .
~
~"
, 15'
K76-63
Mt Discovery A
g: 5
~1V'l'i
0 I" l j
5
10
t
I
Sco~e
15km ~
K7S-57
~'~i~h O
~...,~'--~\
KTe-ss K78-MB •
,~ 78 °
I
30"
I
Fig.1. Map of the Ross Sea region showing the location of the McMurdo Ice Shelf, localities mentioned in the text, and sampling sites on the McMurdo Ice Shelf (solid triangles indicate samples deleted from our second factor analysis).
79 ter, and a field party can cover perhaps 5-10 miles a day across its treacherous terrain on foot. The MIS is approximately 10-50 m thick north of the strait between Black Island and Brown Peninsula, and up to 110 m thick north of Minna Bluff (Swithinbank, 1968), in contrast to the main body of the Ross Ice Shelf, where the thinnest ice (>100m) occurs at the ice shelf margin. In contrast to the Ross Ice Shelf which is maintained largely by precipitation, basal freezing of sea water contributes significantly to the mass balance of the McMurdo Ice Shelf. This mechanism was hypothesized by Debenham (1919) to explain the presence of fossil remains on the surface. He suggested that basal freezing in shallow water permitted the ice to pick up marine sediments and organisms, and that through surface ablation this material would, in time, move to the ice shelf surface and accumulate there. Support for the hypothesis that ice accretion occurs at the base of the MIS comes from several additional lines of evidence. Gow and Epstein (1972) showed that the isotopic composition of ice in cores drilled north of Koettlitz Glacier was consistent with a sea-water origin. Heine (1968) and Kovacs and Gow (1975) described pockets of brine in the ice shelf at selected locations. Isotopic analyses of over 100 surface ice samples we collected at widely scattered localities show that the ice is composed almost entirely of frozen sea water (Stuiver et al., 1981a; Kellogg, T. et al., in prep.). What is still not known is whether marine sediments and organisms are picked up only in shallow areas where the ice shelf is grounded, as along the shores of Black Island and Brown Peninsula, or if the anchor ice mechanism (Dayton et al., 1969) permits addition of marine material in deeper water. Oliver (pers. comm., 1984) has observed anchor ice only in shallow water ( < 50 m) in McMurdo Sound. This observation is in agreement with occurrences of anchor ice noted by Scott (1905, p. 380). Because most of the shelf is floating on water
deeper than 500 m, assuming that isobaths in McMurdo Sound are representative for that part of the Sound covered by the MIS, we suspect that most of the debris and fossil material is incorporated into the ice shelf in shallow water. In this paper, we present the results of our diatom analyses of sediment samples collected from the surface of the MIS during four field seasons. These data constitute evidence for a new mechanism for producing mixed marine and non-marine diatoms assemblages in highlatitude marine sediments.
Objectives and methods Our field work was conducted during parts of the 1975-1976, 1976-1977, 1978-1979, and 1981-1982 austral summers. We concentrated our efforts on the band trending north from Black Island during the first two seasons, and expanded our efforts to include other portions of the MIS during later years. In all, we spent over three months on the MIS. Sample locations are shown in Fig.1. At each location, we sampled sediment, ice, macrofossil remains (when present), algal material (both desiccated and in melt ponds, when accessible), sieved sediment to concentrate microfossils, and searched for radiocarbon-datable shell material. Sample locations were determined by triangulation. Our original objective was to collect microfossil data from radiocarbon dated samples for interpretation of the past history of McMurdo Sound (Kellogg, T. et al., 1977), but we did not realize that non-marine species dominate in MIS sediments. We were aware from the start that non-marine diatoms should also be present in many of our sediment samples, complicating our effort to distinguish marine assemblages, because thousands of small melt ponds cover the ice-shelf surface. In order to obtain a clear picture of the nature of modern nonmarine assemblages to which to compare our fossil material, we also collected melt-water samples and both fresh and desiccated algae at many locations.
80 Diatom preparations of sediment samples were made using the method of Schrader (1974) and slides were examined with a Leitz Dialux20 microscope at 1000 x. For each slide, we made a census of diatom species present. Nonoverlapping traverses of the slide were analyzed until 300 specimens had been observed or, when diatom abundances were very low, until approximately 20 traverses (approximately the entire slide) had been examined. Because of the large number of both samples and species, it was necessary to use factor analysis to resolve which species consistently occurred together. We used the Q-mode program CABFAC (Klovan and Imbrie, 1971) for clustering purposes. Marine diatoms
Fragments of centric marine diatom species are present in all our samples, but complete specimens are rare and marine species dominate the total diatom assemblage at only 4 of the 59 sites (K76-22, in which 64% of 91 specimens counted were marine; K76-55, in which 97% of the 116 specimens counted were marine; K76-63, in which 100% of 12 specimens counted were marine; and K76-69, in which 81% of the 74 specimens counted were marine). Non-marine species dominate at the remaining sites, comprising more than 90% of each sample with few exceptions (Fig.2). Our results expand on the work of Brady and Batts (1981), who also studied diatoms from the MIS. They reported (p. 13) "...many fragments of marine diatoms and sponge spicules...and some fragments belong to the Thalassiothrix/Thalassionema genera." They also identified N. kerguelensis fragments from salt beds near Black Island. Melosira sol, Nitzschia kerguelensis, and fragments of Thalassionema nitzschiodes are present in most of our samples (Table I). Other commonly occurring marine species include: Nitzschia curta, Nitzschia obliquecostata, Nitzschia sublineata, Eucampia antarctica, and Actinocyclus actinochilus. Most of the marine species we report here live today in the Ross Sea
(Truesdale and Kellogg, 1979), and some (e.g. N. curta and A. actinochilus) were thought to be Quaternary stratigraphic indicators (McCollum, 1975; Kellogg, D. and Kellogg, T., 1986). Diatoms are plants and require light for photosynthesis. Ice thicknesses for the MIS range from 20 to over 100 m (Swithinbank, 1968), which should be sufficient to prevent, or at least inhibit, photosynthetic activity. Thus, we suspect these common marine species were carried beneath the ice shelf by ocean currents, and deposited in marine sediments which were later transported to the ice shelf surface by the basal freezing and surface ablation mechanism which is also apparently responsible for transporting marine macrofossils to the upper surface of the MIS (Debenham, 1919). Another possible, but probably less-important, mechanism for introducing marine diatoms to MIS sediments is winds blowing across diatombearing deposits in adjacent snow- and icefree areas, which are widespread in the Dry Valleys and on Black Island and Brown Peninsula. Single specimens of the extinct marine species Denticulopsis hustedtii, Nitzschia praeinterfrigidaria, and Trinacria excavata were identified positively at sites K76-2, K76-6, K768, K76-22, K76-23, K76-24, K76-59, K76-69, and K78-79. Probable fragments of these extinct species were also noted at K76-11 and K76-19. The presence of extinct species on the ice-shelf surface suggests that in-situ sediments older than Pleistocene may be exposed on the sea floor beneath the Pinnacled Ice, or that Quaternary sediments beneath the ice shelf contain reworked older material. We prefer this latter alternative because our previous studies have repeatedly documented reworked sediments in Ross Sea cores and in cores from beneath the Ross Ice Shelf (Kellogg, T. and Truesdale, 1979; Truesdale and Kellogg, 1979; Kellogg, T. and Kellogg, D., 1981; Kellogg, D. and Kellogg, T., 1986). In either case, the extinct species are probably transported to the ice shelf surface along with younger marine sediments and organic matter during the basal freezing process.
81
McMurdo Ice Shelf. Small ponds, rivulets, and pools are widely distributed over its surface during the austral summer months of December and January. Mats of green and
Non-marine diatoms Non-marine diatoms are abundant and wellpreserved in samples from the surface of the I
166°E
~65 °
McMurdo
;67 °
tayloren$/s (J7)
Sound
77o45 '-
N glbbula (9)
0.0
N muhcops;s 1 ~ N1
L
f
evolu/a (77)
N mutlcop51$ f reducto (52)
mutlcops/forme (52) N curta (12)
/~ mullcops/s (14)
.....
,o
J
_~0/
(51
Cyclofe//a spp. (15)
\
k
Melosira sp "A" ~64)
% ~ N quoternorzo (66), ~ .
! ,\
/
'4~Melos/ra sp "8" (20)
T n,tzschz?/des (il)!>~.__~, bt
/""~ p"" ;'" :'~o, ,',/ cymofopleure ( ) \'~"
7 '1~ ~1' ~N confento (23) 78 ° -
N shockleton/ {143) N
gauss/
(59)
1.3 ~' Melos/ra SO/ (11) !/;!< N kefgulens/s (22)
O.£) q "~' N rnuticops/s f murray/ (5)~ t Brawl
/j
0.0
, "
wesil~ (222)
N muhcops/s f. copl'tafo (25)] F/Peninsula ~/ confenfa f. pafal/e/o (43) J"~ . . . . . -::-
i White
0.0
"--~
.:,
Island 0.0
.~
-/
q.1' I M d, stons (S)
~,a'~
/
Island
~C~o~.
~" 15'-
I00 rN. obliquecq$tata (43) I~ subl]nearis (18) E1 ontarch~a (IO) ~A lactinochHu$ (9)
M/' Discovery &
c~
,-~-r-~
_
I
Scole
.-
i
~
o.o
t~I I'IDO
~
O.O•
~ 78
J
"-"-
,"~
° 30'
-
I
Fig.2. Percentage of marine diatoms in McMurdo Ice Shelf samples. Also shown are sample localities at which individual diatom taxa are most abundant (number counted in parentheses).
Diatom
Sample
K76-12
7
K76-53
4
14
K76-66
K76-67
K76-64
K76-63
K76-62
K7~60
K76-59
K76-58
K76-57
-
5
K76-52
8
2
K76-51
K76-55
8
2
2
1
1
K76-50
K76-31
K76-30
K76-26
K76-25
K76-24
K76-23
K76-22
K76-21
K76-20
K76-19
K76-18
K76-17
K76-16
K76-14
-
1
16
K76-11
1
8
K76-10
K76-13
3
K76-9
.
--
-
.
-
-
-2
.
1
1
.
1
1
5
-
4
K76-7
K7~8
2
2
K76-6
.
-
--
_
1
1
1
-
2
-
3
-
2
K76-4
3
4
K76-5
1
3
17
-
2
data"
K76-3
K76-2
census
1
I
TABLE
.
.
A
-
1
1
2
1
1
6
15
6
2
6
1
5
.
-
-
-
-
-
-
-
-
.
-27
-
6
.
6
2
17
3
1
1
1
2
1
6
1
2
11
3
9
1
1
2
1
2
3
3
7
1
-
.
1
1
2
2
1
8
.
.
3
9
-
.
10
.
6
1
4 1
5
1
1
5
1
10
20
1
9
1
2
11
6
6
7
5
3
1 4
3
1
-
2
8
1
-
3
5
4
11
1
43
5
10
3
10
3
12
2
3
5
2
32
1
6
4 12
9
23
2
14
!
6
4
2
1
36
30
2
8
11
16
16
10
26
7
2
12
60
34
23
4
21
16
29
12
20
23
8
4
14
-
18
5
-
2
2
16
8
4
8
7
4 -
-
-
2
9
13
15
14
1 2
3
2 2
4
2
-
3 6
13
12
3
1
8
11
8
7
43
15
2
2
2
2
10
5
22
2
13
20
3
22
5
43
10
9
7
17
36
4
1
36
8
6
2
3
20
-
42
20
15
-
.
3
2
1
1
1
1
1
1
1
1
1
2
9
7
16
.
-
-
.
17
.
1
2
18
1
2
1
2
19
2
3
4
5
2
1
1
2
6
52
1
2
44
1
5
25
-
-
20
-
-
6
4
1
3
1
1
5
1
3
6
4
3
7
3
1
1
1
14
4
2
5
21
3
1
3
1
1
1
1
9
2
1
25
-
-
-
-
-
-
-
-
23
-
-
-
22
47
9
3
41
1
17
35
1
3
4
2
3
2
3
5
4
1
20
5
1
77
5
7
12
25
5
23
-
-
-
-
-
-
-
-
-
-
-
1
1
2
3
1
1
1
2
1
2
1
24
12
4
3
7
2
3
2
8
15
2
5
5
1
14
3
2
1
6
9
8
1
16
1
6
1
2
52
2
2
11
14
8
25
8
8
1
1
3
2
2
27
61
1
10
44
39
19
5
18
8
2
46
38
18
37
5
60
31
25
22
41
32
13
3
15
-
-
-
26
-
-
-
15
1
3
11
22
33
29
44
12
38
87
35
65
48
47
143
19
47
18
7
1
24
4
51
1
11
4
1
7
21
1
27
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3
1
28
24
7
3
62
32
1
98
92
68
167
33
16
222
139
26
99
8
153
Iii
77
81
137
145
53
17
129
28
160
17
43
45
45
16
33
113
220
29
20
21
4
24
19
21
8
21
9
1
23
61
29
32
3
18
23
64
22
18
18
11
3
58
8
23
6
6
14
6
3
1
26
8
30
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1
1
31
5
3
K78-79 K78-100
1
3
-
-
-
1
1
-
K76-24
K76-23
4
5
3
6
1
2
3
1 1
1
3
11
1
--
2
44
32
23 2
41
25
3
2
9
5
3
11
10
-
-
-
-
1
1
1
1
45
-
12
26 1
17
18
15
15
46
2
2
7
1
71
8
1
4
5
3
2
1
4
1
1
1
1
3
46
1
5
37
47
59
49
59
34
45
1
2
7
36
1 1
22
1
1
4 1
3
2
2
1
6
43
6
2
3
4
1
4
2
4
4
1 9
K76-22
1
1
1
1
2
5
42
4
1
3
2
3
1
2
1
2
1
1
1
2 12
K76-21
K76-20
K76-19
K76-18
K76-17
K76-16
K76-14
K76-13
K76-12
K76-11
K76-10
K76-9
4
2
K76-7
K76-8
1
K76-6
K76-5
1
1
41
6
8
1
2
5
1
64
K76-4
1
-
-
-
1
1
39 40
1
3
1
1
6 1
6
1
2
4
4
2
1
1
2 1
3
1
2
1
2
37 38
1
-
-
1
1
K76-3
1
5
K76-2
--
33 34 35 36
32
Sample
3
3
2
-
1
1 1
4 -
3
K78-SM
-
2 1
-
1
1
1
-
1
2
1
K78-MB
-
9
K78-65
K78-66 K78-76
K78-108
7
12
K78-63
7
K78-56
K78-59
-
3
K78-53
1
2
1
K78-30
K78-10
-
-
1
-
K78-3
K78-2
1
K76-68
K76-69
11
12
K76-67A
~
.
5
8
1
2
3
2
47
4
1
2
1
1
--
48
-
-
-
1
-
49
1 2
1
-
.
--
1
50
.
-
-
1 2
-
1
4
1
51
-
2
2 1
5
14
-
2
7
17
.
-
1
1
52
4
3 1
1
3
7
7
5
.
53
1
1
4
2
.
3
54
1
1
6 2
5
17
22
6
7
1
1
4
4
5
5
8
1
2
5
5
2
1
55
-
3
.
.
1
-
1
1
56
2
7
15
10
5
15
8
2
30
1
57
3
7
15
16
15
8
40
23
3
1
12
2
61
1
2
1
-
-
1
5
16
22
32
15
18
61
64
2
4
12
58
-
4 19
1
1
59
-
-
-
10
-
1
1
1
2
-
60
1
1
2
177
38 5
63
28
36
46
30
1
29
23
41
2
32
1
1
1
1
3
6
23
61
-
64
85
85
101
85
76
3
18
-
-
-
1 -
48
.
K78-30
.
.
-
.
.
.
.
.
.
.
-
1
2
1
1
1
1
7
6
.
.
.
.
.
.
-
.
-
1
.
3
1
1
aSee Table III for names
K78-SM
K78-MB
K78-108
K78-100
of taxa
.
-
1
1
.
1
.
.
9
3
-
3
2
K78-66
.
.
-
.
.
.
1
1
K78-79
2
K78-65
1
1
2
2
1
.
.
.
.
.
1
1
1
.
.
.
.
.
-
-
2
1
.
1
10
1
7
2
37 38
K78-76
1
K78-63
K78-59
K78-56
.
-
K78-10
K78-53
-
K78-3
K78-2
1
2
K76-68
K76-69
1
3
K76-67A
K76-67
K76-66
K76-64
K76-63
.
.
1
.
.
2
.
.
K76-62
.
-
3
1
1
33 34 35 36
K76-60
K76-59
K76-58
K76-57
K76-55
K76-53
K76-52
K76-51
.
1
K76-50
1
-
32
(continued)
K76-31
I
K76-30
K76-26
K76-25
Sample
TABLE
.
.
.
-
.
1
-
2
1
1
.
-
-
-
.
.
.
1
1
3
.
1
.
4
1
10
-
39 40
.
.
.
.
2
1
2
.
1
6
2
1
6
3
3
6
6
1
1
6
41
.
.
. .
-
.
.
.
1
1
42
.
.
.
.
.
.
2
.
43
.
.
.
1
1
1
44
.
.
.
.
1
1
1
1
1
5
12
1
1
9
45
.
-
-
-
-
-
.
-
-
1
1
1
3
1
46
.
.
.
.
2
2
1
1
2
6
2
7
1
2
.
43
1
1
1
47
. .
.
.
.
.
1
48
. .
.
.
.
.
.
.
1
1
49
.
.
.
.
.
.
.
1
1
1
1
50
.
.
.
.
.
.
.
.
.
1
2
18
1
51
.
.
.
.
.
1
52
.
.
.
.
.
.
1
53
.
.
. .
. .
1
2
.
54
.
.
-
.
.
1
1
55
.
.
-
-
.
2
56
.
-
-
-
1
58
1 -
2
ii
57
5
59
1
1
60
-
-
1
61
O0
85 blue-green algae are present in most of these ponds, even when only the edges are melted, and desiccated algal material is common among the sediment and marine-fossil debris of the MIS. Melt ponds in the adjacent Dry Valleys support a similar rich non-marine diatom flora associated with algal remains (Kellogg, D. et al., 1980). Brady (1978) first reported non-marine diatoms on the McMurdo Ice Shelf. Brady and Batts (1981) reported Pinnularia cymatopleura, Navicula shackletoni, Nitzschia westii (reported as N. antarctica, but not the homonym described by Okuno, 1954), Navicula quaternaria (reported as N. seminulum), and Tropidoneis laevissima, all associated with salt beds on the MIS near Black Island. We identified 53 taxa of non-marine diatoms in our MIS samples (Table I). The most important species (i.e., having high abundances and/or occurring in most samples) include: Nitzschia westii, Pinnularia cymatopleura, the Navicula species deltaica, gaussii, and quaternaria, and shackletoni, the Navicula muticopsis formae evoluta and reducta, Hantzschia amphioxys, and species of Melosira and Cyclotella.
Diatom assemblages Although MIS diatom data are not ideal for factor analytical analysis, because samples have different 14C ages, we performed Q-Mode factor analysis (Imbrie and Kipp, 1971) on our data (59 samples containing 61 species). Our objective was to establish common geographic or age groupings of sampling sites based on species assemblages. Although this analysis did resolve five assemblages, communalities for samples with low total diatom counts were very low and the cumulative variance was only 75%. We therefore reran the factor analysis after removing 14 samples with low abundances and communalities (see sample localities marked with solid triangles in Fig.l). Factor analysis with 45 samples containing 61 species resolved four assemblages with a cumulative variance of 90.316%. These four assemblages have the same species composi-
tions as the first four factors in the previous analysis. The main difference is deletion of the fifth factor (composed of marine species), which was apparently an artifact of a single sample (K76-63) with a low total count but dominated by marine species. Factor loadings for each assemblage are shown in Figs.3-6, and Table II. The sum of squares of loading values for each sample is termed the communality and equals unity for samples having a perfect fit into the assemblages calculated by this QMode varimax solution (Imbrie and Kipp, 1971). To determine the significance of individual loadings (i.e. the percentage of information explained by a particular assemblage), the values should be squared and multiplied by 100. A convenient rule is that a loading of 0.5 corresponds to 25%, 0.7 is ~50%, and 0.86 is 75%. Factor scores, explaining how each species contributes to each assemblage, are given in Table III. Assemblage 1 has a variance of 53% and is dominated by Nitzschia westii (Fig.3). Highest factor loadings occur in sites along the debris band that extends northwards from Black Island (especially the northern half of this band where only one sample has a loading less than 0.7), and in sites from the strait between Black Island and Brown Peninsula. Assemblage 2 has a variance of 23% and is comprised of Pinnularia cymatopleura and the Navicula species deltaica, gaussii, shackletoni, and quaternaria (Fig.4). Highest factor loadings (all >0.7) occur in samples from sites along the debris band separating Koettlitz Glacier ice from the McMurdo Ice Shelf. Elsewhere, loadings are mostly 0.5 or less. Assemblage 3 has a variance of 9.3~o and is dominated by the Navicula muticopsis formae evoluta, capitata, and reducta, as well as by the closely related Navicula muticopsiforme (Fig.5). Loadings >0.5 are restricted to 5 samples (K76-7, K76-62, K76-63, K76-67, and K78-100) from scattered localities in the strait between Black Island and Brown Peninsula, near the northern edge of the MIS, and on White Island. A scattered distribution of samples with high loadings would normally
86 TABLE II V a r i m a x factor m a t r i x Site
Communality
Assemblage 1
Assemblage 2
Assemblage 3
K76-2 K76-3 K76°4 K76-5 K76-6 K76-7 K76-8 K76-9 K76-10 K76-11 K76-12 K76-13 K76-14 K76-16 K76-17 K76-18 K76-19 K76-20 K76-21 K76-23 K76-24 K76-25 K76-26 K76-30 K76-31 K76-50 K76-51 K76-52 K76-53 K76-60 K76-62 K76-64 K76-67 K76-68 K78-2 K78-3 K78-53 K78-56 K78-59 K78-63 K78-65 K78-66 K78-79 K78-100 K78-108
0.979 0.914 0.891 0.856 0.959 0.772 0.898 0.813 0.968 0.849 0.947 0.946 0.969 0.962 0.986 0.934 0.851 0.950 0.945 0.975 0.901 0.979 0.980 0.611 0.960 0.982 0.954 0.931 0.953 0.749 0.780 0.838 0.802 0.732 0.954 0.929 0.955 0.946 0.953 0.936 0.934 0.958 0.972 0.728 0.862
0.978 0.844 0.822 0.841 0.934 0.361 0.928 0.833 0.931 0.769 0.823 0.925 0.912 0.913 0.889 0.875 0.442 0.854 0.853 0.746 0.416 0.737 0.952 0.751 0.838 0.929 0.894 0.783 0.863 0.519 0.570 0.389 0.177 0.271 0.810 0.923 0.240 0.348 0.258 0.230 0.443 0.374 0.943 - 0.076 0.164
0.090 0.368 0.224 0.155 0.230 0.214 0.161 0.308 0.250 0.458 0.488 0.251 0.306 0.310 0.331 0.365 0.624 0.382 0.378 0.519 0.711 0.561 0.215 0.097 0.388 0.288 0.288 0.339 0.306 0.566 0.096 0.227 0.505 0.727 0.505 0.122 0.919 0.878 0.911 0.890 0.816 0.884 0.248 - 0.043 0.884
Variance
53.003
23.908
9.300
4.105
Cumulative variance
53.003
76.911
86.211
90.316
0.108 0.255 0.401 0.353 0.184 0.770 0.081 0.132 0.078 0.152 0.177 0.151 0.177 0.159 0.106 0.091 0.004 0.106 0.141 0.119 0.068 0.067 0.069 0.190 0.209 0.119 0.111 0.394 0.213 0.102 0.667 0.797 0.702 0.242 0.173 0.239 0.065 0.083 0.070 0.226 0.238 0.167 0.133 0.847 0.174
Assemblage 4 - 0.041 - 0.040 0.063 0.001 - 0.020 - 0.051 - 0.069 - 0.083 0.180 0.156 - 0.020 0.076 0.110 0.082 0.273 0.164 0.516 0.253 0.235 0.368 0.466 0.340 0.153 - 0.039 0.251 0.150 0.244 0.217 0.262 0.385 0.038 0.013 0.151 0.266 0.115 0.080 0.221 0.218 - 0.225 - 0.201 - 0.118 - 0.090 - 0.053 0.048 0.154
87
TABLE
III
Scaled varimax No. a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
factor scores
Taxon
1
2
3
4
Achnanthes taylorensis Cyclotella comta Cyclotella glomerata Cyclotella stelligera Cyclotella s p p . Eunotia s p p . Hantzschia s p p . Melosira distans Melosira sulcata Melosira sp. " A " Melosira sp. " B " Navicula contenta N. contenta v. parallela Navicula deltaica Navicula gaussi Navicula gibbula Navicula minuscula Navicula mutica N. mutica v. cohnii Navicula muticopsiforme Navicula muticopsis N. muticopsis f. capitata N. muticopsis f. evoluta N. muticopsis f. murrayi N. muticopsis f. reducta Navicula quaternaria Navicula shackletoni Navicula s p p . Nitzschia westii Pinnularia cymatopleura Pinnularia s p p . Stauroneis anceps TabeUaria s p p . Tropidoneis s p p . Actinocyclus ingens Actinocyclus actinochilus Asteromphalus parvulus Coscinodiscus s p p . Denticulopsis sp. Eucampia antarctica Melosira sol Melosira sulcata Nitzschia closterium Nitzschia cylindra Nitzschia curta Nitzschia kerquelensis Nitzschia obliquecostata Nitzschia praeinterfrigidaria Nitzschia ritscheri Nitzschia separanda Nitzschia sublineata Rhabdonema s p p . Rhizosolenia s p p . Rouxia antarctica
- 0.184
- 0.419
- 0.460
0.900
- 0.000
0.000
0.000
0.000 - 0.249
0.111
0,007
- 0.011
- 0.105
- 0.125
0.850
0.150
0.420
- 0.027
- 0.042
- 0.895 - 0.015
- 0.001
0.012
- 0.002
- 0.104
- 0.314
2.113
0.311
0.045
0.191
- 0.002
- 0,218
0.050
- 0.025
- 0.001
- 0,055
- 0.046
0.230
- 0.032
- 0,240
0.282
0.053
0.137
0.115
0.395
- 0.054
0.129
0.138
0.417
0.556
0.910
- 1.244
0.237
1.725
1.036
1.933
- 0.094
3.470
0,988
- 2.767 - 0.134
0.081
0.008
0,111
- 0.003
0.006
0,001
0.029
0.015
0.006
- 0.007
- 0.041
0.062
-0.030
0,064
- 0.596
0.054
3.174
0.021
0.132
0.619
-0.107 0.890 -0.061
- 0.057
- 0.155
1.198
0.038
- 0.351
- 0,246
5.836
- 0.293
- 0.007
0,155
- 0.003
- 0.114
0.042
0.380
2.071
0.123
1.698
0.496
0.208
2.567
- 0.447
5.15{}
- 0,031
0.083
0.024
0.015
-0.456 3.743
7.7{}9
0.040
0,462
- 0.128
- 0,253
5.958
- 0.702
- 2.{}15
- 0,038
- 0.033
0,279
0.018
0,080
0.017
0.033
- 0.054
- 0.176
- 0.073
0.913
0.293
- 0.004
0.005
- 0.004
0.040
0.002
- 0.002
0.001
0.008
- 0.001
0,083
0.090
0.105
0.022
- 0.011
0.002
- 0.036
0.053
0.001
0.021
- 0.103
0.006
0.000
0.034
0.078
0.020
0.000
0.006
0.088
0.389
- 0.103
0.058
- 0,424
0,002
- 0.002
0.001
0.008
0.011
0.007
- 0.007
- 0.004
- 0.000
0.001
0.015
0.003
- 0.073
- 0.018
0.502
0.080
0.162
- 0.045
0.014
- 0.041
- 0.037
0.086
0.121
0.211
- 0.000
0.000
- 0.000
0.000
0.002
- 0.001
- 0.002
0.001
0.003
- 0.002
0.019
0.014
0.014
- 0.003
0.024
0.018
0.036
0.003
- 0.022
- 0.092
0.005
- 0.003
0.002
0.021
0.057
- 0.011
- 0.025
- 0.103
88
TABLE III
(continued)
No."
Taxon
55
Stephanopyxis turris Thalassionema bacillaris Thalassionema nitzschioides Thalassionema spp. Thalassiosira s p p . Thalassiothrix s p p . Trinacria excavata
56 57 58 59 60 61
1
2
3
4
0.000
- 0.004
0.018
0.007
- 0.010
0.025
0.005 0.009
- 0.017
- 0.041
0.296
0.054
- 0.000
0.000
- 0.000
0.000
- 0.005
0.019
0.012
0.024
0.002
- 0.002
0.001
0.008
0.006
- 0.002
- 0.003
- 0.010
aSee T a b l e I.
suggest an invalid assemblage. We suspect that Assemblage 3 may be meaningful because all these samples (except K78-100) have 14C ages between 2100 and 3500 yr B.P., or are adjacent to sites having this age range. This suggests that environmental conditions were similar at these locations during that interval. Assemblage 4 has a variance of 4.1% and is characterized by Navicula shackletoni and N. quaternaria and by the absence of other species in Assemblage 2 (Fig.6). Factor loadings for this assemblage exceed 0.5 at only one sample site (K76-19) and any positive value >0.2 ( ~ 4 % ) may be considered high. These "high" values are restricted to the northern half of the Koettlitz debris band and the middle and southern part of the Black Island band.
Factors controlling diatom assemblages An unequivocal explanation for the distributions of the four diatom assemblages is not currently possible but at least four considerations deserve attention. These are, differences in age, geographic locations of individual sites, micro-geochemical environments, and seasonal blooms. Age Nearly 50 radiocarbon dates are now available from shell and algal samples collected from the surface of the MIS (Stuiver et al., 1981b; Stuiver and Braziunas, 1985; Kellogg, T. et al., in prep.). Uncorrected ages range from
570__+60 yr B.P. to 45,500_+ 1500 yr B.P., with the majority between 1200 and 6600 yr B.P. For radiocarbon-dated sites where diatoms were studied (Table IV), the dates should be considered minimum ages for marine diatom specimens, because dates were obtained on macrofossil remains associated with marinediatom bearing sediment. For these same sites, 14C ages should be considered as maximum ages for non-marine diatoms because these species were introduced to MIS sediments after the dated marine biota and sediments melted out at the ice-shelf surface. At least four factors complicate interpretation of ages for the diatom assemblages: (1) The non-marine diatom assemblage at any particular location potentially represents an accumulation of specimens from all the melt ponds that have existed in the immediate area since formation of the ice comprising the ice shelf at the locality (i.e., each sample contains a mixture of non-marine specimens of different ages). (2) The sample site will have moved during this time (e.g., ice now at sites near the northern end of the Black Island debris band probably formed along the north coast of Black Island), possibly causing changes in diatom species' dominance related to changing location and/or local geochemical environment. (3) Diatom assemblages are probably modified by wind action, resulting in both removal of specimens, and addition from adjacent snowand ice-free diatom-bearing locations. An indication that wind is a significant transport mechanism in this area is the occurrence of
89
sand-sized and smaller material scattered widely over large areas of the McMurdo Ice Shelf that are free of debris bands. (4) Some non-marine (and marine) specimens present in our samples may be considerably older than 165 °
radiocarbon ages for the sites where they occur. These reworked specimens may be derived from older deposits exposed to wind action on land, or they may be mixed in with marine species in the sediments beneath the ice 166°E
-
McMurdo
Sound
77045 `
'%
-,
,
k~..
\x
~'\ .75
78 °
-,08
'!it / ~, I,r
.Z~p ~ Brow .44
.37 ~/%0
Peninsula
•%,
i
,16 .52
i '.g4
C<-
3,oc~IsLond
1.39
(/: ,15'
L.,..J-~' Mt Discovery
ASSEMBLAGE
z~
I
2
co
5
0
5
Sca~e
~0
15km
Mir)r)a ~
~
~
/"~,"~•
Q'~ 7 8 ° 30'"
I
Fig.3. Factor loadings for Diatom Assemblage 1 (Nitzschia
i
westii). Note highest loadings along the Black Island debris band.
90
late Wisconsin glaciation (Stuiver et al., 1981b), and containing marine macrofossil fragments dated at 35,4004-1500 yr B.P. (Kellogg, T. et al., in prep.). The diatom assemblage
shelf. Possible evidence for this last effect comes from site K78-100 on White Island. The sediment here is glacial drift deposited by ice moving westward from the Ross Sea during the i
I
165 °
166°E
McMurdo
Sound
77o45
'-
I/ i , 78o
-
.ss -.0,4 .91
.89 .SZ • 88 i
Peninsulo
.S8
[".2 5
3lock Islond
)
.22,
5,.
Mr.
ASSEMBLAGE
Discovery
A
2 a~
5 .-.
0 ,--I
5 I
10
i
15krn
I
Sco~e
78 ° 30'i
,g~l
Fig.4. Factor loadings for Diatom Assemblage 2 (Pinnularia cymatopleura, Navicula gaussii, and Navicula deltaica). Note highest loadings along the debris band marking the end of Koettlitz Glacier.
91
White Island, we suspect that they were present in Ross Sea sediments prior to the glacial advance because our analyses of Recent sediments and older diamicton at numerous loca-
consists of only 6 specimens, all of non-marine species. Although we can not rule out the possibility that these specimens were introduced after deposition of the drift on northern I
I
166°E
165 °
McMurdo
Sound
77o45
' -
', ,,
, .~::'-~..,~. ,,
7B °-
l, .OS .85 .0~
i
Ii' 'J' 1
.17
I' i'
Peninsula
.IT~
I
',.J\ . j , .10
31ac~ Island ~° 78
Mt Discovery
ASSEMBLAGE
z~
15'.
5 q~
# 5
0
5
Scale
I0
15km
Mi~
o 78 ° 30"
I
I
Fig.5. Factor loadings for Diatom Assemblage 3 (Navicula muticopsis f o r m a e evoluta, reducta, and capitata, and Navicula
muticopsiforme).
92
Geographic factors
tions on the Ross Sea continental shelf commonly reveal rare non-marine specimens (Kellogg, T. and Truesdale, 1979; Truesdale and Kellogg, 1979; Kellogg, T. and Kellogg, D., 1981).
The debris-covered Surface of the McMurdo Ice Shelf is an ablation zone which has its
I
I
165 °
~
166°E
McMurdo
i67o
Sound
77 °
45' -
/'-'V'--,,~\, ~'..\ o:o- ~\-.o5
<
/.o
,~...~.--.
,
-
,V
'~ \ , ,~'
~
, , :,
~\ I [ .26 .-,02 " .08
,, la, ' ~",,~
~. '...t t.
" ' '
!1~,
78 ~ . ,
.22
.52 .05
-.2,.
".2O~[Bro~ -.t2 A /
-ogF//~
O
i IIi
~/Peninsulo
.15
.39
-r", :' - , 0 5
Islond o15,_
Mr.
ASSEMBLAGE
Discovery
4
A %.
5
t--, ,7
0
,-t
5
I0
I
J
15km
t
Scale
78 ° 301-~
i
"-'~---~-.,.
,l""ll
I
Fig.6. Factor loadings for Diatom Assemblage 4 (Navicula quaternaria and Navicula shackletoni). Note highest loadings near the southern end of the Black Island debris band.
93 TABLE IV Radiocarbon dates of samples analysed for diatoms Site
Lab. no.
Date and error
K76-4 K76-18 K76-26 K76-55 K76-58 K76-59 K76-62 K76-64 K76-66 K76-67A K78.100 K78-108
QL-1126 6510 __50 QL-1127 4630+80 QL-1128 1260 + 30 QL-1222 3590-4-_80 QL-1129 > 51,000 QL-1132 3610_+40 QL-1130 3770__40 QL-1224 4410__90 QL-1225 1340 ___30 QL-1226 3280_300 QL-1451 35,400_ 1500 QL-1453 720 + 60
Ref.a
Material
Lat. (S)
Long. (E)
Comments
1 1 1 1 2 1 1 3 1 3 3 3
Barnacle Barnacle Barnacle Barnacle Barnacle Serpulids Barnacle Barnacle Bryozoa Shells Barnacle Bryozoa
77o50.5' 77°59.9' 78°05.9' 78°11.0' 78o29.2' 78°12.0' 78°08.8' 78°13.5' 77°52.0' 77o53.5' 78°02.0' 78009.0'
165°47.0' 166°02.5' 166°05.0' 166o46.0' 167004.0' 166°45.0' 165o44.0' 165°40.0' 165°15.0' 165°04.0' 167o24.0' 165°10.0'
On Black Island band On Black Island band Near N. Black Is. on Black Is. Band North of east end of Black Island North of end of Minna Bluff Off east side of Black Island Between Brown Pen. and Black Island Between Brown Pen. and Black Island Near eastern Dailey Island Dailey Islands White Island, Speden (1962) Site 19 West of Brown Pen. on Koettlitz band
al =Stuiver et al. (1981); 2 = Kellogg and Truesdale (1979); 3= Kellogg et al. (in prep.). Barnacle = Bathylasma corolliforme (Hoek); "Shells" = mixed bryozoa, serpulids, barnacles, etc. e a s t e r n b o u n d a r y n e a r a l i n e r u n n i n g from t h e s o u t h e r n e n d of H u t P o i n t P e n i n s u l a t h r o u g h the strait between Black and White islands ( S w i t h i n b a n k , 1968). S t u i v e r et al. (1981b) a t t r i b u t e d t h i s a b l a t i o n to a d i a b a t i c a l l y w a r m e d k a t a b a t i c w i n d s f l o w i n g d o w n a n d to t h e n o r t h from Mt. D i s c o v e r y a n d M i n n a Bluff. W i n d p a t t e r n s m a y be p a r t l y r e s p o n s i b l e for t h e d i s t r i b u t i o n s of A s s e m b l a g e s 1 a n d 2, w h i c h a p p e a r a l m o s t as o p p o s i t e s . S a m p l e s i t e s a l o n g the debris band marking the boundary with K o e t t l i t z G l a c i e r ice s h o u l d be a f f e c t e d by winds flowing down Koettlitz Glacier and the w e s t f l a n k s of Mt. D i s c o v e r y ; t h o s e from t h e d e b r i s b a n d n o r t h of B l a c k I s l a n d s h o u l d experience winds blowing down the east flanks of Mt. D i s c o v e r y a n d M i n n a Bluff a n d t h r o u g h the strait between Black Island and Brown P e n i n s u l a . In a d d i t i o n , t h e B l a c k I s l a n d d e b r i s b a n d is c l o s e r to s e a s o n a l l y o p e n w a t e r in McMurdo Sound than the Koettlitz Glacier band. G e o g r a p h i c s e t t i n g s of i n d i v i d u a l s a m p l e s m a y a l s o i n f l u e n c e d i a t o m s p e c i e s i n d i r e c t l y by modifying temperature. Areas with continuous and/or thick debris cover should absorb more solar radiation than areas with patchy or thin d e b r i s cover. T h i s m e c h a n i s m m i g h t c a u s e t h e differences noted between Assemblages 1 and
2. T h e f o r m e r a s s e m b l a g e o c c u r s p r i m a r i l y on t h e B l a c k I s l a n d d e b r i s b a n d w h e r e s e d i m e n t is c o n t i n u o u s a n d u s u a l l y s e v e r a l c e n t i m e t e r s in t h i c k n e s s . S e d i m e n t is p a t c h y a n d t h i n a l o n g the Koettlitz debris band. Sediment distribut i o n m a y a l s o a f f e c t d i a t o m d i s t r i b u t i o n s by p r o v i d i n g a h a b i t a t for a e r o p h i l i c s p e c i e s (e.g.
Hantzschia amphioxys, Navicula mutica, N. gibbula, a n d N. contenta f. parallela: see K e l l o g g , D. et al., 1980), w h i c h live in soils.
Chemical factors A l l n o n - m a r i n e s p e c i e s e n c o u n t e r e d in M I S s e d i m e n t s o c c u r l i v i n g t o d a y in l a k e s a n d p o n d s in t h e D r y V a l l e y s , w h e r e s a l i n i t y c o n d i t i o n s r a n g e from fresh to s u p e r s a l i n e ( K e l l o g g , D. et al., 1980). T h e y a r e n o t k n o w n to l i v e in m a r i n e w a t e r s . T h e g e o c h e m i s t r y of D r y V a l l e y l a k e s a n d p o n d s is e x t r e m e l y v a r i a b l e but, as yet, no e v i d e n c e is a v a i l a b l e on h o w t h i s g e o c h e m i c a l v a r i a b i l i t y affects individual diatom species' distributions. No g e o c h e m i c a l s t u d i e s of M I S m e l t p o n d s have been made, but v a r i a b i l i t y does occur. B r a d y a n d B a t t s (1981) d e s c r i b e d beds of m i r a b i l i t e s a l t s (Na2SO4.10H20), w i t h associa t e d n o n - m a r i n e d i a t o m s , on t h e ice s h e l f n e a r B l a c k I s l a n d . T h e y a t t r i b u t e d t h e s e b e d s to
94 uplift of sub-ice-shelf brines, by Debenham's (1919) basal freezing mechanism. We visited these salt beds (near K76-60), but have not observed salt deposits elsewhere on the MIS. Nevertheless, pockets of brine were noted within the ice (Heine, 1968; Kovacs and Gow, 1975), and could influence the geochemistry of melt ponds even if the salts are not sufficiently concentrated to precipitate beds. Another possible factor controlling the geochemistry of MIS melt ponds is their position relative to seasonally open water in McMurdo Sound. The eastern side of McMurdo Sound becomes ice free during most austral summers. Ice on the western side does not break up every year, and when it does, the interval with ice free conditions is considerably shorter than for the eastern side. The possibility thus exists for introduction of marine salts by atmospheric transport of aerosols during the short austral summers at times with northerly winds. This factor should be variable from east to west because of variations in ice extent, and from north to south because of distance from open water. We suspect that this mechanism is partly responsible for the differences in diatom dominance between the Koettlitz and Black Island debris bands (Assemblages 1 and 2) and along these same debris bands (Assemblage 4). Each tiny pond may have its own peculiar chemical characteristics, related to location and to introduction of salts from below, which may be more conducive to blooms of one of the diatom assemblages than to the others. Seasonal blooms
Diatom assemblages in our sediment samples may reflect seasonal blooms [e.g., Assemblage 1 might dominate in the early austral summer; Assemblage 2, mid-summer; etc. (and/or vice versa)]. In this case, our samples might represent fortuitous collections representing short bloom intervals rather than average annual or multi-year composite floras. While this explanation is possible, we regard it as unlikely because of the complex factors described above that should result in multi-season samples.
Discussion Because of the co-occurrence of locally abundant marine macrofossils in many of our samples, we doubt that significant numbers of marine diatoms are introduced to MIS sediments by winds blowing across adjacent diatom-bearing deposits. Thus, the presence of marine diatoms on the MIS is consistent with the hypothesis of a marine origin for this ice shelf, and with Debenham's (1919) mechanism of basal freezing and surface ablation to transport sediment and biota to the ice shelf surface. Because marine diatoms are usually rare in MIS sediment samples, they are unsuited for detailed and quantitative studies of the Holocene paleo-oceanography of southern McMurdo Sound and of waters beneath the ice shelf. Non-marine diatoms, known to tolerate the wide salinity variations of the dry valley lakes and ponds, are abundant and diverse in surface melt ponds of the ablating MIS. Distributions of the four diatom assemblages are probably controlled by some combination of local geochemical variability, seasonal blooms, geographic position on the ice shelf surface, and sediment (and diatom) age. The mixed biota of MIS sediments provides a source for introduction of marine and nonmarine material to the Ross Sea floor. This occurs in two ways: First, and important only locally, mixed biota and sediment on the iceshelf surface may be introduced (and/or reintroduced) to the marine environment through the agent of melt streams flowing off the front of the ice shelf or through cracks and crevasses. Second, and important regionally, icebergs calved from the McMurdo Ice Shelf release their load of mixed biota and sediments when they melt. Recent history shows that while this mechanism is not of great importance at present, because few icebergs are calving from the McMurdo Ice Shelf today, it may have been important at times in the past and may occur in other locations today. This study therefore provides evidence for the action of a new mechanism for producing
95
mixed marine and non-marine floras: transport of marine material upward through an ice shelf by the combined agents of basal freezing and surface ablation, augmented by the addition of non-marine diatoms and algae growing in melt ponds on the ice-shelf surface. This mechanism should also operate on the surfaces of other present and former antarctic ice shelves where the basal-freezing process occurs (e.g. Terra Nova Bay and George VI Sound). We suspect that similar processes of sediment mixing may be responsible for the origin of analogous mixed diatom floras we have begun to describe from sediments drilled recently in the Dry Valleys (Kellogg, D. and Kellogg, T., 1984), and on the origin of diatom-bearing deposits of the Vestfold Hills (Setty et al., 1984; Pickard, 1985). MIS sediments are obviously unique. Indeed, it is hard to imagine any other geologic setting which consists of a mixture of marine and nonmarine micro- and macrofossils of different age, which moves laterally through time into areas with possibly different environments, and is subject to the addition of reworked and unreworked specimens from below and above and to removal of specimens from above. We can say with certainty only that these competing factors should be less significant for nonmarine species in samples with the youngest radiocarbon ages. For marine species, because the time for upward transport of material through the ice (by basal freezing and surface ablation) is geologically instantaneous ( ~ 1 0 0 y r for 50-m thick ice at a measured ablation rate of 0.5m y r - l : Gow, 1967), corrected radiocarbon ages should represent the residence time of the marine material at that site on the ice shelf surface.
Acknowledgements We thank the helicopter pilots and crew of Antarctic Development Squadron VXE-6 for their enthusiastic support of our field work. George Denton, and Terry Hughes read drafts of the manuscript and provided encouragement and stimulating ideas throughout the course of this work. This project was supported by
National Science Foundation grants OPP7515524, DPP77-21083, and DPP-8020000. References Brady, H. T., 1978. Geological studies in southern Victoria Land, on Black Island, and on the McMurdo Ice Shelf. Antarct. J. U.S., 13: 13-14. Brady, H. T. and Batts, B., 1981. Large salt beds on the surface of the Ross Ice Shelf near Black Island, Antarctica. J. Glaciol., 27: 11-18. Dayton, P. K., Robilliard, G. A. and DeVries, A. L., 1969. Anchor ice formation in McMurdo Sound, Antarctica, and its biological effects. Science, 163: 273-274. Debenham, F., 1919. A new mode of transportation by ice. Q. J. Geol. Soc. London, 75(2): 51 76. Gow, A. J., 1967. Antarctic glaciological studies. Antarct. J. U.S., 2: 121-122. Gow, A. J. and Epstein, S., 1972. On the use of stable isotopes to trace the origins of ice in a floating ice tongue. J. Geophys. Res., 77: 6553-6557. Heine, A. J., 1968. Brine in the McMurdo Ice Shelf, Antarctica. N.Z. J. Geol. Geophys., 11:829 839. Imbrie, J. and Kipp, N. G., 1971. A new micropaleontological method for quantitative paleoclimatology: application to a late Pleistocene Caribbean core. In: K. K. Turekian (Editor), Late Cenozoic Glacial Ages. Yale Univ. Press, New Haven, Conn., pp. 71-181. Kellogg, D. E. and Kellogg, T. B., 1984. Preliminary diatom results from Eastern Taylor Valley drill cores. Antarct. J. U.S., 19: 45-47. Kellogg, D. E. and Kellogg, T. B., 1986. Diatom biostrati. graphy of sediment cores from beneath the Ross Ice Shelf. Micropaleontology, 32:74 94. Kellogg, D. E., Stuiver, M., Kellogg, T. B. and Denton, G. H., 1980. Non-marine diatoms from Late Wisconsin perched deltas in Taylor Valley, Antarctica. Palaeogeogr., Palaeoclimatol., Palaeoecol., 30: 157-189. Kellogg, T. B. and Kellogg, D. E., 1981. Pleistocene sediments beneath the Ross Ice Shelf. Nature, 293:130 133. Kellogg, T. B. and Truesdale, R. S., 1979. Late Quaternary paleoecology and paleoclimatology of the Ross Sea: the diatom record. Mar. Micropaleontol., 4: 137-158. Kellogg, T. B., Stuiver, M., Kellogg, D. E. and Denton, G. H., 1977. Marine microfossils on the McMurdo Ice Shelf. Antarct. J. U.S., 12:82 83. Kellogg, T. B., Kellogg, D. E. and Stuiver, M., in prep. Origin and significance of debris bands on the McMurdo Ice Shelf. Klovan, J. E. and Imbrie, J., 1971. An algorithm and FORTRAN-IV program for large-scale Q-mode factor analysis and calculation of factor scores. Math. Geol., 3: 61 77. Kovacs, A. and Gow, A. J., 1975. Brine infiltration in the McMurdo Ice Shelf. J. Geophys. Res., 80: 1957-1961. McCollum, D. W., 1975. Diatom biostratigraphy of the Southern Ocean. In: Initial Reports of the Deep Sea Drilling Project, Leg 28. U.S. Government Printing Office, Washington, D.C., pp. 515-571.
96 Okuno, H., 1954. Electron microscopical study on Antarctic diatoms. J. Jap. Bot., 29: 18-25. Pickard, J., 1985. The Holocene fossil marine macrofauna of the Vestfold Hills, East Antarctica. Boreas, 14: 189-202. Schrader, H. J., 1974. Proposal for a standardized method of cleaning diatom-bearing deep-sea and land-exposed marine sediment. Nova Hedwigia, Beih., 45: 403-409. Scott, R. F., 1905. "The Voyage of the 'Discovery'." Smith and Elder, London, 2 vols. Scott, R. F., 1914. "Scott's Last Expedition" Arranged by Leonard Huxley. Smith and Elder, London, 2 vols. Setty, M. G. A. P., Kellogg, D. E. and Kellogg, T. B., 1984. Preliminary report on diatoms from the Deep Lake terraces, Vestfold Hills, Antarctica. Antarct. J. U.S., 19: 88-90. Speden, I. G., 1962. Fossiliferous Quaternary deposits in the McMurdo Sound region, Antarctica. N . Z . J . Geol. Geophys., 5: 746-777. Stuiver, M. and Braziunas, T. F., 1985. Compilation of isotopic dates from Antarctica. Radiocarbon, 27(2A), 304 pp.
Stuiver, M., Yang, I. C., Denton, G. H. and Kellogg, T. B., 1981a. Oxygen isotope ratios of antarctic permafrost and glacier ice. (Antarct. Res. Ser., 33) Am. Geophys. Union, pp. 131-139. Stuiver, M., Denton, G. H., Hughes, T. J. and Fastook, J. L., 1981b. History of the marine ice sheet in West Antarctica during the last glaciation: a working hypothesis. In: G. H. Denton and T. J. Hughes (Editors), The Last Great Ice Sheets. Wiley, New York, N.Y., pp. 319-346. Swithinbank, C. W. M., 1968. Ice movement in the McMurdo Sound area of Antarctica. In: Int. Syrup. Antarctic Glaciological Exploration (ISAGE), Int. Assoc. Sci. Hydrol. SCAR, Publ., 86: 472-487. Taylor, G., 1914. The Western Journeys I: Koettlitz, Ferrar, and Taylor Glaciers. In: R. F. Scott, Scott's Last Expedition. Smith and Elder, London, vol. 2, pp. 182221. Truesdale, R. S. and Kellogg, T. B., 1979. Ross Sea diatoms: modern assemblage distributions and their relationship to ecologic, oceanographic, and sedimentary conditions. Mar. Micropaleontol., 4: 13-31.
77
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
DIATOMS OF THE McMURDO ICE SHELF, ANTARCTICA: IMPLICATIONS FOR SEDIMENT AND BIOTIC REWORKING 1 DAVIDA E. KELLOGG and THOMAS B. KELLOGG Institute for Quaternary Studies and Department of Geological Sciences, University of Maine, Orono, ME 04469 (U.S.A.) (Received M a r c h 25, 1986; revised and accepted October 29, 1986)
Abstract Kellogg, D. E. and Kellogg, T. B., 1987. Diatoms of the McMurdo Ice Shelf, Antarctica: implications for sediment and biotic reworking. Palaeogeogr., Palaeoclimatol., Palaeoecol., 60: 77-96. In the course of investigations of the origin of the McMUrdo Ice Shelf (MIS), we analyzed diatoms in 59 sediment samples collected from its upper surface. Fragments of centric marine species, most of which occur in the Ross Sea today, were found in all samples, but identifiable marine taxa dominated the total diatom flora in only two samples. In contrast, non-marine diatoms are a b u n d a n t and diverse in most MIS samples. These non-marine species probably live in the numerous melt ponds which are distributed widely over the shelf surface during the warmest few weeks of each austral summer. Non-marine diatoms are clustered in four assemblages, which are probably related in a complex way to geographic position on the ice shelf, sediment age, local geochemical variations, and seasonal blooms. The presence of marine diatoms in MIS sediments is consistent with isotopic evidence t h a t the MIS consists of frozen seawater, and with Debenham's hypothesis t h a t transport of marine macrofossils to its surface is accomplished by freezing on at the base and upward movement t h r o u g h the shelf caused by surface melting. The combined marine and non-marine diatom floras of the MIS suggest a new mechanism to explain mixed diatom floras and sediments, observed in antarctic cores. These composite floras and sediments may be introduced or reintroduced to the marine environment during austral summers by melt water streaming off the front of the MIS or through crevasses. Additionally, icebergs calved from the MIS release their load of mixed biota and sediment when they melt. These observations may pertain to other present and former antarctic ice shelves t h a t are characterized by surface ablation and basal freezing.
Introduction The McMurdo Ice Shelf, located in the southwest corner of the Ross Sea (Fig.l), is bounded by Minna Bluff and the mountains of Southern Victoria Land on the west and south, and by Ross Island and the Ross Ice Shelf on the northeast and east. The waters of McMurdo Sound, which become partly or
~This paper is dedicated to the memory of F r a n k Debenham, who first proposed uplift of marine organisms by basal freezing and surface melting of the Pinnacled Ice. 0031-0182/87/$03.50
completely free of pack ice during the austral summer, lie to the north. Brown Peninsula, snow-covered White Island, and bare volcanic Black Island, are surrounded by the MIS. To the first-time observer, the most notable features of the MIS are the distinctive bands and patches of debris that mantle much of its surface (Fig.l). Scott (1905, p. 154) first described the McMurdo Ice Shelf, and the term "Pinnacled Ice" was used during his last expedition to describe the entire area and the lower Koettlitz Glacier (Scott, 1914). Although Scott's term has priority and conveys a more evocative description of the area, we use "McMurdo Ice Shelf" (MIS) because ice pin-
© 1987 Elsevier Science Publishers B.V.
78
nacles are developed only locally (Kellogg, T. et al., in prep.). Perhaps the most evocative image of the surface was given by Taylor (1914, p. 205):
"Most picturesque in appearance, but as a sledging proposition it can only be described as infernal!" Today this area is approached only by helicopr
i 165 °
166°E
McMurdo
Sound
77o45 '
K78-SM
ROSS
SEA
K76-7
ROSS
.K76-9
K76-12~ K76-13 K76-14
L>.V %. '.\
YY
• LK76-16 \/ K 7 6 - 1 7 K76-1e
7 8 ~-
. K76-19
K78-56
K78-100 KTB-59 --
K78-63 K7S-65 K78-66
K76-24
,
"I,
kjK~s-2~ ~rs-~ Ii,l
K78-1C
Peninsula
White K78-108
Island
K76-60
K76-59
/ K76- 64
Black Island ¢~"~a .
~
~"
, 15'
K76-63
Mt Discovery A
g: 5
~1V'l'i
0 I" l j
5
10
t
I
Sco~e
15km ~
K7S-57
~'~i~h O
~...,~'--~\
KTe-ss K78-MB •
,~ 78 °
I
30"
I
Fig.1. Map of the Ross Sea region showing the location of the McMurdo Ice Shelf, localities mentioned in the text, and sampling sites on the McMurdo Ice Shelf (solid triangles indicate samples deleted from our second factor analysis).
79 ter, and a field party can cover perhaps 5-10 miles a day across its treacherous terrain on foot. The MIS is approximately 10-50 m thick north of the strait between Black Island and Brown Peninsula, and up to 110 m thick north of Minna Bluff (Swithinbank, 1968), in contrast to the main body of the Ross Ice Shelf, where the thinnest ice (>100m) occurs at the ice shelf margin. In contrast to the Ross Ice Shelf which is maintained largely by precipitation, basal freezing of sea water contributes significantly to the mass balance of the McMurdo Ice Shelf. This mechanism was hypothesized by Debenham (1919) to explain the presence of fossil remains on the surface. He suggested that basal freezing in shallow water permitted the ice to pick up marine sediments and organisms, and that through surface ablation this material would, in time, move to the ice shelf surface and accumulate there. Support for the hypothesis that ice accretion occurs at the base of the MIS comes from several additional lines of evidence. Gow and Epstein (1972) showed that the isotopic composition of ice in cores drilled north of Koettlitz Glacier was consistent with a sea-water origin. Heine (1968) and Kovacs and Gow (1975) described pockets of brine in the ice shelf at selected locations. Isotopic analyses of over 100 surface ice samples we collected at widely scattered localities show that the ice is composed almost entirely of frozen sea water (Stuiver et al., 1981a; Kellogg, T. et al., in prep.). What is still not known is whether marine sediments and organisms are picked up only in shallow areas where the ice shelf is grounded, as along the shores of Black Island and Brown Peninsula, or if the anchor ice mechanism (Dayton et al., 1969) permits addition of marine material in deeper water. Oliver (pers. comm., 1984) has observed anchor ice only in shallow water ( < 50 m) in McMurdo Sound. This observation is in agreement with occurrences of anchor ice noted by Scott (1905, p. 380). Because most of the shelf is floating on water
deeper than 500 m, assuming that isobaths in McMurdo Sound are representative for that part of the Sound covered by the MIS, we suspect that most of the debris and fossil material is incorporated into the ice shelf in shallow water. In this paper, we present the results of our diatom analyses of sediment samples collected from the surface of the MIS during four field seasons. These data constitute evidence for a new mechanism for producing mixed marine and non-marine diatoms assemblages in highlatitude marine sediments.
Objectives and methods Our field work was conducted during parts of the 1975-1976, 1976-1977, 1978-1979, and 1981-1982 austral summers. We concentrated our efforts on the band trending north from Black Island during the first two seasons, and expanded our efforts to include other portions of the MIS during later years. In all, we spent over three months on the MIS. Sample locations are shown in Fig.1. At each location, we sampled sediment, ice, macrofossil remains (when present), algal material (both desiccated and in melt ponds, when accessible), sieved sediment to concentrate microfossils, and searched for radiocarbon-datable shell material. Sample locations were determined by triangulation. Our original objective was to collect microfossil data from radiocarbon dated samples for interpretation of the past history of McMurdo Sound (Kellogg, T. et al., 1977), but we did not realize that non-marine species dominate in MIS sediments. We were aware from the start that non-marine diatoms should also be present in many of our sediment samples, complicating our effort to distinguish marine assemblages, because thousands of small melt ponds cover the ice-shelf surface. In order to obtain a clear picture of the nature of modern nonmarine assemblages to which to compare our fossil material, we also collected melt-water samples and both fresh and desiccated algae at many locations.
80 Diatom preparations of sediment samples were made using the method of Schrader (1974) and slides were examined with a Leitz Dialux20 microscope at 1000 x. For each slide, we made a census of diatom species present. Nonoverlapping traverses of the slide were analyzed until 300 specimens had been observed or, when diatom abundances were very low, until approximately 20 traverses (approximately the entire slide) had been examined. Because of the large number of both samples and species, it was necessary to use factor analysis to resolve which species consistently occurred together. We used the Q-mode program CABFAC (Klovan and Imbrie, 1971) for clustering purposes. Marine diatoms
Fragments of centric marine diatom species are present in all our samples, but complete specimens are rare and marine species dominate the total diatom assemblage at only 4 of the 59 sites (K76-22, in which 64% of 91 specimens counted were marine; K76-55, in which 97% of the 116 specimens counted were marine; K76-63, in which 100% of 12 specimens counted were marine; and K76-69, in which 81% of the 74 specimens counted were marine). Non-marine species dominate at the remaining sites, comprising more than 90% of each sample with few exceptions (Fig.2). Our results expand on the work of Brady and Batts (1981), who also studied diatoms from the MIS. They reported (p. 13) "...many fragments of marine diatoms and sponge spicules...and some fragments belong to the Thalassiothrix/Thalassionema genera." They also identified N. kerguelensis fragments from salt beds near Black Island. Melosira sol, Nitzschia kerguelensis, and fragments of Thalassionema nitzschiodes are present in most of our samples (Table I). Other commonly occurring marine species include: Nitzschia curta, Nitzschia obliquecostata, Nitzschia sublineata, Eucampia antarctica, and Actinocyclus actinochilus. Most of the marine species we report here live today in the Ross Sea
(Truesdale and Kellogg, 1979), and some (e.g. N. curta and A. actinochilus) were thought to be Quaternary stratigraphic indicators (McCollum, 1975; Kellogg, D. and Kellogg, T., 1986). Diatoms are plants and require light for photosynthesis. Ice thicknesses for the MIS range from 20 to over 100 m (Swithinbank, 1968), which should be sufficient to prevent, or at least inhibit, photosynthetic activity. Thus, we suspect these common marine species were carried beneath the ice shelf by ocean currents, and deposited in marine sediments which were later transported to the ice shelf surface by the basal freezing and surface ablation mechanism which is also apparently responsible for transporting marine macrofossils to the upper surface of the MIS (Debenham, 1919). Another possible, but probably less-important, mechanism for introducing marine diatoms to MIS sediments is winds blowing across diatombearing deposits in adjacent snow- and icefree areas, which are widespread in the Dry Valleys and on Black Island and Brown Peninsula. Single specimens of the extinct marine species Denticulopsis hustedtii, Nitzschia praeinterfrigidaria, and Trinacria excavata were identified positively at sites K76-2, K76-6, K768, K76-22, K76-23, K76-24, K76-59, K76-69, and K78-79. Probable fragments of these extinct species were also noted at K76-11 and K76-19. The presence of extinct species on the ice-shelf surface suggests that in-situ sediments older than Pleistocene may be exposed on the sea floor beneath the Pinnacled Ice, or that Quaternary sediments beneath the ice shelf contain reworked older material. We prefer this latter alternative because our previous studies have repeatedly documented reworked sediments in Ross Sea cores and in cores from beneath the Ross Ice Shelf (Kellogg, T. and Truesdale, 1979; Truesdale and Kellogg, 1979; Kellogg, T. and Kellogg, D., 1981; Kellogg, D. and Kellogg, T., 1986). In either case, the extinct species are probably transported to the ice shelf surface along with younger marine sediments and organic matter during the basal freezing process.
81
McMurdo Ice Shelf. Small ponds, rivulets, and pools are widely distributed over its surface during the austral summer months of December and January. Mats of green and
Non-marine diatoms Non-marine diatoms are abundant and wellpreserved in samples from the surface of the I
166°E
~65 °
McMurdo
;67 °
tayloren$/s (J7)
Sound
77o45 '-
N glbbula (9)
0.0
N muhcops;s 1 ~ N1
L
f
evolu/a (77)
N mutlcop51$ f reducto (52)
mutlcops/forme (52) N curta (12)
/~ mullcops/s (14)
.....
,o
J
_~0/
(51
Cyclofe//a spp. (15)
\
k
Melosira sp "A" ~64)
% ~ N quoternorzo (66), ~ .
! ,\
/
'4~Melos/ra sp "8" (20)
T n,tzschz?/des (il)!>~.__~, bt
/""~ p"" ;'" :'~o, ,',/ cymofopleure ( ) \'~"
7 '1~ ~1' ~N confento (23) 78 ° -
N shockleton/ {143) N
gauss/
(59)
1.3 ~' Melos/ra SO/ (11) !/;!< N kefgulens/s (22)
O.£) q "~' N rnuticops/s f murray/ (5)~ t Brawl
/j
0.0
, "
wesil~ (222)
N muhcops/s f. copl'tafo (25)] F/Peninsula ~/ confenfa f. pafal/e/o (43) J"~ . . . . . -::-
i White
0.0
"--~
.:,
Island 0.0
.~
-/
q.1' I M d, stons (S)
~,a'~
/
Island
~C~o~.
~" 15'-
I00 rN. obliquecq$tata (43) I~ subl]nearis (18) E1 ontarch~a (IO) ~A lactinochHu$ (9)
M/' Discovery &
c~
,-~-r-~
_
I
Scole
.-
i
~
o.o
t~I I'IDO
~
O.O•
~ 78
J
"-"-
,"~
° 30'
-
I
Fig.2. Percentage of marine diatoms in McMurdo Ice Shelf samples. Also shown are sample localities at which individual diatom taxa are most abundant (number counted in parentheses).
Diatom
Sample
K76-12
7
K76-53
4
14
K76-66
K76-67
K76-64
K76-63
K76-62
K7~60
K76-59
K76-58
K76-57
-
5
K76-52
8
2
K76-51
K76-55
8
2
2
1
1
K76-50
K76-31
K76-30
K76-26
K76-25
K76-24
K76-23
K76-22
K76-21
K76-20
K76-19
K76-18
K76-17
K76-16
K76-14
-
1
16
K76-11
1
8
K76-10
K76-13
3
K76-9
.
--
-
.
-
-
-2
.
1
1
.
1
1
5
-
4
K76-7
K7~8
2
2
K76-6
.
-
--
_
1
1
1
-
2
-
3
-
2
K76-4
3
4
K76-5
1
3
17
-
2
data"
K76-3
K76-2
census
1
I
TABLE
.
.
A
-
1
1
2
1
1
6
15
6
2
6
1
5
.
-
-
-
-
-
-
-
-
.
-27
-
6
.
6
2
17
3
1
1
1
2
1
6
1
2
11
3
9
1
1
2
1
2
3
3
7
1
-
.
1
1
2
2
1
8
.
.
3
9
-
.
10
.
6
1
4 1
5
1
1
5
1
10
20
1
9
1
2
11
6
6
7
5
3
1 4
3
1
-
2
8
1
-
3
5
4
11
1
43
5
10
3
10
3
12
2
3
5
2
32
1
6
4 12
9
23
2
14
!
6
4
2
1
36
30
2
8
11
16
16
10
26
7
2
12
60
34
23
4
21
16
29
12
20
23
8
4
14
-
18
5
-
2
2
16
8
4
8
7
4 -
-
-
2
9
13
15
14
1 2
3
2 2
4
2
-
3 6
13
12
3
1
8
11
8
7
43
15
2
2
2
2
10
5
22
2
13
20
3
22
5
43
10
9
7
17
36
4
1
36
8
6
2
3
20
-
42
20
15
-
.
3
2
1
1
1
1
1
1
1
1
1
2
9
7
16
.
-
-
.
17
.
1
2
18
1
2
1
2
19
2
3
4
5
2
1
1
2
6
52
1
2
44
1
5
25
-
-
20
-
-
6
4
1
3
1
1
5
1
3
6
4
3
7
3
1
1
1
14
4
2
5
21
3
1
3
1
1
1
1
9
2
1
25
-
-
-
-
-
-
-
-
23
-
-
-
22
47
9
3
41
1
17
35
1
3
4
2
3
2
3
5
4
1
20
5
1
77
5
7
12
25
5
23
-
-
-
-
-
-
-
-
-
-
-
1
1
2
3
1
1
1
2
1
2
1
24
12
4
3
7
2
3
2
8
15
2
5
5
1
14
3
2
1
6
9
8
1
16
1
6
1
2
52
2
2
11
14
8
25
8
8
1
1
3
2
2
27
61
1
10
44
39
19
5
18
8
2
46
38
18
37
5
60
31
25
22
41
32
13
3
15
-
-
-
26
-
-
-
15
1
3
11
22
33
29
44
12
38
87
35
65
48
47
143
19
47
18
7
1
24
4
51
1
11
4
1
7
21
1
27
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
3
1
28
24
7
3
62
32
1
98
92
68
167
33
16
222
139
26
99
8
153
Iii
77
81
137
145
53
17
129
28
160
17
43
45
45
16
33
113
220
29
20
21
4
24
19
21
8
21
9
1
23
61
29
32
3
18
23
64
22
18
18
11
3
58
8
23
6
6
14
6
3
1
26
8
30
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
1
1
31
5
3
K78-79 K78-100
1
3
-
-
-
1
1
-
K76-24
K76-23
4
5
3
6
1
2
3
1 1
1
3
11
1
--
2
44
32
23 2
41
25
3
2
9
5
3
11
10
-
-
-
-
1
1
1
1
45
-
12
26 1
17
18
15
15
46
2
2
7
1
71
8
1
4
5
3
2
1
4
1
1
1
1
3
46
1
5
37
47
59
49
59
34
45
1
2
7
36
1 1
22
1
1
4 1
3
2
2
1
6
43
6
2
3
4
1
4
2
4
4
1 9
K76-22
1
1
1
1
2
5
42
4
1
3
2
3
1
2
1
2
1
1
1
2 12
K76-21
K76-20
K76-19
K76-18
K76-17
K76-16
K76-14
K76-13
K76-12
K76-11
K76-10
K76-9
4
2
K76-7
K76-8
1
K76-6
K76-5
1
1
41
6
8
1
2
5
1
64
K76-4
1
-
-
-
1
1
39 40
1
3
1
1
6 1
6
1
2
4
4
2
1
1
2 1
3
1
2
1
2
37 38
1
-
-
1
1
K76-3
1
5
K76-2
--
33 34 35 36
32
Sample
3
3
2
-
1
1 1
4 -
3
K78-SM
-
2 1
-
1
1
1
-
1
2
1
K78-MB
-
9
K78-65
K78-66 K78-76
K78-108
7
12
K78-63
7
K78-56
K78-59
-
3
K78-53
1
2
1
K78-30
K78-10
-
-
1
-
K78-3
K78-2
1
K76-68
K76-69
11
12
K76-67A
~
.
5
8
1
2
3
2
47
4
1
2
1
1
--
48
-
-
-
1
-
49
1 2
1
-
.
--
1
50
.
-
-
1 2
-
1
4
1
51
-
2
2 1
5
14
-
2
7
17
.
-
1
1
52
4
3 1
1
3
7
7
5
.
53
1
1
4
2
.
3
54
1
1
6 2
5
17
22
6
7
1
1
4
4
5
5
8
1
2
5
5
2
1
55
-
3
.
.
1
-
1
1
56
2
7
15
10
5
15
8
2
30
1
57
3
7
15
16
15
8
40
23
3
1
12
2
61
1
2
1
-
-
1
5
16
22
32
15
18
61
64
2
4
12
58
-
4 19
1
1
59
-
-
-
10
-
1
1
1
2
-
60
1
1
2
177
38 5
63
28
36
46
30
1
29
23
41
2
32
1
1
1
1
3
6
23
61
-
64
85
85
101
85
76
3
18
-
-
-
1 -
48
.
K78-30
.
.
-
.
.
.
.
.
.
.
-
1
2
1
1
1
1
7
6
.
.
.
.
.
.
-
.
-
1
.
3
1
1
aSee Table III for names
K78-SM
K78-MB
K78-108
K78-100
of taxa
.
-
1
1
.
1
.
.
9
3
-
3
2
K78-66
.
.
-
.
.
.
1
1
K78-79
2
K78-65
1
1
2
2
1
.
.
.
.
.
1
1
1
.
.
.
.
.
-
-
2
1
.
1
10
1
7
2
37 38
K78-76
1
K78-63
K78-59
K78-56
.
-
K78-10
K78-53
-
K78-3
K78-2
1
2
K76-68
K76-69
1
3
K76-67A
K76-67
K76-66
K76-64
K76-63
.
.
1
.
.
2
.
.
K76-62
.
-
3
1
1
33 34 35 36
K76-60
K76-59
K76-58
K76-57
K76-55
K76-53
K76-52
K76-51
.
1
K76-50
1
-
32
(continued)
K76-31
I
K76-30
K76-26
K76-25
Sample
TABLE
.
.
.
-
.
1
-
2
1
1
.
-
-
-
.
.
.
1
1
3
.
1
.
4
1
10
-
39 40
.
.
.
.
2
1
2
.
1
6
2
1
6
3
3
6
6
1
1
6
41
.
.
. .
-
.
.
.
1
1
42
.
.
.
.
.
.
2
.
43
.
.
.
1
1
1
44
.
.
.
.
1
1
1
1
1
5
12
1
1
9
45
.
-
-
-
-
-
.
-
-
1
1
1
3
1
46
.
.
.
.
2
2
1
1
2
6
2
7
1
2
.
43
1
1
1
47
. .
.
.
.
.
1
48
. .
.
.
.
.
.
.
1
1
49
.
.
.
.
.
.
.
1
1
1
1
50
.
.
.
.
.
.
.
.
.
1
2
18
1
51
.
.
.
.
.
1
52
.
.
.
.
.
.
1
53
.
.
. .
. .
1
2
.
54
.
.
-
.
.
1
1
55
.
.
-
-
.
2
56
.
-
-
-
1
58
1 -
2
ii
57
5
59
1
1
60
-
-
1
61
O0
85 blue-green algae are present in most of these ponds, even when only the edges are melted, and desiccated algal material is common among the sediment and marine-fossil debris of the MIS. Melt ponds in the adjacent Dry Valleys support a similar rich non-marine diatom flora associated with algal remains (Kellogg, D. et al., 1980). Brady (1978) first reported non-marine diatoms on the McMurdo Ice Shelf. Brady and Batts (1981) reported Pinnularia cymatopleura, Navicula shackletoni, Nitzschia westii (reported as N. antarctica, but not the homonym described by Okuno, 1954), Navicula quaternaria (reported as N. seminulum), and Tropidoneis laevissima, all associated with salt beds on the MIS near Black Island. We identified 53 taxa of non-marine diatoms in our MIS samples (Table I). The most important species (i.e., having high abundances and/or occurring in most samples) include: Nitzschia westii, Pinnularia cymatopleura, the Navicula species deltaica, gaussii, and quaternaria, and shackletoni, the Navicula muticopsis formae evoluta and reducta, Hantzschia amphioxys, and species of Melosira and Cyclotella.
Diatom assemblages Although MIS diatom data are not ideal for factor analytical analysis, because samples have different 14C ages, we performed Q-Mode factor analysis (Imbrie and Kipp, 1971) on our data (59 samples containing 61 species). Our objective was to establish common geographic or age groupings of sampling sites based on species assemblages. Although this analysis did resolve five assemblages, communalities for samples with low total diatom counts were very low and the cumulative variance was only 75%. We therefore reran the factor analysis after removing 14 samples with low abundances and communalities (see sample localities marked with solid triangles in Fig.l). Factor analysis with 45 samples containing 61 species resolved four assemblages with a cumulative variance of 90.316%. These four assemblages have the same species composi-
tions as the first four factors in the previous analysis. The main difference is deletion of the fifth factor (composed of marine species), which was apparently an artifact of a single sample (K76-63) with a low total count but dominated by marine species. Factor loadings for each assemblage are shown in Figs.3-6, and Table II. The sum of squares of loading values for each sample is termed the communality and equals unity for samples having a perfect fit into the assemblages calculated by this QMode varimax solution (Imbrie and Kipp, 1971). To determine the significance of individual loadings (i.e. the percentage of information explained by a particular assemblage), the values should be squared and multiplied by 100. A convenient rule is that a loading of 0.5 corresponds to 25%, 0.7 is ~50%, and 0.86 is 75%. Factor scores, explaining how each species contributes to each assemblage, are given in Table III. Assemblage 1 has a variance of 53% and is dominated by Nitzschia westii (Fig.3). Highest factor loadings occur in sites along the debris band that extends northwards from Black Island (especially the northern half of this band where only one sample has a loading less than 0.7), and in sites from the strait between Black Island and Brown Peninsula. Assemblage 2 has a variance of 23% and is comprised of Pinnularia cymatopleura and the Navicula species deltaica, gaussii, shackletoni, and quaternaria (Fig.4). Highest factor loadings (all >0.7) occur in samples from sites along the debris band separating Koettlitz Glacier ice from the McMurdo Ice Shelf. Elsewhere, loadings are mostly 0.5 or less. Assemblage 3 has a variance of 9.3~o and is dominated by the Navicula muticopsis formae evoluta, capitata, and reducta, as well as by the closely related Navicula muticopsiforme (Fig.5). Loadings >0.5 are restricted to 5 samples (K76-7, K76-62, K76-63, K76-67, and K78-100) from scattered localities in the strait between Black Island and Brown Peninsula, near the northern edge of the MIS, and on White Island. A scattered distribution of samples with high loadings would normally
86 TABLE II V a r i m a x factor m a t r i x Site
Communality
Assemblage 1
Assemblage 2
Assemblage 3
K76-2 K76-3 K76°4 K76-5 K76-6 K76-7 K76-8 K76-9 K76-10 K76-11 K76-12 K76-13 K76-14 K76-16 K76-17 K76-18 K76-19 K76-20 K76-21 K76-23 K76-24 K76-25 K76-26 K76-30 K76-31 K76-50 K76-51 K76-52 K76-53 K76-60 K76-62 K76-64 K76-67 K76-68 K78-2 K78-3 K78-53 K78-56 K78-59 K78-63 K78-65 K78-66 K78-79 K78-100 K78-108
0.979 0.914 0.891 0.856 0.959 0.772 0.898 0.813 0.968 0.849 0.947 0.946 0.969 0.962 0.986 0.934 0.851 0.950 0.945 0.975 0.901 0.979 0.980 0.611 0.960 0.982 0.954 0.931 0.953 0.749 0.780 0.838 0.802 0.732 0.954 0.929 0.955 0.946 0.953 0.936 0.934 0.958 0.972 0.728 0.862
0.978 0.844 0.822 0.841 0.934 0.361 0.928 0.833 0.931 0.769 0.823 0.925 0.912 0.913 0.889 0.875 0.442 0.854 0.853 0.746 0.416 0.737 0.952 0.751 0.838 0.929 0.894 0.783 0.863 0.519 0.570 0.389 0.177 0.271 0.810 0.923 0.240 0.348 0.258 0.230 0.443 0.374 0.943 - 0.076 0.164
0.090 0.368 0.224 0.155 0.230 0.214 0.161 0.308 0.250 0.458 0.488 0.251 0.306 0.310 0.331 0.365 0.624 0.382 0.378 0.519 0.711 0.561 0.215 0.097 0.388 0.288 0.288 0.339 0.306 0.566 0.096 0.227 0.505 0.727 0.505 0.122 0.919 0.878 0.911 0.890 0.816 0.884 0.248 - 0.043 0.884
Variance
53.003
23.908
9.300
4.105
Cumulative variance
53.003
76.911
86.211
90.316
0.108 0.255 0.401 0.353 0.184 0.770 0.081 0.132 0.078 0.152 0.177 0.151 0.177 0.159 0.106 0.091 0.004 0.106 0.141 0.119 0.068 0.067 0.069 0.190 0.209 0.119 0.111 0.394 0.213 0.102 0.667 0.797 0.702 0.242 0.173 0.239 0.065 0.083 0.070 0.226 0.238 0.167 0.133 0.847 0.174
Assemblage 4 - 0.041 - 0.040 0.063 0.001 - 0.020 - 0.051 - 0.069 - 0.083 0.180 0.156 - 0.020 0.076 0.110 0.082 0.273 0.164 0.516 0.253 0.235 0.368 0.466 0.340 0.153 - 0.039 0.251 0.150 0.244 0.217 0.262 0.385 0.038 0.013 0.151 0.266 0.115 0.080 0.221 0.218 - 0.225 - 0.201 - 0.118 - 0.090 - 0.053 0.048 0.154
87
TABLE
III
Scaled varimax No. a
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54
factor scores
Taxon
1
2
3
4
Achnanthes taylorensis Cyclotella comta Cyclotella glomerata Cyclotella stelligera Cyclotella s p p . Eunotia s p p . Hantzschia s p p . Melosira distans Melosira sulcata Melosira sp. " A " Melosira sp. " B " Navicula contenta N. contenta v. parallela Navicula deltaica Navicula gaussi Navicula gibbula Navicula minuscula Navicula mutica N. mutica v. cohnii Navicula muticopsiforme Navicula muticopsis N. muticopsis f. capitata N. muticopsis f. evoluta N. muticopsis f. murrayi N. muticopsis f. reducta Navicula quaternaria Navicula shackletoni Navicula s p p . Nitzschia westii Pinnularia cymatopleura Pinnularia s p p . Stauroneis anceps TabeUaria s p p . Tropidoneis s p p . Actinocyclus ingens Actinocyclus actinochilus Asteromphalus parvulus Coscinodiscus s p p . Denticulopsis sp. Eucampia antarctica Melosira sol Melosira sulcata Nitzschia closterium Nitzschia cylindra Nitzschia curta Nitzschia kerquelensis Nitzschia obliquecostata Nitzschia praeinterfrigidaria Nitzschia ritscheri Nitzschia separanda Nitzschia sublineata Rhabdonema s p p . Rhizosolenia s p p . Rouxia antarctica
- 0.184
- 0.419
- 0.460
0.900
- 0.000
0.000
0.000
0.000 - 0.249
0.111
0,007
- 0.011
- 0.105
- 0.125
0.850
0.150
0.420
- 0.027
- 0.042
- 0.895 - 0.015
- 0.001
0.012
- 0.002
- 0.104
- 0.314
2.113
0.311
0.045
0.191
- 0.002
- 0,218
0.050
- 0.025
- 0.001
- 0,055
- 0.046
0.230
- 0.032
- 0,240
0.282
0.053
0.137
0.115
0.395
- 0.054
0.129
0.138
0.417
0.556
0.910
- 1.244
0.237
1.725
1.036
1.933
- 0.094
3.470
0,988
- 2.767 - 0.134
0.081
0.008
0,111
- 0.003
0.006
0,001
0.029
0.015
0.006
- 0.007
- 0.041
0.062
-0.030
0,064
- 0.596
0.054
3.174
0.021
0.132
0.619
-0.107 0.890 -0.061
- 0.057
- 0.155
1.198
0.038
- 0.351
- 0,246
5.836
- 0.293
- 0.007
0,155
- 0.003
- 0.114
0.042
0.380
2.071
0.123
1.698
0.496
0.208
2.567
- 0.447
5.15{}
- 0,031
0.083
0.024
0.015
-0.456 3.743
7.7{}9
0.040
0,462
- 0.128
- 0,253
5.958
- 0.702
- 2.{}15
- 0,038
- 0.033
0,279
0.018
0,080
0.017
0.033
- 0.054
- 0.176
- 0.073
0.913
0.293
- 0.004
0.005
- 0.004
0.040
0.002
- 0.002
0.001
0.008
- 0.001
0,083
0.090
0.105
0.022
- 0.011
0.002
- 0.036
0.053
0.001
0.021
- 0.103
0.006
0.000
0.034
0.078
0.020
0.000
0.006
0.088
0.389
- 0.103
0.058
- 0,424
0,002
- 0.002
0.001
0.008
0.011
0.007
- 0.007
- 0.004
- 0.000
0.001
0.015
0.003
- 0.073
- 0.018
0.502
0.080
0.162
- 0.045
0.014
- 0.041
- 0.037
0.086
0.121
0.211
- 0.000
0.000
- 0.000
0.000
0.002
- 0.001
- 0.002
0.001
0.003
- 0.002
0.019
0.014
0.014
- 0.003
0.024
0.018
0.036
0.003
- 0.022
- 0.092
0.005
- 0.003
0.002
0.021
0.057
- 0.011
- 0.025
- 0.103
88
TABLE III
(continued)
No."
Taxon
55
Stephanopyxis turris Thalassionema bacillaris Thalassionema nitzschioides Thalassionema spp. Thalassiosira s p p . Thalassiothrix s p p . Trinacria excavata
56 57 58 59 60 61
1
2
3
4
0.000
- 0.004
0.018
0.007
- 0.010
0.025
0.005 0.009
- 0.017
- 0.041
0.296
0.054
- 0.000
0.000
- 0.000
0.000
- 0.005
0.019
0.012
0.024
0.002
- 0.002
0.001
0.008
0.006
- 0.002
- 0.003
- 0.010
aSee T a b l e I.
suggest an invalid assemblage. We suspect that Assemblage 3 may be meaningful because all these samples (except K78-100) have 14C ages between 2100 and 3500 yr B.P., or are adjacent to sites having this age range. This suggests that environmental conditions were similar at these locations during that interval. Assemblage 4 has a variance of 4.1% and is characterized by Navicula shackletoni and N. quaternaria and by the absence of other species in Assemblage 2 (Fig.6). Factor loadings for this assemblage exceed 0.5 at only one sample site (K76-19) and any positive value >0.2 ( ~ 4 % ) may be considered high. These "high" values are restricted to the northern half of the Koettlitz debris band and the middle and southern part of the Black Island band.
Factors controlling diatom assemblages An unequivocal explanation for the distributions of the four diatom assemblages is not currently possible but at least four considerations deserve attention. These are, differences in age, geographic locations of individual sites, micro-geochemical environments, and seasonal blooms. Age Nearly 50 radiocarbon dates are now available from shell and algal samples collected from the surface of the MIS (Stuiver et al., 1981b; Stuiver and Braziunas, 1985; Kellogg, T. et al., in prep.). Uncorrected ages range from
570__+60 yr B.P. to 45,500_+ 1500 yr B.P., with the majority between 1200 and 6600 yr B.P. For radiocarbon-dated sites where diatoms were studied (Table IV), the dates should be considered minimum ages for marine diatom specimens, because dates were obtained on macrofossil remains associated with marinediatom bearing sediment. For these same sites, 14C ages should be considered as maximum ages for non-marine diatoms because these species were introduced to MIS sediments after the dated marine biota and sediments melted out at the ice-shelf surface. At least four factors complicate interpretation of ages for the diatom assemblages: (1) The non-marine diatom assemblage at any particular location potentially represents an accumulation of specimens from all the melt ponds that have existed in the immediate area since formation of the ice comprising the ice shelf at the locality (i.e., each sample contains a mixture of non-marine specimens of different ages). (2) The sample site will have moved during this time (e.g., ice now at sites near the northern end of the Black Island debris band probably formed along the north coast of Black Island), possibly causing changes in diatom species' dominance related to changing location and/or local geochemical environment. (3) Diatom assemblages are probably modified by wind action, resulting in both removal of specimens, and addition from adjacent snowand ice-free diatom-bearing locations. An indication that wind is a significant transport mechanism in this area is the occurrence of
89
sand-sized and smaller material scattered widely over large areas of the McMurdo Ice Shelf that are free of debris bands. (4) Some non-marine (and marine) specimens present in our samples may be considerably older than 165 °
radiocarbon ages for the sites where they occur. These reworked specimens may be derived from older deposits exposed to wind action on land, or they may be mixed in with marine species in the sediments beneath the ice 166°E
-
McMurdo
Sound
77045 `
'%
-,
,
k~..
\x
~'\ .75
78 °
-,08
'!it / ~, I,r
.Z~p ~ Brow .44
.37 ~/%0
Peninsula
•%,
i
,16 .52
i '.g4
C<-
3,oc~IsLond
1.39
(/: ,15'
L.,..J-~' Mt Discovery
ASSEMBLAGE
z~
I
2
co
5
0
5
Sca~e
~0
15km
Mir)r)a ~
~
~
/"~,"~•
Q'~ 7 8 ° 30'"
I
Fig.3. Factor loadings for Diatom Assemblage 1 (Nitzschia
i
westii). Note highest loadings along the Black Island debris band.
90
late Wisconsin glaciation (Stuiver et al., 1981b), and containing marine macrofossil fragments dated at 35,4004-1500 yr B.P. (Kellogg, T. et al., in prep.). The diatom assemblage
shelf. Possible evidence for this last effect comes from site K78-100 on White Island. The sediment here is glacial drift deposited by ice moving westward from the Ross Sea during the i
I
165 °
166°E
McMurdo
Sound
77o45
'-
I/ i , 78o
-
.ss -.0,4 .91
.89 .SZ • 88 i
Peninsulo
.S8
[".2 5
3lock Islond
)
.22,
5,.
Mr.
ASSEMBLAGE
Discovery
A
2 a~
5 .-.
0 ,--I
5 I
10
i
15krn
I
Sco~e
78 ° 30'i
,g~l
Fig.4. Factor loadings for Diatom Assemblage 2 (Pinnularia cymatopleura, Navicula gaussii, and Navicula deltaica). Note highest loadings along the debris band marking the end of Koettlitz Glacier.
91
White Island, we suspect that they were present in Ross Sea sediments prior to the glacial advance because our analyses of Recent sediments and older diamicton at numerous loca-
consists of only 6 specimens, all of non-marine species. Although we can not rule out the possibility that these specimens were introduced after deposition of the drift on northern I
I
166°E
165 °
McMurdo
Sound
77o45
' -
', ,,
, .~::'-~..,~. ,,
7B °-
l, .OS .85 .0~
i
Ii' 'J' 1
.17
I' i'
Peninsula
.IT~
I
',.J\ . j , .10
31ac~ Island ~° 78
Mt Discovery
ASSEMBLAGE
z~
15'.
5 q~
# 5
0
5
Scale
I0
15km
Mi~
o 78 ° 30"
I
I
Fig.5. Factor loadings for Diatom Assemblage 3 (Navicula muticopsis f o r m a e evoluta, reducta, and capitata, and Navicula
muticopsiforme).
92
Geographic factors
tions on the Ross Sea continental shelf commonly reveal rare non-marine specimens (Kellogg, T. and Truesdale, 1979; Truesdale and Kellogg, 1979; Kellogg, T. and Kellogg, D., 1981).
The debris-covered Surface of the McMurdo Ice Shelf is an ablation zone which has its
I
I
165 °
~
166°E
McMurdo
i67o
Sound
77 °
45' -
/'-'V'--,,~\, ~'..\ o:o- ~\-.o5
<
/.o
,~...~.--.
,
-
,V
'~ \ , ,~'
~
, , :,
~\ I [ .26 .-,02 " .08
,, la, ' ~",,~
~. '...t t.
" ' '
!1~,
78 ~ . ,
.22
.52 .05
-.2,.
".2O~[Bro~ -.t2 A /
-ogF//~
O
i IIi
~/Peninsulo
.15
.39
-r", :' - , 0 5
Islond o15,_
Mr.
ASSEMBLAGE
Discovery
4
A %.
5
t--, ,7
0
,-t
5
I0
I
J
15km
t
Scale
78 ° 301-~
i
"-'~---~-.,.
,l""ll
I
Fig.6. Factor loadings for Diatom Assemblage 4 (Navicula quaternaria and Navicula shackletoni). Note highest loadings near the southern end of the Black Island debris band.
93 TABLE IV Radiocarbon dates of samples analysed for diatoms Site
Lab. no.
Date and error
K76-4 K76-18 K76-26 K76-55 K76-58 K76-59 K76-62 K76-64 K76-66 K76-67A K78.100 K78-108
QL-1126 6510 __50 QL-1127 4630+80 QL-1128 1260 + 30 QL-1222 3590-4-_80 QL-1129 > 51,000 QL-1132 3610_+40 QL-1130 3770__40 QL-1224 4410__90 QL-1225 1340 ___30 QL-1226 3280_300 QL-1451 35,400_ 1500 QL-1453 720 + 60
Ref.a
Material
Lat. (S)
Long. (E)
Comments
1 1 1 1 2 1 1 3 1 3 3 3
Barnacle Barnacle Barnacle Barnacle Barnacle Serpulids Barnacle Barnacle Bryozoa Shells Barnacle Bryozoa
77o50.5' 77°59.9' 78°05.9' 78°11.0' 78o29.2' 78°12.0' 78°08.8' 78°13.5' 77°52.0' 77o53.5' 78°02.0' 78009.0'
165°47.0' 166°02.5' 166°05.0' 166o46.0' 167004.0' 166°45.0' 165o44.0' 165°40.0' 165°15.0' 165°04.0' 167o24.0' 165°10.0'
On Black Island band On Black Island band Near N. Black Is. on Black Is. Band North of east end of Black Island North of end of Minna Bluff Off east side of Black Island Between Brown Pen. and Black Island Between Brown Pen. and Black Island Near eastern Dailey Island Dailey Islands White Island, Speden (1962) Site 19 West of Brown Pen. on Koettlitz band
al =Stuiver et al. (1981); 2 = Kellogg and Truesdale (1979); 3= Kellogg et al. (in prep.). Barnacle = Bathylasma corolliforme (Hoek); "Shells" = mixed bryozoa, serpulids, barnacles, etc. e a s t e r n b o u n d a r y n e a r a l i n e r u n n i n g from t h e s o u t h e r n e n d of H u t P o i n t P e n i n s u l a t h r o u g h the strait between Black and White islands ( S w i t h i n b a n k , 1968). S t u i v e r et al. (1981b) a t t r i b u t e d t h i s a b l a t i o n to a d i a b a t i c a l l y w a r m e d k a t a b a t i c w i n d s f l o w i n g d o w n a n d to t h e n o r t h from Mt. D i s c o v e r y a n d M i n n a Bluff. W i n d p a t t e r n s m a y be p a r t l y r e s p o n s i b l e for t h e d i s t r i b u t i o n s of A s s e m b l a g e s 1 a n d 2, w h i c h a p p e a r a l m o s t as o p p o s i t e s . S a m p l e s i t e s a l o n g the debris band marking the boundary with K o e t t l i t z G l a c i e r ice s h o u l d be a f f e c t e d by winds flowing down Koettlitz Glacier and the w e s t f l a n k s of Mt. D i s c o v e r y ; t h o s e from t h e d e b r i s b a n d n o r t h of B l a c k I s l a n d s h o u l d experience winds blowing down the east flanks of Mt. D i s c o v e r y a n d M i n n a Bluff a n d t h r o u g h the strait between Black Island and Brown P e n i n s u l a . In a d d i t i o n , t h e B l a c k I s l a n d d e b r i s b a n d is c l o s e r to s e a s o n a l l y o p e n w a t e r in McMurdo Sound than the Koettlitz Glacier band. G e o g r a p h i c s e t t i n g s of i n d i v i d u a l s a m p l e s m a y a l s o i n f l u e n c e d i a t o m s p e c i e s i n d i r e c t l y by modifying temperature. Areas with continuous and/or thick debris cover should absorb more solar radiation than areas with patchy or thin d e b r i s cover. T h i s m e c h a n i s m m i g h t c a u s e t h e differences noted between Assemblages 1 and
2. T h e f o r m e r a s s e m b l a g e o c c u r s p r i m a r i l y on t h e B l a c k I s l a n d d e b r i s b a n d w h e r e s e d i m e n t is c o n t i n u o u s a n d u s u a l l y s e v e r a l c e n t i m e t e r s in t h i c k n e s s . S e d i m e n t is p a t c h y a n d t h i n a l o n g the Koettlitz debris band. Sediment distribut i o n m a y a l s o a f f e c t d i a t o m d i s t r i b u t i o n s by p r o v i d i n g a h a b i t a t for a e r o p h i l i c s p e c i e s (e.g.
Hantzschia amphioxys, Navicula mutica, N. gibbula, a n d N. contenta f. parallela: see K e l l o g g , D. et al., 1980), w h i c h live in soils.
Chemical factors A l l n o n - m a r i n e s p e c i e s e n c o u n t e r e d in M I S s e d i m e n t s o c c u r l i v i n g t o d a y in l a k e s a n d p o n d s in t h e D r y V a l l e y s , w h e r e s a l i n i t y c o n d i t i o n s r a n g e from fresh to s u p e r s a l i n e ( K e l l o g g , D. et al., 1980). T h e y a r e n o t k n o w n to l i v e in m a r i n e w a t e r s . T h e g e o c h e m i s t r y of D r y V a l l e y l a k e s a n d p o n d s is e x t r e m e l y v a r i a b l e but, as yet, no e v i d e n c e is a v a i l a b l e on h o w t h i s g e o c h e m i c a l v a r i a b i l i t y affects individual diatom species' distributions. No g e o c h e m i c a l s t u d i e s of M I S m e l t p o n d s have been made, but v a r i a b i l i t y does occur. B r a d y a n d B a t t s (1981) d e s c r i b e d beds of m i r a b i l i t e s a l t s (Na2SO4.10H20), w i t h associa t e d n o n - m a r i n e d i a t o m s , on t h e ice s h e l f n e a r B l a c k I s l a n d . T h e y a t t r i b u t e d t h e s e b e d s to
94 uplift of sub-ice-shelf brines, by Debenham's (1919) basal freezing mechanism. We visited these salt beds (near K76-60), but have not observed salt deposits elsewhere on the MIS. Nevertheless, pockets of brine were noted within the ice (Heine, 1968; Kovacs and Gow, 1975), and could influence the geochemistry of melt ponds even if the salts are not sufficiently concentrated to precipitate beds. Another possible factor controlling the geochemistry of MIS melt ponds is their position relative to seasonally open water in McMurdo Sound. The eastern side of McMurdo Sound becomes ice free during most austral summers. Ice on the western side does not break up every year, and when it does, the interval with ice free conditions is considerably shorter than for the eastern side. The possibility thus exists for introduction of marine salts by atmospheric transport of aerosols during the short austral summers at times with northerly winds. This factor should be variable from east to west because of variations in ice extent, and from north to south because of distance from open water. We suspect that this mechanism is partly responsible for the differences in diatom dominance between the Koettlitz and Black Island debris bands (Assemblages 1 and 2) and along these same debris bands (Assemblage 4). Each tiny pond may have its own peculiar chemical characteristics, related to location and to introduction of salts from below, which may be more conducive to blooms of one of the diatom assemblages than to the others. Seasonal blooms
Diatom assemblages in our sediment samples may reflect seasonal blooms [e.g., Assemblage 1 might dominate in the early austral summer; Assemblage 2, mid-summer; etc. (and/or vice versa)]. In this case, our samples might represent fortuitous collections representing short bloom intervals rather than average annual or multi-year composite floras. While this explanation is possible, we regard it as unlikely because of the complex factors described above that should result in multi-season samples.
Discussion Because of the co-occurrence of locally abundant marine macrofossils in many of our samples, we doubt that significant numbers of marine diatoms are introduced to MIS sediments by winds blowing across adjacent diatom-bearing deposits. Thus, the presence of marine diatoms on the MIS is consistent with the hypothesis of a marine origin for this ice shelf, and with Debenham's (1919) mechanism of basal freezing and surface ablation to transport sediment and biota to the ice shelf surface. Because marine diatoms are usually rare in MIS sediment samples, they are unsuited for detailed and quantitative studies of the Holocene paleo-oceanography of southern McMurdo Sound and of waters beneath the ice shelf. Non-marine diatoms, known to tolerate the wide salinity variations of the dry valley lakes and ponds, are abundant and diverse in surface melt ponds of the ablating MIS. Distributions of the four diatom assemblages are probably controlled by some combination of local geochemical variability, seasonal blooms, geographic position on the ice shelf surface, and sediment (and diatom) age. The mixed biota of MIS sediments provides a source for introduction of marine and nonmarine material to the Ross Sea floor. This occurs in two ways: First, and important only locally, mixed biota and sediment on the iceshelf surface may be introduced (and/or reintroduced) to the marine environment through the agent of melt streams flowing off the front of the ice shelf or through cracks and crevasses. Second, and important regionally, icebergs calved from the McMurdo Ice Shelf release their load of mixed biota and sediments when they melt. Recent history shows that while this mechanism is not of great importance at present, because few icebergs are calving from the McMurdo Ice Shelf today, it may have been important at times in the past and may occur in other locations today. This study therefore provides evidence for the action of a new mechanism for producing
95
mixed marine and non-marine floras: transport of marine material upward through an ice shelf by the combined agents of basal freezing and surface ablation, augmented by the addition of non-marine diatoms and algae growing in melt ponds on the ice-shelf surface. This mechanism should also operate on the surfaces of other present and former antarctic ice shelves where the basal-freezing process occurs (e.g. Terra Nova Bay and George VI Sound). We suspect that similar processes of sediment mixing may be responsible for the origin of analogous mixed diatom floras we have begun to describe from sediments drilled recently in the Dry Valleys (Kellogg, D. and Kellogg, T., 1984), and on the origin of diatom-bearing deposits of the Vestfold Hills (Setty et al., 1984; Pickard, 1985). MIS sediments are obviously unique. Indeed, it is hard to imagine any other geologic setting which consists of a mixture of marine and nonmarine micro- and macrofossils of different age, which moves laterally through time into areas with possibly different environments, and is subject to the addition of reworked and unreworked specimens from below and above and to removal of specimens from above. We can say with certainty only that these competing factors should be less significant for nonmarine species in samples with the youngest radiocarbon ages. For marine species, because the time for upward transport of material through the ice (by basal freezing and surface ablation) is geologically instantaneous ( ~ 1 0 0 y r for 50-m thick ice at a measured ablation rate of 0.5m y r - l : Gow, 1967), corrected radiocarbon ages should represent the residence time of the marine material at that site on the ice shelf surface.
Acknowledgements We thank the helicopter pilots and crew of Antarctic Development Squadron VXE-6 for their enthusiastic support of our field work. George Denton, and Terry Hughes read drafts of the manuscript and provided encouragement and stimulating ideas throughout the course of this work. This project was supported by
National Science Foundation grants OPP7515524, DPP77-21083, and DPP-8020000. References Brady, H. T., 1978. Geological studies in southern Victoria Land, on Black Island, and on the McMurdo Ice Shelf. Antarct. J. U.S., 13: 13-14. Brady, H. T. and Batts, B., 1981. Large salt beds on the surface of the Ross Ice Shelf near Black Island, Antarctica. J. Glaciol., 27: 11-18. Dayton, P. K., Robilliard, G. A. and DeVries, A. L., 1969. Anchor ice formation in McMurdo Sound, Antarctica, and its biological effects. Science, 163: 273-274. Debenham, F., 1919. A new mode of transportation by ice. Q. J. Geol. Soc. London, 75(2): 51 76. Gow, A. J., 1967. Antarctic glaciological studies. Antarct. J. U.S., 2: 121-122. Gow, A. J. and Epstein, S., 1972. On the use of stable isotopes to trace the origins of ice in a floating ice tongue. J. Geophys. Res., 77: 6553-6557. Heine, A. J., 1968. Brine in the McMurdo Ice Shelf, Antarctica. N.Z. J. Geol. Geophys., 11:829 839. Imbrie, J. and Kipp, N. G., 1971. A new micropaleontological method for quantitative paleoclimatology: application to a late Pleistocene Caribbean core. In: K. K. Turekian (Editor), Late Cenozoic Glacial Ages. Yale Univ. Press, New Haven, Conn., pp. 71-181. Kellogg, D. E. and Kellogg, T. B., 1984. Preliminary diatom results from Eastern Taylor Valley drill cores. Antarct. J. U.S., 19: 45-47. Kellogg, D. E. and Kellogg, T. B., 1986. Diatom biostrati. graphy of sediment cores from beneath the Ross Ice Shelf. Micropaleontology, 32:74 94. Kellogg, D. E., Stuiver, M., Kellogg, T. B. and Denton, G. H., 1980. Non-marine diatoms from Late Wisconsin perched deltas in Taylor Valley, Antarctica. Palaeogeogr., Palaeoclimatol., Palaeoecol., 30: 157-189. Kellogg, T. B. and Kellogg, D. E., 1981. Pleistocene sediments beneath the Ross Ice Shelf. Nature, 293:130 133. Kellogg, T. B. and Truesdale, R. S., 1979. Late Quaternary paleoecology and paleoclimatology of the Ross Sea: the diatom record. Mar. Micropaleontol., 4: 137-158. Kellogg, T. B., Stuiver, M., Kellogg, D. E. and Denton, G. H., 1977. Marine microfossils on the McMurdo Ice Shelf. Antarct. J. U.S., 12:82 83. Kellogg, T. B., Kellogg, D. E. and Stuiver, M., in prep. Origin and significance of debris bands on the McMurdo Ice Shelf. Klovan, J. E. and Imbrie, J., 1971. An algorithm and FORTRAN-IV program for large-scale Q-mode factor analysis and calculation of factor scores. Math. Geol., 3: 61 77. Kovacs, A. and Gow, A. J., 1975. Brine infiltration in the McMurdo Ice Shelf. J. Geophys. Res., 80: 1957-1961. McCollum, D. W., 1975. Diatom biostratigraphy of the Southern Ocean. In: Initial Reports of the Deep Sea Drilling Project, Leg 28. U.S. Government Printing Office, Washington, D.C., pp. 515-571.
96 Okuno, H., 1954. Electron microscopical study on Antarctic diatoms. J. Jap. Bot., 29: 18-25. Pickard, J., 1985. The Holocene fossil marine macrofauna of the Vestfold Hills, East Antarctica. Boreas, 14: 189-202. Schrader, H. J., 1974. Proposal for a standardized method of cleaning diatom-bearing deep-sea and land-exposed marine sediment. Nova Hedwigia, Beih., 45: 403-409. Scott, R. F., 1905. "The Voyage of the 'Discovery'." Smith and Elder, London, 2 vols. Scott, R. F., 1914. "Scott's Last Expedition" Arranged by Leonard Huxley. Smith and Elder, London, 2 vols. Setty, M. G. A. P., Kellogg, D. E. and Kellogg, T. B., 1984. Preliminary report on diatoms from the Deep Lake terraces, Vestfold Hills, Antarctica. Antarct. J. U.S., 19: 88-90. Speden, I. G., 1962. Fossiliferous Quaternary deposits in the McMurdo Sound region, Antarctica. N . Z . J . Geol. Geophys., 5: 746-777. Stuiver, M. and Braziunas, T. F., 1985. Compilation of isotopic dates from Antarctica. Radiocarbon, 27(2A), 304 pp.
Stuiver, M., Yang, I. C., Denton, G. H. and Kellogg, T. B., 1981a. Oxygen isotope ratios of antarctic permafrost and glacier ice. (Antarct. Res. Ser., 33) Am. Geophys. Union, pp. 131-139. Stuiver, M., Denton, G. H., Hughes, T. J. and Fastook, J. L., 1981b. History of the marine ice sheet in West Antarctica during the last glaciation: a working hypothesis. In: G. H. Denton and T. J. Hughes (Editors), The Last Great Ice Sheets. Wiley, New York, N.Y., pp. 319-346. Swithinbank, C. W. M., 1968. Ice movement in the McMurdo Sound area of Antarctica. In: Int. Syrup. Antarctic Glaciological Exploration (ISAGE), Int. Assoc. Sci. Hydrol. SCAR, Publ., 86: 472-487. Taylor, G., 1914. The Western Journeys I: Koettlitz, Ferrar, and Taylor Glaciers. In: R. F. Scott, Scott's Last Expedition. Smith and Elder, London, vol. 2, pp. 182221. Truesdale, R. S. and Kellogg, T. B., 1979. Ross Sea diatoms: modern assemblage distributions and their relationship to ecologic, oceanographic, and sedimentary conditions. Mar. Micropaleontol., 4: 13-31.