Diatoms in a South Pole ice core: Serious implications for the age of the Sirius Group

• Methods and results
• Sources and atmospheric transport of diatoms
• Diatom deposition: Implications for the Sirius Group
• References

One of the most controversial topics of the past decade for paleoclimatologists has been the hypothesized existence of a Pliocene warm interval in Antarctica around 3.0­2.5 million years ago (Webb and Harwood 1991). Resolution of this controversy has been linked to the validity of two competing explanations for the presence of marine diatoms in glacigenic Sirius Formation (now called Sirius Group; McKelvey et al. 1991) deposits sampled from high-elevation locations (mostly higher than 1,500 meters) along a 1,000-kilometer portion of the Transantarctic Mountains (figure 1).

Figure 1. Map of Antarctica showing locations mentioned in text: B, ice stream B; DG, David Glacier; DV, dry valleys; J-9, Ross Ice Shelf Project site J-9; KG, King George Island; LH, Larsemann Hills; MIS, McMurdo Ice Shelf; OI, Ongul Islands; RG, Reedy Glacier; SD, Siple Dome; SI, Signy Island; TD, Taylor Dome; VH, Vestfold Hills. The Sirius Group outcrops at scattered locations in the Transantarctic Mountains from David Glacier southward to near Reedy Glacier. Wilkes and Pensacola subglacial basins are located west of the Transantarctic Mountains.

 

• According to the "dynamic" hypothesis (Webb et al. 1984; Harwood 1986a,b; Harwood and Webb 1995), Sirius Group sediments contain reworked Pliocene marine diatoms that are thought to have been deposited originally west of the Transantarctic Mountains in the Wilkes and Pensacola subglacial basins during a Pliocene warm interval, when the east antarctic ice sheet retreated leaving a narrow ice-free seaway. Subsequent cooling and glacial expansion resulted in grounded ice overriding the basins, incorporating marine sediments and diatoms, and subsequently, emplacing them at Transantarctic Mountains locations as the Sirius Group. This may have occurred at a time when Transantarctic Mountains elevations were 1­3 kilometers lower than they are today (Webb and Harwood 1991).
• The contrasting "stable" hypothesis argues that the east antarctic ice sheet has remained relatively unchanged for millions of years (Denton, Prentice, and Burckle 1991). Supporting data include geomorphic analyses of dry valleys landforms (Marchant et al. 1993, 1994), the preservation of delicate argon-40/argon-39­dated features in the dry valleys (McIntosh and Wilch 1995), evidence for less than 300 meters of Transantarctic Mountains uplift since the early Pliocene (Wilch et al. 1993a,b), stable isotope records from deep-sea cores that show an absolute maximum of 25 meters of sea-level increase during and since the Pliocene (Kennett and Hodell 1993, 1995), and the nature of the antarctic marine biota which suggests a stable environment over millions of years (Kennett 1995).

The diatoms in the Sirius Group represent the single key to resolving this controversy. Were these diatoms incorporated in the Sirius soon after they lived, hence providing maximum ages for Sirius emplacement, or do they represent aeolian contamination, possibly introduced long after the Sirius sediments were deposited? Here, we report on aerially transported diatoms in ice-core samples from the South Pole.

Methods and results

Material for this study comes from the 227-meter ice core drilled at the South Pole by the Polar Ice Coring Office during the 1980­1983 field seasons (Kuivinen et al. 1982). The core spans the last years between samples (stratigraphy based on information from Gow personal communication). We also sampled snow from pits at Siple and Taylor Domes.

At the National Ice Core Facility (NICL) in Denver, Colorado, the melted ice samples, which ranged in volume from 250 to 2,000 milliliters, were filtered using a Millipore system having 1.2-millimeter perforated MF "Nuclepore" filters. Dried filters were cut into six wedges, two of which were kept for archive purposes. The remaining four were placed, sample side down, on glass cover slips and cleared (made transparent) with acetic acid. Cover slips were dried and mounted on standard glass slides. Each slide was examined in its entirety at 1,000 x, and tallies from multiple slides for each sample were combined. In addition to recording diatoms, we also noted sponge spicules, silicoflagellates, pollen grains, opal phytoliths, inorganic particulates, plant fragments, and other organic fibers.

Some workers may wonder whether our samples are contaminated and, therefore, unreliable indicators of atmospheric diatom transport. We recognize three possible stages in the processing of our samples when contaminants might be introduced:

• during drilling or core packing in the field,
• during melting and filtering at NICL, and
• in our laboratory when filters were prepared for examination.

At the South Pole, no source for diatoms is near the drilling or core-packing site. If contamination occurred at the latter times, one would expect to see a significant extra-antarctic component in the diatom assemblage. Because our samples are all dominated by typical antarctic species, we conclude that contamination is not a problem for this study.

Diatoms are a small but pervasive constituent of snow falling at the South Pole (and at Siple and Taylor Domes), although in a patchy pattern through both space and time (figure 2). Over 40 marine and nonmarine taxa were recorded (table). Abundances are extremely variable, ranging from nil to over 260 specimens in individual samples. Of 136 samples

Figure 2. Diatom abundance fluctuations (specimens per liter) and percentage of nonmarine specimens in the South Pole core. Ages are calendar years based on correlation with the adjacent 1981 core at South Pole (Gow personal communication).

• 34 percent contain more than 75 percent marine specimens,
• 4 percent are more than 75 percent nonmarine,
• 35 percent have intermediate mixtures of marine and nonmarine taxa, and
• the remainder are barren or are dominated by species of uncertain provenance.

Most recorded species have been reported by us or other workers from a variety of antarctic sites (table). Not all taxa we report have yet been associated with antarctic source areas and may represent transport from remote locations such as the other southern continents. Census data for individual samples will be available in a separate publication (D. Kellogg in preparation).

Taxon	Sample/a	A/b	Habitat/c	Reported locations/d	Notes
Achnanthes lanceolata	SP981 (2)	1	FR	EO, KG, SI
Achnanthes sp.	SP 981 (2)	1	FR
Actinochlus ehrenbergi	SP 440, 1460 (1)	1	MAR	A
Actinoptychus senarius	SP 1460 (1)	1	MAR	R1, RP
Chaetoceros diadema	TD pit D, 0 cm (1)	3	MAR		Polar waters
Chaetoceros sp.	TD pit K, 93 cm (1)	3	MAR	A, W
Coscinodiscus marginatus	SP 1532 (10)	1	MAR	R1, RP
Coscinodiscus radiatus	SP 1532 (6)	1	MAR	K (in red snow)
Cyclotella comta	SP 1704 (3)	1,3	BR	A1, M, TV
Cyclotella comta v. oligactis	SP 981 (24)	1	C	A`, TV
Cyclotella glomerata	SP 223 m(3)	1	FR	A, M
Cyclotella pseudostelligera ?	SP 224.5 m(3)	1	BR		Often counted as C. stelligera
Cyclotella stelligera	SP 981 (184)	1,3	C	A1, LM, M
Cyclotella striata ?	SP 223 m(11)	1	BR
Cyclotella sp.	SP 726 (21)	1,3	C	A, M, R1	Probably C. stelligera
Cymbella lunata	SP 1265 (1)	1	FR	SO	As Encyconema gracilis
Denticulopsis hustedtii	SP 1449 (1)	1	MAR	A, R1, RP, W	Miocene
Diploneus smithii	SP 440 (1)	1	BR	LH
Diploneus sp.	SP 182 (2)	1		A1
Fragilaria pinnata	SP 981 (1)	1	FR	KG, LG, SI
Fragilaria virescens	SP 981 (6)	1	FR	DV
Grammatophora sp.	SP-37 (1)	1	MAR	A, RP
Melosira distans	SP 223 m (17)	1,3	FR	A1, DV, M
Melosira granulata	SP-37 (28)	1	FR	LG	=Aulasoseira granulata
Melosira sp.	SP 981, 458 (2)	1	FR	A, M, R1	Probably M. granulata
Navicula festiva	SP 223 m (2)	1	FR	KG	NZ (Harper, personal communication)
Navicula muticopsis	SD camp (2)	2	FR	DV, LM, M, RO, TV
N. muticopsis v. evoluta	TD Pit 50S, 84 cm (1)	3	FR	M, TV
Navicula muticopsis n.v.	SD S50 W50 (15)	2	FR		Possible new variety?
Navicula sp.	SP 1637 (2)	1,2	FR?	A1, M, TV
Nitschia aricularis ?	SD S50 W50 (3)	2	FR	EO
Nitschia amphibia	TD pit E, 120 cm (1)	3	FR		Arctic
Nitschia closterium	TD pit I, 0 cm (1)	3	BR	M
Nitschia curta	SD S50 W50 (1)	2	MAR	A, M, R, R1, R2, RP, TV
Nitschia cylindra	TD pit 50S, 0 cm (11)	3	MAR	A, R1, R2
Nitschia gracilis	SD S50 W50 (1)	2	FR	SI
Nitschia microcephala ?	SD S50 W50 (3)	2	FR		Europe
Nitschia obliquecostata	TD pit 50S, 84 cm (1)	3	MAR	A, M, R1
Nitschia sublineata	TD pit D 0 cm (1)	3	MAR	A, M, R1
Nitschia sp.	SP 1460, 213 m (4)	1,2	MAR/FR	A, M, R1, RP, TV
Paralia sulcata	SP (5 samples) (1)	1	MAR	M, RP	=Melosira sulcata
Pinnularia nodosa	SP 1677 (1)	1	FR		NZ (Cassie 1984)
Pinnularia maior	SP 981 (2)	1	FR		Tierra del Fuego (Frenguelli 1923) (as Navicula maior); NZ (Cassie 1984)
Pinnularia sp.	SP 1637 (8)	1	FR?	M
Pseudoneunotia doliolus	SP 1449, 1460 (1)	1	MAR		Subtropics, Pleistocene
Rhabdonema sp.	SP 1440 (1)	1	MAR	M, RP
Stephanodiscus astraea	SP (6 samples) (1)	1	FR/BR		W. Europe
Stephanopyxis turris	TD pit 50S, 0 cm (1)	3	MAR	M, R1, RP
Synedra fasciculata	SP 981 (5)	1	FR/BR
Tabellaria flocculosa	SP 981 (2)	1	FR	A1, TV
T. fenestrata/quadriseptata	SP 223 m (2)	1,2	FR	A1, DV
Thalassionema nitzschiodes	SP 458, 1532 (4)	1	MAR	A, M, R1, RP, TV, W	See T. longissima
Thalassiosira eccentrica	TD pits B&D (1)	3	MAR	A, R1
Thalassiosira occulus-iridis	TD pits D&G(1)	3	MAR	A
Thalassiosira sp.	SP 1662 (14)	1,2	MAR	A, M, R1, RP
Thalassiothrix longissima	SD N50 W50 (119)	2,3	MAR	A, M, R1, RP, W	Includes T. nitzschiodes fragments
Trachyneis aspera	TD pit E, 120 cm (1)	3	MAR	A
Trachyneis sp.	TD pit D, 80 cm (1)	3	MAR
Centric diatom fragments	SP 1460 (53)	1,2,3		A, M, R1, RP, TV	Probably mostly marine taxa
Pennate diatom fragments	SP 981 (7)	1
Unidentified	SP 1704 (3)	1,3

aSP=South Pole; numbers are calendar age in years A.D. or depth in meters if older than 37 B.C.; TD=Taylor Dome, pit number and depth; SD=Siple Dome pit number; numbers in parantheses are maximum value recorded for the taxon in the sample listed.

bAssemblage: 1=South Pole; 2=Siple dome; 3=Taylor dome.

cMAR=Marine; FR=nonmarine; BR=brackish; C=possibly nonmarine but common in antarctic marine samples.

dA=Amundsen Sea marine sediments (Kellogg and Kellogg 1987a), A1=sediments and/or water on Amundsen Sea islands (Kellogg and Kellogg 1987a), DV=lakes and ponds in dry valleys (Seaburg et al. 1979), EO=East Ongul Islands (Karaswa and Fukushima 1977), K=Kerguelen Island, red snow (Fritsch 1912b), KG=King George Island (Schmidt et al. 1990), M=McMurdo Ice Shelf (Kellogg and Kellogg 1987b), LG=Lake Glubokoye (Lavrenko 1966), LH=Larsemann Hills (L. Heidi) (Gillieson 1991), LM=Lake Miers, Dry Valleys (Baker 1967), R=Ponds and sediments on Ross Island (Fritsch 1912a, and/or West and West 1911), R1=Ross Sea sediments (Truesdale and Kellogg 1979), R2=Ross Sea sediments (Barron and Burckle 1987), RP=Ross Ice Shelf Project site J-9 (Kellogg and Kellogg 1986), SI=Signy Island (Oppenheim 1990), SO=South Orkney Islands (Frenguelli 1923), TV=Taylor Valley deltas (Kellogg et al. 1980), W=west antarctic ice sheet beneath ice stream B (Scherer 1991).

Sources and atmospheric transport of diatoms

Diatoms are extremely light and easily transported by winds (e.g., the well-known diatom deposits in the equatorial Atlantic derived from Saharan Africa; Folger 1970), and winds in Antarctica are known to reach very high velocity. The antarctic surface windfield is dominated by katabatic flow, outward and down from high ice domes toward the sea (Parish and Bromwich 1987). Storms tend to track around the continent. Occasional large storms break through the circumflow and penetrate to the South Pole (Bromwich and Robasky 1993). Our diatoms were probably carried by these episodic events, which occur today at most a few times annually. An alternative transport mechanism, stratospheric return (poleward) flow, is unlikely because most of our diatoms are antarctic endemics whereas most stratospheric particles are entrained in tropical areas. Terrestrial sediments containing marine and nonmarine diatoms probably serve as the most important diatom sources. We envision diatom entrainment as episodic, perhaps occurring only a few times in a decade, and responsible for the low background level of less than 20 diatoms per liter of melted ice typical for approximately 70 percent of our samples.

Samples with higher diatom concentrations may represent short periods during which higher than normal surface winds occurred in a particular source area, or in more than one area of the coastal zone.

Specific provenances for our diatoms cannot be identified because most individual species have been reported from a number of locations (table). Marine diatom-bearing sediments are widespread in the dry valleys area of the Transantarctic Mountains, especially where Late Wisconsin Ross Sea Drift (Stuiver et al. 1981; Denton et al. 1989) is exposed. The marine species reported here are present in virtually every sample of this drift that we have examined. Similar diatom-bearing sediments are probably widespread elsewhere around the continent. That most marine specimens have been reworked from subaerially exposed sediments is further suggested by the high degree of dissolution and breakage exhibited by the marine specimens. Nonmarine diatoms are also widespread in the dry valleys, in subaerially exposed deposits, and in virtually every lake, pond, or seasonal melt pool. Many of these water bodies are ephemeral or display fluctuating water levels. Complete or partial desiccation exposes fossil material for transport by winds as described above.

Diatom deposition: Implications for the Sirius Group

Diatoms settling on the polar plateau are buried and trapped in the snow. As the snow compresses to ice and flows gradually down and outward toward the ice sheet margin, the diatoms are carried along until they reach either the glacial bed or come to the surface in an area with surface ablation (where flowlines outcrop). In the former case, diatoms from many years of deposition may become concentrated at the ice bed in morainal material. Thus, atmospherically transported diatoms have the potential to result in a reworked assemblages containing diatoms of different ages.

Not all diatoms carried through the atmosphere end up in the ice. If they land on an ice- or snow-free area, they may be retransported unless they fall in cracks or crevices protected from the wind. Evidence for this diatom-trapping mechanism was presented by Burckle (1995, in preparation) who found Pliocene/Pleistocene diatoms in cracks and crevices of antarctic sedimentary rocks. Most atmospherically transported diatoms trapped in cracks and crevices of glacigenic sedimentary deposits should remain near the surface (Stroeven and Prentice 1995), but penetration is also possible, even in compact sediments such as the Sirius Group. A thin layer of snow falling on such a sediment often melts because of heat retention by the relatively dark surface, carrying small amounts of meltwater deep into the sediment by capillary action, entraining the tiny (mostly less than 100 micrometers), delicate diatoms. Penetration should be enhanced by the presence of frost cracks in the compact Sirius sediments. We have no data suggesting how deep such penetration may go but a meter or more seems possible. We conclude that atmospheric transport routinely distributes marine and nonmarine diatoms across the antarctic ice sheet. Our data demonstrate that Sirius Group contamination by younger diatoms is unavoidable because of the pervasive and widespread effects of this atmospheric transport.

Together with our work, studies by Burckle (1995, in preparation) and Burckle and Potter (1996) of diatoms in sedimentary and igneous antarctic rocks cast serious doubts on the validity of presumed in situ Pliocene marine diatoms in the Sirius Group because the Pliocene diatoms are not demonstrably associated with the glacial sediments in which they occur. Hence, the entire construct of a warm Pliocene event in Antarctica is in doubt. A more complete presentation of ideas and data presented in this paper may be found in Kellogg and Kellogg (1996).

We thank Eric Steig, Pieter Grootes, Ken Taylor, Joan Fitzpatrick, Ellen Mosely-Thompson, Jeff Hargreaves, and Todd Hinckley for assistance in ice-core sampling. Tony Gow kindly provided the stratigraphy of the 1981 South Pole core. Ben Carter of Corning-Costar supplied modified Nuclepore filters. Lloyd Burckle and Terry Hughes discussed these results and read drafts of the manuscript. Margaret Harper called our attention to a number of reports of individual species listed in table. Financial support was provided by National Science Foundation grant OPP 93-16306 to D.E. Kellogg.

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Davida E. Kellogg and Thomas B. Kellogg, Institute for Quaternary Studies and Department of Geological Sciences, University of Maine, Orono, Maine 04469