DAVIDA E. KELLOGG and THOMAS B. KELLOGG, Institute for Quaternary Studies and Department of Geological Sciences, University of Maine, Orono, Maine 04469
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 3.0-2.5 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).
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.
Material for this study comes from the 227-meter ice core drilled at the South Pole by the Polar Ice Coring Office during the 1980-1983 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 ¥, 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:
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
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).
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.
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 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 Mosley-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 the table. Financial support was provided by National Science Foundation grant OPP 93-16306 to D.E. Kellogg.
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Reprinted from the September 1997 online issue of Antarctic Journal of the United States (volume 32, number 1).