THOMAS B. KELLOGG, Department of Geological Sciences and Institute for Quaternary Studies, University of Maine, Orono, Maine 04469
LLOYD H. BURCKLE, Lamont-Doherty Earth Observatory, Palisades, New York 10964
DAVIDA E. KELLOGG, Department of Geological Sciences and Institute for Quaternary Studies, University of Maine, Orono, Maine 04469
JAMES L. FASTOOK, Department of Computer Sciences and Institute for Quaternary Studies, University of Maine, Orono, Maine 04469
Recent studies report diatoms in a variety of glacigenic and subglacial deposits. Some interpretations of these occurrences are controversial and require ice-sheet collapse. Harwood (1986a) found freshwater diatoms in the base of the Greenland Ice Sheet. Similarly, diatoms were found in sediments of the Sirius Group (Kellogg and Kellogg 1984; Webb et al. 1984; Harwood 1986b), which crops out along the Transantarctic Mountains; on the surface of the east antarctic ice sheet at Elephant Moraine (Faure and Harwood 1990); and beneath the west antarctic ice sheet at Upstream B (Scherer 1991, 1993). Here, we summarize an alternative mechanism to account for the presence of diatoms in glacigenic sediments, one which does not require collapse of an ice sheet.
Assuming no major ice-sheet collapse, two independent processes could combine to deliver diatoms to subglacial sediments: eolian transport (onto the ice sheet) followed by glacial transport (to the base of the ice sheet or to the ice-sheet margin). Both freshwater and marine diatoms have been recovered from ice and snow samples on present-day ice sheets (Burckle et al. 1988; Ram, Gayley, and Petit 1988; Ram and Gayley 1991, 1994; Kellogg and Kellogg 1996). These occurrences are all due to surface winds carrying diatoms, and even diatomaceous sediment clasts, to central regions of Antarctica as well as to other ice sheets from sources beyond the ice-sheet margins. Kellogg and Kellogg (1996, in preparation) demonstrated that winds transport diatoms, opal phytoliths, and sponge spicules to the South Pole. Burckle et al. (1988) found diatoms in ice at Dome C (figure 1), whereas Burckle and Potter (1996) reported diatoms and diatomaceous clasts in cracks in Paleozoic and Mesozoic igneous and sedimentary rocks from Marie Byrd Land and the McMurdo Dry Valleys (figure 1).
Ice melting off the bed or along the margin of an ice sheet is ultimately derived from snow accumulation on the ice sheet. Diatoms deposited on an ice sheet by eolian processes are buried and trapped in the snow. Snow is compressed to ice and moves vertically and horizontally through the ice column along a flowband until it either reaches the ice bed or the ice margin, where it is melted off or otherwise removed by ablation (Hooke and Hudleston 1978; Paterson 1994; Gow and Meese 1996). Solid particles in the ice are also transported in this manner (figure 2). Thus, atmospherically transported diatoms have the potential to occur in reworked assemblages containing diatoms of different ages and habitats (Kellogg and Kellogg 1996). If basal melting causes the flowlines to intersect the glacier bed, the diatoms are delivered to the subglacial bed (figure 2 A ). Note that deposition takes place without collapse of the overlying ice sheet.
If flowlines do not move to the ice bed because it is frozen (figure 2 B ) or freezing (figure 2 C ), diatoms should crop out below the equilibrium line near the ice-sheet periphery. An example might be Elephant Moraine in northern Victoria Land, where flowlines intersect the surface because of local surface ablation. If flowlines crop out at the ice-sheet margin, diatoms may accumulate in end moraines rather than ground moraines (basal tills) as in the melting-bed scenario. Our last example is of a marine-based ice sheet feeding an ice shelf (figure 2 D ). Beneath grounded portions of the ice sheet, basal melting should be accentuated under ice streams because of frictional heating. Thus, diatom deposition should be concentrated in ice-stream beds, as at Upstream B. If basal melting occurs beneath an ice shelf, flowlines intersecting the bed should release diatoms and other particles to the underlying water where they settle to the sea floor, contaminating other sedimentary components being deposited contemporaneously (e.g., Ross Ice Shelf Project site J-9).
These pathways for diatoms are described by a simplified model of a flowband using reasonable assumptions for the dome height, velocity field, surface accumulation rates, and ice hardness for various basal melting rates (Burckle et al. 1997). The model estimates residence times for diatoms deposited along the top surface of the flowband. At a basal melt rate of 0.01 meter per year, the trajectories of all flowlines but those nearest the margin intersect the bed; diatoms deposited near the dome reach the bed about halfway down the flowband. Larger values of basal melting lead to the diatoms reaching the bed even faster and closer to the point of origin.
In view of these results, we suggest alternate interpretations of some reported diatom occurrences in glacially derived sediments in Antarctica (Burckle et al. 1997). We thank T. Hughes, J. Kleman, N. Reeh, and A. Stroeven for helpful comments during various stages of this study. Research was supported by grants from the National Science Foundation: OPP 92-20216 to Lloyd H. Burckle, OPP 94-16306 to Davida E. Kellogg, and EPSCoR grant R11-8922105 to Thomas B. Kellogg and James L. Fastook. Lloyd H. Burckle also acknowledges support from the Swedish Natural Science Research Council.
Burckle, L.H., R.I. Gayley, M. Ram, and J.-R. Petit. 1988. Diatoms in antarctic ice cores: Some implications for the glacial history of Antarctica. Geology , 16(4), 326-329.
Burckle, L.H., D.E. Kellogg, T.B. Kellogg, and J. Fastook. 1997. A new mechanism for emplacement and concentration of diatoms in subglacial deposits. Boreas , 26(1), 55-60.
Burckle, L.H., and N. Potter, Jr. 1996. Pliocene-Pleistocene diatoms in Paleozoic and Mesozoic sedimentary and igneous rocks from Antarctica: A Sirius problem solved. Geology , 24(3), 235-238.
Faure, G., and D.M. Harwood. 1990. Marine microfossils in till clasts of the Elephant Moraine on the east antarctic ice sheet. Antarctic Journal of the U.S. , 25(5), 23-25.
Gow, A.J., and D.A. Meese. 1996. Nature of basal debris in the GISP2 and Byrd ice cores and its relevance to bed processes. Annals of Glaciology , 22, 134-140.
Harwood, D.M. 1986a. Do diatoms from beneath the Greenland Ice Sheet indicate interglacials warmer than present? Arctic , 39(4), 304-308.
Harwood, D.M. 1986b. Diatom biostratigraphy and paleoecology and a Cenozoic history of antarctic ice sheets. (Ph.D. Thesis, Ohio State University. Columbus, Ohio.)
Hooke, R.L., and P.J. Hudleston. 1978. Origin of foliation in glaciers. Journal of Glaciology , 20(83), 285-299.
Kellogg, D.E., and T.B. Kellogg. 1984. Non-marine diatoms in the Sirius formation. Antarctic Journal of the U.S. , 19(5), 44-45.
Kellogg, D.E., and T.B. Kellogg. 1996. Diatoms in South Pole ice: Implications for eolian contamination of Sirius Group deposits. Geology , 24(2), 115-118.
Kellogg, D.E., and T.B. Kellogg. In preparation. Diatoms in antarctic ice cores.
Paterson, W.S.B. 1994. Physics of glaciers (3rd edition). Oxford: Elsevier.
Ram, M., and R.I. Gayley. 1991. Long-range transport of volcanic ash to the Greenland Ice Sheet. Nature , 349, 401-404.
Ram, M., and R.I. Gayley. 1994. Insoluble particles in polar ice: Identification and measurement of the insoluble background aerosol. Geophysical Research Letters , 21(6), 437-440.
Ram, M., R.I. Gayley, and J.-R. Petit. 1988. Insoluble particles in antarctic ice: Background aerosol size distribution and diatom concentration. Journal of Geophysical Research , 93(D7), 8378-8382.
Scherer, R.P. 1991. Quaternary and Tertiary microfossils from beneath Ice Stream B: Evidence for a dynamic west antarctic ice sheet history. Palaeogeography, Palaeoclimatology, Palaeoecology (Global Planetary Change Section) , 90(4), 395-412.
Scherer, R.P. 1993. There is direct evidence for Pleistocene collapse of the west antarctic ice sheet. Journal of Glaciology , 39(133), 716-722.
Webb, P.-N., D.M. Harwood, B.C. McKelvey, J.H. Mercer, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice-volume variation on the east antarctic craton. Geology , 12(5), 287-291.