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Recycled marine microfossils in glacial tills of the Sirius Group at Mount Fleming: Transport mechanisms and pathways
The documentation of marine microfossils in consolidated glacial sediments of the Sirius Group (McKelvey et al. 1991), radically altered the range of glacial and climatic interpretations of this deposit (e.g., Mercer 1968; Brady and McKelvey 1979; Barrett and Powell 1982; Harwood 1983; Webb and Harwood 1991; Stroeven et al. 1994; Stroeven, Prentice, and Kleman in press). A resolution of these disparate viewpoints depends critically on the inferred transport mechanism of marine microfossils from their source areas to these glacial sediments (Sugden 1992; Stroeven and Prentice 1994).
We tested marine diatom transport to the Sirius Group by considering the microfossil distribution within one key glacial deposit in the dry valleys reported to contain Neogene marine diatoms: the Sirius Group at Mount Fleming (Harwood 1986a) (figure 1). We assumed that the microfossils were recycled by the ice depositing the till and expected to find a random occurrence of diatoms in samples from the investigated deposits.
Figure 1. Index map of the dry valleys in the McMurdo Sound region with surface elevation contours. Mount Fleming is situated at the head of Wright Valley, southwest of Wright Upper Glacier. UF and LF refer to the locations of the upper and lower Fleming tills the Sirius Group at Mount Fleming (Stroeven 1994; Stroeven and Prentice in preparation).
The lithostratigraphic subdivision was threefold: consolidated, unweathered dark gray sediments overlain by moderately consolidated, weathered light gray sediments, and capped by yellow-reddish unconsolidated sediments (figure 2). We interpret the bottom two units as lodgement till emplaced by alpine ice and consider them Sirius Group equivalents (Stroeven, Prentice, and Borns 1992; Stroeven et al. 1994; Stroeven and Prentice in preparation). Because hallmark characteristics of lodgement till are absent for the surface unit, however, it could be a lag deposit from the underlying lodgement till or a glacial or nonglacial deposit unrelated to the underlying Sirius Group till. At excavations 91-001 and 91-002, dark-gray unweathered till cropped out (figure 2). At least one microfossil sample was collected from all lithostratigraphic units present in each excavation.
Figure 2. Pit stratigraphies for excavations 91-001, 91-002, 91-020, 91-031, and 91-038 pertain to site UF, whereas excavations 91-053, 91-054, and 91-055 pertain to site LF in figure 1. Given are the location of the excavations; the approximate depth of the excavations (depth scale is in 2.5-centimeter increments); the lithostratigraphy of the excavations; and sample location and number. In all but two excavations, three units could be distinguished: from the top-down, a loose yellow-reddish layer, capping a light-gray, massive, structureless, moderately consolidated layer, and a massive, structureless, consolidated dark-gray layer. Also given are the locations of sediment and microfossil samples (ASF 91-).
Diatom extraction followed improved standard procedures (Harwood 1986b; Harwood, Grant, and Karrer 1986; Stroeven 1994). The diatom extraction technique relies on the specific size and hydrodynamic properties of diatoms for extraction. Samples averaging 0.5 kilogram were dispersed in Calgon solution for 48 hours and introduced in a 1.2-meter settling tube. Deionized water that was filtered at 0.45 microns entered at the base of this settling tube through a plastic rod and agitated the sediment. We calculated that a water column having an upward velocity of 0.04 meters per second will float most common diatoms (i.e., diatoms that have a specific gravity less than 2.25 and a diameter less than 100 microns). The suspended material was siphoned off near the top of the tube and sieved at 25 microns. Further separation of the fraction larger than 25 microns occurred through heavy-liquid separation and centrifuging at 500 and 2,000 revolutions per minute.
We ran all samples twice through the microfossil extraction unit. The first run was on a dissolved split of the raw sample. Following these preliminary results (Stroeven and Prentice 1994), we considered the possibility that diatoms remained "stuck-together" in diatomaceous sediment microclasts. To remove all lingering organic material that could bind these microclasts, we treated the coarse (i.e., larger than 100 microns) fraction of the first run with 30 percent hydrogen peroxide and repeated the improved standard procedure. Improvements in diatom-yield were negligible (table), however, indicating that diagenetic processes played no significant role in observed diatom abundance distributions.
Occurrence of microfossils in samples from the Sirius Group at Mount Fleming. Samples in bold typeface have been taken in the loose surface unit, whereas others have been taken at depth, and refer to lodgement till samples. We ran all samples twice through the microfossil extraction unit. Improvements in diatom-yield during the second run were negligible (bottom line), indicating that diagenetic processes played no significant role in observed diatom abundance distributions. Refer to figures 1 and 2 for excavation locations (ASE) and sample locations (ASF) within each pit. Given are B=barren, B-R=barren-rare (1 diatom fragment/slide), R=rare (2-9 diatom fragments/slide), A=abundant (10 diatom fragments/slide), P=present, and HW=Harwood (1986a) samples from Mount Fleming.
In all, 23 samples from eight pits were examined for and yielded marine and nonmarine diatoms, diatomaceous sediment microclasts, radiolarian fragments, and one sponge spicule (figure 3, table). Our results indicate the existence of a marked microfossil abundance decline from the surface unit into the semiconsolidated till. This distribution is best illustrated in pits 91-031, 91-053, and 91-054 (figure 2, table). Moreover, microfossils are better preserved in the surface unit than in the till units, where they were only identified to the genus-level.
Figure 3. Diatoms recovered from surface sediments overlying the Sirius Group at Mount Fleming. Enlargement is 1,000 x, and the diatoms were photographed from sample ASF 91-041 (figure 3), except where indicated otherwise. A. Thalassiosira vulnifica. B. Eucampia antarctica, sample ASF 91-011; C. Thalassiosira vulnifica. D. Coscinodiscus marginatus. E. Thalassiosira kolbeii. F. Thalassiosira inura. G. Stellarima microtrias, enlarged 750x. H. Actinocyclus ingens. Identifications as given by D. Harwood, University of Nebraska-Lincoln.
We regard it unlikely that better preserved marine diatoms at the surface were derived from the poorly preserved marine diatoms in the lodgement till units by lag processes. Instead, we suggest that the lodgement till was barren of biological material and that few diatom fragments at depth indicate a recycling-downward process in polar-desert conditions.
Critical evidence that has been used to support the subglacial reworking of diatoms into the Sirius Group till is the presence of diatomaceous sediment microclasts in the matrix of these tills (e.g., Harwood 1983, 1986a,c; Harwood, Grant, and Karrer 1986). Diatomaceous sediment microclasts were absent, however, in the matrix of the lodgement till units, except at pit 91-055 (figure 2, table). These diatomaceous sediment microclasts range in size from 25 to 40 microns, however, and do not preclude eolian transport. We suggest that the inverse stratigraphy observed in pit 91-055 is a function of either nonrepresentative surface sampling or the disintegration of fewer and larger diatomaceous sediment microclasts into a multitude of smaller ones by the extraction procedure, or both. Similarly, we suggest that the apparent abundance of microfossil fragments in sample 91-009 (pit 91-020; figure 2, table) signifies the breakdown of one or few intact diatoms into a number of unrecognizable fragments.
These results indicate that for one deposit on which the dynamicists built their viewpoint, the Sirius Group at Mount Fleming, the glacial conveyor mechanism appears erroneous. We suggest that if the microfossils in the surface unit arrived by glacial transport mechanisms, they ought to occur within a glacial deposit of younger age than the underlying lodgement tills. If the microfossils arrived by eolian processes, they should occur in surface deposits of disparate origin but of unknown age, both glacial and nonglacial, given the ability for the deposit to trap fine-grained, wind-blown material (McFadden, Wells, and Jercinovich 1987; Wells et al. 1995).
Denton, Prentice, and Burckle (1991) proposed scenarios by which airborne diatoms were incorporated into Sirius Group glacial sediments. For these diatoms to become airborne, however, at least two requirements must be met:
The latter is important because eolian transport at present recycles varying proportions of marine and nonmarine diatoms to east antarctic ice sheet plateau locations (Burckle et al. 1988; Kellogg and Kellogg 1996). In addition, the source area from which these marine diatoms were scoured by wind remained enigmatic, because the preponderance of planktic taxa over benthic taxa in Sirius Group sediments seemingly invalidates uplifted near-shore marine sediments (e.g., Webb and Harwood 1991) and because a stable cryosphere and marine diatom source areas appear to be conditions in contradiction. This contradiction arises because the stabilists melt-down mechanisms cannot account for the necessary ice recession (Denton et al. 1993). Finally, the absence in Sirius Group samples from Mount Fleming of marine diatom species such as Nitzschia curta that dominate today in circumantarctic waters (Burckle 1984) requires that the eolian microfossil conveyor operated before such species became dominant.
We advocate alternative mechanisms for deglaciation and outline one plausible scenario with marine diatom source areas and transport pathways. This scenario highlights eolian transport of Plio-Pleistocene diatoms to high-elevation deposits and is constrained by ice-volume fluctuation during the early and middle Pliocene. An, albeit short-lived, ice-volume reduction of between 10 and 40 percent of the present ice volume appears reasonable (Kennett and Hodell 1993). Denton et al. (1993) present argon-39/argon-40 constraints on the upper limit of glaciation in Taylor Valley during the last 2.97 million years. Therefore, ice is an unlikely transporting agent for diatoms in those deposits in the dry valleys situated above the last 2.97 million years maximum ice limit and having early-middle Pliocene diatom assemblages reported in them (Stroeven et al. in press).
We regard Wilkes Basin as a prime candidate for partial deglaciation because at its margin facing the open ocean, the ice sheet is partly grounded at depths in excess of 1,000 meters below present-day sea level (Drewry 1983). During proposed periods of ice recession, with associated delayed isostatic recovery, marine sedimentation occurred in these basins. Upon emergence of basin floors, strong katabatic winds from the large but shrunken east antarctic ice sheet caused deflation of the exposed marine sediments, and airborne marine diatomaceous microclasts were blown to presently high-altitude Transantarctic Mountain sites (Stroeven et al. in press). The eolian source was closed when these basins were overrun by a reexpanded east antarctic ice sheet. For marine diatom transport by glaciers, the whole length of the Wilkes and Pensacola basins has to be deglaciated to explain the spatial distribution of Pliocene marine diatom-bearing tills. For eolian diatom transport (which is directionally variable and turbulent), however, only partial deglaciation of these basins and subsequent uplift above contemporaneous sea-level, is required.
We are indebted to our co-investigators M. Helfer and C. Schlüchter and to G. Simonds, S. Dunbar, S. Iversen, and D. Rosenthal for fieldwork. D. Harwood supervised the diatom extraction procedures, identified recovered diatom species, and provided us with helpful student assistance. Helpful comments by L.H. Burckle, D. Goodwillie, and G.C. Rosqvist improved the manuscript. H. Drake drew some of the figures. We thank VXE-6 for excellent field support. This work is supported by National Science Foundation grant OPP 90-20975 to Michael L. Prentice and Harold W. Borns, Jr., by the Swedish Natural Science Research Council, and by the Swedish Society for Anthropology and Geography André grant to Arjen P. Stroeven.
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Arjen P. Stroeven, Department of Physical Geography. Stockholm University, S-106 91 Stockholm, Sweden
Michael L. Prentice, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire 03824-3525
Johan Kleman, Department of Physical Geography, Stockholm University, S-106 91 Stockholm, Sweden