Submitted to Journal of Nanoscience and Nanotechnology (communication)
Frustules to fragments, diatoms to dust: how degradation of microfossil
shape and microstructures can teach us how ice sheets work
Reed P. Scherer Geology and Environmental Geosciences, Northern Illinois University,
DeKalb, IL 60115, reed@geol.niu.edu
Charlotte M. Sjunneskog Geology and Environmental Geosciences, Northern Illinois
University, DeKalb, IL 60115, cmsjunne@geol.niu.edu
Neal R. Iverson Geological and Atmospheric Sciences, Iowa State University, 253 Science I
Hall, Ames, IA 50011, niverson@iastate.edu
Thomas S. Hooyer Wisconsin Geological Survey, University of Wisconsin, 3817 Mineral
Point Road, Madison, WI 53705, tshooyer@facstaff.wisc.edu
√ = confirmed
Received December 12, 2003; revised version received February 13, 2004.
Abstract
In a laboratory experiment we investigated micro and nano-scale changes in fossil diatom
valves and in the texture of diatomaceous sediments that result from ice sheet overburden
and subglacial shearing. Our experiment included compression and shearing of Antarctic
diatom-rich sediments in a ring shear device, and comparison of experimental samples
with natural glacial sediments from the Antarctic continental shelf. The purpose of the
experiment is to establish objective criteria for analyzing subglacial processes and
interpreting the origin of glacial-geologic features on the Antarctic continental shelf. We
find distinct changes resulting from different glacial settings, with respect to whole
diatom frustules, diatom micromorphology, and micro-textural properties of sedimentary
units. By providing constraints on subglacial shearing, these observations of genetically
controlled micro and nano-scale diatom structures and architecture are contributing to
the understanding of large-scale glacial processes, aiding the development of models of
modern ice sheet processes, and guiding interpretation of past ice sheet configurations.
Microfossils in sedimentary sequences are the physical expression of past biological
processes, but as fossils they are no longer themselves biological entities. As mineralized
fossils, diatoms and other microfossils have become sedimentary particles subject to physical
transport and mechanical degradation. Unlike most “sand” or “silt” grains, fossils are particles
with known initial conditions, including genetically controlled micro and nano-scale structures.
We know their original size, shape, geologic age, and environment of origin. Furthermore, submicron sized fragments can be unequivocally identified as diatom silica. Consequently, these
morphological properties make it possible to use diatom micromorphology to provide unique
insights into physical processes of depositional and post depositional forces.
Large-scale ice sheet processes, including mass balance and controls on fast glacier
flow, often hinge on a subglacial shear zone within unconsolidated, waterlain sediments (till)
that may be as little as a few cm thick1. Understanding these basal conditions is critical to
modeling ice sheet behavior, and to interpreting past ice sheet configurations from analysis of
glacial sedimentary deposits2. Using scanning electron and light microscopy we evaluated
micromorphological changes in diatom valves and diatomaceous sediments caused by
compaction and shearing under conditions similar to those beneath the West Antarctic ice
sheet.
Sediments from the Antarctic continental shelf include abundant but variably
fragmented diatoms. Diatom valves have been shown to be able to withstand high direct
pressure (up to 560 N mm-2)3, but hardness and elastic modulus is highly variable within a
single species, depending on region within the frustule4. Despite their strength, mechanical
fragmentation of diatoms in sedimentary deposits is widespread. Partial dissolution greatly
increases susceptibility to fragmentation, but simple mechanical degradation can occur in
several ways: (1) initially through predation, (2) through compaction by overlying sediments or
grounded ice, or (3) via shearing of subglacial sediment caused by basal sliding of glaciers
over diatomaceous deposits. Fragmentation by compaction and shearing coincide with
dramatic loss of pore space in diatomaceous sediments. Unique fragmentation patterns
produced by these processes are investigated in an attempt to establish a quantitative index that
might distinguish compaction from shearing. Such an index will greatly aid the interpretation
of active and relict glacial sedimentary deposits.
We report results of a laboratory experiment using a large ring shear device (Fig. 1)
designed to mimic subglacial conditions5, into which was loaded ~20 l of modern
diatomaceous sediment recovered from the Ross Sea, Antarctica. The purpose of the
experiment was to use degradation of fossil diatoms to evaluate, quantify, and distinguish
between strains due to both compaction from glacial overburden and to shearing to high strain
due to ice sheet basal sliding, under conditions comparable to beneath the West Antarctic ice
sheet. We used a normal load of 85 kPa, and internal shear strain reaching a maximum of 628,
following 4 successive shearing episodes (EP1 – EP4). We convert natural hemipelagic
diatomaceous sediments typical of the Antarctic continental shelf into an artificial till, then
compare the results with glacial facies deposited during Pleistocene glacial maxima6.
Understanding the transition from diatom-rich sediment to diatom-poor diamicton will greatly
aid in interpreting polar continental shelf sedimentary facies. Furthermore, these observations
provide insights into ice stream basal processes, contributing to our understanding of ice sheet
behavior7. Both of these objectives are important for modeling current and past ice sheet
stability and behavior.
Loss of diatoms in terms of absolute abundance (diatoms per gram dry sediment) is
high with simple compaction, but is dramatic with both compaction and shearing6.
Furthermore, we recognize distinct trends in breakage of whole diatoms based on their original
shape and degree of silicification. We observe that the ratio of centric (discoid) to pennate
(elongate) diatom valves (C/P) correlates well with shear strain: the higher the C/P ratio, the
higher the shear strain (Fig.3). Heavily silicified diatoms (e.g. Eucampia antarctica) are more
resistant to breakage, thus their relative abundance (percent) increases with shearing. Not
surprisingly, smaller diatoms have a higher preservation potential under shear. The relative
abundance of reworked Tertiary age diatoms also increases with increasing shear, but our
observations show that these tend to be small and heavily silicified examples (e.g.,
Denticulopsis spp. and small Actinocyclus ingens), which have already survived multiple
shearing events (ie., multiple glacial advance-retreat cycles).
The C/P ratio from the experiment compares well with data from Ross Sea glacigenic
sediments. Low C/P ratios are found in true hemipelagic diatomaceous mud, whereas high C/P
ratios characterize diamictons beneath active ice streams and those deposited in areas
interpreted as having been overridden and sheared by ice streams during the last (late
Pleistocene) glacial maximum. Intermediate ratios characterize diamicton from areas that had
experienced loosely coupled, or slow-moving grounded ice. Our results imply that, at least
with respect to Ross Sea deposits, a high C/P ratio (>10.0) indicates sheared till, and a low C/P
ratio (<1.0) indicates a hemipelagic deposit. This observation holds even if general
fragmentation is high due to compaction by ice or sediment overburden6 (Fig. 2).
Figure 3 illustrates the effects of compaction and shearing on micro and nano-scale
morphological features of a diatomaceous sedimentary matrix. We show that the nano-scale
architecture of diatoms is retained despite significant consolidation and fragmentation of the
frustules due to compression under normal load of 90 kPa for an extended period of time. In
contrast, effectively complete degradation of nano-scale diatom structures in the fine matrix is
evident in the highly sheared samples. Shearing to strains well under our maximum shear strain
of 628, resulted in little recognizable diatomaceous debris at high magnification, although
some large, robust centric diatoms do persist. It is still possible to generate statistically
significant C/P ratios on sheared samples, though only when a large volume of processed
material is observed. Once a diatom fractures, the fine fragments, which make up the bulk of
the sediment matrix (Fig 3-b, 3-c), apparently rapidly degrade with shearing. High
magnification analysis reveals that particles within the sheared matrix become highly aligned
with the plane of shear, and porosity becomes dramatically reduced (Figs. 3-h, 3-j).
These observations can be directly applied to glacial deposits in an effort to interpret
their origin and, thus, reconstruct past glacial conditions. As such, the results of this study have
specific application to interpreting glacial sedimentary deposits, and for modeling past and
present ice stream processes.
Although the strength of diatom frustules has been investigated(3, 4), no such work has
been performed to date on fossil material. The loss of incorporated organic material may make
diatoms more brittle. However, time and burial result in increased crystallization and reduced
hydration in opaline skeletons, thus increasing their strength. Consequently, certain
applications of microfossils as nanomachine parts may be better served using mined fossil
material than cultured diatoms. Such an investigation is a worthy next step.
Acknowledgements. Support came from NSF grants OPP-9980364 and EAR-0116385 to RS
and OPP-9530814 to NI. We thank the editors of this volume for taking a broad view of the
potential of diatoms in diverse practical applications in micro-science.
References
1. B. Kamb, Basal zone of the West Antarctic ice streams and its role in lubrication of
their rapid motion, in The West Antarctic Ice Sheet: Behavior and Environment,
Antarctic Res. Ser., edited by R. B. Alley and R. A. Bindschadler, AGU, Washington,
D. C. (2001), v. 77, p. 157.
2. R.B. Alley, D.E. Lawson, G.J. Larson, E.B. Evenson, and G.S. Baker, Stabilizing
feedbacks in glacier-bed erosion. Science 424, 758 (2003).
3. C.E. Hamm, R. Merkel, O. Springer, P. Jurkojc, C. Maier, K. Prechtel, and V.
Smetacek, Architecture and material properties of diatom shells provide efficient
mechanical protection. Nature 421, 41 (2003).
4. N. Almqvist, Y. DelAmo, B. L. Smith, N. H. Thomson, Å. Bartholdsson, R. Lal, M.
Brzezinski and P. K. Hansma. Micromechanical and structural properties of a pennate
diatom investigated by atomic force microscopy. Jour. Microscopy 202, 518 (2001).
5. N.R. Iverson, R.W. Baker, and T.S. Hooyer, A ring-shear device for the study of till
deformation: tests on tills with contrasting clay contents: Quatern. Sci. Rev., 16, 1057
(1997).
6. R. P. Scherer, C. M Sjunneskog, N. R. Iverson and T. S. Hooyer, Diatomaceous mud to
till: laboratory constraints on interpreting Antarctic continental shelf diamictons.
Geology (in press).
7. S. Tulaczyk, R. P Scherer and C. D. Clark, A ploughing model for the origin of weak
tills beneath ice streams: a qualitative treatment. Quaternary International 86, 59
(2001).
Key words: Antarctica, diatom, fragmentation, ice sheet models, micromorphology, ring shear,
Ross Sea, West Antarctic ice sheet
Figure Captions
Fig. 1. Ring shear device5. Shearing experiment in progress (left) and with the chamber
opened, following the experiment (right).
Fig. 2. Pennate/centric ratios plotted against centric/pennate ratios of variably sheared
sediments, illustrating the transformation from hemipelagic mud to sheared till. The inset (b)
illustrates results of the ring shear experiment, compared to a range of natural sediments from
Ross Sea deposits (a), plotted to the same scale (data presented with further discussion in
Scherer et al., in press6). We plot P/C against C/P in order to highlight the transition between
diatomaceous mud and sheared till endmembers.
Fig. 3. Scanning electron micrographs (JEOL JSM5610-LV) of the diatom-rich box core
sample from the Ross Sea used in the ring shear experiment. (a-c) Original uncompacted,
unsheared sediment. (a) The persistence of genetically controlled siliceous structures is evident
down to the sub-micron scale. (b, c) Diatom fragments strongly dominate the matrix in the
original sample. (d-e) Fragmentation from compaction without shearing occurs along natural
lines of weakness in diatom valves. (f, g) Although few diatoms are unbroken, identifiable
diatom debris persists after extended compaction under 90 kPa without shearing. (h-i) Once
sheared to maximum shear strain of 628, particles become highly aligned, and no recognizable
diatom structures remain. Discernable structures are only visible at these high magnifications,
where aligned particles can be recognized. Compare the highly sheared h and i with similarly
magnified c, d, and g. Scale bars = 5 μm, 1 μm and 50 nm, as illustrated.