Hi this is a geology mud assignment I need help with its about reading an article and paraphrasing. Thanks
PERSPECTIVES
Liu et al. conclude that invasions bring
about intense interactions between previously
geographically isolated species. In such asymmetric interactions, the B biotype is competitively superior and indigenous biotypes suffer
more from interactions with the B biotype than
the B biotype suffers from interactions with
the indigenous types. It still would be of interest to compare invasive populations of biotype
B with populations in its indigenous habitats of
the Middle East and Asia Minor to determine
whether biotype B inherently has invasive
characteristics, or whether populations have
been selected for through previous invasions.
Such questions of how invasive populations
compare with their original source populations
are among the most pertinent in invasion biology today (16).
Maintaining a long-term perspective is
important, as the results of Liu et al. show.
Brief snapshots of the event may not have led
to the same conclusions as did their longerterm study. Clearly, invasions provide opportunities for dramatic ecological and evolutionary experimentation. Unfortunately, invasions
come at tremendous environmental and economic costs, yet understanding interactions
between invaders and residents will continue
to be necessary for more effective control of
invasive species (9).
References
1. D. Pimentel, et al., Ecol. Econ. 52, 273 (2005).
2. A. K. Sakai et al., Annu. Rev. Ecol. Syst. 32, 305 (2001).
3. S.-S. Liu et al., Science 318, 1769 (2007); published 8
November 2007 (10.1126/science.1149887).
4. L. M. Boykin et al., Mol. Phylogenet. Evol. 44, 1306 (2007).
5. International Union for the Conservation of Nature and
Natural Resources (IUCN), Invasive Species Specialist
Group, “100 of the World’s Worst Invasive Alien Species”
(www.issg.org).
6. T. M. Perring, Crop Prot. 20, 725 (2001).
7. P. J. De Barro, J. W. H. Trueman, D. R. Frohlich, Bull.
Entomol. Res. 95, 193 (2005).
8. J. K. Brown et al., Annu. Rev. Entomol. 40, 511 (1995).
9. E. A. Dame, K. Petren, Anim. Behav. 71, 1165 (2006).
10. S. R. Reitz, J. T. Trumble, Annu. Rev. Entomol. 47, 435
(2002).
11. J. M. Levine, Science 288, 852 (2000).
12. S. Y. Strauss et al., Ecol. Lett. 9, 357 (2006).
13. M. A. Davis, Bioscience 53, 481 (2003).
14. D. Simberloff, L. Gibbons, Biol. Invasions 6, 161 (2004).
15. D. L. Strayer et al., Trends Ecol. Evol. 21, 645 (2006).
16. P. Alpert, Biol. Invasions 8, 1523 (2006).
10.1126/science.1152124
GEOLOGY
Mudstones can be deposited under
more energetic conditions than widely
assumed, requiring a reappraisal of many
geologic records.
On the Accumulation of Mud
Joe H. S. Macquaker and Kevin M. Bohacs
O
n page 1760 of this issue, Schieber et al.
(1) document a mechanism for depositing mud that is at odds with
perceived wisdom. Geoscientists tend to
assume that most mud accumulates directly
from suspension in the water column, that
mud deposition requires quiet bottom-water
conditions, and that mudstones containing
closely spaced, parallel laminae represent
continuous deposition (see the first figure, top
panel). In contrast, the authors show that mud
can accumulate as current ripples composed
of grain aggregates under currents that can
transport very fine sand (see the first figure,
bottom panel). Thus, a layer of muddy sediment can be eroded and transported laterally
without showing obvious signs of such disturbance and may record surface-water conditions elsewhere in the basin. The results call
for critical reappraisal of all mudstones previously interpreted as having been continuously
J. Macquaker is in the School of Earth, Atmospheric and
Environmental Sciences, The University of Manchester,
Manchester M13 9PL, UK. K. M. Bohacs is with the
ExxonMobil Upstream Research Company, Houston, TX
77027, USA. E-mail: Joe.Macquaker@Manchester.ac.
uk; Kevin.M.Bohacs@exxonmobil.com
1734
Not so simple. Mud deposition via suspension settling (wavy vertical arrows) (top) and the advective
sediment transport processes close to the sediment-water interface (wavy close-to-horizontal
arrows) identified by Schieber et al. (bottom).
Bedding planes are indicated by solid lines, laminae
by dotted lines. The vertical scale is exaggerated
relative to the horizontal scale. In mudstone
successions, the expression of these two very different physical processes can only be distinguished by
detailed inspection of the textures present.
~5 mm
~5 mm
~200 mm
deposited under still waters. Such rocks are
widely used to infer past climates, ocean conditions, and orbital variations.
Fine-grained sedimentary rocks such as
shales or mudstones—with an average grain
size of less than 62.5 µm—are by far the most
common sedimentary rocks preserved close
to Earth’s surface. Most were deposited on
lake or ocean floors, where they provide a
record of Earth’s history. These rocks also play
an important part in the global carbon budget,
groundwater flow, and landfill containment
and contribute important resources such as
oil, shale gas, minerals, and metals.
Mudstones typically consist of various
materials, including clays, quartz, organic
14 DECEMBER 2007
VOL 318
SCIENCE
Published by AAAS
matter, remains of organisms, and chemical
precipitates formed when the sediment was
buried. Because of their very fine grain size,
they appear homogeneous in hand specimens;
moreover, their high clay content makes them
very susceptible to weathering. Thus, they do
not reward casual inspection and are poorly
understood relative to other rock types.
Researchers typically resort to analysis of
attributes such as fossil content, chemical
composition, and electromagnetic characteristics to deduce the conditions under which
the mudstone was deposited.
Patterns of change in these proxy data are
typically attributed to variations in ocean circulation, water chemistry, plankton growth,
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A valuable aspect of the study by Liu et al. is
that they documented the process of establishment and displacement as it occurred over time
in different areas within China and Australia.
Rarely has this approach been possible or
undertaken: Invasions and displacements often
are not detected or studied until they are widespread and complete. Consequently, much of
our information on these historical events is
derived from retrospective studies, which can be
confounded by rapid evolutionary changes in
both invading and indigenous populations (12).
In turn, these displacements should not be
regarded as total victory for the invaders.
Some authors argue that invasive competitors
may cause local extinctions of indigenous
species but are unlikely to cause the complete
extinction of indigenous species (13). Further,
some invasive populations have undergone
seemingly unexplained crashes, which open
opportunities for additional changes in invaded communities (14, 15). It remains to be
seen whether remnant populations of the
indigenous biotypes exist and may respond
evolutionarily to the invasive biotype B.
PERSPECTIVES
result, fine-grained successions recognize traces of bottom currents in very
in the sedimentary record are fine-grained rocks, supported by laboratory,
much less complete than com- modern mud, and ancient rock studies.
monly assumed. Third, most
The study by Schieber et al. enables us to
researchers did not consider it critically reexamine existing databases and to
important that floccules can be extract maximal information from new ones.
stable under traction transport, Such studies will reward us with deeper
although some, including coastal insights into the inner workings of the domiengineers, have recognized the nant sediment type on Earth.
Beyond suspension settling. Thin-section scan of a mudstone collected from the Kimmeridge Clay Formation (Upper Jurassic). The vital role that floccules probably
References
sample is mainly composed of silt and clay and contains a ripple. play (4). Most models of mud1. J. Schieber et al., Science 318, 1760 (2007).
stone
deposition
do
not
incorpoThe existence of this ripple indicates that the sediment was not sim2. C. A. Nittrouer, Marine Geology 154, 3 (1999).
ply delivered by suspension settling, but rather was deposited from rate any of these factors. Geo3. J. H. S. Macquaker, K. G. Taylor, R. L. Gawthorpe,
J. Sediment. Res. 77, 324 (2007).
logists will have to revisit these
traction currents operating close to the sediment-water interface.
4. R. B. Krone, Flume Studies of the Transport of Sediment in
rocks and generate much subtler
Estuarial Shoaling Processes (Final Report, Hydraulic
climate, or Earth-Sun distance. It is com- models to explain their variability.
Engineering Laboratory and Sanitary Engineering Research
Laboratory, University of California, Berkeley, 1962).
monly assumed—but not always explicitly
These results come at a time when mud5. I. N. McCave, J. Sediment. Petrology 41, 89 (1971).
stated—that fine-grained sediment was deliv- stone science is poised for a paradigm shift.
6. R. M. Cluff, J. Sediment. Petrology 50, 767 (1980).
ered more or less continuously from buoyant Observations accumulated over the past 30
7. K. M. Bohacs, in Mudstones and Shales, vol. 1,
Characteristics at the Basin Scale, J. Schieber,
plumes produced by storms and river floods, years (3, 5–9) indicate that deposition and burW. Zimmerle, P. Sethi, Eds. (Schweizerbart’sche
zones of high primary productivity, or turbid- ial of mud is as dynamic and complex as that
Verlagsbuchhandlung, Stuttgart, 1998), pp. 32–77.
ity currents before settling out of suspension of sand or limestone—or possibly even more
8. N. R. O’Brien, in Palaeoclimatology and
Palaeoceanography from Laminated Sediments, A. E. S.
as individual grains in still waters.
so, because of myriad processes—including
Kemp, Ed. (Special Publications v. 116, Geological
This paradigm appears to fit available grain-size changes due to aggregate growth
Society, London, 1996), pp. 23–36.
proxy data and is consistent with the few sedi- and decay, presence of biofilms, reworking,
9. J. Schieber, Sediment. Res. 69, 909 (1999).
mentary structures that are readily visible. It is, and cement precipitation—that occur in mudhowever, at odds with observations in modern stones to control their variability. We can now
10.1126/science.1151980
oceans and lakes (2), where environments and
water-column chemistries can change rapidly
and a variety of sediment transport processes
TRANSCRIPTION
have been observed. Fine-grained sediment is
seldom deposited as individual grains but
commonly organized into grain aggregates.
Doubts about the validity of the paradigm have
also emerged from imaging studies of ancient Jeffry L. Corden
fine-grained rocks (3), which have revealed
the presence of millimeter-scale sedimentary Patterns of phosphorylation in a region of RNA polymerase II may constitute a code that controls
structures, including localized erosion, pro- the recruitment of regulatory factors to control gene expression.
gressively fine-grained beds, and low-angle
ripple laminae (see the second figure).
ukaryotic RNA polymerase II, the Chapman et al. on page 1780 (5) and by
The laboratory investigations reported by
enzyme that converts DNA informa- Egloff et al. on page 1777 (6), provide eviSchieber et al. now provide direct evidence of
tion into RNA, couples this transcrip- dence that expands the number of potential
advective sediment transport of mud-sized tional activity to both modifying the DNA CTD phosphorylation states, supporting the
material, using apparatus designed to main- template (chromatin) and to processing nas- notion of a CTD code. Together, the papers
tain the integrity of the floccules. In the exper- cent RNA transcripts into mature forms. show that CTD phosphorylation is more comiments, clay aggregates formed migrating rip- Proteins that carry out the latter two functions plicated than previously thought and link, for
ples that deposited sediment under much are tethered to the catalytic core of poly- the first time, expression of specific genes
higher current velocities than previously merase II by a flexible carboxyl-terminal with a distinct CTD phosphorylation pattern.
assumed. These floccule ripples have low domain (CTD) that harbors tandem repeats of
CTD heptapeptides are tandemly repeated
crests (2 to 20 mm) and very long spacings the consensus amino acid sequence Tyr1-Ser2- from 17 to 52 times in different eukaryotes and
(300 to 400 mm); they deposit nonparallel Pro3-Thr4-Ser5-Pro6-Ser7 (1–3). Actively tran- these sequences are modified by phosphorylainclined laminae that could be easily misinter- scribing polymerase II is phosphorylated on tion, glycosylation, and proline isomerization
preted as parallel-laminated.
different sites within this heptapeptide (2, 3). In principle, CTD modification could
Together, these studies indicate that many sequence, and the pattern of phosphorylation dictate many aspects of polymerase II function
of our preconceptions about fine-grained has been proposed as a code that controls the including assembly of the multisubunit enzyme,
rocks are naïve. First, mud accumulation can binding of different regulatory factors to the its transport to the nucleus, its localization either
occur in higher-energy conditions than most enzyme (4). Two papers in this issue, by on the DNA template or within subnuclear
researchers had assumed. Second, Schieber
domains, and its eventual destruction.
et al. suggest that advective traction currents
Most work to date has focused on the role
The author is in the Department of Molecular Biology and
commonly erode, transport, and deposit subof CTD phosphorylation during transcription.
Genetics, Johns Hopkins Medical School, Baltimore, MD
21205, USA. E-mail: jcorden@jhmi.edu
stantial volumes of fine-grained sediment; as a
The pattern of phosphorylation is established
5.0 mm
E
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SCIENCE
VOL 318
Published by AAAS
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Seven Ups the Code
1735
On the Accumulation of Mud
Joe H. S. Macquaker and Kevin M. Bohacs
Science 318 (5857), 1734-1735.
DOI: 10.1126/science.1151980
http://science.sciencemag.org/content/318/5857/1734
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CONTENT
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REFERENCES
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ARTICLE TOOLS
REPORTS
12. The sample could be transferred between the reactor and
UHV chamber without exposure to air (5, 6). The UHV
chamber (base pressure ~ 1 × 10−10 torr) was equipped
with instrumentation for XPS, low-energy electron
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111, 3685 (2007).
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(1988).
21. O. Dulub et al., Science 317, 1052 (2007).
22. To estimate the TOF of CeO2-x/Au(111) surfaces, we took
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concentration of O vacancies. It was assumed that all the
atoms of a flat Cu(100) surface are active in the WGS
reaction. This is a common assumption (17, 18).
23. The unrestricted DF calculations were performed with the
DMol3 code, treating molecules, nanostructures, and
extended surfaces with the same level of accuracy (11, 24).
Au(100), Au(111), and TiO2/Au(111) were modeled by
means of the supercell approach with three-layer gold
slabs and an 11 Å vacuum between the slabs (11). The top
layer of the Au substrate, the oxide nanostructures, and
the adsorbates were allowed to fully relax.
24. B. Delley, J. Chem. Phys. 92, 508 (1990); 113, 7756 (1992).
Juergen Schieber,1* John Southard,2 Kevin Thaisen1
Mudstones make up the majority of the geological record. However, it is difficult to reconstruct the
complex processes of mud deposition in the laboratory, such as the clumping of particles into floccules.
Using flume experiments, we have investigated the bedload transport and deposition of clay
floccules and find that this occurs at flow velocities that transport and deposit sand. Deposition-prone
floccules form over a wide range of experimental conditions, which suggests an underlying universal
process. Floccule ripples develop into low-angle foresets and mud beds that appear laminated after
postdepositional compaction, but the layers retain signs of floccule ripple bedding that would be
detectable in the rock record. Because mudstones were long thought to record low-energy conditions of
offshore and deeper water environments, our results call for reevaluation of published interpretations of
ancient mudstone successions and derived paleoceanographic conditions.
A
century ago, Henry Clifton Sorby, one of
the pioneers of geology, pointed to the
study of muds as one of the most challenging topics in sedimentary geology (1). Today,
with our knowledge clearly expanded, muddy
sediments are still considered highly complex systems that may require as many as 32 variables and
parameters for a satisfactory physicochemical
characterization (2). More research may clarify
interdependencies between a number of these parameters and may allow us to consider a smaller
number of variables, but the fundamental complexity of muddy sediments is likely to remain. A
key issue in mudstone sedimentation is flocculation, a phenomenon in which a number of these
parameters, such as settling velocity, floccule size,
grain-size distribution, ion exchange behavior, and
organic content “come together.” A joining of
smaller particles to form larger aggregates, flocculation enhances the deposition rate of fine-grained sediments, and its understanding is critical for modeling
the behavior of mud in sedimentary environments.
Flocculation is affected by particle concentration within the fluid and intensity of turbulence
(3 , 4 ). Over time, floccules enlarge to a maximum
1
Department of Geological Sciences, Indiana University,
Bloomington, IN 47405, USA. 2Department of Earth and
Planetary Sciences, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA.
*To whom correspondence should be addressed. E-mail:
jschiebe@indiana.edu
1760
equilibrium diameter that is related to the intensity
of turbulence (5 ). Floccule deposition is influenced
by turbulence, bed shear stress, sediment concentration, and settling velocity. We currently still miss
critical data on floccule formation and on the influence of floccule structure and turbulence on the
formation of muddy sediments (6 ). We will collect
data concerning these issues with new instrumentation in the near future, although the importance of
our observations will not be affected. The notion is
widely held that slow-moving currents or still water
are a prerequisite for substantial mud deposition
(7 , 8 ) because shear stress in swift-moving currents
disrupts previously formed fragile floccules and
prevents their deposition, but our observations suggest an alternative mode of mud deposition that
apparently left its imprint in the rock record.
Mudstones constitute up to two-thirds of the
sedimentary record and are arguably the most
poorly understood type of sedimentary rocks (9 ).
Mudstone successions contain a wealth of sedimentary features that provide information about
depositional conditions and sedimentary history
(10 –13 ), but presently we lack the information that
would allow us to link features observed in the
rock record to measurable sets of physical variables
in modern environments.
Although various small-scale sedimentary
structures have been described from modern muds,
these have not been observed in the making. This
forces us to infer controlling parameters (e.g., cur-
14 DECEMBER 2007
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4 September 2007; accepted 22 October 2007
10.1126/science.1150038
rent velocity and density of suspension) from temporally and spatially very limited measurements in
the overlying water column (14 –17 ). Such measurements (e.g., flow velocity, sediment concentration) in modern environments are commonly
considered representative of depositional conditions
for the uppermost millimeters to decimeters of the
accumulating deposits. However, upon close examination, modern sediments show considerable heterogeneity at the millimeter to centimeter scale
(16 ), an indication that what we observe in surficial sediments is not a direct response to measured conditions in the overlying water column.
To improve on this situation, it is essential to
conduct experiments that replicate natural conditions and to compare the experimental sediments to the rock record.
Here, we report experimental insights into the
sedimentology of mudstones. In past experimental
studies, centrifugal pumps were used to recirculate
mud suspensions (18 –20 ), destroying the clay
floccules that are of such key importance in mud
transport and deposition. Therefore, to minimize
the risk of shredding clay floccules once formed,
we built a racetrack flume that uses a paddle belt
(21) for moving the mud suspension.
The racetrack flume (fig. S1) used for these
experiments (21) has a 25-cm-wide channel. The
effective flow depth was 5 cm. Powdered kaolinite
clay (Fig. 1A) was mixed with water and added
into the flume, running at 50 cm/s velocity (21).
Sediment concentrations ranging from 0.03 g/l
to 2 g/l were explored, and suspended sediment
concentrations were monitored with an optical
turbidity sensor. Experiments were conducted in
distilled water, fresh (tap) water, and salt water
(35 per mil salinity). In a few experiments, Camontmorillonite and natural lake mud (sieved to
63 mm) was used.
Addition of clay to the flume resulted within
minutes in the formation of “floccule streamers”
that mark boundary-layer streaks (22). Floccules
range in size from 0.1 mm to almost 1 mm (Fig. 1,
B to D) and were sampled and examined with a
scanning electron microscope (SEM). After establishment of a stable suspended clay concentration,
the velocity was stepwise reduced (Fig. 2) until the
critical velocity of sedimentation was reached (23 ).
At that point, a linear decline of sediment concen-
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Accretion of Mudstone Beds from
Migrating Floccule Ripples
25. T. Albaret, F. Finocchi, C. Noguera, Faraday Discuss. 114,
285 (1999).
26. V. A. Bondzie, S. C. Parker, C. T. Campbell, Catal. Lett.
63, 143 (1999).
27. B. K. Min, C. M. Friend, Chem. Rev. 107, 2709 (2007).
28. M. S. Chen, D. W. Goodman, Science 306, 252 (2004).
29. N. C. Hernández, J. F. Sanz, J. A. Rodriguez, J. Am. Chem.
Soc. 128, 15600 (2006).
29. The work performed at Brookhaven National Laboratory
was supported by the U.S. Department of Energy, Office
of Basic Energy Sciences, under contract DE-AC0298CH10886. J.E. and M.P. are grateful to Intevep for
partial support of the work carried out at the Universidad
Central de Venezuela.
REPORTS
ovoid bodies (0.2 to 0.6 mm in size) (fig. S5), the
compacted floccules from which the migrating ripples and the accreting clay bed were constructed.
It appears that irrespective of what drives flocculation in a given experiment, flocculation provides deposition-prone particles without fail over a
wide range of experimental conditions. Formation
of floccule ripples from a variety of clay-size materials (kaolinite, montmorillonite, and lake mud),
and over a range of sediment concentrations and
salinities (distilled, fresh, and salt water) strongly
suggests a universal process at work.
Our observations do not support the notion that
muds can only be deposited in quiet environments
with only intermittent weak currents (8 ). Instead,
bedload transport of flocculated mud and deposition
occurs at current velocities that would also transport
and deposit sand (21). Clay beds can accrete from
migrating floccule ripples under swiftly moving
currents in the 10 cm/s to 26 cm/s velocity range, a
range likely to expand as flows with larger sediment concentrations are explored. Whereas the
clay beds formed in our experiments consist of
downcurrent-inclined laminae, they appear to be
parallel-laminated once fully compacted (Fig. 4A).
Because floccule ripples are spaced 30 to 40 cm
apart, ancient sediments of this origin are likely
to appear parallel-laminated (Fig. 4C) as well.
Detection of ripple-accreted muds in the rock
record will require carefully defined, and yet to
be developed, criteria. Things to look for might
be subtle nonparallel lamina geometry, as well
as basal downlap and top truncation of laminae.
Examination of ancient shale units may, for example, yield low-amplitude bedforms (Fig. 4, D
and E) as indicators of lateral accretion and ripple
migration. Bedding-oblique orientation of larger
flat particles, such as spores, microfossils, plant
debris, and mica flakes, could be another indicator.
If such particles are deposited on the inclined
foresets of floccule ripples, they may record the
gentle inclination of the latter even when compaction has rendered the depositional fabric of
inclined laminae (Fig. 4B) unrecognizable.
Fig. 1. Flume feed and
floccules. (A) SEM image
of typical kaolinite clay
used in the majority of
experiments. Inset shows
size and morphology of
clay flakes. (B) SEM image of floccules (pointed
out by arrows) that were
trapped at the flume bottom with a grooved glass
slide. (C) Close-up of floccules in (B). (D) SEM image of a floccule (outline
marked with white arrows).
This floccule measures
0.12 mm along the long
axis.
Downloaded from http://science.sciencemag.org/ on September 15, 2020
tration was observed and essentially all sediment
settled out of the flow (Fig. 2). By shining strong
lights from above through the flow, we were able
to photograph and film floccule streamers (Fig.
3A), individual migrating floccules, floccule ripples
(Fig. 3B), and fields of floccule ripples (Fig. 3C).
Floccules that give rise to “floccule streamers”
(Fig. 3A) form even at small sediment concentrations (0.03 g/l), in both distilled water and fresh water, and increase in abundance as velocity is lowered.
Below the critical velocity of sedimentation (Fig. 2),
patches of floccules form and organize into streamlined ripples that migrate slowly downcurrent (Fig. 3,
B to D). The critical velocity for sedimentation
depends on initial sediment concentrations and
ranges from ~10 cm/s for small sediment concentrations (0.03 g/l) to at least 26 cm/s for sediment
concentrations in the 1 to 2 g/l range. In several
experiments where flow was stopped suddenly and
water was drained and replaced with clear water,
floccules were observed on the foreset slopes of
floccule ripples (Fig. 3E).
We also conducted experiments in which we
introduced multiple sediment pulses and allowed
the clay to accumulate at the bottom before adding
the next pulse (21). A small amount of pulverized
hematite (a red powder) was added between clay
pulses to mark the tops of successive clay pulses.
This addition of clay increments and hematite spikes
was repeated until a sediment layer of approximately 2 cm (uncompacted thickness) had accumulated. At the end of the flume run, draining of
the water typically revealed that the mud bed
carried at its surface elongated ripples that stood up
to 3 cm above the flume bottom and were spaced
between 30 and 40 cm apart in the downstream
direction (Fig. 3D and fig. S2). These experimental
mud layers were air-dried to the consistency of soft
butter and scraped with a butter knife or spatula to
reveal internal layering outlined by hematite drapes.
These internal layers were inclined in the downcurrent direction (fig. S3), indicating lateral accretion of clays. Once the clay beds have dried
completely, these internal laminae appear to layer
parallel on surfaces perpendicular to bedding (Fig.
4A). Drying out, however, also leads to separation
of the top portions of layers along bottom-parallel
fractures and reveals that the overall deposit is
characterized by low-angle, downcurrent-dipping
cross-strata (Fig. 4B).
These observations show that ripples composed
of clay floccules migrated over the flume bottom at
the onset of deposition (Fig. 3) and that a rippled
bed topography was present at the end of deposition (fig. S2). In addition, the texture produced by
the low-angle, downcurrent-dipping cross-strata in
Fig. 4B has a direct textural analog in sandstones,
where it is known as “rib and furrow” structure
(24 ). The latter is seen on horizontal surfaces cut
through sandstone beds that accumulated from migrating ripples. Closer inspection of the surface
exposed in Fig. 4B shows the deposits of multiple
overriding floccule ripples (fig. S4). Under the microscope, the inclined clay laminae from Fig. 4B
show a “bumpy” surface pattern of closely packed
Fig. 2. Example of
changes in suspended
sediment in the course
of an experiment. Vertical axis shows continuously logged suspended
sediment concentration,
and horizontal axis shows
time elapsed (tick marks
separate successive days).
The critical velocity of sedimentation lies between
25 cm/s and 20 cm/s and
coincides with the onset of
development of floccule
ripples. Its exact determination requires the use of
smaller velocity steps.
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REPORTS
In the course of two decades of detailed studies
of shales and mudstones, one of us (25 –27 ) has
seen comparable low-amplitude bedforms (Fig.
4D) in shale units that were deposited in a wide
variety of environments. Examples can be found in
the Mid-Proterozoic Belt Supergroup, the Devonian of the eastern United States, the Jurassic
Posidonia Shale, the Cretaceous Mancos Shale,
and the Eocene Green River Formation. This suggests that mud accretion from migrating floccule
ripples probably occurred throughout geologic
history. Many ancient shale units, once examined
carefully, may thus reveal that they accumulated in
the manner illustrated here, rather than having
References and Notes
Fig. 4. (A) Laminated
flume sediment (between
white arrows) that was deposited during an experiment with continuous current
flow. Sample was embedded
in epoxy and cut perpendicular to bedding. Sample is
curved due to desiccation.
The darker internal laminae
are hematite markers. (B)
Textural detail from the interior of the clay layer in (A)
(top view, arrow indicates
90° twist). As the layer dried
out, its upper portion formed
concave desiccation polygons,
whereas its lower portion remained attached to the flume
bottom. Removing the upper
portion exposes a bottomparallel surface through the
clay layer. The curved lines
are the upper terminations of broken foreset laminae of floccule ripples. The foreset laminae are inclined to
the right, and the circled numbers indicate a succession of overriding ripples (see also fig. S4). The overall
texture resembles “rib and furrow” structures as known from current rippled sandstones (24). (C) Parallellaminated black shale, New Albany Shale, Devonian, Indiana. Lighter laminae are silt-enriched. (D) Crosslaminated shale collected from the same core box as (C). (E) Tracing of silt laminae visible in (D). Arrow marks
an internal erosion surface. In the center are inclined (to the left) truncated laminae, forming the outline of a
compacted and mud-dominated ripple. The synoptic relief of this ripple is 3 mm, but its original relief would
have been ~20 mm (assuming 85% water content), the same magnitude as observed in our experiments.
1762
14 DECEMBER 2007
VOL 318
SCIENCE
1. H. C. Sorby, Q. Geol. Soc. London 64, 171 (1908).
2. J. Berlamont, M. Ockenden, E. Toorman, J. Winterwerp,
Coast. Eng. 21, 105 (1993).
3. H. A. Einstein, R. B. Krone, Proc. J. Hydr. Div. H12, 51
(1961).
4. I. N. McCave, in Fine-Grained Sediments: Deep Sea
Processes and Facies, D. A. V. Stow, D. J. W. Piper, Eds.
(Geological Society of London, London, 1984), pp. 35–69.
5. E. Parthenaides, Proc. J. Hydraulics Div. 91, 105 (1965).
6. J. C. Winterwerp, C. Kranenburg, Fine Sediment Dynamics
in the Marine Environment, Proceedings in Marine
Science 5 (Elsevier, Amsterdam, 2002).
7. P. E. Potter, J. B. Maynard, W. A. Pryor, Sedimentology of
Shale (Springer Verlag, New York, 1980).
8. P. E. Potter, J. B. Maynard, P. J. Depetris, Mud and
Mudstones: Introduction and Overview (Springer,
New York, 2005).
9. J. Schieber, in Shales and Mudstones (vol. 1): Basin
Studies, Sedimentology and Paleontology, J. Schieber,
W. Zimmerle, P. Sethi, Eds. (Schweizerbart’sche
Verlagsbuchhandlung, Stuttgart, 1998), pp. 131–146.
10. N. R. O’Brien, R. M. Slatt, Argillaceous Rock Atlas
(Springer Verlag, New York, 1990).
11. J. Schieber, Sedimentology 33, 521 (1986).
12. J. Schieber, Sedimentology 36, 203 (1989).
13. J. Schieber, in Shales and Mudstones (vol. 1): Basin
Studies, Sedimentology and Paleontology, J. Schieber,
W. Zimmerle, P. Sethi, Eds. (Schweizerbart’sche
Verlagsbuchhandlung, Stuttgart, 1998), pp. 187–215.
14. S. A. Kuehl, C. A. Nittrouer, D. J. DeMaster, Cont. Shelf
Res. 6, 311 (1986).
15. S. A. Kuehl, C. A. Nittrouer, D. J. DeMaster, J. Sediment.
Petrol. 58, 12 (1988).
16. S. A. Kuehl, T. M. Hariu, M. W. Sanford, C. A. Nittrouer,
D. J. DeMaster, in Microstructure of Fine-Grained
Sediments, R. H. Bennett, W. R. Bryant, M. H. Hulbert,
Eds. (Springer Verlag, New York, 1991), pp. 33–45.
17. M. P. Segall, S. A. Kuehl, Sediment. Geol. 93, 165 (1994).
18. N. Hawley, Sedimentology 28, 699 (1981).
19. K. W. Pasierbiewicz, J. Kotlarczyk, J. Sediment. Res. 67,
510 (1997).
20. J. H. Baas, J. L. Best, J. Sediment. Res. Sect. A 72, 336
(2002).
21. Materials and methods are available as supporting
material on Science Online.
22. J. R. L. Allen, Principles of Physical Sedimentology
(George Allen and Unwin, London, 1985).
www.sciencemag.org
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Fig. 3. (A) Floccule
streamers photographed
through the flume bottom
(flow velocity 48 cm/s,
initial sediment concentration 0.72 g/liter, lake
mud in tap water). (B)
Streamlined floccule ripple
and floccule streamers. (C)
Multiple floccule ripples
begin to coalesce. Flow is
from top to bottom in images (A), (B), and (C). (D)
Oblique view of large
barchan-shaped migrating
floccule ripple. Flow is
toward the right. (E) Side
view of the left horn of
this ripple [pointed out by
black arrow in (D)]. Inset
shows that the surface is
covered with small, round
bodies, the clay floccules
that moved across the
ripple during current activity. Numbers indicate
floccule size.
largely settled from slow-moving or still suspensions. This, in turn, will most likely necessitate the
reevaluation of the sedimentary history of large
portions of the geologic record.
Elucidating the mechanisms of mudstone deposition not only helps to better understand the
rock record but also benefits hydrocarbon exploration, hydrogeology, and coastal and shelf engineering. Managing mud is important for the
maintenance of harbors, shipping lanes, and water
reservoirs, especially given the impact of climate
change. How mudstones act as barriers to fluid
migration (oil and water) is probably linked to
depositional processes that affect mud microfabrics.
For example, if a mud accumulated from currenttransported floccules, one might expect a network
of larger pores, poorer sealing capacity, and easier
release of liquid and gaseous hydrocarbons. Conversely, accumulation in still water from dispersed
clays and low-density floccules should lower permeability and may produce an oil shale that clings
tightly to its generated hydrocarbons. These qualities are also critical for the ability of a mudstone
unit to protect aquifers from contamination and to
compartmentalize groundwater reservoirs.
REPORTS
23. J. Berlamont, M. Ockenden, E. Toorman, J. Winterwerp,
Coast. Eng. 21, 105 (1993).
24. W. L. Stokes, Primary Sedimentary Trend Indicators as
Applied to Ore Finding in the Carrizo Mountains, Arizona
and New Mexico, Part 1 (U.S. Atomic Energy Commission,
Oak Ridge, TN, 1953).
25. J. Schieber, Sedimentology 36, 203 (1989).
26. J. Schieber, in Shales and Mudstones (vol. 1): Basin
Studies, Sedimentology and Paleontology, J. Schieber,
W. Zimmerle, P. Sethi, Eds. (Schweizerbart’sche
Verlagsbuchhandlung, Stuttgart, 1998), pp. 187–215.
27. J. Schieber, J. Sediment. Res. 69, 909 (1999).
28. This research was supported by NSF grants EAR-0318769
and EAR-0617128.
A Madden-Julian Oscillation Event
Realistically Simulated by a Global
Cloud-Resolving Model
Hiroaki Miura,1* Masaki Satoh,1,2 Tomoe Nasuno,1 Akira T. Noda,1 Kazuyoshi Oouchi1
A
Madden-Julian Oscillation (MJO) is an
envelope of active convection thousands of kilometers wide that travels
eastward at an average speed of 5 m/s over the
Indian and Pacific Oceans (1). Given the largescale (103 to 104 km horizontally) coupling between the atmospheric circulation and deep
convection, an MJO influences not only the intraseasonal (30 to 90 days) variability of the
tropics but also tropical cyclone genesis, the
onset and break of the Asian-Australian monsoon, and the evolution of the El Niño–Southern
Oscillation event (2, 3 ). Despite its extensive
effects on weather events and climate variability,
weather prediction and climate models do not
simulate the MJO well (4 ). Even recently, most of
the coupled atmosphere-ocean general circulation models (GCMs) presented in the Fourth
Assessment Report of the Intergovernmental
Panel on Climate Change had difficulty simulating the variance and phase speed of the MJO (5 ).
It is expected that weather forecasts beyond 10
days could be improved if the MJO representations in global weather prediction models were
more realistic (2).
The major difficulty in simulating the MJO
with GCMs involves cumulus parameterization
used to estimate the vertical redistribution of heat
and moisture by unresolved convective clouds in
1
Frontier Research Center for Global Change, Japan Agency
for Marine-Earth Science and Technology, 3173-25 Showamachi,
Kanazawa-ku, Yokohama, Kanagawa 236-0001, Japan. 2Center
for Climate System Research, University of Tokyo, 5-1-5
Kashiwanoha, Kashiwa, Chiba 277-8568, Japan.
*To whom correspondence should be addressed. E-mail:
miurah@jamstec.go.jp
GCMs (4 , 6 ). Computational constraints have
made it almost impossible to run global cloudresolving models (GCRMs) that compute the
effects of clouds explicitly and do not depend on
cumulus parameterizations. However, recent increases in available computer power have begun
to eliminate the models’ artificial gap between
cloud processes and the atmospheric circulation
(7 ). Improved MJO simulations with GCMs
substituting two-dimensional cloud-resolving
models for cumulus parameterizations (8 , 9 ) suggest the importance of representing the variation
in quasi-equilibrium states (10 ); that is, the statistical balance between stabilization by convection and destabilization by external forcing,
which depends on large-scale atmospheric circulation. Therefore, GCRMs may allow realistic
MJO simulations because convective activity can
be linked directly to dynamic and thermodynamic atmospheric conditions of large-scale atmospheric circulation and convection. Here we
report a numerical simulation of an MJO event
that occurred between December 2006 and
January 2007.
On the Earth Simulator, we ran a GCRM
called the Nonhydrostatic Icosahedral Atmosphere Model (11), which has been upgraded as
a result of aquaplanet experiments (12, 13 ) and
a realistic tropical cyclone experiment (14 ),
with horizontal grids with mesh sizes about 3.5
and 7 km. These resolutions are almost fine
enough to resolve the gross behavior of cumulus
ensembles, including heating and moistening, as
a response to large-scale atmospheric conditions.
The 3.5-km grid run covered 1 week, whereas the
7-km grid run covered 30 days, which was long
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SCIENCE
VOL 318
www.sciencemag.org/cgi/content/full/318/5857/1760/DC1
Materials and Methods
Figs. S1 to S5
25 June 2007; accepted 30 October 2007
10.1126/science.1147001
enough to reproduce the eastward migration of
the convective center from the Indian to the
Pacific Ocean. The initial atmospheric conditions
were generated by linear interpolation from the
National Centers for Environmental Prediction
(NCEP) Global Tropospheric Analyses at 00:00
universal time coordinated (UTC) on 25 December
2006 for the 3.5-km grid run and at 00:00 UTC on
15 December 2006 for the 7-km grid run. We did
not use any artificial techniques to nudge the model
atmosphere to realistic atmospheric states during
the numerical integrations (15 ).
The large-scale convectively active region of the
MJO was reproduced approximately by the 3.5-km
(Fig. 1) and 7-km (not shown in Fig. 1) grid runs.
The convective center was near Borneo on 31 December 2006, and upper tropospheric clouds
covered the islands of Southeast Asia and the surrounding seas. The typical multiscale structure of
A
B
Downloaded from http://science.sciencemag.org/ on September 15, 2020
A Madden-Julian Oscillation (MJO) is a massive weather event consisting of deep convection
coupled with atmospheric circulation, moving slowly eastward over the Indian and Pacific Oceans.
Despite its enormous influence on many weather and climate systems worldwide, it has proven
very difficult to simulate an MJO because of assumptions about cumulus clouds in global
meteorological models. Using a model that allows direct coupling of the atmospheric circulation
and clouds, we successfully simulated the slow eastward migration of an MJO event. Topography,
the zonal sea surface temperature gradient, and interplay between eastward- and westwardpropagating signals controlled the timing of the eastward transition of the convective center. Our
results demonstrate the potential making of month-long MJO predictions when global cloudresolving models with realistic initial conditions are used.
Supporting Online Material
Fig. 1. (A) Infrared image from the MultiFunctional Transport Satellite (MTSAT-1R) at 00:30
UTC on 31 December 2006 and (B) outgoing longwave radiation from the 3.5-km run averaged from
00:00 UTC to 01:30 UTC on 31 December 2006.
14 DECEMBER 2007
1763
Accretion of Mudstone Beds from Migrating Floccule Ripples
Juergen Schieber, John Southard and Kevin Thaisen
Science 318 (5857), 1760-1763.
DOI: 10.1126/science.1147001
http://science.sciencemag.org/content/318/5857/1760
SUPPLEMENTARY
MATERIALS
http://science.sciencemag.org/content/suppl/2007/12/12/318.5857.1760.DC1
RELATED
CONTENT
http://science.sciencemag.org/content/sci/318/5857/1734.full
REFERENCES
This article cites 15 articles, 4 of which you can access for free
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