Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
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Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
Ripple marks indicate Mid-Devonian paleo-wind directions in the
Orcadian Basin (Orkney Isles, Scotland)
David De Vleeschouwer a,b,⁎, David Leather c, Philippe Claeys b
a
b
c
MARUM, Center for Marine Environmental Sciences, Leobener Straße, D-28359 Bremen, Germany
Earth System Sciences, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
Woodlands, Panorama Drive, Ilkley, West Yorkshire, UK
a r t i c l e
i n f o
Article history:
Received 13 October 2014
Received in revised form 16 February 2015
Accepted 3 March 2015
Available online 11 March 2015
Keywords:
Devonian
Orkney
Ripple marks
Rousay Flagstone
Monsoon
Paleo-winds
a b s t r a c t
Few climate proxies provide information on paleo-wind directions. However, fossilized bedform elements can
provide insight into the direction of flow that formed them. Wave-formed ripple marks, for example, which developed in shallow waters, extend transversely to the wind direction. In this study, Middle Devonian paleo-wind
directions are reconstructed by measuring the orientation of 511 fossilized wave ripple marks in the Rousay
Flagstone Formation on the island of Westray, Orkney, Scotland. The orientation of ripple marks was measured
in four different localities on the island. A chi-squared test demonstrates that these four ripple-mark subsets
show the same distribution of ripple mark orientation and thus indicates that the studied ripple marks display
the same distribution of paleo-winds. Two dominant ripple mark orientations were observed at all studied
localities. The most abundant ripple mark orientation suggests paleo-winds from the present-day North or
South. Second most abundant are ripple marks that suggest paleo-winds from the present-day ENE or WSW
(70° or 250°). When a 40° clockwise rotation of Orkney since the Middle Devonian is taken into account, the
ripple marks suggest that the dominant wind directions were south-southeast (SSE) and north-northeast
(NNE) in the tropical setting of the Orcadian basin. The SSE wind-direction is represented by the abundant ripple
marks, with a present-day near N–S orientation of the line perpendicular to the crests. These ripple marks are also
characterized by the largest wavelengths. The abundance of the latter ripple marks with long wavelengths is the
result of a lower water level and thus a larger wave-exposed surface during the dry winter season, from April to
July, which was mainly characterized by SSE paleo-trade winds.
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The general circulation of the atmosphere is a key-feature of Earth's
climate. Unfortunately, it is often a difficult task to reconstruct aspects of
atmospheric circulation of past climate systems based on proxy records
extracted from geological archives. Possible ways to reconstruct paleowind direction include (a) measuring the anisotropy of the low-field
magnetic susceptibility (AMS) in loess deposits (Ge et al., 2014 and
references therein), (b) conducting provenance analysis (e.g., Smith
et al., 2003; Rao et al., 2014), and (c) studying the orientation of
fossilized bedform elements (e.g., Poland and Simms, 2012).
In this paper, the orientation of wave-formed ripple marks in the
Rousay Flagstone on Westray, the most northwesterly of the Orkney
Isles, is studied. The aim of this study is to provide insights into the distribution of paleo-winds during the Middle Devonian of southeastern
Euramerica, at a paleolatitude of about 16°S (Fig. 1a; Scotese, 2013).
The studied siltstones and sandstones were laid down as soft sediments
⁎ Corresponding author at: MARUM, Center for Marine Environmental Sciences,
Leobener Straße, D-28359 Bremen, Germany.
E-mail address: ddevleeschouwer@marum.de (D. De Vleeschouwer).
http://dx.doi.org/10.1016/j.palaeo.2015.03.001
0031-0182/© 2015 Elsevier B.V. All rights reserved.
between about 390 and 385 million years ago (Ma) in a vast freshwater
lake known as Lake Orcadie (Donovan et al., 1976; Stephenson et al.,
2006). As will be shown, the study of the ripple marks will provide information on the monsoonal circulation during Middle Devonian
times. Subsequently, the obtained reconstruction of paleo-winds is
compared to climate simulations for the Devonian, carried out using a
general circulation model (GCM; De Vleeschouwer et al., 2014).
Thereby, the validity of this Devonian climate model is tested and an
assessment is made of whether or not one can trust the latter model to
expand the understanding of Middle Devonian atmospheric dynamics.
2. Geological setting
During the Middle Devonian, Scotland was a part of the Euramerican
tropical continent, i.e., the Old Red Sandstone Continent. During the
Early Devonian, a series of north–south strike-slip faults developed
within the Caledonian mountains and a broad half graben rift valley
opened up, in which fresh water collected at its south end to form
Lake Orcadie (Fig. 1). The lake reached its maximum extent during
deposition of the Achanarras/Sandwick fish bed, which extended for
several hundreds of kilometers along the rift (Trewin, 1976, 1986;
D. De Vleeschouwer et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
69
Fig. 1. (a) Middle Devonian global paleogeography after Scotese (Scotese, 2013). The red dot indicates the location of Lake Orcadie. (b) Generalized Middle Devonian paleogeography,
based on the present geography of Scotland (adapted from Trewin and Thirlwall, 2002).
Astin, 1990; Marshall et al., 2007). During that time, the lake probably
drained to the Rheic Ocean to the southeast. During the Givetian, the
size of the lake decreased and water circulation was more restricted
(Trewin and Thirlwall, 2002). Rainfall in this tropical climate was
characterized by strong seasonal variability, resulting in constantly fluctuating water levels (Marshall et al., 2007). The lake also varied in size on
longer time-scales, as both the Lower and Upper Stromness Flagstone
Formations and the Rousay Flagstone Formation consist of rhythmic
units averaging about 12 m (Donovan, 1980). The longer-term changes
in lake level are thought to be the result of astronomically-forced fluctuations in monsoonal rainfall. The driving astronomical parameter of the
lake cycles is climatic precession, with a Middle-Devonian periodicity
of about 18 kyr (Berger et al., 1992), as was suggested by Astin (1990)
and Andrews and Trewin (2010). In this interpretation, monsoon rainfall
is intensified when summer insolation is at a maximum during a
precession maximum (Earth in the perihelion during austral summer).
The studied Rousay Flagstone forms the bedrock of almost the entire
island of Westray and consists of siltstones and fine sandstones. Detailed stratigraphic logging on the island of Westray, carried out in the
framework of this study, indicates that the Rousay Flagstone Formation
comprises at least 50 cycles of between 7 and 20 m and has a minimum
thickness of 600 m. A typical cycle from lower part of the Rousay Flagstone Formation is shown in Fig. 2. Ripple marks were measured from
different cycles within the Rousay Flagstone Formation. Notably, the
ripple marks were observed in the “shallow lake with fluctuating
seasonal level” part of the cyclic lacustrine deposits (association D in
Donovan, 1980; Trewin and Thirlwall, 2002; Fig. 2). Lithological association D consists of an alternation of variously light gray and gray green
shales with siltstones and fine sandstones in discrete beds from 1 to
100 mm in thickness. Multiple surfaces of symmetrical ripple marks
occur in the coarser lithologies (Donovan, 1980).
3. Materials and methods
The orientation of the ripple marks was measured by aligning a compass perpendicular to the ripple crests. Therefore, it should be noted that
in Figs. 3a and 4a and in Tables 1–4, the reported orientations are the orientations of the lines perpendicular to the ripple crests, i.e., the direction
70
D. De Vleeschouwer et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
Fig. 2. A typical cycle in the Rousay Flagstone Formation. The logged cycle outcrops at Stancro in the Rapness peninsula in the south of Westray and contains characteristic fossil fish
Osteolepis panderi (Op) and Thursius pholidotus (Tp). The lithological associations are those of Donovan (1980). The cycle follows climatic fluctuations driven by precessional changes
with a Middle-Devonian periodicity of about 18 kyr (Berger et al., 1992). Other fish present in this cycle are Asterolepis orcadensis (Ao), Glyptolepis paucidens (Gp) and Dipterus valenciennesi
(Dv). Other cycles show fluviatile and deltaic sediments and river channel infills in association D.
of the paleo-wind that formed them. The wavelength of the ripples was
calculated by dividing the full length of the bedding plane with ripple
marks by the number of ripples. Before each measurement, we checked
that ripples were nearly symmetrical in shape with straight crests, to distinguish them from current ripples developed under unidirectional flow.
Wave ripple marks were measured in four different localities on
the Island. A Pearson's chi-squared test is applied to evaluate the likelihood that the frequency distribution of ripple orientations at a
certain locality is consistent with the frequency distribution at the
other three localities. Therefore, ripple marks were subdivided into
Fig. 3. (a) Geological map of the Island of Westray. Red dots indicate the location of the measured ripple marks. Four rose diagrams show the frequency distribution of paleo-wind orientations with respect to present-day North, as suggested by the ripple marks at the four studied localities. (b) Stratigraphic succession in Orkney, with correlation to the global standard
conodont zonation after Marshall et al. (2007).
D. De Vleeschouwer et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
71
Fig. 4. (a) Frequency distribution of paleo-wind orientations with respect to Paleo-North, as suggested by all studied ripple marks, obtained after correction for the 40° clockwise rotation of
Northern Scotland since the Middle Devonian. (b) Simulated climate for Lake Orcadie, using the HadSM3 general circulation model (De Vleeschouwer et al., 2014). The comparison
between (a) data and (b) model indicates that most ripples were formed by SSE paleo-winds during the dry winter season, when lake level was low.
nine categories (each 20° wide), according to their orientation. The
value of the test-statistic is
4. Results
where Oi is the observed frequency at the locality in question, Ei is the
expected frequency based on the frequency distribution at the three
other localities, and n = 9, i.e., the number of categories. The
chi-squared test-statistic can then be used to calculate a p-value by
comparing χ2 to a chi-squared distribution with 7 degrees of freedom. The chi-squared test evaluates the null hypothesis, which states
that there is no significant difference between the observed distributions of ripple mark orientations at a certain locality, compared to the
observed distribution at the three other localities. If this test confirms
the null hypothesis, we can state that the observed frequency
distributions at the four localities are mutually consistent and are
thus likely to carry the same signature of paleo-wind distribution.
Red dots indicate the 511 measured ripple marks on the geological
map of Westray (Fig. 3a). The ripple marks were observed at four localities in the northern, eastern, southern and western corner of the island.
Four rose diagrams give a concise view of the orientation of the paleowinds that formed the studied ripple marks at each locality (Fig. 3a).
The observed orientations range between 0 and 180° with respect to
the present-day North. As the wave-formed ripple marks are symmetrical in shape, a ripple mark solely provides information on the orientation of the wave-forming wind, not on its direction. For that reason,
the 360° rose diagrams in Fig. 3a are perfectly symmetrical. The chisquared test-statistic for each locality is calculated in Tables 1–4. For
the eastern, southern and western localities, the chi-squared testvalue is rather low. As a consequence, the corresponding p-value is
high, suggesting that there is no statistically significant difference in
the frequency distribution of ripple mark orientations among these
three localities. However, for the northern locality, 31 ripple marks
Table 1
Result of chi-squared test for the orientation of ripple marks measured at the northern locality. Listed orientations are measured perpendicular to the ripple's crests.
Table 2
Result of chi-squared test for the orientation of ripple marks measured at the eastern locality. Listed orientations are measured perpendicular to the ripple's crests.
Xn ðO −E Þ2
i
i
χ ¼
i¼1
Ei
2
ð1Þ
Paleo-wind orientation
(with respect to present-day North)
Oi
Ei
ðOi −Ei Þ2
Ei
Paleo-wind orientation
(with respect to present-day North)
Oi
Ei
ðOi −Ei Þ2
Ei
170–9°
10–29°
30–49°
50–69°
70–89°
90–109°
110–129°
130–149°
150–169°
31
17
12
6
8
5
9
2
6
17.86
26.20
10.90
6.72
9.97
7.19
4.87
4.41
7.88
Total
p-Value
9.68
3.23
0.11
0.08
0.39
0.67
3.50
1.31
0.45
19.42
0.0128
170–9°
10–29°
30–49°
50–69°
70–89°
90–109°
110–129°
130–149°
150–169°
17
26
9
5
13
7
4
3
8
17.11
25.11
10.44
6.44
9.56
6.89
4.67
4.22
7.56
Total
p-Value
0.00
0.03
0.20
0.32
1.24
0.00
0.10
0.35
0.03
2.27
0.9714
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D. De Vleeschouwer et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
Table 3
Result of chi-squared test for the orientation of ripple marks measured at the southern
locality. Listed orientations are measured perpendicular to the ripple's crests.
Paleo-wind orientation
(with respect to present-day North)
Oi
Ei
ðOi −E i Þ2
Ei
170–9°
10–29°
30–49°
50–69°
70–89°
90–109°
110–129°
130–149°
150–169°
21
31
20
12
16
12
8
11
16
27.34
40.12
16.69
10.30
15.27
11.01
7.46
6.75
12.07
Total
p-Value
1.47
2.07
0.66
0.28
0.04
0.09
0.04
2.68
1.28
8.61
0.3765
that suggest paleo-winds from the present-day North or South
(between 170° and 9°) are observed, whereas the expected frequency
is only 17.86, based on the frequency distribution at the other localities.
As a consequence, the test-statistic for the northern locality is high. The
corresponding p-value is 1.28%, and the null-hypothesis can be rejected
with confidence. In other words, the chi-squared test suggests that the
northern locality is characterized by a frequency distribution of ripple
mark orientations different from the three other localities. However,
when we compare the rose diagram of this northern locality with the
other three rose diagrams (Fig. 3a), it turns out that the northern
frequency distribution is not completely different from the others. The
northern frequency distribution seems rotated by a few degrees
counterclockwise with respect to the others. Therefore, we interpret
the rejection of the chi-squared test's null-hypothesis as the result of a
local effect on the wind directions that led to the formation of the ripple
marks. Alternatively, some of the strata on which measurements were
made in the north of the island may have been skewed by minor
strike-slip faults. In summary, we conclude that all four localities
contain ripple marks that were formed under the same wind regime.
Ripple mark orientations from the four localities were joined together, to obtain a better perspective on the dominant ripple mark orientations on the island of Westray. Subsequently, the rotation of northern
Scotland since Middle Devonian times had to be taken into account, in
order to interpret the ripple mark orientations in terms of Devonian
paleo-wind directions with respect to the Paleo-North. According
to Table 4 in Torsvik et al. (2012), the apparent polar wander path
(λp, ϕp) for Europe was at 5.9°N, 324.6°E at 380 Ma, and at 5.9°N,
321.7°E at 390 Ma. These paleomagnetic poles translate in declinations
(D) for Westray (present-day coordinates λs = 59.5°N, ϕp = 3°W) of
respectively 38.6° and 42.3°. This means that the Orkney Islands have
rotated clockwise by about 40° since then. The uncertainties on these
values are not well determined because of limited data, but they are
probably about ±10° (Van der Voo, pers. comm.). Hence, to reconstruct
paleo-wind directions, we merge and rotate the rose diagrams (Fig. 3a)
Table 4
Result of chi-squared test for the orientation of ripple marks measured at the western
locality. Listed orientations are measured perpendicular to the ripple's crests.
Paleo-wind orientation
(with respect to present-day North)
Oi
Ei
ðOi −E i Þ2
Ei
170–9°
10–29°
30–49°
50–69°
70–89°
90–109°
110–129°
130–149°
150–169°
39
56
18
12
14
12
9
5
10
32.55
47.77
19.87
12.26
18.18
13.10
8.88
8.03
14.37
Total
p-Value
1.28
1.42
0.18
0.01
0.96
0.09
0.00
1.14
1.33
6.41
0.6017
counterclockwise by 40° and obtain a rose diagram showing the
frequency distribution of Middle Devonian paleo-winds across Lake
Orcadie (Fig. 4a), as recorded by ripple marks on the Rousay Flagstones
of Westray. This paleo-wind rose is clearly dominated by two overriding
directions. First, about 40% of all measured ripple marks are associated
with paleo-winds that were blowing either from the SSE or from the
NNW (Fig. 4a). In the discussion section of this paper, we will advocate
that these ripple marks are the result of SSE paleo-winds. Moreover,
these ripple marks are characterized by the longest average wavelength
and contain the maximal ripple mark wavelength observed (14.3 cm;
Fig. 4a). Secondly, a confined paleo-wind direction with increased frequency (7% of all ripple marks; Fig. 4a) can be noted for paleo-winds
that were blowing from the NNE (see discussion). The ripple marks
that account for these two paleo-wind directions have been found at
all four localities, suggesting paleo-winds from the present-day North
or South, and present-day ENE or WSW (70°–250°), respectively
(Fig. 3a; Tables 1–4).
5. Discussion
In tropical regions, the atmospheric circulation consists of a pair of
large convective cells, i.e., Hadley Cells, featuring an ascending limb
near the thermal equator (InterTropical Convergence Zone, ITCZ) and
descending limbs in the subtropics of both hemispheres. The ascending
and descending limbs are linked by upper-level poleward airflow and
ITCZ-ward flow near the surface. The latter flow from the subtropics
towards the equator is deflected towards the west in both hemispheres
by the Coriolis effect, and is often referred to as trade winds or tropical
easterlies. The Middle Devonian paleolatitude of the study area is
around 16°S (Scotese, 2013) and therefore, one can suggest that the
Middle Devonian wind pattern of Lake Orcadie was dominated by
the tropical easterlies. This characteristic of tropical atmospheric
circulation allows us to deduce the direction of the paleo-winds purely
from symmetrical ripple marks. Hence, Fig. 4a suggests that the
dominant paleo-wind directions were south-southeast (SSE) and
north-northeast (NNE), with respect to the Paleo-North.
A recent numerical simulation of Devonian climate (De Vleeschouwer
et al., 2014) allows for a more detailed interpretation of the reconstructed paleo-wind directions. In that climate modeling study, a Devonian
atmospheric circulation was simulated for different astronomical configurations. Simulated monthly average temperature and precipitation,
among other climatic parameters, are provided for grid boxes with a
latitudinal width of 2.5° and a longitudinal width of 3.75°. In the present
study, the simulated monthly average temperature and precipitation of
the “median-orbit run” (called xacla in De Vleeschouwer et al., 2014)
are used to construct a climate graph for the Devonian paleogeographic
position of the Orcadian basin (Fig. 4b). This climate graph is based
on averaged climate variables from 4 grid boxes, between 13.75°S–
18.75°S and 3.75°W–3.75°E. It demonstrates a climate that is characterized by a dry winter and a wet summer, with rainfall maxima in
February and November, related to the passage of the ITCZ. Winds
blow from the southeast (SE, a bearing of 130° in June) during southern
hemispheric winter and from the North by east (NbE, 14° in December)
in summer. These two main wind directions, as suggested by the
climate model, correspond well with the reconstructed paleo-wind
directions in this study.
Therefore, it is concluded that the most abundant ripple mark orientation suggesting paleo-winds from the present-day North or South, indicate SSE paleo-winds with respect to Paleo-North. These winds are
the result of winter atmospheric circulation, when the ITCZ was situated
north of the study area. However, the ripple marks suggest an overwhelming dominance of these SSE paleo-winds, whereas the climate
model only simulates SSE winds during 3 or 4 months per year. This apparent contrast is explained by the fact that the lake water level was low
during the winter season. Indeed, a larger portion of the lake bottom
gets exposed to ripple-forming wave-action as a consequence of a
D. De Vleeschouwer et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 426 (2015) 68–74
73
Fig. 5. Photograph of ripple mark association at Stancro beach (southern locality, 59°14′44.8″N, 2°52′42.4″W) showing two different ripple mark orientations. The yellow line indicates a
ripple mark that suggests a paleo-wind with a present-day N–S orientation (4–184°). This ripple mark is interpreted to be the result of waves generated by SSE paleo-winds with respect to
Paleo-North. The purple line indicates a ripple mark that suggests a paleo-wind with a present day ENE–WSW orientation (73°–253°). This ripple mark is interpreted to correspond to NNE
paleo-winds with respect to Paleo-North. In the Rousay Flagstone Formation, numerous examples of this specific ripple mark orientation couplet can be found, leading to our interpretation
that both ripple mark orientations have formed within one year. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
lower lake level during winter. Moreover, with lower lake levels, a larger portion of the wave's energy reaches the lake-bottom sediments,
explaining why the longest ripple wavelengths are found in association
with this paleo-wind orientation. The ripple marks that indicate NNE
paleo-winds probably formed during the months of September and October, as suggested by Fig. 4b. During these months, the ITCZ is moving
northward, after it has reached its southernmost position, and is thus
shifting in the direction of the study area. As a consequence, winds are
backing to the NNE, rainfall picks up and the lake level starts to gradually rise after the dry season. However, the lake level is still low enough so
that ripple marks can be formed. This interpretation is largely based on
the fact that the ripple mark orientations that indicate these two wind
directions are often found in association. For example, in Fig. 5, the yellow line marks a ripple mark that is formed by a paleo-wind with a
present-day N–S orientation (4°–184°). According to the above interpretation, this ripple mark matches SSE paleo-winds. On the left side
of Fig. 5, the purple line marks a ripple mark formed by a paleo-wind
with a present-day ENE–WSW orientation (73°–253°). The latter ripple
marks are interpreted to correspond to NNE paleo-winds. As is evident
from the photograph in Fig. 5, both ripple marks were found on the
same slab of Rousay Flagstone, in direct relationship with each other.
The ripple marks on the left (purple line, representing NNE paleowinds) are observed to have formed on a thin lamina, a few millimeters
thick, which formed on top of the ripple mark on the right side (yellow
line, representing SSE paleo-winds). Therefore, we hypothesize that the
ripple marks that are shaped by NNE winds (purple line) formed only a
very short time, a few months, after the SSE winter monsoon paleowinds formed the underlying ripple marks (yellow line). In our interpretation, the powerful summer-monsoon winds, which blew over
Lake Orcadie from November to January, left few ripple marks on the
Rousay Flagstone of Westray because the lake water level was simply
too high during this season and the lake floor was below the wave base.
6. Conclusion
The orientations of wave-generated ripple marks are successfully
used to reconstruct paleo-wind directions and thereby provide a detailed understanding of the atmospheric circulation of the period
under investigation. In the framework of this study, the orientations of
511 ripple marks were measured, fossilized in the Middle Devonian
Rousay Flagstone Formation on Westray, Orkney. These ripple marks
suggest that two paleo-wind directions were dominant in the evolution
of winds over the yearly cycle. If one assumes a 40° clockwise rotation of
northern Scotland since the Middle Devonian, these dominant paleowind directions are SSE and NNE. A comparison of this paleo-wind reconstruction with numerical climate simulations leads to an interaction
between geological data and modeling results. In the one direction, the
climate simulations allow for a more in-depth interpretation of the
geological data. In particular, the SSE paleo-winds suggested by the
ripple marks were attributed to the winter atmospheric circulation,
while the NNE paleo-winds were associated with spring (September
to October) circulation. In the other direction, this data-model comparison can be considered an independent test for the performance of the
Devonian climate model in question. The good agreement between
data and model reinforces the confidence that one can have in the
atmospheric dynamics, as they are simulated by the model.
Acknowledgments
This work is supported by a Ph.D. fellowship awarded by the
Research Foundation-Flanders (FWO; 1148713N) assigned to DDV. PC
thanks FWO (G.A078.11), Hercules Foundation and VUB research
funds for their contribution. We thank Dominique Maes for helping
with the design of the chi-squared test and Rob Van der Voo and an
anonymous reviewer for their constructive reviews. A very warm
“Thank you” goes to Midge Leather, for creating the welcoming and
cozy atmosphere in Stancro.
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