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The Impact of Industrial Pollution on the Rock Art of Murujuga, Western Australia

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MacLeod and Fish have recently suggested that there is no adverse impact on the engraved rock art of Murujuga (the Burrup Peninsula) from industrial pollution. This highly controversial conclusion demands examination because it could influence future government decision-making concerning ongoing applications to expand industrial activity on Murujuga. We, therefore, review the data and arguments underpinning that conclusion. We find the conclusion unsubstantiated, misleading and potentially damaging for the long-term preservation of the Murujuga rock art. Evidence suggests that the petroglyphs are already actively degraded by industrial pollution.
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Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
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KEYWORDS: Petroglyph – Industry emissions – Rock varnish – Acid rain – Murujuga – Scientic integrity
THE IMPACT OF INDUSTRIAL POLLUTION ON THE
ROCK ART OF MURUJUGA, WESTERN AUSTRALIA
Benjamin W. Smith, John L. Black, Stéphane Hœrlé, Marie A. Ferland,
Simon M. Diey, Jolam T. Neumann and Thorsten Geisler
Abstract. MacLeod and Fish have recently suggested that there is no adverse impact on the
engraved rock art of Murujuga (the Burrup Peninsula) from industrial pollution. This highly

decision-making concerning ongoing applications to expand industrial activity on Murujuga.
           
conclusion unsubstantiated, misleading and potentially damaging for the long-term preser-
vation of the Murujuga rock art. Evidence suggests that the petroglyphs are already actively
degraded by industrial pollution.
Introduction
Murujuga and its rock art are of exceptional global

incorporating Murujuga, has over a million rock art
engravings or petroglyphs said to capture more than
50 000 years of Indigenous knowledge and spiritual
beliefs. The Murujuga rock art is among Australia’s
most extraordinary Aboriginal heritage sites and is
of Outstanding Universal Value (Australian Heritage
Council 2012; DBCA 2021). The rock art is thought to
contain some of the earliest known representations
in the world of the ‘human face’. It includes some of
the world’s oldest complex geometric designs, extinct
animals including the ‘fat-tailed kangaroo’ and ‘thy-
lacine’, as well as recording the evolving fauna of the
area through global climate change during and after
the last Ice Age (McDonald and Veth 2009; Mulvaney
   
both to current Aboriginal Custodians and to the global
community.
From the 1960s, sections of the Burrup Peninsula
have been increasingly industrialised, initially with
an iron ore port and salt production facility and, from
the late 1970s, with large petrochemical industries,
including natural gas plants and others that use the
by-products from gas liquefaction. The area is now

in the Southern Hemisphere’ (Mulvaney 2011b: 17).
Aside from the unconscionable direct destruction of
petroglyphs during the construction of this industry

upon the rock art of polluting emissions have been pre-
sented and discussed in many reports and publications
(e.g. MacLeod 2000, 2011; Bednarik 2002, 2007a, 2007b,
2009; Lau et al. 2008; Mulvaney 2011b; Moodie 2016;
Parliament
of Australia 2018; González Zarandona 2020). Conclu-
sions from governmental reports have been shown
to understate the evidence of damage to the rock art

Black et al. 2017a).
Ian MacLeod and Warren Fish have recently pub-
lished a conference paper (MacLeod and Fish 2021) in

to the factors controlling the decay mechanisms on
engraved rocks in the Pilbara region of Western Aus-
tralia’. Given the massive volume of published and
grey literature on this topic, this is a big statement.
The paper concludes that ‘the present monitoring data
shows that there is presently no adverse impact on the
rock engravings from industrial pollution …’. This is

expectations of decades of studies (Bednarik 1994, 2002,
2007b, 2009; MacLeod 2005, 2011; Mulvaney 2011b;

Moodie 2016; Black et al. 2017b; Ramanaidou et al.
2017; Data Analysis Australia et al. 2018). The basis of
this claim is a set of pH, nitrate, chloride ion and redox
measurements on rock surfaces at eight sites across
Murujuga, as well as some rock surface colour mea-
surements. We review the current understanding of
factors controlling the synthesis and decay of Murujuga
rock surfaces to assess whether the extraordinary claim
made by MacLeod and Fish (2021) can be considered

Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
4
The red-brown rocks of Murujuga – the living rocks
The rich red-brown colour that characterises the
visible faces of the granophyre and gabbro rocks of
Murujuga is not the original natural colour of the rocks,
which is blue-grey. Tectonic forces and temperature
   

new rock faces degrade extremely slowly (Pillans and
   
weathering rind, or leach zone, comprising a mixture
        
5–10 mm in 30 000 years depending on the rock type
(Bednarik 1979, 2007b; Donaldson 2011; Ramanaidou
and Fonteneau 2019). The weathering rind is covered
by the hard, red-brown surface outer layer that is
only 1 to 200 microns in thickness (Liu and Broecker
2000; Ramanaidou and Fonteneau 2019). It comprises
a ferromanganeous crust, known as the patina or rock
varnish. The rock art was made by breaking
through that patina and into the weathering rind
to provide a colour and contour contrast (Fig. 1).
Evidence from studies into the growth of
the ferromanganeous patina on rocks in simi-
lar environments to Murujuga shows that it is
formed by biomineralisation processes, where
budding bacteria and micro-fungi concentrate
manganese and iron compounds and form the
hard, cement-like components of the patina
(Miller et al. 2012; Dorn 2020; Lingappa et al.
2021). These microbes have evolved in extremely
harsh, low moisture, high-temperature rock
surface environments and have developed
strategies to protect themselves against radiation
and oxidative stress.
Dust is the primary source of manganese
and iron oxide on rock surfaces (Bednarik 2002;
Macholdt et al. 2019). Manganese oxide arriving
on the rock surface is reduced to H-Mn++ ions by pho-
tochemical or biological processes when moisture from
dew is present (Lingappa et al. 2021). The H-Mn++ ions

cyanobacteria, Chroococcidiopsis, with manganese being
concentrated up to 50 times or more from that in the
local environment. Chroococcidiopsis is the dominant
organism living on rock surfaces in extremely dry and
harsh desert environments, whether in the Antarctic or
hot deserts (Mishra 2020). This bacterium has evolved
to tolerate extreme desiccation, ionising radiation and
ultraviolet light and can maintain normal metabolic
processes following the loss through desiccation of
more than 50% of its weight (Kvíderová et al. 2011).
Lingappa and colleagues (2021) report that the
manganese compound in the patina is predominantly
in the Mn4+
mixtures of Mn4+and Mn3+. These authors consider that
this mixing of manganese
ion types is consistent with
the oxidation-reduction
recycling process occurring
within the patina, where
it is part of an ecosystem.
Figure 2 shows examples of
electron microscope images
of budding bacteria in rock
varnish and the concen-
tration of manganese and
iron within the bacteria.
The rock varnish-forming
microbes are thought to lie
dormant for much of their
lives, becoming active only
when moisture concen-
trations are sufficient for
active metabolism. When
the organisms die, their
biomass is oxidised, and
the magnesium and iron-
Figure 1. An example of a rock art panel showing part of the
Murujuga landscape (photograph: BWS).
Figure 2. Electron microscope images of rock varnish showing the bacteria sheaths
(hyphae) concentrating manganese-iron-rich material in rock patina (left) and a
budding hypha is emerging from a cocci bacterial form (right). The centre boom
image is an elemental spectral analysis of the hyphae (left) compared with the adjacent
patina (centre top), showing marked concentration of manganese (Mn) and iron (Fe)
in the bacteria (right image adapted from Dorn 2020: Fig. 13.3(a); left and centre
images from Dorn 2020: Fig. 13.2 originally from Krinsley et al. 2009: Fig. 5(B).
Images with permission from R. I. Dorn – The American Geographical Union grants
permission to use gures in academic works).
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
5
rich components are incorporated
with clays into the rock patina (Dorn
2020; Lingappa et al. 2021). Bacterial
remains and fossilised fungi have
been observed within the patina of
rocks collected from the Pilbara in
Western Australia, with Murujuga
being part of this region (Flood et al.
2003). Krinsley et al. (2017) suggest
that as few as one bacterium every
~400 years may be incorporated into
rock varnish under warm desert con-
ditions. Consequently, the rock patina
grows at extraordinarily slow rates of
1 to 10 microns per 1000 years (Dorn
and Meek 1995; Liu and Broecker
2000; Dorn 2009).
The manganese and iron oxides
and hydroxides in the patina are
formed only under near-neutral
and alkaline conditions (Goldsmith et al. 2014). The
ratio of manganese to iron compounds varies with

local acidity-alkalinity (Dorn 1990; Broecker and Liu
2001). Proportionately more iron compounds form in
drier conditions with a more alkaline environment
(Dorn 1990). The colour of the patina varies with the
proportion of darker manganese compounds relative
to the proportion of redder ferrous oxide compounds.
When suitable conditions exist, the patina is generally
formed in layers, but it can also be largely amorphous
and then is known as ‘rubbly varnish’ as illustrated in
Figure 3 (Garvie et al. 2008).
Over periods of 10 000 to 30 000
years, the outer patina can grow over
the weathering rind of the original
petroglyph to reduce the colour con-
trast and show primarily a contour or
texture contrast (Fig. 4). Critically, the
petroglyphs are lost if the outer patina
is removed either by being dissolved
or detached from the underlying
weathering rind (Fig. 4).
The inuence of pH
on Murujuga rock art decay
Published studies agree that the
rich red-brown patina of the Muruju-
ga rocks, as with other forms of rock
varnish, is dissolved with increasing
acidity (Bednarik 2002, 2007b; Ma-
cLeod 2005, 2011; Black et al. 2017b;
cf. Dorn 2020). Even a ‘[s]ubtle change
[…] in surface pH of only 0.3’ (Ma-

future survival of the rock art.
The impact of increasing acidity on
the chemical changes occurring within
the patina is most clearly illustrated by
consideration of the Pourbaix diagrams for manganese
and iron compounds (Black et al. 2017b). Figure 5
shows the Pourbaix diagram for manganese. The
diagram shows the three equations from MacLeod
and Fish (2021) for the synthesis of Mn3O4 (equation 1,
circled 16), MnO2 (equation 2, circled 20) and Mn(OH)22+
(equation 3, circled MF3). The range of rock surface pH
values reported in MacLeod (2005), Black et al. (2017b),
and MacLeod and Fish (2021) are also shown on the
diagram, as well as the domain of stability of water
where water is thermodynamically stable.
As acidity increases (i.e., pH falls), the manganese
Figure 3. Optical micrographs (A and B) of ultrathin sections of rock varnish
showing layering (LV) with dierent colours and more amorphous ‘rubbly
varnish’ (RV) overlaying a quar (Q) rock surface (from Garvie et al.
2008, Fig. 1. ‘Fair use’ permission presumed from The Geological Society
of America).
Figure 4. An ancient petroglyph where the patina has grown over the weath-
ering rind to provide only a contour contrast from the background rock
(centre-left) and where the patina has detached from the weathering rind
(centre-right) and the portion of the petroglyph removed (from Pillans
and Field 2013; with permission from B. Pillans, and through Elsevier
License 5163861131736).
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
6
compounds in the patina are altered through a series
of chemical transformations accompanied by colour
changes (brown-black Mn3O4, black Mn2O3, brown or
black MnO2
as pale pink) to Mn++,+++ soluble ions. The soluble
manganese ions may then be washed from the rocks
in rainfall events.
There is ample evidence that a high concentration
of natural organic acids will dissolve the manganese
and iron compounds that harden rock patina. The
outer patina is completely removed from rock surfaces
at Murujuga with natural acid from trees (Bednarik

2017) and also from areas of rock surfaces under lichens
(Dorn 1990; Dragovich 1987) and microcolonial fungi
(Dragovich 1993) at other desert locations in Australia
and southern United States of America (U.S.A.). For
this reason, it is not a surprise that acidic industrial
emissions have been found to degrade or destroy rock
surfaces (Dorn 2020; Giesen et al. 2014).
Industrial NOx emissions also cause an increase in
nitrite and nitrate concentrations on Murujuga rock
surfaces. MacLeod (2005) showed that there was a log
(10-fold) increase in the growth of bacteria, yeasts,
fungi, and lichens for each unit increase in rock surface
nitrate. These organisms release organic acids that
reduce the pH of the rock surface. There is a strong
relationship between nitrate and pH on Murujuga
rock surfaces (MacLeod 2005; MacLeod and Fish 2021).
Gleeson and colleagues (2018) speculate that changes
to the microenvironment of the rock patina on Muru-

species and metabolism of the microorganisms on the
rock surface. Large numbers of invasive opportunistic

      -
cient quantity will dissolve the manganese and iron
compounds in the patina (Dorn 1990). Second, these
organisms will outcompete the specialised patina form-
ing Chroococcidiopsis strains, which evolved to survive
in extremely low nitrogen rock surface environments

2019).
Bednarik (2009) states that ferromanganeous patina
will not occur in areas where the pH of the rain is 5.6
or less, which is consistent with the observation that
manganese ions leach from dust or rock varnish when
pH falls to 5.7 (Goldsmith et al. 2014;.
Osborn and colleagues (1981) measured the pH of rain
in the Arizona rangelands, where rock varnish is nota-
Figure 5. Pourbaix diagram of Mn (manganese) compounds. With increasing acidity, the dierent Mn oxides
progressively become unstable (from Mn3O4 to Mn2O3 to MnO2) until only dissolved Mn ions can exist.
Any change from the natural pH (blue dashed line) results in surface loss or fresh mineral deposits, which
changes the chemical composition of the rock surface and, thus, the appearance of the rock art (MacLeod and
Fish 2021: 4). Equation 3 (MacLeod and Fish 2021:5) is at about MF3; however, it does not normally appear
on the Pourbaix diagram (Hem 1963). Diagram adapted from Pourbaix (1974) and Black et al. (2017b) with
additional data from Black et al. (2017b); and MacLeod and Fish (2021).
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
7
bly widespread, to have a mean of 6.8 in places where
no polluting industries were operating. Bednarik (2002:
36) states that the pH of natural rain in the Pilbara re-
gion of Western Australia was ‘in the order of 6.8 prior
to industrial development, and reached about pH 7.0
to 7.2 at Murujuga’.
   -
nation of how pH changes on rock surfaces following
a rain event with a lower pH due to the dissolution of
atmospheric carbon dioxide. They propose that rain,
-
ly low to dissolve manganese from dust, but when it
lands on rocks, reactions occur that remove protons
from the carbonic acid (occurring naturally in rain) to
form alkaline minerals. As the sun dries the rocks, the
carbon dioxide from the carbonic acid is released, while
the alkaline metals remain and further increase pH
(i.e., raise alkalinity) with manganese hydroxides from
Mn2+ to Mn4+ continuing to form until pH 8 is reached.
Similarly, Goldsmith and colleagues (2014) report that
moist dust from the Negev desert in Israel has a pH of
about 8 and an EH of about 0.6 V. Dorn (1990) found a
strong correlation between soil pH and the pH of rock
varnish at 67 sites across Arizona and California in the
U.S.A. The pH of the rock varnish was predominantly
in the range 6.7 to 9, with values lower than these being


produce organic acids.
MacLeod and Fish (2021: 4) assert that ‘the natu-
ral pH of local weathered gabbro and granophyre is
5.5±0.2’. This stands in striking contrast to MacLeod’s
own recordings (Black et al. 2017b) and the known
pH of rocks upon which ferromanganeous patinas
survive in other parts of Australia and the world. Such
an allegedly low pre-industrial rock surface pH cannot
be reconciled with the observations made by Bednarik
(1979), who found that the mean pH was 5.9 across 30
sites on Murujuga where bird droppings had dissolved
the rock patina. Bednarik’s evidence suggests that,
contrary to MacLeod and Fish (2021: 4), rock art will
be completely degraded over time on any rock that

With this broader knowledge of the natural for-
mation processes of rock patinas and the deleterious

       
MacLeod and Fish (2021: 6). From the eight sites that
they monitored between 2017 and 2019, all of the rocks
       
event on 6 June 2018, average pH levels had reached
an acidity in which manganese ions are known to be
leached (all falling to between pH 5.2 and 4.4). Whilst
MacLeod and Fish place much emphasis on the impact

they present shows that rock surface acidity returns to
unacceptable levels soon after even a major rain event.
The pH of rocks taken from Murujuga to the Western
Australian Museum before industrialisation was found
to be 6.8±0.2 (Black et al. 2017b), which corresponds

by organic acids in the U.S.A. (Dorn 1990). By contrast,
the mean surface pH of the eight rock sites monitored
by MacLeod and Fish was 4.71 in 2019. This change in
average pH from pre-industrial times is dramatic and,
according to the Pourbaix diagram and other evidence

cause the patina to dissolve.
A rain event does not truly ‘reset’ the decay clock,
as MacLeod and Fish (2021) suggest. It dissolves some
of the dry, acidic dust pollution from industry that has
been deposited on the rock surfaces. As the dust mixes
with water, the resulting solution bathes the surfaces
in highly corrosive acids that will dissolve the rock
patina in the same way that acid rain eats away at stone
buildings and monuments in polluted cities. Very light
rains, the most common type at Murujuga, as well as
morning dews (which can be heavy during winter
months), react with the dry SO2 and NOX pollution
particles, oxygen and other chemicals to form sulphuric
(pKa, or acidity, -3.0) and nitric (pKa -1.37) acid. These
acids can reach very low pH and be highly corrosive to
ferromanganeous patinas. In most years, the amount
   
remove the acidic dust from most surfaces (Clark et al.
2018). It is only during heavy rains, typically cyclonic
events, when this acidity is truly washed away. Even
this is context-dependent. Smooth granophyre rocks
may be washed clean, whilst the rough gabbros may
   

pores. Major cyclonic events have been recorded on
average twice a decade over the past forty years at
Murujuga (Sudmeyer 2016; Holmes 2021). Whilst this
cyclonic removal of acidic particles assists the survival
of the rock surface patinas, such events happen too

   
away dissolved manganese and iron ions from the
pores in the rock surface and diminishes the integrity
of the patina (Andreae et al. 2020).
As a result, for the bulk of the time, the rocks at
Murujuga are currently subjected to a level of acidity
known to be corrosive to ferromanganeous patinas.
Our own commissioned pH testing in 2017, conducted
by MacLeod and reported in MacLeod and Fish (2021),
shows that rocks at site 4, adjacent to the Woodside
petrochemical plants, have an average pH of below 4.

to cause loss of iron and manganese particles from the
types of patinas that exist at Murujuga and that this
damage becomes extreme below a pH of 5.7 (Bednarik

follows that the average rock surface measurements of
pH 5.29 (2017), pH 5.51 (2018), pH 4.71 (2019) reported
by MacLeod and Fish (2021), even though these years
included a cyclonic event and a massive industrial
ammonia leak (alkaline) (MacLeod and Fish 2021: 4),
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
8

rock surfaces on the majority of days in the majority of

leads to surface loss’ (2021: 4).
Colour changes on the rocks
Colour change should be an important proxy for
patina decay, since any leaching of manganese and
iron compounds from the rocks will cause the surface
to become lighter in colour (Black et al. 2017b). In prac-
tice, the measurement of colour change on the rocks
of Murujuga has proven to be challenging. CSIRO at-
tempted it as part of a Western Australian government
monitoring program at Murujuga from 2004 until 2014.
The highly variable results were heavily criticised, both
for the method by which they were generated and
for how they were analysed and interpreted (Black et
al. 2017a; Data Analysis Australia 2016, 2017). There
were repeated inconsistencies in results from year to
year that arose from changing the colour measuring
-
ditions under which the readings were taken
(Black et al. 2017a). Black and colleagues
(2017a) also demonstrated that precise colour
change monitoring is almost impossible;
they provided the example of a close-up
photograph of a Murujuga rock surface and
demonstrated that the displacement of the
colour instrument measuring head by only
0.5 mm from the previous measurement
location could result in a substantial change
in measured colour (Fig. 6 and Table 1).

(2016) reanalysed the CSIRO 2004 to 2014
data (Markley et al. 2015) and found that
70% of all spots measured (engraving and

(L* increased) and that none were darker
over the decade of measurement. CSIRO
themselves had claimed that there was no
consistent trend in colour change, but these
claims were made without statistical analysis

the method, the extent of colour degradation
  
enough over a decade for it to be evident,
even in a colour measuring program that has
been shown to be imprecise.
   
one of the colourimeters formerly used by CSIRO and
that they measured average colour across the surface,
    
the same rock. Therefore, their work is subject to the

(Black et al. 2017a). MacLeod and Fish (2021: 7, Fig 8)
show colour contrast between background rock and
engraved areas for 2019 but present results for only
four of the eight sites because 
the regression (site 7), were claimed to be statistically
-


Figure 8 does 

pH of the surface. Indeed, they conclude that ‘[w]ithout
pH monitoring data, it is unwise to base conservation
   
Figure 6. Three square areas (A, B, C) with the same area as a 3 mm
diameter aperture positioned on a photograph of the surface patina
of a Murujuga rock (Bednarik 2007b: 224, Figure 19) and moved
0.5 mm to the right in the lower panels (reproduced from Black et
al. 2017a: Fig. 6).
Table 1. L*, a* and b* colour space values predicted for each square with the same area as a 3 mm diameter aperture
using the Image Color Summarizer (Krzywinski 2017) (reproduced from Black et al. 2017a: Table 6)
Square Position L* a* b*
Avg Median Min Max Avg Median Min Max Avg Median Min Max
A 0, 0 51 51 29 85 26 26 10 44 22 22 6 50
B 0, 0 46 46 17 80 25 25 1 47 19 17 -8 47
C 0, 0 59 60 22 79 34 35 16 52 29 29 1 49
A 0.5, 0 50 51 14 75 24 24 8 41 19 19 -2 46
B 0.5, 0 48 48 17 78 25 25 1 49 19 18 -8 51
C 0.5, 0 55 56 22 76 30 31 8 52 24 26 0 46
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
9
between the engraved and background areas of
the images.’ It is clearly not on an alleged colour
change that MacLeod and Fish base their extraor-
dinary conclusion that industrial pollution does
not impact the rock art.
Acidic emissions
MacLeod and Fish (2021) assert that ‘Wood-
side Petroleum introduced low NOx burners
between 2005 and 2015, which resulted in great-
ly reduced nitrate levels’, and this seems to be
one reason for their optimism that ‘the present
monitoring data shows that there is presently
no adverse impact on the rock engravings from
industrial pollution …’. However, they provide
no NOx     
reduction. The claim appears to be based on the
supposition that rock surface nitrate concentra-
tions are lower on average in 2017–19 than measured by
MacLeod (2005) in 2003–04. However, this supposition
is invalid because:
i)    

2017–19;
ii) MacLeod and Fish (2021) chose the highest value of
6.53±5.1 ppm recorded in 2003–04 for comparison;
iii) MacLeod and Fish (2021) report only a mean val-
ue of 0.7±0.5 ppm, when there was a wide range
in nitrate concentration sufficient to develop a
regression equation relating nitrate concentration
to pH; and
iv) MacLeod and Fish (2021) did not mention that ni-
trate measured pre-industrialisation was 0.3 ppm
(MacLeod 2005).
The emissions data reported by the industry at
Murujuga gives less room for optimism. Woodside

8900 tonnes of NOx annually. On top of the
Woodside and ongoing shipping emissions, since
2017, there have also been new and increasing
NOx emissions from Yara Pilbara Nitrates. Yara
Pilbara reported recently that their fertiliser plant
     
x
annually (Yara Pilbara 2019).
Contradictory to statements by MacLeod and
Fish (2021), the industry emissions data reported
to the National Pollution Inventory (2020) illus-
trate that NOx emissions rose steadily from 2000
until 2017 (Fig. 7). Woodside explained the appar-
ent drop in 2014 of Karratha Onshore Gas Plant
emissions as a change in the way emissions were
calculated rather than a real change in emissions
(Woodside 2019b). The reported tenfold drop in
Pluto onshore gas treatment emissions from 2018
to 2020 has not been explained. Given that there
were no reported major adjustments to any of
the main industrial processes or facilities in the
year of this ten-fold reduction (National Pollution
Inventory 2020), the reduction needs explanation.
Even if the NOx emissions have declined over the
last three years, the emission rate across Murujuga re-
mains high enough to damage the rock art, as seen by
the continuing low rock surface pH. A reduction in NOx
concentrations from earlier years is not the criterion for
assessing whether the rock art is being damaged. The
criterion is the quantities of acidic compounds from
industrial emissions deposited on the rock surfaces,
which will dissolve the outer rock patina. Clearly,
from the discussion above, when rock surface pH falls
below 5.9, the long-term survival of the rock art is in
jeopardy (Bednarik 1979).
The distribution of atmospheric nitrogen dioxide
across Murujuga is clearly illustrated in the data
from the Copernicus satellite for a week in 2021 (Fig.
8). Although the highest concentration is located
directly over the Woodside facilities and notably in
Figure 7. NOx emissions as reported annually by industry at
Murujuga (from National Pollution Inventory 2020).
Figure 8. Mean values of NO2 concentration over seven days
(26/4/2021 – 2/5/2021) derived from satellite data (Copernicus
data hub 2021). Analysis and map by Christopher Swain.
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
10
the immediate vicinity of various sampling sites, the
impact is evident across a wide area of Murujuga. A
similar distribution of NOx deposition due to industry
across Murujuga and adjacent islands was reported
by Parsons et al. (2021) in their study for the Western
Australian Government into the impact of cumulative
air emissions across Murujuga.
As we have noted above, nitrogen oxides form acid
rain when removed from the atmosphere during a rain-
fall event, and they also undergo dry deposition as dust
falling on the rocks at other times. This nitrous-rich
dust then forms nitric acid during a period of light
rainfall and dew. Results from chemical deposition
monitoring in 2012–2014 provided by Woodside show
     
deposited on the rocks (Woodside 2019a, Appendix
E – Air Quality, Table 3-8).
We have presented evidence on NOx emissions, but
emissions of SOx, particularly from shipping, are also
extremely high (Black et al. 2017a), and these will be
having an equally profound impact on the rock patinas
of Murujuga, forming sulphuric acid during periods
of light rainfall and dew.
MacLeod and Fish’s (2021) claim that these massive
and highly acidic industrial emissions are having no
impact on Murujuga rock art is, therefore, simply not
    -
dence presented. What then is the basis of the claim?
We are genuinely unsure, but it seems to relate to Ma-
cLeod and Fish’s (2021) term ‘precipitation’.
‘Precipitation’
There is a suggestion by MacLeod and Fish (2021)
that somehow rock patinas, whilst currently etched by
acidity during the bulk of the time, are quickly recuper-
ating during the brief interludes (presently only a few

level above 6.5. Unlike previous studies, MacLeod and
Fish (2021) emphasise that ‘alkalisation (from sea salt
or ammonia leaks) causes fresh minerals to deposit and
thereby change the appearance of the engravings’ (p. 4).
They report observing from photographs ‘an increased
amount of the purple-black patina’ on the sample
rock art from Deep George (Nganjarli) after a massive
ammonia leak event from Yara Fertilisers that had
turned rock surfaces in this area unusually alkaline.
The following year, they found that the reverse side
of the rock ‘had a deep-purple patina associated with
the presence of Mn3O4
redox (voltage) measurements. Unfortunately, no L*,
a* and b* data to quantify the colour measurements are
presented. They explain the colour changes in terms of

that this new ‘patina’ was dissolving (p. 5), presumably
as the rock acidity returned to below a pH of 6.5. They
describe having seen the same process operating at
two other sites among the eight they were monitoring.
They conclude that ‘dry deposition of ammonia and
wind-borne sea salts works to mitigate the release of
iron and manganese minerals from the Murujuga rock
engravings’ (p. 8). This claim seems to lie at the heart
of MacLeod and Fish (2021) assertion that ‘there is
presently no adverse impact on the rock engravings
from industrial pollution …’.
As we have described in this paper, the ferroman-
ganeous rock patinas of Murujuga were formed by bio-
mineralisation processes where microbes concentrate
manganese and, to a lesser extent, iron compounds
so as to form the hard cement-like components of the
patina (Miller et al. 2012; Dorn 2020; Lingappa et al.
2021). They do this only under alkaline conditions,
and it happens at extraordinarily slow rates of 1 to 10
microns per 1000 years (Dorn and Meek 1995; Liu and
Broecker 2000; Dorn 2009). Dorn and Krinsley (1991)
showed clearly that reprecipitation of manganese and
ferric compounds following acidic dissolution removes
the natural layering of the rock varnish, increasing
porosity and redistributing the manganese and ferric
deposits along rock wall fractures and not as a new
form of varnish. Thus, even if reprecipitation of man-
ganese and iron compounds occurred, it would not be
within the hyphae of dead microbes, and the structure
and morphology of the patina would be changed with
a detrimental impact on the long-term survival of the
rock art. We doubt that MacLeod and Fish (2021) have
observed examples of this process, and they certainly
do not provide evidence of it. Whatever the cause of the
colour changes observed by MacLeod and Fish (2021),
their data shows it was for a short period, and the
re-deposition was already being dissolved by acidity
within just a year.
Therefore, the bigger picture is what is important
here, namely that the industrial emissions at Murujuga
   and thereby
pose a constant threat to the integrity and survival
of the rock art. Occasional industrial ammonia leaks
and extreme weather events that temporarily alleviate

     
and cannot reverse the ongoing deterioration of the
rock art. The cause for genuine concern is real and
cannot be wished away, particularly as new industrial
developments are currently proposed that will result
in increased acidic emissions.
Conclusion
A detailed understanding of the rock decay pro-
cesses at Murujuga has evolved over more than 20
years from painstaking local research, and this has
    
the formation processes of ferromanganeous patinas in
low rainfall environments. The evidence we have pre-
sented clearly shows that, with the currently recorded
acidity levels, the rock patina and associated art will
degrade and disappear over time, as has occurred on
rocks with continual bird droppings (Bednarik 1979).

because the Western Australian State Government is
Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
11
considering development applications for a further
50 years of natural gas processing and from two new
industries seeking to construct plants at Murujuga.
We caution in the strongest possible terms that the
       
not acceptable as an excuse to allow the placement of
further polluting industry at Murujuga. The statement
is not supported by current science.
 
is of such concern that existing industry should be
subjected to far more stringent emission controls to
bring Western Australia into line with other global
leaders. Excellent technology is available and utilised
successfully elsewhere that substantially reduces
emissions by industries of the type located at Muru-
juga. One of the parent companies of Murujuga’s
own Yara Pilbara is a market leader in manufac-
turing emissions control technology. The Western
Australian Government should heed the warnings of
contemporary science and ensure that new industry
is placed away from Murujuga, in one of the many

strict emissions control requirements are enacted to
reduce pollution at Murujuga and bring the State in

pollution and climate change. In direct contrast, the
Norwegian Government placed a tax on emissions
of NOx in 2007 (recently 23.48 NOK or AUD 3.40 per
kg NOx   
x in
forming acid rain and damaging human health. A tax
of this magnitude would raise approximately $30 m
annually from Woodside operations alone and would
undoubtedly change behaviour and help to ensure the
long-term preservation and protection of the Indige-
nous cultural heritage contained in the irreplaceable
Murujuga rock art.
Given that the bigger claims of MacLeod and Fish
(2021) are at odds with existing science, the onus is on

upon which they base their claims (including their
statistical methodology) and secondly, they should
make their data publicly available. MacLeod and Fish
acknowledge in their paper (2021: 3) that they both
work as contractors to the petrochemical industry
producing the acidic emissions at Murujuga. The tide
is turning against those that seek to play down the
impacts of industrial pollution upon the world. Even
within the industry, big vested interests are changing
tack and, for example, supporting the shift to sustain-
able energy and net zero-carbon emissions (Murray
2020; Samios and Harris 2021).
Acknowledgments
 -
original Corporation for their support. We are grateful for
the institutional support that we receive from the Centre for
Rock Art Research and Management in the School of Social
Sciences at the University of Western Australia and the

publication is produced as a part of the Murujuga Rock Art

of industrial pollution on Murujuga rock art, was funded by
ordinary citizens who gave an amount in excess of $285 000
through crowdfunding. This extraordinary level of public
support shows the extent of public concern that exists on this
topic. We thank each of the many hundreds of people that
supported this research. The views expressed in this paper


insightful comments of Dr Ilona Box. We thank Christopher
Swain for analysing the Copernicus satellite data and the
production of the NO2 map in Figure 8.
Conicts of interest
JLB and MAF are members of Friends of Australian Rock
-
lished in 2006 to protect, preserve and promote Australia’s
Aboriginal rock art, particularly the ancient petroglyphs of
the Dampier Archipelago, including Murujuga. We are all
scientists from a range of disciplines, determined to uphold

paper were voluntary.
Prof. Benjamin W. Smith
Centre for Rock Art Research and Management
University of Western Australia
Perth, WA 6009
and Rock Art Research Institute, Wits University, South
Africa
Benjamin.Smith@uwa.edu.au
Dr John L. Black
Centre for Rock Art Research and Management
University of Western Australia
Perth, WA 6009
Australia
John.Black@uwa.edu.au
Dr Stéphane Hœrlé
CNRS UMR 5199 PACEA
Université de Bordeaux
CS 50023, 33615
Pessac
France
and Rock Art Research Institute
School of Geography
Archaeology and Environmental Studies
University of the Witwatersrand
Johannesburg
South Africa
stephane.hoerle@sfr.fr
Dr Marie A. Ferland
P.O. Box 3309, East Perth, WA 6892
ferlandmarie@gmail.com

Apex Biometry Pty Ltd,
South Fremantle, WA 6162
Australia
simon@apexbiometry.com
Jolam Neumann and Prof. Dr Thorsten Geisler
Institut für Geowissenschaften (ehemals Steinmann-Institut)

Rock Art Research 2022 - Volume 39, Number 1, pp. 3-14. B. W. SMITH et al.
12
Universität Bonn
Meckenheimer Allee 169
D-53115 Bonn
Germany
jolam@uni-bonn.de, tgeisler@uni-bonn.de
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RAR 39-1370
... The observation of accelerated degradation would not be surprising, considering that in its National Pollutant Inventory the Australian Government lists at least 320 t of ammonia, nearly 9000 of nitrogen oxides, and more than 300 t of sulphur dioxide that were released into the air by the industrial complexes on the peninsula in 2019 alone [23]. Smith et al. showed, using observations from the Copernicus satellites, that nitrogen dioxide was widely distributed across the archipelago, and highly concentrated over the industrial area [24]. It is known that nitrogen oxides released into the air lead to acid rain [25] which could result in accelerated dissolution of the rock varnish on Murujuga and thus a faster decay of the petroglyphs [24]. ...
... Smith et al. showed, using observations from the Copernicus satellites, that nitrogen dioxide was widely distributed across the archipelago, and highly concentrated over the industrial area [24]. It is known that nitrogen oxides released into the air lead to acid rain [25] which could result in accelerated dissolution of the rock varnish on Murujuga and thus a faster decay of the petroglyphs [24]. Bednarik reported that the pH of natural rain in the Pilbara region of Western Australia was in the order of 6.8 prior to industrial development and reached values between 7.0 and 7.2 at Murujuga [18], which is essentially consistent with observed neutral pH values for rain in the Arizona rangelands, where desert varnish is notably widespread and no polluting industries were operating [26]. ...
... In particular, a recent publication by MacLeod and Fish concludes that industrial emissions have no adverse impact on the petroglyphs [32]. Smith et al. claim that this conclusion is based on unsubstantiated, misleading and scientifically unsound data [24]. Therefore, there is an urgent need to design and perform studies that can ultimately answer the question as to whether industrial emissions are leading to accelerated weathering of the rock varnish. ...
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The petroglyphs of the Murujuga (Burrup Peninsula and Dampier Archipelago) in Western Australia are of exceptional cultural value for the nation. It is Australia’s largest and most significant collection of aboriginal rock art essentially as petroglyphs, with the number estimated at more than one million engravings. The Murujuga is a textbook example of ancient and modern times colliding as it also hosts potentially polluting, major industrial complexes such as iron ore and salt ports; liquefied natural gas, liquid ammonia and ammonium nitrate plants; railway lines, pipelines and rock quarries. The work presented in this paper is underpinned by an annual monitoring study of 10 selected sites including two control sites located on Dolphin and Gidley islands and eight test sites located closer to the industrial areas. The main rock types of the Murujuga, gabbro and granophyre, have been affected by weathering consisting of a cm-thick leached zone capped by a skin of orange and red iron and manganese oxides. The petroglyphs were engraved by removing the few top millimetres of the iron oxide-rich layer and the contrast with the paler leached zone clearly showing the carved motifs. Phosphorus is abnormally enriched in the leached zone and the surface coating, with manganese exclusively on the surface coating, demonstrating the impact of bird droppings and ‘desert varnish’ on the rocks. The colour difference and the hematite–goethite ratio, measured with a field portable reflectance spectrometer, between the background and engraving provides a tool to estimate the relative age of the petroglyphs and the selection of the backgrounds. As the fine-grained granophyres were preferentially chosen from the coarser-grained gabbros to host the petroglyphs, the goethitic (or yellower) backgrounds were prepared as canvas before being engraved. Many petroglyphs register the food items of the area, which changed as the last ice age ended and sea levels rose. We hypothesise that based on their locations and colour difference the constraint for the oldest age of the petroglyphs exceeds 17 000 years BP.