Conveying the importance of stromatolites to self-guided tourists in Nettle Cave, Jenolan, NSW

Abstract

Stromatolites span all of geological time; they are the oldest evidence of life on Earth and are important in understanding how life began and first started to diversify. They also played a critical role in shaping the atmosphere as we know it today, causing an irreversible increase in atmospheric oxygen at ~2.4 Ga. Significantly, stromatolitic speleothems in Nettle Cave, Jenolan, resemble these truly ancient forms. Cyanobacteria growing on the surface of these stromatolites use hydrogen carbonate from roof seepage drip-waters as a source of carbon, and light from four entrances allows for photosynthesis and liberation of oxygen. Calcium from the limestone drip-water is deposited as calcium carbonate. The overall shape of these stromatolite structures is most influenced by wind, which causes drip-water to fall at an oblique angle, forming the asymmetric, segmented shapes. Wind also blows-in insects and sediment which become trapped on the surface of the stromatolites by biofilm produced by the microbes. One sample analysed previously shows growth over a period of 20,000 years, capturing an excellent source of information about recent past climates and could reveal environmental fluctuations over this time span. In order to ensure their continued protection, it is important to promote an understanding of the scientific value of the stromatolitic speleothems in Nettle Cave to both visitors and guides at Jenolan. Linking these speleothems with information about the earliest life on Earth and the persistence of stromatolites through time plays a key part in engaging and educating the public about these interesting structures. Currently, tourists can take a self-guided tour of Nettle Cave using the Jenolan mobile application; however, there is minimal information available on the scientific value of stromatolites to our understanding of past climate and the early Earth. The proposed new visitor information project would involve: updating the stromatolite section of the current mobile application; placing a concise and attractive display board in front of the most prominent stromatolites; creating a pamphlet with more explanation, including pictures and diagrams, available from the guides office; and, updating the Jenolan Caves website to give further detail, including examples of other key stromatolites through time and links to scientific literature, for those wanting up-to-date and more in-depth information.

Full text

Conveying the importance of stromatolites to self-guided tourists in Nettle
Cave, Jenolan, NSW.
E. V. Barlow1
Affiliation: 1Australian Centre for Astrobiology, and School of Biological, Earth and Environmental Sciences
(University of New South Wales, Kensington, NSW, Australia). e.barlow@unsw.edu.au
Abstract
Stromatolites span all of geological time; they are the oldest evidence of life on Earth and are important in understanding how life
began and first started to diversify. They also played a critical role in shaping the atmosphere as we know it today, causing an
irreversible increase in atmospheric oxygen at ~2.4 Ga. Significantly, stromatolitic speleothems in Nettle Cave, Jenolan, resemble
these truly ancient forms. Cyanobacteria growing on the surface of these stromatolites use hydrogen carbonate from roof seepage
drip-waters as a source of carbon, and light from four entrances allows for photosynthesis and liberation of oxygen. Calcium from
the limestone drip-water is deposited as calcium carbonate. The overall shape of these stromatolite structures is most influenced
by wind, which causes drip-water to fall at an oblique angle, forming the asymmetric, segmented shapes. Wind also blows-in
insects and sediment which become trapped on the surface of the stromatolites by biofilm produced by the microbes. One sample
analysed previously shows growth over a period of 20,000 years, capturing an excellent source of information about recent past
climates and could reveal environmental fluctuations over this time span.
In order to ensure their continued protection, it is important to promote an understanding of the scientific value of the stromatolitic
speleothems in Nettle Cave to both visitors and guides at Jenolan. Linking these speleothems with information about the earliest
life on Earth and the persistence of stromatolites through time plays a key part in engaging and educating the public about these
interesting structures. Currently, tourists can take a self-guided tour of Nettle Cave using the Jenolan mobile application; however,
there is minimal information available on the scientific value of stromatolites to our understanding of past climate and the early
Earth. The proposed new visitor information project would involve: updating the stromatolite section of the current mobile
application; placing a concise and attractive display board in front of the most prominent stromatolites; creating a pamphlet with
more explanation, including pictures and diagrams, available from the guides office; and, updating the Jenolan Caves website to
give further detail, including examples of other key stromatolites through time and links to scientific literature, for those wanting
up-to-date and more in-depth information.
Keywords: Stromatolites, Nettle Cave, Early life, Science Communication, Cave Management.
1. Introduction
Nettle Cave, part of the greater Jenolan Caves system, has
been open on and off as a show cave for the best part of 200
years, since 1838 (Cox 1984). It contains large, blue-green
stromatolitic speleothems, which are able to grow and flourish
in this particular cave due to the balance of microbial activity
with sunlight, wind and the rate of cave drip-waters. These
relatively rare speleothems were first described, although not
directly by name, by Cook (1889): “One prominent stalagmite
is like the back of a newly-shorn sheep, with shear-marks in
the wool.” However, it was not until a century later that these
structures, more commonly known as ‘lobsters’ or ‘craybacks’
due to their asymmetric mound-shape and overall segmented
appearance (Fig. 1A, B), were studied in more detail (Cox et
al. 1989a). Cox et al. (1989a) looked at the morphology and
mode of formation of the blue-green speleothems and
concluded that they fit the definition of cryptalgal stromatolites
after Aitken (1967): “(those structures which) originate through
the sediment-binding and/or carbonate-precipitating activities
of nonskeletal algae.”
2. Formation and Morphology of Stromatolites
Stromatolites are most commonly found submerged in shal-
low-water environments, including; marine (e.g. the hyper-
saline lagoon of Shark Bay, Western Australia), fresh-water
(e.g. Lake Pavilion, Canada), and hot spring environments
(e.g. Rotorua, New Zealand). Stromatolites also occasionally
occur in subaerial evaporitic settings, such as those from
Nettle Cave, and also Wombeyan Caves (James et al. 1982).
However, there are relatively few examples of stromatolites of
this type worldwide. Cox et al. (1989a) proposed a model of
formation of the stromatolitic speleothems in Nettle Cave,
which involved a complex interplay between the photosyn-
thetic cyanobacteria, rate of drip-waters, wind speed and
direction, humidity and level of evaporation (Fig. 1C). These
stromatolites primarily grow from “biologically driven inor-
ganic calcite precipitation” (Cox et al. 1989b), enhanced by
evaporation, on the surfaces where water falls from above.
Depending on which stage a stromatolite is at in this cycle, its
colour can change dramatically from deep blue-green (Fig.
2A), to dusty blue (Fig. 2B), to cream, as new calcite precipi-
tates covering the cyanobacteria (Fig. 1A).
There are a number of differences between the Nettle Cave
stromatolitic speleothems and other forms of stromatolites
(Table 1). The evaporitic cave stromatolites are unique in that
they are subaerial, gaining moisture and a source of carbon
from the cave drip-waters (Cox et al. 1989b), whereas typical
stromatolites are submerged in water and are only peri-
odically, if ever, exposed. The morphology of stromatolites,
whether exposed in air or submerged in water, is controlled by
both the microbes and the prevailing environmental con-
ditions. In the case of the Nettle Cave stromatolites, the cave
environment is the primary control of overall morphology. The
asymmetric, elongated shape of the stromatolitic spe-
leothems is caused by the wind, which blows between the
110 Proceedings of the 17th International Congress of Speleology

Figure 1. (A), (B) Sub-aerial stromatolitic speleothems in Nettle Cave were
known as ‘lobsters’ or ‘craybacks’ due to their asymmetric and segmented
appearance. (B) Shows blue-green cyanobacteria visible on opposite side of
same stromatolite as in (A). (C) Mode of stromatolitic speleothem formation in
Nettle Cave, from Cox et al. (1989a).
southern entrance to Nettle Cave and the large void opening
into the Devils Coach House to the north (Fig. 3). In cross
section, the tallest point (i.e. left-hand side of Fig. 1C) is where
the drips fall straight down, whilst the tapering ‘tail’ is caused
by the wind blowing the drips sideways to varying degrees.
This effect is nicely displayed on the concrete walkway of the
Devils Coach House (Fig. 4A). A similar phenomenon is
observed with the modern stromatolites in Hamelin Pool
(Shark Bay, Western Australia), where the stromatolites are
elongated in the direction of the tides (Fig. 4B). However,
unlike the submerged stromatolites in Shark Bay which are
subject to erosion and sediment deposition from rough waves
and storms, the stromatolitic speleothems are comparatively
protected from the outside environment. This results in a rela-
tively uninterrupted record of the environmental and climatic
conditions being preserved in these cave examples.
3. Value of Stromatolites
Fossilised stromatolites are important clues in understanding
how life began and first started evolving on Earth. They are
the oldest evidence of life, with good examples preserved from
the c. 3.5 Ga Dresser Formation in Western Australia (Walter
et al. 1980), and new, controversial stromatolites reported
from c. 3.7 Ga in Isua, Greenland (Nutman et al. 2016). Stro-
matolites are known from throughout the global geological
record, persisting through to the present day. Interestingly, it
is not until c. 1.8 Ga that individual fossilised microorganisms
are large enough to be visible with the naked eye (Grypania,
see: Walcott 1899; Han 1992; Sharma & Shukla 2009, among
others); microbial life apparently persevered alone for over 1.7
billion years.
Microbes also played a huge role in shaping the Earth as we
know it today. The Archean atmosphere was very oxygen
poor, with <10-5 of the present atmospheric level (PAL)
(Pavlov & Kasting 2002). It was not until the Great Oxidation
Table 1.
Comparison between shallow-water stromatolites and stromatolitic speleothems:
Shallow-water stromatolites
Stromatolitic speleothems
Fully or partially submerged in water
Completely subaerial
Source of carbon from surrounding sea/fresh water
Source of carbon from cave drip waters (freshwater)
Water-borne sediment is trapped by microbial biofilm
Wind-blown sediment, along with dust and small insects are
trapped by microbial biofilm
Primary influence on shape is sunlight, and tide strength and
Primary influence on shape is wind (Cox et al. 1989a) and sunlight
direction
from cave entrances
Range of morphologies present with increasing water depth
One broad morphology present, just different sizes
Growth of stromatolite primarily controlled by sediment input,
Growth of stromatolite primarily controlled by inorganic calcite
as well as carbonate precipitation by microbes
precipitation driven by microbes, as well as evaporation
Affected by erosion from wind and wave action, as well as poten-
Primarily affected by erosion from rate of drip water, but very pro-
tial to be partly or wholly smothered by sediment, and damaged
tected from storms, wind erosion and potential to be completely
by storms
smothered by sediment
Most common form of stromatolite at present day and through-
No known fossilised examples from geological record, and only
out geological record
rare examples exist today
Proceedings of the 17th International Congress of Speleology
111

(A) (B)
Figure 2. Then and now:
(A) Stromatolitic speleothem from Cox et al. (1989a), wet with drip waters, highlighting vibrant blue-green colour of
cyanobacteria.
(B) Same stromatolite in 2017 with cyanobacteria still present, but much less colour due to cessation of drips. In foreground is
current, temporary A3-sized poster board on display to public.
Event (GOE) at c. 2.4 Ga that there was a significant rise in
levels of atmospheric oxygen (Farquhar 2000; Holland 2002;
Bekker et al. 2004). This irreversible rise is regularly attributed
to oxygenic photosynthesis by stromatolites (Schopf 2014).
Thus, through research into – and comparison with – modern
analogues, fossilised stromatolites provide key information in
understanding and reconstructing the original environment of
deposition (Table 2). For example, Barlow et al. (2016)
determined the relative water depth of different fossilised
stromatolites from 2.4 Ga and was able to reconstruct the
position of these different morphologies within the carbon-ate
reef system, which allowed analysis of transgression and
regression cycles. In a similar way, stromatolitic speleothems
preserve a neat record of the paleoclimatic conditions during
formation. Cox et al. (1989b) dated a piece of cyanobacteri-
ally-covered flowstone from Nettle Cave at over 20,000 years
old, and estimated some of the larger structures to be at least
100,000 years old. Studying layer thickness and variability, as
well as carbon and oxygen isotope data of stromatolitic
speleothems such as these can reveal terrestrial paleoclimate
information such as temperature, pH, rainfall and changes in
overlying vegetation (Blyth et al. 2016; Wong & Breecker
2015).
4. Promoting Education
Since first use of the term stromatolite more than a century
ago (Kalkowsky 1908), much research has gone into the dif-
ferent environments stromatolites inhibit and the array of
microbes associated with each, resulting in an understand-ing
of their importance in being able to unravel information about
the past. In order to manage and protect the stromatolites at
Nettle Cave, this needs to be communicated effectively to both
the guides and visitors to the cave. The aim is to com-
municate scientific knowledge in an engaging way through a
mutli-layered approach, using different media. This would
include:
Table 2. Environmental and depositional information available from studying stromatolites:
Type of environment (lagoon, carbonate reef, lake, etc.)
Tide direction and subsequent shoreline
Approximate water depth
Location within the carbonate platform system (intertidal, subtidal, etc.)
Compositional information of water
Relative stability of environment (thickness of units)
Other environmental influences on stromatolite shape: relative tide/current strength, presence of storms, changes in
amount of sediment input, changes in water depth.
By compiling information about stromatolites, inferences can be made about the changing and potentially increasing
diversity over time, both in the form of morphology and microbial make-up.
112 Proceedings of the 17th International Congress of Speleology

Figure 3. Nettle Cave map (© Jenolan Survey project, 2014),
with stromatolitic speleothems shown by the elongated,
segmented ovals in the central right-hand side of the cave
(e.g. arrow).
1. Updating some of the content of the mobile application,
linking stromatolites with evidence of early life in the
Archean, whilst keeping it easy to understand.
2. A poster board displayed in front of the most prominent
stromatolite, where the current temporary sign is now (Fig.
2B), with more detailed information than is currently there.
Proposed size would be A2 and it would cover: how
stromatolites grow; that these particular stromatolites are at
least 20,000 years old and by studying the layers, scien-
tists can get information on past climates; stromatolites
more commonly grow in shallow marine environments (e.g.
Shark Bay, WA), which is why these cave examples are so
special; the oldest evidence of life on Earth are fossilised
stromatolites (from at least 3.5 Ga). The board would refer
visitors who want more information to a pamphlet avail-able
at the Guides’ Office.
3. A pamphlet, with further details on the above information
including more diagrams and pictures, would be made
available at the Guides’ Office. This could be in the form of
a folded, double-sided A4 sheet of paper, for ease of repro-
ducibility. The pamphlet would direct those seeking even
more information to the Jenolan Caves website.
4. Updating the stromatolitic speleothem page on the Jeno-
lan Caves website, with extra information made available
through links to articles and journal papers on stroma-
tolites, both modern and fossilised. Using this method
means it would be easy to update and add to the page as
new research is reported, keeping the information current.
Using this multi-layered approach, where each of these media
would contain slightly more information than the last, means
visitors to Nettle Cave can gain as much, or as little, infor-
mation as they would like about the stromatolites, allowing
them to easily investigate further if they wish. Furthermore, the
pamphlet and website would be a good resource for the
guides to use, and to direct people towards more information.
Most importantly, the proposed approach will have minimal
impact on Nettle Cave and the stromatolites.
(A)
(B)
Figure 4. (A) New stromatolitic speleothems growing on the
concrete walkway in Devils Coach House, Jenolan,
highlighting the effect of wind on stromatolite form (arrows).
(B) Submerged, shallow-water stromatolites in Shark Bay,
Western Australia, showing elongation with tide direction
(arrow).
5. Summary
It is extraordinary that the stromatolite form has persisted for
at least the last 3,500 million years, across a range of differ-
ent environments. To accurately encompass this range, and
include both modern and fossilised stromatolite examples, an
updated and more widely accepted definition is now used:
“Stromatolites are... layered, early lithified, authigenic micro-
bial structures… that develop at the sediment water interface
in freshwater, marine and evaporitic environments” Riding
(2011). The stromatolitic speleothems in Nettle Cave are an
uncommon but important example, as they preserve a con-
sistent paleoclimatic record from the last 20,000, and possibly
up to 100,000 years (Cox et al. 1989b). The introduction of the
self-guided tour and installation of floating walkways has
allowed visitors to view the stromatolites and surrounding
cave with minimal impact, whereas previously, they could walk
right up to and around the speleothems. It is proposed the
value and importance of stromatolites be further conveyed in
a series of engaging and multi-layered media for the public,
including a poster board, pamphlet and updated webpage, to
go alongside the existing mobile application.
Proceedings of the 17th International Congress of Speleology 113

Acknowledgements
Thank you to the Jenolan Caves Reserve Trust, and A/Prof.
J. James and Dr C. Barnes for assistance with visiting Nettle
Cave. All photographs taken by author, unless otherwise
stated.
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