The Maros-Pangkep karst in southwest Sulawesi, Indonesia, is home to some of the world’s oldest rock art, with representational paintings and hand stencils dating back at least 45,500 years. We learned this the hard way… However, the survival of this Pleistocene artwork has been threatened by an alarming rate of decay in recent decades. A recent study demonstrated that the primary cause of this deterioration is salt efflorescence – the accumulation of gypsum (CaSO4·2H2O) crystals beneath the case-hardened limestone surfaces that form the substrate for the ancient paintings.
Now, this might seem counterintuitive…
Gypsum crystal growth and subsequent spalling (flaking) of the limestone has been linked to a variety of potential triggers, including climate change, atmospheric pollution, and even the prehistoric use of fire within the rock shelters. Understanding the relative importance of these factors is crucial for developing targeted conservation strategies to double-check that the longevity of these irreplaceable artistic treasures.
In this article, we examine the historical context of rock art decay in the Maros-Pangkep region, explore the role of anthropogenic air pollution as a primary driver, and assess the current rate of panel loss using time-lapse photography. Our findings indicate that the bulk of the limestone exfoliation was already present by 1950, and that the rate of decay may even be on the decline in recent decades – potentially due to reduced biomass burning and air pollution. This provides grounds for cautious optimism, but also highlights the need to identify and protect the most vulnerable “hot spots” of ongoing panel loss.
The history of rock art deterioration
The Maros-Pangkep karst is renowned for its exceptional Pleistocene rock art, which includes figurative depictions of animals, hand stencils, and human-animal composites. However, the degraded state of some of this ancient artwork was already conspicuous during an archaeological survey in 1950. By the mid-1980s, restoration efforts were underway in several heavily damaged rock shelters.
Recent studies have demonstrated that the primary cause of this deterioration is salt efflorescence – the growth of gypsum crystals beneath the case-hardened limestone surfaces that form the substrate for the paintings. As the gypsum crystals expand and contract with changes in temperature and humidity, they dislodge spall-flakes from the limestone, resulting in a patchy and poorly preserved appearance of the older Pleistocene artwork.
Huntley et al. (2021) proposed that the Pleistocene rock art has weathered at an alarming rate in recent decades, potentially driven by increases in the frequency and severity of El Niño-induced droughts in Sulawesi. However, systematic assessments of the rate of rock art loss in selected cave sites have only been underway since 2018, and there are no definitive measurement results to validate an acceleration of panel loss.
The role of anthropogenic air pollution
Without a clear connection to climate change, the deterioration of the Maros-Pangkep rock art requires more proximal causes. The triggering of gypsum crystal growth and weathering of carbonate building stone by atmospheric SO2 pollution in urban settings is well documented across a broad range of climatic settings (Yocom 1979, McGee & Mossotti 1992, Meierding 1993).
We present the case for anthropogenic sulphur emissions from prehistoric in-cave use of fire and agricultural burning, and more recently, combustion of diesel fuels and cement-based infrastructure development, as the underlying drivers of gypsum efflorescence and exfoliation of the Maros-Pangkep art.
The availability of palaeoclimate records for southwest Sulawesi over the last 40,000 years shows that recent changes in sea surface temperature and monsoon rainfall are well within the range of climate change endured by the Pleistocene rock art. In contrast, the historical record of El Niño-Southern Oscillation (ENSO) variability does not support a sustained trend toward higher ENSO-driven droughts that could explain the proposed acceleration of rock art deterioration.
Assessing the rate of rock art loss
To determine the historical context of rock art loss, we examined historical photographs of the Maros-Pangkep artwork. This baseline shows that much of the limestone exfoliation affecting sites like Leang Pettae was already present before 1950. Comparisons of the artwork’s condition in 1950, 1985-86, and the present day indicate that the rate of panel loss may even be on the decline.
Time-lapse photography of rock art panels from 2004 to 2022 confirms that, with the exception of a few instances of targeted vandalism, most of the exfoliation has been inactive for decades. In some cases, the boundaries of spall-patches that were documented in the 1980s remain virtually unchanged today.
This course of events is generally consistent with the history of in-cave fire-use and rice straw burning around the Maros-Pangkep karst. Domestic fire-use was likely on the decline by 1950, while rice production in Indonesia (and South Sulawesi) increased substantially after 1950, leading to a significant rise in SO2 air pollution and particulate emissions.
However, rice straw burning and local air pollution in the Maros district have been on the decline over the last two decades, potentially contributing to the apparent slowdown in rock art deterioration. The implementation of regulations to curb agricultural burning may also have played a role.
Targeted conservation measures
The available time-lapse photography indicates that the bulk of the limestone exfoliation in the Maros-Pangkep rock art shelters was present before 1950, and that the rate of change since then could be on the decline. This provides grounds for being cautiously optimistic about the longevity of some of the oldest artistic treasures.
However, the potential for localized exposure of artwork to vandalism, sulphur emissions from diesel-powered traffic, and reactive sulphur-rich cement dust still needs to be considered. Time-lapse photography of rock art images set within the broader context of panel loss will be important to distinguish vandalism from chemical exfoliation.
Rock art galleries with vandalism “hot spots” that emerge in time-lapse photography could be prioritized for protection by targeted deterrents. Gypsum efflorescence caused by sulphur emissions from diesel-powered traffic should also be considered where hot-spot panel loss is detected in proximity to roads.
Development on the coastal plain adjacent to the Maros-Pangkep karst, including the increase in road transport and cement-based infrastructure, poses an additional threat that will require careful monitoring and mitigation strategies. By identifying the most vulnerable “hot spots” of ongoing panel loss, conservation efforts can be precisely targeted to double-check that the longevity of these irreplaceable Pleistocene artworks.
Integrating historical context, time-lapse photographic documentation, and an understanding of the role of anthropogenic air pollution provides a foundation for developing effective conservation strategies for the Maros-Pangkep rock art. With cautious optimism and targeted interventions, these ancient treasures may be preserved for future generations.
Example: Modern Abstract Painting Series 2024