The Tonasa Formation is one of the most widely distributed rock units in the southern arm of Sulawesi, reaching thicknesses of 1,750 to 3,000 meters. It consists of interbedded limestone and Globigerina marl deposited on a carbonate platform that flourished as the Indo-Australian, Pacific, and Eurasian plates collided, eventually sculpting Sulawesi into its distinctive K-shaped outline. Yet despite its grandeur and its role in creating spectacular karst landscapes, the formation’s stratigraphic record remains incomplete in places. In the Jeneponto area, Oligocene to Early Miocene strata were long considered poorly exposed and poorly known, leaving a gap in scientists’ understanding of how this tropical carbonate platform responded to global climate change roughly 34 million years ago.

To fill that gap, the research team turned to the Karama area, where the formation is beautifully exposed along the coast as alternating beds of limestone and marl. They sampled 23 layers using measured stratigraphy, preparing each sample as a smear slide sealed under a 24 by 24 millimeter cover glass. Under a polarizing microscope at 1,000 times magnification, the fossil content of the rocks came into view: a constellation of coccoliths and related structures produced by nannoplankton, the primary producers of ancient seas. Because these organisms evolved rapidly and were hugely abundant, their first and last appearances in the rock record provide datums, time markers that allow geologists to correlate strata across entire ocean basins with remarkable precision.

The analysis identified 20 species in the Karama A section, including members of the genera Coccolithus, Cyclicargolithus, Reticulofenestra, Dictyococcites, Discoaster, Sphenolithus, and Zygrhablithus, along with the distinctive Braarudosphaera bigelowii. From the vertical distribution of these species, the researchers recognized three key biostratigraphic datums. The first is the First Occurrence of Sphenolithus pseudoradians, which marks the boundary between zones NP19 and NP20 in the standard zonation of Martini from 1971. The second is the First Occurrence of Sphenolithus distentus, corresponding to the CP16/CP17 boundary of Okada and Bukry’s 1980 scheme, equivalent to NP22/NP23, found in layer 12 of the section. The third is the Last Occurrence of Sphenolithus predistentus in layer 18, defining the NP23/NP24 boundary.

Together, these datums establish that the studied part of the Tonasa Formation at Karama A spans the Late Eocene to the Middle Oligocene. That conclusion matters because it means the Eocene/Oligocene boundary itself, the dividing line between the epochs, is captured within this section. This was precisely the interval when Earth’s climate flipped from the Cenozoic’s greenhouse state toward icehouse conditions, as massive ice sheets grew on Antarctica and global temperatures plummeted. Previous work on the Karama area using planktonic foraminifera had identified only a latest Eocene age, so the new nannofossil data extend the known record substantially and help place this section in the middle part of the formation’s regional stratigraphic framework.

The story the fossils tell is not only about time but also about temperature. Calcareous nannoplankton distribution is strongly tied to surface water temperature and nutrient availability, making their assemblages natural thermometers for ancient oceans. Certain genera carry well-established climatic affinities: Discoaster, a lower photic zone dweller, is considered an indicator of warm, oligotrophic waters, as are Sphenolithus and Zygrhablithus bijugatus. Coccolithus pelagicus, by contrast, is a classic cold-water species that becomes rare or absent in low latitudes. The relative abundance of these taxa through the section therefore allows researchers to track how conditions in the basin changed as global climate deteriorated.

In the Karama A section, warm-water indicators such as Discoaster, Sphenolithus, and Zygrhablithus bijugatus are present from the bottom to the top, which the team interprets as evidence that the basin remained in warm-water conditions overall. But the details are revealing. Species diversity and specimen counts tended to decline from the Eocene into the Oligocene, mirroring the long-term global cooling documented in marine and terrestrial records worldwide. Discoaster tanii decreased steadily and disappeared by layer 18, while the numbers of Discoaster and Sphenolithus individuals dropped through the interval. Poor preservation, including diagenetic overgrowth, complicated species identification in places, but the overall pattern of decline is clear.

Intriguingly, the trend did not end in a simple collapse. In the upper part of the Middle Oligocene, the warm-water taxa rebounded, with Discoaster, Sphenolithus, and Zygrhablithus becoming more abundant again, while Cyclicargolithus floridanus and Reticulofenestra bisectus remained abundant and even increased toward the top of the section. The researchers interpret this recovery as evidence that although the carbonate environment stayed fundamentally warm, water temperatures dipped during the Oligocene cooling and then partially recovered. The assemblage changes are gradual rather than a sharp turnover, suggesting the tropical platform experienced the global transition as a gentle but persistent decline in temperature, reducing both diversity and abundance of the most heat-loving species.

The findings also resolve a lingering puzzle in Sulawesi geology. Earlier research had reported that Oligocene strata of the Tonasa Formation in the Jeneponto area were unknown, yet the carbonate succession there was thought to range from the Late Eocene to the Middle Miocene. By demonstrating a Late Eocene to Middle Oligocene age for the Karama A section, the study shows that Oligocene rocks are indeed present and datable in Jeneponto, and it complements previous nannofossil work in the Barru area that yielded a Middle Eocene to Early Miocene age. The result strengthens stratigraphic correlations between the Pangkajene and Jeneponto regions and refines the picture of how the Tonasa Carbonate Platform, whose accommodation space was controlled largely by subsidence, evolved through a time of global climatic upheaval.

More broadly, the study underscores why nannofossils remain among the most powerful tools in carbonate sediment research. These primary producers responded sensitively to the surface water conditions of their time, and their rapid evolutionary turnover allows biostratigraphers to slice the Eocene and Oligocene into fine zones using the standard Martini and Okada and Bukry frameworks. For Indonesia, a nation whose geology was forged by the collision of three major plates, each such study adds a piece to the intricate puzzle of island arc evolution and tropical paleoceanography. For climate scientists, the Karama A section offers another ground-truth record of how marine ecosystems in low-latitude carbonate settings registered the last great transition from greenhouse to icehouse Earth, a transition whose lessons resonate strongly in an era of rapid warming.

Subject of Research: Biostratigraphy and paleoclimate of the Eocene to Oligocene Tonasa Formation in South Sulawesi, Indonesia, based on calcareous nannofossils

Article Title: The Eocene to Oligocene boundary and paleoclimatic indications based on calcareous nannofossils of Tonasa Formation, South Sulawesi, Indonesia

Article References: Farida, M., Jaya, A., Ahmad, A., & Nugraha, J. (2024). The Eocene to Oligocene boundary and paleoclimatic indications based on calcareous nannofossils of Tonasa Formation, South Sulawesi, Indonesia. Fossil Record, 27(1), 221-231. https://doi.org/10.3897/fr.27.96985

Image Credits: AI Generated

DOI: 10.3897/fr.27.96985

Keywords: calcareous nannofossils, Eocene Oligocene boundary, Tonasa Formation, South Sulawesi, biostratigraphy, paleoclimate, Sphenolithus, Discoaster, carbonate platform, Oligocene cooling, Indonesia, stratigraphy