"The limestone layers themselves are millions of years old, but the iconic sea stacks formed more recently."
A new study published in the Australian Journal of Earth Sciences provides a detailed geological history of the Twelve Apostles, the famous limestone sea stacks off the coast of Victoria, Australia. The research clarifies the age of the rock layers forming the stacks and the timeline of their emergence due to coastal erosion.
Key Findings: Age and Formation
The study, led by Associate Professor Stephen Gallagher from the University of Melbourne, determined that while the limestone layers are ancient, the stacks themselves are much younger.
Rock Layer Ages: The limestone and marl layers that compose the cliffs and stacks were deposited over a 16-million-year period.
Specific Sediment Layers: The researchers identified three main layers:
- Gellibrand marl: Deposited between 16 and 14.1 million years ago.
- Grey limestone: Formed until approximately 11.6 million years ago.
- Yellow limestone: Deposited as the sea level shallowed.
- The landmass rose from the ocean 8.6 million years ago, leading to the formation of the Hesse clay, sand dunes, and gravel on top.
Sea Stack Formation: The stacks themselves emerged within the last few thousand years after the last Ice Age, due to coastal erosion caused by rising sea levels.
Number of Stacks: The site currently has eight stacks remaining. Seven stacks formed in the last 20,000 years. Five "Drowned Apostles" lie approximately 6 km offshore and are estimated to be about 60,000 years old, but are now submerged.
Research Methodology
The research team combined unpublished data from the 1960s with contemporary fieldwork to date the rock layers.
- Tools Used: Scientists used a Jacob's staff measuring tool and a spectrometer to measure gamma radiation of the layers at the Gibson Steps.
- Sampling: Each layer was measured in meter segments, with samples taken every 25 cm.
- Fossil Analysis: The analysis included the study of micro-fossils within the limestone and marl layers.
The findings were published in the Australian Journal of Earth Sciences. This is the first published study on the sea stacks since 1944.
Climate Record and Implications
The rock layers of the Twelve Apostles are described by researchers as a record of climate history from the Miocene period.
- CO2 Levels: Carbon dioxide (CO2) levels at the base formation were estimated at 480 parts per million (ppm). Current levels are around 430 ppm.
- Temperature Change: The study suggests that temperatures have cooled by approximately 4°C since the Miocene.
- Context: Researchers are using the site to study past climate and sea level changes.
Statements
Stephen Gallagher (University of Melbourne): Said the layers are "one of the best preserved and most accessible records of climate and sea level from the Miocene period."
Milo Barham (Curtin University): Not involved in the study, stated that the research integrates detailed coastal mapping and age relationships.
Matej Lipar (Physical Geographer): Not involved in the study, noted the importance of studying the stacks as they preserve evidence of past environments.
Additional Context
- The largest Apostle contains approximately 760 trillion microfossils.
- Researchers note that further erosion may cause the remaining stacks to collapse, while new stacks may form from other cliffs.
- Gallagher's next research step is to detail the climate history of the region.