Well, caves can offer a unique and detailed window into the distant past through the fossil deposits found within them. Many caves worldwide preserve the remains of long extinct plants, animals, and even our early human relatives. Animals and hominins lived in caves and sometimes fell in from the surrounding landscape. After they died, their remains became fossilized within layers of sediment. Since these records accumulate gradually over very long periods of time caves form excellent archives of changes in biodiversity. However, importantly to our story, fossils can only accumulate once caves are open to the surface. Therefore, understanding when caves first started to form and open can shed light on the potential antiquity of important fossils deposits.
The World Heritage Naracoorte Caves and its deposits
The Naracoorte Cave Complex in southern Australia is an extraordinary cave system and one of world’s richest fossil sites. It hosts an incredibly diverse range of fossil vertebrates, including the now extinct, iconic Australian megafauna, for which they are World Heritage listed. Palaeontologists have identified fossils within sandy layers of the extinct large marsupial and reptilian predators (Thylacoleo carnifex, Wonambi naracoortensis), giant browsing marsupials (Diprotodon optatum, Zygomaturus trilobus), and enormous, short-faced kangaroos.
We know that these giants became extinct about 48,000-37,000 years ago but the conditions that ultimately led to their demise remain a subject of intense debate. The older the fossils that we can find, the better chance we have of understanding how long-term drivers such as Australia’s changing climate may have influenced their evolution and possibly their extinction.
But how can we know how old the caves and their fossils are when they are well beyond the commonly used radiocarbon dating method?
Fortunately, like many caves worldwide, the Naracoorte Caves is beautifully decorated with speleothems (calcite formations such as stalagmites, stalactites and flowstones). Speleothems can provide important information about the age of the caves themselves and the fossils found therein, providing a key Australian site for understanding how biodiversity responds to changing climates.
The Naracoorte caves formed within the late Eocene to Miocene Gambier Limestone as a result of groundwater percolating through cracks, dissolving the rock and forming cavities. The caves are overlain by a fossil dune ridge, which formed when the coastline was much further inland than present. The cavities gradually drained as the coastline moved seawards and the water table lowered due to uplift of the southern part of Australian continent.
It was previously thought that the caves formed around the same time as the overlying dune. Based on the geomagnetic reversal recorded in the dune sediments, it was estimated to be at least 780,000 years old. Other ages from the dune are ~720,000 years and 938,000 years old based on Optical Stimulated Luminescence (OSL) and Amino Acid Racemization (AAR) techniques. However, none of these techniques are well suited to this task: OSL is far beyond its dating limit and AAR is best used as a relative dating tool. The most recent studies hypothesized that the caves developed sometime between 0.8 to 1.1 million years ago, but these estimates are derived from extrapolation of the palaeomagnetic, OSL and AAR ages. As such, precise and reliable ages for the onset of cave formation have not been obtained. Until now.
Dating a speleothem
In our new study, we directly dated speleothems, which is a much more reliable approach than dating the overlying dune sediments. We measured the decay of minute amounts of Uranium locked up inside speleothems when they form. As Uranium is naturally radioactive, it decays into the element Lead over time. We can therefore use Uranium within speleothems as a natural clock to date them. To do so, the Uranium and Lead must be extracted from the speleothem in a laboratory. We start with drilling out a small sample from speleothem using a dental drill. Next, we then dissolve this, remove the matrix and extract the Uranium and Lead by using chromatography. We measure each element on a mass spectrometer and calculate the sample’s age. The laboratory work is fun, but also quite labour intense: it usually takes a week or more to get from original sample to an age.
What can speleothem ages tell us about the antiquity of a cave and its fossils?
Because speleothems only start to grow once a subterranean cavity is formed and above the groundwater table, the oldest speleothem age obtained reveals the minimum age of the cave itself. Using the Uranium-Lead dating technique, we demonstrated for the first time how old the Naracoorte Caves really are.
Our new data reveal that they began to form at least 1.34 million years ago, exceeding previous estimates by at least 250 to 500 thousand years. This intricate work has taken five years but it was worth the wait, greatly improving our understanding of the antiquity of this important site.
We also used microscopic charcoal and pollen, captured within the speleothems as they grew, as indicators of when the caves first opened to the surface. Only then can aerosols and animals enter the caves and later become fossilized. We show that charcoal and pollen first became trapped in growing speleothems around 600,000 years ago, recording the time when the caves started to open to the surface, first allowing animals in and fossils to accumulate.
This age has important implications: while the currently known fossil deposits extend back to about 400,000-500,000 years, our new results suggest that there is great potential for finding older vertebrate fossil material at Naracoorte, up to at least 600,000 years in age. These results will help palaeontologists target new excavation sites to find older fossils, providing valuable evidence of how our continent’s unique biodiversity developed.
Understanding when caves opened is of course not only relevant to the caves in Naracoorte. There are many cave sites worldwide that host both speleothems and vertebrate fossils, such as Tight Entrance Cave in southwestern Australia, Schwabenreith Cave in Austria and Porcupine Cave in North America. For all such settings, our new approach can help to unravel how old a fossil deposit might be, and more importantly, how biodiversity changed through time.
Read our article here: https://www.nature.com/articles/s43247-022-00538-y
Cover image: Speleothem decorations in Whale Bone Cave, one of the oldest caves within the Naracoorte Cave Complex, Australia. Source: Steve Bourne.