Margret Steinthorsdottir and Helen K Coxall (Sweden)
Near the small town of Clarkia in Shoshone County, Idaho in the USA, exists a rich and unique fossil deposit. The Clarkia fossils, or Clarkia Flora, as the deposit is mostly called due to the abundance of fossil plants, is so well preserved that the assemblage is referred to as a “lagerstätte”, a scientific term reserved for the world’s very finest fossil deposits. The Clarkia fossils are found in sediments that are now known to be about 16 million years old and belong to a period in Earth history called the Miocene. By this time, the (non-avian) dinosaurs were long extinct (the last of these dinosaurs disappeared about 66 million years ago), the Earth’s continents were more or less in the same position as today, and many of the animals and plants would have started looking familiar to modern humans (who emerged much later, about 200,000 years ago).
Among the Clarkia fossils can be found various insects, fish and occasionally the remains of small mammals. However, most striking is the wealth of plant fossils in the form of exceptionally well-preserved leaves, nuts, seeds and wood. Impressively, one can find leaves of oak, laurel, pine and birch that look virtually identical to those we find today. If you look quickly when a new fossil is newly exposed from within the host sediments, you may occasionally even see the original green or autumn red leaf colour. This, and many other original features of the leaves, were locked in to the rock record due to special conditions that have continued to protect the leaves through the millennia.
Although initially discovered 45 years ago, scientists have still not fully capitalised on these ancient floral treasures and much more research needs to be done. The authors of this article, who are from the Swedish Museum of Natural History and Stockholm University in Sweden, recently visited Idaho to study the Clarkia Flora. Returning with new fossil material, they hope to kick-start a new wave of research into life and climate 16 million years ago.
The sediments containing the Clarkia fossils were deposited in a lake that formed when massive outpourings of lava, known as the Columbia River Basalts, blocked the mouth of a large valley, thereby damming several rivers to create a lake. The Miocene Lake Clarkia was a long, narrow body of water, surrounded by forested hills. The lake filled quickly with sediments – or quickly in geological terms that is – in about 10,000 to 100,000 years. The sediments were mostly fine-grained silts and clays. With the water and suspended sediments came plant and animal remains, which, together with the seasonal fall of autumn leaves, created a supply of natural treasures descending to the lake bottom.
The rather deep (about 100 to 150m) Lake Clarkia did not get fully stirred by wind, and anaerobic (devoid of oxygen) and toxic conditions developed at the bottom. These conditions were perfect for preserving fossils – sinking plant and animal remains lay undisturbed, because the scavengers and bacteria that usually decompose organic remains could not survive there. The fast sedimentation rates and close vicinity of the forest to the lake contributed to the unique preservation, by minimising transport-related damage to incoming plant or animal remains, and quickly buried potential fossils in a muddy, lake floor made up of layer-cake of sediments.
Today, the Miocene Lake Clarkia is long gone, replaced by grazing cattle, forestry plantations and garnet mines. However, the sediments that once filled the lake are now above water and can be accessed as a series of exposures of soft, crumbly rock.
However, not all the sediment exposures contain exceptional fossil beds, so you have to know where to look. The best-known and most intensively studied exposure occurs at the initial discovery site called “Fossil Bowl”, off Highway 3, near Clarkia. Located on private land, but made accessible to collectors and public for a small fee, here one can combine fossil hunting with all-terrain motor sport, as the site also hosts a motocross racetrack. The original discovery was made by Mr Francis Kienbaum, while he was excavating for a snowmobile racetrack (ten years later, converted to the motocross racetrack).
The second most productive and well-studied site is located in the Emerald Creek valley, on private land belonging to eminent palaeobotanist, Dr William (Bill) Rember, emeritus at the University of Idaho, who has dedicated a large part of his life to unearthing and cataloguing Clarkia fossil treasures. Both sites contain extremely abundant and well-preserved plant megafossils. Fossils are easily found by excavating blocks of clay from the cliff face, then gently cleaving the clay layers using a thin-bladed kitchen knife.
Original leaf colour and cellular structure preserved
The truly amazing fact about the Clarkia Flora is that many of the fossil leaves are preserved almost completely intact. They are essentially unaltered from the time of deposition, to the point that sometimes the original colour of the leaf is preserved, as mentioned above, most often as autumnal browns and reds, but occasionally green leaves are also found. The dominance of autumn colours is easily explained by the abundance of leaves that were shed by the Clarkia vegetation every autumn, just as happens every autumn today. The original colours quickly turn to dark brown or black on contact with oxygen.
The leaves have been compressed and fossils such as these are also called compressions, in contrast to impressions – fossils where the original material is gone, but the imprint remains on the rock surface. Incredibly, leaf compressions can be removed in one piece from the rocks using special laboratory techniques involving strong acids. When the fossil leaves have been removed from the rocks, they can be examined in detail under the microscope. The preservation allows the study and analysis of cellular details on leaf surfaces, which can provide information about the environmental conditions in which the forest grew – such as CO2 concentration.
The amazing preservation of the rich Clarkia Flora has allowed the identification of many species, most of which are now extinct, but belong to groups that are familiar to most people. These include pines, sequoias and other conifers, magnolias, maples, cypresses, cedars, willows, hazels, laurels, poplars, oaks, beeches, alders and birches, as well as members of the bean family, vines, ferns, horsetails and mosses.
A window into a warmer past
When palaeobotanists analysed the Clarkia Flora, it became evident that the taxonomic composition of the ancient forest is most similar to that of modern forests found in eastern Asia and the south-eastern USA – that is, forests that grow in a considerably warmer climate (subtropical) than exists in central Idaho today. The modern vegetation in the Clarkia area is now dominated by conifers, some of which (the characteristic Ponderosa Pine) are drought tolerant. Idaho climate today is temperate and influenced by continental extremes, including summer droughts. In contrast, the Clarkia fossil flora – together with Clarkia insect and fish fossil fauna – indicates a warm and humid climate, with less seasonal extremes.
These findings constitute another piece of the geological and palaeontological puzzle indicating that Miocene global climate was considerably warmer than that of today – in particular during the so-called Mid Miocene Climatic Optimum (MMCO), a period of particularly warm climate 18 to 14 million years ago.
One of the still unanswered questions about the MMCO is which factors exactly caused this global warming. Likely, atmospheric CO2 concentration rose, driving the warming due to greenhouse forcing, just like our modern concerns about manmade CO2 increase and climate, but it is not known by how much. The specimens collected during the August 2017 fieldwork will help shed light on this question by using cellular-level features on the fossil leaves to estimate MMCO CO2.
Understanding how the climate has varied in the past, and the consequences of these variations for climate patterns locally and globally, allows us to predict how future climate change may take shape. That is, application of this and other modern palaeontological methods to the incredible Clarkia Flora promises exceptional new insights into ancient (and future?) greenhouse worlds.
The authors sincerely thank Bill Rember for hosting us, sharing his immense knowledge of the Clarkia Flora and facilitating the collection of fossils, as well as Professor Paul Pearson of Cardiff University in the UK, for invaluable help in the field. The Swedish Research Council starting grant to MS (VR NT-7 2016 04905) is acknowledged and the Bolin Centre for Climate Research, Stockholm University in Sweden is gratefully acknowledged for its generous support of the fieldwork.