Rancho La Brea: A Look into Pleistocene Biota Through Exceptionally Preserved Fossils University of Southern California, Department of Earth SciencesKimberly Morales [[email protected]] I. IntroductionWithin the heart of downtown Los Angeles and near the foot of the Santa Monica Mountains sits Rancho La Brea, an Ice Age excavation site with asphalt pools rich in preserved fossil deposits of late Pleistocene biota. Extensive exploration of the tar seeps over the past century has led to the discovery of over three million fossil specimens ranging from freshwater mollusks to large land mammals such as the Columbian Mammoth (McDonald et al., 2015; \cite{Spencer_2003}) (Figure 1). To date, more than 700 taxa have been identified, with the majority being arthropods and birds (McDonald et al., 2015). This exceptionally preserved fauna also includes reptiles such as lizards and snakes, species of fish and numerous plants\cite{Nudds_2008} Equally striking as the diversity of biota is the bias and the fine state of fossil preservation. The La Brea deposits show a predominance in carnivore specimens, including wolves and the now extinct sabretooth cat \cite{McHorse_2012}Spencer et al., 2003). This occurrence can be explained by understanding how the organisms were preserved. The asphalt pits found at Rancho La Brea are remnants of crude oil seeping from the ground and onto the surface as heavy and sticky asphalt pools, where organisms get trapped and are unable to escape. The active asphalt pools then serve as a burial sites as the remains of organisms get rapidly buried and preserved. It has long been explained that the dominance of carnivorous species found in the tar pits is due to a repeated cycle that begins its course when herbivores become trapped in the asphalt which then are preyed on by larger carnivores that would also get stuck in the pools (\cite{Akersten_1983}; Spencer et al., 2003). The trapping of these large organisms in turn attracted additional carnivores and scavengers (\cite{Campbell_2012}). This cycle over a duration of more than 40,000 years can explain not only the concentration of carnivorous land mammals but also the substantial amount of fossils that have been preserved at Rancho La Brea. The quality of preservation is of great wonder as well. The exceptional preservation of the organisms is said to result from two factors: rapid burial and the seeping of asphalt into the bones (Nudds and Selden, 2008). This finely tuned preservation within a fairly concentrated region has allowed for Rancho La Brea to harness a truly unique collection of fossils that has no parallel to other records of past life. The continuous excavation of the tar pits at Rancho La Brea along with the rapid innovation of data analysis techniques and enhanced geological knowledge, have allowed for Rancho La Brea to become a model system or reference that is commonly used for the interpretation of other asphalt-preserved biota sites around the world (McDonald et al., 2015). With over 100 years of research, Rancho La Brea provides essential insight into understanding the the taphonomy and origin of biota from the late Pleistocene- of which knowledge remained largely scarce to due to lack of fossils present throughout the rock record (\cite{Deevey_1949}(Deevey 1949); McDonald et al., 2015). Such analysis of the remains has proven useful for deciphering the extinction wave that hit more than half of large mammals at the very end of the Pleistocene Epoch; a thriving mystery within the field of paleontology. The Rancho La Brea deposits are also a useful tool for paleoclimate examination as the late Pleistocene was a time of extensive global cooling with intermediate warming periods, which has much relevance to the climate change that is taking place today (Akersten et al., 1983). As the world’s most famous asphaltic fossil deposit, Rancho La Brea and the discoveries made by its excavation has proven to be a milestone for the field of Pleistocene paleoecology. II. History and Discovery of Rancho La BreaThe tar pits have been used for thousands of years- serving as a useful tool for both indigenous tribes and then for Spanish settlers that arrived in the eighteenth century. The sticky sticky tar was commonly used to waterproof materials and as an adhesive for caulking canoes and housing structures (McDonald et al., 2015). The first recorded description of the tar springs was made in 1769 by a Spanish explorer, Gaspar de la Porta, who conducted a survey of what was then a Native American village called Yang-na. A more expansive description was by a European travel named Jose Longinos Martinez in 1792, who alluded to the thick and viscous asphalt as “A great lake of pitch with many pools in which bubbles are constantly forming and exploding...” (\cite{Stock_1929}). Jose Longinos Martinez was also one of the very first individuals to describe the remains of organisms that were found within the tar springs along with an explanation of their presence. He predicted that “In hot weather, animals have been seen to sink in it and when they tried to escape…the lake swallowed them. After many years, their bones have come up through the holes, as if petrified” (Stock 1956). Despite this and other early notions that the seeps contained animal bones, it was not until 1875 that such deposits were determined to be prehistoric fossils (Harris, 2015) (Figure 2). By the early 20th century, the excavation of the fossils began. The site was then brought to the attention of both domestic and foreign institutions, including University of California, Berkeley, leading to large-scale extraction of over 2 million fossil specimens collected between 1906 and 1915 (Akersten et al., 1983). Currently the muesuem is working on recovering a pit that may possibly double their collection size. III. North America During the Pleistocene EpochA. Paleoclimate The application of Carbon-14 radiometric dating to the bone assemblages and floral remains found in the tar pits has constrained the age of the Rancho La Brea fossils to range from 55,000 (the oldest fossil found in Pit 92) to 11,000 years old (Harris, 2015; Brannick et al, 2015). These dates mark the Pleistocene epoch which began approximately 2.58 millions years ago, at the start of the Quaternary period. North American during the Pleistocene was molded by the fluctuating change in climate which supported some fauna that we are familiar with today and other t The climate during the Quaternary is commonly characterized by repeated glacial cycles or “Ice Ages” as a direct result of rapid decrease in global temperatures (\cite{Lawrence_2010}). During these glacial periods, ice sheets advanced over much of the northern continents, including Europe, Asia and North America (Nudds and Selden, 2008) (Figure 3). It is estimated that over 13 million square km on the North American continent was covered with ice cap (Nudds and Selden, 2008). Four major cold periods were experienced in North America during the Quaternary, with the coldest one being the Wisconsin glaciation that took place over the duration of the Pleistocene which in when most of the fossils found in Rancho La Brea are dated back to (Nudds and Selden, 2008). This extensive range of ice coverage caused for a decrease in global sea levels of approximately 130 m, which in turn exposed land masses like the Bering Straight which then connected Siberia with Alaska (Guthrie, 2001). The Bering Isthmus served as a bridge for mammalian species, like the mammoth and bison, to migrate eastward from Eurasia to northwestern North America (Guthrie, 2001; Nudds and Selden, 2008). Though prolonged glacial events did persist throughout the Pleistocene, there were brief interglacial warm periods that caused for the glaciers in North America to retreat (Lawrence et al., 2010). As the Quaternary was broken up into glaciation events the Pleistocene can also be divided by its climate transitions. Oxygen isotope data that tracks paleoclimate temperatures by using proxies such as pollen and plankton from deep sea core samples has split the Pleistocene into stages with one representing the last glacial maxima (LGM; 59-24 ka) and the other is the glacial-interglacial transition (14-12 ka) (Coltrain et al., 2004). Possible explanations for the temperature swings in the Pleistocene include variations in Earth’s orbit, albedo and atmospheric CO2 (Dansgaard and Tauber, 1969). Both changes in Earths orbit and albedo cause different distributions in solar radiation received by the Earth’s surface. However, such changes alone are unlikely to have caused the frequent climate changes that are diagnostic of the Pleistocene epoch. Past concentration of CO2 in the atmosphere have been measured from bubbles trapped in ice, allowing scientists to correlate surface temperature with atmospheric CO2 (Barnola et al., 2003). The records show that the warm interglacial periods had decreases in temperature that were accompanied by increases in CO2 concentrations (Barnola et al., 2003) (Figure 4). The fluctuations in climate throughout the Pleistocene were not always gradual. From 20,000 to 60,00 years ago, there was abrupt warming periods that are now referred to as Dansgaard-Oeschger events (Lawrence et al., 2010). One of these abrupt increases in temperature is believed to be a causing factor of the large mammalian fauna extinction that marks the end of the Pleistocene.B. Pleistocene Ecosystem The local climate of the the west coast through out the Pleistocene was indeed affected by the alternating warm and cold periods that are diagnostic of the Quaternary period. During the glacial cycles, the winters and summers were cooler and more moist with temperatures around 5-10°C (modern ~ 16°C ) and seasonal precipitation more than twice of what it is today (Coltrain, et al., 2004; Lindsey and Seymour, 2015 à Harris). The increase in rainfall is linked to more frequent and stronger El Niño events that brought in unusually warm water to the Peruvian coast which caused for moist unstable air masses to cover the North American west coast, resulting in above-average annual rainfall (Minnich, 2007). The greater precipitation allowed for a multitude of lakes, streams and ponds to cover the plains and foster thriving freshwater communities (Lindsey and Seymour, 2015 à Harris; Nudds and Selden, 2008). The interglacial (warmer) periods had climates comparable to the present which may explain why some of the organisms that are found in Rancho La Brea are similar to those that are present in Southern Californian today (Minnich, 2007). Despite cycles of heavy rainfall, the environment of Rancho La Brea during the Pleistocene is commonly depicted as open country, with no true forest (Campbell and Boncheski, 2015). Previous studies of the California landscape during the Pleistocene claim that the now Rancho La Brea site was covered in open grasslands with a few scattered plant species and trees, including coastal sage scrub, herbs, cone pines and cypress trees (Nudds and Selden, 2008). Many of these hypotheses are derived from investigating the preferred habitats of the species found in the asphalt pools. For example, remains of owls found in the seeps have been identified as species that prefer semi-open landscapes (Campbell and Boncheski, 2015). Mountainsides, such as those of the Santa Monica mountains, had more concentrated wood bushes but still were fairly open in terms of space. Unlike the surrounding landscape, the canyons had a forest-like density of trees that included redwood, dogwood and bay (Nudds and Selden, 2008). The Pleistocene southern Californians ecosystem was a habitat favored by fauna and flora that could thrive in vast open spaces and relatively cooler temperatures. The wide grassland was ideal for large herbivores like bison, horses, ground sloths, camels and mammoths to roam. The herbivores would in turn attract carnivores- large cats, bears and wolves. The moderately cooler temperatures diagnostic at the time could explain the presence of both oxen, which are usually found in areas with very cold climates and smaller mammals that we find today such as raccoons and rabbits (Nudds and Selden, 2008). In addition to mammals, a large number of birds and arthropods were present in the Pleistocene ecosystem (Harris, 2015). Freshwater bivalves and and plants have also been excavated from the pits, which supports the claims that there were more bodies of water distributed throughout the landscape than there are today. P P P IV. Types of Organisms Preserved at Rancho La Brea Rancho La Brea and its occurrence of fossils have served as a standard locality of the Rancholabrean North American Land Mammal Age because many of the organisms found at the site are cold-adapted fauna, dominated by large mammals such as the mammoth, woolly rhinoceros, and dire wolf (SavageàHarris, 1951). V. Asphalt Preservation Many of the remains excavated from the site have been preserved in their original state but with a brown staining which is caused by the asphalt (Nudds and Selden, 2008). The terrain in which the asphalt deposits occur lies near an oil field that was once an important industrial petroleum producer, the Salt Lake Oilfield (Quinn, 2001). The rock formations over the oil field are highly fractured because of the intermittent earthquakes resulting from tectonic movements in the Los Angeles Basin. Under pressure, natural petroleum finds its way to the surface along fracture zones until it reaches the surface through vents, or fissures, of varying sizes. The locations of modern vents are known to change after earthquakes, and occasionally changes occur for no apparent, visible reason. After exiting the vent at the surface, the liquid petroleum flows over the surface of the ground, flowing down any slope (Fig. 1). Occupying an alluvial plain less than 4 km south of the Santa Monica Mountains, the area of the asphalt seeps is not perfectly flat, and before human occupation, the ground surface and topography was probably in constant flux because of deposition and erosion resulting from outflow from the nearby mountains.Asphalt seeps provide an unusual opportunity to examine the palaeoecology of plant and animal com- munities. Located in the Los Angeles Basin south of the Santa Monica Mountains, RLB seeps are fed by petroleum originating in vertically tilted oil sands. Since f 40 ka, methane pressure has moved trapped asphalt to the surface through fissures in Pleistocene alluvium, forming seeps that can reach several square meters in area and 9 – 11 m in depth (Stock and Harris, 1992, pp. 11 – 13). The flow of seepage is episodic and a temporary reduction in pressure can cause surface asphalt to regress down the chimney from which it exuded. Once pooled, asphalt develops a viscous crust which is ‘‘stickier’’ during warm months and diurnal hours, increasing the potential for entrapment (Coltrain et al.) Why asphalt is good à back of notesStress the quality of preservation à NuddsVI. Megafaunal ExtinctionPaper: A Multispecies Overkill Simulation of the End-Pleistocene Megafaunal Mass Extinction
