Rancho La Brea:
A Look into Pleistocene
Biota Through Exceptionally Preserved Fossils
University of Southern California,
Department of Earth Sciences
I. Introduction
Within 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 Brea
The
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 Epoch
A. 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.
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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 notes
Stress the quality of preservation à Nudds
VI. Megafaunal Extinction
Paper: A Multispecies Overkill Simulation
of the End-Pleistocene Megafaunal Mass Extinction