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Tar pits, sometimes referred to as asphalt pits, are large
asphalt Asphalt, also known as bitumen (, ), is a sticky, black, highly viscous liquid or semi-solid form of petroleum. It may be found in natural deposits or may be a refined product, and is classed as a pitch. Before the 20th century, the term a ...
deposits. They form in the presence of
oil An oil is any nonpolar chemical substance that is composed primarily of hydrocarbons and is hydrophobic (does not mix with water) & lipophilic (mixes with other oils). Oils are usually flammable and surface active. Most oils are unsaturated ...
, which is created when decayed
organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...
is subjected to pressure underground. If this crude oil seeps upward via fractures, conduits, or porous sedimentary rock layers, it may pool up at the surface. The lighter components of the crude oil evaporate into the atmosphere, leaving behind a black, sticky asphalt. Tar pits are often excavated because they contain large fossil collections. Tar pits form above oil reserves, and these deposits are often found in anticlinal traps. In fact, about 80 percent of petroleum found on Earth has been found in anticlinal traps.
Anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is the ...
s are folds in stratigraphic layers in which each half of the fold dips away from the crest. Such structures are usually developed above
thrust fault A thrust fault is a break in the Earth's crust, across which older rocks are pushed above younger rocks. Thrust geometry and nomenclature Reverse faults A thrust fault is a type of reverse fault that has a dip of 45 degrees or less. If ...
s or in tectonic regions where the land is bending and folding. If the structure above the concave-down fold (arch) is a non-porous rock or
aquitard An aquifer is an underground layer of water-bearing, permeable rock, rock fractures, or unconsolidated materials (gravel, sand, or silt). Groundwater from aquifers can be extracted using a water well. Aquifers vary greatly in their characteris ...
, such as shale, it is considered an anticlinal trap. The figure in this section is a cartoon cross-section diagram that shows oil stuck in an anticlinal trap. If there is a fault or fracture in the overlying strata above the oil reserve, the oil may migrate to the surface. This is possible by
capillary fringe The capillary fringe is the subsurface layer in which groundwater seeps up from a water table by capillary action to fill pores. Pores at the base of the capillary fringe are filled with water due to tension saturation. This saturated portion of ...
and because oil is less dense than water.


Chemistry

Tar pits are pools of asphalt. However, at the beginning of their formation, they were not always sticky and dense. The pools were composed of crude oil that originated below Earth's surface. Crude oil is a mixture of heteroatom compounds, hydrocarbons,
metal A metal (from Greek μέταλλον ''métallon'', "mine, quarry, metal") is a material that, when freshly prepared, polished, or fractured, shows a lustrous appearance, and conducts electricity and heat relatively well. Metals are typica ...
s, and
inorganic compound In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as ''inorganic chemistr ...
s. Heteroatom compounds are organic molecules that contain elements that are not carbon or hydrogen, while hydrocarbons contain only carbon and hydrogen. Crude oil is less
viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the in ...
than asphalt because it contains a higher percentage of light hydrocarbons. Light hydrocarbons include the following
alkane In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in whi ...
s: methane, ethane, propane, and
butane Butane () or ''n''-butane is an alkane with the formula C4H10. Butane is a gas at room temperature and atmospheric pressure. Butane is a highly flammable, colorless, easily liquefied gas that quickly vaporizes at room temperature. The name bu ...
. These molecules have very low
molecular weight A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioche ...
s. Crude oils may also contain some inorganic impurities, such as CO2, H2S, N2, and O2. At the surface, these light molecules may evaporate out of the crude oil, leaving behind the heavier, stickier molecules. Asphalt, or bitumen, usually contains hydrocarbon molecule chains with 50+ carbon atoms. The longer the hydrocarbon chain, the more viscous it becomes, and the boiling point increases.   Evaporation is an important process in the formation of tar pits. A reservoir of light crude oil on Earth's surface can be reduced by up to 75% of the initial volume just after a few days, forming asphalt as the resulting product. For medium crude oils, the volume may be reduced by 40%. Crude oils will evaporate differently depending on their chemical composition. The average composition of a bitumen sample by weight is 80.2% carbon, 7.5% hydrogen, 7.6% oxygen, 1.7% nitrogen, and 3.0% sulfur.


Notable tar pits


La Brea Tar Pits

The
La Brea Tar Pits La Brea Tar Pits is an active paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural asphalt (also called asphaltum, bitumen, or pitch; ''brea'' in Spanish) has seeped up from the gr ...
are located in Southern California. The petroleum that is seen on the surface is sourced from the
Salt Lake Oil Field The Salt Lake Oil Field is an oil field underneath the city of Los Angeles, California. Discovered in 1902, and developed quickly in the following years, the Salt Lake field was once the most productive in California; over 50 million barrels of o ...
reservoir and the oil sands in the Repetto and Pico formations. These oil deposits were formed during the Miocene Epoch when marine plankton organisms accumulated in an ocean basin. Over time, sediments buried the organisms 300 to 1000 meters below Earth's surface, subjecting them to high pressures. This process turned the organic matter into oil. The 6th Street Fault that cuts through the Salt Lake Oil Field is the conduit that feeds the La Brea Tar Pits. Petroleum migrated to the surface over time, trapping and preserving animals and plants for the past 50,000 years.


Carpinteria Tar Pits

The Carpinteria Tar Pits are located in Tar Pits Park in Carpinteria, California. These tar pits were predicted to have formed during the Pleistocene. During an excavation project, 25 plant species were recovered along with 55 species of birds and 26 species of mammals. Springs of tar still ooze to the surface through fractures in the underlying stratigraphic layers of marine shale.


Binagadi Asphalt Lake

The
Binagadi Asphalt Lake The Binagadi asphalt lake (or Binagadi tar pits) are a cluster of tar pits in urban Baku, Azerbaijan. Asphaltum or tar has seeped up from the ground in this area for tens of thousands of years. The tar is often covered with dust, leaves, or wate ...
is located in
Azerbaijan Azerbaijan (, ; az, Azərbaycan ), officially the Republic of Azerbaijan, , also sometimes officially called the Azerbaijan Republic is a transcontinental country located at the boundary of Eastern Europe and Western Asia. It is a part of t ...
, or in the Caucasus, a region between the Black Sea and the
Caspian Sea The Caspian Sea is the world's largest inland body of water, often described as the world's largest lake or a full-fledged sea. An endorheic basin, it lies between Europe and Asia; east of the Caucasus, west of the broad steppe of Central Asi ...
. This tar pit is known for preserving the heads and bodies of multiple cave lions, a mammal that flourished in the Pleistocene. A well-preserved horse skull was also found in the Binagadi asphalt lake. It is estimated to be 96-120 thousand years old. It is on display at the Natural-Historical Museum of Azerbaijan in
Baku Baku (, ; az, Bakı ) is the capital and largest city of Azerbaijan, as well as the largest city on the Caspian Sea and of the Caucasus region. Baku is located below sea level, which makes it the lowest lying national capital in the world ...
,
Azerbaijan Azerbaijan (, ; az, Azərbaycan ), officially the Republic of Azerbaijan, , also sometimes officially called the Azerbaijan Republic is a transcontinental country located at the boundary of Eastern Europe and Western Asia. It is a part of t ...
.


Pitch Lake

Pitch Lake in Trinidad and Tobago is a large, bowl-like depression filled with bitumen. The lake has a maximum depth of 250 feet with an area of 100 acres, making it the largest deposit of solid bitumen on Earth. The lake is cold and dense near the shores, and it has a top layer that can be walked on. Underneath this skin, the asphalt is continuously churning. The lake gets gradually softer and hotter near the center where the bitumen begins to bubble. The gas being released in the middle of the lake is largely methane and an ample amount of carbon dioxide. Pitch Lake formed thousands of years ago in the presence of tectonic activity. Movement along a fault created a fracture that tapped into an oil and gas reservoir deep within the crust. The oil and gas seeped upward to Earth's surface through the fracture over time, creating Pitch Lake. Due to this constant replenishment of the oil and gas, the lake has a slight current.  The current is largely unnoticed because the top layer of Pitch Lake is mostly solid.


Fossils

Tar pits are characteristic of their many fossils. This is the case because the thick, sticky asphalt traps animals.  Once animals step into the tar, they become immobilized and begin sinking immediately if the asphalt is warm and sticky enough. Predators that see these helpless animals usually would advance into the tar pits with the hope of catching their next meal. As a result, prey are usually found beneath the predator during excavation projects. The bones and hard parts of the animals are well preserved because they are buried rapidly after the organism's death. Beneath the surface, the hard parts are engulfed with asphalt, and they are protected from climate variations like rain, wind, or snow that may accelerate weathering processes. Asphalt also lacks oxygen and water, so major decomposing organisms like aerobic fungi and bacteria are absent. In the La Brea Tar Pits, more than one million bones have been recovered since 1906. 231 vertebrate species, 234 invertebrate species, and 159 plant species have been identified. The most frequent large mammal found in the La Brea Tar Pits is the dire wolf, one of the most famous prehistoric carnivores that lived during the Pleistocene. Fossils from
saber-toothed cat Machairodontinae is an extinct subfamily of carnivoran mammals of the family Felidae (true cats). They were found in Asia, Africa, North America, South America, and Europe from the Miocene to the Pleistocene, living from about 16 million unt ...
s and
coyote The coyote (''Canis latrans'') is a species of canine native to North America. It is smaller than its close relative, the wolf, and slightly smaller than the closely related eastern wolf and red wolf. It fills much of the same ecological n ...
s were also abundant. Additional fossils are constantly being discovered through continued excavation projects.


Living organisms

Life was found in a ca. 28,000-year-old sample of natural asphalt in the
La Brea Tar Pits La Brea Tar Pits is an active paleontological research site in urban Los Angeles. Hancock Park was formed around a group of tar pits where natural asphalt (also called asphaltum, bitumen, or pitch; ''brea'' in Spanish) has seeped up from the gr ...
. Hundreds of new species of bacteria were discovered that have the ability to thrive in environments with little to no water or air. They contain special enzymes that can break down hydrocarbons and other petroleum products. The origin of the bacteria in these natural asphalt pits is unknown, but it is believed that they evolved from preexisting soil microorganisms that survived an asphalt seepage event thousands of years ago. The soil microorganisms had to adapt and undergo genetic changes to help tolerate the harsh, new environment, which ultimately gave rise to new bacterial species.   In one study, the predominant bacteria found in the La Brea Tar Pits were of the '' Gammaproteobacteria'' class in the '' Chromatiales'' order, more simply referred to as 
purple sulfur bacteria The purple sulfur bacteria (PSB) are part of a group of Pseudomonadota capable of photosynthesis, collectively referred to as purple bacteria. They are anaerobic or microaerophilic, and are often found in stratified water environments including h ...
. Purple sulfur bacteria do not use water as their reducing agent, so oxygen is not produced during respiration. Instead, they use sulfur in the form of
sulfide Sulfide (British English also sulphide) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to chemical compounds lar ...
s as their reducing agent. Other bacteria discovered in the tar pits were of the '' Rubrobacteraceae'' family. These bacteria are known for being some of the most radiation-resistant organisms on the planet.   Pitch Lake, another asphalt pit in Trinidad and Tobago, is also a habitat for microbial communities of archaea and bacteria. Bacterial microorganisms from the orders ''
Burkholderiales The Burkholderiales are an order of Pseudomonadota.George M. Garrity: '' Bergey's Manual of Systematic Bacteriology''. 2. Auflage. Springer, New York, 2005, Vol. 2: ''The Proteobacteria Part C: The Alpha-, Beta-, Delta-, and Epsilonproteabacteria ...
'' and ''
Enterobacteriales Enterobacterales is an order of Gram-negative, non-spore forming, facultatively anaerobic, rod-shaped bacteria with the class Gammaproteobacteria. The type genus of this order is ''Enterobacter.'' The name Enterobacterales is derived from the ...
'' have been found living in microliter-sized droplets of water recovered from the lake. The biomass in Pitch Lake lake was reported up to 107 cells per gram of asphalt. Many of these microbes survive on sulfur, iron, methane, or other hydrocarbons. Ongoing research is being conducted in Pitch Lake because it mimics the environment found on the surface of Saturn’s largest moon,
Titan Titan most often refers to: * Titan (moon), the largest moon of Saturn * Titans, a race of deities in Greek mythology Titan or Titans may also refer to: Arts and entertainment Fictional entities Fictional locations * Titan in fiction, fictiona ...
. The discovery of extremophiles in Pitch Lake provides insight into the possibilities for microbial life in the hydrocarbon lakes that are found on Titan.


Contributions to greenhouse gases

Tar pits are formed by the fractionation of crude oil at the surface. The lighter hydrocarbons of the crude oil, which include methane (CH4), ethane (C2H6), and propane (C3H8), evaporate, leaving behind larger hydrocarbons that make up the chemical composition of asphalt. This is a concern because methane, ethane, and propane are either major
greenhouse gas A greenhouse gas (GHG or GhG) is a gas that absorbs and emits radiant energy within the thermal infrared range, causing the greenhouse effect. The primary greenhouse gases in Earth's atmosphere are water vapor (), carbon dioxide (), methane ...
es and/or photochemical
pollutant A pollutant or novel entity is a substance or energy introduced into the environment that has undesired effects, or adversely affects the usefulness of a resource. These can be both naturally forming (i.e. minerals or extracted compounds like o ...
s. The La Brea Tar Pits emit about 500 kg of methane per day. The emissions are the highest along the 6th Street Fault, which is the conduit that feeds the tar pits with crude oil from the sediments underneath Earth's surface. It was also discovered that methane is evaporating out of the nearby soil, affecting the physiology of the native grasses. The La Brea Tar Pits have the highest natural gas flux measured for any onshore seepage zone in the United States. On a global scale, geologic CH4 and C2H6 emissions estimates from gas seepage in sedimentary rock are in the order of 50-70 Tg/year and 2-4 Tg/year respectively. These values are roughly half of the global CH4 and C2H6 emissions from
anthropogenic Anthropogenic ("human" + "generating") is an adjective that may refer to: * Anthropogeny, the study of the origins of humanity Counterintuitively, anthropogenic may also refer to things that have been generated by humans, as follows: * Human im ...
fossil fuel
combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...
, which are approximately 100-150 Tg CH4/year and 6-8 Tg C2H6/year. These hydrocarbon emissions can be contributed to oil biodegradation and methanogenesis within the tar pits.   The natural geologic sources of methane and other hydrocarbons should be considered when modeling atmospheric greenhouse gases. Not all sources of hydrocarbons in the atmosphere are a result of anthropogenic emissions.


Dangers of tar pits

Hydrocarbon seepage in urban or industrialized areas present a geologic hazard due to the explosive nature of hydrocarbons. On March 24, 1985, a pocket of methane gas passed through a small opening between the floor slab and foundation walls of a Ross clothing department store in Los Angeles, only about a mile north of the La Brea Tar Pits. This methane pocket created an explosion that injured 21 people. This event increased awareness of the potential dangers of methane pockets and hydrocarbon seepage in the area.


Key to paleoplant behavior

Tar pits are excellent preserving agents, and they also have the ability to provide
carbon isotope Carbon (6C) has 15 known isotopes, from to , of which and are stable. The longest-lived radioisotope is , with a half-life of years. This is also the only carbon radioisotope found in nature—trace quantities are formed cosmogenically by ...
data for trees that have fallen into the asphalt. Looking at carbon isotope data in prehistoric trees can reveal information about plant responses to different amounts of carbon dioxide in the paleoatmosphere. Samples of '' Juniperus'' trees from the Last Glacial Period were recovered from the La Brea Tar Pits, and they revealed that the ratio of intercellular and atmospheric CO2 was similar between glacial and modern trees. Since the amount of carbon dioxide during the Last Glacial Period was between 180 and 200 ppm (409.8 ppm today), there was less carbon available for photosynthesis. The ''Juniperus'' trees had to enhance CO2 uptake to survive under limiting carbon conditions. It is likely that the trees’ stomatal conductance and
chloroplast A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it in ...
demand for CO2 was higher during this period to increase their carbon consumption. When moving into the following Interglacial Period, the ''Juniperus'' trees’ stomatal conductance and chloroplast demand for CO2 decreased as a result of higher temperatures and higher CO2 concentrations in the atmosphere. This response to fluctuating carbon levels is seen in plants over time. For example, increased stomatal conductance is observed in modern C3 plants grown in low CO2 environments. It is also hypothesized that the wetter climate during the Last Glacial Period may have increased the nitrogen availability to plants, which therefore increased the concentration of nitrogen in leaves. This change may have increased the ''Juniperus'' trees’ photosynthetic capacities.


History of tar pits and humans

A woman's body was recovered from the La Brea Tar Pits in 1914.  Only the skull and parts of the skeleton were preserved, and she was determined to have died about 9,000 years ago.  She was between the ages 18–24 at death, and she was 4 feet and 8-10 inches tall. This is the only reported instance of human remains found within tar pits. For thousands of years, Native Americans used tar from the La Brea Tar Pits as an adhesive and binding agent. They would use it as waterproof caulking to line their boats and baskets. When Westerners arrived at the tar pits, they began mining and extracting the tar for roofing material in nearby towns.


References

{{Reflist Petroleum geology Sedimentology