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Fattorini

trickles to larger, more episodic bursts. 5,55 These seeps dioxide and hydrogen sulfide. 61,62 The stable isotopic
create distinct ecological communities characterized by composition of methane (δ C and δD) is often used
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chemosynthetic organisms (i.e., tube worms, mussels, to distinguish between thermogenic (geological) and
bacterial mats) that utilize methane and sulfide as biogenic (microbial) methane sources. 61
energy sources, forming the base of complex food webs In addition to typical cold seeps, hydrothermal vents,
independent of sunlight. The Santa Barbara Channel which are found predominantly along mid-ocean ridges
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off the coast of California is another well-known and back-arc basins, also release hydrocarbons into
example, mainly characterized by a prolific shallow- the deep ocean. 19-21,60 These systems involve seawater
water oil seep system, such as Coal Oil Point. 5,56 circulation through hot, fractured oceanic crust, leading
These seeps have been active for millennia, releasing to chemical reactions and the discharge of superheated,
crude oil that frequently washes to the shoreline as a chemically rich fluids. While the primary constituents
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form of spherical aggregates, a natural phenomenon of hydrothermal fluids are typically trace elements and
long recognized by indigenous populations. Other sulfur compounds, significant amounts of hydrocarbons,
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significant seep areas include the Caspian Sea, the including methane and heavier alkanes and aromatics,
North Sea, and various regions along the Atlantic and have been detected in various vent systems globally.
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Pacific continental slopes. Partly similar phenomena The origin of these hydrocarbons in hydrothermal
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to submarine seepages are represented by pockmarks; systems can be varied. Some are abiogenic, meaning
these are depressions or craters found on the seabed, they are formed by inorganic chemical reactions
typically formed by the episodic or continuous involving carbon dioxide, hydrogen, and minerals at
expulsion of fluids, such as methane gas and other high temperatures and pressures within the Earth’s
hydrocarbons, from the subsurface. 22,23,57,58 Unlike crust, such as serpentinization reactions. 60,63 The “Lost
seepages, pockmarks have also been found in areas with City” hydrothermal field in the Atlantic, for instance, is
relatively shallow water and are not directly related to known for its abiogenic methane and light hydrocarbon
fractures in the Earth’s crust, but rather to geological production. Other hydrocarbons found in vent fluids
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formations with hydrocarbons close enough to the can have a thermogenic origin, such as conventional
surface of the seabed to produce slow and episodic petroleum, formed by heating deeply buried organic
upwelling. 23,57 Although the hydrocarbon contribution matter in sediments that are subsequently incorporated
from pockmarks is lower than that of natural seepages, into the hydrothermal circulation. A third potential
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their presence may help shape the characteristics of source is biogenic methane produced by thermophilic
relatively shallow ecosystems, such as some regions of and hyper-thermophilic microorganisms residing in
the Mediterranean. 22,57,58 the subsurface or within the vent structures. 19,20,63 The
The hydrocarbons released from geological seeps relative contribution of each origin varies among
exhibit a wide range of geochemical characteristics different fields, but the presence of hydrocarbons in
depending on their source rock, thermal maturity, these extreme environments underscores the diverse
and migration pathways. Naturally seeping oil often geological and chemical processes that can generate
contains a complex mixture of alkanes (saturated them. 19,20,60,63
hydrocarbons), cycloalkanes, and polycyclic aromatic Beyond geological processes, marine organisms
hydrocarbons (PAHs), along with non-hydrocarbon themselves represent a continuous and diffuse source of a
compounds, such as sulfur, nitrogen, and oxygen- wide range of hydrocarbons to seawater. These biogenic
containing molecules. 1,2,34,50 Notably, crude oils from hydrocarbons are fundamentally different in their genesis
natural seeps often contain distinct compounds, hopanes, and often in their chemical structure compared to fossil
steranes, and terpanes that are resistant to degradation fuels. 64,65 Algae, particularly phytoplankton, are prolific
and reflect the original organic matter from which the producers of hydrocarbons. Many species of green algae
oil was produced. 59,60 These hydrocarbons constitute (Chlorophyta) and cyanobacteria produce short-chain
fingerprint compounds, allowing them to differentiate volatile hydrocarbons, such as isoprene and longer-
natural seepage oil from anthropogenically sourced oil chain alkanes (e.g., n-heptadecane, n-octadecane).
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in environmental samples. The gaseous components These compounds are typically synthesized as metabolic
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of natural seeps are predominantly methane, with byproducts, signaling molecules, or components of cell
the presence of small amounts of heavier gaseous membranes. For instance, n-alkanes are commonly
hydrocarbons (ethane, propane, butane) and trace found in the waxes and cuticles of many organisms.
quantities of non-hydrocarbon gases, such as carbon The large biomass and rapid phytoplankton turnover

Volume 22 Issue 6 (2025) 24 doi: 10.36922/AJWEP025290224