Tar on beaches often appears as dark, sticky lumps, commonly referred to as tar balls. These vary in size, from small pebbles to larger patties, and are a frequent sight along coastlines worldwide. Their presence indicates a broader environmental phenomenon, stemming from both natural geological processes and various human activities.
What Are Tar Balls?
Tar balls are weathered crude oil transformed by the ocean environment. When oil enters the water, it undergoes weathering, a process including evaporation of lighter compounds, dissolution, oxidation, and microbial degradation. This concentrates the heavier, more persistent oil components into a sticky, semi-solid mass. The resulting tar balls range from dark brown to black, often with a gritty texture from incorporated sand, shells, or other debris.
Their size varies significantly, from tiny specks to large mats several centimeters in diameter. Their density can change, allowing them to float on the surface, remain suspended, or sink to the seafloor. This resilience enables them to persist in the marine environment for extended periods.
Natural Ocean Seeps
A source of tar on beaches comes from natural oil and gas seeps on the ocean floor. These geological formations continuously release crude oil and gas from subsurface reservoirs into the marine environment. This process has occurred for millions of years, predating human industrial activity. Active natural seeps are found in the Gulf of Mexico and off the coast of California, where oil visibly bubbles to the surface.
When crude oil is released from these seeps, it weathers, and ocean currents and wave action break it into smaller fragments. These naturally occurring tar balls then float on the ocean’s surface, carried by currents. This natural seepage contributes oil to the marine environment globally.
Human Activities and Discharges
Human activities are another major contributor to tar on beaches. Large-scale accidental oil spills from tankers, offshore drilling platforms, or pipelines release quantities of crude oil into the ocean. While episodic, these events can lead to widespread tar ball contamination over extensive coastal areas. The oil from these incidents forms tar balls that disperse throughout the marine environment.
Routine shipping operations also contribute to oil pollution and tar ball formation. Ships often discharge oily bilge water or release residual oil during tank cleaning. These operational discharges, though smaller than major spills, collectively introduce a continuous influx of oil into the ocean, becoming a chronic source of beach contamination.
Land-based sources also contribute through urban and industrial runoff. Oil and grease from roads, industrial facilities, and wastewater treatment plants can be carried by stormwater into rivers and eventually into the ocean. This diffuse pollution introduces petroleum hydrocarbons that form tar-like substances. These human-derived inputs often combine with naturally occurring tar, making it challenging to pinpoint the exact origin of every tar ball found on a beach.
Ocean Currents and Transport
Once oil, from natural seeps or human activities, forms tar balls, ocean currents become the primary mechanism for their transport. Surface currents, driven by global wind patterns and thermal differences, can carry these buoyant masses across vast distances. For instance, tar balls from natural seeps in the Gulf of Mexico can be transported by the Loop Current and Gulf Stream to beaches along the southeastern United States or even across the Atlantic.
Wind also plays a role in pushing floating tar balls across the ocean surface. Persistent winds direct these oil particles towards specific coastlines, influencing where they eventually wash ashore. As tar balls approach coastal areas, local tides and wave action further dictate their final deposition. Strong onshore winds and high tides can push tar balls higher up the beach, where they become stranded. The interplay of these oceanographic forces determines the ultimate fate and distribution of tar balls found on shorelines.