Back when this all first started, after explaining that Deepwater Horizon had been sabotaged by militant environmentalists, everyone's favorite conservative radio host had the following to say about America's worst oil spill:

"The ocean will take care of this on its own if it was left alone and left out there.  It's natural.  It's as natural as the ocean water is."  -Rush Limbaugh, 5/3/10.

Before we dive into the content, I must correct a misconception on ignorant display: "natural" does not equal "good." It can, and in advertisements it always does, but in reality, the two words are not synonymous. There are a lot of terrible things in nature: Ebola; Infanticide (e.g. in lions); Rape (e.g. in dolphins); Murder (e.g. in chimps); Cannibalism (e.g. all over the place). All natural, but not good.  Sorry, pet peeve.

Back to oil, Limbaugh has also made the claim that more oil seeps into the Gulf of Mexico naturally, every year, than has spilled from Deepwater Horizon.  As it turns out, this claim is actually true.

Oil seeps are fairly common around the world both underwater and aboveground.  Oil seeps occur when enough cracks and fissures form above a reservoir to enable a small quantity of oil to escape naturally.  The La Brea Tar Pits in Los Angeles (pictured below) are a large terrestrial oil seep, and oil seeps have long been used to help identify submarine oil reserves.  Oil seeps are prevalent in many bodies of water, and the Gulf of Mexico is no exception.

Oil seeps are more common than you think, both on land and underwater.

A satellite survey published in January of 2000 counted at least 600 natural oil seeps within the Gulf.  And they release a lot of oil.

As you know, it is difficult to calculate underwater spill rates.  Especially for 600+ sites.  So the numbers here are pretty wide ranges, but the scale of the estimates is impressive.

A 2003 National Academies study estimated that about 980,000 barrels of oil, or about 41 million gallons, seep into the Gulf - every year.  Recall that the Exxon Valdez is estimated to have spilled about 250,000 barrels.

This link will take you to a table of annual petroleum releases into U.S. waters by source, but it is confusing to read.  If you are willing to wade through the paper, though, the pages that follow do explain each of the individual sources.

So if that much oil is released every year, why isn't the Gulf covered in oil slicks?  It actually is. You just can't see them (and it doesn't really matter).  Oil can spread out very, very thinly.  In fact, a gallon of oil can spread out to cover more than a full square mile, forming the tiniest film on the surface, one-hundredth of a millimeter thick.  At that dose, oil is not dangerous.  Note: in an oil spill, a lot of oil is released into the same place at the same time.  It is still hydrophobic and all wants to sit on top of the water, so it forms a thicker slick than that.

While invisible up close, microscopic oil slicks from natural seeps are visible from space because cohesion between oil molecules flattens wave action to form smooth areas on the water.

Close up. Image Source: Jesse Allen, NASA.

Because seeps are dispersed and oil only seeps from them instead of gushing, areas around seeps are still able to support thriving biological communities. Scientists don't even think the animals living near seeps have needed to evolve any adaptations*; seeping oil simply doesn't have that great an effect.

*One cool exception to this statement: you may have seen pictures or videos of the giant red tubeworms etc. that leave near deep ocean hydrothermal vents.  Those vents don't just expel superheated water; some are actually gas seeps too.  The chemosynthesis that supports those ecosystems actually uses methane as a feedstock.  So those animals have not adapted to natural gas as a toxin they can tolerate, they've adapted to natural gas as a food source they can eat, and gas seeps as a habitat they need to survive!  ...but oil is bad!

Strong oil seeps can lead to increased microbial productivity (as those bacteria break down more abundant oil) and result in some local hypoxia (lack of oxygen) on the ocean floor, but not to the point of causing large dead zones.  Further, individual seeps are not always active and the release rate can even vary considerably during a single day and from day to day.  As a result, only a small area around a seep is ever actually exposed to "fresh," un-degraded oil, and that is when it is most toxic.

What we know as "oil" is actually a varying combination of thousands of different compounds.  Many of these react differently and have different fates when released into the water: some molecules evaporate, others degrade in sunlight ("photolysis"), some dissolve in seawater, some get eaten by microbes, and others sink and end up in sediments.  That is, if they don't wash up on a beach or become entrained in the biosphere first.

A study published in May 2009 found that oil from natural seeps normally stays in the water for between 10 hours and 5 days.  In that time, those molecules that easily can be broken down are, leaving behind the remaining, heavier oil - consisting mostly of larger compounds that are more difficult to dissolve, evaporate or be digested by microbes.  These molecules sink to the floor.

Oil from natural seeps stays in the water for less than 5 days.

An analysis of sediment samples from different areas around a natural seep revealed a consistent rate of hydrocarbon loss in the oil that eventually sank.  This indicates that there is an upper limit to how much oil can be broken down by natural forces in the ocean.  This appears to be the key finding for us.

The question we are trying to answer here is, "how are oil seeps different from oil spills?"  Oil seeps occur constantly, throughout the Gulf.  Although they do release a lot of oil together over time, their individual spill rates are far, far lower than the Deepwater Horizon gusher.  What's more, these much smaller seeps are dispersed around the Gulf, so each seep's oil can be degraded quickly.

That is not what happens in an oil spill.  It is true that the amount of oil that has spilled from this gusher so far is less than the ANNUAL AGGREGATE of all 600+ seeps in the Gulf.  But it's all coming out at the same time, in the same place.  The water in one location can only degrade so much oil at one time; an oil spill goes far beyond overwhelming the ocean's natural oil-coping mechanisms.

And remember, the oil from all those natural seeps escapes year-round.  Yes, the Gulf can degrade small amounts of oil within 5 days, continuously.  But that oil-disposal capacity is always already in use, year-round.  So any additional oil spilled does not follow that time line.  It lasts much longer and has a much greater impact.

So, in conclusion, the Gulf has a limited ability to deal with oil that seeps out slowly and is widely dispersed.  But those capabilities are constantly in use.  This spill is gushing massive amounts of oil into one place.  Marine ecosystems cannot cope with that assault.  And don't forget the toxic dispersants that are accompanying the toxic oil, and the fact that most of the oil is still underwater, where it remains "fresh" (which, like "natural," does not mean good here) longer because it weathers more slowly there.

This spill is and will continue to be devastating.  There's a reason why one gambling website is already letting users bet on which Gulf species will go extinct.

Rush Limbaugh and friends are using true facts to reach false conclusions.

This post was born not just out of Rush Limbaugh's deceit, but also some good questions from matyra.  Congratulations, you two - it's a blog!