Local photographer Greg Nelsen was running on the beach at La Jolla Shores March 14 when he encountered black blobs near Scripps Pier, which coated the soles of his shoes with a sticky residue.
The tar balls — globs or lumps of solidified petroleum — were anywhere from fist- to plate-sized.
Peter Lonsdale, a professor of marine geology at Scripps Institution of Oceanography, said there are three possible explanations for the tar balls’ appearance — including a fuel spill from a ship or a leak from an offshore oil-drilling rig.
“There were oil wells in the 1960s off Oceanside,” Lonsdale said. “They were supposed to be capped, but they might still leak.”
However, the most plausible explanation, he said, is that the tar comes from a natural seepage in the bottom of the ocean floor.
It is a fairly common occurrence, Lonsdale said, and mostly likely the source of a deluge of tar balls that washed ashore on North County beaches in July of 2010.
“You can’t tell from casual observation, but most of the ones that have been analyzed here turn out to be from a natural (source) — offshore seepages under the ocean,” he said. “There are a lot of natural seeps off California. This happens a lot in the Santa Barbara channel.”
Lonsdale believes a similar seepage area exists on the ocean floor about six miles west of Scripps Pier.
“It’s what we informally call ‘The La Jolla oil seep,’ ” he said, noting that Scripps’ graduate students plan to study the potential leak in July.
“We’re planning to go down there with an unmanned, remote-operating vehicle — basically a robot — that can take pictures and collect samples, so we’ll know for sure,” he said.
“Data suggests pretty strongly that there’s a hydrocarbon seep there. We have sonar records of it that have characteristic features of an oil seep.”
Whatever the source, Lonsdale advises beachgoers who encounter tar balls not to touch them.
“Not only is it black and sticks to you, but like all hydrocarbons it’s got some very nasty chemicals in it, some carcinogenic,” he said. “It would be very bad for kids to think they can play with this stuff.”