Environmental scientists have known that high levels of the toxic element, mercury, have been accumulating in the Arctic Ocean for some time. It was believed to be mostly caused by atmospheric sources stemming from the combustion of coal. However, a new study from the Harvard School of Engineering and Applied Sciences and the Harvard School of Public Health has found that the great majority of Arctic mercury arrives via circumpolar rivers. Some of the largest rivers in the world flow north into the Arctic in Eurasia and North America.

The oil giant Shell filed suit in federal court in Alaska last week against a dozen environmental groups, employing a rare — and rarely successful — legal gambit in an effort to pre-empt anticipated legal challenges to its plans to begin exploration in the Arctic Ocean this summer.

Was the unusual maneuver an act of bravado, even desperation, by a company fearful that it might be thwarted again in its efforts to begin drilling in the seabed off Alaska’s North Slope?

Sea-ice coverage across the Arctic Ocean has dwindled to its second-lowest level since satellite records started in 1979, according to the National Snow and Ice Data Center.

Areas of the Arctic with at least 15 percent sea-ice as of Saturday totaled 1.68 million square miles, slightly above the record-low of 1.61 million square miles recorded in 2007, the center said.

Yet to be determined is whether the reported sea-ice cover will be the lowest for the year. Annual minimums are usually reached around mid-September.

The minimum summertime volume of Arctic sea ice fell to a record low last year, researchers said in a study, suggesting that thinning of the ice had outweighed a recovery in area. The study estimated that last year broke the previous, 2007 record for the minimum volume of ice, which is calculated from a combination of sea ice area and thickness.