Scientists dig deep, find silt is culprit

CANBERRA, Australia — To persuade people to protect the environment, scientists' future projections can fall short. Often they need clues from the past.

As Queensland was debating whether to protect the Great Barrier Reef by restricting river runoff, coral researchers could point to historical records — not to mention contemporary recollections — that the state's rivers had grown more silt-filled since European settlement, threatening the reef. But they had very little data to back it up.

"We knew things were happening," says Malcolm McCulloch, a geochemist at Australia National University. "But we didn't know the true scale of what went out to the reef."

In what many here see as a seminal piece of sleuthing, McCulloch and colleagues from the Australian Institute of Marine Science took core samples of coral, which develop annual growth bands like trees.

Looking for chemical signatures of soil runoff, particularly the element barium, they found that from 1750 to about 1870, sediments from the Burdekin River — the country's second-largest when it floods — reached the inner portions of the reef "only occasionally." After about 1870, the amount of soil disgorged to the inner reef grew five- to 10-fold as land upriver was cleared for ranching and grazing began.

Queensland passed runoff regulations last year.