When Islands Rose, Australian Rainforests Fell

What killed the lush rainforests that covered northwestern Australia until about 3 million years ago? A new study points the finger at the islands that now make up Indonesia. When this archipelago rose from the ocean floor during the past few million years, the study suggests, it reduced the flow of warm water spilling southward from the equator along the western coast of Australia. That robbed the region of the abundant precipitation needed to maintain the forests.

Today, the western portions of Australia are relatively arid. Yet previous studies, including analyses of windblown pollen trapped in sea-floor sediments that accumulated off northwestern Australia before 3 million years ago, reveal that the area once hosted rainforests, says Cyrus Karas, a paleoceanographer at the University of Kiel in Germany. Now, analyses of ocean-bottom sediments collected off the Australian coast and elsewhere in the Indian Ocean, the researchers report in an upcoming issue of Paleoceanography, may explain why those forests disappeared.

Using sea-floor sediments that had accumulated between 6 million and 2 million years ago, Karas and his colleagues measured the chemical composition and the ratio of oxygen isotopes in fossil shells of the surface-dwelling plankton Globigerinoides sacculifer. That allowed the researchers to estimate sea-surface temperatures during that interval. Although surface temperatures in the western and central Indian Ocean remained relatively stable during the last days of the Australian rainforests between 3.5 million and 3 million years ago, the temperature a few hundred kilometers off northwestern Australia dropped between 2° and 3°C during the same time period. That drop in temperature—a decrease that has lasted at least since 2.95 million years ago—would have reduced the amount of warm, moist air blowing onto the continent, the researchers speculate. That, in turn, would have reduced precipitation, and when rainfall declined, so did the rainforests.

Because the sea-surface temperatures didn't change much in the Indian Ocean, cooling along the coast of northwestern Australia probably isn't due to a global chill. Instead, the researchers contend, changing currents are to blame. When the Indonesian archipelago—including large islands such as Timor, which is about the same size as the Czech Republic—rose from the ocean floor, the new land blocked or diverted much of the warm water traveling southward along the Australian coast, triggering the local cooling. Previous studies have suggested that islands in the archipelago diverted the region's ocean currents when they rose, the researchers note.

Karas and his colleagues "make a good case for significant changes in ocean currents around 3.5 million years ago, and there's a plausible link to tectonic changes in the region then," says Robert Hall, a geologist at Royal Holloway, University of London. "This is indeed a tectonically complicated area that has changed dramatically in the last 5 million years." Nevertheless, scientists can't yet draw maps with enough detail to determine precisely how ocean currents may have shifted during this time span, Hall notes.