By Jordan Joseph

A new study pulls a 5,000-year thread from mud off East Java and shows that people began causing soil erosion by reshaping ancient tropical soils much earlier than believed.

The evidence comes from a marine sediment core that captured what rivers carried seaward: eroded soil, microscopic plant waxes, and faint chemical fingerprints of fire.

Lead author Dr. Yanming Ruan, of MARUM, Center for Marine Environmental Sciences, University of Bremen, and colleagues in Germany, the Netherlands, and China, traced how those fingerprints rose and fell through time.

Their goal was simple, and bold: separate natural climate swings from the mark of human land use across the late Holocene.

Ocean sediments reveal farming

Sediments raining into the Indian Ocean off East Java build thin layers year after year. Over centuries, those layers store signals of what happened on land, including soil loss during storms and ash and soot after fires.

One class of molecules, brGDGTs (branched glycerol dialkyl glycerol tetraethers), comes mainly from soils and serves as a tracer of terrigenous input to the sea.

Another, levoglucosan, forms when vegetation burns and is widely used to infer past fire activity, including low intensity, managed burns.

In humid tropical forests, natural fires are uncommon, so most fire signals point to people, not lightning or volcanism.

Early farming fires and erosion

“Such early farming practices likely made soils more susceptible to erosion,” said Ruan.

The team reports a sharp rise in fire markers about 3,500 years ago without matching shifts in regional vegetation or rainfall, pointing to early swidden cultivation in East Java.

Around 2,000 to 1,500 years ago, the isotopes point to a punchier wet season paired with a longer dry season.

That pattern raises rainfall erosivity, which can turn disturbed hillslopes into sediment sources and drive heavier river loads.

Farming shift drove soil erosion

The most intense soil erosion in the last five centuries coincides with a transition from swidden to more permanent agriculture across many tropical frontiers, a shift that numerous case studies link to faster, more persistent soil loss.

Independent pollen evidence from northern Java’s offshore sediments documents stronger land use signals during this late interval, including crop indicators and plantation pollen.

Finding an early human fingerprint changes the baseline we use to judge modern impact.

Models project more frequent and intense positive Indian Ocean Dipole events as greenhouse warming proceeds, a setup that can supercharge regional rainfall extremes.

Projections also point to greater hydroclimate variability tied to the El Ni?o Southern Oscillation, which can toggle between punishing droughts and deluges across Indonesia.

Human activities have already reorganized Earth’s sediment cycle by accelerating hillslope erosion and altering river sediment delivery, so heavier rain on more exposed soils stacks the deck for even faster losses.

Why this matters today

Long dry seasons raise the risk that managed burns escape and escalate. During 1997, Indonesian peatland fires released vast amounts of carbon, a stark reminder that fire use can flip from tool to hazard under the wrong conditions.

Levoglucosan is a powerful tracer of burning, yet it is not a perfect, passive clock. Its production, transport, and preservation vary with fire temperature, fuel, and environment, so multiproxy approaches, as used here, are key to robust reconstructions.

Soils thin slowly and can vanish quickly. The IPCC reports that land degradation and erosion undermine food security, especially where extreme rainfall and intensive land use collide, which places parts of the Maritime Continent high on the watch list.

The record from Java says the Anthropocene’s deep root in this region is not a recent sprout, and the combination of intensive agriculture with sharper rainfall extremes is exactly the mix that accelerates erosion.

Global impact of soil erosion

Soil erosion in the tropics not only strips away fertile land but also damages river systems and coastal zones that depend on steady sediment supplies.

As sediments decline or arrive in pulses, estuaries and reefs can suffer, weakening ecosystems that provide food and coastal protection.

Historical studies suggest that accelerated soil loss has played a role in local declines of farming communities by lowering yields and increasing vulnerability to floods and droughts.

The evidence from Java shows that these pressures were present centuries ago, long before modern agricultural intensification.

The study is published in Geophysical Research Letters.

(Source: https://www.earth.com/news/amthropocene-humans-began-causing-global-soil-erosion-much-earlier-than-believed/)