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Understanding how Earth's climate has naturally fluctuated during the Holocene—the current geological epoch spanning the last 11,700 years—is crucial for contextualizing modern human-driven warming and improving future climate projections. However, the climate history of tropical Australasia has remained unclear, with scientists often divided over interpretations of paleoclimate records.

To investigate this, a research team led by Prof. Zhang Enlou from the Nanjing Institute of Geography and Limnology at the Chinese Academy of Sciences analyzed "molecular fossils"—branched glycerol dialkyl glycerol tetraethers (brGDGTs)—preserved in the sediments of Girraween Lagoon. These organic compounds serve as precise paleothermometers when properly calibrated. This study was in the journal CATENA.

Using a specific calibration for tropical lakes, the researchers reconstructed mean annual air temperatures over the past 10,400 years. Their results revealed a steady warming trend of 2°C throughout the Holocene. This warming trend aligns with other land temperature records, alkenone-derived ocean temperature data, and climate models for the region.

Interestingly, it diverges from estimates based on magnesium-to-calcium ratios in planktonic fossils, highlighting ongoing debates about the reliability of different proxy methods.

In addition to temperature changes, the sediments indicated an environmental transformation. Over millennia, the lagoon became increasingly acidic. This acidification resulted from two main factors: first, declining rainfall in northern Australia reduced the inflow of alkaline groundwater from limestone sinkholes beneath the lake; second, decaying organic matter in the sediments increased acid production. Together, these processes altered the lagoon's chemistry.

Furthermore, by correlating their data with regional climate records, the researchers traced these changes to intensifying El Niño-Southern Oscillation (ENSO) activity. As ENSO variability increased, tropical Australasia transitioned from cool and wet conditions to the warmer, drier climate we see today, characterized by stark seasonal contrasts.

This dual reconstruction of temperature and pH not only resolves long-standing scientific contradictions but also provides a comprehensive framework for understanding how ocean-atmosphere interactions, hydrology, and biogeochemical cycles collectively shaped the region's climate history. These insights are critical for refining models that predict how tropical climates may respond to future global warming.

"Lake sediments are like nature's history books," said Prof. Zhang. "By interpreting their chemical signatures, we are better prepared for the climate challenges ahead."