Chemical Weathering and Erosional Response of Northern New Guinea to Orbital-Scale Climate Variability

Abstract

The island of New Guinea comprises arc-ophiolite units tectonically imbricated with continental rocks offscraped from the colliding Australian plate and contributes large amounts of sediment to the ocean. A sequence deposited close to the north shore and sampled at International Ocean Discovery Program (IODP) Site U1485 is largely formed from sediment delivered by the Sepik River. We reconstruct changing intensities of chemical weathering and source bedrock contributions for 330 ka to assess how they are influenced by orbitally driven climate change. Higher smectite/kaolinite ratios indicate a more seasonal chemical weathering during glacial times, with interglacial periods marked by more tropical weathering. Nd and Sr isotopes show that erosion of continental bedrock is at its maximum during interglacial periods when rainfall was more intense and penetrated deeper into the Highlands, where silicic units are preferentially exposed. During colder/drier time, erosion is more focused in arc-ophiolite lowland regions. The Chemical Index of Alteration (CIA) and several other major element proxies imply a gradual increase in the alteration intensity of sediments due to chemical weathering. Comparing the bulk sediment and source rock compositions shows long-term variability in the consumption rates of CO₂. Weathering is most effective at removing atmospheric CO₂ during glacial times when ΔCO₂ values (mol/kg) reach around three times those seen in major mainland Asian river systems. Conversely, CO₂ consumption is reduced during interglacial maxima, implying that weathering in New Guinea, controlled by orbital cycles, may amplify global climate variations.