Search for: All records

Abstract Orbital‐scale Indian Summer Monsoon variability is often interpreted as a direct response to northern hemisphere summer insolation. Here we present a continuous (0–640 kyr) orbital scale precipitation isotope (δD_precip) record using leaf wax δD from the core monsoon zone of India. The δD_preciprecord is quantitatively coherent with, and δD_precipminima in phase with, greenhouses gas maxima, and ice volume minima across all orbital bands. The δD_preciprecord is also coherent and in phase with the two existing orbital‐scale Indian speleothem δ¹⁸O records, demonstrating a consistent regional response among independent proxies. These findings preclude interpretation of Indian precipitation isotope records as a direct response to northern hemisphere summer insolation. Rather, they dominantly reflect changes in moisture source and transport paths associated with changes in greenhouse gases and ice volume. The orbital‐scale precipitation isotope responses of the Indian and East Asian monsoon systems are uncoupled and are driven by different forcings. 

Abstract Earth's orbital geometry exerts a profound influence on climate by regulating changes in incoming solar radiation. Superimposed on orbitally paced climate change, Pleistocene records reveal substantial millennial‐scale variability characterized by abrupt changes and rapid swings. However, the extent to which orbital forcing modulates the amplitude and timing of these millennial variations is unclear. Here we isolate the magnitude of millennial‐scale variability (MMV) in two well‐dated records, both linked to precession cycles (19,000‐ and 23,000‐year periodicity): composite Chinese speleothemδ¹⁸O, commonly interpreted as a proxy for Asian monsoon intensity, and atmospheric methane. At the millennial timescale (1,000–10,000 years), we find a fundamental decoupling wherein precession directly modulates the MMV of methane but not that of speleothemδ¹⁸O, which is shown to be strikingly similar to the MMV of Antarctic ice coreδ²H. One explanation is that the MMV of methane responds to changes in midlatitude to high‐latitude insolation, whereas speleothemδ¹⁸O is modulated by internal climate feedbacks. 

Abstract. The International Ocean Discovery Program (IODP) conducted a series of expeditions between 2013 and 2016 that were designed to address thedevelopment of monsoon climate systems in Asia and Australia. Significantprogress was made in recovering Neogene sections spanning the region fromthe Arabian Sea to the Sea of Japan and southward to western Australia. Highrecovery by advanced piston corer (APC) has provided a host ofsemi-continuous sections that have been used to examine monsoonal evolution. Use of the half-length APC was successful in sampling sand-rich sediment in Indian Ocean submarine fans. The records show that humidity and seasonality developed diachronously across the region, although most regions show drying since the middle Miocene and especially since ∼ 4 Ma, likely linked to global cooling. A transition from C3 to C4 vegetation oftenaccompanied the drying but may be more linked to global cooling. WesternAustralia and possibly southern China diverge from the general trend inbecoming wetter during the late Miocene, with the Australian monsoon beingmore affected by the Indonesian Throughflow, while the Asian monsoon is tied more to the rising Himalaya in South Asia and to the Tibetan Plateau in East Asia. The monsoon shows sensitivity to orbital forcing, with many regions having a weaker summer monsoon during times of northern hemisphericGlaciation. Stronger monsoons are associated with faster continentalerosion but not weathering intensity, which either shows no trend ora decreasing strength since the middle Miocene in Asia. Marine productivityproxies and terrestrial chemical weathering, erosion, and vegetation proxiesare often seen to diverge. Future work on the almost unknown Paleogene isneeded, as well as the potential of carbonate platforms as archives ofpaleoceanographic conditions. 

Abstract Most of Earth’s rain falls in the tropics, often in highly seasonal monsoon rains, which are thought to be coupled to the inter-hemispheric migrations of the Inter-Tropical Convergence Zone in response to the seasonal cycle of insolation. Yet characterization of tropical rainfall behaviour in the geologic past is poor. Here we combine new and existing hydroclimate records from six large-scale tropical regions with fully independent model-based rainfall reconstructions across the last interval of sustained warmth and ensuing climate cooling between 130 to 70 thousand years ago (Marine Isotope Stage 5). Our data-model approach reveals large-scale heterogeneous rainfall patterns in response to changes in climate. We note pervasive dipole-like tropical precipitation patterns, as well as different loci of precipitation throughout Marine Isotope Stage 5 than recorded in the Holocene. These rainfall patterns cannot be solely attributed to meridional shifts in the Inter-Tropical Convergence Zone. 

South Asian precipitation amount and extreme variability are predicted to increase due to thermodynamic effects of increased 21st-century greenhouse gases, accompanied by an increased supply of moisture from the southern hemisphere Indian Ocean. We reconstructed South Asian summer monsoon precipitation and runoff into the Bay of Bengal to assess the extent to which these factors also operated in the Pleistocene, a time of large-scale natural changes in carbon dioxide and ice volume. South Asian precipitation and runoff are strongly coherent with, and lag, atmospheric carbon dioxide changes at Earth’s orbital eccentricity, obliquity, and precession bands and are closely tied to cross-equatorial wind strength at the precession band. We find that the projected monsoon response to ongoing, rapid high-latitude ice melt and rising carbon dioxide levels is fully consistent with dynamics of the past 0.9 million years.