Abstract
In climates with strongly seasonal rainfall, speleothem-based paleoclimate reconstructions are often thought to reflect wet season conditions, assuming a bias toward the season with greater water supply. This is particularly true in monsoon regions, where speleothem records are interpreted to document monsoon strength changes on multiple timescales. Dry season infiltration variability and rainfall seasonality are not typically considered in these reconstructions, even though cave ventilation could bias speleothem growth toward the cooler season. To investigate the influence of dry season infiltration on speleothem geochemistry, we combine a modern, sub-seasonally resolved trace element record from Mawmluh Cave in Northeast India with forward modeling experiments. We find that variations in the amplitude of seasonal signals in speleothem Mg/Ca, which reflects prior carbonate precipitation, are more sensitive to dry season rather than monsoon season infiltration. This sensitivity may be enhanced by dry season cave ventilation. The Mawmluh speleothem Mg/Ca record is consistent with increased dry season rainfall during the 1976–1998 warm phase of the Pacific Decadal Oscillation relative to 1964–2013. Our work demonstrates the importance of considering non-monsoon season rainfall when interpreting speleothem paleoclimate records and suggests that trace elements could provide insight into periods of enhanced dry season infiltration in monsoonal climates.
Highlights
The Indian Summer Monsoon (ISM) is a critical component of the climate system in South Asia, delivering 70–80% of annual rainfall between June and September (JJAS)
Similar patterns are seen in the δ18O Continuous wavelet transforms (CWTs), where El Niño-Southern Oscillation (ENSO)-scale periodicities are insignificant when the Pacific Decadal Oscillation (PDO) index is positive[6]
The positive correlation between U/Ca and the other trace element ratios suggests that U/Ca is sensitive to PCP and that calcite is the primary form of PCP in the Mawmluh Cave epikarst system
Summary
The Indian Summer Monsoon (ISM) is a critical component of the climate system in South Asia, delivering 70–80% of annual rainfall between June and September (JJAS). Acute water shortages occur across India during the dry season following years of weak ISM10. Isotope-enabled climate modeling suggests that speleothem δ18O in the broader Asian monsoon region primarily records changes in large scale atmospheric processes[19,20,21], and does not provide unequivocal www.nature.com/scientificreports/. Additional, independent proxies are required to investigate and relate rainfall amount changes to moisture transport history It is not known how variations in rainfall seasonality are recorded in speleothems from this region or what might drive seasonality on decadal or longer timescales[24]. Pinpointing the cause(s) of changes in dry season rainfall and how this might be reflected in dripwater chemistry could significantly improve our understanding of proxy seasonality in speleothem records from monsoonal settings. We further examine the relationships between Pacific climate variability and year-round rainfall in this water-sensitive region
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