RESUMO
The Ganges-Brahmaputra-Meghna (G-B) river system transports >1 × 109 t/yr of sediment, with an estimated 0.7 × 109 t/yr reaching the Bay of Bengal (BoB). This discharge represents a major input of sediment and associated elements to the global ocean, but quantification of the sediment-element mass reaching the BoB has yet to be fully explored. Published geochemical and suspended sediment data are used to calculate a first-order budget for the modern sediment supply of geochemical elements to the BoB. River profile bulk sediment-element concentrations are calculated based on suspended sediment and element measurements taken in the Ganges and Brahmaputra rivers. A Monte Carlo analysis is applied to account for variable sediment and geochemical contributions from each river. Results show that on average, the G-B system contributes ~5% of the global riverine discharge of solid-phase elements from sediment to the oceans. G-B sediments transport >10% of the global element supply of Hf and Zr. For others, like As and Cu, contributions from the G-B are <5%. Results also show that sediment reaching the BoB is relatively enriched in Hf, Zr, Th, REEs, Sn, and Bi, and majorly depleted in Na and Sr compared to UCC elemental concentrations. While limited by data availability and necessary simplifying assumptions, this study nevertheless provides a reasonable first-order budget for the modern discharge of solid-phase elements to the BoB. Insights from this work are significant for understanding the role of the G-B river system in global elemental cycling, and for providing a basis of comparison for future sediment-element discharge in light of rapid environmental change taking place in the region.
RESUMO
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.
