Monsoon Variability

Numerous palaeoenvironmental and archaeological studies from southern Arabia (Yemen and Oman) have revealed strong relations between phases of human settlements and climate change linked to the Indian monsoon system. Analyses on speleothems, cave fills, lacustrine deposits and palaeo-mangroves have shown that during the Early to Mid-Holocene, a humid Optimum culminated around 9000-8000 cal BP. New results on inland speleothems and cave sediments fromthe Jebel Qara (southern Oman) are crucial in our depiction of Early and Mid-Holocene climatic evolution and cultural dynamics of the region. These aspects are discussed here, based on new archaeological surveys, excavations, geoarchaeological and micromorphological studies, aiming to better understand connections with Terminal Pleistocene and Early Holocene autochthonous cultures of southern Arabia. Our results suggest that the final Pleistocene was marked by strong aridity, which promoted a widespread thermoclastism within rock shelter and deposition of aeolian sand; in contrast, the transition towards the Holocene is marked (since c.12,000 cal BP) by a progressive increasing in environmental humidity, which permitted the formation of thick strata of peridesert loess. After this phase, the environmental humidity of the Jebel increased and permitted the existence of a large community of land snails; the latter were exploited by Early Holocene hunter-gatherers who lived in the rock shelters between c. 10,500e9500 cal BP and left consistent accumulations of land shells (escargotières). The maximum of Holocene humidity was reached between 9000 and 8000 cal BP; regional aquifer were recharged and the deposition of calcareous tufa at the entrance of caves started, lasting up to c. 4500 cal BP. C and O stable isotopes from calcareous tufa highlights, in accordance with several regional records, the progressive decline of the intensity of the Indian Ocean monsoon and the transition towards arid conditions. In this phase, the area was abandoned and archaeological communities possibly relocated along the coast of central and southern Oman, where they exploited the mangrove environment.

As the economy and livelihoods of Bangladesh heavily depends on agriculture, any changes in monsoon rainfall have severe implications for the country. There is a growing concern on monsoon rainfall pattern change in Bangladesh in recent years like other parts of Indian summer monsoon region. A study has been carried out in this paper to analyze the monsoon rainfall time series of Bangladesh to decipher if there any shift in monsoon rainfall regime of Bangladesh. Sixty four years rainfall data recorded at twenty-nine locations distributed over Bangladesh were analyzed using a sequential regime shift detection method for this purpose. The proposed method employed Student's t-test to detect difference between two subsequent regimes with a cutoff length of one to determine the regime shift. The result shows that monsoon rainfall has increased, mostly in recent years in many locations of Bangladesh. Though increased monsoon rainfall will be helpful for rain-fed agriculture in Bangladesh, at the same time it will also cause more frequent floods, urban water logging, water-borne diseases, etc.

Denitrification is the process by which nitrate and nitrite are reduced to nitrogen gas when the oxygen concentration falls below 1 μM in the oxygen minimum zones (OMZ; oxygen concentration <20 μM) of the world’s oceans. The process of denitrification plays an important role in the global nitrogen cycle. Evidence of denitrification has been reported from various ocean basins, including the Arabian Sea, but there is still a lack of information about the long-term evolution of OMZ and denitrification during the Quaternary or even further in the geologic past. Here we present nitrogen and carbon isotope ratios (δ15N, δ13C), total organic carbon (%C), total nitrogen (%N), and the C/N ratio of the squeeze cake samples retrieved during IODP Expedition 355, which are dated to the Quaternary to middle Miocene. The average sample resolution is ~140 kyr. The results show variations of denitrification associated with paleoproductivity changes in the eastern Arabian Sea over the time interval studied. The δ15N values vary from ~2.4‰ to ~8.2‰ during quaternary to middle Miocene, whereas the average value is 5.3‰. From 10.15 Ma to 3.09 Ma, the δ15N values vary from 6.4‰ to 3.9‰. Overall, the trend of increasing δ15N values indicates enhanced denitrification from the Pliocene to present. The %N and %C values, which represent the degree of productivity, exhibit similar variability, suggesting a strong coupling between denitrification and productivity. The δ13C and C/N values average -22.3‰ and 6.7, which confirm a marine origin for the sedimentary organic matter.