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In the pursuit of identifying improved amorphous oxide coatings for gravitational wave detector test masses, 44% Ti-doped GeO2 has emerged as a potential candidate due to its low mechanical loss [Vajente et al. Phys. Rev. Lett. 127, 071101 (2021)]. It has been proposed and experimentally demonstrated that a reduction in mechanical loss in amorphous oxides is associated with medium-range order (MRO) dominated by cornersharing links in the amorphous network. In this work, we describe a combined experimental and theoretical approach to investigate the MRO of Ti-doped GeO2. Using atomistic modeling and simulations, we calculate the Raman spectrum of 44% Ti-doped GeO2, and we show that it predicts the main features observed in the experiments and further provides insight into the MRO, which would otherwise be hidden. It is shown that the amorphous oxide mixture exhibits a distribution of predominantly large (>4 member) rings formed by corner-shared tetrahedral Ge-O-Ti connections. It is also shown that the addition of Ti to a-GeO2 does not largely compromise corner-sharing connections, as the smaller populations of Ti-O-Ti and Ge-O-Ge links in the network are predominantly corner-shared. The mainly covalently bonded representation of the MRO of Ti-doped GeO2 revealed by this analysis validates the importance of the MRO network organization in influencing mechanical loss in amorphous oxides. 

Speleothem δ18O records from central southern China have long been regarded as a key benchmark for Asian summer monsoon intensity. However, the similar δ18O minima observed among precession minima and their link to seasonal precipitation mixing remains unclear. Here, we present a 400,000-y record of summer precipitation δ18O from loess microcodium, which captures distinct precession cycles similar to those seen in speleothem δ18O records, particularly during glacial periods. Notably, our microcodium δ¹⁸O record reveals very low-δ¹⁸O values during precession minima at peak interglacials, a feature absent in speleothem δ¹⁸O records from central southern China. This discrepancy suggests that the mixed summer and nonsummer climatic signals substantially influence the speleothem δ¹⁸O records from central southern China. Proxy-model comparisons indicate that the lack of very low-δ¹⁸O values in speleothem δ¹⁸O records is due to an attenuated summer signal contribution, resulting from a lower summer-to-annual precipitation ratio in southern China at strong monsoon intervals. Our findings offer a potential explanation for the long-standing puzzle of the absence of 100- and 41-kyr cycles in speleothem δ¹⁸O records and underscore the critical role of seasonality in interpreting paleoclimatic proxies in central southern China. These insights also have broader implications for interpreting speleothem δ¹⁸O records globally, advocating for a more multiseason interpretive framework. 

Abstract A better understanding of the relative roles of internal climate variability and external contributions, from both natural (solar, volcanic) and anthropogenic greenhouse gas forcing, is important to better project future hydrologic changes. Changes in the evaporative demand play a central role in this context, particularly in tropical areas characterized by high precipitation seasonality, such as the tropical savannah and semi-desertic biomes. Here we present a set of geochemical proxies in speleothems from a well-ventilated cave located in central-eastern Brazil which shows that the evaporative demand is no longer being met by precipitation, leading to a hydrological deficit. A marked change in the hydrologic balance in central-eastern Brazil, caused by a severe warming trend, can be identified, starting in the 1970s. Our findings show that the current aridity has no analog over the last 720 years. A detection and attribution study indicates that this trend is mostly driven by anthropogenic forcing and cannot be explained by natural factors alone. These results reinforce the premise of a severe long-term drought in the subtropics of eastern South America that will likely be further exacerbated in the future given its apparent connection to increased greenhouse gas emissions.