Yao Wu

Coral skeletons exhibit a complex composition of mineral and organic components, with the water content playing a significant role in their structure. Water associated with organic matrices constitutes the major share, whereas nonstructural water (skeleton inclusion water) is present in much smaller amounts, typically less than 0.5 wt% of the coral skeleton. The isotopic composition of this water may reflect biomineralization processes or environmental conditions during skeleton formation. Here, we present fluid inclusion hydrogen and oxygen isotopic compositions of cold‐water coral skeletons from Angola and Iceland. We confirmed the kinetic isotope fractionation when coral skeletons were crushed at a temperature of 120°C using a cavity ring‐down spectroscopy analyzer. δ ¹⁸ O and δ ² H values are not reproducible across analytical setups with varying temperature parameters. When coral fragments are encapsulated in a glass tube, the amount of released water and its isotopic signature are far more reproducible. δ ¹⁸ O and δ ² H values of coral skeleton inclusion water from Angola show no significant differences between the Holocene and the last glacial period. The δ ² H values are characterized by strong isotopic fractionation compared to seawater. Our study highlights that current methods do not adequately capture the variability in the initial δ ¹⁸ O and δ ² H of the “quasi‐free” skeleton inclusion water in coral aragonite. It is also possible that there are varying degrees of exchange of skeleton inclusion water with seawater. A better understanding of the coral calcification process is still necessary to establish a clearer link between the isotopic compositions of seawater and skeleton inclusion water.

The interpretation of δ18Oc and δ13Cc values of stalagmites within China is still complex, although numerous cave archives have been reported in this region. Present challenges include distinguishing between natural and anthropogenic influences on regional hydrological and environmental changes, in particular due to the increasing human activities during the mid-to late Holocene. Here, we report for the first time the δ18Ofi and δ2Hfi records of fluids entombed as inclusions during the Holocene (6290 to 690 yr BP) from a stalagmite from southwest China. We excluded measurement-induced artefacts using Rayleigh fractionation models and improved measurement methods, producing reliable results. We observed very high δ18Ofi and δ2Hfi values during a weak Asian summer monsoon (ASM). Our record reveals six drought events during the mid- and late-Holocene (~950, 1360, 2260, 3450, and 5600 yr BP), which coincide with the weakening of ASM intensity and variations in low latitude forcing, such as tropical sea surface temperature, El Niño/Southern Oscillation, and intertropical convergence zone. In 950-1100 A.D., the dramatic enrichment of δ18Ofi (magnitude ~7‰) corresponds with the increase in regional population density due to large-scale population migration at this time (historically known as the Jingkang event). The overall coefficient of variation (C.V = standard deviation/mean) of the δ18Ofi sequence is 125% compared to only 5% for δ18Oc. Hence, δ18Ofi seems to exhibit a greater sensitivity to regional environment wet/dry variations than traditional carbonate isotope proxies.

The interpretation of stalagmite δ¹⁸O in terms of reflecting Asian summer monsoon (ASM) precipitation is still elusive. Here, we present high‐resolution stalagmite trace element ratios (X/Ca, X = Mg, Sr, Ba) records from southwest China covering 116.09 to 4.07 ka BP. δ¹⁸O, δ¹³C, and X/Ca values exhibit clear precessional cycles, with δ¹⁸O values reflecting ASM circulation/intensity, while X/Ca ratios capture local precipitation or evapotranspiration variations. Our results show that Northern Hemisphere summer insolation (NHSI) is the main driver of ASM intensity and precipitation phase variation, but global ice volume modulates the response magnitude of summer precipitation to insolation. During the Last Glacial Maximum, high ice volumes caused significant monsoon precipitation to decrease. In contrast to modern observations of the tripolar distribution of precipitation in China, our record is consistent with paleo‐precipitation records in southern and northern China.