A new low-latitude loess record unveils the rainfall cycle of the East Asian summer monsoon

A low-latitude loess profile from China revealed the dominant ~20,000-year cycle in rainfall change in the core zone of the East Asian summer monsoon, driven by variation in insolation controlled by precession, and precipitation and temperature variation may not follow the same orbital-scale cycles.
Published in Earth & Environment
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Background

The formation of the monsoon is related to the seasonal temperature difference between land and sea. A large amount of water vapour is transported inland with the summer monsoon, leading to abundant rainfall, which plays a crucial role in sustaining the ecosystem in East Asia. There is considerable controversy about how changes in the Earth's orbit drive variations in the East Asian summer monsoon (EASM). While reconstructions based on loess deposits in northern China show a distinct ~100,000-year cycle in summer monsoon precipitation, cave stalagmite records in southern China exhibit a clear ~20,000-year cycle in EASM variations. The difference in the dominant cycles of the EASM revealed by different records remains the unresolved "Chinese 100 kyr problem." Compared to the mid-latitude regions such as the Chinese Loess Plateau (CLP), the low-latitude loess records from the core area of the EASM, such as those in Pengze County, play a crucial role in solving the "Chinese 100 kyr problem". In response to this controversy, we focused on the Madang profile in Pengze County and obtained the continuous high-resolution loess paleoclimate record from the core area of the EASM.

 

Main findings

Due to the influence of post-depositional intense weathering and local hydrological factors, the magnetic susceptibility (MS) of loess in southern China often struggles to accurately reflect climate changes. However, the Madang profile, due to its unique location with excellent ventilation and drainage conditions, exhibits a remarkable similarity in MS variation to a classic profile from the CLP located 900 km away—the Luochuan profile. This suggests that the Madang profile serves as a rare and valuable archive of paleoclimate records in low-latitude regions. The research team established a chronological scale for the profile through climate-stratigraphic comparison and validated it through OSL dating. They then systematically measured proxy indicators of the Madang loess to reconstructed a climate change sequence spanning about 350,000 years.

 

Wavelet and spectral analyses indicate that the MS chronology of the Madang profile exhibits a 100,000-year variation cycle, consistent with the dominant cycle of global ice volume. In contrast, the FeD/FeT (ratio of dithionite−citrate−bicarbonate extractable iron to total iron) chronology shows a variation cycle of approximately 20,000-year, closely resembling the precession cycle of Earth's orbital parameters. The FeD/FeT ratio is an indicator that typically reflects the intensity of chemical weathering of soils, revealing the proportion of iron released during the chemical weathering process of iron-bearing silicate minerals. In the southern regions of China, where annual precipitation is relatively high, its variation in modern topsoil shows a clear dependence on precipitation, suggesting that FeD/FeT can be used as a proxy indicator to reflect precipitation levels.

 

The FeD/FeT record in the Madang profile is negatively correlated with the δ18O record of the Sanbao Cave stalagmite in China and significantly positively correlated with the low-latitude summer solar insolation gradient between the northern and southern hemispheres. It is hypothesized that the precession-driven enhancement of solar insolation increases the land-sea thermal gradient, strengthens the EASM, increases land precipitation, promotes enhanced soil chemical weathering, and consequently leads to an increase in the FeD/FeT ratio in the soil.

 

MS typically indicates the intensity of pedogenesis and is generally influenced by both precipitation and temperature. If the MS in the Madang profile primarily serves as a proxy indicator for precipitation, it necessitates the introduction of post-depositional processes acting as signal smoothers to explain the elimination of the 20,000-year cycle. A more plausible interpretation is that the loess MS in the Madang profile predominantly reflects temperature variations. It shows a distinct positive correlation with Antarctic temperature records and a significant negative correlation with deep-sea δ18O records (glacial-interglacial cycles) (Figure 4). This study speculates that the increase in temperature favours evapotranspiration, thereby reducing soil moisture and preventing loess from undergoing reduction due to excessive wetness, leading to a decrease in MS.

 

Significance

The new findings in this paper confirm that precession-dominated changes in low-latitude summer solar insolation are a primary driver of the variability in the EASM. It provides new geological evidence to address the 'Chinese 100 kyr problem' and deepens our understanding of the processes and mechanisms underlying changes in the EASM. The study also suggests that, on an orbital scale, precipitation and temperature in the monsoon core region may not vary synchronously (a 20,000-year precipitation cycle and a 100,000-year temperature cycle). Therefore, in addition to the commonly accepted cold-dry and warm-wet modes, the paleoclimate in the monsoon zone has cold-wet and warm-dry modes (e.g., cold-dry for MIS 2 and cold-wet for MIS 3). According to the calculated insolation variation, EASM precipitation began to decline gradually about 10,000 years ago, and this trend is expected to continue until about 3,700 years later.

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