Research Express@NCKU - Articles Digest

The Asian summer monsoon has been precisely reconstructed from the high-resolution record from the speleothem, but reconstruction of the Asian winter monsoon is less satisfactory. Yancheva et al. provide such a reconstruction for the last 16,000 years from the titanium (Ti) content of the sediments of Lake Huguang Maar in coastal South China. However, we argue that the Ti is likely to have come mainly from the catchment and so the Ti content may instead be related to the hydrology of the lake.
Considering the small catchment and densely vegetated inner slope of the lake, Yancheva et al. overlook the Ti from the catchment through runoff. However, weathering and erosion in coastal South China are intensive, owing to high temperatures and heavy precipitation. The steep slopes surrounding the lake would promote surface erosion further. Sandy sediments in the lake are strong evidence for significant terrigenous input through runoff. This is strengthened by records from Tianyang Maar in the Leizhou Peninsula and Shuangchi Maar in the north of Hainan, which indicate that terrigenous input through runoff may be common for Maars in coastal South China. The basalts and laterite in the Leizhou peninsula, which are dominant in the catchment, have a high Ti content (Table 1) and so should be important sources of Ti for the lake.

More importantly, the flux of the lithogenic materials to the lake, 5–10 and 10–20 mg cm^-2 yr^-1 during the Holocene and glacial times, respectively, is too high to be explained by a wind-blown mechanism. Although it is comparable with an estimated modern dust flux of 4.6 mg cm^-2 yr^-1 in coastal South China, the dust flux was probably overestimated, given the atmospheric dust concentration in coastal South China and its settling velocity. In addition, modern observation indicates that mineral dust flux on the Loess plateau ranges between 15 and 36 mg cm^-2 yr^-1 at sites close to dust sources and between 9 and 10 mg cm^-2 yr^-1 at other sites, but only about 25% of the particles are smaller than 6 μm. Particles larger than 6 μm generally have short atmospheric lifetimes owing to gravitational settling. Lake Huguang Maar is about 1,500 km south of the Loess plateau and is not on a main dust trajectory. It should have a dust deposition rate much smaller than 2.5 mg cm^-2 yr^-1. This is consistent with model-derived deposition rates of <0.2 to 0.5 mg cm^-2 yr^-1 at this site. This evidence disproves wind-blown dust from remote North China as the main source of Ti to Lake Huguang Maar.

The Ti content of local basalts and laterite is apparently higher than in the loess deposit in the Loess plateau (Table 1). A source of Ti from the catchment is therefore more reasonable. This explanation is supported by the correspondence of high Ti content with the occurrence of sandy sediments during two cold, dry phases, the Younger Dryas episode and the period before the Bølling–Allerød warming. It would also account for some puzzles arising from the connection between Ti content and the Asian winter monsoon. For example, although the Ti content indicates a southward movement of the intertropical convergence zone 7.8 kyr ago, some places further north experienced a stronger summer monsoon until about 6 kyr ago; the Ti content during the Holocene is comparable with those during the Younger Dryas and the pre-Bølling–Allerød periods, but other records indicate that the summer monsoon was much stronger and that the winter monsoon was much weaker during the Holocene than during the two cold, dry periods.

The correspondence between the higher Ti content and occurremce of coarser sediments during the Younger Dryas and the pre-Bølling–Allerød periods indicates a connection between the Ti content and the hydrology of the lake. The higher Ti content during the period 5.2–7.8 kyr ago may have been caused by a similar mechanism because precipitation may have been lower for this period. Precipitation in coastal South China during 7.2–2.7 kyr ago may also have been reduced in Shuangchi Maar. If Ti was transported into the lake mainly in particle and colloidal phases, increased input through runoff and reduced water depth could have contributed to the higher Ti content recorded in these cores.

Our alternative interpretation is therefore that the Ti content may have been controlled by the hydrology of the lake, rather than by the Asian winter monsoon. Whether, and how, this mechanism could influence proxies such as total organic carbon and magnetic properties, however, needs further investigation.

Table 1 Ti contents in basalt and laterite and in loess-palaeosol.

The Ti contents are calculated according to the TiO₂ contents in the basalt and laterite on the Leizhou Peninsula, coastal South China, and in the loess-palaeosol deposition at Luochuan, North China.