Abstract
High-resolution environmental records from the Tibetan Plateau are essential to understand past global climatic and environmental changes. Magnetic minerals in lake sediments are important proxies to reconstruct environmental and climatic changes. Nam Co (lake) is a typical great lake in the transitional region of southwest monsoon in the Tibetan Plateau. Previous studies have extensively focused on geochemistry, microfossils, sedimentology and biochemistry analysis of Nam Co, which provides sound interpretation of paleoclimatic and paleoenvironmental changes. However, up to now, no systematic environmental magnetic studies have been carried out. Therefore, high-resolution and systematic magnetic studies combined with geochemical parameters were carried on lake sediments of core NC 08/01 from Nam Co for the Holocene period (11.3 cal ka BP) in order to explore how magnetic properties of the sediments respond to climatic changes. Based on variations of magnetic proxies, the sequence can be separated into 3 units. Unit 1 (236–199 cm, 11.3–7.8 cal ka BP) contains dominantly coarse-grained magnetite with homogeneous grain size. A positive correlation between magnetite and Ti strongly suggests that these coarse-grained detrital magnetites reflect detrital input signals due to insignificant effects of postdepositional dissolution processes on these coarse-grained magnetite particles. For Unit 2 (198–102 cm, 7.8–2.1 cal ka BP), magnetic grain size is finer and the corresponding concentration of magnetite is also reduced. This is mainly due to significant dissolution of these fine-grained detrital magnetite particles, which were transported under reduced water flow conditions during this period. For Unit 3 (101-0 cm, 2.1-0 cal ka BP), the bulk magnetic properties are dominated by a mixture of single domain biogenic magnetite and detrital magnetite. The concentration of magnetic minerals is not correlated with the Ti content. In conclusion, the preservation of magnetic minerals in the lake sediment and thus the corresponding magnetic properties are related to the initial grain size. Combination of magnetic properties (including variation of grain size and concentration) and other proxies of detrital inputs (e.g. Ti) can be used to infer the variation of redox conditions in Nam Co. These results provide a viable framework for reconstructing the paleoenvironmental changes of this lake.
Highlights
Mechanism of variations in environmental magnetic proxies of lake sediments from Nam Co, Tibet during the Holocene
Susceptibility is low in the whole Holocene lake sediment for two reasons: on one hand, the water of Nam Co is mainly supplied from south and southwest shore mainly composed of sandstone, which contains less magnetic minerals and is weakly magnetic; on the other hand, there are probably iron sulfides in the whole core, which is suggested by 3-axis thermal demagnetization curves and -T curves
Magnetic properties of lake sediment are influenced by many factors, detrital input, reductive dissolution process and postdepositional magnetotactic bacteria activity can be distinguished by detailed rock magnetic study
Summary
Mechanism of variations in environmental magnetic proxies of lake sediments from Nam Co, Tibet during the Holocene. Magnetic minerals in lake sediments are important proxies to reconstruct environmental and climatic changes. High-resolution and systematic magnetic studies combined with geochemical parameters were carried on lake sediments of core NC 08/01 from Nam Co for the Holocene period (11.3 cal ka BP) in order to explore how magnetic properties of the sediments respond to climatic changes. For Unit 2 (198–102 cm, 7.8–2.1 cal ka BP), magnetic grain size is finer and the corresponding concentration of magnetite is reduced This is mainly due to significant dissolution of these fine-grained detrital magnetite particles, which were transported under reduced water flow conditions during this period. Pollen, total organic carbon, grain size and microelements are important proxies to reveal that Holocene climate in the TP deteriorated from warm and wet to cold and dry within certain periods, and with periodically abrupt shifts [2,3]
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