Magnetofossils are geologically preserved magnetic minerals produced
intra-cellularly by magnetotactic bacteria and have been proposed as
distinctive environmental indicators. Particularly, links
between magnetofossils and geological thermal events have been reported
in both marine and haemipelagic sediments.
|Biomagnetic response to the Holocene Warm Period in semi-arid East Asia
Hyperthermal events in geological times, such as the Palaeocene-Eocene
Thermal Maximum, are believed to have profoundy changed global and
regional hydrological cycles, and thus significantly reshaped
sedimentary and biological processes in both marine and terrestrial
environments. The Holocene Warm Period (HWP) provides valuable insights
into the climate system and biotic responses to environmental
variability during geological thermal events and thus serves as an
excellent analogue for future global climate changes.
magnetic concentration (represented by ARM in a) and grain size
(represented by the ratio in b) have nice correlations with TOC and
TIC, used as indicator of nutrition supply into Dali Lake. Therefore,
it is inferred that high magnetofossil abundance during the Holocene Warm Period (HWP)
is fostered by better nutrient availability, which can further be
linked to warm and wet conditions in the lake region. Our magnetic
record documents clearly the beginning of the HWP at ~9.8 ka and a
sudden termination of the HWP at ~5.9 ka. A two-stepped increase at
~7.7 ka coincides with inferred water source change: from snow/ice melt
to monsoonal precipitation.
One theme of Suzhen's PhD thesis is the identification and
environmental interpretation of magnetofossils in Dali Lake (inner
Mongolia, China) sediments during the HWP. The main conclusions were:
- The warm and wet HWP conditions were highly favourable for magnetofossil proliferation in the semi-arid Asian interior.
- The increase and termination of biomagnetic
signal at ~9.8 ka and ~5.9 ka coincided respectively with the beginning
and end of the HWP, and are respectively linked to increased nutrient
supply due to postglacial warming and poor nutrition due to drying at
~6 ka in the Asian interior. The two-stage transition at ~7.7 ka
correlates well with inferred change of source water into the lake from
ice/snow melt to monsoonal precipitation (therefore, ameliorating
climate conditions in the East Asian Monsoon area).
- Our biomagnetic record correlates well with
changes in summer insolation at high northern latitudes, which reflects
the response of biotic systems in semi-arid lakes to insolation-driven
climate changes through magnetic mineral production and deposition.
This record may also represents a good example of magnetofossil
abundance during geological thermal periods.
Orientaton of Precambrian extrusive rocks in North China Craton
environmental magnetic record of the last deglaciation: contrasting
magnetic sources during warm and cold periods
Preservation and alternation of magnetic minerals in lake sediments
provide implication of the redox conditions and hydrology during lake
Detailed analysis of magnetic concentration, mineralogy, grain size and
domain state can thus assistant in identification of magnetic mineral
sources and perhaps lake deposition process.
For instance, allochthonous magnetic minerals in lake sediments may
origin from bedrock and soil in the catchment, eolian dust, vocanic
ash, while authigenic contribution includes magnetosome (geologically
magnetofossils) produced intracellularly by magnetotactic bacteria and
iron sulphide as byproduct of iron-reducing bacteria. Moreover,
post-depositional alternation such as dissolution, oxidation and
dilution can bring ambiguities in environmental interpretation of the
Field work in Linxia
Another topic of Suzhen’s
PhD research is detailed evalution of magnetic sources in Dali Lake
since the last deglaciation. Techniques involved are systematic rock
magnetic analysis, TEM and SEM observation and XRD. The aim is to
constrain the source of magnetic signal during different stage, and to
explore the implication of lake depositon process and regional surface
and climate changes during this period.
Autumn view of the Dali Lake catchment area
Publications of Suzhen Liu
S., Deng, Chenglong, Xiao, Jule, Li, Jinhua, Paterson, Greig, Chang,
Liao, Yi, Liang, Qin, Huafeng, Pan, Yongxin & Zhu, Rixiang (2016).
High-resolution enviromagnetic records of the last deglaciation from
Dali Lake, Inner Mongolia. Palaeogeography, Palaeoclimatology,
Palaeoecology, 454, (pp. 1-11).
- Li, S., Deng, Chenglong, Dong,
Wei, Sun, Lu, Liu, S., Qin, Huafeng, Yin, Jiyun, Ji, Xueping & Zhu,
Rixiang (2015). Magnetostratigraphy of the Xiaolongtan Formation
bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China -
Constraint on the initiation time of the southern segment of the
Xianshuihe–Xiaojiang fault. Tectonophysics, 655, (pp. 213-226).
- Liu, S., Deng, Chenglong, Xiao, Jule, Li,
Jinhua, Paterson, Greig, Chang, Liao, Yi, Liang, Qin, Huafeng, Pan,
Yongxin & Zhu, Rixiang (2015). Insolation driven biomagnetic
response to Holocene Warm Period in semi-arid East Asia. Scientific
Reports, 5 (8001).