Figure 2. Age-depth plot for the Maar Lake Huguangyan. The
calibrated AMS14C dates are from Wang et al., (2016) (red) and Chu et
al., (2017) (blue).
2.3 GDGTs analysis
This study builds upon previous work of Huguangyan Maar Lake (Chu et
al., 2017). For GDGTs analysis, freeze-dried samples
(~1.5 g) were extracted using a dichloromethane (DCM):
methanol (9:1, v/v) mixture with an accelerated solvent extractor (ASE
350) at 120 ℃ and 1500 psi for two cycles. The extracts were dried under
N2 and separated into apolar and polar fractions over an
activated Al2O3 column using hexane: DCM (9:1, v/v) and DCM: methanol
(1:1, v/v) as the respective eluents.
The branched GDGTs were analyzed using a Shimadzu high-performance
liquid chromatography triple quadrupole mass spectrometry system
(HPLC-MS), with an autosampler and Analyst software (modified by Hopmans
et al., 2004; Weijers et al., 2007). Separation was achieved using a
Grace Prevail Cyano column (150 mm × 2.1 mm; 3 μm); ion scanning was
performed in a single ion monitoring mode at m/z 1050, 1048, 1046, 1036,
1034, 1032, 1022, 1020 and 1018. The brGDGTs were quantified using a C46
internal standard.
MAAT from brGDGT for the Huguangyan Maar Lake was obtained using the
degree of methylation (MBT) and the cyclisation ration of branched
tetraether (CBT) according to the proxy calibration for the lacustrine
sediment from China and Nepal (Sun et al., 2011):
MAAT=3.949 – 5.593 × CBT + 38.213 × MBT (n=100,
r2=0.73)
3 Result and Discussion
3.1 Reconstruction temperature vs. instrumental data and historical
documentary.
All proxy data are indirect measurements of climate change, and
therefore it is necessary to calibrate or validate them against
instrumental or other independent data(Jones & Mann, 2004). Hence, we
compared the brGDGTs-based MAAT record from Huguangyan Maar Lake with
several other independent climate datasets: instrumental annual mean air
temperature data from Zhanjiang meteorological station,
~15 km from Huguangyan Lake; historical documents from
140 local chronicles and 14 monographs from southern China (south of
23.5ºN), and a synthetic temperature reconstruction for China (Figure
3c).
The reconstructed MAAT is 22.5℃ for the period of 2010 CE–1951 CE
(based on 137Cs-dating of the uppermost part of the sediment column).
This result is close to the instrumental annual mean air temperature
(23.2℃) for the same period obtained from nearby Zhanjiang
meteorological station, however, there is a 0.7℃ offset between the
instrumental data and the reconstructed temperatures. Although
temperatures in this tropical lake are favorable for biological activity
throughout the year, the brGDGTs-based temperature is slightly biased to
winter and spring temperatures because of the turnover of the water
column, which causes nutrient-rich bottom water to reach the surface and
support the growth of aquatic organisms. This was confirmed by the
seasonal flux of brGDGTs estimated from sediment traps (Hu et al.,
2016), together with monthly observations of planktonic diatoms (Wang et
al., 2012). The occurrence of lower temperatures during the 1970s is
confirmed by both instrumental data for 1921 CE–1939 CE and 1951
CE–2010 CE, as well as by the MAAT reconstruction (Figure 3a).
Historical documents have often been used for paleoclimatic
reconstruction. In tropical China, historical documents are widely
available and there are hundreds of local chronicles. Direct
descriptions using terms such as “rivers frozen” and “snow, sleet and
frost” in tropical China (south of 23º30’ N) are interpreted as clear
evidence of cold events. These abnormal phenomena are related to cold
surges (cold waves). Meteorologically, a cold surge has been defined as
a very large temperature decrease (exceeding 8℃) within 24 hours, and
they are proposed as a surrogate for winter monsoon strength in China
(Ray et al., 1991). Cold surges have a relatively short duration, often
less than one week. We used historical documents to produce a
compilation of evidence of “rivers frozen, snow, sleet and frost in the
tropical plains” (south of 23.5º N; see Supplementary Table S1). The
sources are from ”Natural disasters in historical documents in Guangdong
province” (including Hainan province) and ”Natural disasters in
historical documents in Guangxi province” (Institute of Literary and
History, Guangdong Province, unpublished data), which consist of 140
local chronicles and 14 monographs.