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Fig. 1 a Summer (JJA) insolation at 20
°
N(blue line) (Laskar et al.
2004
) and paleoclimatic
records of core SO 188 17286-1 versus age for the last 135 ka: planktonic foraminiferal G. ruber
δ
◄
18
O record (black solid line);
13
C records of the four most abundant n-alkanes. More
D and
δ
δ
13
C
records indicate higher proportions of C
3
vegetation, whereas enriched values indicate increase in
the contribution of C
4
vegetation. b Simulated difference between early Eemian and early
Holocene precipitation minus evaporation balance for July (upper panel) and difference in total
precipitation (JJA) for the early Holocene (left bottom) and Eemian (right bottom) relative to
Present
negative values of
D indicate higher precipitation and vice versa. More negative values of
δ
δ
Fig. 2 Sensitivity of the vegetation C
4
/C
3
ratio to changes in temperature (left), atmospheric CO
2
(middle), and precipitation (right). Red dots represent the modern situation
systems (Fig.
1
b, upper panel). The C
4
/C
3
ratio of vegetation is determined by
climate (temperature, precipitation) and atmospheric CO
2
. Our model results
highlight a non-linear response in the abundance of C
4
vegetation expressed as a
logarithmic increase in C
4
/C
3
plant proportions with rising temperatures and an
exponential decrease under higher atmospheric CO
2
concentrations (Fig.
2
). The
response to precipitation change is rather linear with drier conditions favoring a
higher C
4
vegetation ratio (Fig.
2
). Any shift in vegetation C
4
/C
3
ratios inferred
from
13
C of sedimentary n-alkanes is thus a combination of potentially opposing
effects and cannot straightforwardly be interpreted as response to humid or dry
conditions only. As a consequence, also more efforts have to be spent to regional
calibration and application of the
δ
δ
D of individual n-alkanes, because it is still
dif
D signal even of individual n-alkanes from
plant waxes as a quantitative measure for past changes in precipitation independent
of vegetation changes (Wang et al.
2013
). Obviously, combinatory studies
including multiproxy records for past precipitation and river runoff, as well as
coupled ocean-atmosphere-vegetation models provide the best approach to gain
better insights to the complexity of and the past variations in the Indian summer
monsoon system.
cult to interpret the sedimentary
δ
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