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Three drier periods (lower rainfall) (i.e., before ~17, ~1513.5, and 7–3 ka BP) and three wetter periods (higher rainfall) (i.e., ~17–15, ~13.5–7, and after ~3 ka BP) were detected on Southern Indonesia (off southwest Sumba) based on geochemical element (terrigenous input) proxies (ln Ti/Ca and K/Ca). During the Last Deglaciation, AISM rainfall responded to high latitude climatic events related to the latitudinal shifts of the austral summer ITCZ. Sea level rise, solar activity, and orbitally-induced insolation were most likely the primary driver of AISM rainfall changes during the Holocene, but the driving mechanisms behind the latitudinal shifts of the austral summer ITCZ during this period are not yet understood.
Three drier periods (lower rainfall) (i.e., before ~17, ~1513.5, and 7–3 ka BP) and three wetter periods (higher rainfall) (i.e., ~17–15, ~13.5–7, and after ~3 ka BP) were detected on Southern Indonesia (off southwest Sumba) based on geochemical element (terrigenous input) proxies (ln Ti/Ca and K/Ca). During the Last Deglaciation, AISM rainfall responded to high latitude climatic events related to the latitudinal shifts of the austral summer ITCZ. Sea level rise, solar activity, and orbitally-induced insolation were most likely the primary driver of AISM rainfall changes during the Holocene, but the driving mechanisms behind the latitudinal shifts of the austral summer ITCZ during this period are not yet understood.
- paleoclimate
- Australian-Indonesian monsoon
- elemental ratio
- Southern Indonesia
During the Last Deglaciation, the rainfall changes were most likely connected to the high latitude climate events, i.e., early stage of Deglaciation (ED) (before ~17 ka BP), HS1 (~17–15 ka BP), ACR (~15–13.5 ka BP), and Younger Dryas (~13.5–11 ka BP) (Figure 1). Wetter periods coincided with the NH cold events (i.e., HS1 and YD) while drier periods indicated NH warm events (ED) and southern hemisphere (SH) cold events (ACR). Higher rainfall during HS1 and YD might be induced by the southward shift of the austral summer ITCZ while lower rainfall during ED and ACR were linked to the northward shift of the austral summer ITCZ
(Figure 2). The NH cooling (during HS1 and YD) enhanced the boreal winter cold surges, which pushed the ITCZ southward
[1]
. This mechanism also resulted in a southward shift of the boreal summer ITCZ, indicated by the lower EASM rainfall
. The lower rainfall during ACR could be explained by its co-occurrence with B-A
[5]
hence the northward shift of austral summer ITCZ was most likely linked to the NH warming (during ED and B-A)
. The Last Deglaciation rainfall records of ST08 were similar to d
18
O records from Bali Gown Cave (Northwestern Australia)
[7]
(Figure 1), indicating the southern boundary of the austral summer ITCZ during wetter periods (HS1 and YD)
[1]
(Figure 2). On the other hand, D
18
O (d
18
O
Gs.ruber
−d
18
O
G.bulloides
) records (which indicated AIWM changes) from off south Java
[1]
and terrigenous input proxies (ln Ti/Ca) from off southwest Java
[8]
showed the opposite rainfall changes during the ACR (Figure 1). This indicated that the records from Java region responded to B-A instead, so they hinted a considerable influence of NH cross-equatorial moisture transport and the southern boundary of the austral summer ITCZ
[8]
(Figure 2).
Figure 1.
Drier (yellow highlight) and wetter (green highlight) periods inferred from terrigenous input proxies (
a
,
b
)
[9]
. Other paleoclimate records are presented for comparison: (
c
). Terrigenous input proxy (ln Ti/Ca) record of
GeoB10043-3
[8]
, (
d
). Terrigenous input proxy (ln Ti/Ca) record of
GeoB10065-7
[10]
, (
e
). d
18
O record of stalagmites from Bali Gown cave (Northwestern Australia)
[7]
, (
f
). d
18
O
Globigerinides
(
Gs.
)
ruber
–d
18
O
Globigerina
(
G.
)
bulloides
(D
18
O) record of GeoB10053-7
[11]
, (
g
). C
30 n
-alkanoic fatty acids d
13
C (d
13
C
FA
) record of GeoB10069-3
[12]
, (
h
). d
18
O record of Antarctic (EPICA Dronning Maud Land/EDML) ice core
[13]
, (
i
). d
18
O record of Greenland (GISP2) ice core
[14]
, (
j
). Reconstructed relative sea level from d
18
O of Red sea benthic foraminifera
[15]
[16]
, (
k
). 10-years averaged reconstructed sunspot numbers
[17]
, and (
l
).
20° S Dec. (austral summer) insolation (red) and 30°
N Jun. (boreal summer) (blue)
[18]
. Data are plotted against the mean ages obtained from the age model. Black curves indicate smoothed values (exponential smoothing, df: damping factor).
Figure 2.
The southern limit of the austral summer ITCZ during the drier (lower rainfall) periods (red dashed line) and during the wetter (higher) rainfall periods (blue dashed line) as suggested by this study and other previous studies
. Numbers show the location of ST08 ((
a
), this study) and other studies used for comparison i.e., GeoB10043-3 (
b
)
[8]
, GeoB10053-7 (
c
)
[11]
, GeoB10065-7 (
d
)
[10]
, Liang Luar cave (Flores) (
e
)
[20]
, GeoB10069-3 (
f
)
[12]
, and Bali Gown cave (Northwestern Australia) (
g
)
[6]
.
The YD wetter period continued until Early Holocene (EH). The abrupt rainfall increase in EH, which was inferred in off southwest Java
[8]
, not detected on ST08. This could be linked to the relatively constant terrigenous input in off southwest Sumba due to the considerable distance from the recently drowned-Sunda Shelf, as opposed to off southwest Java
[8]
. δ
18
O records on off south Java, which changes closely followed boreal summer insolation, increased during YD–EH
[1]
(Figure 1). This indicated the simultaneous increase of AISM and AIWM, but the effect of the strengthening AIWM and lower austral summer insolation (which should result in AISM weakening) was most likely distressed by the enhanced moisture supply related to abrupt sea-level rise
(Figure 1).
During Mid-Holocene, the rainfall records of ST08 were similar to the ln Ti/Ca records from off northwest Sumba, which inferred drier (wetter) Mid (Late) Holocene
[10]
(Figure 1). Lower rainfall during the Mid Holocene (MH) (~7–3 ka BP) was most likely linked to the decrease in solar activity (hence lower sunspot numbers)
, which suppressed the effect of increasing austral summer insolation
. During the Late Holocene (LH) (after ~3 ka BP), an increase in austral summer insolation and solar activity resulted in the enhancement of rainfall
. The southern boundary of the austral summer ITCZ during MH was most likely located around its position during ED and ACR [1,19,20] and shifted southward during LH to around its position during HS1, YD, and EH
(Figure 2), but their relation to solar activity and orbitally-induced austral summer insolation is still not understood
[21]
. The carbon isotope composition of the C
30 n
-alkanoic fatty acids (d
13
C
FA
) records from the southwestern Savu Sea
[12]
showed contradictive rainfall records (Figure 1). This contradiction might be related to the differences in climate signals recorded on terrigenous input and d
13
C
FA
proxies. d
13
C
FA
reflects the dry season (AIWM) water stress connected to the amount of rainfall during AIWM (AIWM rainfall)
[12]
. We suggest a joint analysis of terrigenous input and d
13
C
FA
proxies from the same site in future studies to reconstruct the past changes of both AISM and AIWM rainfall, so more robust AIM rainfall records are produced.
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