Elsevier

Quaternary Science Reviews

Volume 93, 1 June 2014, Pages 142-154
Quaternary Science Reviews

Mid- to Late-Holocene Australian–Indonesian summer monsoon variability

https://doi.org/10.1016/j.quascirev.2014.04.006Get rights and content

Highlights

  • Australian–Indonesian summer monsoon (AISM) rainfall changes over the past ∼6000 years.

  • Abrupt increase in rainfall at ∼2800 yr BP coincides with a grand solar minimum.

  • Increased rainfall in response to a solar minimum is consistent with climate model simulations.

  • Combined effect of orbital and solar forcing on AISM rainfall.

  • No significant control by long-term variations in ENSO.

Abstract

The Australian–Indonesian monsoon has a governing influence on the agricultural practices and livelihood in the highly populated islands of Indonesia. However, little is known about the factors that have influenced past monsoon activity in southern Indonesia. Here, we present a ∼6000 years high-resolution record of Australian-Indonesian summer monsoon (AISM) rainfall variations based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). The record suggests lower riverine detrital supply and hence weaker AISM rainfall between 6000 yr BP and ∼3000 yr BP compared to the Late Holocene. We find a distinct shift in terrigenous sediment supply at around 2800 yr BP indicating a reorganization of the AISM from a drier Mid Holocene to a wetter Late Holocene in southern Indonesia. The abrupt increase in rainfall at around 2800 yr BP coincides with a grand solar minimum. An increase in southern Indonesian rainfall in response to a solar minimum is consistent with climate model simulations that provide a possible explanation of the underlying mechanism responsible for the monsoonal shift. We conclude that variations in solar activity play a significant role in monsoonal rainfall variability at multi-decadal and longer timescales. The combined effect of orbital and solar forcing explains important details in the temporal evolution of AISM rainfall during the last 6000 years. By contrast, we find neither evidence for volcanic forcing of AISM variability nor for a control by long-term variations in the El Niño-Southern Oscillation (ENSO).

Introduction

The Asian and Australasian monsoons affect roughly half the world's population, which largely depends on the monsoonal climate for their food and livelihood (Webster et al., 1998, Christensen et al., 2007). Despite the importance to so many, the monsoon is difficult to predict and model, making its future development in a changing global climate uncertain (Webster et al., 1998). Hence, it is vital to reconstruct monsoon variations beyond the instrumental record in order to improve our understanding of the mechanisms that may act on monsoonal rainfall variability. Most proxy evidence on multi-decadal to multi-millennial scale changes in monsoonal rainfall intensity during the Holocene has been deduced from continental and marine archives located in the Indian and East Asian monsoon domains. These studies on the Indian (Fleitmann et al., 2003, Gupta et al., 2005) and the East Asian (Wang et al., 2005, Zhang et al., 2008) summer monsoon rainfall indicate a strong response to orbital and solar forcing during the Holocene. In contrast to its Northern Hemisphere counterparts, very little is known about the development of the Australian-Indonesian summer monsoon (AISM) rainfall during the Holocene.

With respect to the AISM rainfall during the Holocene, Australian proxy records suggest wetter conditions during the Early and Mid Holocene compared to present (Nott and Price, 1994, Magee et al., 2004) which has been explained by a Northern Hemisphere insolation control (Magee et al., 2004, Miller et al., 2005), a response to regional sea-surface temperatures (Liu et al., 2003) or human-induced changes in vegetation cover during the Late Holocene (Miller et al., 2005). In contrast, two recent studies from southern Indonesia indicate reduced rainfall during the Mid Holocene compared to the Late Holocene (Griffiths et al., 2009, Mohtadi et al., 2011), highlighting the complex and contrasting patterns of AISM development in southern Indonesia and northern Australia. Another study relates the changes in AISM rainfall during the past two millennia to shifts in the mean position of the Inter-Tropical Convergence Zone (ITCZ) forced by variations in Northern Hemisphere climate mean state (Tierney et al., 2010). A recent study from the Australian tropics suggests that El Niño-Southern Oscillation (ENSO) may have played a dominant role in driving AISM variability since the middle Holocene (Denniston et al., 2013). Taken together, mechanisms that influence the AISM during the Holocene are far from being understood because of the limited number of AISM records and the strong disagreement on the nature and causes of rainfall changes in the existing AISM records.

Here, we present a ∼6000 years high-resolution record of southern Indonesian rainfall based on bulk sediment element analysis in a sediment archive retrieved offshore northwest Sumba Island (Indonesia). This new record allows us to study the history of monsoonal rainfall in southern Indonesia at multi-decadal to multi-millennial time scales, and to explore the role of various potential forcing mechanisms in driving AISM rainfall through the Holocene.

Section snippets

Modern climate

At present, south and central Indonesia from south Sumatra to Timor Island, parts of Kalimantan, Sulawesi, and Irian Jaya as well as the northern portions of Australia experience a monsoonal climate, with the majority of the annual rainfall occurring in austral summer (December–March) when the northwest monsoon carries humid air and heavy rainfall as the ITCZ-related rainbelt migrates southward (Fig. 1a). During austral winter (June–September), the southeast monsoon winds are relatively cool

Strategy and proxy variables used for rainfall reconstruction

We use the logarithmic ratio between titanium (Ti) and calcium (Ca) as a proxy for riverine terrestrial input as in Mohtadi et al. (2011). The linkage between riverine detrital input and AISM rainfall has been demonstrated previously for the study area by Rixen et al. (2006). The supply of terrigenous material as monitored by the ratio between the lithogenic particles and calcium carbonate in a sediment trap off South Java (JAM; see Fig. 1a for location) shows pronounced seasonal variations

Core GeoB10065-7

Sediment core GeoB10065-7 (9°13.39′S; 118°53.58′E; 1296 m water depth; core length 9.75 m) was recovered from the eastern Lombok Basin – northwest off Sumba Island and southeast off Sumbawa Island (Indonesia), during the RV SONNE SO-184 “PABESIA” expedition in 2005 (Fig. 1a). Sedimentological description of the core revealed dark olive grey to olive grey nannofossil/diatom-bearing clay (Hebbeln et al., 2006). A turbidite layer has been identified between 37 and 40 cm depth in the core. Age

Age control

The three individual depth profiles of radionuclides of the two multi-cores at station GeoB10065-9 (henceforth MUC-A, and MUC-B) and the gravity core at station GeoB10065-7 were depth aligned based on minimizing the sum of squared residuals (Fig. 2; Pittauerova et al., 2009). It was estimated that MUC-A is shifted by 2 cm downwards relative to MUC-B and that there is 20 cm missing on top of gravity core GeoB10065-7 relative to MUC-B. 137Cs values were mostly below the decision threshold and the

Discussion

Our AISM record bears similarities with Holocene changes in rainfall as inferred from a ln-ratio of Ti/Ca of a core located south off Java (Mohtadi et al., 2011) and a cave stalagmite stable oxygen isotope record from Flores, Indonesia (Griffiths et al., 2009, Griffiths et al., 2010), which also show a change from a drier Mid Holocene to a wetter Late Holocene (Fig. 5c–d). The southern Indonesian proxy records for changes in AISM rainfall show a similar overall increase in AISM rainfall until

Conclusions

In the light of our new record and previously published data from the Northern Hemisphere Indian and East Asian monsoon (Fleitmann et al., 2003, Gupta et al., 2005, Wang et al., 2005, Zhang et al., 2008), a picture emerges suggesting that the Asian-Australian monsoon system as a whole is responding to variations in the solar output. The combined effect of orbital and solar forcing explains important details in the temporal behaviour of AISM rainfall over southern Indonesia during the last 6000

Acknowledgements

S. Steinke and M. Mohtadi acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG grants STE1044/4-1 and HE3412/15-1) and the German Bundesministerium für Bildung und Forschung (BMBF grant PABESIA). We thank J. Southon and D. Oppo for valuable comments on the age model and on an earlier draft of the manuscript, U. Röhl and V. Lukies for technical support, and K. B. Olafsdottir for statistical analyses. This research used data acquired at the XRF Core Scanner Lab at the MARUM

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