Previous studies suggested the multi-millennial scale changes of Australian-Indonesian monsoon (AIM) rainfall, but little is known about their mechanism. Here, AIM rainfall changes since the Last Deglaciation (~18 ka BP) are inferred from geochemical elemental ratios (terrigenous input) and palynological proxies (pollen and spores). Pollen and spores indicate drier Last Deglaciation (before ~11 ka BP) and wetter Holocene climates (after ~11 ka BP). Terrigenous input proxies infer three drier periods (i.e., before ~17, ~15–13.5, and 7–3 ka BP) and three wetter periods (i.e., ~17–15, ~13.5–7, and after ~3 ka BP) which represent the Australian-Indonesian summer monsoon (AISM) rainfall changes. Pollen and spores were highly responsive to temperature changes and showed less sensitivity to rainfall changes due to their wider source area, indicating their incompatibility as rainfall proxy. 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.

No abstract

Sari-Foraminifera merupakan mikrofauna/mikrofosil yang sering diaplikasikan dalam geologi terutama untuk analisis biostratigrafi dan paleoekologi. Wilayah lepas pantai barat daya Sumba sangat dipengaruhi oleh pergantian muson barat lautmuson tenggara dan berada di dekat salah satu jalur keluar ITF yaitu Selat Ombai-Laut Sawu yang akirnya berperan besar dalam membentuk kondisi ekologinya. Penelitian ini bertujuan untuk membuat biozonasi dan analisis salah satu parameter ekologi yaitu kedalaman termoklin di wilayah lepas pantai barat daya Sumba berdasarkan kumpulan foraminifera planktonik. Inti sedimen laut dalam sepanjang 235 cm dengan kode S8 akan digunakan sebagai bahan penelitian..Determinasi foraminifera dilakukan secara kuantitatif dengan mengamati kirakira 300 spesimen dalam satu bagian sampel. Masing-masing takson yang ditemukan pada bagian tersebut dihitung jumlah individunya sedangkan taksa berbeda yang ditemukan di luar bagian tersebut dianggap berjumlah satu. Biozonasi dibuat berdasarkan biozonasi Blow serta biozonasi Bolli dan Saunders. Berdasarkan biozonasi Blow diperoleh satu zona yaitu zona N23 yang setara dengan Pleistosen-Holosen. Sementara berdasarkan biozonasi Bolli dan Saunders diperoleh dua zona yaitu zona Globigerinella calida-Clavarotella bermudezi (di bawah kedalaman 147 cm) yang setara Pleistosen Akhir dan zona Globorotalia fimbriata (kedalaman 0-147 cm) yang setara Holosen. Analisis paleoekologi difokuskan pada parameter kedalaman termoklin yang ditentukan dari kelimpahan relatif taksa thermocline dweller yang terdiri dari Neogloboquadrina dutertrei, Puleniatina obliqueloculata, dan Globorotalia menardii. Selama Pleistosen kelimpahan thermocline dweller relatif lebih tinggi (49,76-75,66%, rata-rata 60,01%) yang mengindikasikan pendangkalan termoklin (kondisi eutropik) sedangkan selama Holosen kelimpahannya lebih rendah (33,90-57, 17%, rata-rata 45,77%) yang mengindikasikan mendalamnya termoklin (kondisi lebih oligotropik). Kondisi tersebut berkaitan dengan dominasi muson tenggara (ITF menguat) selama Pleistosen yang kemudian melemah pada Holosen dan memicu penguatan muson barat laut (ITF melemah).

Changes of the thermocline depth (DOT) at the Sumba Island offshore are not well-known compared to the DOT changes in the Timor Sea, the main exit passage of the Indonesian Through-flow (ITF). Planktonic foraminiferal assemblages in cores collected from the southwest Sumba offshore (ST08) and Sumba Strait (ST12, ST13, and ST14) were used as a tool to infer the DOT and paleoproductivity changes at the Sumba Island offshore. The DOT changes were indicated from the thermocline and mixed layer dwellers’ relative abundance while the paleoproductivity changes were indicated from the relative abundance of Neogloboquadrina dutertrei. This study suggests a contrast between the DOT pattern at the Sumba Island offshore and the DOT pattern in the Timor Sea during the Last Deglaciation–Holocene. The contrast DOT pattern indicated that the multi-millennial changes of the Australian-Indonesian monsoon (AIM) during the Last Deglaciation– Holocene were the main factors behind the DOT changes in this region while the effects of El Niño Southern Oscillation (ENSO) –like, Indian Ocean Dipole (IOD) –like, and ITF to DOT changes were minimal. Paleoproductivity enhancement at the Sumba Island offshore was not solely related to the monsoon-driven coastal upwelling intensification, which resulted in the DOT shoaling and eutrophic condition. The increase of nutrient availability in surface water due to river runoff increase and changes in the lifted water mass nature were also able to enhance productivity in this region.