PALEOCLIMATE RECONSTRUCTIONS BY MULTIPROXY APPROACHES IN THE HALMAHERA SEA SINCE THE LATE PLEISTOCENE - HOLOCONE

• Main Author: GUSTIANTINI , LULI
• Format: Dissertations
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/28656
• Institution: Institut Teknologi Bandung

Indonesian Archipelago or the maritime continent geographically is situated along the equator, within the Intertropical Convergence Zone (ITCZ) displacement, and acts as the main source of convection. It is within the Asia-Australia monsoon region and the Indo-Pacific Warm Pool, as a result rainfall in Indonesia is strongly influenced by the interaction between monsoon and Pacific dynamics. Oceanic circulation also gives a great impact to the climate variability in Indonesia, given the only seaway in the low latitude that passing the branch of thermohaline circulation (known as Indonesian Through flow, ITF), connecting water from the Pacific Ocean to the Indian Ocean. <br /> <br /> Halmahera Sea is in the eastern part of Indonesian Archipelago that is classified as an anti-monsoonal region because monsoon circulation in this zone is interrupted by ENSO and ocean circulation. Halmahera Sea plays a role as the eastern path of Indonesian Throughflow, which renewed by the South Pacific Subtropical Water (SPSW) in the thermocline layer and Antarctic Intermediate Water (AAIW) at the deeper layer. The mass water is carried by South Equatorial Current (SEC) that has been transformed into New Guinea Coastal Current (NGCC) and New Guinea Coastal Undercurrent (NGCUC). <br /> <br /> In order to determine the Halmahera Sea paleoclimate, multi proxy analyses have been applied to MD10-3339, a 39.8m marine sediment core, retrieved from 00°26.67'S, 128°50.33' E in 1919m water depth. Multi-proxies analyses have been conducted including geochemistry of foraminiferal shells (oxygen isotope, carbon isotope and Mg/Ca composition) from Globigerinoides ruber and Pulleniatina obliqueloculata, sediment elemental composition by scanned XRF analysis, magnetic properties conducted by continuous measurement on U-channel and low field Anisotropy Magnetic Susceptibility, inorganic grain meansize of sortable silt, and microfaunal assemblage (foraminifera and cocolith). <br /> <br /> According to G. ruber 14C radiocarbon AMS dating, MD10-3339 is spanned to the last 65 ka. Sea surface temperature (SST) reconstruction indicates glacial-interglacial oscillation with 29.5ºC is the Holocene SST average, and 26.6º C is the average of glacial SST. This suggests a close link between low latitude and high latitude. The SST of LGM is 26.3º C, giving a 3.2ºC different between the SST of Holocene and LGM. The SST record is dominated by precession and modulated obliquity scale control, suggests that variability in the Halmahera Sea is not just a response to the northern hemisphere, but also an impact of the regional variability such as monsoon and ENSO. Local dynamics also play a role and regulate climate condition in the Halmahera Sea. <br /> <br /> A glacial period is also indicated by high nutrient content compared to that during interglacial. High nutrient content during glacial might be due to increase productivity when thermocline shoaled, initiated by the weaken stratification in the Western Pacific during the SST decrease relative to the intermediate water temperature. Due to low SST, the precipitation and freshwater inputs are reduced. High nutrient content might be also related to the inflow of cool and rich-nutrient thermocline water from the West Pacific. <br /> <br /> Near bottom current in the Halmahera Sea is identified in a persistently SSW– NNE preferential current alignment. The intensity of the bottom current is relatively small variation during glacial, in general, it is stronger compared to that during interglacial. This is either due to the increase erosion strength that transport more and coarser material from exposed shelves during sea level stand, or it might related to the increase AAIW production in the Southern Ocean, and possibly supported by the intensification of NGCUC. <br /> <br /> Unlike the SST record, precipitation in the Halmahera Sea does not demonstrate significant glacial – interglacial oscillation and more dominated by modulation precession ~33 ka and semi precession ~16 ka scales. This suggests that regional impact such as monsoon and ENSO play a role driving the Halmahera climate. Close correlation to the SON insolation that is similar to other WPWP area corroborates the occurrence of ENSO influence. Local forces might influence the climate in this area including tidal mixing. <br /> <br /> Paleoclimatic changes in the Halmahera Sea can be resumed as: (64-34 ka), characterized by rapid millennial variability, demonstrate alternated warm and cold period, most related to the changes in Asian Winter monsoon intensity; 34 – 22 ka, characterized by relatively warmer SST, higher precipitation and terrigenous input, freshener sea surface salinity, and higher nutrient content. It most related to the more frequent La Niña-like condition, due to the westward replacement of the warm pool. The possibly water mixing process revealed; LGM (15-22 ka), characterized by cooler SST, lower precipitation, more saline SSS, increase wind strength lead to increase terrigenous input, more intense upwelling, high nutrient content, and water mixing. Possibly due to the asymmetric east-west heating that leads to decrease east-west pressure gradient, weakening the trade wind, more frequent of El Niño like condition; 13.5-11 ka, indicated by abruptly decrease terrigenous input and slightly decrease SST, suggests cool and dry condition, driven by intensification of Asian winter monsoon. This event is coherent with YD in the Northern Hemisphere; 8.2 ka, indicated by abruptly decrease terrigenous input and slightly SST decrease, suggests cool and dry condition. This event is coherent with 8.2 ka event that related to the North Atlantic freshening; Holocene, characterized by warmer SST, higher precipitation and terrigenous input, freshener salinity, and lower nutrient content. <br /> <br /> Final of this study has arrived at the conclusion that Halmahera Sea is sensitive to the global climate changes and regional dynamics such as ENSO and monsoon circulation. The climatic pattern in Halmahera is particularly influenced by the changes in global ice volume reflected in glacial-interglacial oscillation, oceanic circulation, regional forces ENSO and monsoon, and local dynamics such as tidal mixing.