Hydroclimatic variations in the Makassar Strait over the past 5000 years

Abstract:

The Indonesian Throughflow (ITF) is the only low-latitude connection between two oceans and an important part of the global conveyor belt influencing global climate. About 80% of the ITF flows through the Makassar Strait between the islands of Borneo and Sulawesi and can be modified by the complex climate system over the Maritime continent characterized by interacting climate phenomena like e.g. El Niño Southern Oscillation and the Australasian monsoon system. To assess changes in the hydroclimate of the Makassar Strait in relation to dominant climatic forcing over the past 5,000 years, sediment core SO 217-18517 (1°32.198' S 117°33.756' E, 698 m water depth) collected off the Mahakam Delta, eastern Borneo, was studied. We use shell Mg/Ca ratio in planktic foraminifera to reconstruct sea surface temperature (SST), sedimentation rates and Ti/Ca ratios to reconstruct changes in terrigenous runoff, and seawater δ¹⁸O (δ¹⁸O_sw) as a measure of past changes in sea surface salinity. Zr/Rb ratios are interpreted to indicate changes in grain size distribution.

SST shows small-scale variations around 28.5°C during the mid Holocene, a decreasing trend between 3,000 and 1,700 years BP, and thereafter minor variations around 27.5°C. Sedimentation rates were higher between 3,400 and 1,000 years BP. Runoff increased during the past 5,000 years while our data indicate no change in sea surface salinity from mid to late Holocene. Grain size decreased until 1,700 years BP, and remained stable thereafter towards the present.

The partly inconsistent timing and pattern of our data indicate the different degree to which our proxies are influenced by different forcings. We will explore the complex connection between local insolation and climate phenomena such as ENSO, dynamic and thermodynamic changes in ITCZ and monsoonal rainfall, their possible relation to high-latitude forcing including an (inter)hemispheric insolation gradient.