Past dynamics of the Indonesian Throughflow

Jerry F. McManus and Joanne Muller, Geology and Geophysics Department, WHOI
John M. Toole, Physical Oceanography Department, WHOI

Recent studies have referred to the tropical latitudes as a driver for past climate cycles through changes in atmospheric and oceanic circulation patterns. Several have suggested that the tropical ocean may have played an important role in high-latitude warming during the past. Model simulations also support this connection, suggesting that variations in tropical Pacific sea surface temperatures linked to insolation, as the possible trigger. Given the obvious importance of the tropical region for past climate there is a great need for additional high-quality, high-resolution paleoclimate records. For this particular study we focus on paleo-reconstructions of the strength of  the Indonesian Throughflow (ITF).

In the modern ocean the ITF is a key component of the global thermohaline circulation (THC). It serves as an important conduit for the return flow of warm shallow waters that balances the spreading of deep waters that form at high latitudes. The ITF is the only tropical connection between major ocean basins, and serves to transport large amounts of heat from the Pacific to the Indian Ocean. In the modern day the ITF is believed to wield an influence on a number of climatic modes, including the El Niño Southern Oscillation (ENSO). Although the importance of the ITF is widely acknowledged for the tropical and global climate, we have only a modest understanding of  how it operated in the past.

During the Last Glacial Maximum (LGM) the Indonesian Seas experienced significant sea level regression with consequent changes in boundary conditions. Previous paleo-studies within the Indonesian Seas implied a weakening of the ITF during the LGM. These interpretations are unavoidably inconclusive, as they were focused on indirect measures, such as paleoproductivity and sedimentology, rather than more direct indicators of paleocirculation. We propose to apply the 231Pa/230Th paleocirculation tracer to document the ITF paleo history. With the addition of this kinematic proxy we will address the recent debate concerning past ITF strength. We will use the results to test this hypothesis: The strength of the Indonesian Throughflow was weaker during the Last Glacial Maximum than the present day.

We will generate a detailed down core 231Pa/230Th profile from the Flores Sea (VM33-80, 7° 51’ S, 123° E) during the last LGM, deglaciation, and Holocene (0-25 kyr). In addition to past ocean circulation there are a number of other factors, such as particle rain and productivity, that may affect the 231Pa/230Th record. Since this is the first application of a relatively new proxy to the Indonesian Seas it is imperative that we undertake detailed sedimentology, including sediment lithology, grain size, and mass flux in order to address these potentially complicating influences.

The paleocirculation record from the Flores Sea will be compared with records that show past ENSO, Western Pacific Warm Pool (WPWP) and Asian Monsoon dynamics, in order to explore the relationship between the ITF and these important phenomena. We will also compare  our down core record with other paleocirculation records from the North Atlantic to look into the role the ITF plays within the THC system. This will provide a low-latitude perspective on the THC.

Our record will provide first order insights into past dynamics of the ITF, centered on the LGM. This record will serve as the first step in a larger, multi-core project designed to constrain past variations in the entire ITF. With this information we hope to improve understanding of the role of the ITF in past climate change and the influence it exerts on present-day tropical and global climate.