PLIOCENE DEEP OCEAN CIRCULATION IN THE SOUTHWEST PACIFIC

The circulation of water masses originating in high latitudes plays an important role in the global distribution of heat, salts, gasses, and nutrients. Deep western boundary currents (DWBC) play a fundamental role in transporting these water masses from higher latitudes, toward the equator. Currently, deep ocean water masses originate in the North Atlantic and around the Antarctic margin as North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) respectively. It has been considered that during periods of sustained warmth, such as the mid-Pliocene warm period (~3Ma), the production and export of NADW was increased. Alternatively, records show that AABW production was greater during times of enhanced Southern Hemisphere cooling such as the late Pliocene (3.3 to 2.6 Ma).

Here we examine the exact downstream response of high-latitude climate variability on oceanic overturning along the southwest Pacific DWBC at Ocean Drilling Program (ODP) Site 1123 during the mid-Pliocene to early Pleistocene. We present an integrated record of oxygen (δ18O) and carbon (δ13C) isotopes, inorganic geochemical measurements derived from x-ray fluorescence (XRF) core scanning data, a mass accumulation rate record of terrigenous vs. carbonate sedimentation, and a sortable silt record composed of particle size data from the 10–63μm grain size range of sediment samples. This data highlights a very important threshold not previously identified, such that a warming of >2℃ results in an overall more sluggish deep ocean circulation regime along the southwest Pacific DWBC compared to today.