Assessing Surface Ocean Variability in the Western Pacific Warm Pool Using Nitrogen Isotopes of Sedimentary Gravity Cores From Kau Bay, Indonesia

Kau Bay (1°N, 128°E) is located in eastern Indonesia between the northern arms of the island of Halmahera. The island is situated within the Western Pacific Warm Pool (WPWP). The bay is shallow (max. depth = 470m, sill depth = 40m) and has low dissolved oxygen levels in bottom waters with high levels of productivity and high and variable deposition rates. Previous oceanographic studies in Kau Bay determined that it was periodically flushed with oxygen-rich surface water from the Pacific Ocean in the last 60-80 years and the bottom waters changed from anoxic to dysaerobic. During the last century, the flushing of the basin was determined by the temperature and salinity of open WPWP waters just outside the basin. Flushing occurred during times of maximum salinity in October-November. Previous studies of sediment cores from Kau Bay reveal a complex sediment record with high deposition rates and continuous undisturbed sediments only deposited in certain areas of the basin. In an attempt to examine the late Holocene ventilation history of the basin we collected gravity- and multi-cores from Kau Bay during the summer of 2003. Four of these cores were analyzed for δ15N of the organic fraction at 8 cm intervals and pteropod rich sections of the cores were 14C dated to begin the reconstruction of the ventilation history of the basin. Previous studies of sedimentary δ15Nbulk sediment have shown that in basins with high productivity, such as Kau Bay, higher δ15Nbulk sediment reflects periods when oxygen levels were sufficiently low in the water column to allow denitrification to take place. When oxygen-rich waters flush the bay through its shallow sill, oxygen levels become high enough to squelch water column denitrification and δ15Nbulk sediment decreases. Flushing occurred during times of maximum salinity in October-November. Our best cores reveal that the basin was relatively well ventilated from 3Ka to ~2Ka and became progressively more oxygen poor until ~700 years B.P. Denitrification rates have decreased (and δ15N as decreased) since 700 B.P. to relatively more oxygenated conditions today. We are currently analyzing pteropod and benthic foraminifera δ18O to refine our interpretation of the δ15Nbulk sediment records.