Kau Bay, Halmahera, a late quaternary palaeoenvironmental record of a poorly ventilated basin

Abstract

Kau Bay is a small marine basin in between the two northern arms of the Island of Halmahera (Indonesia), separated from the adjacent Philippine Sea by a sill of 40 m deep. Microfossil and radiocarbon data of piston cores indicate that Kau Bay was a freshwater lake in Weichselian times. At 10 ka BP, the Bay became reconnected with the open ocean. Present-day deep water in Kau Bay is poorly ventilated because the sill limits water exchange with the adjacent ocean. Ventilation occurs through occasional inflow of denser oceanic water, but when this fails oxygen depleted conditions rapidly arise. The permanent presence of oxygen tolerant deep water benthic foraminifers over the past 10 ka indicates that dysoxic conditions prevalied and that anoxic conditions, as observed during the Snellius-I Expedition (May 1930), apparently existed for a short time only and left no trace that could be resolved with our time-resolution of 35 years. Slight and long-term variations in the degree of oxygen deficiency most likely are caused by long-term changes in the density of inflowing oceanic water, possibly in consequence of climatic oscillations. The homothermal surface water of Kau Bay shelters a low-diversity planktonic foraminiferal fauna of primarily shallow-dwelling species. This specific association extends back over the past 8 ka indicating that surface waters remained homothermal over much of the Holocene. Surface water productivity most likely changed with variations in river discharge.