Oxygen and carbon isotopic composition of Recent larger and smaller foraminifera from the Madang Lagoon (Papua New Guinea)

Abstract

Tests of Recent benthic foraminifera from the Madang Lagoon (Papua New Guinea) were analyzed for their oxygen and carbon isotopic composition. Analyses were performed on scuba collected species from backreef and low-oxygenated harbor environments covering a depth range of 0 to 45 m. Both rotaliid and miliolid symbiont-bearing species were found to exhibit increasing δ18O with depth, while only the rotaliids reveal a comparable trend in their δ13C composition. The general trend of depletion in the heavier isotopes, 13C and 18O appears to be depth- and light-dependant. Neither the symbiont type nor the nature of the microhabitat appear to affect the magnitude of 18O16O fractionation in larger foraminifers from this environment.The carbon isotope patterns of symbiont-bearing species revealed distinct differences between rotaliids and miliolids. Consistently lower δ13C values in rotaliid foraminifers may be related to a higher rate of photosynthetic activity. The photosynthetic effect of carbon isotope fractionation may be overprinted by the presence/absence of an internal carbon pool and/or specific modes of biomineralization which may act as isotopic filters.Foraminifera collected from low-oxygenated, organic-rich sediments in harbor inlets at Madang were found to exhibit particularly low δ13C values. Depletion (δ13C) is suggested to be due to oxidative processes in the surrounding organic-rich sediment coupled with preferential incorporation of isotopically lighter 12C.