Stable isotope geochemistry of a late Quaternary coral reef sequence, New Guinea : application of high resolution data to palaeoclimatology

Abstract

Using physiographic evidence, geologists have endeavoured for a long time to employ fossil coral reefs as ancient sea-level gauges. A further step in this direction has been the production of a numerical chronology of sea-level changes from reefs, through radiometric dating of corals and molluscs that preserve their original mineralogy. Recent oxygen and carbon isotope abundance studies in reef material from a sequence of uplifted late Quaternary coral reefs on the Huon Peninsula, New Guinea, reported in this study, now provide a record of changes in ocean chemistry and temperature. The modern and ancient coral reef environments are isotopically monitored by the giant clam Tridacna gigas which is associated with the coral reefs. 18o;16o measurements in the exoskeleton of the giant clams constitute a record of combined temperature and shifts in the oceanic oxygen isotope composition induced by Pleistocene ice volume changes. Since variations in ice volumes are established by an independent line of evidence from the same site (i.e. stratigraphic sea-level data), the temperature 18 factor alone can be isolated from the 80 record. Apart from detecting a rapid ocean temperature fluctuation late during the last interglacial phase, the palaeotemperature estimates suggest near-interglacial conditions following each of the early Wisconsinan glacial stages, and general decline of the ocean temperature late during the last ice-age stages. The isotopic data also indicate a slow recovery of the ocean temperature to climatic optimum levels during the mid-Holocene. The fluctuations in surface ocean temperature estimated from the coral reef record conflict in magnitude with the palaeotemperature record obtained from deep-sea cores in the west equatorial Pacific. However, they support the cooling at sea-level inferred from the Late Pleistocene lowering of the snow and tree line in the New Guinea Highlands reported by Bowler et al.(1976) Climatic variations during the last 5 years at high latitudes, indirectly inferred from the Huon Peninsula sea-level data, when coupled with tropical ocean temperatures recorded by the u18o data, provide the essential definition of a sequence of events bearing on ice-age initiation, evolution and termination, and allow a critical evaluation of various models of ice ages. Recorded ∂13c changes in the coral reefs from New Guinea indicate variations in the carbon cycle over the last 5 years. In addition, the 18 distinct positive correlation observed between ∂13c and ∂18o records implies a coupling between sea water carbon cycle and sea-level (or climate) itself. It is concluded that only the formation and destruction of a reservoir of organic-rich shelf sediment caused by climatically-induced sea-level oscillations (Niitsuma-Broecker model) are consistent with the data presented. Marked skeletal growth bands in Tridacna allow detailed investigation of seasonal variations in modern and past marine environments by isotopic measurements. In addition to identifying the last interglacial stage (reef VIIa) as probably the wettest period, the variations observed in seasonal isotopic range indicate that important changes occurred in the oceanic and atmospheric circulation late during the last ice-age stages, implied by reduced seasonality. The separation of local and regional dynamic effects from longer-term global changes is an important problem in the coral-reef palaeoclimatic record. To verify the conclusions from the Huon Peninsula reefs, it is vital to extend the same analytical techniques to other reef sites in the tropical Pacific. Show more