Time-series delta-O-18 and delta-C-13 records from cohabiting massive coral Porites australiensis and giant clam Tridacna gigas from the Great Barrier Reef of Australia, and from calcareous green algae in a core through modern Halimeda bioherm accreting in the eastern Java Sea, provide insights into the complex links between environmental factors and stable isotopes imprinted in these reef skeletal materials. The aragonitic coral and giant clam offer 20 years and 15 years of growth history, respectively. The giant clam yields mean delta-O-18 and delta-C-13 values of -0.5 +/- 0.5 parts-per-thousand and 2.2 +/- 0.2 parts-per-thousand (n = 67), which agree well with the predicted equilibrium values. The coral yields mean delta-O-18 and delta-C-13 values of -5.6 +/- 0.5 parts-per-thousand and -1.8 +/- 0.7 parts-per-thousand (n = 84), offering a striking example of kinetic and metabolic fractionation effects. Although both the coral and giant clam harbor symbionts and were exposed to a uniform ambient environment during their growth histories, their distinct isotopic compositions demonstrate dissimilar calcification pathways. The delta-O-18 records contain periodicities corresponding to the alternating annual density bands revealed by X-radiography and optical transmitted light. Attenuation of the delta-O-18 seasonal amplitudes occurring in the giant clam record 8 years after skeletal growth commenced is attributed to a changeover from fast to slow growth rates. Extreme seasonal delta-O-18 amplitudes of up to 2.2 parts-per-thousand discerned in both the coral and giant clam records exceed the equivalent seasonal temperature contrast in the reef environment, and are caused by the combined effect of rainfall and evaporation during the monsoon and dry seasons, respectively. Thus in addition of being useful temperature recorders, reef skeletal material of sufficient longevity, such as Porites and Tridacna, may also indicate rainfall variations. Changing growth rates, determined from the annual growth bands, may exert a primary control on the coral delta-C-13 record which shows a remarkable negative shift of 1.7 parts-per-thousand over its growth history, by comparison with only 0.15 parts-per-thousand negative shift in the contemporaneous giant clam record. Use of coral delta-C-13 records as proxies of fossil fuel CO2 uptake by the ocean must be regarded with caution. The delta-O-18 and delta-C-13 records from Halimeda are remarkably uniform over 1000 years of bioherm accretion history (delta-O-18 = -1.7 +/- 0.2 parts-per-thousand; delta-C-13 = 3.9 +/- 0.1 parts-per-thousand, n = 28), in spite of variable Mg-calcite cements present in the utricles. Most of the cement infilling is probably syndepositional, and both the Halimeda aragonite and the Mg-calcite cements containing 12.3 mole % MgCO3 are deposited in isotopic equilibrium. Therefore, in favorable circumstances these algal skeletal remains may act as the shallow water analogs of benthic foraminifera in deep sea sediments in recording ambient sea water isotopic composition and temperature.
