パプアニューギニア, ヒュオン半島の更新世後期サンゴ礁段丘に関するαスペクトル^<230>Th/^<234>U年代の再評価 Re-evaluation of α-spectrometric^<230>Th/^<234>U Ages for Late Pleistocene Coral Reef Terraces of Huon Peninsula, Papua New Guinea
Total number of fifty-four Pleistocene corals collected during the 1988 and 1992 international expeditions, were dated by the α-spectrometric <SUP>230</SUP>Th/<SUP>234</SUP>U method, and the similar dates reported previously were critically reevaluated after the five-step screening by the following criteria : (1) The sample has no evidence of recrystallization of coral skeleton and/or cementation with the secondary calcite or aragonite ; (2) Skeletal textures have not changed through their diagenetic history ; (3) The sample should be free of the initial <SUP>230</SUP>Th ; (4) The uranium concentration should be the same (2 to 4 ppm) as those in the present-day counterparts ; and (5) The initial <SUP>234</SUP>U/<SUP>238</SUP>U activity ratio should be consistent with that in the present sea water, 1.144±0.002 (Chen <I>et al., </I> 1986). Most of the α-spectrometric dates evaluated to be fully reliable, were consistent with the TIMS (thermal ionization mass-spectrometer) <SUP>230</SUP>Th/<SUP>234</SUP>U date of the same sample.<BR>One of the most salient observations is that many corals occurring in the Pleistocene terraces of Huon Peninsula have not necessarily preserved the closed system for uranium isotopes, even though they are composed of 100% aragonite without any textural alteration of original skeleton. For the reason, only half of α-spectrometric <SUP>230</SUP>Th/<SUP>234</SUP>U dates were proven to be reliable in this study.<BR>On the basis of the precise α-spectrometric <SUP>230</SUP>Th/<SUP>234</SUP>U dating, ages of terraces correlated with oxygen isotope stage 3 (Shackleton and Opdyke, 1973) become a little older than the former assignment (<I>e.g., </I> Bloom <I>et al., </I> 1974). Terrace IV of Chappell (1974) was assigned to <I>ca</I>. 73 ka ; and Terrace IIIa, IIIa middle, IIIIa lower, IIIb, IIIc lower, to <I>ca</I>. 52 ka, ca. 51 ka, <I>ca</I>. 46 ka, <I>ca</I>. 45 ka, <I>ca</I>. 44 ka, respectively. Ages of Terrace II b (<I>ca</I>. 38 ka) and II c (<I>ca</I>. 33 ka) are newly revised to <I>ca</I>. 41 ka of Terrace II a. Such increase of the age-values for Terraces II to IV works to lower the paleo sealevel estimated by Chappell and Shackleton (1986) to that from the oxygen isotopic records (Shackleton, 1987). Three α-spectrometric and one TIMS <SUP>230</SUP>Th/<SUP>234</SUP>U dates corresponding to the last interglacial maximum were obtained from the basal part of coral limestone beneath Terrace VIb. This observation indicates decidedly that lower Pleistocene terraces such as Terraces VI to N are underlain by the limestone formed during the isotope stage 5e.
地學雜誌 104(5), 758-776, 1995-10-25
Tokyo Geographical Society