Toshio Kawana

Variations in Mg/Ca, Na/Ca, and Sr/Ca ratios of coral skeletons with chemical treatments: Implications for carbonate geochemistry

We made a systematic examination into the effects of chemical treatments on Mg/Ca, Na/Ca, and Sr/Ca ratios of coral skeletons. Five skeletal samples were cut from modern and fossil corals of the genus Porites. Each sample was ground into powder, and replicate subsamples were taken and split into four groups. One group was left untreated as the control group. The other three groups were treated cumulatively with distilled/deionized water (DDW), 30% H2O2, and weak HNO3, with one group separated after each treatment step. The H2O2 and HNO3 treatments incurred partial dissolution of the skeletal powder and thus resulted in considerable sample loss. All the groups were determined for the elemental ratios. The Mg/Ca and Na/Ca ratios showed decreases or little changes with the DDW treatment, and then increased with the H2O2 and HNO3 treatments. The Mg/Ca and Na/Ca variations were closely parallel throughout the treatment sequence. The Sr/Ca ratio showed slight or little variation throughout the treatment sequence. These results reflect fine-scale elemental distribution in the skeletons. The Mg/Ca and Na/Ca decreases with the DDW treatment can be ascribed to removal of adsorptive Mg and Na from the skeletal surface. The Mg/Ca and Na/Ca increases with the latter treatments reflect enrichments in Mg and Na at the innermost portion of the skeletons (i.e., around the center of calcification). The Mg/Na ratio of the adsorptive phase is approximately the same as that of the skeletons. The covariation of Mg and Na in the adsorptive and skeletal phases indicates that Mg and Na behave similarly both in adsorption onto the skeletal surface and in the skeletogenesis. Sulfate ion (SO42−) may participate in the Mg and Na behaviors. The Sr/Ca variation indicates that Sr is distributed almost homogeneously in the skeletons with little adsorptive fraction. Attention should be paid to the effects of chemical treatments associated with the fine-scale elemental heterogeneity, especially if coral Mg/Ca and Na/Ca ratios are used for paleoenvironmental analysis.

Mangrove habitat formation and response to Holocene sea-level changes on Kosrae Island, Micronesia

Mangrove habitats on Kosrae are divided into three types, i.e., an estuary or delta type, a backmarsh or lagoon type and a coral reef or tidal-flat type. Most of the mangrove forests of Kosrae have been developed during the last 2000 years by accumulating mangrove peat with the gradual sea-level rise of 1 to 2 mm/yr except the landward part of the estuary or delta type. On the other hand, during the period of rapid sea-level rise of about 10 mm/yr between 4100 and 3700 yr B.P., the mangrove forests ceased peat accumulation and retreated landward. Until 3500 yr B.P., mangrove forests were distributed only in narrow bands in the inlets. Therefore, the critical rate of mangrove peat accretion with sea-level rise is estimated at more than 2 mm/yr and less than 10 mm/yr. If the anticipated sea-level rise exceeds this critical rate, all of the mangrove forests of Kosrae will retreat landward and reduce rapidly.

Thermoluminescence dating of the Ryukyu Limestone

Abstract Thermoluminescence [TL] dating of calcite is useful for corals in the Pleistocene Ryukyu Limestone in Okinawa Island, in which no aragonite is found. Fossil calcite corals in the Ryukyu Limestone have a variety of induced TL glow curves with peaks at about 150, 210, 250 and 310°C. The TL growth of a group of them exhibits supralinear behavior, while that of another group exhibits saturation behavior. Almost all of the TL ages of the corals which show the supralinear behavior are roughly consistent with ages estimated from geology and uranium-series and ESR ages.