RESUMO
Abundant fossil specimens of Scaphechinus mirabilis, now occurring mostly in temperate waters, have been found in the Toukoshan Formation (Pleistocene) in Miaoli County, Taiwan. Environmental changes leading to its extirpation (local extinction) have thus far been elusive. Here, we reconstruct past environmental and oceanic conditions off northwest Taiwan by analyzing clumped isotopes, as well as stable oxygen isotopes, of well-preserved fossil echinoid tests collected from the Toukoshan Formation. Radiocarbon dates suggest that these samples are from Marine Isotope Stage 3 (MIS 3). Paleotemperature estimates based on clumped isotopes indicate that fossil echinoids were living in oceanic conditions that range from 9 to 14 °C on average, comparable with the estimate derived for a modern sample from Mutsu Bay, Japan. Notably, this temperature range is ~ 10 °C colder than today's conditions off northwest Taiwan. The substantially lower temperatures during ~ 30 ka (MIS 3) compared to the modern conditions might be due to the rerouting of surface currents off northwest Taiwan when the sea level was ~ 60 m lower than today, in addition to the cooling caused by a lower atmospheric CO2 level during the Last Glacial Period. Colder waters brought here by the China Coastal Current (CCC) and the existence of shallow subtidal zones termed "Miaoli Bay" (mainly located in the present-day Miaoli county) during MIS 3 plausibly sustained generations of S. mirabilis, yielding tens of thousands of fossil specimens in the well-preserved fossil beds. The likely extirpation driver is the drastic change from a temperate climate to much warmer conditions in the shallow sea during the Pleistocene-Holocene transition.
Assuntos
Mirabilis , Fósseis , Isótopos de Oxigênio , Taiwan , TemperaturaRESUMO
Humic acid (HA) has been shown to be a toxic factor for many mammalian cells, however the specific mechanism of the cytotoxicity induced by HA remains unclear. From the assessment of its redox properties, HA has been shown to be capable of reducing iron(III) to iron(II) in aqueous conditions over a broad range of pH values (from 4.0 to 9.0). By using thiobarbituric acid-reactive substances as an index, the presence of HA was noted to increase the extent of lipid peroxidation both for linoleic acids and within rat liver microsomes. In addition, the increase in HA-induced lipid peroxidation is partially inhibited by sodium azide (a singlet oxygen scavenger) or disodium 4,5-dihydroxy-1,3-benzene-disulfonic acid (a superoxide scavenger), reflecting the involvement of singlet oxygen and superoxide in the process of lipid peroxidation. The addition of HA into a reaction system has been shown to generate superoxide in a dose-dependent manner by the superoxide dismutase-inhibitable cytochrome c reduction assay. In addition, HA is able to reduce and release iron from ferritin, but this process is partially inhibited by superoxide scavengers. Subsequently, the iron released from ferritin was shown to accelerate the HA-induced lipid peroxidation. From our results we conclude that HA has the ability to reduce and release iron from ferritin storage as well as to promote lipid peroxidation. Therefore, HA coupled with released iron can disturb the redox balance and elicit oxidative stress within a biological system. This may be one of the most important mechanisms for HA-induced cytotoxicity.