Glycerol dialkyl glycerol tetraethers in the Ohno River estuary, Japan during the Holocene

Abstract

We investigated glycerol dialkyl glycerol tetraethers (GDGTs) in sediments taken from a 97 m long borehole core in the estuary of the Ohno River, Kyushu, Japan. Our aim was to understand the influence of estuary-produced branched GDGTs on MBT′/CBT indices and the terrestrial contribution on TEX86 in estuary sediments by comparing with soils in its drainage basin and offshore marine sediments.In borehole sediments covering the last 8000 years, TEX86 and MBT′/CBT records are inconsistent with other paleoclimate records from western and central Japan, implying that the application of TEX86 and MBT′/CBT as paleotemperature proxies to this sample set was unsuccessful. The isoprenoid GDGT compositions of the study site were closer to those of the surface soils in the Ohno River drainage area rather than those in Beppu Bay sediments. TEX86 reflects a mixing ratio of estuary and soil GDGTs. In contrast to isoprenoid GDGTs, the branched GDGT compositions were closer to those of Beppu Bay sediments rather than those of surface soils, implying that the branched GDGTs were mainly of estuary origin.Variation in BIT shows a consistent pattern with that of water depth reconstructed from a molluskan fossil record. The input of terrestrial labile organic matter could contribute to the production of branched GDGTs. In turn, the BIT indirectly reflects the contribution of terrestrial organic matter.