Occurrence and distribution of hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers(OH-GDGTs) in the Han River system, South Korea

We investigated the occurrence and distribution of terrestrial-derived hydroxylated isoprenoid glycerol dialkyl glycerol tetraethers(OH-GDGTs) in the Han River system and their potential impact on the application of the ring index of OH-GDGTs(RI-OH) as a sea surface temperature(SST) proxy in the eastern Yellow Sea. Thereby, we analyzed various samples collected along the Han River and from its surrounding areas(South Korea, n = 34). The OHGDGTs were found in all samples investigated. OH-GDGT-0 was the dominant OH-GDGT component in the estuary and marine samples while OH-GDGT-2 was generally dominant in the soils, the lake sediments and the river suspended particulate matter(SPM). Our results thus suggests a possible warm bias of the RI-OH-derived summer SSTs in the coastal zone to which a large amount of terrestrial organic matter is being supplied. Further studies are necessary to better assess the applicability of the RI-OH proxy in the eastern Yellow Sea.

Paleoaltimetry proxies based on bacterial branched tetraether membrane lipids in soils

The MBT/CBT （Methylation Index of Branched Tetraethers/Cyclisation ratio of Branched Tetraether） proxy, a terrestrial paleothermometer based on bacterial branched glycerol dialkyl glycerol tetraethers CoGDGTs）, was employed to indicate altimetry; however, the mechanistic control on this proxy is still ambiguous. Here, we investigated the bGDGTs＇ distribution and associated environmental factors along an altitude transect of Mt. Shennongjia in China in order to determine the applicability of bGDGT-based proxies to altimetry reconstruction. The MBT index exhibits only a weak correlation with estimated mean annual air temperature （MATe, estimated according to the meteorological record and lapse rate） or altitude. Likewise, MBT shows weak or no relationship with temperature or altitude at four other mountains （Mts. Meghalaya, Jianfengling, Gongga, and Rungwe）. It is notable that mean annual air temperature （MAT） or altitude estimated by the MBT/CBT proxy largely relies on CBT, rather than on MBT, which was generally acknowledged. The poor relationship between MBT and MATe for Mt. Shennongjia can be ascribed to the insensitive response of bGDGT-I to temperature. Our data fxom this mountain imply that care should be taken if the MBT/CBT proxy is employed as an indication of paleoaltimetry. We propose that the fractional abundance of bGDGTs may be a better paleoaltimeter than the MBT/ CBT proxy, because specific bGDGT subsets that might show the most sensitive response to temperature can be preferentially selected using a statistical method and used to establish local calibration. This local calibration was applied to Mt. Shennongjia and apparently improves the accuracy of temperature and altimetry reconstruction. The differential response of bGDGTs to temperature among mountains suggests that local calibrations are needed to better constrain the altimetry.

Flooding impact on the distribution of microbial tetraether lipids in paddy rice soil in China

Isoprenoid and branched glycerol dialkyl glycerol tetraethers （GDGTs） lipids were studied in flooded and non-flooded paddy soil in Wuhan, central China, to examine the response of the GDGTs distribution to the soil flooding. Samples were collected before and after the soil flooding in four specific months. Both core （CL） and intact polar （IPL） GDGTs were quantified. Increase in the abundance of archaeol and caldarchaeol may be indicative of the occurrence of methanogens in the flooded soil. A negative correlation was observed between the ratio of IPL branched GDGT-IIa to GDGT-Ia and the soil pH. The rise of the soil pH in the acid soil is known to be controlled by the redox conditions resulting from flooding. Thus, the branched GDGTs distribution may be controlled by the water content in the paddy soil. In addition, we suggest that the anoxic conditions resulting from flooding may also control the abundance of branched GDGTs relative to crenarchaeol, which in turn results in the increase of branched and isoprenoidal tetraethers （BIT） values, the index for the terrestrial input to the marine sediments.