Advances in GDGT research in Chinese Marginal Seas: A review

Archaeal and bacterial glycerol dialkyl glycerol tetraethers(GDGTs) play a unique role in discerning the biogeochemical processes and climate change in terrestrial(e.g. soil, peat, stalagmites, lakes, rivers, hot springs) and marine environments. Organic proxies based on GDGTs(e.g. TEX_(86), MBT/CBT and BIT) have made impressive applications in the open ocean and terrestrial environments. However, the applicability of these proxies in marginal seas has not been thoroughly evaluated, which is necessary given the complexity and dynamics of these systems, such as organic matter(OM) flux, hydrodynamic conditions, and human impact. This review aims to summarize recent studies of GDGTs and GDGT-based proxies in the Chinese marginal seas(CMS), which are characterized by diverse gradient in terrigenous supplies and ocean productivity and hold rich information on climate and sea level changes, ocean current dynamics, sedimentary evolution and biogeochemical processes.

Evolution of the East China Sea sedimentary environment in the past 14 kyr: Insights from tetraethers-based proxies

We reconstruct the environmental evolution of the East China Sea in the past 14 kyr based on glycerol dialkyl glycerol tetraethers(GDGTs) in a sediment core from the subaqueous Yangtze River Delta. Two primary phases are recognized. Phase I(13.8–8 cal kyr BP) reflects a predominantly continental influence, showing distinctly higher concentrations of branched GDGTs(averaged 143 ng/g dry sediment weight, dsw) than isoprenoid GDGTs(averaged 36 ng/g dsw), high BIT index(branched vs. isoprenoid tetraethers) values(>0.78) and a fluctuating GDGT-0/crenarchaeol ratio(R_(0/5), varied from 0.52 to 3.81). Within this interval, temporal increases of terrestrial and marine influence are attributed to Younger Dryas(YD)(ca. 12.9–12.2 cal kyr BP) cold event and melt-water pulse(MWP)-1B(11.5–11.1 cal kyr BP), respectively. The prominent transition from 8 to 7.9 cal kyr BP shows a sharp decrease in BIT index value(<0.4) and increase in crenarchaeol, which marks the beginning of phase II. Afterwards, the proxies remain relatively constant, which indicates that phase II(7.9 cal kyr BP-present) is a shelf sedimentary environment with high stand of sea level. Overall, the BIT index in our record serves as a good marker for terrestrial influence at the site, and likely reflects the flooding history of the region. The TEX_(86)(Tetra Ether Index of tetraethers consisting of 86 carbons) proxy is not applicable in phase I because of an excess terrestrial influence; but it seems to be valid for revealing the annual SST in phase II(21.6±0.9°C, n=49). In contrast, the MBT'/CBT(Methylation of Branched Tetraethers and Cyclization of Branched Tetraethers) proxy appears to faithfully record the annual mean air temperature(MAT)(14.3±0.63°C, n=68) and presents an integrated signal over the middle and lower Yangtze River drainage basin.

Prediction of Cold Dwell-Fatigue Crack Growth of Titanium Alloys

The dwell effect of the material can reduce the fatigue lives of titanium alloys at room temperature. A unified fatigue life prediction method developed by the authors＇ group is modified in this paper to predict dwell-fatigue crack growth taking into account the effects of dwell time and maximum stress. The modified model can be successfully used to predict the crack growth rate and calculate the fatigue life of different titanium alloys under pure fatigue and dwell-fatigue conditions. It is validated by comparing prediction results with the experimental data of several titanium alloys with different microstructures, dwell time, hydrogen contents, stress ratios and stress levels.