•Up to 21 MAGE compounds were identified in soils, river and marine sediments.•Their ubiquitous presence suggests MAGEs not specific for sulfate reducing bacteria.•Terrestrial inputs of MAGEs are minor compared to in situ production in Bohai Sea.•The MAGEs distributional patterns varied with environmental conditions.The 1-O-monoalkyl glycerol ethers (MAGEs) are traditionally considered as biomarkers for sulfate-reducing bacteria (SRB), but recent studies suggest other biological sources are likely. In this study, we examined the MAGEs in the Yellow River and Bohai Sea. The results show the ubiquitous occurrence of MAGEs in soils, river and marine sediments with the mean concentration one order of magnitude higher in Bohai Sea sediments (91.8 μg/g OC) than in soils (9.8 μg/g OC) and river sediments (12.9 μg/g  OG). The distributional patterns of MAGEs differ between soils and aquatic environments with the maximum at i-C15:0 and n-C16:0, respectively. Comparison of the MAGEs and phospholipid fatty acids reveals that Gram-negative and Gram-positive bacteria and even fungi are all possible biological sources for MAGEs. The abundance ratio of (n-C16:0 + m-C17:0)/(i-C15:0 + n-C16:0 + m-C17:0) MAGEs gradually increases from soil, river, lake, marginal sea to open ocean, suggesting that the MAGEs can be used for discerning different environmental conditions.

Recently, 6-methyl branched glycerol dialkyl glycerol tetraethers (brGDGTs) were separated from 5-methyl brGDGTs, which are used in brGDGT-based proxies. Here we analyzed brGDGTs in 27 soil samples along the 400 mm isoline of mean annual precipitation in China by using tandem 2D liquid chromatography. The fractional abundance of 6-methyl brGDGTs showed a positive correlation with soil pH, while that of 5-methyl brGDGTs decreased with increasing soil pH. The abundance ratio of 6-/5-methyl brGDGTs, namely the isomerization of branched tetraethers (IBT), was calculated. The correlation of IBT with pH (pH = 6.33 − 1.28 × IBT; R2 0.89; root mean squared error, RMSE, 0.24) was much stronger than that of the traditionally used cyclization index of branched tetraethers (CBT) with pH (R2 0.52; RMSE 0.49) and comparable with that of CBT′ with pH (R2 0.88; RMSE 0.25). Compiling all available data from 319 soil samples resulted in a global calibration: pH = 6.53 − 1.55 × IBT (R2 0.72; RMSE 0.65), which has a better correlation than the CBT5ME-pH proxy (R2 0.63; RMSE 0.78), but a weaker correlation than the CBT′-pH proxy (R2 0.85; RMSE 0.52). Our result suggests that the IBT is a promising indicator for soil pH, particularly in cases when some compounds in the CBT′ index cannot be determined.

•Different sources for bGDGTs and iGDGTs in Yellow River dominated margin•bGDGT-temperature in Bohai Sea agrees with annual MAT of Chinese loess plateau.•The BIT index is controlled by aquatic inputs.•Crenarchaeol and other iGDGTs are influenced by both terrestrial and aquatic inputs.To assess the source of glycerol dialkyl glycerol tetraethers (GDGTs) and their usefulness as proxies for terrestrial organic matter inputs and temperature in the Yellow River-dominated margin, we measured isoprenoid and branched GDGT concentrations in surface sediments along a lower Yellow River-estuary–coast transect. Branched GDGTs dominated over isoprenoid GDGTs in the riverbed sediments and had similar compositions from river to coast. In contrast, isoprenoid GDGTs displayed an increasing abundance and a decreasing GDGT-0 to crenarchaeol ratio (1.6 to 0.6) toward the sea. Such distribution patterns of GDGTs, combined with the result from a principal component analysis (PCA), confirmed the different origin of branched and isoprenoid GDGTs with branched GDGTs being primarily from soil erosion of the Chinese loess plateau (CLP) whereas, in addition to allochthonous terrestrial inputs, aquatic Thaumarchaeota partially contributes to the isoprenoid GDGT pool in estuarine and coastal areas. The branched GDGT-derived temperature (avg. 11 °C) is consistent with the annual mean air temperature (MAT) of the CLP in the middle river basin, a major source region for the Yellow River sediments, whereas the isoprenoid-derived temperature (12.7 to 28.4 °C) deviated widely from the annual mean temperature in the study region. Application of a binary mixing model based on δ13C, the branched and isoprenoid tetraether (BIT) index and branched GDGT concentrations showed consistent decreases in the relative amount of terrestrial organic carbon toward the sea, but estimates from the latter two proxies were lower than those from the δ13C.

•Better preservation of aromatic and alkyl carbon than celluloses and carbohydrates.•CPI and ACL of n-alkanes remained relative constant despite 90% C degraded.•< 2‰ positive δ13C shift of long chain n-alkane despite 90% C degraded.A litterbag method was used for studying the variability in chemical and carbon isotopic compositions of four grasses during litter decomposition. After the 300 d degradation, > 90% of litter mass was lost for three C4 species (Setaria viridis, Eleusine indica, Amaranthus retroflexus) and one C3 species (Erigeron speciosus). The solid state 13C NMR spectra showed that mean proportion of aromatic and alkyl carbon increased from ca. 10% to 15% and ca. 10% to 20%, respectively, whereas that of O-alkyl carbon substantially decreased from ca. 70% to 50%. The carbon preference index and average chain length of n-alkanes remained relatively constant, whereas the carbon isotopic compositions of long chain n-alkanes varied < 2‰. Our results demonstrate that the degradation of litters alone does not significantly change the n-alkane chemical and carbon isotopic proxies. Compared to open plant–soil systems, our litterbag experiments present much less variability in chemical and carbon isotopic compositions of n-alkanes. Based on these facts, we recommend a combined measurement of chemical and carbon isotopic properties in evaluation of carbon sources, dynamics and paleoenvironments.

•Relatively high abundance of perylene detected in Okinawa Trough over the past 20 kyr.•Correlation of perylene was much stronger with long chain n-alkanes than alkenones.•Substantially different δ13C between perylene and long chain n-alkanes.•Perylene in Okinawa Trough is of a terrigenous origin likely from soil fungi.Perylene, a five-ring nuclear polycyclic aromatic hydrocarbon (PAH), has a different distribution pattern from anthropogenic PAHs. Its source, however, remains obscure, limiting its application as an environmental indicator. Here we examine perylene in the upper 85 m sediments of the ODP Hole 1202B from the southern Okinawa Trough. Over the past 20,000 years, the mass accumulation rate of perylene is substantially higher in the last deglaciation than the Holocene and last glacial maximum. Perylene presents a strong correlation with C31n-alkane (r = 0.55; p < 0.001), a biomarker for terrigenous organic matter, and a weak correlation with C37:2 alkenone (r = −0.04; p = 0.23), a biomarker for marine organic matter. The mean δ13C value of perylene (−22.8‰ ± 0.6‰) is substantially higher than that of long chain n-alkanes (−30.7 ± 0.4‰), but close to that of fluvial organic matter in adjacent areas (−24.0‰ to −25.0‰). Our results strongly suggest that the perylene is of a terrigenous origin likely from soil fungi, and may be a useful tracer for soil organic carbon in marine environments.

Bulk geochemical characterization (total organic carbon, grain size distribution, carbon isotope composition) and molecular biomarkers (lignin phenols, straight chain aliphatic hydrocarbons, glycerol dialkyl glycerol tetraethers) were analyzed for a 21 m core from the Bohai Sea (North China), spanning ca 21 ka BP. These paleo-proxies presented remarkable differences between the late glacial period and the Holocene, reflecting continental and coastal environments, respectively. Two peat layers were deposited during the period of ca 9000–8460 yr BP. Thereafter the core site has been consistently covered by seawater until recent reclamation of land from the sea. The occurrence of a total organic carbon maximum from ca 6000–3800 yr BP was attributed to delivery of organic carbon enriched sediments via the Yellow River, consistent with increased vegetation density and higher development of soil under warm and humid mid-Holocene climate conditions. The distributions of lignin phenol compositions and C31/C29n-alkane ratio suggested the largest expansion of woody plants between ca 5300 and 4000 yr BP, corresponding to the extremely favorable climatic conditions. Since ca 3800 yr BP, an abrupt increase in the C31/C29n-alkane ratio suggested higher abundance of grasses, consistent with a drying climate trend after the mid-Holocene. Since our coastal sediments close to the Yellow River outflow contain catchment-integrated environmental signals of the river basin, molecular proxies demonstrate that the variability of vegetation distributions in the Holocene is a widespread phenomenon in those areas adjacent to Yellow River Basin.

•High resolution sea surface temperature in Okinawa Trough was reconstructed for the past 15,000 years.•Abrupt climate changes such as the B/A warming, Older Dryas, YD and 8.2 ka events were detected in Okinawa Trough.•Holocene sea surface temperature in Okinawa Trough was mediated by a complex sun–ocean–atmosphere coupling.The study of long-chain alkenones in the Ocean Drilling Program (ODP) core 1202B reveals a sub-centennial resolution record of sea surface temperature (SST) in the southern Okinawa Trough for the past 15 thousand years (kyr). From the Deglaciation to Holocene, SST varied from 21.1 to 26.5 °C. The presence of Bølling (15.0–14.2 kyr BP) and Allerød (13.7–13.4 kyr BP) warming phases, Older Dryas (14.2–13.7 kyr BP) and Younger Dryas (12.8–11.6 kyr BP) cold periods reflects a tight teleconnection of climate between the Okinawa Trough and the North Atlantic region in the last Deglaciation. After rapidly increased and reached the maximum at ~ 7.4 kyr BP, SST in the southern OT gradually decreased, corresponding with the lowering of northern hemisphere summer solar insolation. However, a series of abrupt SST drops were identified at ca. 8.6–8.1, 5.8–4.8, 4.1–3.9, 3.0–2.5, 1.6–1.3 and 0.6–0.5 kyr BP, which cannot be explained by solar insolation changes alone, and instead are mediated by a complex sun–ocean–atmosphere coupling.