The occurrence of "pre-aged" organic carbon (OC) in continental margin surface sediments is a commonly observed phenomenon, yet the nature, sources, and causes of this aged OC remain largely undetermined for many continental shelf settings. Here, we present the results of an extensive survey of the abundance and radiocarbon content of OC in surface sediments from the northern Chinese marginal seas. Pre-aged OC is associated with both coarser (>63 µm) and finer (<63 µm) sedimentary components, with measurements on specific grain size fractions revealing that it is especially prevalent Page 2 of 17 within the 20-63 µm fraction of inner shelf sediments. We suggest that organic matter associated with this "sortable silt" fraction is subject to protracted entrainment in resuspension-deposition loops during which it ages, is modified, and is laterally dispersed, most likely via entrainment within benthic nepheloid layers. This finding highlights the complex dynamics and pre-depositional history of organic matter accumulating in continental shelf sediments, with implications for our understanding of carbon cycling on continental shelves, development of regional carbon budgets, and interpretation of sedimentary records.

No abstract

Glycerol ether lipids have been developed as proxies to reconstruct past environmental changes or in their intact polar form to fingerprint the viable microbial community composition. However, due to the structural complexity, the full characterization of glycerol ether lipids requires separate protocols for the analysis of the polar head groups and the alkyl chain moieties in core ether lipids. As a consequence, the valuable relationship between core ether lipid composition and specific polar head groups is often lost; this limits our understanding of the diversity of ether lipids and their utilities as biogeochemical proxies. Here, we report a novel reverse-phase liquid chromatography-electrospray ionization-mass spectrometry (RP-ESI-MS) protocol that enables the simultaneous analysis of polar head groups (e.g., phosphocholine, phosphoglycerol, phosphoinositol, hexose, and dihexose) and alkyl moieties (e.g., alkyl moieties modified with different numbers of cycloalkyl moieties, hydroxyl and alkyl groups, and double bonds) in crude lipid extracts without further preparation. This protocol greatly enhances the detection of archaeal intact polar lipids (IPLs) and core lipids (CLs) with double-bond-and hydroxyl-group-bearing alkyl moieties. With these improvements, widely used ratios that describe relative distribution of the core lipid, such as the TEX 86 and ring index, can now be directly determined in specific intact polar lipids (IPL-specific TEX 86 and ring index). Since IPLs are the putative precursors of the environmentally persistent core lipids, their detailed examination by this protocol can potentially provide new insights into diagenetic and biological mechanisms inherent to these proxies. In a series of 12 samples from diverse settings, core and IPL-specific TEX 86 values follow the order: 2G-GDGTs > core GDGTs > 1G-GDGTs > 1G-GDGT-PI; and the ring indices follow: 1G-GDGTs ≈ core GDGTs > 2G-GDGTs > 1G-GDGT

Three-component difunctionalization of gem-difluoroalkenes via the generation and transformation of a α-fluoroalkylated carbanion, a carbon–metal species, a radical, and a carbocation intermediate.

In recent decades, microbial membrane lipids have become a focus of geoscientific research because of their proxy potential. The aim of this study was to develop new methods for ultra high performance liquid chromatography (UHPLC) separation of isomers of archaeal and bacterial membrane ether lipids, in particular glycerol dialkyl glycerol tetraethers (GDGTs), because of their tendency to co-elute with related but incompletely characterized derivatives. Our newly developed protocol, involving analysis using two Acquity BEH HILIC amide columns in tandem, enables chromatographic separation of several of these co-eluting compounds, such as the isoprenoid GDGT with four cyclopentyl moieties and other chromatographic shoulders often observed in GDGT analysis. Additionally, resolved peaks were observed for isoprenoid GDGTs, branched GDGTs and isoprenoid glycerol dialkanol diethers (GDDs); these have typically the same molecular mass as the corresponding major compound. Multiple stage mass spectrometry (MS 2) indicated that the shoulder peaks represent either regioisomers or other structural isomers with different ring or methyl positions. In some samples, these isomers can be even more abundant than their "regular" counterparts, suggesting that previously hidden clues regarding source organisms and/or community response to environmental forcing factors may be encoded in the distributions.