An Early Miocene Lowland on the Northeastern Tibetan Plateau

The northeastern Tibetan Plateau (NE TP) has long been thought to be the last part of the Plateau to be raised, but this assumption has been challenged by recent analyses of fossil leaf energy, which have pointed to the possibility that the present surface altitude of ∼3,000 m above sea level (asl) in the Qaidam Basin (QB) was attained during the Oligocene. Here, for the first time, we present a record of glycerol dialkyl glycerol tetraethers (GDGTs) from a well-dated Cenozoic section in the QB. This record appears to demonstrate that the mean annual average paleotemperature of the QB was 28.4 ± 2.9°C at ∼18.0 Ma. This would suggest that the paleoelevation of the QB was only ∼1,488 m asl at that time and that a ∼1,500 m uplift was attained afterwards, in agreement with the massive shortening of the QB and the rapid drying of inland Asia since the late Miocene.

Millennial-age glycerol dialkyl glycerol tetraethers (GDGTs) in forested mineral soils: ¹⁴C-based evidence for stabilization of microbial necromass

Understanding controls on the persistence of soil organic matter (SOM) is essential to constrain its role in the carbon cycle and inform climate–carbon cycle model predictions. Emerging concepts regarding the formation and turnover of SOM imply that it is mainly comprised of mineral-stabilized microbial products and residues; however, direct evidence in support of this concept remains limited. Here, we introduce and test a method for the isolation of isoprenoid and branched glycerol dialkyl glycerol tetraethers (GDGTs) – diagnostic membrane lipids of archaea and bacteria, respectively – for subsequent natural abundance radiocarbon analysis. The method is applied to depth profiles from two Swiss pre-Alpine forested soils. We find that the Δ14C values of these microbial markers markedly decrease with increasing soil depth, indicating turnover times of millennia in mineral subsoils. The contrasting metabolisms of the GDGT-producing microorganisms indicates it is unlikely that the low Δ14C values of these membrane lipids reflect heterotrophic acquisition of 14C-depleted carbon. We therefore attribute the 14C-depleted signatures of GDGTs to their physical protection through association with mineral surfaces. These findings thus provide strong evidence for the presence of stabilized microbial necromass in forested mineral soils.

Predominance of hexamethylated 6-methyl branched glycerol dialkyl glycerol tetraethers in the Mariana Trench: source and environmental implication

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are useful molecular indicators for organic carbon (OC) sources and the paleoenvironment. Their application in marine environments, however, is complicated because of a mixed terrestrial and marine source. Here, we examined brGDGTs in sediments from the Mariana Trench, the deepest ocean without significant terrestrial influence. Our result shows a strong predominance of hexamethylated 6-methyl brGDGT (IIIa′) (73.40±2.39 % of total brGDGTs) and an absence of 5-methyl brGDGTs, different from previously reported soils and marine sediments that comprised both 5-methyl and 6-methyl brGDGTs. This unique feature, combined with high δ13COC (-19.82±0.25 %), low OC∕TN ratio (6.72±0.84), low branched and isoprenoid tetraether (BIT) index (0.03±0.01), and high acyclic hexa- ∕ pentamethylated brGDGT ratio (7.13±0.98), support that brGDGTs in the Mariana Trench sediments are autochthonous rather than terrestrial products. The compiling of literature data shows that the enhanced fractional abundance of hexamethylated 6-methyl brGDGTs is a common phenomenon in continental margins when the marine influence was intensified. The cross plot of acyclic hexa- ∕ pentamethylated brGDGT ratio and fractional abundance of brGDGT IIIa′ provide a novel approach to distinguish terrestrial and marine-derived brGDGTs.

Radiocarbon analysis of isoprenoid and branched Glycerol Dialkyl Glycerol Tetraethers in soils and fluvial sediments

<p>Glycerol dialkyl glycerol tetraethers (GDGTs), membrane lipids synthesized by archaea (isoprenoid GDGTs) and bacteria (branched GDGTs), form the basis of a suite of molecular proxies used in terrestrial as well as marine environments. Compound-specific radiocarbon analysis has provided valuable insights into the sources and yielded constraints on transport dynamics of different biomarkers in the context of carbon cycle processes. To complement the existing biomarker radiocarbon toolbox, and to shed new light on the sources and fate of GDGTs, we developed a new method to measure GDGT radiocarbon compositions in natural samples.</p><p>Isoprenoid and branched GDGTs are isolated using two UHPLC silica columns in series coupled to a fraction collector set to eluent recovery at different time intervals. The accuracy of the method was tested using a modern and a radiocarbon-dead reference material. Procedural blanks show that the separation procedure adds less than 3 µg carbon with a Fm of 0.64.</p><p>The method is first applied to determine the Δ<sup>14</sup>C composition of isoprenoid and branched GDGTs in two soil core profiles from a temperate and subalpine forest ecosystem in order to explore the range of typical values encountered in natural systems.  The cores, which reach a depth of 80 cm and 40 cm respectively, have previously been analyzed with respect to radiocarbon characteristics of long-chain n-alkanes and fatty acids as well as bulk particulate and dissolved organic carbon (OC) [1]. For each core, GDGTs were separated and analyzed from 3 different depth intervals. The Δ<sup>14</sup>C of both isoprenoid and branched GDGTs decreases, at a similar rate as the bulk, by -350‰ and -200‰ along the temperate and the subalpine core respectively, hence confirming their potential for constraining transport-dynamics of soil-derived matter in rivers.</p><p>The radiocarbon age of GDGTs in a suite of fluvial sediments is older than expected under the assumption that topsoil-derived organic matter is the main source of the compounds. Potentially, this offset could be caused by rapid degradation of the compounds during transport and therefore alter the proxy signal on the way to sedimentary archives.</p><p> </p><p>[1] van der Voort, T. S., et al., 2017 - Geophysical Research Letters 44, 23</p>