Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are widely distributed in marine and terrestrial settings. In the past decade, several indicators based on brGDGT compositions have been developed and widely used for the reconstruction of paleoenvironments.

The application of brGDGT-based proxies in marine settings is founded on the presumption that brGDGTs are only derived from land. However, mounting evidence suggests that brGDGTs can also be produced by marine organisms in marine environments. It remains vague whether they are produced in the water column or the sediments. This source appropriation is important for the application of brGDGT-based proxies in modern and paleo-environments since they may record the signal where they were biosynthesized.

Researchers from the Department of Ocean Science and Engineering at the Southern University of Science and Technology (SUSTech) recently made a breakthrough in discovering the origins of brGDGTs in global marine sediments. Their findings are considerably applicable in using brGDGTs as environmental proxies for ocean bottom conditions in global oceans.

Their paper, entitled “Global Scale Production of BrGDGTs by Benthic Marine Bacteria: Implication for Developing Ocean Bottom Environmental Proxies,” was published in Global and Planetary Change.

In this paper, the researchers compiled an expansive dataset composed of globally distributed marine sediments from continental shelves, slopes, abyssal plains, and hadal trenches, aiming to determine the source of marine-derived brGDGTs preserved in global marine sediments (allochthonous water column and in-situ sediment), and assessing their environmental implications. They first used the ΣIIIa/ΣIIa index to distinguish the marine vs. terrestrial signals of brGDGT production.

For marine-derived brGDGTs, the cyclization of branched tetraethers (CBT) index, related to ambient pH, displays a large variability. This suggests that marine-derived brGDGTs in sediments are mainly produced within sediments where the pH of sediment porewater is highly variable, rather than in the water column where pH is relatively constant. This is also supported by good correlations between the bottom water temperature and the methylation of branched tetraethers (MBT’) index, between the water depth and the ΣIIIa/ΣIIa index, as well as between the brGDGT-based proxies and sediment redox conditions during glacial/interglacial cycles.

Figure 1. World map showing the location of marine sediments included in the dataset of this study

Based on the above findings, the researchers proposed that the brGDGTs in marine environments are derived from terrestrial input and marine in-situ production in the sediments and the water column. The contribution of benthic production to the brGDGTs’ pool becomes dominant towards the deep-sea sediments. The shallow area is greatly influenced by terrestrial input, and the sedimentary brGDGTs record the signatures of the continental temperature and soil pH where the brGDGTs were produced. While in the deep ocean dominated by marine in-situ production, sedimentary brGDGTs are potential tools for recording environmental conditions of the ocean bottom.

Figure 2. Schematic displaying the distribution of brGDGTs in marine environments

Dr. Wenjie Xiao is the first author of this paper. Prof. Chuanlun Zhang, Prof. Jian Lin, and master’s student Zhiyu Zeng from SUSTech, alongside Dr. Hongrui Zhang from ETH Zurich, are the co-authors of the paper. Dr. Wenjie Xiao and Prof. Yunping Xu from the Shanghai Ocean University (SHOU) are the corresponding authors.

This work was supported by the National Natural Science Foundation of China (NSFC), China Postdoctoral Science Foundation, Shenzhen Key Laboratory of Marine Archaea Geo-Omics, SUSTech, Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Shanghai Sheshan National Geophysical Observatory, and State Key Laboratory of Estuarine and Coastal Research. Dr. Wenjie Xiao also acknowledges the support of the SUSTech Presidential Postdoctoral Fellowship.

Paper link: https://www.sciencedirect.com/science/article/abs/pii/S0921818122000509