Isoprenoid glycerol dialkyl glycerol tetraethers (GDGTs) are chemical compounds produced by archaea, a type of microorganism. Scientists use these compounds to study past climate and environmental conditions through different indicators, such as the Methane Index (MI) and TEX86. TEX86 helps estimate past ocean temperatures, but its accuracy depends on whether the GDGTs in the sediment mainly come from a specific group of marine archaea. The MI is often used to check if TEX86 data has been influenced by methane-related processes and, more recently, to study past methane activity. However, there is still uncertainty about how GDGT-based indicators vary across different locations and how MI and TEX86 are connected.
To address this gap, faculty members from the Division of Marine Geology and Geophysics at the Institute of Oceanography, National Taiwan University—including associate professors Sze Ling Ho, Pei-Ling Wang, and Chih-Chieh Su, assistant professor Tzu-Ting Chen, and their PhD and MSc students Pei-Ting Lee, Hui-Hsin Wang, Tzu-Jung Cheng, and Yun-Ju Wang—collaborated with researchers from National Sun Yat-sen University and National Taiwan Ocean University to carry out an interdisciplinary study based on samples collected using a remotely operated vehicle (ROV). The sampling area is located in a cold seep environment at a depth of several thousand meters, which presents the challenge of navigating the rugged carbonate rock terrain. The ROV’s high-precision positioning and sampling technology ensure the quality of sediment samples, providing a reliable foundation for microbial biomarker analysis. The team analyzed sediment samples from offshore southwest Taiwan, where methane activity varies. They also examined gas, porewater, sediment chemistry, archaeal cell abundance, and GDGT composition to understand how these factors influence MI and TEX86. The results showed that methane-related chemical changes occurred at different depths in the sediments. Certain GDGTs were strongly linked to methane activity, but MI values did not always change predictably with methane levels. This suggests that the duration of methane activity may play a role in shaping MI values. The team also found a strong connection between MI and TEX86 in areas affected by methane activity, though the relationship varied by location. Despite this variation, the ocean temperature estimates from TEX86 remained consistent across sites (within 1.5°C of each other) and aligned well with modern climate data. The findings from this study suggest that even when MI values are high—above the usual threshold of 0.3—the impact on TEX86-derived temperature estimates may not be as significant as previously thought.
These findings may enhance the understanding of how ocean changes and methane activity in Taiwan responds to climate change. The results show that the MI has the potential to track the occurrence of gas hydrate offshore Taiwan. This study further demonstrates the technical prowess of the Taiwanese oceanographic community in sampling and biogeochemical analysis.
This work was published in the journal of The Geochemical Society Geochimica et Cosmochimica Acta:
Ho, S.L., Lin, Y.-S., Wang, P.-L., Chen, T.-T., Lee, P.-T., Wang, H.-H., Cheng, T.-J., Wang, Y.-J., Su, C.-C., Chen, M.-T., 2025. Methane Index and TEX86 values in cold seep sediments: Implications for paleo-environmental reconstructions. Geochimica et Cosmochimica Acta 391, 262–276. https://doi.org/10.1016/j.gca.2024.12.033
Figure 1: Location and seabed images at study sites offshore southwest Taiwan.
