Liang Dong^1*, Manyu Kang², Xiaoxiao Zhao^3†, Yunge Jing³, Taisi Li¹, Xiaobo Jin², Yun Zhao¹, Xuyuan E. Ai⁴, Yeqiang Shu^5,6, Taoliang Zhang¹, Yunru Chen³, Guodong Jia<sup>2*

1</sup> State Key Laboratory of Submarine Geoscience; Key Laboratory of Polar Ecosystem and Climate Change, Ministry of Education; and School of Oceanography, Shanghai Jiao Tong University, Shanghai 200030, China
²State Key Laboratory of Marine Geology, Tongji University, Shanghai 200092, China
³State Key Laboratory of Microbial Metabolism, School of Life Science and Biotechnology, Shanghai Jiao Tong University, Shanghai 200240, China
⁴Department of Geosciences, Princeton University, Princeton, NJ, USA
⁵State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China
⁶Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou), Guangzhou, China

 Marine N₂-fixation fuels new production in oligotrophic oceans, yet how diazotroph community structure regulates carbon export across glacial–interglacial cycles remains unclear. Because sedimentary δ¹⁵N integrates multiple nitrogen sources, the specific role of different diazotroph groups in the biological carbon pump is often obscured. Here, we present a ~100 ka multi-proxy record from the South China Sea that combines specific lipid biomarkers for diatom–diazotroph associations (DDAs; C5 heterocyst glycolipids) and ammonia-oxidizing archaea (AOA; crenarchaeol) with geochemical indicators of export and burial. Under strongly stratified conditions between 100 and 60 ka, TOC-normalized C5 heterocyst glycolipids and total organic carbon (TOC) remained low, whereas crenarchaeol was high, consistent with stronger upper-ocean recycling and weak burial. From 62 to 25 ka, TOC-normalized C5 heterocyst glycolipids increased stepwise and TOC also rose, indicating that DDAs accounted for an increasing share of the exported organic-carbon pool and that the DDA-associated carbon sink strengthened toward the last glacial maximum. Over the last 21 ka, DDA-associated export remained high, but carbon burial became more variable, probably reflecting stronger upper-ocean recycling. Our results show that shifts in diazotroph community composition can modulate low-latitude carbon sequestration in ways not captured by sedimentary δ¹⁵N alone.

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