Zhongya Hu

The Mid‐Brunhes Event (MBE, ∼430 ka) represents a pivotal climatic transition in the late Pleistocene, while monsoon‐driven impacts on low‐latitude regions during this interval remain debated. Sediments archived in the MD05‐2901 gravity core from the South China Sea (SCS) record the influence of the East Asian summer monsoon (EASM) on regional climate dynamics. In this study, we employ major and trace element systematics and magnesium isotope analyses (δ ²⁶ Mg) of clay‐sized (26 Mg values is observed, correlating with reduced smectite and chlorite proportions in the total mineral assemblage. This coupled trend deviates from expectations for silicate weathering sequences, necessitating a re‐evaluation of sediment provenance. Provenance analysis demonstrates that smectite and chlorite predominantly derive from mafic rock weathering in the southern SCS. We propose that the observed mineralogic and isotopic shifts reflect a temporal decrease of highly weathered siliciclastic materials from southern provenances presumably related to a weakened clockwise oceanic circulation system, which is a coupled response to the long‐term weakening of EASM and intermittent inundation of the Sunda Shelf over the past 430 Ka. Our results underscore the interconnected marine‐terrestrial climatic feedbacks modulating tropical monsoon systems on orbital timescales.

The surficial cycling of Mg is coupled with the global carbon cycle, a predominant control of Earth’s climate. However, how Earth’s surficial Mg cycle evolved with time has been elusive. Magnesium isotope signatures of seawater (δ ²⁶ Mg _sw ) track the surficial Mg cycle, which could provide crucial information on the carbon cycle in Earth’s history. Here, we present a reconstruction of δ ²⁶ Mg _sw evolution over the past 2 billion years using marine halite fluid inclusions and sedimentary dolostones. The data show that δ ²⁶ Mg _sw decreased, with fluctuations, by about 1.4‰ from the Paleoproterozoic to the present time. Mass balance calculations based on this δ ²⁶ Mg _sw record reveal a long-term decline in net dolostone burial (NDB) over the past 2 billion years, due to the decrease in dolomitization in the oceans and the increase in dolostone weathering on the continents. This underlines a previously underappreciated connection between the weathering-burial cycle of dolostone and the Earth’s climate on geologic timescales.

Microbes are known to mediate dolomite precipitation in laboratory experiments; however, the linkage of specific microbes to ancient dolomites remains poorly constrained due to scarce diagnostic biogeochemical signatures and mineralized microbial relics in the rock record. Here, we report the occurrence of methanogen-mediated dolomite in the Lower Permian lacustrine Lucaogou Formation in northwestern China. The clumped isotope (Δ₄₇) temperature provides direct evidence of a low-temperature origin (typically 26Mg_DSM3 (up to +0.44‰) and δ¹³C_VPDB (up to +19‰) values in the dolomite indicate authigenic precipitation in methanogenic lake sediments. Micron-sized spheroidal bodies and filamentous and sheetlike structures are interpreted as mineralized coccoid methanogenic archaea and extracellular polymeric substances (EPSs), respectively. Dolomite nanoglobules (primarily 40−100 nm in diameter) are interpreted as mineralized viruses attached to the archaea and EPSs and between the cells. A combination of geochemical and microscale evidence confirms the microbial origin of the dolomite induced by methanogens and their associated bacteriophages. Furthermore, dolomite nanoglobules initially nucleated on the surfaces of methanogen cells, EPSs, and viruses and then merged into larger aggregates. The formation of microbial dolomite is characterized by a metabolic incubation, heterogeneous nucleation, and aggregative growth pathway. These findings provide valuable clues to decipher the biosignatures of these particular ancient dolomites.

The growth and morphology of stromatolitic carbonates, i.e., lithified microbial mats, are tightly linked to complex microbial activities. However, the role of microbial metabolism in carbonate nucleation and formation during stromatolite growth remains controversial. Magnesium isotopes are a promising tool to trace carbonate formation processes due to significant isotopic fractionations associated with carbonate precipitation. This study reports Mg isotope compositions of Holocene, ca. 4000 years old stromatolites from the Lagoa Salgada lagoon (Brazil) with different morphologic features. Petrological and C-S isotopic evidence indicates microbial metabolisms varied during the formation of morphologically different stromatolites. However, the direction and magnitude of Mg isotope fractionation between Mg-calcite and ambient fluid are relatively constant despite the changes of microbial activities in ecosystems and consistent with the typical Mg isotope behavior during abiotic calcite precipitation. As a result, we propose that microbial metabolism do not influence the dehydration pathway of aqueous Mg2+ into calcite lattices. Our study corroborates earlier findings indicating that metabolism-related fractionation of Mg isotopes is limited or even absent during stromatolite growth. This finding also promotes understanding of Mg isotopic records in widespread Precambrian stromatolites for reconstructing seawater chemistry in deep time.