Animals depend on oxygen for respiration. Thus, the emergence and proliferation of early animals from the late Neoproterozoic to the early Paleozoic era (~600-500 million years ago) have been traditionally ascribed to significant increases in marine oxygen levels. However, recent evidence challenges this view. Geochemical tracers and numerical models suggest that both atmospheric and marine oxygen levels during the late Neoproterozoic to early Paleozoic were considerably lower than present-day levels, with highly fluctuating marine redox states. This period saw marine oxygenation occurring under conditions of low atmospheric oxygen, attributed to an intensified marine biological pump. However, the role of other factors, such as the mineral carbon pump—promoting organic matter burial through mineral protection—has not been thoroughly explored for this era.
In the modern ocean, organic matter particles precipitate and undergo remineralization in the water column, depleting dissolved oxygen in deeper waters. Consequently, only a small fraction of organic matter (< 0.03 to 13%) formed in surface waters ultimately finds its way to burial on the seafloor due to aerobic respiration. Minerals, including clays and oxides, play a crucial role in this process by absorbing and physically protecting organic matter from reoxidation in the water column, thereby enhancing burial efficiency in marine seafloor sediments.
While previous studies have suggested that increased preservation of continental clays in marine sediments may have triggered oxygenation in the later Neoproterozoic oceans, doubts remain regarding methodology and data interpretation. Robust evidence linking continental clay formation to global marine oxygenation during the Neoproterozoic to the earliest Paleozoic is lacking.
To address this gap, Associate Professor Guangyi Wei from Nanjing University and Professor Mingyu Zhao from the Institute of Geology and Geophysics (CAS), along with their collaborators, developed the lithium isotope system (δ7Li) in marine siliciclastic sediments, such as mudstones, to track the proportions of continentally derived clays preserved in continental shelf sediments. By combining this approach with the Sedimentary Geochemistry and Paleoenvironments Project (SGP) database and a global biogeochemical model, new δ7Li data offer direct insights into the correlation between continental clay export and marine oxygenation under conditions of low atmospheric oxygen levels.
During chemical weathering, isotopically light lithium is preferentially taken up by secondary silicate minerals (i.e., clays), resulting in more negative δ7Li values in strongly weathered products. In contrast, primary silicate minerals and weakly weathered products (e.g., feldspar, mica) exhibit higher δ7Li signatures, resembling those of the average upper continental crust (≥ 0‰). Additionally, marine authigenic clays formed through marine reverse weathering generally display higher δ7Li values due to lithium sourced from seawater. Therefore, the δ7Li values of marine mudstones serve as potentially reliable indicators for the proportions of different types of silicate minerals preserved in ancient continental shelf sediments.
The authors collected approximately 600 mudstone samples spanning the late Neoproterozoic to the middle Cambrian (ca. 660–500 Ma) and analyzed their lithium isotope compositions alongside K/Al ratios to track changes in continental silicate weathering and clay mineral composition during this interval. Significant decreases in δ7Li and K/Al ratios of marine mudstones post-early Cambrian (ca. 525 Ma) indicate enhanced continental silicate weathering and clay mineral preservation in marine sediments.
Furthermore, the new mudstone δ7Li database was compared with phosphorus (P), total organic carbon (TOC), and uranium (U) contents in marine siliciclastic sediments from the SGP compilations. Synchronous occurrences of decreased δ7Li and increased P, TOC, and U in marine sediments provide compelling evidence for correlations between continental clay formation, marine P reservoir, organic carbon burial flux, and marine oxygenation levels. Results from a global biogeochemical model suggest that increased continental clay influx to marine sediments could notably drive increased atmospheric and deep marine oxygen levels under initial conditions of low atmospheric oxygen (< 20–40% PAL).
This research was published online in Science Advances on March 29th. The first author of the article is Assoc. Prof. Guangyi Wei, while the corresponding authors are Assoc. Prof. Guangyi Wei and Prof. Mingyu Zhao. This study was funded by the National Key Research and Development Program of China and the National Natural Science Foundation of China. (Link)
Figure 1: The schematic diagram of marine organic matter production and burial in the continental shelf ocean, regulated by marine nutrient cycling and mineral protection.(Image by IGGCAS)
