Decadal-scale shifts in diatom assemblages since ∼1760 CE and their implications for biogenic silica recycling in the northern Yellow Sea

Marine diatoms on continental shelves have faced significant challenges from climate change and human activities over the last century. Changes in diatom species composition and abundance can greatly affect the cycling of biogenic silica (BSi) and organic carbon (OC) in the ocean. Here, we reconstruct decadal-scale patterns in diatom fossil assemblages and geochemical parameters in the northern Yellow Sea, and analyze the associated environmental triggers and their potential influences on BSi/OC ratios. Sequential t-test and CONISS analysis identify two significant shifts. One shift occurred in the 1850s, marked by the proliferation of species preferring low salinity (Actinocyclus ehrenbergii, Cyclotella stylorum), coinciding with the diversion of the Yellow River from the southern Yellow Sea to the Bohai Sea. The second shift occurred in the 1970s, marked by increases of small and heavily silicified species (Paralia sulcata, Thalassiosira eccentrica), consistent with eutrophication in the northern Yellow Sea. After this second shift, BSi/OC ratios initially decreased, which might have been driven by increased terrestrial OC inputs. However, a subsequent increase in the BSi/OC ratio within this phase is attributed to enhanced diatom silicification. These findings suggest that shifts towards heavily silicified diatoms might substantially alter the marine silica cycle.