LAUSR

Abstract Recent analytical advances enable stable potassium (K) isotopes to serve as a promising geochemical tracer of paleoceanography. Preserved calcified fossils in marine carbonate sediments have the potential to act as important archives of ancient seawater. However, little is known about the magnitude, direction, and mechanism of K isotope fractionation between marine carbonates and seawater. To investigate isotope fractionation between biogenic carbonates and modern seawater, we measured K concentration, phase, and isotopic composition in calcified skeletons from a variety of calcifying species. Samples included deep-sea corals, hermatypic corals, bivalves, gastropods, brachiopods, and planktonic foraminifera recovered globally over the past ten years in habitats with temperatures varying from 2 to 29 ⁰C. Our results show that the δ41K values of the calcified organisms varied significantly, ranging from -0.72±0.11 to 0.94±0.04‰. Among studied samples, deep-sea corals exhibit the largest isotopic variability and the lowest δ41K, ranging from -0.72±0.11 to 0.28±0.09‰. Hermatypic corals display a moderate δ41K range from -0.20±0.07 to 0.37±0.10‰. Bivalves display δ41K values falling within a wide range from 0.04±0.05 to 0.94±0.04‰, including the highest δ41K observed. Gastropods exhibit δ41K values between -0.42±0.06 and -0.12±0.06‰, while brachiopods express δ41K values from -0.30±0.05 to 0.24±0.06‰. Limited foraminifera samples (n=2) reveal δ41K values of 0.15±0.06 to 0.21±0.06‰. Based on synchrotron-based atomic analyses, K in biogenic carbonates is dominantly hosted in amorphous K2CO3, calcite-like and aragonite-like K phases, and intracrystalline organic matrices of varying proportions. The K isotopic composition in studied marine biogenic carbonates does not exhibit strong temperature dependence in general but is linked to skeletal K phases. This phase-control potentially indicates a first-order biological control on skeletal K incorporation, partitioning, and associated K isotope fractionation. This appears to reflect a substantial “vital effect” role; i.e., physiological modification of the environmental information recorded in calcifying organisms. Observed substantial variations in δ41K call for additional attentions before using marine biogenic carbonates to interpret ancient seawater δ41K composition. In sum, physiological modulation substantially complicates the interpretation of marine carbonate δ41K records through time. Future studies should include species-specific calibration with complementary synchrotron data to refine K isotope applications for paleoceanography.