Potassium phases and isotopic composition in modern marine biogenic carbonates

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 degrees C. The delta K-41 values of the calcified organisms vary significantly, ranging from -0.72 +/- 0.11 to 0.94 +/- 0.04 parts per thousand. Deep-sea corals exhibit the largest isotopic variability and the lowest delta K-41, ranging from -0.72 +/- 0.11 to 0.28 +/- 0.09 parts per thousand. Hermatypic corals display a moderate delta K-41, ranging from -0.20 +/- 0.07 to 0.37 +/- 0.10%e. Bivalves display widely variable delta K-41 values from 0.04 +/- 0.05 to 0.94 +/- 0.04 parts per thousand, including the highest delta K-41 observed. Gastropods exhibit delta K-41 values between -0.42 +/- 0.06 and -0.12 +/- 0.06 parts per thousand, while brachiopods have delta K-41 values from -0.30 +/- 0.05 to 0.24 +/- 0.06 parts per thousand. Limited foraminifera samples (n = 2) reveal delta K-41 values of 0.15 +/- 0.06 to 0.21 +/- 0.06 parts per thousand. Synchrotron-based atomic analyses show that 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 of marine biogenic carbonates is not strongly temperature-dependent, in general, but correlates with skeletal K phases. This phase-control may indicate a first-order biological control on skeletal K incorporation, partitioning, and associated K isotope fractionation. This appears to reflect a substantial vital effect; i.e., physiological modification of the environmental information recorded in calcifying organisms. Substantial variations in delta K-41 call for additional scrutiny before using marine biogenic carbonates to interpret ancient seawater delta K-41 composition as physiological modulation substantially complicates the interpretation of marine carbonate delta K-41 records through time. Future studies should include species-specific calibration with complementary synchrotron data to refine K isotope applications for paleoceanography. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study investigated isotope fractionation between biogenic carbonates and modern seawater by measuring K concentration, phase, and isotopic composition in calcified skeletons from various calcifying species. Deep-sea corals showed the largest isotopic variability while the K isotopic composition of marine biogenic carbonates is not strongly temperature-dependent but correlates with skeletal K phases.