Potassium isotope signatures in modern marine sediments: Insights into early diagenesis

The sedimentary fluxes controlling potassium (K) budget of the oceans and the isotope composition of K (delta K-41) potentially yield valuable information about the global K cycling. However, at present sedimentary diagenesis in the oceans is one of the least well-known components of the seawater K budget. This study presents a dataset of the modern (<50 cm) sediments from a range of hydrographic regimes including continental margin settings (the Peru Margin, California Borderland, and Mexican Margin) and deep-sea environments (the Equatorial Pacific). We determine the K isotope compositions and origins of K in the sediments and compare them with the results of elemental abundances and spectroscopic analyses. The wide variability in sedimentary K/Al and Rb/K ratios, K-phase distributions, and K isotope compositions support the interplay between continental weathering, which provides clastic inputs, and sedimentary diagenesis, which constitutes an important sink of seawater K. The net result of these processes potentially alters the K elemental and isotopic budgets in sediment strata of various marine environments. We demonstrate that K is dominantly hosted by illite, glauconite, and feldspars, and regional K-phase partitioning modulates marine sedimentary K budget. For example, considerable amounts of K hosted in illite (up to 90%) primarily of terrestrial origins increase Rb/K ratios, producing low delta K-41 in California Borderland sediments (-0.57 to -0.40 parts per thousand), the Pescadero Slope from the Mexican Margin (-0.42 to -0.39 parts per thousand), and Equatorial Pacific deep-sea basins (-0.56 to -0.44 parts per thousand). In contrast, glauconite authigenesis causes decreases in the Rb/K ratio and relatively high delta K-41 at a site along the Peru Margin (-0.35 to -0.29 parts per thousand), as well as at two other sites along the Mexican Margin (-0.39 to -0.27 parts per thousand). Despite the presence of feldspar-K (<20%), delta K-41 in the sediments reflects dominant contributions of detrital illite (delta K-41(illite) similar to -0.56 parts per thousand) and authigenic glauconite (delta K-41(glauconite) similar to -0.18 parts per thousand). We conclude that the formation of glauconite in the sediments takes up isotopically light K from seawater and serves as an important sink of seawater K (including K in seawater-derived porewater) (similar to 1 to 16, average similar to 6 Tg K.yr(-1)) during early diagenesis, especially on continental margins. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the importance of potassium budgets and isotope compositions in marine sedimentary rocks for the global potassium cycling, highlighting the interplay between continental weathering and marine sedimentary diagenesis, which influences the distribution of potassium in seawater through the mineral phases, origins, and isotopic compositions in rocks.