Boron and magnesium isotopic composition of seawater

TLDR

Boron and magnesium isotopic systems are increasingly used as environmental proxies, and seawater isotopic composition serves as a key interlaboratory standard, yet systematic studies of its homogeneity are lacking despite the elements’ long residence times. The study reports B and Mg isotopic compositions from 26–28 seawater samples across diverse ocean basins, salinities, temperatures, and depths. The authors measured B and Mg isotopic ratios in 26–28 seawater samples collected from multiple ocean basins covering salinities 32–38 psu, temperatures −0.3 to +25.9 °C, and depths 0–1240 m. No significant variation was observed in B or Mg isotopic composition, supporting their homogeneity, and the authors provide mean values (δ11B = 39.61 ± 0.04 ‰, δ25Mg = −0.43 ± 0.01 ‰, δ26Mg = −0.82 ± 0.01 ‰) for future use.

Abstract

The isotopic systems of boron and magnesium are increasingly being used as proxies for a number of environmental variables and processes. The isotopic composition of seawater for both systems plays a central role in these studies and is an important interlaboratory standard. Given the long residence times of both elements (∼10 7 years) it is commonly assumed that seawater is isotopically homogenous for these systems, yet no systematic studies currently exist. Here we present the B and Mg isotopic composition of 26–28 seawater samples from a number of ocean basins that encompass a significant range in salinity (32 to 38 psu), temperature (−0.3 to +25.9°C) and water depth (0 to 1240 m). We find no significant or systematic variation for either system in accordance with their long residence times. We recommend that the mean values we report ( δ 11 B = 39.61 ± 0.04 ‰ (2 s.e.; n = 28), δ 25 Mg = −0.43 ± 0.01 ‰ (2 s.e.; n = 26), δ 26 Mg = −0.82 ± 0.01 ‰ (2 s.e.; n = 26)) be used in future studies involving Mg and B isotopes.