Boron and magnesium isotopic composition of seawater

Abstract

[1] 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 (d 11 B = 39.61 ± 0.04 ‰ (2 s.e.; n = 28), d 25 Mg = −0.43 ± 0.01 ‰ (2 s.e.; n = 26), d26 Mg = −0.82 ± 0.01 ‰ (2 s.e.; n = 26)) be used in future studies involving Mg and B isotopes.

δ<sup>11</sup>B as monitor of calcification site pH in marine calcifying organisms

“…(39.94 ‰, Table 3) similar to the average 11 BSW value (i.e., comprising the 11 B composition of both dissolved borate and boric acid; 39.61 ‰) determined by Foster et al (2010), raising the possibility that coralline red alga incorporate both 365 species of dissolved inorganic boron during calcification. In support of this argument, Cusack et al (2015) provide NMR data indicating that 30 % of the B incorporated into the coralline red alga Lithothamnion glaciale was present as boric acid.…”

Boron Isotopes in the Ocean Floor Realm and the Mantle

“…Boron has a long residence time in the ocean of approximately 10-20 million years and is isotopically very well mixed (Lemarchand et al 2002;Foster et al 2010). The B isotopic composition of modern ocean water is well defined at +39.61 ± 0.04‰ and the long residence time suggests that the rate of change was low in the Cenozoic at approximately 0.1‰/Ma ( Fig.…”

“…The B isotopic composition of modern ocean water is well defined at +39.61 ± 0.04‰ and the long residence time suggests that the rate of change was low in the Cenozoic at approximately 0.1‰/Ma ( Fig. 8.7; Lemarchand et al 2000;Foster et al 2010). The reconstruction of the B isotopic composition of ocean water is, however, a matter of debate, and especially the more distant history of its secular evolution is poorly constraint.…”

pH homeostasis during coral calcification in a free ocean CO 2 enrichment (FOCE) experiment, Heron Island reef flat, Great Barrier Reef

“…The pH of the calcifying fluid were derived from measured skeletal δ 11 B carb values according to the following equation (48): where pK B is the dissociation constant dependent on temperature and salinity, δ 11 B sw = 39.61 (49), and α B3-B4 is the boron fractionation factor for the pHdependent equilibrium of B(OH) 4 − and B(OH) 3 in seawater equal to 1.0272 (50).…”