The influence of rivers on marine boron isotopes and implications for reconstructing past ocean pH

Abstract

Ocean pH is particularly sensitive to atmospheric carbon dioxide content. Records of ocean pH can therefore be used to estimate past atmospheric carbon dioxide concentrations. The isotopic composition of boron (delta11B) contained in the carbonate shells of marine organisms varies according to pH, from which ocean pH can be reconstructed. This requires independent estimates of the delta11B of dissolved boron in sea water through time. The marine delta11B budget, however, is still largely unconstrained. Here we show that, by incorporating the global flux of riverine boron (as estimated from delta11B measurements in 22 of the world's main rivers), the marine boron isotope budget can be balanced. We also derive ocean delta11B budgets for the past 120 Myr. Estimated isotope compositions of boron in sea water show a remarkable consistency with records of delta11B in foraminiferal carbonates, suggesting that foraminifera delta11B records may in part reflect changes in the marine boron isotope budget rather than changes in ocean pH over the Cenozoic era.

Exploring B/Ca as a pH proxy in bivalves: relationships between <I>Mytilus californianus</I> B/Ca and environmental data from the northeast Pacific

“…The residence time of Ca in the ocean is 1.1 million years (myr) (Broecker and Peng, 1982) and that of B is estimated between 14 and 20 myr (Pagani et al, 2005;Spivack and Edmond, 1987;Lemarchand et al, 2000). Thus, the total concentration of B in the modern ocean is constant over the timescales relevant to this study at around 4.52 ppm in open marine environments (Lee et al, 2010).…”

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

“…Boron has a residence time in seawater of ca. 14 million years (Lemarchand et al, 2000), which is much longer than the mixing time of oceans (ca. 1000 years), suggesting that it behaves conservatively in time and space (Foster et al, 2010), making 11 B an attractive palaeo-proxy for pH.…”

Boron Isotopes in the Ocean Floor Realm and the Mantle

“…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.…”

“…More recent reconstructions of the seawater d 11 B value are based on box models that take into account the sources and sinks of boron in the oceans (Lemarchand et al 2000(Lemarchand et al , 2002Joachimski et al 2005;Simon et al 2006). The sources are riverine input, hydrothermal vents, and fluids expelled from continental margins, while the sinks are low-T alteration of the oceanic crust, formation of clay, and carbonate precipitation.…”