Articles | Volume 13, issue 2
Clim. Past, 13, 149–170, 2017
Clim. Past, 13, 149–170, 2017

Research article 24 Feb 2017

Research article | 24 Feb 2017

A record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera

Rosanna Greenop1,2, Mathis P. Hain1, Sindia M. Sosdian3, Kevin I. C. Oliver1, Philip Goodwin1, Thomas B. Chalk1,4, Caroline H. Lear3, Paul A. Wilson1, and Gavin L. Foster1 Rosanna Greenop et al.
  • 1Ocean and Earth Science, National Oceanography Centre Southampton, University of Southampton, Waterfront Campus, European Way, Southampton SO14 3ZH, UK
  • 2School of Geography & Geosciences, Irvine Building, University of St Andrews, North Street, St Andrews, KY16 9AL, UK
  • 3School of Earth & Ocean Sciences, Cardiff University, Cardiff, CF10 3AT, UK
  • 4Department of Physical Oceanography, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA

Abstract. The boron isotope composition (δ11B) of foraminiferal calcite reflects the pH and the boron isotope composition of the seawater the foraminifer grew in. For pH reconstructions, the δ11B of seawater must therefore be known, but information on this parameter is limited. Here we reconstruct Neogene seawater δ11B based on the δ11B difference between paired measurements of planktic and benthic foraminifera and an estimate of the coeval water column pH gradient from their δ13C values. Carbon cycle model simulations underscore that the ΔpH–Δδ13C relationship is relatively insensitive to ocean and carbon cycle changes, validating our approach. Our reconstructions suggest that δ11Bsw was  ∼  37.5 ‰ during the early and middle Miocene (roughly 23–12 Ma) and rapidly increased during the late Miocene (between 12 and 5 Ma) towards the modern value of 39.61 ‰. Strikingly, this pattern is similar to the evolution of the seawater isotope composition of Mg, Li and Ca, suggesting a common forcing mechanism. Based on the observed direction of change, we hypothesize that an increase in secondary mineral formation during continental weathering affected the isotope composition of riverine input to the ocean since 14 Ma.

Short summary
Understanding the boron isotopic composition of seawater (δ11Bsw) is key to calculating absolute estimates of CO2 using the boron isotope pH proxy. Here we use the boron isotope gradient, along with an estimate of pH gradient, between the surface and deep ocean to show that the δ11Bsw varies by ~ 2 ‰ over the past 23 million years. This new record has implications for both δ11Bsw and CO2 records and understanding changes in the ocean isotope composition of a number of ions through time.