Hain Mathis P.
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ArticleA record of Neogene seawater δ11B reconstructed from paired δ11B analyses on benthic and planktic foraminifera(Copernicus Publications on behalf of the European Geosciences Union, 2017-02-24) Greenop, Rosanna ; Hain, Mathis P. ; Sosdian, Sindia M. ; Oliver, Kevin I. C. ; Goodwin, Philip ; Chalk, Thomas B. ; Lear, Caroline H. ; Wilson, Paul A. ; Foster, Gavin L.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.
ArticleCauses of ice age intensification across the Mid-Pleistocene Transition(National Academy of Sciences, 2017-11-27) Chalk, Thomas B. ; Hain, Mathis P. ; Foster, Gavin L. ; Rohling, Eelco J. ; Sexton, Philip F. ; Badger, Marcus P. S. ; Cherry, Soraya G. ; Hasenfratz, Adam P. ; Haug, Gerald H. ; Jaccard, Samuel L. ; Martínez-García, Alfredo ; Pälike, Heiko ; Pancost, Richard D. ; Wilson, Paul A.During the Mid-Pleistocene Transition (MPT; 1,200–800 kya), Earth’s orbitally paced ice age cycles intensified, lengthened from ∼40,000 (∼40 ky) to ∼100 ky, and became distinctly asymmetrical. Testing hypotheses that implicate changing atmospheric CO2 levels as a driver of the MPT has proven difficult with available observations. Here, we use orbitally resolved, boron isotope CO2 data to show that the glacial to interglacial CO2 difference increased from ∼43 to ∼75 μatm across the MPT, mainly because of lower glacial CO2 levels. Through carbon cycle modeling, we attribute this decline primarily to the initiation of substantive dust-borne iron fertilization of the Southern Ocean during peak glacial stages. We also observe a twofold steepening of the relationship between sea level and CO2-related climate forcing that is suggestive of a change in the dynamics that govern ice sheet stability, such as that expected from the removal of subglacial regolith or interhemispheric ice sheet phase-locking. We argue that neither ice sheet dynamics nor CO2 change in isolation can explain the MPT. Instead, we infer that the MPT was initiated by a change in ice sheet dynamics and that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Southern Ocean as a consequence of larger ice sheets.