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SUMMARY:Biomineralisation and boron proxies: the possible\, the impossible
  and the likely - Oscar Branson (University of Cambridge)
DTSTART:20200225T120000Z
DTEND:20200225T130000Z
UID:TALK129157@talks.cam.ac.uk
CONTACT:Oliver Shorttle
DESCRIPTION:Carbonate biominerals play a key role in the ocean carbon cycl
 e\, and preserve vital archives of past climate change in their geochemist
 ry. The abundance and isotopic content of boron in carbonate biominerals p
 rovide our best records of ocean carbon chemistry and pH\, which have prov
 ed instrumental in studying past episodes of CO2-induced climate change [1
 ]. \n\nThe boron proxies are based on the theory that carbonates solely in
 corporate B(OH)4- in proportion to seawater B(OH)4-/HCO3- or B(OH)4-/CO32-
 \, capturing both the state of the ocean C system and the pH-dependent iso
 topic composition of B(OH)4-. However\, significant modification of intern
 al carbon chemistry is required to facilitate calcification\, and substant
 ial proton export has been observed during carbonate formation [2]. The pH
 \, carbon and boron chemistry at the site of calcification cannot be the s
 ame as that of external seawater. How\, then\, do biominerals appear to re
 cord seawater B(OH)4-? While unanswered\, this question raises serious pro
 blems for our use and interpretation of the B proxies.\nThis question may 
 be explored using a quantitative model of B transport processes during bio
 mineralisation. Three key fluxes dominate biomineral formation: CaCO3 prec
 ipitation\, the exchange of seawater with the external environment\, and i
 on transport across membranes by diffusion or active pumping [3]. By reduc
 ing the problem to the balance between these three key fluxes\, it is poss
 ible to explore a wide range of biomineralisation scenarios with minimally
  restrictive assumptions. Including both the transport of B(OH)4-\, and th
 e transport and passive diffusion of membrane-permeable B(OH)3 within this
  framework captures the full range of potential biomineralisation scenario
 s and B transport processes. Sets of B geochemical data from biominerals g
 rown in known conditions then provide crucial constraints that reveal: (1)
  A mechanism that allows biomineral boron to be sensitive to seawater pH a
 nd carbon chemistry\, despite significant differences in chemistry and pH 
 at the site of calcification\, and (2) the ion transport dynamics of the c
 alcification environment (e.g. ‘closed’ vs. ‘open’ or Rayleigh- vs
 . transport-dominated system). Together\, this adds confidence to the use 
 of the B palaeo-proxies in all carbonate biominerals\, and provides a new 
 lens through which B geochmistry can be used to understand biomineralisati
 on mechanics\, which is particularly relevant to the resilience of coral c
 alcification to changing ocean carbon chemistry [4].\n\n[1] Foster and Rae
  (2016) Reconstructing Ocean pH with Boron Isotopes in Foraminifera. Ann. 
 Rev. Earth. Plan. Sci. 44\, 207-237\n[2] Toyofuku et al. (2017) Proton pum
 ping accompanies calcification in foraminifera. Nature Comms. 8\, 14145\n[
 3] Gagnon\, Adkins and Erez (2012) Seawater transport during coral biomine
 ralization. EPSL 329\, 150-161\n[4] McCulloch et al (2017) Coral calcifica
 tion in a changing World and the interactive dynamics of pH and DIC upregu
 lation. Nature Comms. 8 15686
LOCATION:Tilley Lecture Theatre\, Department of Earth Sciences
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