Validation of the B/Ca Proxy for Surface Seawater pH and Application to Measure Anthropogenic Ocean Acidification

Lead PI: Dr. Baerbel Hoenisch

Unit Affiliation: Geochemistry, Lamont-Doherty Earth Observatory (LDEO)

March 2008 - February 2014
Global ; Catalina Island, CA ; Isla Magueyes, Puerto Rico
Project Type: Research

DESCRIPTION: A better knowledge of past natural fluctuations in the earth’s climate and its relation to greenhouse gases will enhance the ability to constrain future impacts on ocean and atmospheric chemistry, and thus global climate change from anthropogenic additions of CO2. Particular concern exists regarding the potential impact of ocean acidification on marine calcifiers such as corals, pteropods, foraminifers and calcareous algae. The actual extent of ocean acidification since the industrial revolution is still poorly quantified. This proposal seeks support for a female graduate student to quantify anthropogenic ocean acidification in a high-resolution sediment core from the Gulf of Mexico. This will be the first time to directly measure surface open ocean acidification with a carbonate chemistry proxy, the B/Ca ratio in planktic foraminifer shells. A pilot study has shown that this proxy has the potential to reflect glacial/interglacial changes of surface seawater pH and atmospheric pCO2. However, the incorporation of boron into planktic foraminifer shells also seems to be affected by temperature, and salinity variations change the seawater boron concentration. Because pH, temperature and salinity change in unison in the ocean, quantification of the respective effects on B/Ca can only be quantified in the laboratory, were parameters can be singled out. The first step of the proposed research is therefore the proxy validation under controlled conditions in the laboratory. If the quality of the B/Ca proxy is confirmed in laboratory culture and effects can be quantified, B/Ca could have several advantages over the boron isotope/paleo-pH proxy: Analyses are much faster, require less sample material, and the proxy may not be affected by dissolution in the sediment. -resolution sediment core will be used to determine anthropogenic acidification of the oceans using a carbonate chemistry proxy: the B/Ca ratio in planktic foraminifer shells.

OUTCOMES: Culture experiments have been performed with the subtropical O. universa and temperate G. bulloides on Catalina Island in 2008 and with the tropical G. sacculifer and G. ruber in Puerto Rico in 2010. The effects of temperature, pH, salinity, boron concentration and varying carbonate ion at constant pH were tested and a range of trace element proxies was analyzed. The experiments have shown that the sensitivity of the B/Ca proxy in planktic foraminifers is insufficient for the study of anthropogenic ocean acidification.


National Science Foundation (NSF)




Katherine Allen


Australian National University, Rutgers University New Brunswick



Hönisch, B., K.A. Allen*, A.D. Russell, S.M. Eggins, J. Bijma, H. J. Spero, D.W. Lea, and J. Yu, “Planktic foraminifers as recorders of seawater Ba/Ca”, Marine Micropaleontology 79, p. 52-57. Allen, K.A., B. Hönisch, J. Yu, S. Eggins, H. Spero, H. Elderfield, “Controls on boron incorporation in cultured tests of the planktic foraminifer Orbulina universa, Earth and Planetary Science Letters 309, p. 291-301

*Allen, K.A., B. Hönisch, S.M. Eggins, Y. Rosenthal, “Environmental controls on B/Ca in tests of the tropical planktic foraminifer species Globigerinoides ruber and G. sacculifer”, EPSL 351-352 (2012) 270–280, doi: 10.1016/j.epsl.2012.07.004.

*Allen, K.A., B. Hönisch, “The planktic foraminiferal B/Ca proxy for seawater carbonate chemistry: A critical evaluation”, EPSL 345-348, p. 203-211, doi: 10.1016/j.epsl.2012.06.012.

Hönisch, B., K.A. Allen*, D.W. Lea, H.J. Spero, S.M. Eggins, J. Arbuszewski, P. deMenocal, Y. Rosenthal, A.D. Russell, “The influence of salinity on Mg/Ca ratios of cultured and core-top planktic foraminifers”, Geochimica et Cosmochimica Acta 121, p. 196-213,

*Allen, K.A., B. Hönisch, S.M. Eggins, *L.L. Haynes, Y. Rosenthal, J. Yu, “Trace element proxies for surface ocean conditions: A synthesis of culture calibrations with planktic foraminifera”, Geochimica et Cosmochimica Acta 193, p. 197-221,


paleoclimate foraminifera marine calcifiers boron ocean acidification climate change ocean and climate physics calcium (ca)


Modeling and Adapting to Future Climate