Culture-Based Calibration of the Benthic Foraminiferal Mg/Ca Thermometer


OCCI Project Funded: 2004


What are the primary questions you are trying to address with this research? (Or, if more appropriate, is there a hypothesis or theory that you are trying to prove or disprove?)
We are setting up a laboratory culturing system that will let us grow deep-sea benthic foraminifera under controlled physical and chemical conditions. We will then use this system to carry out the first culture-based calibration of the benthic foraminiferal magnesium/calcium (Mg/Ca) thermometer. This “thermometer” is a geochemical method of estimating deep-water temperatures in the past, using the temperature dependence of the shell chemistry of benthic foraminifera, a group bottom-dwelling microorganisms that make calcium carbonate shells. To date, calibrations of the benthic foraminferal shell chemistry “thermometer” have been based on field samples. These field calibrations are often ambiguous, because many environmental and biological factors can co-vary with temperature in the deep sea. Laboratory cultures provide one way to eliminate these ambiguities. 

What is the significance of this research for others working in this field of inquiry and for the broader scientific community?

Estimates of past changes in deep-water temperature are an essential part of efforts to understand the oceans’ role in, and response to, climate change. Temperature estimates derived from the shell chemistry of bottom-dwelling organisms are the best available method of estimating deep-water temperatures over the last 20,000 years - e.g., since the last glacial maximum. However, these geochemical estimates are inherently uncertain, in large part because of uncertainties in modern-ocean calibrations using core-top sediment sampless. The improved temperature calibration we hope to obtain from laboratory culturing will improve paleoceanographers’ understanding the history of deep-water temperature, and of the oceans’ role in climate.

What is the significance of this research for society?
A good understanding of past climates and natural climate variability is an essential foundation for efforts to anticipate future climate change, and in particular, to understand the likely effects of anthropogenic increases in greenhouse gases (such as carbon dioxide) and atmospheric aerosols.

When and where will this investigation be conducted? (For instance, is this new fieldwork, or a new analysis of existing data?)

The culturing work will be carried out here at WHOI, starting this summer (2005).  We will collect live foraminifera during an NSF-funded cruise in early June; culturing experiments will be carried out both here at WHOI, and by colleagues at the University of South Carolina.

T Anolmaly

USC bulk culture system close up  

What are the key tools or instruments needed to conduct this research?
The culturing system will include three refrigerators, and a seawater circulation system. This system will consist of a single large (100 L) seawater reservoir, connected via peristaltic pumps to a set of small Plexiglas microcosms in which the foraminifera will actually be grown.

The analytical work will use Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) instrumentation at WHOI and at USC for the magnesium:calcium analyses, and stable isotope mass spectrometry at WHOI to determine carbon and oxygen isotopic compositions.

What are the greatest challenges - physical or intellectual - to conducting this investigation?
Simply getting the foraminifera to thrive (calcify and reproduce in culture) requires both a green thumb and some amount of luck. The most technically challenging aspect of the project will be maintaining the water chemistry of the culture system - particularly the carbonate chemistry (pH) of the system - within tight limits throughout the several-month growth period.

Is this research part of a larger project or program?
The temperature-calibration study supported by the OCCI complements our ongoing culturing work with colleagues at USC, which is focused on carbonate chemistry and growth rate influences on foraminiferal shell chemistry, at constant temperature.

If you have conducted previous/similar work on this subject, please suggest any web links or citations that might help others better understand the background to your line of research. If appropriate and readily available, please suggest or provide photographs, illustrations, tables, and charts, as well.

Our previous culturing work, at USC, is described in a paper by (then) graduate student Christopher Hintz. Hintz is now continuing this research as a post-doc at the University of South Carolina. (Hintz, C.J. G.T. Chandler, J.M. Bernhard, D.C. McCorkle, S.M. Havach, J.K. Blanks, T.J. Shaw (2004)  A physicochemically-constrained seawater culturing system for production of viable, calcite-producing, paleoceanographically-important benthic foraminifera. Limnology & Oceanography Methods, 2: 160-170. )

Biographical Information


Daniel C. McCorkle - Associate Scientist, Geology and Geophysics

B.A. Columbia University, 1978, Geology;
M.S. University of Washington, 1983, Chemical Oceanography;
Ph.D. University of Washington, 1987, Chemical Oceanography

My research interests include:
Benthic geochemistry - pore water and solid phase studies of organic matter decomposition, calcium carbonate dissolution, and trace metal remobilization.

Ocean paleochemistry - calibration studies of benthic foraminiferal shell chemistry, and isotopic and elemental estimates of changes in ocean circulation and the oceanic carbon cycle.

Land-sea groundwater interactions - chemical and isotopic studies of groundwater discharge into estuaries and the coastal ocean.

Joan M. Bernhard - Associate Scientist, Geology and Geophysics

B.A. Colgate University, 1982, Geology
M.S. Univ. of California, Davis, 1984, Geology
Ph.D. Scripps Institution of Oceanography,
Univ. of California, San Diego, 1990, Biological Oceanography

My research interests include:
Benthic foraminiferal ecology and paleoecology; Biogeochemistry of redox boundary sediments in both modern and ancient environments; Microaerophilic and anaerobic protists from Oxygen Minimum Zones and methane and cold seeps; Protistan-prokaryote symbioses; Culturing of deep-sea benthic foraminifera to ground truth paleoceanographic proxies; Microbial communities of laminated sediments; Sub-millimeter life positions of microorganisms in sediments; Application of various microscopic techniques (TEM, SEM, LSCM) to marine microbial ecology