235 Comer

61 Route 9W - PO Box 1000

Palisades

NY

10964-8000

US

Phone: 

(845) 365-8828

Fax: 

(845) 365-8155

hoenisch@ldeo.columbia.edu

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My main interest is focussed on the understanding and reconstruction of past marine carbonate chemistry. Specifically, I seek to validate and apply the boron isotope proxy for past seawater pH. The methods used for the validation work are culture experiments with living planktic foraminifers, as well as observations on planktic and benthic foraminifers from coretop sediments. Specifically I studied the influence of symbiont photosynthesis on the pH recorded by planktic foraminifers (Hönisch et al., 2003) and scleractinian corals (Hönisch et al., 2004). The boron isotopic composition of symbiont-bearing planktic foraminifers also changes with foraminiferal shell size, as it appears that larger individuals spend relatively more time closer to the seasurface where light levels are higher and symbiont photosynthesis is stronger (Hönisch and Hemming, 2004). In the latter study we also describe a significant post-depositional dissolution effect on the boron isotopic composition in foraminifera shells. Although these side effects are significant, the detailed knowledge of these proxy limitations now allows us to carefully select sample material for paleoreconstructions. I am now applying the boron isotope proxy to reconstruct surface seawater pH across the Pleistocene glacial/interglacial cycles.

In collaboration with Ann Russell we also established the U/Ca ratio of planktonic foraminifer shells as a proxy for seawater carbonate ion concentration (Russell et al., 2004). In 2004 we carried out another set of temperature, salinity and carbonate experiments in order to identify and quantify potential side effects on U/Ca. If the observed proxy relationships prove robust, we will be able to use the combination of the two proxies to reconstruct pH and carbonate ion concentration and then to estimate any other parameter of the carbonate system in the ocean.

Some of my projects include:

• Ground-truthing the boron isotope paleo-pH proxy
• Reconstructing deep ocean acidification at the PETM
• Validating the boron isotope proxy in deep-sea corals and tracing anthropogenic CO2 invasion