D/H Paleoclimate Records

Mojave Desert, near Joshua Tree N.P. This is one of the field sites where we have studied the potential for hydrogen isotopes in plant leaf waxes to record aridity.

Scientists understand past changes in the Earth's climate is through the use of 'proxies.' These are things that we can measure, such as tree-ring size or ice layer thickness or carbonate O-18 content, that are well correlated with changes in some aspect of the Earth's climate. The deuterium (2H, or D) content of organic leaf waxes in terrestrial plants is one such proxy, and we have had a series of projects both study this proxy and exploiting it for understanding terrestrial climate change.

We usually think of plants as fixing carbon, but in a sense they also fix hydrogen. They take H (in the form of H2O) from their environment, and reduce it to yield carbon-bound organic H. It happens that the relative abundance of D in environmental water varies rather dramatically with climate: the D/H ratio increases when its hot and dry, and decreases when its cold and wet. This isotopic 'fingerprint' of environmental water is then recorded by the organic hydrogen in plants. It is not recorded perfectly, because there are large biologic effects that lead to systematic offsets in the deuterium content of water versus organic compounds. But if we can understand those systematic effects, and calibrate the relationship between organic compounds and water, then we would have a useful paleoclimate proxy. In particular, the long-chain hydrocarbons that make up the waxy coatings of leaves are key targets for study. They are not volatile, degrade very slowly, and are abundantly preserved in most lake sediments.

Projects in my group on this subject have ranged from analyzing the leaves of desert plants in the Mojave Desert, to studying sediment cores from the Sierra Nevada, Santa Barbara Basin, West Africa, Argentina, South China Sea, Indonesia, the Swiss Alps, and — most recently (as of 2015) — New Zealand.

Recent papers on this subject:

Niedermeyer EA, Sessions AL, Feakins SJ, Mohtadi M (2014) Hydroclimate of the western Indo-Pacific Warm Pool during the past 24,000 years. PNAS 111, 9402-9406.

Sachse D, Billault I, Bowen GJ, Chikaraishi Y, Dawson TE, Feakins SJ, Freeman KH, Magill CR, McInerney FA, van der Meer MTJ, Polissar P, Robins RJ, Sachs JP, Schmidt HL, Sessions AL, White JWC, West JB, Kahmen A (2012) Molecular paleohydrology: Interpreting the hydrogen-isotopic composition of lipid biomarkers from photosynthesizing organismsAnnual Reviews of Earth and Planetary Science 40, 221-249.

Li C, Sessions AL, Thiagarajan N, Valentine D (2011) D/H variations of terrestrial lipids from Santa Barbara Basin over the past 1400 years: A preliminary assessment of paleoclimatic relevance. Organic Geochemistry 42, 15-24.

Feakins SJ and Sessions AL (2010b) Crassulacean acid metabolism influences D/H ratio of leaf wax in succulent plants. Organic Geochemistry41, 1269-1276.

Niedermeyer E, Schefuss E, Sessions AL, Mulitza S, Mollenhauer G, Schulz M, Wefer G (2010) Orbital- and millennial-scale changes in the hydrologic cycle and vegetation in the western African Sahel: insights from individual plant wax δD and δ13C. Quaternary Science Reviews 29, 2996-3005.

Feakins SJ and Sessions AL (2010) Controls on the D/H ratios of plant leaf waxes in an arid ecosystem. Geochimica et Cosmochimica Acta 74, 2128-2141.

Sessions AL (2006) Seasonal changes in lipid D/H fractionation by Spartina alternifloraGeochimica et Cosmochimica Acta70, 2153-2162.

Sessions AL, Sylva SP, Summons RE, and Hayes JM (2004) Isotopic exchange of carbon-bound hydrogen over geologic timescales. Geochimica et Cosmochimica Acta 68,1545-1559.

Copyright 2015 by Alex Sessions. All rights reserved.