A biogeochemist ponders muddy molecules and past climates.
I am amazed by how humble fossil lipids in muddy sediments can yield insight into Earth's history. The structures and relative abundances of these marine biomarkers, which originate from cellular membranes, provide records of physiological and ecological responses to changing ocean chemistry and temperature. They help to quantify ancient climates, and may, for example, offer a peek at the future by providing clues to ocean temperatures when the poles were free of ice.
Shifting abundance ratios of membrane lipids from marine Archaea — a proxy called TEX86 — faithfully indicate modern sea-surface temperatures. Yet ancient temperatures signalled by TEX86 can be significantly higher than those indicated by other proxies, making TEX86 hard to interpret.
Julius Lipp and Kai-Uwe Hinrichs at the Center for Marine Environmental Sciences in Bremen, Germany, show that the constituent compounds in TEX86 may be a mixture derived from ancient microbes and those living in muddy sediments today (J. S. Lipp and K.-U. Hinrichs Geochim. Cosmochim. Acta 73, 6816–6833; 2009).
The authors identified the mud-dwellers' lipids from their polar functional groups; ancient lipids lack these groups because they are quickly lost after burial. The core hydrocarbons waving the polar flags probably account for the proxy's overestimation of temperature. By identifying contributions from organisms living in sediments, the researchers provide a powerful means to discern which environments preserve the primary TEX86 signature and thus under which conditions we can reliably use this important proxy.
Climate scholars should take note and take heart, because this work will ultimately strengthen our interpretations of these muddy molecules to help us better understand Earth's past and future climate.
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