University of Cambridge > > Institute for Energy and Environmental Flows (IEEF) > Why and how does organic matter in sediment matter to climate change science?

Why and how does organic matter in sediment matter to climate change science?

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At geological time scale (Ma and beyond), the Earth’s surface has recorded temperature changes in the order of a couple of 10ºC. The strong negative feedback that is responsible for such a narrow change in the Earth’s surface temperature (by comparison with what is known for Venus or Mars) is the temperature sensitivity of the kinetic of atmospheric CO2 consumption. For an inorganic reaction, such as the chemical weathering of silicate rocks and the precipitation of carbonate, such temperature sensitivity is the Arrhenius’ law. Initiated by Robert Garrels in the 1960’s, the modelling of such reaction at global scale led to a first order understanding of climatic changes found in the geological record for the Phanerozoic. However, a pure inorganic reaction fails explaining some of the key periods, like the sudden cooling during the upper Paleozoic (almost 300 Myrs ago) or the cooling during the Neogene (the last 25 Myrs). Most recent model are linking global rates of silicate chemical weathering to biologically enhanced weathering through the health of terrestrial ecosystems. However, we do not have direct evidences for this. The cycling of element by the vegetation can be monitored using stable isotopes. For instance, the vegetation preferentially selects the light isotopes of Ca leading to a measurable effect of the biological cycling of Ca in the 44/42Ca of soils under forest. In some cases, the extremely low 44/42Ca values witness the recycling of the Ca from the litter in the upper soil horizons. However inorganic processes can also led to Ca isotopic fractionation. For instance, the precipitation of secondary carbonate scavenges more of the light isotopes of Ca. The biological cycling of Ca becomes only witnessed by 44/42Ca in river water at small scale and during wet periods. At a larger scale than few km2, the biological cycling of Mg and Ca have no measurable impact on the isotopic composition of the dissolved load. The biological cycling is simply too fast to leave a mark on the dissolved load of sizeable rivers. An alternative feedback mechanism involves the rate of burial of organic material that is sensitive to the temperature (complex effect, with both positive and negative feedbacks) but even more directly to atmospheric CO2 levels (negative feedback). If the organic sub-cycle of the carbon cycle has been taken in consideration in any modeling attempt (primarily via the modeling of the 13C of carbonate), its direct study (both production and fate in the sediments) remained to be properly done at global scale. Recent studies in active tectonic areas (Taiwan, Himalaya) suggest a significant recycling of fossil organic matter associated with the burial of newly photosynthetised organic matter and both are climatically controlled, adding another negative feedback that partly accounts for the cooling during the Carboniferous but could also for the recent icehouse period.

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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