U23C-0071
Volatile Cycling and Planetary Evolution
The thermal history of terrestrial planets is investigated using a parameterized model that couples thermal convection and volatile cycling. The model breaks volatile inventory into surface and interior reservoirs and calculates the exchange between them. A volatile-dependent mantle rheology is incorporated. We focus on water at this stage. Water recycling into the mantle is assumed to occur principally through subduction of a hydrated serpentine layer whose thickness is self-determined based on serpentine stability. Degassing occurs through magmatism which is parameterized using a volatile dependent solidus. We present preliminary results that explore model parameter space. We have also coupled a radiative-convective climate model to our mantle-volatile model to explore the feedback effects between changing climate conditions, due to mantle degassing, and volatile cycling through the mantle. The model stress state is monitored for all parameter sweeps so as to evaluate the potential of changes in the tectonic mode of a planet (plate tectonics or a single plate mode).
U23C-0072
Variations in planetary convection via the effect of climate on damage
The generation of plate tectonics on Earth and its absence on the other terrestrial planets (especially Venus) remains a significant conundrum in geophysics. We propose a model for the generation of plate tectonics that suggests an important interaction between a planet's climate and its lithospheric damage behavior; and thus provides a simple explanation for the tectonic difference between Earth and Venus. We propose that high surface temperatures will lead to higher healing rates (e.g. grain growth) in the lithosphere that will act to suppress localization, plate boundary formation, and subduction. This leads to episodic or stagnant lid convection on Venus because of its hotter climate. In contrast, Earth's cooler climate promotes damage and plate boundary formation. The damage rheology presented in this paper attempts to describe the evolution of grain size by allowing for grain reduction via deformational work input and grain growth via surface tension- driven coarsening. We present results from convection simulations and a simple "drip-instability" model to test our hypothesis. The results suggest the feasibility of our proposed hypothesis that the influence of climate on damage may control the mode of tectonics on a planet.
U23C-0073
Selective Recycling of Graphite During Continental Erosion: a Long-Term Stabilization of C in the Crust
Organic C (OC) exported by rivers is a mix of recent OC (e.g. plant debris and soil OC) and petrogenic OC derived from erosion of rocks. Burial of petrogenic OC is a simple recycling of reduced C and has no effect on atmospheric CO2 and O2 levels. Conversely, its oxidation consumes O2 from and returns CO2 to the atmosphere. Addressing the role of continental erosion on the global C cycle thus requires assessing the fate of petrogenic OC during erosion. We showed that the Himalayan system is characterised by a very efficient burial of OC (Galy et al. 2007, Nature). In this study, we present a structural characterisation of petrogenic OC contained in source rocks, river sediments and marine sediments using Raman Microspectrometry (RM) and High-Resolution Transmitted Electron Microscopy (HRTEM). We use radiocarbon dating of bulk OC to quantify the proportion of petrogenic and recent OC in river sediments. Petrogenic OC has been detected by RM and HRTEM in Himalayan river and Bengal Fan sediments. In Himalayan rivers, structural organization of petrogenic OC is similar to that observed in the source rocks, from highly disorganised C to pure graphite. In the delta and Bengal Fan, sediments contain almost only highly graphitized petrogenic OC. Disorganised C particles are thus oxidised during transport in the floodplain, while highly graphitised C is selectively preserved and buried in Bengal Fan sediments. Radiocarbon dating of river sediments collected along vertical depth-profiles allows the quantification of petrogenic OC in river sediments. 30 to 50% of the OC contained in the Himalayan rocks appears to be preserved and recycled during the erosion cycle, mostly through burial of highly graphitic carbon in the Bengal fan sediments. Selective preservation of graphite during erosion and weathering indicates that graphitization occurring during metamorphism subtracts C from the external cycle (atmosphere-biosphere-ocean) and locks it into the geological cycle (crust-mantle). At geological timescales, the graphitization thus maintains an imbalance between photosynthesis and respiration by locking reduced C into the crust, hence promoting both consumption of CO2 and accumulation of O2 in the atmosphere.
U23C-0074
Coupled Atmosphere-Hydrosphre-Mantle Evolution: Exploring Mantle Feedbacks
Venus is, in bulk, nearly Earth's twin, yet the tectonic styles of the two planets are radically different. Suggested explanations for this extreme difference have invoked the lack of liquid water on Venus, the greater degree of crustal differentiation on Venus, and recently, the effect of the elevated surface temperature on convective stresses. These explanations generally involve an interaction between the state of the atmosphere/hydrosphere and the state of the mantle. Atmospheric evolution models have long considered the interaction between different reservoirs, including the mantle, but have never considered the feedback between rates of mantle processes and volatile recycling rates. Coupled atmospheric- hydrosphere-mantle dynamics models will be presented that explore the relationships between water recycling, mantle viscosities, lithospheric stresses, melt production rates and plate yielding. The models include two-dimensional solutions of mantle flow that include water-dependent viscosity and solidus behavior, volatile recycling and outgassing, and a parameterized greenhouse atmosphere with both carbon dioxide and water reservoirs. The complex system of feedbacks yields multiple equilibira and quasi-stable oscillatory states.