Browsing by Author "Jiang, Hehe"
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Item A Framework for Understanding Whole-Earth Carbon Cycling(Cambridge University Press, 2019) Lee, Cin-Ty A.; Jiang, Hehe; Dasgupta, Rajdeep; Torres, Mark; Orcutt, Beth N.; Daniel, Isabelle; Dasgupta, RajdeepBuilding from the base of knowledge presented in the preceding chapters, this chapter explores how the cycling of carbon in subduction zones and orogenic belts varies with supercontinent cycles and mountain building. It discusses the processes that link short-term and long-term carbon cycling and the timescales of these processes, such as the response times of weathering and atmospheric drawdown at periods of enhanced volcanism. This chapter covers topics of potential fluctuations in the long-term CO2 content of Earth’s atmosphere because of mantle–climate feedback, again taking advantage of the modeling platforms available for further exploration of these topics.Item Deep mantle roots and continental emergence: implications for whole-Earth elemental cycling, long-term climate, and the Cambrian explosion(Taylor & Francis, 2018) Lee, Cin-Ty A.; Caves, Jeremy; Jiang, Hehe; Cao, Wenrong; Lenardic, Adrian; McKenzie, N. Ryan; Shorttle, Oliver; Yin, Qing-zhu; Dyer, BlakeElevations on Earth are dominantly controlled by crustal buoyancy, primarily through variations in crustal thickness: continents ride higher than ocean basins because they are underlain by thicker crust. Mountain building, where crust is magmatically or tectonically thickened, is thus key to making continents. However, most of the continents have long passed their mountain building origins, having since subsided back to near sea level. The elevations of the old, stable continents are lower than that expected for their crustal thicknesses, requiring a subcrustal component of negative buoyancy that develops after mountain building. While initial subsidence is driven by crustal erosion, thermal relaxation through growth of a cold thermal boundary layer provides the negative buoyancy that causes continents to subside further. The maximum thickness of this thermal boundary layer is controlled by the thickness of a chemically and rheologically distinct continental mantle root, formed during large-scale mantle melting billions of years ago. The final resting elevation of a stabilized continent is controlled by the thickness of this thermal boundary layer and the temperature of the Earth’s mantle, such that continents ride higher in a cooler mantle and lower in a hot mantle. Constrained by the thermal history of the Earth, continents are predicted to have been mostly below sea level for most of Earth’s history, with areas of land being confined to narrow strips of active mountain building. Large-scale emergence of stable continents occurred late in Earth’s history (Neoproterozoic) over a 100–300 million year transition, irreversibly altering the surface of the Earth in terms of weathering, climate, biogeochemical cycling and the evolution of life. Climate during the transition would be expected to be unstable, swinging back and forth between icehouse and greenhouse states as higher order fluctuations in mantle dynamics would cause the Earth to fluctuate rapidly between water and terrestrial worlds.Item Interplays between magmatism, crustal deformation, surface erosion and long-term climate on the rise and fall of continental arcs(2018-04-16) Jiang, Hehe; Lee, Cin-Ty AContinental arc is one of the most dominant topographic features on Earth with complicated petrogenesis, active volcanism and rapid erosion. Interplays between the deep and surface processes in continental arcs play an important role in shaping the earth. The overall objective of this study is to develop a better understanding of the interconnections between various processes such as magmatism, uplift, surface erosion in continental arcs, and their influences on long term development of the atmosphere and hydrosphere. Continental arcs are characterized by episodic voluminous magmatic flare-ups in the time scale of tens to hundreds of millions of years. Due to the interaction of magma with sedimentary carbonates stored in the continental upper plate, flare-ups in continental arc might result in enhanced CO2 concentration in the atmosphere and greenhouse climate. However, development of continental magmatic arc often accompanied by rapid arc unroofing and erosion due to magmatic/tectonic thickening of the crust, thus continental arcs likely enhance the chemical weathering flux, in turn increasing the CO2 sink. With these feedbacks, it is not clear how long-term climate responds to the variability in the tectonic forcings in the continental arcs. In the first two chapters, I explored the dynamic linkage between magmatism and erosion in continental arcs and evaluated the carbon input and output associated with these processes. I found that during the rise and fall and continental arcs, the arc serves as either a carbon source or sink depending on the stage of the arc life. I propose that the development of continental arcs increases weatherability through mountain building processes, and therefore may increase the strength of the global negative feedback between silicate weathering and climate. In the rest studies, I investigated other processes associated with continental arc development. In Chapter 4, I investigated geochemistry of pseudotachylite from an ancient shear zone in a continental arc. This study attempts to obtain insights of the physics behind earthquake such as thermodynamic conditions and stress state in the shear zone from a geochemical aspect. As the rise and fall of the continental arc can drive changes in the subsurface hydrologic conditions, groundwater products can serve as proxies for the tectonic and climatic forcings in tectonic basins. In Chapter 5, I explored the prospects of using U-Pb dating of petrified (silicified) wood as means of quantifying ancient continental hydrology.Item Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation(Springer Nature, 2019) Tang, Ming; Lee, Cin-Ty A.; Chen, Kang; Erdman, Monica; Costin, Gelu; Jiang, HeheThe surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.Item Nb/Ta systematics in arc magma differentiation and the role of arclogites in continent formation(Springer Nature, 2019) Tang, Ming; Lee, Cin-Ty A.; Chen, Kang; Erdman, Monica; Costin, Gelu; Jiang, HeheThe surfaces of rocky planets are mostly covered by basaltic crust, but Earth is unique in that it also has extensive regions of felsic crust, manifested in the form of continents. Exactly how felsic crust forms when basaltic magmas are the dominant products of melting the mantles of rocky planets is unclear. A fundamental part of the debate is centered on the low Nb/Ta of Earth’s continental crust (11–13) compared to basalts (15–16). Here, we show that during arc magma differentiation, the extent of Nb/Ta fractionation varies with crustal thickness with the lowest Nb/Ta seen in continental arc magmas. Deep arc cumulates (arclogites) are found to have high Nb/Ta (average ~19) due to the presence of high Nb/Ta magmatic rutiles. We show that the crustal thickness control of Nb/Ta can be explained by rutile saturation being favored at higher pressures. Deep-seated magmatic differentiation, such as in continental arcs and other magmatic orogens, is thus necessary for making continents.Item Volcanic ash as a driver of enhanced organic carbon burial in the Cretaceous(Springer Nature, 2018) Lee, Cin-Ty A.; Jiang, Hehe; Ronay, Elli; Minisini, Daniel; Stiles, Jackson; Neal, MatthewOn greater than million year timescales, carbon in the ocean-atmosphere-biosphere system is controlled by geologic inputs of CO2ᅠthrough volcanic and metamorphic degassing. High atmospheric CO2ᅠand warm climates in the Cretaceous have been attributed to enhanced volcanic emissions of CO2ᅠthrough more rapid spreading at mid-ocean ridges and, in particular, to a global flare-up in continental arc volcanism. Here, we show that global flare-ups in continental arc magmatism also enhance the global flux of nutrients into the ocean through production of windblown ash. We show that up to 75% of Si, Fe and P is leached from windblown ash during and shortly after deposition, with soluble Si, Fe and P inputs from ash alone in the Cretaceous being higher than the combined input of dust and rivers today. Ash-derived nutrient inputs may have increased the efficiency of biological productivity and organic carbon preservation in the Cretaceous, possibly explaining why the carbon isotopic signature of Cretaceous seawater was high. Variations in volcanic activity, particularly continental arcs, have the potential of profoundly altering carbon cycling at the Earth's surface by increasing inputs of CO2ᅠand ash-borne nutrients, which together enhance biological productivity and burial of organic carbon, generating an abundance of hydrocarbon source rocks.