Earth, Atmosphere & Environment Sciences
Title : | Magnesium Isotope History of Cenozoic Seawater: Continental Chemical Weathering vs. Hydrothermal Sequestration |
Area of research : | Earth, Atmosphere & Environment Sciences |
Principal Investigator : | Dr. Sambuddha Misra, Indian Institute Of Science, Bangalore, Karnataka |
Timeline Start Year : | 2023 |
Timeline End Year : | 2026 |
Contact info : | sambuddha@iisc.ac.in |
Equipments : | "PFA Acid Distillation Unit
PFA coated graphite hotplate" |
Details
Executive Summary : | Earth’s climate is controlled by the concentration of the greenhouse gas CO2 in the atmosphere (pCO2) and the quasi-periodic variations in the planet’s orbital motion (Milankovitch Cycle). On Glacial-Interglacial timescale (thousands of years) pCO2 is modulated by the exchange of CO2 between atmosphere and deep-ocean. On geological time scales (millions of years) pCO2 is moderated by a balance between CO2 supply by mantle degassing as volcanism and metamorphism, and CO2 consumption by chemical weathering of tectonically uplifted continental rocks. However, the hypothesis that continental chemical weathering is the prime feedback that has maintained equable climatic conditions over Earth history remains controversial. For example, the transition from the early Cenozoic hothouse to the modern glaciated world is caused by a decrease in pCO2; however, two contrasting hypotheses can explain this Cenozoic CO2 drawdown: (i) an increase in the rate of continental chemical weathering due to uplift of Himalayas, or (ii) a decrease in the rates of seafloor spreading and thus mantle degassing at mid ocean ridges. I will utilize Mg isotope (∂26Mg) history of seawater to deconvolve these two contrasting mechanisms for CO2 drawdown during the Cenozoic. The chemical weathering of continental rocks controls the delivery of cations to the oceans. Records of seawater chemistry preserved in carbonate fossils provide a powerful archive of the interplay and feedback between climate and chemical weathering fluxes. Magnesium is the 6th and 2nd most abundant cation in Earth’s crust and seawater respectively. Long-term changes in Mg concentration of seawater, especially the ratio of Mg to Ca is intricately linked to the global carbon cycle as this ratio controls the dominant form of biogenic and inorganic CaCO3 formation, with high ratios favoring the dissolution prone aragonite precipitation over the more dissolution resistant calcite. In seawater, Mg has a long residence time (13 My) and is homogeneous in both concentration and isotopic composition (∂26Mg-SW = -0.82‰). The concentration and isotopic composition of Mg in seawater are primarily controlled by its riverine supply (∂26Mg-Riv = -1.1‰), and by the two removal processes, high-temperature hydrothermal circulation at mid ocean ridges (∆∂26Mg-SW–HT = 0‰) and dolomitisation of marine carbonate sediments (∂26Mg-SW–Dol~-4‰). Low-temperature off-axis hydrothermal circulation and authigenic sediment formation are minor sinks of Mg from seawater. Present day ∂26Mg-SW is isotopically enriched than continental crust implying that seawater Mg budget is removal limited. A high-resolution ∂26Mg-SW record for the last 60Ma will allow us to answer three key questions: (1) did an increase in continental chemical weathering flux consume pCO2 to cause global cooling during the Cenozoic? (2) did the uplift of Himalaya modulate the increase in continental chemical weathering? (3) did the rates of mid ocean ridge spreading change? |
Total Budget (INR): | 54,59,360 |
Organizations involved