Introduce the strontium isotope (87Sr/86Sr, δ88/86Sr) system
——— a more comprehensive means to understand ocean chemistry
Sr has four isotopes, 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%). Among them, only 87Sr is radiogenic and is formed from radioactive decay of 87Rb with a half-life of 4.88×10^9 years. Traditionally, people use the term “strontium isotope” to represent “radiogenic 87Sr/86Sr”. Recent advancements in mass spectrometry have enabled the measurement of stable Sr isotope ratios (δ88/86Sr).
Radiogenic Sr (87Sr/86Sr)
Seawater 87Sr/86Sr ratios recorded in marine carbonate rocks has long been used to reconstruct changes in the balance between hydrothermal and weathering fluxes to the ocean.
Elevated continental discharge = higher 87Sr/86Sr (more radiogenic)
Elevated hydrothermal/diagenitic fluids = lower 87Sr/86Sr (less radiogenic)
Traditionally, the normalization procedure employed to account for instrumental fractionation during radiogenic Sr isotope analysis eliminates any naturally occurring mass-dependent 87Sr isotope fractionation.
The stable Sr isotope variations are reported in delta notation (per mil, ‰) relative to the NIST SrCO3 standard reference material 987:
δ88/86Sr = [(88Sr/86Sr)smp /(88Sr/86Sr)std -1] ×1000.
Carbonate precipitation preferentially incorporates lighter strontium isotopes (carbonates have relatively low δ88/86Sr values, e.g. Fietzke and Eisenhauer, 2006). Unlike internally normalized 87Sr/86Sr ratios, the δ88/86Sr signatures not only differentiate between riverine, hydrothermal, and diagenetic inputs, but also the flux of carbonate burial.
The strontium isotopes system (87Sr/86Sr and δ88/86Sr) enable us to construct a more comprehensive evaluation of global carbon cycle, thereby, the interactions between continental weathering, carbonate burial rates, ocean chemistry and their relationship to massive environmental change.
Paleo-ocean chemistry change during Permian-Triassic Mass Extinction
——— Understanding one of “the big five” (click image for project description)
A plot of genus extinction intensity throughout the past 540Ma (from Wikipedia)
Aftermath of the Neoproterozoic Snowball Earth – Cap carbonates
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Pore evolution within shale using Neutron Scattering
The origin of interbeded carbonate layer within Marcellus shale