Lithium Isotope Evolution of Cenozoic Seawater

This study presents the first high-resolution long-term history of seawater lithium isotope ratio (δ7LiSW) reconstructed from analyses of chemically cleaned planktonic foraminifera. The lithium isotope ratio of seawater (δ7LiSW ~31.0‰) is at secular equilibrium with its input sources via chemical weathering of the silicate continents (δ7LiRiv ~23‰), hydrothermal weathering of seafloor silicate basalts (δ7LiHT ~5.6‰) and removal by reverse weathering of authigenic sediments and seafloor basalts (δ7LiSED ~15‰). The δ7LiSW preserved in marine calcitic planktonic foraminifera provides a unique time tracer of changes in the global silica cycle. The Cenozoic 7Li/6Li record of seawater was constructed by analyzing over 300 age and species overlapping foraminifera samples, including both individual species and 'reverse picked' bulk foraminifera samples, from eight DSP/ODP Sites (588, 757, 758, 926, 1262, 1263, 1265, and 1267) with existing high resolution strontium isotope record. To meet the analytical requirements of foraminiferal δ7Li analyses an improved quadrupole-ICP-MS method for 7Li/6Li determination with low total lithium consumption (<0.2 ng/quintuplicate analyses) and high isotope ratio precision (≤ ±0.8‰, 2σ) was developed. A refined single step ion-exchange chromatographic method for quantitative lithium seperation, characterized by low blanks (1.0 ± 0.5 pg-Li) and high column yields (>99.98%), was also developed. The effects of foraminifera cleaning on calcite bound lithium concentrations and isotopic compositions were evaluated. A new analytical ICP-MS method for simultaneous determination of lithium, magnesium, manganese, vanadium, strontium, and barium ratios to calcium in chemically cleaned planktonic foraminifera was also developed. The newly developed analytical method and the improved foraminiferal cleaning technique was applied to late Cretaceous and Cenozoic samples to investigate the correlation between lithium isotopic composition and their lithium, magnesium, and strontium concentrations of the calcite shell as a guide to the to better explain the δ7Li record of seawater. The 68 Ma history of δ7LiSW spanning Late Cretaceous to Holocene demonstrates that the δ7LiSW decreased sharply by ~5‰ at the Cretaceous-Tertiary boundary and during the rest of the Cenozoic δ7LiSW increased by 8-9‰ over the last 60 Ma. Unlike the 87Sr/86Sr and 187Os/186Os isotope history of seawater, the rise in δ7LiSW during Cenozoic is not monotonous in nature. Plateaus and quasi-linear increases in δ7LiSW punctuate the 8-9‰ rise in seawater δ7Li during the Cenozoic. The sharp drop in δ7LiSW across K-Pg boundary was probably due to rapid supply large masses of isotopically light lithium to seawater from the congruent weathering of freshly erupted continental flood basalts (CFB's of Deccan Traps). The 8-9‰ rise in δ7LiSW during the rest of the Cenozoic suggests that hydrothermal contribution of isotopically light lithium to seawater has decreased over the Cenozoic, whereas, both lithium flux (FRiv) and isotopic composition (δ7LiRiv) of rivers have increased over the same period. Our findings suggest that neither δ7LiSW nor [Li]SW are buffered to oceanic basalts.