Postdoctoral Research Associate
Climate Change and Earth-Ocean-Atmosphere Systems
Chemical evolution of cenozoic seawater
Earth is sustained as a habitable planet through close interactions and feed-backs among the lithosphere, hydrosphere, atmosphere, and biosphere. These interactions occur through material and energy transfer between Earth's reservoirs, referred to as global biogeochemical cycles, and result in chemical and physical changes within each reservoir. One such interaction is the chemical weathering of uplifted continental rocks which consumes carbon dioxide and transports cations to the oceans, thereby playing a critical role in controlling both seawater chemistry and climate. There are few archives of seawater chemical change that reveal shifts in global tectonic forces connecting Earth-ocean-climate processes. Past records of seawater chemistry, as recorded by calcite bearing foraminifera and preserved in marine sediments, is one such powerful archive for reconstructing how the Earth-ocean-climate interactions and feedbacks have changed over time. My work, as a part of the Godwin Laboratory for Paleoclimate Research, tracks the changes in seawater chemistry during Earth's history in response to changes in global climate and tectonics though the use of stable isotope (Li, B, and Mg) and trace element composition of foraminifera.
(a) Past Rapid Climate Change: Using foraminiferal boron isotope ratio (δ11B) and B/Ca to reconstruct changes in seawater pH and it's application as a proxy for pCO2 reconstruction for the Plio-Pleistocene with special emphasis to Glacial - Inter Glacial transitions.
(b) Silicate Weathering Driven Climate Control Over Geologic Timescales: Studying changes in continental silicate weathering and reverse weathering over the geologic past through the application of foraminiferal lithium, boron, and magnesium (δ7Li, δ11B, δ25Mg) isotope systematics.
(c) New Climate Proxies: Development and application of new isotope proxies for past ocean chemistry changes and paleoclimate studies. Simultaneously, I work on creating and improving established mass spectrometric (HR-ICP-MS; High Resolution Inductively Coupled Plasma Mass Spectrometry) determination methods for stable isotope ratio and trace element determination.
Sixty-eight million year (Late Cretaceous to Holocene) Li isotope record reconstructed from triple cleaned (Reductive-Oxidative-Reductive) planktonic foraminifera and published values of seawater records for 87Sr/86Sr and 187Os/186Os. Lithium isotope values (δ7Li), expressed as per mil (‰) variation from NIST L-SVEC standard (SRM 8450), are plotted on the top left Y-axis. The error bars represent 2s uncertainty associated with each quintuplicate measurement. The gray line represents 5-point running mean of δ7LiForam record and the two parallel black lines are the corresponding ± 2s uncertainty based on the average standard deviation of all d7LiForam measurements (s = ± 0.55‰, n = 301). Analyses of age overlapping samples from different ODP/DSDP sites were done to prove the absence of site-specific offset in δ7Li values. Foraminiferal Li and Sr data are color coded according to drill sites and are plotted on the same age chronology. The Cretaceous - Tertiary (K-Pg) boundary is set at 65.68. The Cenozoic marine Os isotope record (187Os/186Os) is plotted on the bottom left Y-Axis. Because of osmium's short residence time in the ocean and its isotopic sensitivity to impacts and mantle sources (LIP's), the 187Os/186Os record reflects large abrupt shifts that are not discernable in either the 87Sr/86Sr or 7Li/6Li records. The rise in d7LiSW during the Cenozoic is nonlinear, punctuated by transient flat steady-states and quasi-linear rises that may coincide with major climatic and tectonic events. The last 60 Ma history of 7LiSW can be divided into periods of quasi-linear rise and periods of flat steady state. Between 52 - 47 Ma, 35 - 31 Ma, and 14 - 6 Ma, the average rate of increase in δ7LiSW (∆δ7LiSW / ∆t) is ~ 0.4‰ ± 0.1‰ / Ma (2s). The net result is a 9‰ rise in d7LiSW during the past 60 Ma. This increase in d7LiSW is caused primarily due to large changes in continental weathering and seafloor reverse weathering consistent with increased tectonic uplift, more rapid continental denudation, increasingly incongruent continental weathering (lower chemical weathering intensity) and more rapid CO2 drawdown. A 5‰ drop in δ7LiSW across the Cretaceous-Paleogene boundary cannot be produced by an impactor nor by Deccan trap volcanism, suggesting large-scale continental denudation.