Colloquium: Nivedita Thiagarajan

Nivedita Thiagarajan, Postdoctoral scholar in Geochemistry, Division of Geological and Planetary Sciences, Caltech

Abstract: Deep-sea corals are a relatively new and powerful archive for reconstructing past oceanic conditions.  Here I show the results of our efforts towards developing them as a temperature and circulation proxy as well as an application during the last deglaciation. 

I calibrated the carbonate clumped isotope thermometer in 11 specimens of modern deep-sea corals spanning a total range in growth temperature of 2–25 ^oC.  I find that deep-sea corals exhibit a temperature-dependent trend in clumped isotope value that is indistinguishable from inorganic calcite and can resolve temperatures with sub-degree precision.  I then use a new dating technique, called the Reconnaissance ¹⁴C Dating Method to investigate the ecological response of deep-sea coral communities in the North Atlantic and Southern Ocean to both glaciation and rapid climate change.  I find that the deep-sea coral populations in both the North Atlantic and the Southern Ocean expand at times of rapid climate change.  The most important factors for controlling deep-sea coral distributions are likely climatically driven changes in productivity, [O₂] and [CO₃^2-].

Finally, I apply clumped isotope and radiocarbon measurements in deep-sea corals to tackle one aspect of the problem of how the structure of the deep ocean changes during glacial/interglacial transitions.  The last deglaciation started at ~19ka in the Southern hemisphere but full deglacial warming only began in the high latitudes of the Northern hemisphere at 14.7ka, coinciding with the start of the Bolling/Allerod.  At the start of the Bolling/Allerod, the strength of whole ocean overturning dramatically increased, however, the mechanism for the restart of ocean circulation is still being debated.  I measured clumped isotope temperatures and Δ¹⁴C in a depth transect of deep-sea corals from the North Atlantic, which grew during this transitional time-period.  My measurements showed the existence of a poorly ventilated and warm water mass underneath colder waters for several hundred years prior to the restart of ocean circulation and the start of the Bolling/Allerod.  I propose that the gravitational potential energy stored in this warm deep water below cold water ultimately becomes unstable and leads to the largest reorganization of climate during the last deglaciation, a new idea for how the system moves from glacial to interglacial states.