PhD Defense: Carlye Peterson

Event Date: 

Tuesday, March 8, 2016 - 8:00am

Carlye Peterson will present her PhD Defense on Tuesday March 8, 2016 at 8:00 AM in the Webb 1006 Conference Room.  Her talk is entitled Compilation of deglacial benthic stable carbon isotope data for reconstructing ocean circulation change, terrestrial biosphere change, and 3D visualizations.



Quaternary ice age cycles are characterized by fluctuations in atmospheric CO2, global temperature changes, ice volume changes, global vegetation changes, and physical and chemical changes in the ocean, including changes in deep ocean ventilation and circulation patterns. The ocean is the largest carbon reservoir and is likely a sink for atmospheric CO2 during glacials, but no one hypothesis can account for all the observed CO2 changes. A better understanding of global mean δ13C changes through time can help constrain the roles of vegetation and circulation changes on atmospheric CO2. By amassing a benthic δ13C dataset covering as many glacial cycles as geologic and coring constraints allow, we can systematically improve estimates of global mean δ13C, water mass boundary movements, deep ocean carbon storage changes, and past changes in ocean circulation. Additionally, regional and global stacks from a large benthic δ13C dataset spanning the last 150,000 years would improve the signal-to-noise ratio and reduce the uncertainty of mean δ13C estimates and water mass boundary changes further back in time.

            At the Last Glacial Maximum (LGM, 23-19 ka), most of the carbon released by the terrestrial biosphere was stored in the ocean, where the light isotopic signature of terrestrial carbon is observed as a 0.32–0.7‰ depletion in mean benthic foraminiferal δ13C values. We present a more precise estimate of glacial-interglacial δ13C change of marine dissolved inorganic carbon using benthic Cibicidoides wuellerstorfi δ13C records from 480 core sites (more than 3 times as many sites as previous studies). We divide the ocean into eight regions to generate linear regressions of regional δ13C versus depth for the Late Holocene (6–0 ka) and Last Glacial Maximum (23–19 ka) and estimate a mean δ13C decrease of 0.38 ± 0.08‰ (2σ) for 0.5–5 km. Estimating large uncertainty ranges for δ13C change in the top 0.5 km, below 5 km, and in the Southern Ocean, we calculate a whole-ocean change of 0.34 ± 0.19‰. This implies a terrestrial carbon change that is consistent with recent vegetation model estimates of 330–694 Gt C. Additionally, we find that a well-constrained surface ocean δ13C change is essential for narrowing the uncertainty range of estimated whole-ocean δ13C change.

Deglacial ventilation of respired carbon represents an important feedback in the climate system that impacts the carbon cycle, global temperatures, sea level, etc. Therefore, reconstructing deglacial changes in benthic δ13C can improve estimates of changes in deep ocean carbon storage and the global carbon cycle. Here we estimate the mean ocean δ13C in 1-kyr time steps across the deglaciation (20-6 ka) by placing 118 benthic δ13C records on regional age models, and averaging these nine regional δ13C stacks. Additionally we volume-weight intermediate and deep δ13C stacks, and a global ocean (0.75-5 km) mean δ13C stack.

We perform empirical orthogonal function (EOF) on 118 benthic δ13C records to identify the principle modes of variability in benthic δ13C. We find the first component of the EOF explains 39% of the variance in benthic δ13C, and describes the long-term deglacial trend in the reduction in deep ocean carbon storage related to the deglacial increase in atmospheric CO2. The second component in benthic δ13C explains 20% of the variance, and resembles proxies of changes in the strength of North Atlantic deep water formed and millennial scale (1000's of years) variability in North Atlantic iceberg calving event-sourced fresh water. This suggests a fifth of the variability in benthic δ13C is due to rapid changes in the formation of deep water in the North Atlantic. A simulated mixing experiment with Atlantic δ13C records from deep water formation regions indicates that the secondary signal in benthic δ13C is most consistent with a millennial-scale reduction in percent North Atlantic Deep Water at 2-4 km in the North Atlantic.

            Similar to PC1, the volume-weighted Deep δ13C stack minus Intermediate δ13C stack tracks deep ocean carbon storage and covaries with atmospheric CO2. Our volume-weighted global mean δ13C stack suggests the terrestrial biosphere expanded from 19-6 ka while atmospheric CO2 reached its interglacial value around 8 ka. This 4D δ13C data set is suitable for model-data comparisons and time-stepping 3D visualizations.

Can 3D visualizations improve undergraduate students’ understanding of fundamental Earth science concepts like plate tectonics? Our simple 3D Atlantic seafloor movie helps non-STEM students more than STEM students understand what the seafloor looks like and how ocean basins are formed. Simple 3D visualizations potentially benefit novice science students and closes the gap between STEM and non-STEM 



Carlye Peterson