Person: Petersen, Sierra Victoria
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Petersen
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Sierra Victoria
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Petersen, Sierra Victoria
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Publication Rapid Climate Change in the Cenozoic: Insights from Geochemical Proxies(2014-10-21) Petersen, Sierra Victoria; Schrag, Daniel P.; Johnston, David; Tziperman, EliStudying the mean state and variability of past climate provides important insight into the dynamically coupled climate system, directly aiding projections of future climate. Reconstruction of past climate conditions can be achieved using geochemical proxies including the novel clumped isotope paleothermometer. In this thesis I use multiple proxies to study climate variability during the last glacial period and at the onset of Antarctic glaciation. Greenland ice cores record repeated millennial-scale fluctuations in climate during the last glacial period known as Dansgaard-Oeschger (DO) cycles. We measure 18O of bulk sediment and planktonic foraminifera (Neogloboquadrina Pachyderma) in sediment cores from the North Atlantic to investigate fluctuations in sediment properties on the timescale of DO cycles. We find evidence of episodic deposition of carbonate ice-rafted debris near Iceland. Integrating these observations with published data and modeling studies, we propose a new hypothesis to explain DO cycles. We suggest that a large ice shelf in the Nordic Seas acted in concert with sea ice to set the slow and fast timescales of DO cycles. The ice shelf was periodically removed by subsurface warming with the timescale of shelf regrowth setting the duration of each interstadial. We utilize the clumped isotope proxy to reconstruct the climate history during a key period of the Cenozoic - the onset of Antarctic glaciation. To facilitate this work, a new inlet is developed to streamline sample preparation and reduce sample size requirements. We decrease the required sample size from 5-8mg to 1-2mg per replicate, while still achieving external precision of 0.005-0.010o/oo, equivalent to previous methods. This new capability increases the range of possible applications for the clumped isotope paleothermometer, specifically in the field of paleoceanography. We apply the clumped isotope paleothermometer to thermocline-dwelling foraminifera (Subbotina angiporoides and Subbotina utilisindex) from the Southern Ocean core ODP689 across the Eocene-Oligocene transition. With the clumped isotope paleothermometer we separate the contributions of near- surface temperature change and ice sheet growth on the ~1o/oo increase in 18O observed in planktonic foraminifera from this site. We measure no change in temperature, and 0.8+-0.2o/oo change in delta 18Osw, equivalent to 124-140% of the modern Antarctic ice sheet volume.Publication Clumped Isotope Measurements of Small Carbonate Samples Using a High-Efficiency Dual-Reservoir Technique(Wiley-Blackwell, 2014) Petersen, Sierra Victoria; Schrag, DanielRationale: The measurement of multiply substituted isotopologues of CO2 derived from carbonate has allowed the reconstruction of paleotemperatures from a single phase (CaCO3), circumventing uncertainty inherent in other isotopic paleothermometers. Traditional analytical techniques require relatively large amounts of carbonate (3–8 mg per replicate), which limits the applicability of the clumped isotope proxy to certain geological materials such as marine microfossils, commonly used for paleoclimate reconstructions. Methods: Clumped isotope ratio measurements of small samples were made on a new, high-efficiency, dual-reservoir sample-preparation inlet system attached to a Thermo-Finnigan MAT 253 mass spectrometer. Sample gas produced on the inlet is introduced from a 10 mL reservoir directly into the source via a capillary. Reference gas fills an identical 10 mL reservoir installed between the reference bellows and the capillary. The gas pressures in the two reservoirs are initially balanced, and are allowed to decrease together over the run. Results: Carbonate samples from 1 mg to 2.6 mg produced Δ47 values equivalent to those from the traditional two-bellows method with identical single-sample precision (1 SE = 0.005–0.015‰) and external standard error (SE = 0.006–0.015‰, n = 4–6). The size of sample needed to achieve good precision is controlled by the sensitivity of the mass spectrometer and the size of the fixed reservoirs and adjacent U-trap installed on our inlet. Conclusions: The high-precision clumped isotope measurements of small aliquots of carbonate obtained in this method allows for the application of this proxy to a wider range of geological sample materials, such as marine microfossils, that until now have been nearly impossible to analyze given sample size limitation.Publication A New Mechanism for Dansgaard-Oeschger Cycles(Wiley-Blackwell, 2013) Petersen, Sierra Victoria; Schrag, Daniel; Clark, P. U.[1] We present a new hypothesis to explain the millennial-scale temperature variability recorded in ice cores known as Dansgaard-Oeschger (DO) cycles. We propose that an ice shelf acted in concert with sea ice to set the slow and fast timescales of the DO cycle, respectively. The abrupt warming at the onset of a cycle is caused by the rapid retreat of sea ice after the collapse of an ice shelf. The gradual cooling during the subsequent interstadial phase is determined by the timescale of ice-shelf regrowth. Once the ice shelf reaches a critical size, sea ice expands, driving the climate rapidly back into stadial conditions. The stadial phase ends when warm subsurface waters penetrate beneath the ice shelf and cause it to collapse. This hypothesis explains the full shape of the DO cycle, the duration of the different phases, and the transitions between them and is supported by proxy records in the North Atlantic and Nordic Seas.