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You searched for +publisher:"Harvard University" +contributor:("Mitrovica, Jerry xmlui.authority.confidence.description.cf_uncertain"). Showing records 1 – 2 of 2 total matches.

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Harvard University

1. Masterson, Andrew Laurence. Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records.

Degree: PhD, 2016, Harvard University

Many of the long-term geochemical fluxes influencing the surface sulfur cycle are microbially catalyzed and a substantial portion of active S cycling occurs in organic-rich continental margin sediments. Stable S isotopes historically provide the most powerful analytical tool for understanding these small and large scale fluxes and for relating them back to laboratory characterizations of microbial metabolisms—particularly that of dissimilatory sulfate reduction. A more recent expansion of stable S isotope geochemistry to include the minor isotopes (33S and 36S) has demonstrated the capacity to diagnose the presence of additional S metabolic processes and to further characterize the response of microbial sulfate reduction to environmental forcing. In particular, emerging work suggests that multiple S isotope signatures in laboratory experiments are dictated by the physiological rate of a metabolic process. This is especially true for sulfate reduction. In this thesis we expand the scope of minor S isotope geochemistry to include early diagenetic processes, exploring the fidelity of laboratory calibrations and how they translate both to the modern marine sediments as well as an S isotope proxy records of seawater sulfate through the Cretaceous and Cenozoic. Early diagenesis of organic carbon in marine sediments via sulfate reduction is a dominant microbial process, and leaves a characteristic isotopic imprint in pore water and solid phase S-bearing species. To place those S isotope signatures into a physical context, we construct reactive transport models that take sulfur, carbon, and in one case, iron cycling into account for two geochemically well characterized sedimentary environments: Alfonso basin and Aarhus bay. Alfonso basin is an anoxic-silled marginal basin in the Gulf of California and Aarhus bay is a well-oxygenated, shallow coastal system. We demonstrate in both cases that large S isotope fractionations during microbial sulfate reduction (34εSR = 70‰) are required to explain the pore water isotope signatures, and there is no need for a depth or rate-dependent fractionation relationship. Furthermore, in the case of Aarhus Bay, it is clear that that isotopic contribution from oxidative S processes is negligible. Both these results – an apparent fixed 34εSR in sediments and little to no isotopic sensitivity to oxidative reactions – should hold true across similar, common shallow water marginal sediments. In parallel to work targeting the behavior of modern marine sediments, we revisit a well known δ34S proxy record for Cretaceous and Cenozoic seawater sulfate. Using minor isotope techniques, we demonstrate that Δ33S and Δ36S values are isotopically homogeneous (Δ33SSO4 = 0.043±0.016‰ and Δ36SSO4 = -0.39±0.15‰) despite δ34S variability. These observations, the first of their kind, place upper limits on pyrite burial and evaporite dissolution over the last 120 million years. Together, this thesis highlights analytical advances in stable isotope geochemistry and complements those measurements with reactive… Advisors/Committee Members: Mitrovica, Jerry xmlui.authority.confidence.description.cf_uncertain (advisor), Berelson, William M. (committee member), Pearson, Ann (committee member), Schrag, Daniel (committee member).

Subjects/Keywords: Geochemistry; Geology

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Masterson, A. L. (2016). Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records. (Doctoral Dissertation). Harvard University. Retrieved from http://nrs.harvard.edu/urn-3:HUL.InstRepos:33840718

Chicago Manual of Style (16th Edition):

Masterson, Andrew Laurence. “Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records.” 2016. Doctoral Dissertation, Harvard University. Accessed January 22, 2020. http://nrs.harvard.edu/urn-3:HUL.InstRepos:33840718.

MLA Handbook (7th Edition):

Masterson, Andrew Laurence. “Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records.” 2016. Web. 22 Jan 2020.

Vancouver:

Masterson AL. Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records. [Internet] [Doctoral dissertation]. Harvard University; 2016. [cited 2020 Jan 22]. Available from: http://nrs.harvard.edu/urn-3:HUL.InstRepos:33840718.

Council of Science Editors:

Masterson AL. Multiple Sulfur Isotope Applications in Diagenetic Models and Geochemical Proxy Records. [Doctoral Dissertation]. Harvard University; 2016. Available from: http://nrs.harvard.edu/urn-3:HUL.InstRepos:33840718

2. Robel, Alexander Abram. Ice Stream Variability and Links to Climate.

Degree: PhD, 2015, Harvard University

This dissertation explores the variability and climatic importance of ice streams, regions of fast flow in ice sheets. Observations indicate that ice stream variability plays an important role in the current mass balance of the West Antarctic Ice Sheet and may be related to periods of rapid climatic change in the past, such as Heinrich Events and glacial-interglacial transitions. We first explore a thermal-regulation mechanism to explain centennial- to millennial-scale ice stream temporal variability based on a simple model which couples ice stream dynamics to subglacial meltwater production. High geothermal heat flux or warm ice surface temperature lead to steady streaming flow, while low geothermal heat flux or cold ice surface temperature lead to thermally-regulated oscillations in ice stream flow. There is a hysteretic transition between these two regimes associated with a subcritical Hopf bifurcation. This simple model can reproduce the time scale and amplitude of ice stream variability associated with Heinrich Events, as well as modern Siple Coast ice streams which appear to be in an oscillatory parameter regime near the transition to a steady-streaming mode. To understand how this thermally-regulated ice stream variability is manifested at the grounding line, we use a purpose-built flowline model with lateral shear stresses and freely-varying bed properties. Unforced internal variability in ice streams causes rapid migrations in grounding line position with amplitude over 100 km at rates that can exceed 1 km/yr. An ice stream with net positive mass balance may still undergo retreat over a prograde slope due to thinning near the grounding line as part of unforced oscillatory behavior. Ice streams exhibiting unforced internal variability are far from a steady-state and their behavior cannot be explained with conventional theories of grounding line stability. The grounding line of a stagnant ice stream may persist on a retrograde slope for hundreds to thousands of years before reversing direction on the same slope. This behavior indicates that identifying whether an ice stream is in an oscillatory regime is critical for evaluating whether a grounding line that is observed to retreat onto a retrograde slope is likely to undergo irreversible retreat. Ice streams may have played a role in the rapid deglaciations which ended past glacial cycles. We use an idealized configuration of a 3D thermomechanical ice sheet model, which explicitly resolves ice streams, to simulate deglaciation in response to a change in climate motivated by Milankovitch forcing. We show that a large ice sheet, which is able to develop ice streams, is more sensitive to a change in climate forcing. This explains why ice sheets experience several precession and obliquity cycles before responding with a full deglaciation only when they reach a sufficiently large size. The rapid deglaciation of large ice sheets is primarily caused by accelerated calving at ice stream marine margins, which is, in turn, caused by enhanced driving stresses in ice… Advisors/Committee Members: Mitrovica, Jerry xmlui.authority.confidence.description.cf_uncertain (advisor), Schoof, Christian (committee member), Rice, James (committee member).

Subjects/Keywords: Geophysics; Environmental Sciences

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APA · Chicago · MLA · Vancouver · CSE | Export to Zotero / EndNote / Reference Manager

APA (6th Edition):

Robel, A. A. (2015). Ice Stream Variability and Links to Climate. (Doctoral Dissertation). Harvard University. Retrieved from http://nrs.harvard.edu/urn-3:HUL.InstRepos:23845475

Chicago Manual of Style (16th Edition):

Robel, Alexander Abram. “Ice Stream Variability and Links to Climate.” 2015. Doctoral Dissertation, Harvard University. Accessed January 22, 2020. http://nrs.harvard.edu/urn-3:HUL.InstRepos:23845475.

MLA Handbook (7th Edition):

Robel, Alexander Abram. “Ice Stream Variability and Links to Climate.” 2015. Web. 22 Jan 2020.

Vancouver:

Robel AA. Ice Stream Variability and Links to Climate. [Internet] [Doctoral dissertation]. Harvard University; 2015. [cited 2020 Jan 22]. Available from: http://nrs.harvard.edu/urn-3:HUL.InstRepos:23845475.

Council of Science Editors:

Robel AA. Ice Stream Variability and Links to Climate. [Doctoral Dissertation]. Harvard University; 2015. Available from: http://nrs.harvard.edu/urn-3:HUL.InstRepos:23845475

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