PhD Thesis Research

PhD Abstract: A long-standing controversy in the geochemical community is whether the oxygen isotope composition of the ocean has changed significantly over Earth history.  Resolving this conflict is essential for understanding the ocean's paleochemistry and temperature evolution.  This thesis proposed to address whether the ocean was isotopically depleted by around 7‰ in the Proterozoic and has been increasing to today's value of 0‰ through the Phanerozoic by measuring oxygen isotopes in marine sulfate in well preserved marine barium sulfate (barite).  A new procedure for the extraction of marine barite from pelagic sediments for the measurement of its oxygen isotopic composition was developed.  The oxygen isotopic composition of marine barite showed unexpected variability over the past 60 million years, including excursions of approximately 6‰.   This suggested a reevaluation of the marine sulfur cycle, which highlighted the importance of sulfate reduction and sulfide reoxidation in organic-rich sediments.  Although the residence time of marine sulfate for sulfur isotopes is over 10 million years, because of the rapid cycling of sulfur in organic rich sediments, the residence time for oxygen isotopes is around 1 million years.  Coupled conservation equations were used to model temporal variability in the oxygen isotopic composition of marine sulfate, while simultaneously tracking sulfate concentrations.   Because the oxygen isotopic composition of marine sulfate does not correlate with the d34S of sulfate, the primary driver of temporal variations in the oxygen isotopic composition of marine sulfate must be changes in the sulfide reoxidation pathway in organic rich sediments.  This suggests that the redox variability of the oceans over the Cenozoic may be significantly greater than previously thought.   The oxygen isotopic composition of marine sulfate may be a powerful tool to use at times in Earth’s history when we know that the redox state of the oceans has changed dramatically, such as Cretaceous Ocean Anoxic Events (OAE).  Initial results of oxygen isotopic composition of marine sulfate measurements around one of the largest OAEs suggest there were significant changes in the redox state of the oceans prior to the onset of whole-ocean anoxia.  Unexpected variations in oxygen isotopic composition of marine sulfate profiles in porewaters of organic-rich sediments highlight the complexity of the pathways of sulfate reduction and sulfide reoxidation, but suggest that the oxygen isotopic composition of marine sulfate may help resolve redox coupling and pathways of organic matter remineralization. 

Chapter 1: Methods

In the first chapter of my thesis I detailed the method I developed for extracting marine barite from ocean sediments to measure its oxygen isotopic composition. This chapter was published in shorter form in both Chapters 2 and 3. You can download it here.

Chapter 2: Oxygen Isotope Constraints on the Sulfur Cycle over the Past 10 Million years

Published in Science, 2004. Download Paper. Abstract: Oxygen isotopes in marine sulfate measured in marine barite show variability over the past 10 million years including a 5‰ decrease during the Plio-Pleistocene with near constant values during the Miocene that are slightly enriched over the modern ocean.  A numerical model suggests that sea level fluctuations during Plio-Pleistocene glacial cycles impacted the sulfur cycle by reducing the area of continental shelves and increasing the oxidative weathering of pyrite.  The data also require that sulfate concentrations were 10-20% lower in the late Miocene than today.

Chapter 3: Evolution of the Sulfur cycle over the Cenozoic: Insight from Oxygen isotopes in marine sulfate

Published in EPSL, 2006. Download Paper. Abstract: We report new data on oxygen isotopes in marine sulfate measured in marine barite (BaSO4), over the Cenozoic. The oxygen isotopic composition of marine sulfate varies by 6‰ over the Cenozoic, with major peaks 3, 15, 30 and 55 Ma. The oxygen isotopic composition of marine sulfate does not co-vary with the sulfur isotopic composition of marine sulfate, emphasizing that different processes control the oxygen and sulfur isotopic composition of sulfate. This indicates that temporal changes in the oxygen isotopic composition of marine sulfate over the Cenozoic must reflect changes in the isotopic fractionation associated with the sulfide reoxidation pathway. This suggests that variations in the aerial extent of different types of organic-rich sediments may have a significant impact on the biogeochemical sulfur cycle and emphasizes that the sulfur cycle is less sensitive to net organic carbon burial than to changes in the conditions of that organic carbon burial. The oxygen isotopic composition of marine sulfate also does not co-vary with oxygen isotopes measured in benthic foraminifera, emphasizing that oxygen isotopes in water and sulfate remain out of equilibrium over the lifetime of sulfate in the ocean. A simple box model was used to explore dynamics of the marine sulfur cycle with respect to both oxygen and sulfur isotopes over the Cenozoic. We interpret variability in the oxygen isotopic composition of marine sulfate to reflect changes in the aerial distribution of conditions within organic-rich sediments, from periods with more localized, organic-rich sediments, to periods with more diffuse organic carbon burial. While these changes may not impact the net organic carbon burial, they will greatly affect the way that sulfur is processed within organic-rich sediments, impacting the sulfide reoxidation pathway and thus the oxygen isotopic composition of marine sulfate. Our qualitative interpretation of the record suggests that sulfate concentrations were probably lower earlier in the Cenozoic.

Chapter 4: Oxygen isotope variations in Sulfate over the Cretaceous

Work in progress. Not currnelty available for download. Part of this work was presented as a talk at the Earth Systems Processes Meeting in Calgary, Alberta, August 2005. Abstract: We present measurements of the oxygen isotopic compoition of carbonate associated sulfate (CAS) and trace barite, carbonand oxygen isotopes of carbonate, and major element concentrations for the late Albian, Cenomanian, and early Turonian from the pelagic limestones of the Scaglia Bianca formation in the Umbria-Marche Apennines.  The oxygen isotopic composition of sulfate of CAS and trace barite show wide variability over the interval, with excursions of over 5‰.  This may be further evidence of lower marine sulfate concentrations in the Cretaceous.  While the oxygen isotopic composition of CAS and trace barite track each other over the section, the oxygen isotopic composition of trace barite is, on average, 1 to 3‰ lighter.  The sulfur isotopic composition of marine sulfate is on average lower and the oxygen isotopic composition of marine sulfate is on average higher in the Cretaceous compared to the Cenozoic.  Reevaluation of the sulfur cycle suggests that these differences may be explained through changes in the dynamics of sulfur cycling in warm shallow oceans consistent with conditions in the Cretaceous. High-resolution isotopic and elemental measurements were made before and after the Bonarelli shale horizon (representative of global Oceanic Anoxic Event -2, OAE-2), which lies near the top of the section.  Oxygen isotopes in sulfate are a sensitive indicator of changes in redox conditions in organic rich sediments because they track subtle changes in the sulfide reoxidation pathway.  Before the Bonarelli the oxygen isotopic composition of marine sulfate rises by 6‰, and remains enriched for several meters above the shale. The rise precedes the shale by a few meters, indicating sulfate oxygen may highlight processes causal to the OAE. 

Work that has grown from the PhD

• Oxygen isotopes in sulfate in Porewater Profiles through organic rich sediments. This data was presented as an appendix in my Ph.D., was presented as a poster at Fall AGU 2005 (Abstract/AGU Poster (5Mb)) and is currently being prepared for publication. Once it has been submitted (anticipate late January 2006) I will post the title and abstract here.

• Comparison of oxygen isotopes in various sulfate minerals. Matt Hurtgen (Northwestern, webpage - link opens in new window) and I are working on comparing oxygen isotopes in carbonate associated sulfate and trace barite extracted from the same rocks. We used four experiments to explore the efficacy of these two paleoceanographic proxies for the oxygen isotopic composition of marine sulfate and the sulfur cycle. This work is currenlty in progress and being prepared for publication. Once it has been submitted (anticipate April 2006) I will post the title and abstract here.

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