---

Biography

Although I have a fulfilling career as a professional student, when I finally grow up, I want to be an astronaut. My illustrious "career" began at the University of Miami (Go ‘Canes!), and continued at the to New Zealand (University of Otago), where I completed my Masters in Chemistry. Now, I find myself working in the Oceanography group at Stanford University where I am involved in several interdisciplinary projects. My scientific interests include a broad range of topics, including global climate models, trace metal speciation and biogeochemistry, the role of the Southern Ocean and Antarctica in global C cycling, coral reef ecosystem dynamics, as well as the analytical application of geochemical and isotopic proxies for paleoceanographic studies. Currently, I am using Sr/Ca ratios from marine barite as a paleoweathering proxy, and modeling the data in terms of GCMs. I am also involved in a collaborative project with workers in NZ (University of Otago). Here, we are attempting to use Sr, O and Nd isotopes from marine fossils (Eocene-Oligocene sequences) to provide information about paleoceanographic environments at the initiation and intensification of Antarctic circulation. Some of my previous research projects include:

Cu Speciation in Fresh Waters; with Keith Hunter and Jon Kim, University of Otago, NZ

The Interaction of Fe Species and Trichodesmium (a diazotrophic cyanobacteria); with Jim Moffett and John Waterbury, WHOI

Adsorption and Desorption of Phosphate onto Calcium Carbonate: Implications for Florida Bay; with Frank Millero, RSMAS

*Curriculum Vita*

---

Project Descriptions

Sr and Ca Substitution in Marine Barite: Implications for Reconstructing Seawater Sr and Ca Concentrations

Evidence suggests that atmospheric chemistry, ocean circulation, and climate patterns have changed significantly with time in response to tectonic movement; and changes in paleochemistry are strongly related to geo-oceanographic events.  In order to determine the resulting fluxes of weathered silicate and carbonate materials, records of seawater Sr and Ca are necessary. Barite (BaSO4) and celestite (SrSO4), and barite and anyhydrite (CaSO4), have been shown to exhibit solid-solution, such that Sr and Ca substitute for Ba in barite precipitating in marine environments. If this substitution occurs in proportion to seawater Sr and Ca concentrations, then a record of Sr/Ba and Ca/Ba ratios from marine barite may be used to interpret changes in chemical weathering regimes through time. This project constitutes the bulk of my PhD research, and involves investigating the in situ processes possibly affecting the Sr/Ba and Ca/Ba ratios of marine barite include temperature, pressure, salinity, saturation state, and how the microenvironments in which barite precipitates affects incorporation of these trace elements. The ultimate goal of this project is to produce an accurate, high-resolution, seawater Sr and Ca curve for the past 120 myr (1 myr resolution) using marine barite extracted from deep-sea sediments. High-resolution data sets from the last 500kyr  at two sites in the equatorial Pacific, through several glacial-interglacial cycles, shows fluctuations in seawater Sr that are coherent with changes in sea level, as determined from O isotope records.  This is substantial evidence we have that barite is likely recording seawater Sr, although the influence of other processes have not yet been constrained.  A long-term seawater Sr and Ca curve will allow for development of a comprehensive, global carbon model that will include carbonate versus silicate weathering rates and hydrothermal flux.  In addition, it is intended that periods of abrupt climate change will be emphasized.  Seawater Sr data from across the PETM have already been acquired, and the results indicate that the saturation state of the oceans with respect to barite did not change globally, as has been previously postulated. High-resolution data from across the Eocene/Oligocene Boundary are also expected, and will be compared with other proxies of chemical weathering (i.e. Os isotopes).
 

Reconciling Conflicting Results Among Proxies of Export Production

This work is intended to reconcile differences among commonly-used paleoproductivity proxies (marine barite, biogenic Ba, Al/Ti and Ba/Ti ratios) by evaluating the validity of associated assumptions and limitations. Export production fluxes (Cexport) can be calculated from previously developed, proxy-specific algorithms. Comparison of data from the same sediment sections in the equatorial Pacific (through several glacial-interglacial cycles) yields conflicting results even though the calibration for each proxy is based on the same core top samples.  The periodicity of Cexport using marine barite and biogenic Ba records is similar, although the relative magnitude of Cexport, particularly during glacial intervals, is significantly different.  Bulk sedimentary Al/Ti and Ba/Ti ratios co-vary; however, these ratios do not correspond with the downcore records of their respective excess concentrations (and their accumulation rates), or with contemporaneous records based on marine barite accumulation.  Although records based on accumulation rates may be compromised by limitations associated with age model and linear sedimentation rate calculations, or by sediment focusing, the marine barite and biogenic Ba records, when plotted as wt% (on a carbonate free basis), which is not dependent on accumulation rate calculations, agree with accumulation rate trends.  The differences among these data imply that some or all of these proxies do not exclusively respond to changes in export production. The contradictions also highlight the importance of addressing inconsistencies among paleoproxies and re-examining assumptions imbedded in proxy fundamentals--prior to applying paleoproductivity proxies and interpreting paleoceanographic records.

Productivity and Carbonate Deposition in the Pacific Basin through the Eocene/Oligocene Boundary

The relative percentage of barite and CaCO3 (wt %) can be used to determine spatial and temporal fluctuations in carbon export production and CaCO3 preservation. Accurate records of these indices may be used to de-convolute the relationship between export productivity, the delivery of carbonate to the seafloor, and fluctuations in the CCD affecting carbonate preservation.  Our intention is to use the relative quantities of barite and CaCO3 occurring across the Eocene/Oligocene Boundary at select locations at Shatsky Rise (Western Pacific).  During Leg 198, complete Eocene/Oligocene (E/O) Boundary sections were acquired at several sites.  Although oceanographic paleoproductivity changes associated with the E/O Boundary have been documented in terrestrial sequences, and deep-sea sediments from the Atlantic, Indian, and Southern Oceans (e.g. Barrera, 1993; Lazarus, 1993; Diester-Haas, 1993; Diester-Haas, 1996; Diester-Haas, 1995; Salamy, 1999; Latimer, 2002; Sarkar, 2003; Nilsen, 2003), to our knowledge there are no reported records from the Pacific Ocean.  The sedimentary sections from Shatsky Rise have the potential to contribute to our knowledge of how the Pacific Ocean responded to formation of a Circum-Antarctic Current compared to other oceanographic regimes. Ultimately, the sedimentary record of marine barite and carbonate distribution may reveal fluctuations in water mass characteristics and paleo-oceanographic conditions occurring in the Pacific Ocean across this climatically important interval.

---