Geocheminar - spring-2012

April 10, 2012
noon - 1 p.m.
Slichter 3853

Presented By:

• Daniella Cusack - UCLA

Atmospheric nitrogen deposition is increasing in tropical regions, with potential to alter carbon storage and loss in these ecosystems. The driving question of this study was: 1) Does increased nitrogen have the potential to alter carbon storage and loss in tropical forests, despite high background nitrogen availability to plants? This study utilized a long-term nitrogen fertilization experiment in a lower elevation rainforest and an upper elevation cloud forest in Puerto Rico. Nitrogen fertilization increased bulk soil carbon storage for both of these tropical forests, with decreased losses of carbon via respiration from soils. A laboratory soil incubation experiment showed that soil respiration had higher temperature sensitivity in nitrogen fertilized soils. This study clearly indicates that carbon biogeochemical cycles can be highly sensitive to increased nitrogen availability in nitrogen-rich tropical forests. Atmospheric nitrogen deposition in tropical forests has the potential to increase soil carbon cycling, with implications for global carbon cycling and climate change.

April 17, 2012
noon - 1 p.m.
Slichter 3853

Presented By:

• Sumit Chakraborty - Institut für Geologie, Mineralogie und Geophysik

Perhaps the most fascinating aspect of Geological processes is the range of timescales they span. From developing the awareness of "deep time" (many millions of years) to the appreciation of massive forces that can be exerted within minutes (earthquakes, volcanic eruptions, Tsunamis, meteorite impacts), the study of geological processes has profoundly shaped our perception of time itself. More recently, discussion on topics such as climate change have underscored the convergence of time scales of human experience and geological processes. Yet, there remains a general perception that non-catastrophic processes in the interior of the Earth occur over timescales that are far too long to be of relevance for everyday human experience. Recent advances in technology and our improved understanding of kinetics (i.e. time dependence) of processes are now allowing us to recognize that such geological processes operate over a hierarchy of timescales. Geological events that last many millions of years are an amalgamation of episodes that occur over days, months or years. It is now possible to resolve these episodes. This talk will discuss some of the methods, and selected examples from the study of currently erupting volcanoes and mountain ranges that are forming today.

April 24, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Bob Steele - UCLA

The Solar System is a mixture of many different nucleosynthetic sources. Numerous studies have investigated the nucleosynthetic origins of the Solar System by examining isotopic anomalies in meteorites. Isotopic anomalies in the most neutron-rich isotopes of Ca, Ti, Cr in primitive meteorites have been interpreted as incomplete-mixing of a highly neutron enriched nucleosynthetic source into the early Solar System. This source has been variously hypothesised to be a type Ia supernova, type II supernova or an AGB star. However, nearly all of the reported measurements of these anomalies have employed normalisation to a pair of lighter-isotopes, assumed not to be anomalous. Hence, in these cases the inference of a neutron-rich anomaly is a point of interpretation since the anomaly could equally well reside on one of the normalising isotopes. More recently, Ni has been added to the list of elements exhibiting apparent anomalies in its most neutron-rich isotopes, 62Ni and 64Ni. We have made further measurements of Ni isotopes in bulk meteorites, both mass-independently (internally-normalised) and mass-dependently (double-spike), to yield absolute Ni isotope ratios that will better constrain the nucleosynthetic environment from which this incompletely mixed component arose. Our results show that, in fact, the anomalies reside on 58Ni and not on 62Ni or 64Ni.

May 1, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Carolyn Crow - UCLA
• Ellen Harju -

Although CR chondrites have experienced little or no thermal metamorphism, they have undergone varying degrees of parent-body aqueous alteration. O-isotopic measurements of CR chondrites show increasing ?17O and ?18O in more aqueously altered meteorites but these measurements have limitations because of terrestrial weathering and sample heterogeneity. Needed is a petrologic scale for aqueous alteration in CR chondrites analogous to the one constructed for CM chondrites. We examined 30 CR chondrites and used petrographic and compositional parameters to create a CR alteration index. At the onset of alteration mesostasis is converted to phyllosilicate. As alteration continues metal grains and sulfides are oxidized and then alteration of phenocrysts begins until all mafic silicates have been converted to phyllosilicates. The CRs show the full range of aqueous alteration with type 3.0 being the least altered and type 2.0 being the most altered; most have had minimal interaction with water and belong to type 2.9 or 3.0. ++++++++++++++++++++++++++++++++++ It is generally agreed that there was a steep decline in the impact rate on the Moon after 3.9 Ga, but there the nature of bombardment before this time is controversial. Zircons are ideal for investigating the early lunar bombardment because (1) low initial Pb results in high precision U-Pb age measurements, (2) the crystallization ages of lunar zircons all predate the proposed cataclysm, (3) zircons incorporate both U and Pu, so we can measure fissiongenic Xe degassing ages and Pb-Pb crystallization ages for the same crystal. We measured Xe isotopic abundances of three large (~300 ?m) individual Apollo 14 zircons using the University of Manchester Refrigerator Enhanced Laser Analyser for Xenon. Two of the samples produced sufficient xenon for precise xenon isotope ratios to be determined. All releases from these samples are consistent with the 238U end member suggesting little or no Xe contributed from 244Pu fission, which places a upper limit on the xenon degassing age of ~3.9 Ga.

May 8, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Jeanine Ash - UCLA

Extending the temperature record beyond the modern instrumental record is critical to understanding past climate variability and ecosystem response. Lake Tanganyika, Africa is one of the world’s largest rift lakes and is a hotspot of endemism. Our ability to predict the lake’s ecological response to climate change is of the utmost importance for fishery economy in eastern Africa. Here, we use ostracode trace metal geochemistry as a paleoenvironmental indicator in Lake Tanganyika. The trace element compositions of ostracode valves reflect discriminatory element uptake. This uptake reflects ambient environmental conditions and has previously shown promise for quantitative paleotemperature determination. In this study, HR-ICP-MS geochemical data are compared to an existing TEX-86 temperature record from Lake Tanganyika. Two ostracode species were chosen for analyses from core LT-98-58 (1759 +/- 133 AD-modern). Mecynocypria opaca is a near bottom swimming species whereas the lifestyle of Romecytheridea ampla is uncertain. Molar Mg/Ca ratios for M. opaca range from .04 to .16, and a trend towards increased Mg/Ca begins ~1880 AD. Molar Mg/Ca ratios for R. ampla range from .05 to .2, increase beginning at 1880 AD and decrease within the most modern sediments. Sr/Ca ratios in both species range from .003-.006 and remain relatively stable, indicating that changes in Mg/Ca are likely the result of temperature rather than salinity. The M. opaca Mg/Ca record closely resembles the existing TEX-86 paleotemperature record of Tierney et al. (2010) for the past ~240 yr. The variations within R. ampla Mg/Ca may be due to pore water geochemistry or some other unknown parameters. These preliminary results demonstrate that future research is necessary if the use of ostracode trace metal geochemistry is to be used as a paleotemperature proxy in lakes dominated by endemic species. Finally, preliminary data from an initial calibration set for the use of carbonate clumped isotope thermometry in lakes are shown.

May 15, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Matt Armentrout - UCLA
• Jeanine Ash - UCLA

The phase stability and high pressure and temperature equation of state of cobalt oxide was measured up to 65 GPa and 2600 K using the laser heated diamond anvil cell. We find that the B1 structure is stable relative to the low density rhombodedral phase at high temperature throughout this pressure range. We fit a Mie-Grüneisen-Debye model to the B1 data and use it in conjunction with existing thermoelastic parameters of cobalt metal to extend the CoO/Co oxygen fugacity buffer to high pressures and temperatures and to predict relative redox behavior in the deep Earth. ++++++++++++++++++++ Lake Tanganyika, Africa is one of the world’s largest rift lakes and is a hotspot of endemism. Our ability to predict the lake’s ecological response to climate change is of the utmost importance for fishery economy in eastern Africa. Here, we use ostracode trace metal geochemistry as a paleoenvironmental indicator. The trace element composition of ostracode valves reflects ambient environmental conditions and has previously shown promise for quantitative paleotemperature determination. In this study, HR-ICP-MS geochemical data are compared to an existing TEX-86 temperature record from Lake Tanganyika. Two ostracode species were chosen for analyses from core LT-98-58 (1759 +/- 133 AD to modern). Molar Mg/Ca ratios for M. opaca range from .04 to .16, and a trend towards increased Mg/Ca begins ~1880 AD. Molar Mg/Ca ratios for R. ampla range from .05 to .2 but lack a clear anthropogenic warming signal. Sr/Ca ratios in both species range from .003-.006 and remain relatively stable, indicating that changes in Mg/Ca are likely the result of temperature rather than salinity. The M. opaca Mg/Ca record closely resembles the existing TEX-86 paleotemperature record of the past ~240 years. Finally, preliminary data from an initial calibration set for the use of carbonate clumped isotope thermometry in lakes are shown.

May 22, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Cam Macris - UCLA

Stable isotope geochemistry and experimental petrology have been combined in a novel way through the implementation of the three-isotope technique in piston-cylinder experiments. First done by Matsuhisa et al. in 1978, this technique allows the experimentalist to determine the equilibrium fractionation between two phases by extrapolation from experiments run at varying lengths of time, freeing one from having to actually reach equilibrium, which might be unrealistic in the lab. The method was revived at UCLA in 2008, where we continue to integrate isotope geochemistry with experimental petrology to explore new tools for tracing chemical pathways relevant to Earth's formation and evolution. I will survey the methods and applications of the technique, including past, present, and future studies at UCLA and beyond. The isotopic systems discussed here will be Fe, Ni, Si, and the most recent system investigated, Mg.

May 29, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Veronika Heber - UCLA

Seminar Description coming soon.

June 4, 2012
noon - 1 p.m.
3853 Slichter

Presented By:

• Junko Isa - UCLA
• Michelle Jordan - UCLA

Fe and Si are common elements in both planetary cores and the silicate portions of planetary bodies, making them useful tools for understanding differentiation of the chemically distinct reservoirs. Equilibrium Fe and Si isotope fractionation between metal and silicate phases in differentiated meteorites can be used as a proxy for understanding the isotope fractionation that accompanies core formation in a planetary body. I will review previous theoretical calculations and experimental work used to quantify equilibrium Fe and Si isotope fractionation. Additionally, I will discuss earlier measurements of fractionation in meteoritic samples as well as current work being done to definitively determine the equilibrium metal-silicate Fe isotope fractionation in a meteorite. ********************** Rumuruti (R) chondrites have the highest D17O bulk values (3 ‰) among meteorite groups and are one of the two most oxidized groups. R chondrites show textural similarities to ordinary chondrites (OC) but have 3´ higher matrix contents. In past papers it was suggested that R-chondrite chondrules were the same as OC chondrules and that the high bulk D17O values were associated with the large fraction of phyllosilicate-rich (hydrated) fine matrix. However, new data show that the O-isotopic compositions of relict olivine phenocrysts in chondrules from unequilibrated R chondrites are diverse: some have with very low D17O values (-4‰) but some others have D17O values reaching +3‰, similar to those in bulk R chondrites. Thus R chondrules are not identical to OC chondrules, but are sampling a different nebular reservoir.