Thesis Defense - Spring 2009 - Robert Gallardo

Seismic Anisotropy in the Julian Schist, San Diego CA

Robert Gallardo
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Robert Mellors

Wednesday, May 13th, 2008
CSL 422, 9:00am



ABSTRACT
P-wave velocity measurements of a Julian Schist rock sample collected in the Laguna Mountains region of East San Diego County show significant P-wave velocity anisotropy at near surface conditions. Multiple P-wave velocity measurements were taken from orthorhombic cut sample of Julian Schist using the Tico Ultrasonic Instrument with varying parameters of length. A maximum anisotropy of 22.1% was obtained from the P-wave measurements gathered in the X, Y, and Z axes of the Julian Schist sample. Cracks within orthorhombic schist sample demonstrate the need to take measurements with increased pressure parameters as measurements made within the same axis on different locations show p-wave velocity differences of up to 54.0%. This is based on observations of varying numbers of visible cracks on the external surface of rock sample within the same axis of measurement. Increasing metamorphic grade has been known to represent increasing anisotropy within rock units and is an important factor in identifying proximity to faults, general lithologic and structural descriptions of regional rock types, and could have an effect on the Southern California Communal Velocity Model (SCCVM).

Thesis Defense - Spring 2009 - Jared Warner

Biotic Response of Ostracodes to the Middle Eocene Climatic Optimum (MECO; 40.0 Ma) in the Southern Indian Ocean (ODP Site 748)

Jared Warner
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Stephen Schellenberg

Wednesday, May 13th, 2008
CSL 422, 11:30am



ABSTRACT
The long-term cooling trend from “greenhouse” to “icehouse” conditions through the middle-to-late Eocene (49.0 – 33.7 Ma) was interrupted by the middle-Eocene Climatic Optimum (MECO; ~40.0 Ma), a transient (~500 kyr) global warming of ~4-6 C° widely recorded in marine and terrestrial stratigraphic sections. The broad biotic response to the MECO is poorly known, with most published studies focused on planktonic protists such as foraminifera, calcareous nannoplankton, and dinoflagellates. To initiate a complementary benthic biotic perspective, we conducted a high-resolution ostracode faunal analysis (>150 mm size-fraction) through the MECO at Ocean Drilling Program Site 748 on the southern margin of the Kerguelen Plateau (~725 m paleodepth) in the southern Indian Ocean. Ostracodes are the only readily preserved deep-ocean metazoans and thereby provide a unique biotic perspective on this geographically vast ecosystem.
The ostracode faunal assemblages were assigned to three stratigraphic intervals (pre-MECO, MECO, and post-MECO) based on the nominal onset and termination of the oxygen-isotope excursion. Ostracode generic richness, Margalef’s (d), and Simpson’s Index (1/l) were all significantly lower (ANOVA; p <0.001) within the MECO interval, indicating a decrease in ostracode diversity. A Likelihood Ratio Test (LRT) was used to further evaluate the diversity loss via an ecological response versus a sampling artifact. Decline in ostracode diversity is also denoted by the disappearance of Bairdia during the MECO interval, a significant pattern revealed by the LRT. In addition, within the MECO interval, the whole-faunal percentage of smoother, less ornate genera was significantly lower while some genus-specific adult:juvenile valve ratios were significantly higher; both patterns are consistent with a hypothesized decrease in carbonate saturation during the MECO. The absence of faunal turnover and pronounced changes in faunal percentages through the MECO stands in stark contrast to ostracode assemblages previously documented during other intervals of rapid climate change (i.e., PETM)

Thesis Defense - Spring 2009 - Dale Burns

The Recent Prehistoric Geochemical Evolution of Summit Lavas From Kilauea Volcano, Hawaii

Dale Burns
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Aaron Pietruszka

Wednesday, May 13th, 2008
CSL 422, 1:45pm



ABSTRACT
Lavas from Kilauea Volcano display rapid geochemical and isotopic variations (e.g., 206Pb/204Pb or Nb/Y ratios) on a time scale of decades to centuries. The wall of Kilauea Caldera at Uwekahuna Bluff exposes a sequence of recent prehistoric, caldera-filling lavas (erupted mostly between AD 900-1400). Here we present a detailed geochemical study (major element abundances, and Pb, Sr, and Nd isotope ratios) for lavas from the lower portion of the Uwekahuna Bluff section as well as trace element abundances for the entire section. The 206Pb/204Pb ratios of the lavas from the lower Uwekahuna Bluff display small variations that merge with an isotopic excursion towards low 206Pb/204Pb ratios in the lavas from the upper portion of the section. At least four distinct source components within the Hawaiian mantle plume are required to explain the variations in Pb, Sr, and Nd isotope ratios. Two of these components were previously thought to be restricted to the neighboring active volcanoes, Mauna Loa and Loihi Seamount. The occurrence of Mauna Loa- and Loihi-like mantle sources in Kilauea’s melting region (in addition to Kilauea’s typical mantle source) suggests that (1) the Hawaiian plume contains three large-scale compositional heterogeneities and (2) all three heterogeneities extend into Kilauea’s melting region. However, the source region of Kilauea lavas must also be heterogeneous on a small scale to explain the rapid variations in the Pb, Sr, and Nd isotope ratios of the lavas. The fourth component appears to be common to all three of the active Hawaiian volcanoes, and thus, might represent the plume matrix. The Uwekahuna Bluff lavas also reveal a systematic temporal excursion in ratios of highly over moderately incompatible trace elements (e.g., Nb/Y) towards the lowest values yet observed at Kilauea. These low Nb/Y ratios (which correlate with the low 206Pb/204Pb ratios) are thought to result from relatively high degrees of partial melting of the plume matrix. Based on a correlation between high inferred degrees of partial melting and high eruption rates historically at Kilauea, the low Nb/Y ratios of the Uwekahuna Bluff lavas suggest that the eruption rate at Kilauea’s summit may have been unusually high in recent prehistoric times.