Thesis Defense - Spring 2008 - Cynthia Earl

Three dimensional elemental mass transfer during development of saprolite from corestone: Santa Margarita Ecological Reserve, Temecula, California

Cynthia Earl
B.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Gary Girty

Friday, May 9th

ABSTRACT
Our understanding of the characteristics of chemical weathering in a Mediterranean (hot summer) climate has increased significantly over the last few years. For example, we now know that as quartz dioritic corestone is converted to saprolite in this climate, biotite expands and weathers to mixed-layer clays while plagioclase and amphibole play much less significant roles. However, there have been no studies that have investigated the three dimensional aspect of this weathering trend. Hence, I undertook a study that utilized the previous work of Chris Martinez to constrain the horizontal aspects while I focused on assessing the relevance of this trend along a vertical sampling traverse through the same corestone-saprolite pair. I collected 9 specimens, 5 from the corestone and 4 from the saprolite, spaced approximately equally, along a 1.2 m vertical traverse. Each sample was thin sectioned for petrological work and was analyzed for major and trace elements using standard XRF methods. Samples of the corestone are hypidomorphic granular quartz diorites that exhibit little evidence of chemical alteration. In thin sections, euhedral to subhedral biotite exhibits deep brown pleochroism, sharp traces of the {001} cleavage, and typical “birds eye” extinction. Similarly subhedral plagioclase exhibits well developed polysynthetic twinning and concentric compositional zones, while amphibole commonly displays cleavage parallel to {110} at angles of 56o and 124o. In contrast, in saprolite overlying the corestone, the {001} cleavage in biotite becomes less distinct while pleochroism takes on pale orange hues. S. Johnson showed that these characteristics reflect the transformation of biotite to mixed layer expandable clays. Unlike biotite, both plagioclase and amphibole exhibit only mild indications of weathering, while quartz remains unweathered. The average Chemical Index of Alteration (CIA) for corestone samples analyzed during this study is 60.0 ± 0.1 (95% confidence interval) and for saprolite is 60.9 ± 0.5 (95% confidence interval). In A-CN-K ternary space, samples of the corestone plot below and more toward the K-apex than do specimens of saprolite which plot close to the A-CN join. Moreover, τ, the transport function, suggests that the masses of Si, Fe, Ca, Na, Mg, and Mn changed little relative to their masses in the average corestone. In contrast, the masses of K, Rb, and Ba are depleted significantly in the saprolite. Volumetric strain, ε, for the four samples of saprolite ranges from 16% to 21% while porosity ranges from 14.5 % to 16.5%. In contrast, there is little significant change in bulk mass in going from the corestone to the saprolite. The above weathering trends are similar to those documented by C. Martinez, and thus suggest that saprolite development from corestone is a three dimensional process that in Mediterranean (hot summer) climates depends primarily on the expansion and alteration of biotite to mixed layer expandable clay. Apparently, during this process stresses are produced that are sufficiently large enough to break or weaken adjacent intercrystalline bonds. Given sufficient time, this process will render solid and hard quartz diorite to saprolite, a weak material that breaks easily under moderate finger pressure.

Thesis Defense - Spring 2008 - Amelinda Webb

Quantifying the Ecological Response of Brachiopods during the Ordovician Extinction

Amelinda Webb
M.S. Candidate
Department of Geological Sciences
San Diego State University
Advisor Dr. Lindsey Leighton

Friday, May 9th

ABSTRACT
Extinction events are morbidly fascinating to both scientists and the public alike, with catastrophic events and death occurring on vast scales. The current biodiversity crisis is being hailed as the next mass extinction, and the need to understand the processes and patterns of extinction grow as the rate of species loss rises. Extinction events are recognized by taxonomic loss; however the ecological impact of these events is more difficult to quantify. This study applies rank-abundance curves (RACs), a modern ecological tool used in measuring ecosystem health, to quantify the changing community structure of brachiopods during the two pulses of the end Ordovician mass extinction. By applying RACs and measuring the shape with kurtosis, communities can be compared before and after the extinction event to recognize any early warning signs as well as the point of ecological recovery for communities which may be offset from the recovery of taxonomic diversity.
Brachiopod communities were sampled from Paul Copper’s collection from Anticosti Island located at the Geological Survey of Canada’s Ottawa office. Anticosti Island is the most complete, shallow water, fossiliferous record of the Ordovician- Silurian boundary in North America. Upper Ordovician through lowermost Silurian samples were collected from the surface of limestone slabs that showed no evidence of taphonomic overprinting. Stratigraphic position (within 10 members), lithology (Dunham carbonate classification scheme), and surface area of all samples were noted. Individuals were identified to the lowest taxonomic level (species in most cases), and both abundance and biovolume data were collected. RACs were generated for every sample with taxonomic richness greater than two. Samples dominated by one taxon were noted, and the frequency of these communities was compared to the kurtosis of the RACs from that member.
The average kurtosis for each member shows a distinct pattern through both pulses of the extinction. The members directly below each extinction pulse show increasing stress, and the members following the extinction pulse have significantly lower stress (t-test, p<0.05). The frequency of communities dominated by one taxon shows the same pattern as the average kurtosis. While the taxonomic recovery has not been identified until the Middle Silurian, there is an ecological recovery relatively quickly after the increased stress levels before each extinction pulse. These findings support previous qualitative work that suggests that the ecological impact of the end Ordovician extinction is not as severe as the taxonomic impact which ranks this extinction as the second worst in the history of life on Earth. RACs have been successfully used in modern systems, and now this technique can be applied to the fossil record to better understand the progression of and recovery from extinction events.

SEMINAR - R. Mark Leckie

Closure of the Indonesian Seaway during the Miocene: Early History of the Western Pacific Warm Pool

R. Mark Leckie
Department of Geosciences
University of Massachusetts, Amherst

Wednesday, May 7th, 2008

We document the waxing and waning of a ‘proto-Western Pacific Warm Pool’ and Equatorial Undercurrent (EUC) based on a study of multi-species planktic foraminiferal isotope ratios and census data spanning the 13.2-5.8 Ma interval at ODP Site 806 in the western equatorial Pacific (WEP). We hypothesize that the presence or absence of a proto-warm pool in the WEP, caused by the progressive tectonic constriction of the Indonesian Seaway and modulated by sea level fluctuations, created El Niño/La Niña-like alternations of hydrographic conditions across the equatorial Pacific during the late Miocene. This hypothesis is supported by the general antithetical relationship observed between carbonate productivity and preservation in the western and eastern equatorial Pacific, a phenomenon caused by these alternating ocean-climate states in the modern ocean. The two-step development and intensification of a proto-warm pool 11.6-9.6 Ma coincides with ice sheet expansion associated with Miocene isotope events Mi5 and Mi6, resulting in a cumulative sea level fall of 50 m and production of Northern Component Water. It also marks the initiation of a more modern equatorial current system as La Niña-like conditions became established across the tropical Pacific. This situation sustained carbonate and silica productivity in the eastern equatorial Pacific (EEP) at a time when carbonate preservation sharply declined in the Caribbean. Proto-warm pool weakening after 10 Ma may have contributed to the nadir of a similar ‘carbonate crash’ in the EEP. Decay of the proto-warm pool and resultant El Niño-like conditions brought higher productivity to the WEP, particularly 9.0-8.8 Ma coincident with a major perturbation in tropical nannofossil assemblages. This interval of increased productivity records the initial phase of the widespread ‘biogenic bloom’. Resurgence of a later proto-warm pool in the WEP 6.5-6.1 Ma may have spurred renewed La Niña-like conditions, which contributed to a strong late phase of the ‘biogenic bloom’ in the EEP.