Kurt M. Ferguson

The origin and evolution of magmas from the San Pedro-Pellado volcanic complex, S. Chile: multicomponent sources and open system evolution

The San Pedro-Pellado volcanic complex is located at 36° S in the Chilean Andes. The eruptive rocks of the complex record the development and collapse of a caldera, followed by voluminous, largely basaltic andesite, volcanism. At each stage of evolution, crystal fractionation was accompanied by variable degrees of contamination and mixing. Large variations in incompatible element ratios cannot be produced by closed system evolution. Correlations between indices of differentiation and incompatible element ratios, together with high δ18O values, indicate that basaltic andesites have assimilated crust to generate the evolved volcanic rocks at San Pedro-Pellado. Even in the most mafic rocks, however, incompatible element characteristics are variable as a result of source heterogeneity and deep level processes. The restricted ranges in isotope ratios of Sr, Nd and Pb among San Pedro-Pellado rocks are due to the small contrast in isotopic compositions between magma and wallrock. Three source components are recognized as contributing to parental magmas at San Pedro-Pellado. Although the relative contributions of each cannot be quantified, the volumetrically dominant source component is the sub-arc asthenospheric mantle (MORB source). The major source of LILE is thought to be slab-derived fluids which modified the sub-arc mantle. Other incompatible elements may also have been enriched by interaction with the continental lithosphere (mantle and/or lower crust) during ascent.

Crust-magma interactions and the evolution of arc magmas: The San Pedro–Pellado volcanic complex, southern Chilean Andes

Field, petrographic, and trace-element data indicate that volcanic rocks of the Quaternary San Pedro–Pellado complex (lat 36°S, Chile) have incorporated a silicic component that may be derived from the crust. Isotope ratios of Sr, Nd, and Pb vary little among the volcanic rocks. This is due primarily to the limited isotopic contrast between the Mesozoic-Tertiary crust and the young magmas. Isotope and element compositions of San Pedro–Pellado rocks are similar to those reported from other localities in the southern volcanic zone of the Chilean Andes. We suggest that crustal assimilation is a significant process in the genesis of differentiated volcanic rocks in the southern volcanic zone. The San Pedro–Pellado example illustrates the importance of considering all types of evidence, not just isotopic data, in the evaluation of crustal contributions to arc magmas.

Stable Isotopes from Marine Shells, Ancient Environments, and Human Subsistence on Middle Holocene Santa Rosa Island, California, USA

ABSTRACT We report the analysis of stable isotopes on a suite of marine and estuarine shellfish, along with faunal data, from a Middle Holocene red abalone midden (CA-SRI-191) on Santa Rosa Island, California. Our research suggests that people harvested red abalones, mussels, and estuarine shellfish at about 6000 cal BP. Later in time around 4300 cal BP, people relied on California mussel and sea urchin with only trace amounts of red abalone and no estuarine taxa. Analyses of stable oxygen isotopes from California mussel, red and black abalone, and Venus clam shells indicate that a significant change in sea surface temperatures (SST; ∼2°C) between the two occupations is the primary cause for this discrepancy. Differences between the isotopic values of California mussel and black and red abalone shells from the same strata also suggest that people were probably diving in fairly shallow waters for red abalone rather than obtaining them from the intertidal zone. Our research and other studies in the region suggest that environmental and cultural variables (e.g., human harvest pressure) may have worked together to cause changes in the composition of local shellfish habitats. These data demonstrate the importance of conducting stable isotope analysis on a variety of different shellfish species (e.g., rocky coast, estuarine, and other taxa) in concert with traditional midden analysis to help understand ancient environmental, biological, and cultural developments in coastal, island, and other aquatic settings.

Fossil mice and rats show isotopic evidence of niche partitioning and change in dental ecomorphology related to dietary shift in Late Miocene of Pakistan.

Stable carbon isotope analysis in tooth enamel is a well-established approach to infer C3 and C4 dietary composition in fossil mammals. The bulk of past work has been conducted on large herbivorous mammals. One important finding is that their dietary habits of fossil large mammals track the late Miocene ecological shift from C3 forest and woodland to C4 savannah. However, few studies on carbon isotopes of fossil small mammals exist due to limitations imposed by the size of rodent teeth, and the isotopic ecological and dietary behaviors of small mammals to climate change remain unknown. Here we evaluate the impact of ecological change on small mammals by fine-scale comparisons of carbon isotope ratios (δ13C) with dental morphology of murine rodents, spanning 13.8 to ∼2.0 Ma, across the C3 to C4 vegetation shift in the Miocene Siwalik sequence of Pakistan. We applied in-situ laser ablation GC-IRMS to lower first molars and measured two grazing indices on upper first molars. Murine rodents yield a distinct, but related, record of past ecological conditions from large herbivorous mammals, reflecting available foods in their much smaller home ranges. In general, larger murine species show more positive δ13C values and have higher grazing indices than smaller species inhabiting the same area at any given age. Two clades of murine rodents experienced different rates of morphological change. In the faster-evolving clade, the timing and trend of morphological innovations are closely tied to consumption of C4 diet during the vegetation shift. This study provides quantitative evidence of linkages among diet, niche partitioning, and dental morphology at a more detailed level than previously possible.

A 7000-year sea-surface temperature record from CA-SRI-147, Santa Rosa Island, California, USA

California mussels (Mytilus californianus) excavated from archaeological site CA-SRI-147 on the southern side of Santa Rosa Island, California, provided a discontinuous environmental record spanning about 7000 years. An earlier report of the midden remains at this site found a weak inverse correlation between changes in mussel shell length and sea surface temperature (SST), but not productivity, and a decline in mussel size from ~5800 to 5100 calendar years before present during a period of low productivity and increasing SST. This study uses oxygen isotopes from the site to directly compare environmental change to shifts in faunal assemblage. Four intervals of low temperatures are revealed in the nearshore environmental data, each corresponding to a significant change in faunal assemblage at the site. The first occurs c. 6600 calendar years before present (cal. BP) and coincides with the greatest amount of mussel shell mass and size in the site. The second (c. 5500 cal. BP) occurs during an interval of variable mussel contribution and increasing red abalone (Haliotis rufescens) contribution. The third (c. 4800 cal. BP) occurs during a time when platform mussels (Septifer bifurcatus) exceed California mussels in meat weight, while the final interval (c. 3700 cal. BP) occurs prior to a period of probable site hiatus. The overall pattern of decreasing shell contribution and size at the site is not found in the mollusk isotope data, suggesting that environmental change had a profound impact on short-term variability, while human predation of key resources provided the long-term primary stress on nearshore biota.