Michael T. Hren

Re¿Os isotopic evidence for long-lived heterogeneity and equilibration processes in the Earth's upper mantle

The geochemical composition of the Earths upper mantle is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re–Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re–Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a ‘plum pudding’ distribution in the upper mantle, rather than from individual melt depletion events.

Biomarker reconstruction of the early Eocene paleotopography and paleoclimate of the northern Sierra Nevada

We reconstruct ancient temperature and elevation gradients across the early Eocene (52– 49 Ma) northern Sierra Nevada (California, United States) using organic molecular proxies that record atmospheric and ground-level effects of topography. Paleoelevation was determined by reconstructing the change in the hydrogen isotopic composition of precipitation (Δδ ΔδD precip ) and mean annual temperature (ΔT GDGT ) (glycerol dialkyl glycerol tetraethers) from the isotopic composition of fossil angiosperm leaf n-alkanes and the distribution of microbially produced soil tetraethers preserved in leaf-bearing sediments. Organic molecular data produce equivalent range-scale (δD n-alkane ) and channel (T GDGT ) paleoelevation estimates that show the northern Sierra Nevada was a warm (>6–8 °C warmer than modern), high-elevation (>2 km), and moderate- to low-relief landscape at the Eocene Climatic Optimum. Modern northern Sierra Nevada topography likely refl ects post-Paleocene reduction of mean surface elevation and late Cenozoic increases in relief.

Are high 3He/4He ratios in oceanic basalts an indicator of deep-mantle plume components?

The existence of a primordial, undegassed lower mantle reservoir characterized by high concentration of ^3He and high ^3He/^4He ratios is a cornerstone assumption in modern geochemistry. It has become standard practice to interpret high ^3He/^4He ratios in oceanic basalts as a signature of deep-rooted plumes. The unfiltered He isotope data set for oceanic spreading centers displays a wide, nearly Gaussian, distribution qualitatively similar to the Os isotope (^(187)Os/^(188)Os) distribution of mantle-derived Os-rich alloys. We propose that both distributions are produced by shallow mantle processes involving mixing between different proportions of recycled, variably aged radiogenic and unradiogenic domains under varying degrees of partial melting. In the case of the Re–Os isotopic system, radiogenic mid-ocean ridge basalt (MORB)-rich and unradiogenic (depleted mantle residue) endmembers are constantly produced during partial melting events. In the case of the (U+Th)–He isotope system, effective capture of He-rich bubbles during growth of phenocryst olivine in crystallizing magma chambers provides one mechanism for ‘freezing in’ unradiogenic (i.e. high ^3He/^4He) He isotope ratios, while the higher than chondritic (U+Th)/He elemental ratio in the evolving and partially degassed MORB melt provides the radiogenic (i.e. low ^3He/^4He) endmember. If this scenario is correct, the use of He isotopic signatures as a fingerprint of plume components in oceanic basalts is not justified.

The relationship between tectonic uplift and chemical weathering rates in the Washington Cascades: Field measurements and model predictions

Tectonic uplift is one of the key factors controlling the supply of material to weathering environments. At present however, there is debate about whether tectonics or climate plays a larger role in controlling chemical weathering rates on a global scale over geologic time. We measured riverine weathering fluxes from twelve catchments along the Skykomish River in the Washington Cascades, where long-term exhumation rates have been constrained by (U-Th)/He thermochronometry, to examine the effect of tectonic uplift on chemical weathering rates. We show that in the western Washington Cascades, dissolved Si fluxes increase from approximately 1900 to 3000 mol ha−1 yr−1 from west to east across the range and are correlated with rock exhumation rates. We use dissolved Si flux data from these catchments, depth to bedrock measurements, and long-term exhumation rates to test a steady-state weathering model that describes the relative importance of reaction kinetics and rock supply on weathering rates. Characterization of study basins with respect to two key variables in this model, weathering zone depth and exhumation rate, allows us to examine the relative effects of climate and tectonics on chemical weathering by analyzing the ratio of the timescale of reaction kinetics and the residence time in the weathering zone. Evaluation of this model shows that chemical weathering rates in the Cascades are controlled by the rate of rock erosion and factors that influence the depth of the active weathering zone. However, the model predicts that in tectonically inactive areas, the supply of material limits the overall rate of chemical weathering while in active tectonic or erosional environments, the rate of supply exceeds the ability of a system to weather that material resulting in a landscape in which weathering rates are expected to respond strongly to changes in climatic conditions.

Stable isotope food-web analysis and mercury biomagnification in polar bears (Ursus maritimus)

Mercury (Hg) biomagnification occurs in many ecosystems, resulting in a greater potential for toxicological effects in higher-level trophic feeders. However, Hg transport pathways through different food-web channels are not well known, particularly in high-latitude systems affected by the atmospheric Hg deposition associated with snow and ice. Here, we report on stable carbon and nitrogen isotope ratios, and Hg concentrations, determined for 26, late 19th and early 20th century, polar bear (Ursus maritimus) hair specimens, collected from catalogued museum collections. These data elucidate relationships between the high-latitude marine food-web structure and Hg concentrations in polar bears. The carbon isotope compositions of polar bear hairs suggest that polar bears derive nutrition from coupled food-web channels, based in pelagic and sympagic primary producers, whereas the nitrogen isotope compositions indicate that polar bears occupy > fourth-level trophic positions. Our results show a positive correlation between polar bear hair Hg concentrations and δ15N. Interpretation of the stable isotope data in combination with Hg concentrations tentatively suggests that polar bears participating in predominantly pelagic food webs exhibit higher mercury concentrations than polar bears participating in predominantly sympagic food webs.