Michael A. Poage

Crustal reworking at Nanga Parbat, Pakistan: Metamorphic consequences of thermal‐mechanical coupling facilitated by erosion

Within the syntaxial bends of the India-Asia collision the Himalaya terminate abruptly in a pair of metamorphic massifs. Nanga Parbat in the west and Namche Barwa in the east are actively deforming antiformal domes which expose Quaternary metamorphic rocks and granites. The massifs are transected by major Himalayan rivers (Indus and Tsangpo) and are loci of deep and rapid exhumation. On the basis of velocity and attenuation tomography and microseismic, magnetotelluric, geochronological, petrological, structural, and geomorphic data we have collected at Nanga Parbat we propose a model in which this intense metamorphic and structural reworking of crustal lithosphere is a consequence of strain focusing caused by significant erosion within deep gorges cut by the Indus and Tsangpo as these rivers turn sharply toward the foreland and exit their host syntaxes. The localization of this phenomenon at the terminations of the Himalayan arc owes its origin to both regional and local feedbacks between erosion and tectonics.

Reconstructing the paleotopography of mountain belts from the isotopic composition of authigenic minerals

The paleorelief of mountain belts can be estimated from the δ 18 O value of authigenic minerals. Development of relief during mountain building often creates lee-side rain shadows in which precipitation is depleted in 18 O and D. The magnitude of this rain-shadow effect is strongly correlated to relief. A compilation of δ 18 O data from surface waters throughout the globe shows a linear relationship between net elevation change and Δδ 18 O ( R 2 = 0.79). Through the use of this relationship, we investigated the timing and magnitude of elevation change in the Southern Alps of New Zealand and the Sierra Nevada of California. The δ 18 O values of kaolinites from New Zealand show an ∼6‰ decrease in the early Pliocene that corresponds to an ∼2 km elevation change in the Southern Alps. The δ 18 O of smectites from the Sierra Nevada show little change since 16 Ma, suggesting that these mountains have been a long-standing topographic feature.

Topographic development of the Southern Alps recorded by the isotopic composition of authigenic clay minerals, South Island, New Zealand

Abstract The Southern Alps are developing as a consequence of oblique collision between the Pacific and Australian plates. The Southern Alps lie on the west side of the South Island of New Zealand and create a massive rain shadow where greater than 12 m/year of rain falls on the west coast and semiarid conditions exist to the east. The rain-out effect across the mountains causes precipitation west of the Southern Alps to have δD and δ 18 O values averaging −30‰ and −5.5‰, whereas precipitation in the rain shadow to the east is isotopically lighter (δD=−72‰ and δ 18 O =−9.8‰). Such large differences in the isotopic composition of precipitation would not have existed prior to the development of significant topography. We have examined the topographic evolution of the Southern Alps using oxygen isotope analyses of authigenic kaolinites formed in the rain shadow to the east of the mountains between the Cretaceous (low topography) and the Pleistocene. Changes in the isotopic composition of authigenic clay minerals forming in equilibrium with meteoric water in the stratigraphic sequence record the development of Southern Alps topography and the resultant rain shadow. Our oxygen isotope analyses of authigenic kaolinites show a 5–6‰ decrease in the early Pliocene, from ∼18.2‰ in older rocks, to ∼12.3‰ in younger rocks. In addition, smectite is abundant in all samples from the Late Miocene to Recent, but is conspicuously absent in most older rocks, suggesting a change to a generally drier climate roughly coincident with the isotopic shift in kaolinites. This method may be useful in unraveling timing of development of mountain belts elsewhere in the world.

Ca/Sr and Sr isotope systematics of a Himalayan glacial chronosequence: Carbonate versus silicate weathering rates as a function of landscape surface age

We explored changes in the relative importance of carbonate vs. silicate weathering as a function of landscape surface age by examining the Ca/Sr and Sr isotope systematics of a glacial soil chronosequence located in the Raikhot watershed within the Himalaya of northern Pakistan. Bedrock in the Raikhot watershed primarily consists of silicate rock (Ca/Sr 0.20 mol/nmol, 87 Sr/ 86 Sr 0.77 to 1.2) with minor amounts of disseminated calcite (Ca/Sr 0.98 to 5.3 mol/nmol, 87 Sr/ 86 Sr 0.79 to 0.93) and metasedimentary carbonate (Ca/Sr 1.0 to 2.8 mol/nmol, 87 Sr/ 86 Sr 0.72 to 0.82). Analysis of the exchangeable, carbonate, and silicate fractions of seven soil profiles ranging in age from 0.5 to 55 kyr revealed that carbonate dissolution provides more than 90% of the weathering-derived Ca and Sr for at least 55 kyr after the exposure of rock surfaces, even though carbonate represents only 1.0 wt% of fresh glacial till. The accumulation of carbonate-bearing dust deposited on the surfaces of older landforms partly sustains the longevity of the carbonate weathering flux. As the average landscape surface age in the Raikhot watershed increases, the Ca/Sr and 87 Sr/ 86 Sr ratios released by carbonate weathering decrease from 3.6 to 0.20 mol/nmol and 0.84 to 0.72, respectively. The transition from high to low Ca/Sr ratios during weathering appears to reflect the greater solubility of high Ca/Sr ratio carbonate relative to low Ca/Sr ratio carbonate. These findings suggest that carbonate weathering controls the dissolved flux of Sr emanating from stable Himalayan landforms comprising mixed silicate and carbonate rock for tens of thousands of years after the mechanical exposure of rock surfaces to the weathering environment. Copyright

The Influence of Soil Geochemistry on Nematode Distribution, Mcmurdo Dry Valleys, Antarctica

ABSTRACT Soils of the McMurdo Dry Valleys are among the most extreme terrestrial environments, hosting low-diversity food webs of microbes, protozoa, and metazoan invertebrates. Distribution of soil invertebrates, particularly nematodes, is related to the highly variable soil geochemistry of the valleys. Bull Pass is a glacially carved area within the McMurdo Dry Valleys where a broad range of geochemical conditions occurs along a continuous soil gradient. This site provides the opportunity to investigate how soil geochemistry controls nematode distribution on a local scale, and to establish correlations that may also be relevant at regional scales. At Bull Pass, two nematode species were present, with the dominant Scottnema lindsayae occurring in >30% of the samples. There were significant negative correlations between live nematode abundance and soil nitrate concentration and salinity, consistent with experiments showing strong salinity effects on nematode survival. A logistic regression model based on data sets from across the McMurdo Dry Valleys showed a strong negative relationship between soil salinity and the probability of live nematodes occurring. Soil chemistry and nematode distribution from the Bull Pass transect are compared with model results and suggest that the larger-scale distribution of nematodes across the McMurdo Dry Valleys may be reflected in the smaller-scale chemical and biological gradients at Bull Pass.

Isotopic evidence for Holocene climate change in the northern Rockies from a goethite-rich ferricrete chronosequence

Abstract Climate proxy records often show regional heterogeneity due to the complex interplay of large-scale atmospheric circulation patterns and local topographic features. This is especially prevalent in the mountainous regions of the western United States where the interaction of two seasonally distinct weather systems can result in locally abrupt climate boundaries. Here we establish oxygen isotope ratios of goethite from ferricrete chronosequences formed by natural acid rock drainage as a new source of spatially small-scale Holocene climate information. Systematic study of these deposits may prove useful in understanding regional variations in response to global Holocene climate change. Oxygen isotope ratios of goethites from the mountainous northeastern Yellowstone region show an increase of ∼3‰ over the past 9000 years. We attribute this isotopic change to a local increase of isotopically heavier summer precipitation since the early Holocene, possibly augmented by seasonal temperature differences brought on by orbitally forced insolation changes over the past 9000 years. Our interpretation is compatible with palynology studies as well as Holocene climate simulations, which indicate a decrease in monsoon intensity from 9000 years B.P. to the present. Unlike the southwestern United States, this change may have resulted in wetter summers in our study area.

OXYGEN ISOTOPE STUDIES OF ILLITE/SMECTITE AND CLINOPTILOLITE FROM YUCCA MOUNTAIN : IMPLICATIONS FOR PALEOHYDROLOGIC CONDITIONS

Abstract Illite/smectite (I/S) and clinoptilolite mineral separates from drill holes in 11 Ma old altered volcanic tuffs of Yucca Mountain, Nevada were analyzed for δ 18 O values. We reconstruct the diagenetic and paleohydrologic conditions using the isotopic composition of clay minerals combined with other independent geological observations. Our isotopic data show that the δ 18 O values of clay minerals preserve paleohydrologic information from a Middle to Late Miocene episode of hydrothermal alteration at Yucca Mountain. We provide additional evidence for a dual decoupled groundwater circulation system at Yucca Mountain ca. 11 Ma ago. A near-surface system dominated by downward percolation of groundwater is separated from a deeper hydrothermal system by profound thermal and isotopic discontinuities near the R0–R1 I/S transition. Given the set of inferred formation temperatures and the isotopic composition of formation water, we show that the current set of isotopic compositions of clinoptilolite is significantly lower than those at the time of formation, and are not significantly different from the equilibrium values defined by the current groundwater δ 18 O and geothermal gradient. We conclude that, unlike I/S, clinoptilolite has not preserved its original isotopic composition over the past 11 million years, but we do not know how long is necessary to reset the isotopic signature of clinoptilolite under near-surface conditions. The current groundwater at Yucca Mountain is 4%. more enriched in 18 O than the paleogroundwater 10–11 Ma ago as inferred from our reconstruction. This shift may be attributed to a change in the pattern of atmospheric circulation, such as by surficial uplift of the Sierra Nevada, which caused it to become an orographic barrier to moisture penetrating inland from the west.