Travis W. Horton

The Cenozoic climatic and topographic evolution of the western North American Cordillera

Herein we present oxygen isotope records from Cretaceous to Recent terrestrial sediments in the western North American Cordillera. The purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the Cenozoic. To do this we constructed δ18O maps of surface waters for time intervals that trace the development of topography of western North America. These maps are based on 4861 oxygen isotope analyses from both published (4478) and new (383) data. We determined the δ18O values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors. These data suggest that in the late Cretaceous to early Eocene the Sevier hinterland formed a plateau of unknown height. Around 50 Ma, a topographic wave developed in British Columbia and eastern Washington that swept southward reaching northeastern Nevada at ∼40 to 38 Ma, and southern Nevada ∼23 Ma. This southward encroachment of an Eocene Plateau (SWEEP) caused reorganization of drainage patterns such that the intraforeland basins of Wyoming and Utah drainages extended deep within the Sevier hinterland as the wave swept southward. The landscape within the Sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of ∼4 km and relief of ∼1.5 km. This Eocene highland was bordered on the west by a high Sierra Nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands. The spatial and temporal evolution of this highland correlates with the timing of volcanism and extension. These observations support tectonic models that call for north to south removal of the Farallon slab or piecemeal removal of mantle lithosphere. The isotopic data show that prior to growth of this highland the North American Monsoon (NAM) penetrated much farther north in the Paleocene/Eocene than today. The combined effects of global cooling, increasing latitudinal temperature gradients, and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the NAM front. By the Oligocene the hydrologic regime that we observe today was in place. It has been modified since then as a result of Basin and Range extension and collapse of the highlands in the mid-Miocene. This collapse allowed the NAM to penetrate farther north into the Great Basin of Nevada and Utah.

Straight as an arrow: humpback whales swim constant course tracks during long-distance migration

Humpback whale seasonal migrations, spanning greater than 6500 km of open ocean, demonstrate remarkable navigational precision despite following spatially and temporally distinct migration routes. Satellite-monitored radio tag-derived humpback whale migration tracks in both the South Atlantic and South Pacific include constant course segments of greater than 200 km, each spanning several days of continuous movement. The whales studied here maintain these directed movements, often with better than 1° precision, despite the effects of variable sea-surface currents. Such remarkable directional precision is difficult to explain by established models of directional orientation, suggesting that alternative compass mechanisms should be explored.

Chemical weathering and lithologic controls of water chemistry in a high‐elevation river system: Clark's Fork of the Yellowstone River, Wyoming and Montana

Seasonal analyses of surface water geochemistry were conducted in the Clarks Fork of the Yellowstone watershed to determine whole-rock weathering rates. The Clarks Fork of the Yellowstone is a high-elevation catchment with distinct bedrock lithologies. Using dissolved solute concentrations and stream flow data, we calculated cation denudation rates of 119 g m−2 yr−1 (65,900 eq ha−1 yr−1) for carbonate-rich sedimentary rocks, 16.6 g m−2 yr−1 (8200 eq ha−1 yr−1) for andesitic volcanics, and 9.8 g m−2 yr−1(5300 eq ha−1 yr−1) for granitic gneisses. Ca/Na ratios indicate that chemical weathering of disseminated calcite in granitic rocks contributes to the total solute load in these subcatchments. Removal of this c(2100 eq ha−1 yr−1).

Leaf respiration and alternative oxidase in field‐grown alpine grasses respond to natural changes in temperature and light

• We report the first investigation of changes in electron partitioning via the alternative respiratory pathway (AP) and alternative oxidase (AOX) protein abundance in field-grown plants and their role in seasonal acclimation of respiration. • We sampled two alpine grasses native to New Zealand, Chionochloa rubra and Chionochloa pallens, from field sites of different altitudes, over 1 yr and also intensively over a 2-wk period. • In both species, respiration acclimated to seasonal changes in temperature through changes in basal capacity (R₁₀) but not temperature sensitivity (E₀). In C. pallens, acclimation of respiration may be associated with a higher AOX : cytochrome c oxidase (COX) protein abundance ratio. Oxygen isotope discrimination (D), which reflects relative changes in AP electron partitioning, correlated positively with daily integrated photosynthetically active radiation (PAR) in both species over seasonal timescales. Respiratory parameters, the AOX : COX protein ratio and D were stable over a 2-wk period, during which significant temperature changes were experienced in the field. • We conclude that respiration in Chionochloa spp. acclimates strongly to seasonal, but not to short-term, temperature variation. Alternative oxidase appears to be involved in the plant response to both seasonal changes in temperature and daily changes in light, highlighting the complexity of the function of AOX in the field.

Holocene climate change in arid Australia from speleothem and alluvial records

New high-resolution MC-ICPMS U/Th ages and C and O isotopic analyses from a Holocene speleothem in arid south-central Australia provide evidence for increased effective precipitation (EP) relative to present at c. 11.5 ka and c. 8—5 ka, peak moisture at 7—6 ka, and onset of an arid climate similar to present by c. 5 ka. δ18O and δ13C time-series data exhibit marked (>+1‰) contemporaneous excursions over base-line values of −5.3‰ and −11.0‰, respectively, suggesting pronounced moisture variability during the early middle Holocene ‘climatic optimum’. Optically stimulated luminescence and 14C ages from nearby terraced aggradational alluvial deposits indicate a paucity of large floods in the Late Pleistocene and at least five large flood events in the last c. 6 kyr, interpreted to mark an increased frequency of extreme rainfall events in the middle Holocene despite overall reduced EP. Increased EP in south-central Australia during the early to middle Holocene resulted from (1) decreased El Niño-Southern Oscillation (ENSO) variability, which reduced the frequency of El Niño-triggered droughts, (2) the prevalence of a more La Niña-like mean climatic state in the tropical Pacific Ocean, which increased available atmospheric moisture, and (3) a southward shift in the Intertropical Convergence Zone (ICTZ), which allowed tropical summer storms associated with the Australian summer monsoon (ASM) to penetrate deeper into the southern part of the continent. The onset of heightened aridity and apparent increase in large flood frequency at c. 5 ka is interpreted to indicate the establishment of an ENSO-like climate in arid Australia in the late Holocene, consistent with a variety of other terrestrial and marine proxies. The broad synchroneity of Holocene climate change across much of the Australian continent with changes in ENSO behavior suggests strong teleconnections amongst ENSO and the other climate systems such as the ASM, Indian Ocean Dipole, and Southern Annular Mode.

Hydrological effects of the M W 7.1 Darfield (Canterbury) earthquake, 4 September 2010, New Zealand

Abstract The M W7.1 Darfield (Canterbury) earthquake on 4 September 2010 generated widespread hydrological effects ranging from near-instantaneous coseismic liquefaction and changes of groundwater levels in boreholes, to more sustained (days to weeks) post-seismic changes in spring flow, river discharge and groundwater piezometric levels, to longer term shifts in groundwater level one year after the earthquake. Groundwater piezometric responses include local groundwater level increases of >20 m around the Greendale Fault, particularly in deep aquifers (>80 m), whereas decreases occurred in coastal confined aquifers beneath Christchurch city. Increases of up to 5 m persisted within 20 km of the fault 12 h after the earthquake. Groundwater levels and springs were affected throughout New Zealand, from >350 km away in Southland to nearly 1000 km away in Northland, even where shaking intensities were less than Modified Mercalli Intensity (MM) 3–4 (weak to largely observed) and peak ground acceleration was much <0.01 g. Release of artesian groundwater pressure and groundwater flow are postulated to have played pivotal roles in Christchurch liquefaction.

Tectonically driven fluid flow and gold mineralisation in active collisional orogenic belts: comparison between New Zealand and western Himalaya

Abstract Hydrothermal activity and mesothermal-styled gold mineralisation occurs near the main topographic divide of most active or young collisional mountain belts. The Southern Alps of New Zealand is used in this study as a model for the mineralising processes. The collisional tectonics results in a two-sided wedge-shaped orogen into which rock is transported horizontally. Upper crustal rocks pass through the orogen and leave the orogen by erosion, whereas lower crustal rocks are deformed into the mountain roots. High relief drives meteoric water flow to near the brittle–ductile transition. Lower to upper greenschist facies metamorphic reactions, driven by deformation at the crustal decollement and in the root, release water-rich fluids that rise through the orogen. Intimate chemical interaction between fluid and rock results in dissolution and later precipitation of gold, arsenic and sulphur. Fluid flow and mineralisation in the topographic divide region is facilitated by a network of steeply dipping faults and associated rock damage zones where oblique strike-slip faults intersect the thrust faults that strike subparallel to the main mountain range. The Nanga Parbat massif of the western Himalaya is an example of an active collisional zone which hosts hydrothermal activity but no gold mineralisation. The lack of gold mineralisation is due to the following factors: CO 2 -dominated rising metamorphic fluid in dehydrated amphibolite-granulite facies metamorphic rocks does not dissolve gold and arsenic; hot (up to 400 °C) meteoric water confined to fractures in the gneiss limits dissolution of gold and arsenic; low density of hot water/dry steam, and low reduced sulphur content of fluid, restrict solubility of gold and arsenic; absence of fracture networks in the core of the massif and the small volumes of circulating fluid limit metal concentration; and lack of reactive rock compositions limits chemically mediated metal deposition.

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.

Hydrothermal arsenic enrichment in an active mountain belt: Southern Alps, New Zealand

Abstract Torlesse terrane greywackes and hydrothermal calcite associated with active fault systems in the Main Divide Fault Zone of New Zealands South Island exhibit anomalous As concentrations relative to background values of similar rock types. Mass-balance analysis of greywacke compositional data from the upper Wilberforce valley of the Main Divide region demonstrate an average mass gain of As on the order of a factor of 6 above typical Torlesse greywackes (Wilberforce: mean=35.3 ppm; σ =31.3 ppm; n =44; Torlesse: mean=5.6 ppm; σ =4.4; n =31). In addition, hydride generated ICP-MS analyses of the weak acid soluble fraction of individual calcite-rich veins and fault gouges in the Main Divide Fault Zone exhibit As concentrations (mean=81.2 ppm; σ =113 ppm; n =17) in excess of a factor of 100 over hydrothermal calcite from other fault systems (mean=1.7 ppm; σ =4.0; n =29). Stable isotopic analyses of these calcite veins show a wide range for both oxygen ( δ 18 O=9.9‰ to 25.6‰) and carbon ( δ 13 C=−12‰ to −1.1‰). This observation suggests that the hydrothermal fluid source of these veins was of a mixed meteoric and highly exchanged fluid over a wide range of temperatures. We suggest that the presence of high As concentrations in young hydrothermal calcite veins in the Main Divide region results from active transport of As-rich deep crustal fluids upward along the fault zone. This interpretation is consistent with both fluid flow/tectonic models and geophysical observations, which suggest that expansion in the Main Divide Fault Zone provides a conduit for the escape of deep exchanged fluids into the upper crust.

Gold mineralisation near the Main Divide, upper Wilberforce valley, Southern Alps, New Zealand

Abstract Veins up to 8 m wide fill extensional fractures in Torlesse Terrane metasediments near the Main Divide in the upper Wilberforce valley, Canterbury, New Zealand. The upper Wilberforce veins are part of a prominent 40 km long, NNE‐trending swarm of gold‐bearing veins formed across the Main Divide during the late Cenozoic rise of the Southern Alps. The veins occur within, and near, a prominent set of faults which constitute the Main Divide Fault Zone. The veins are irregular in shape due to contrasting host rock properties, and have been only weakly sheared and deformed. Veins cut across greywacke beds and follow irregularly along argillite beds, on the 1–10 m scale. Quartz dominates vein mineralogy, but albite forms up to 45% of some veins, and minor chlorite, pyrite, arsenopyrite, chalcopyrite, and gold occur sporadically, especially in breccias near vein margins. Fluid inclusions in vein quartz homogenise at 180–253°C, and arsenopyrite composition (28.3–30.8 at.% As) suggest formation temperatures of 250–350°C. Elevated arsenic levels (up to 200 ppm above a background of 10 ppm) in some host greywackes and argillites suggest that hydrothermal activity pervaded host rocks as well as forming veins, but there is no textural evidence for this fluid flow. Late‐stage carbonates in faults adjacent to the quartz veins, but which postdate the quartz veins, have δ18O ranging from 11.1 to 25.6‰, and δ13C ranging from ‐12.5 to ‐1.1‰. These carbonates were deposited by a mixture of meteoric and crustally isotopically exchanged fluid as a shallow‐level manifestation of the same hydrothermal system which deposited the quartz veins. The upper Wilberforce veins structurally and mineral‐ogically resemble some late Cenozoic gold‐bearing vein systems in the Mt Cook area, 100 km to the southwest along the Southern Alps.