ALBERT

Description: Hydraulic conductivity is one of the most critical and at the same time one of the most uncertain parameters in many groundwater models. One problem commonly faced is that the data are usually not collected at the same scale as the discretized elements used in a numerical model. Moreover, it is common that different types of hydraulic conductivity measurements, corresponding to different spatial scales, coexist in a studied domain, which have to be integrated simultaneously. Here we address this issue in the context of Image Quilting, one of the recently developed multiple-point geostatistics methods. Based on a training image that represents fine-scale spatial variability, we use the simplified renormalization upscaling method to obtain a series of upscaled training images that correspond to the different scales at which measurements are available. We then apply Image Quilting with such a multi-scale training image to be able to incorporate simultaneously conditioning data at several spatial scales of heterogeneity. The realizations obtained satisfy the conditioning data exactly across all scales, but it can come at the expense of a small approximation in the representation of the physical scale relationships. In order to mitigate this approximation, we iteratively apply a kriging-based correction to the finest scale that ensures local conditioning at the coarsest scales. The method is tested on a series of synthetic examples where it gives good results and shows potential for the integration of different measurement methods in real-case hydrogeological models. This article is protected by copyright. All rights reserved.

Description: The extent to which atmospheric nitrogen (N) deposition reflects land use differences and biogenic vs. fossil fuel reactive N sources remains unclear, yet represents a critical uncertainty in ecosystem N budgets. We compared N concentrations and isotopes in precipitation-event bulk (wet + dry) deposition across nearby valleys in northern Utah with contrasting land use (highly urban vs. intensive agriculture/low-density urban). We predicted greater nitrate (NO 3 - ) vs. ammonium (NH 4 + ) and higher δ 15 N of NO 3 - and NH 4 + in urban valley sites. Contrary to expectations, annual N deposition (3.5–5.1 kg N ha -1 y -1 ) and inorganic N concentrations were similar within and between valleys. Significant summertime decreases in δ 15 N of NO 3 - possibly reflected increasing biogenic emissions in the agricultural valley. Organic N was a relatively minor component of deposition (~13%). Nearby paired wildland sites had similar bulk deposition N concentrations as the urban and agricultural sites. Weighted bulk deposition δ 15 N was similar to natural ecosystems (-0.6 ± 0.7‰). Fine atmospheric particulate matter (PM 2.5 ) had consistently high values of bulk δ 15 N (15.6 ± 1.4‰), δ 15 N in NH 4 + (22.5 ± 1.6‰), and NO 3 - (8.8 ± 0.7‰), consistent with equilibrium fractionation with gaseous species. δ 15 N in bulk deposition NH 4 + varied by more than 40‰, and spatial variation in δ 15 N within storms exceeded 10‰. Sporadically high values of δ 15 N were thus consistent with increased particulate N contributions as well as potential N source variation. Despite large differences in reactive N sources, urban and agricultural landscapes are not always strongly reflected in the composition and fluxes of local N deposition—an important consideration for regional-scale ecosystem models.

Description: Climate variability affects the capacity of the biosphere to assimilate and store important elements, such as nitrogen and carbon. Here we present biogeochemical evidence from the sediments of tropical Lake Titicaca indicating that large hydrologic changes in response to global glacial cycles during the Quaternary were accompanied by major shifts in ecosystem state. During prolonged glacial intervals, lake level was high and the lake was in a stable nitrogen-limited state. In contrast, during warm dry interglacials lake level fell and rates of nitrogen concentrations increased by a factor of 4–12, resulting in a fivefold to 24-fold increase in organic carbon concentrations in the sediments due to increased primary productivity. Observed periods of increased primary productivity were also associated with an apparent increase in denitrification. However, the net accumulation of nitrogen during interglacial intervals indicates that increased nitrogen supply exceeded nitrogen losses due to denitrification, thereby causing increases in primary productivity. Although primary productivity in tropical ecosystems, especially freshwater ecosystems, tends to be nitrogen limited, our results indicate that climate variability may lead to changes in nitrogen availability and thus changes in primary productivity. Therefore some tropical ecosystems may shift between a stable state of nitrogen limitation and a stable state of nitrogen saturation in response to varying climatic conditions.

Description: Lakes within fluvial networks may affect dissolved organic matter (DOM) dynamics in streams by dampening spring DOM snowmelt flushing responses and/or by increasing summer DOM production. We assessed the temporal variability of dissolved organic carbon (DOC) concentration and DOM characteristics (specific ultraviolet absorbance (SUVA254); DOC:dissolved organic nitrogen (DOC:DON)), as well as DOC export in seven paired lake inflows and outflows in the Sawtooth Mountain lake district, Idaho. We hypothesized that lakes would decrease stream DOM temporal variability and increase DOM export as a result of autotrophic production. We correlated DOM variability with landscape factors to evaluate potential drivers of DOM temporal patterns (measured as coefficient of variation). Coefficients of variation were 40–90% higher in lake inflows than outflows for DOC concentrations, characteristics, and DOC:DON. Increases in DOC concentrations on the ascending limb of the snowmelt hydrograph were greater in lake inflows than outflows, and on average mean DOC flux occurred 5.4 days earlier in the inflows than for the outflows. During base flow, mean outflow DOC concentrations were 1.7 times greater than inflows, and six outflows had higher annual export than inflows. Combined, these results illustrate that lakes alter the magnitude, timing and temporal variation of DOM concentration and characteristics exported from subalpine watersheds. This buffering effect results from a seasonal shift in the balance between hydrological versus biological controls on DOC dynamics, where lakes act as a sink during the spring when hydrologic controls dominate watershed DOM transport and act as a DOM source during summer.

Description: We present Mg/Ca data for Globigerina bulloides from 10 core top sites in the southwest Pacific Ocean analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Mg/Ca values in G. bulloides correlate with observed ocean temperatures (7°C–19°C), and when combined with previously published data, an integrated Mg/Ca–temperature calibration for 7°C–31°C is derived where Mg/Ca (mmol/mol) = 0.955 × e0.068 × T (r2 = 0.95). Significant variability of Mg/Ca values (20%–30%) was found for the four visible chambers of G. bulloides, with the final chamber consistently recording the lowest Mg/Ca and is interpreted, in part, to reflect changes in the depth habitat with ontogeny. Incipient and variable dissolution of the thin and fragile final chamber, and outermost layer concomitantly added to all chambers, caused by different cleaning techniques prior to solution-based ICPMS analyses, may explain the minor differences in previously published Mg/Ca–temperature calibrations for this species. If the lower Mg/Ca of the final chamber reflects changes in depth habitat, then LA-ICPMS of the penultimate (or older) chambers will most sensitively record past changes in near-surface ocean temperatures. Mean size-normalized G. bulloides test weights correlate negatively with ocean temperature (T = 31.8 × e−30.5×wtN; r2 = 0.90), suggesting that in the southwest Pacific Ocean, temperature is a prominent control on shell weight in addition to carbonate ion levels.

Description: Biogeochemical reactions associated with stream nitrogen cycling, such as nitrification and denitrification, can be strongly controlled by water and solute residence times in the hyporheic zone (HZ). We used a whole-stream steady state 15N-labeled nitrate (15NO3−) and conservative tracer (Cl−) addition to investigate the spatial and temporal physiochemical conditions controlling the denitrification dynamics in the HZ of an upland agricultural stream. We measured solute concentrations (15NO3−, 15N2 (g), as well as NO3−, NH3, DOC, DO, Cl−), and hydraulic transport parameters (head, flow rates, flow paths, and residence time distributions) of the reach and along HZ flow paths of an instrumented gravel bar. HZ exchange was observed across the entire gravel bar (i.e., in all wells) with flow path lengths up to 4.2 m and corresponding median residence times greater than 28.5 h. The HZ transitioned from a net nitrification environment at its head (short residence times) to a net denitrification environment at its tail (long residence times). NO3− increased at short residence times from 0.32 to 0.54 mg-N L−1 until a threshold of 6.9 h and then consistently decreased from 0.54 to 0.03 mg-N L−1. Along these same flow paths, declines were seen in DO (from 8.31 to 0.59 mg-O2 L−1) and DOC (from 3.0 to 1.7 mg-C L−1). The rates of the DO and DOC removal and net nitrification were greatest during short residence times, while the rate of denitrification was greatest at long residence times. 15NO3− tracing confirmed that a fraction of the NO3− removal was via denitrification as 15N2 was produced across the entire gravel bar HZ. Production of 15N2 across all observed flow paths and residence times indicated that denitrification microsites are present even where nitrification was the net outcome. These findings demonstrate that the HZ is an active nitrogen sink in this system and that the distinction between net nitrification and denitrification in the HZ is a function of residence time and exhibits threshold behavior. Consequently, incorporation of HZ exchange and water residence time characterizations will improve mechanistic predictions of nitrogen cycling in streams.

Description: Rivers receive and process large quantities of terrestrial dissolved organic carbon (DOC). Biologically available (unstable) DOC leached from primary producers may stimulate (i.e., prime) the consumption of more stable terrestrially-derived DOC by heterotrophic microbes. We measured microbial DOC consumption (i.e., decay rates) from contrasting C sources in ten rivers in the Western and Midwestern United States using short-term bioassays of river water, soil and algal leachates, glucose, and commercial humate. We added inorganic nutrients (ammonium and phosphorus) to a subset of bioassays. We also amended a subset of river, soil, and commercial humate bioassays with glucose or algal leachates to test the hypothesis that unstable DOC primes consumption of more stable DOC. We used prior measurements of source-specific DOC bioavailability, linked with a Bayesian process model, to estimate means and posterior probability distributions for source-specific DOC decay rates in multi-source bioassays. Modeled priming effects ranged from a −130 to +370% change in more stable DOC decay when incubated with unstable DOC. Glucose increased modeled river DOC decay by an average of 87% among all rivers. Glucose and algal leachates increased soil leachate and commercial humate decay by an average of 25% above background rates. Inorganic nutrient additions did not have consistent effects on DOC decay, likely because most of the study rivers had high ambient background nutrients. Our results demonstrate that the priming effect can augment DOC decay in rivers. In addition, Bayesian models can be used to estimate mechanisms driving aquatic ecosystem processes that are difficult to measure directly.

Description: Magma mixing plays a prominent role in the origins of intermediate magmas in subduction zones. However, the conditions and timescales of magma mixing and how these are linked to subsequent eruption are unclear. Mount Tauhara is the largest dacitic volcanic complex in the Taupo Volcanic Zone, New Zealand. Dacites from Tauhara Volcano have a complex petrography (Qtz + Plag + Amph + OPx + CPx + Oxi ± Oli) that can only have been produced by magma mixing and offer an ideal opportunity to investigate the processes and timescales involved in assembling dacite magmas in a continental subduction zone. Here we present whole-rock and mineral-specific major and trace element and isotopic data for the Tauhara dacites in order to identify the magma mixing end-members, constrain the physical conditions of mixing, and estimate the timescales and relationships between magma mixing, ascent and eruption. These data reveal that four separate mixing events between crystal-rich rhyolites (77–80 wt.% SiO 2 ; 40 ppm Sr) and crystal-poor mafic magmas of basaltic (48 wt.% SiO 2 ; 1340 ppm Sr) to andesitic (55–59 wt.% SiO 2 ; 490–580 ppm Sr) composition occurred to produce the Tauhara dacites. Mixing took place in well-stirred magma chambers located at mid-crustal depths (8–13 km) at temperatures from 840 to 900ºC. The timescales of magma mixing obtained from Ti diffusion in quartz appear to be largely dependent on the temperature and viscosity contrast between the end-members as andesite and rhyolite magma mixed on timescales of 2–7 months, whereas basalt and rhyolite magmas mixed on timescales of 1–2 years. The short magma mixing timescales, combined with the physical properties (e.g., viscosity and density) of the mixed dacite magmas, as compared with those of the end-member magmas, facilitated the ascent and eruption of dacite magmas at Tauhara volcano.

Description: Laser ablation inductively coupled plasma mass spectrometry was used to analyze the individual chambers from tests of foraminiferal fossil and plankton tow Globigerinoides ruber from the southwest Pacific Ocean, from latitudes 3°S to 42°S. The variability of Mg/Ca between chambers of an individual (intraindividual) and individuals of the same population (interindividual), is such that when converted to temperature, the extent of intra-individual and interindividual variability appears to exceed that attributable to either calcification or seasonal temperature variability. The pooled mean chamber Mg/Ca from each core top and plankton tow site demonstrates a significant (p 〈 0.05) positive correlation with temperature. We derive chamber-specific calibrations where Mg/CaCh_F-2 = 0.798 exp0.070 T, Mg/CaCh_F-1 = 0.891 exp0.067 T and Mg/CaCh_F = 0.590 exp0.072 T. We do not observe any bias between the two morphotypes Gs. ruber ruber and Gs. ruber pyramidalis. The chamber-specific calibrations potentially offset Mg/Ca-based temperature reconstructions if used on bulk (whole) test Mg/Ca or applied to misidentified chambers. Nevertheless, these calibrations can be used to reliably estimate sea surface temperature. Although there is a general overriding temperature control on Mg/Ca, we show that removal of the effect of temperature at each site reveals a lognormal Mg/Ca distribution. This suggests that Mg/Ca variability at each site is also affected by biological mechanism(s) that may control the distribution of interindividual Mg/Ca. In addition, other TE/Ca data (Al/Ca and Mn/Ca) from laser ablation trace element depth profiles can be used to identify detrital or diagenetic phases that may bias the trace element/Ca signal.

Description: We used an in situ steady state 15N-labeled nitrate (15NO3−) and acetate (AcO−) well-to-wells injection experiment to determine how the availability of labile dissolved organic carbon (DOC) as AcO− influences microbial denitrification in the hyporheic zone of an upland (third-order) agricultural stream. The experimental wells receiving conservative (Cl− and Br) and reactive (15NO3−) solute tracers had hyporheic median residence times of 7.0 to 13.1 h, nominal flowpath lengths of 0.7 to 3.7 m, and hypoxic conditions (