Isotope record of groundwater recharge mechanisms and climate change in southwestern North America

Understanding the response of groundwater systems to changes in climate is crucial at a time when human-caused climate change appears to be increasing in magnitude and rate. Groundwater preserves records of past effects resulting from climate change at the time-scale of late Pleistocene-Holocene climate evolution. Detailed regional datasets provide opportunities for evaluating past changes as a means of anticipating future climate-groundwater relations. Isotope parameters δ18O, δ2H and uncorrected 14C, considered together in large groundwater datasets from southwestern North America, provide evidence of changes in groundwater recharge mechanisms and the climate changes that led to them. The evidence consists in positive shifts in δ18O and δ2H in paleowater recharge related to a 13–15 ka shift recorded in in δ18O of speleothem deposits, and as concurrent changes in recharge seasonality in the core area of the North American monsoon. A negative shift in δ18O and δ2H, most likely contemporaneous with a regional 51–55 ka change speleothem deposits, is recorded in certain basins with deep, confined groundwater. A14C threshold of 10 percent modern carbon serves empirically to distinguish paleowaters before and after the 13–15 ka event. A younger, negative isotope shift has occurred in Baja California. The 13–15 ka shift is regional but is not recorded in all basins studied and appears to be absent at the northwestern and southeastern limits of the study area. Relations among the isotope parameters may be complicated by factors such as isotope altitude effects, delayed melting of Pleistocene ice, changing degrees of evaporation in river water and introduction of anthropogenic 14C. Recharge mechanisms fall into two patterns: (1) dominant winter recharge with varying degrees of evaporation prior to infiltration, and (2) recharge in both summer and winter, but only during the wettest months. Pattern (2) replaced pattern (1) at 13–15 ka in the core area of the North American monsoon. Later, the present-day pattern of recharge from summer-fall rain associated with tropical depressions replaced predominant winter recharge in southern and eastern Baja California. A post-1950 negative shift in δ18O and δ2H, observed in southern Nevada and northern New Mexico, may be of anthropogenic origin and related to development of large-scale irrigation in California.24 month embargo; first published 24 February 2023This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Stable chlorine isotopes in arid non-marine basins: Instances and possible fractionation mechanisms

Stable chlorine isotopes are useful geochemical tracers in processes involving the formation and evolution of evaporitic halite. Halite and dissolved chloride in groundwater that has interacted with halite in arid non-marine basins has a delta Cl-37 range of 0 +/- 3 parts per thousand, far greater than the range for marine evaporites. Basins characterized by high positive (-1 to +3 parts per thousand), near-0%, and negative (-0.3 to -2.6%) are documented. Halite in weathered crusts of sedimentary rocks has delta Cl-37 values as high as +5.6 parts per thousand. Salt-excluding halophyte plants excrete salt with a delta Cl-37 range of -2.1 to -0.8%. Differentiated rock chloride sources exist, e.g. in granitoid micas, but cannot provide sufficient chloride to account for the observed data. Single-pass application of known fractionating mechanisms, equilibrium salt-crystal interaction and disequilibrium diffusive transport, cannot account for the large ranges of delta Cl-37. Cumulative fractionation as a result of multiple wetting-drying cycles in vadose playas that produce halite crusts can produce observed positive delta Cl-37 values in hundreds to thousands of cycles. Diffusive isotope fractionation as a result of multiple wetting-drying cycles operating at a spatial scale of 1-10 cm can produce high delta Cl-37 values in residual halite. Chloride in rainwater is subject to complex fractionation, but develops negative delta Cl-37 values in certain situations; such may explain halite deposits with bulk negative delta Cl-37 values. Future field studies will benefit from a better understanding of hydrology and rainwater chemistry, and systematic collection of data for both Cl and Br.Environmental Isotope Laboratory at the University of Arizona24 month embargo. First available online 27 Aug 2017.This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Is Cl isotope fractionation in the seawater-evaporite system dependent on seawater chemistry?

Published values, near 1.0003, of the fractionation factor for stable Cl isotopes between halite and brine (αhal-br) fail to account for several sets of δ37Cl data for marine evaporitic halite. Clustered negative values of δ37Cl data, including some that appear to correspond to basal halite, would require measurable secular change in δ37Cl of marine chloride during the Phanerozoic if αhal-br is invariant. Such change is inconsistent with understanding of the geochemical cycle of Cl. Alternatively, marine δ37Cl has remained constant, but αhal-br has undergone secular change between values of about 1.0003 and 0.9996 in Phanerozoic marine evaporite settings. Such an interpretation is favored by the occurrence of clustered negative δ37Cl values at times when potash-facies evaporite contained sylvite rather than MgSO4 minerals, reflecting long-term secular change in seawater chemistry. Values of αhal-br may therefore depend on brine composition.24 month embargo; first published 19 November 2023This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

The origins of sulfate in cenozoic non-marine evaporites in the basin and-range province, southwestern north america

Cenozoic evaporites (gypsum and anhydrite) in southwestern North America have wide ranges of δ34 S (−30 to +22‰; most +4 to +10‰) and δ18 OSO4 (+3 to +19‰). New data are presented for five basins in southern Arizona. The evaporites were deposited in playas or perennial saline lakes in closed basins of Oligocene or younger age. Very large accumulations in Picacho, Safford and Tucson Basins have isotope compositions plotting close to a linear δ34 S-δ18 OSO4 relationship corresponding to mixing of two sources of sulfur: (1) sulfate recycled from Permian marine gypsum and (2) sulfate from weathering of Laramide-age igneous rocks that include porphyry copper deposits. In the large evaporites, sulfate with δ34 S > +10‰ is dominantly of Permian or Early Cretaceous marine origin, but has locally evolved to higher values as a result of bacterial sulfate reduction (BSR). Sulfate with δ34 S < −10‰ formed following exposure of sulfides, possibly formed during supergene enrichment of a porphyry copper deposit by BSR, and have values of δ18 OSO4 higher than those of local acid rock drainage because of participation of evaporated water in BSR. Accumulations of 30 to 100 km3 of gypsum in Picacho and Safford Basins are too large to explain as products of contemporaneous erosion of Permian and Laramide source materials, but may represent recycling of Late Cretaceous to Miocene lacustrine sulfate. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Preliminary Assessment of Carbon and Nitrogen Sequestration Potential of Wildfire-Derived Sediments Stored by Erosion Control Structures in Forest Ecosystems, Southwest USA

The role of pyrogenic carbon (PyC) in the global carbon cycle is still incompletely characterized. Much work has been done to characterize PyC on landforms and in soils where it originates or in “terminal” reservoirs such as marine sediments. Less is known about intermediate reservoirs such as streams and rivers, and few studies have characterized hillslope and in-stream erosion control structures (ECS) designed to capture soils and sediments destabilized by wildfire. In this preliminary study, organic carbon (OC), total nitrogen (N), and stable isotope parameters, δ13C and δ15N, were compared to assess opportunities for carbon and nitrogen sequestration in postwildfire sediments (fluvents) deposited upgradient of ECS in ephemeral- and intermittent-stream channels. The variability of OC, N, δ13C, and δ15N were analyzed in conjunction with fire history, age of captured sediments, topographic position, and land cover. Comparison of samples in 2 watersheds indicates higher OC and N in ECS with more recently captured sediments located downstream of areas with higher burn severity. This is likely a consequence of (1) higher burn severity causing greater runoff, erosion, and transport of OC (organic matter) to ECS and (2) greater cumulative loss of OC and N in older sediments stored behind older ECS. In addition, C/N, δ13C, and δ15N results suggest that organic matter in sediments stored at older ECS are enriched in microbially processed biomass relative to those at newer ECS. We conservatively estimated the potential mean annual capture of OC by ECS, using values from the watershed with lower levels of OC, to be 3 to 4 metric tons, with a total potential storage of 293 to 368 metric tons in a watershed of 7.7 km2 and total area of 2000 ECS estimated at 2.6 ha (203-255 metric tons/ha). We extrapolated the OC results to the regional level (southwest USA) to estimate the potential for carbon sequestration using these practices. We estimated a potential of 0.01 Pg, which is significant in terms of ecosystem services and regional efforts to promote carbon storage. © The Author(s) 2021.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Intercomparison of high precision C-14 measurements at the University of Arizona and the Queens University of Belfast radiocarbon laboratories

High-precision measurements were completed concurrently at the University of Arizona and the Queen's University of Belfast on blind samples of Irish oak originally measured for the 1986 radiocarbon calibration curve. Subsequent single-year Sequoindendron results were decadally averaged and compared with published results on decadal Douglas-fir samples. The results of these intercomparisons show that the Arizona high-precision results compare favorably with published values from the University of Washington, but show a systematic offset with published Belfast data

Secular variation of Delta C-14 during the Medieval Solar Maximum : A progress report

The Earth is within the Contemporaneous Solar Maximum (CSM), analogous to the Medieval Solar Maximum (MSM). If this analogy is valid, solar activity will continue to increase well into the 21st century, we have completed 75 single-ring and 10 double-ring measurements from AD 1065 to AD 1150 to obtain information about solar activity during this postulated analog to solar activity during the MSM. Delta(14)C decreases steadily during the period AD 1065 to AD 1150 but with cyclical oscillations around the decreasing trend. These oscillations can be successfully modeled by four cycles. These four frequencies are 1/52 yr(-1), 1/22 yr(-1), 1/11 yr(-1), and 1/5.5 yr, i.e., the 4th harmonic of the Suess cycle, the Hale and Schwabe cycles and the 2nd harmonic of the Schwabe cycle

Origin and residence time of groundwater based on stable and radioactive isotopes in the Heihe River Basin, northwestern China

Study region: The Heihe River Basin (HRB) is one of several arid basins in which runoff from the Qilian Mountain recharges basin aquifers. Study focus: A basin-wide dataset (δ18O, D, 3H and 14C) is used to determine the present and past relationships between precipitation, surface runoff and recharge, to constrain groundwater residence times, and to infer Holocene climate change. New hydrological insights for the region: Groundwater in the upper region (UR) of HRB has (δ18O, δD) clustered near (−8.0, −46‰), consistent with present-day Qilian Mountain precipitation. Tritium of groundwater >26 TU indicates post-bomb recharge. Mountain runoff provides recharge to alluvial-fluvial aquifers in the Middle Region (MR) and Lower Region (LR) along the main river of the HRB. Between 1986 and 2001, anthropogenic tritium releases affected north-central China, affecting HRB precipitation. Irrigation reflux strongly affects isotopes in basin groundwater, generating anomalous samples with low tritium and post-bomb 14C, or high tritium and pre-bomb 14C. Stable isotopes in Qilian Mountain runoff have evolved in response to climate change. A 1‰ shift in δ18O since 1960 coincides with drying of the Aral Sea, possibly affecting moisture advected from the west. A 6–8‰ shift before 12 ka may indicate the former extent of the South Asian monsoon. Keywords: Heihe River Basin (HRB), Groundwater origin, Residence time, Stable and radioactive isotopes, Northwestern Chin

Preliminary O and H isotope data for groundwater from the Hueco Bolson (El Paso, Texas; Ciudad Juárez, Chihuahua): constraints on water sources

Groundwater from municipal and monitoring wells gives δ$^{18}$O and δ$^{2}$H values plotting in several fields that distinguish water sources as follows. Values are presented below as (δ$^{18}$O\permil,δ$^{2}$H \permil). Present-day Rio Grande water (-6 to-7, -62 to -66) is strongly affected by evaporation in New Mexico reservoirs. Groundwater from post-bomb infiltration in the river bed is similar. Groundwater from Texas includes non-evaporated water (-9, -63) associated with the flanks of the 1500 m Franklin Mts., and (-10 to -11, -70 to -78) probably derived from the 2500-3500 m San Andres and Sacramento Mts. in New Mexico. Groundwater with strong evaporation signatures (-5.5 to -9, -41 to -64) occurs east of El Paso, and may represent mixtures of river- and mountain-derived water, or evaporated surface water from lower basin elevations. Groundwater from Ciudad Juárez, (-11 to -12, -80 to -90), did not originate as present-day Rio Grande surface water or groundwater from the Hueco Bolson in Texas. Two hypothetical sources are consistent with isotope data: 1. Pre-dam snow-melt from Colorado, supplied as Rio Grande water; 2 Late Quaternary lake water from pluvial Lake Palomas, supplied by underflow from the adjacent Bolson de los Muertos area of Chihuahua. Historical water level data indicate that significant recharge from the Rio Grande was unlikely before aquifer development