The thing that first stimulated my interest in earth science over 45 years ago was the glacial landscape of the Sierra Nevada in California. When I was in high school I took a University of California extension course in geology and asked the instructor how old were the moraines? His reply was “Nobody really knows” and I thought “Wow! There must be a way to find that out”. But when I was completing my undergrad degree in earth science at the University of California at Santa Cruz I asked my professors in what specialty area I should apply to grad school. One reply was “hydrogeology”; that it was an up-and-coming field with a high demand for new professionals. I liked the idea because of the societal benefits of helping to assure water quality and water supply, so I ended up accepting an offer from the Department of Hydrology and Water Resources at the University of Arizona.
My advisor at Arizona was Stanley N. Davis and I worked on noble gases in ground water as a paleoclimatic indicator. I had managed to get into paleoclimate in spite of working in a pretty applied field! While I was at Arizona my advisor was a coauthor on the first paper on measurement of 36Cl in natural water and it seemed that new technology (accelerator mass spectrometry) was opening up frontier opportunities, so I shifted my focus to application of 36Cl and other environmental tracers to groundwater problems.
While working on the problem of the production rate of 36Cl by cosmic rays, I realized that it was produced in rocks as well as in the atmosphere and that it could potentially be used date the length of time that a rock had been exposed to cosmic rays (i.e., been within about a meter of the earth’s surface). This led to a whole second career in the systematics of cosmogenic nuclides in rocks and their application to dating landforms. One of the first things I applied the method to was –guess what?- the chronology of the glaciation of the Sierra Nevada. I have now dated moraines, lake shorelines, volcanoes, river terraces, fault scarps, and other landforms all the way from the Antarctic to the High Arctic and in every continent of the globe except Africa. As I was writing this text I received an email from a Spanish scientist coordinating a synthesis paper on the deglaciation of North and South America and requesting that I write a section on the deglaciation of the Sierra Nevada. Sometimes one ends up right back where one started.
Meanwhile, I explored various facets of tracers and solute transport in hydrological systems. For a while I worked on the quantification of dispersive processes in groundwater, then moved on to trying to understand the hydrodynamics of thick desert vadose zones and the role of ecohydrology in driving these dynamics. Then I moved on to using tracers to understand the generation of surface water flows in mountainous environments.
Today most of my work is focused on three topics. The first is developing a numerical model to quantify groundwater recharge over the entire state of New Mexico at high spatial and temporal resolution. The recharge is calculated based on a soil-water-capacity model on daily time steps. The soil-water content in turn comes from daily evapotranspiration calculations at 250-m resolution using an Allen-type dual crop coefficient approach. It incorporates a full analysis of topographic effects on the radiation balance. This is a new direction for me since none of my previous projects involved detailed land/atmosphere interactions and I have really enjoyed what I have learned by going in depth about this part of the hydrological cycle.
The second topic is contemporary through Miocene tectonics of the southwestern Great Basin. I am heading up a large NSF-funded project to investigate linkages between the tectonic evolution of this area, the changes in the hydrological systems induced by tectonics, and the resultant impacts on aquatic ecosystems. The southwestern Great Basin is one of the tectonically most active areas in North America and tectonics is the principal driver for landscape evolution there. For about 15 years I’ve been working on the connections between the landscapes we can observe at the surface and the subsurface fault geometries and kinematics that have produced the landscape. My current project incorporates detailed three-dimensional modeling of the Miocene-to-present displacements throughout the region, so I am currently spending a lot of time trying to go from fault outcrops and landscapes to underlying structure.
The third topic is going back to cosmogenic nuclide systematics. From 2000 to 2012 I headed up the international CRONUS Earth Project that attempted to quantify and systematize these systematics. One ambiguity that we did not end up resolving was certain details about the global distribution of cosmogenic nuclide production rates (discriminating between the Lal/Stone and LSD scaling models, if you’re into this stuff). It turns out that high latitude and high elevation calibration sites are the key locality for maximizing the difference between the two models, so we are going to determine the age of lava flows from the top of Mt. Erebus in Antarctica using 39Ar/40Ar dating and then measure cosmogenic 36Cl and 3He in the same samples. We will compare the results of these measurements with the predictions of the two models and find out which one does the best job.
The paths taken during a long scientific career are interesting to look back on. Many of them lead in completely new directions. But many of them also lead back to the same questions that started me out on this quest 40 years ago. It is certainly gratifying to be able to look back and say “Now I know the answers to those apparently unanswerable questions.” Science is an immense edifice and we each can contribute only a tiny bit to the overall structure, but those little bits and pieces are permanent contributions to understanding the world we live in.
Socorro Magma Body: Surface uplift history and crustal dynamics
NSF: $303,000 Period: 07/15-06/17 (continuing)
PI’s: Gary Axen, Fred Phillips, Jolante van Wijk RA’s: Bradley Sion, Shuyou Yao
The SMB provides a unique opportunity to study the effects of an unusually deep magma body that is well imaged seismically and geodetically (in contrast to most much shallower magma bodies). Magma is thought to affect tectonics by causing changes in the stress field, thermal structure and density structure of the mantle lithosphere and crust. The importance of magmatism in continental rifting is increasingly appreciated but most studies focus on effects of shallow dikes in mature rifts (the Rio Grande rift is considered juvenile) or of lithosphere-scale dikes for rupture of continental lithosphere. The SMB may reflect a little-studied mechanism of rift-crust growth: volumes equal to several SMBs are needed to explain the Rio Grande rift heat flow, consistent with the massive amounts of mid- or lower-crust mafic magma known from other rifts (e.g., Salton trough) and from early histories of passive margins. Most models of magma bodies and surface uplift assume an elastic crust but the SMB lies well below the brittle-plastic transition, so, depending upon time scales of intrusion, inelastic processes are also probably important in transmitting strain to the surface.
We will map and correlate terraces formed along the Rio Grande and its tributaries for use as strain markers. Structure contour maps will be prepared for each terrace level. SMB-related and Quaternary fault-related surface deformation will be distinguished based upon differences in expected tilt directions, length of tilting, and magnitude of tilting. Terrace correlation will involve relative heights above streams, characteristics of terrace treads, deposits and straths, and soil analysis. Key, well-preserved terraces will be dated using the 36Cl depth-profile method (or 14C if too young for 36Cl). Geodynamic modeling will test geometry and dynamics of the active uplift source (inflation, thermal expansion, feeder source, contraction of crystallized magmas, etc.) by comparing them to InSAR and GPS results, and will guide interpretation of past episodes of SMB surface deformation revealed by terrace analysis.
New Mexico Statewide Water Assessment Project: Evapotranspiration and Recharge
State of New Mexico: $160,000 Period: 08/15-07/19
PI’s: Talon Newton, Fred Phillips, Jan Hendrickx, Dan Cadol RA’s: David Ketchum, Peter ReVelle, Esther Xu, Gabriel Parrish
This project, funded by the New Mexico State Legislature, seeks to quantify the water balance for the entire state. New Mexico Tech has contributed the evapotranspiration model and the diffuse groundwater recharge model. These are high spatial resolution (250 m2) and temporal resolution (daily time step) water-balance models based on the dual crop coefficient method for calculation of evapotranspiration and multi-layer soil-water-capacity model for recharge. Esther is using empirical data to calculate runoff as a function of precipitation intensity and duration and to convert the calculated runoff to focused recharge.
Tectonic and climatic forcing of hydrological systems in the southern Great Basin: Implications for ancient and future aquatic system resilience
NSF: $3,000,000 (total project), $1,526,000 (NM Tech) Period: 08/15-7/20
PI’s: Fred Phillips, Gary Axen, Jolante van Wijk, John Wilson Postdoc: Erica Emry RA’s: Brandon Lutz, Michael Berry, Kyungdoe Han
The southern Great Basin is among the most arid regions in North America. It has almost no perennial streams, but does have >1,000 springs. These springs are islands of aquatic habitat in an ocean of desert. Remarkably, many of these isolated springs contain diverse aquatic ecosystems and even endemic species of fish, spring snails, and other aquatic organisms. The presence of many aquatic species that can only survive in water is evidence that the springs are remnants of a perennial drainage system, and the presence of endemic species requiring intervals in the million-year range for genetic divergence are evidence that at least some of these springs have never desiccated over the geological time scale. Aquatic biogeographical patterns thus inform the geological and hydrological history of the region.
We propose to expand the already-large regional biogeographical database and to use the combined new and preexisting data to test models of tectonic and paleohydrological evolution of the southern Great Basin. We will focus on two timescales: that of the extensional breakup of the region from the late Miocene to the present and that of glacial/interglacial climate cycles. Extensive work has been done to understand the extensional history of the region, which started in the eastern portion of the study area at ~14 Ma and migrated westward to the Sierra Nevada front, driven by plate-boundary dynamics. We will simulate this evolution using a regional quasi-3D kinematic/tectonic-geomorphic-hydrologic coupled model that fully couples movement along faults, mass distribution, magmatism, isostatic compensation and flexural deformation with hydrology and surface geomorphic processes, including erosion and deposition. The extensional fragmentation of the hydrological system will be studied and groundwater flow, necessary to simulate the resulting development of springs, will be an integral part of the regional tectonic-geomorphic-hydrologic model. This model will be initiated and structured using existing and newly generated geological and paleohydrological data. The model will quantify the discretization of the originally through-flowing hydrological system into isolated springs.
At the glacial/interglacial time scale we will employ telescoped local hydrological models to obtain high-resolution simulations of spring (dis-)connection. Local models will be based on geology, geomorphology, climate, and hydrology (for boundary conditions) taken from the regional model for the time period of interest. Local paleoclimate records, particularly temperature/precipitation change estimates obtained from inverse analysis of glacial and pluvial lake extents at various time slices, will enhance the climate forcing of local hydrology. We will track spring appearance, and desiccation and the extent and timing of discharges, sufficient to establish perennial surface-water flow. Environmental tracers measurements in existing springs constrain groundwater age and inform the models.
Modeled paleohydrologic histories will be tested against biotic data (aquatic biota inventories, microbial and macrofaunal DNA, and genetic divergence times) with island biogeography theory. We will test for relations of hydrologic fragmentation chronology with endemic species and for ecosystem diversity with spring resilience, as inferred from groundwater ages and climatically driven modeling. We will use these results to assess and improve our tectonic/paleohydrologic models.
We will, for the first time, assess rigorous tectonic and paleohydrological histories developed using numerical models in the context of aquatic biogeography and aquatic-species evolution in arid-region spring ecosystems. The results should be applicable to aquatic ecosystems in arid and semiarid regions worldwide.
A test of global and Antarctic models for cosmogenic-nuclide production rates using high-precision dating of 40Ar/39Ar lava flows from Mount Erebus
NSF: $357,000 Period: 07/17-06/20
PI’s: Fred Phillips, Matt Heizler, Philip Kyle RA’s: Christine Burrill
Nuclides produced by cosmic rays in rocks at the surface of the earth are widely used for Quaternary geochronology and geomorphic studies and their use is increasing every year. The recently completed CRONUS-Earth Project (Cosmic-Ray Produced Nuclides on Earth) has systematically evaluated the production rates and theoretical underpinnings of cosmogenic nuclides. However, the CRONUS-Earth Project was not able to discriminate between the two leading theoretical approaches: the original Lal model (St) and the new Lifton-Sato-Dunai model (LSD). Mathematical models used to scale the production of the nuclides as a function of location on the earth, elevation, and magnetic field configuration are an essential component of this dating method. The inability to distinguish between the two models was because the predicted production rates did not differ sufficiently at the location of the calibration sites.
The cosmogenic-nuclide production rates that are predicted by the two models differ significantly from each other at Erebus volcano, Antarctica. Mount Erebus is therefore an excellent site for testing which production model best describes actual cosmogenic-nuclide production variations over the globe. We have recently measured 3He and 36Cl in mineral separates extracted from Erebus lava flows. The exposure ages for each nuclide were reproducible within each flow (~2% standard deviation) and in very good agreement between the 3He and the 36Cl ages. However, the ages calculated by the St and LSD scaling methods differ by ~15-25% due to the sensitivity of the production rate to the scaling at this latitude and elevation. These results lend confidence that Erebus qualifies as a suitable high- latitude/high-elevation calibration site. The remaining component that is still lacking is accurate and reliable independent (i.e., non-cosmogenic) ages, however, published 40Ar/39Ar ages are too imprecise and typically biased to older ages due to excess argon contained in melt inclusions.
Our new 40Ar/39Ar data show that previous problems with Erebus anorthoclase geochronology are now overcome with modern mass spectrometry and better sample preparation. They indicate a high likelihood of success for this proposal in defining an accurate global scaling model. Although encouraging, much remains to be accomplished and we will sample lava flows over 3 km in elevation and determine their 40Ar/39Ar and exposure ages. These combined data will discriminate between the two scaling methods, resulting in a preferred scaling model for global cosmogenic geochronology. The LSD method contains two sub-methods, the ‘plain’ LSD scales all nuclides the same, whereas LSDn scales each nuclide individually. We can discriminate between these models using 3He and 36Cl data from lava flows at different elevations, because the first model predicts that the production ratio for these two nuclides will be invariant with elevation and the second that there should be ~10% difference over the range of elevations to be sampled. Finally, we will provide a local, finite-age calibration site for cosmogenic-nuclide investigations in Antarctica.
Thousands of cosmogenic ages are published every year, but it is still not clear what production-scaling model provides the most accurate ages. Our data from Erebus will answer this important question. This will address a huge need in Antarctica, but more broadly will serve the earth sciences in general. Globally important studies such as paleoclimate, evolution and hazards depend critically on accurate geochronology (i.e., the fundamental underpinnings such as decay constants and/or production rates), yet only few locations allow for calibration between multiple methods. Erebus is ideally suited for this. Our proposed graduate and undergraduate students will use multiple state-of-the-art facilities and will develop groundbreaking advances in methodology towards a common goal of improving Late Quaternary geochronology. It is only now, with recent advancements in technology, that the goals of this proposal are doable.