Greg Balco

Climate, tectonics, and the morphology of the Andes

Large-scale topographic analyses show that hemisphere-scale climate variations are a first-order control on the morphology of the Andes. Zonal atmospheric circulation in the Southern Hemisphere creates strong latitudinal precipitation gradients that, when incorporated in a generalized index of erosion intensity, predict strong gradients in erosion rates both along and across the Andes. Cross-range asymmetry, width, hypsometry, and maximum elevation reflect gradients in both the erosion index and the relative dominance of fluvial, glacial, and tectonic processes, and show that major morphologic features correlate with climatic regimes. Latitudinal gradients in inferred crustal thickening and structural shortening correspond to variations in predicted erosion potential, indicating that, like tectonics, nonuniform erosion due to large-scale climate patterns is a first-order control on the topographic evolution of the Andes.

An isochron method for cosmogenic-nuclide dating of buried soils and sediments

We describe an improved method for dating buried paleosols using measurements of the cosmic-ray-produced radionuclides 10Be and 26Al in quartz grains, and apply it to a sequence of intercalated tills and paleosols in central Missouri, USA, that record Plio-Pleistocene advances of the Laurentide Ice Sheet. A buried paleosol implies a period of surface exposure and nuclide accumulation, followed by burial and a halt to nuclide production. If the paleosol is formed in a sedimentary unit such as till, this unit may also have been emplaced with unknown 26Al and 10Be concentrations inherited from past surface exposure. If the inherited nuclide concentrations are the same at all depths in the soil—as is true for well-mixed sediments such as till—then the 26Al and 10Be concentrations at different depths in the paleosol will show a linear relationship. The slope of this line depends on the duration of burial of the paleosol, but not on the inherited nuclide concentrations or on the sample depths. Thus, one can date strata overlying buried paleosols by measuring 26Al and 10Be at multiple depths in the paleosol and calculating the burial age of the paleosol from the resulting isochron. We focus on applying this approach to till-paleosol sequences, but the basic idea of forming an 26Al-10Be burial isochron with a set of samples that share the same burial age, but differ in other aspects of their exposure history, applies to other stratigraphic settings as well. The method yields ages for four tills in Missouri that are stratigraphically consistent, agree with paleomagnetic age constraints, and show that ice advanced into Missouri near 1.25 Ma, near 0.8 Ma, and twice between ca. 0.4 and 0.2 Ma.

Absolute chronology for major Pleistocene advances of the Laurentide Ice Sheet

We used a 26 Al- 10 Be burial isochron method to date the glacial stratigraphic section in Missouri, USA, that records the largest advances of the Laurentide Ice Sheet. This permits an improved comparison of terrestrial and marine records of glaciation. The first recorded advance of the Laurentide Ice Sheet reached 39°N, near the extreme southern limit of North American glaciation, 2.4 Ma. The next advance to this latitude took place near the beginning of the mid-Pleistocene transition, 1.3 Ma, and three more took place from 0.75 to 0.2 Ma. There is no evidence that the Laurentide Ice Sheet advanced south of ∼45°–47°N between 2.4 and 1.3 Ma. This chronology: (1) shows that North American continental glaciation postdated Cordilleran alpine glaciation; (2) is consistent with the hypothesis that both of these events were threshold responses to tropical cooling; (3) is consistent with the hypothesis that the first advance of the Laurentide Ice Sheet was glaciologically anomalous due to the presence of deformable preglacial regolith; (4) is not consistent with the hypothesis that this deformable regolith persisted until the mid-Pleistocene transition; and (5) indicates that the increase in global ice volume at the mid-Pleistocene transition was at least in part the result of a more extensive Laurentide Ice Sheet.

Thermochronometry reveals headward propagation of erosion in an alpine landscape.

Glacial troughs in New Zealand mountains developed by propagation of erosion up valleys. Glacial erosion of mountain ranges produces spectacular alpine landscapes and, by linking climate with tectonics, influences a broad array of geophysical phenomena. Although the resultant landforms are easily identified, the timing and spatial pattern of topographic adjustment to Pleistocene glaciations remain poorly known. We investigated topographic evolution in the archetypal glacial landscape of Fiordland, New Zealand, using (U-Th)/He thermochronometry. We find that erosion during the past 2 million years removed the entire pre-Pleistocene landscape and fundamentally reshaped the topography. Erosion focused on steep valley segments and propagated from trunk valleys toward the heads of drainage basins, a behavior expected if subglacial erosion rate depends on ice sliding velocity. The Fiordland landscape illustrates complex effects of climate on Earth’s surface morphology.

Sediment supply, base level, braiding, and bedrock river terrace formation: Arroyo Seco, California, USA

In many settings, rivers alternate between carving wide valley bottoms (straths) and cutting narrow gorges over time, thereby creating longitudinally continuous paired bedrock strath terraces along valleys. Strath terraces are used ubiquitously in geomorphology and tectonics; however, how and why they form remain poorly understood. Here, we focus on Arroyo Seco in the central California Coast Ranges, where we test hypotheses for strath planation and subsequent strath terrace formation. Several lines of evidence indicate that strath planation is triggered by braiding in bedrock channels. In particular, hydraulic modeling reveals that the width of Arroyo Seco’s most recently formed terrace is comparable to the width of currently braided channel reaches. Additionally, a comparison of currently braided reaches to abandoned bedrock meander cutoffs shows that braided channels have valleys that are several times wider than single-thread meandering bedrock channel reaches. Lastly, in locations where the modern channel is currently braided, terraces are poorly preserved, suggesting that evidence for past episodes of braiding, in the form of paired strath terraces, is apparently largely destroyed by subsequent episodes of braiding. Field observations combined with mapping of terrace levels using an objective light detection and ranging (LiDAR)–based terrace identification algorithm reveal that temporal variation in tectonic uplift rate, sea level, and/or alluvial cover along the river cannot explain strath planation and subsequent terrace formation in Arroyo Seco. Rather, our results provide evidence that aggradation and degradation of alluvial sediments downstream of the Reliz Canyon fault result in impulsive base-level forcing of Arroyo Seco’s bedrock channel. Strath abandonment and terrace formation apparently occur as incision into downstream alluvial sediments propagates upstream into bedrock. Braiding and planation of straths, in contrast, occur during intervals of low vertical incision rate associated with downstream aggradation or immediately following pulses of vertical lowering triggered by downstream incision of alluvial sediments.

Rapid thinning of Pine Island Glacier in the early Holocene.

Once in a While Many regions at the edge of the Antarctic Ice Sheet have rapidly increased the rates at which they are sliding into the sea and thinning, raising concerns that global warming might cause the sudden collapse of some sections. Johnson et al. (p. 999, published online 20 February) present data from Pine Island Glacier, which has been thinning and retreating rapidly over the past two decades. The glacier experienced another rapid thinning around 8000 years ago, which occurred about as quickly as is happening now, and which lasted for 25 to 100 years. Pine Island Glacier in Antarctica thinned rapidly, as it is doing now, at least once before in the past 8000 years. Pine Island Glacier, a major outlet of the West Antarctic Ice Sheet, has been undergoing rapid thinning and retreat for the past two decades. We demonstrate, using glacial-geological and geochronological data, that Pine Island Glacier (PIG) also experienced rapid thinning during the early Holocene, around 8000 years ago. Cosmogenic 10Be concentrations in glacially transported rocks show that this thinning was sustained for decades to centuries at an average rate of more than 100 centimeters per year, which is comparable with contemporary thinning rates. The most likely mechanism was a reduction in ice shelf buttressing. Our findings reveal that PIG has experienced rapid thinning at least once in the past and that, once set in motion, rapid ice sheet changes in this region can persist for centuries.

Greenland was nearly ice-free for extended periods during the Pleistocene

The Greenland Ice Sheet (GIS) contains the equivalent of 7.4 metres of global sea-level rise. Its stability in our warming climate is therefore a pressing concern. However, the sparse proxy evidence of the palaeo-stability of the GIS means that its history is controversial (compare refs 2 and 3 to ref. 4). Here we show that Greenland was deglaciated for extended periods during the Pleistocene epoch (from 2.6 million years ago to 11,700 years ago), based on new measurements of cosmic-ray-produced beryllium and aluminium isotopes (10Be and 26Al) in a bedrock core from beneath an ice core near the GIS summit. Models indicate that when this bedrock site is ice-free, any remaining ice is concentrated in the eastern Greenland highlands and the GIS is reduced to less than ten per cent of its current volume. Our results narrow the spectrum of possible GIS histories: the longest period of stability of the present ice sheet that is consistent with the measurements is 1.1 million years, assuming that this was preceded by more than 280,000 years of ice-free conditions. Other scenarios, in which Greenland was ice-free during any or all Pleistocene interglacials, may be more realistic. Our observations are incompatible with most existing model simulations that present a continuously existing Pleistocene GIS. Future simulations of the GIS should take into account that Greenland was nearly ice-free for extended periods under Pleistocene climate forcing.

Numerical investigations of apatite ^4He/^3He thermochronometry

Apatite ^4He/^3He thermochronometry has the potential to constrain cooling histories for individual samples provided that several presently untested assumptions are valid. Here we simulate the sensitivity of ^4He/^3He spectra to assumptions regarding geometric model, crystallographic anisotropy, broken grain terminations, parent nuclide zonation, and the accuracy of results obtained from analyses of aggregates of multiple crystals. We find that ^4He/^3He spectra obtained from a cylinder with isotropic diffusion are almost indistinguishable from those obtained from an equivalent sphere with an equivalent initial ^4He distribution. Under similar conditions anisotropic diffusion from the cylinder can greatly bias ^4He/^3He spectra, but only if diffusion is >10 times faster in the axial than the radial direction. Existing data argue against anisotropy of this magnitude. We find that analysis of apatites with broken terminations will also bias ^4He/^3He spectra, but not greatly so. In contrast, we find that zonation of a factor of 3 in parent nuclide concentration produces ^4He/^3He spectra that deviate substantially from the homogeneous model. When parent nuclides are highly concentrated near the grain rim and/or cooling is fast, the resulting ^4He/^3He spectra will be readily identified as aberrant. However, more subtle zonation, higher concentrations in the grain interior, or samples that have cooled slowly regardless of zonation style can yield ^4He/^3He spectra that look acceptable but will lead to inaccurate thermochronometric interpretation if parent homogeneity is assumed. Finally, we find that analysis of an aggregate of crystals with identical ^4He distributions can yield ^4He/^3He spectra (and diffusion Arrhenius arrays) that are very different from those that would be obtained on the individual crystals if even small variations in He diffusion exist among the grains. Overall, our observations suggest that modeling tools that assume spherical geometry and isotropic diffusion are appropriate for interpreting apatite ^4He/^3He spectra. However, it is essential to analyze only individual crystals and to assess the degree of parent nuclide zonation in those crystals.

Basins and bedrock: Spatial variation in 10Be erosion rates and increasing relief in the southern Rocky Mountains, USA

We used measurements of cosmogenic 10 Be in alluvium to estimate erosion rates on a 10 3 –10 4 yr time scale for small (0.01–47 km 2 ), unglaciated basins in northern Colorado, southern Wyoming, and adjacent western Nebraska (western United States). Basins formed in Proterozoic cores of Laramide ranges are eroding more slowly (23 ± 7 mm k.y. –1 , n = 19) than adjacent basins draining weakly lithified Cenozoic sedimentary rocks (75 ± 36 mm k.y. –1 , n = 20). Erosion rates show a relationship to rock resistance and, for granitic rocks, to basin slope, but not to mean annual precipitation. We estimated longer-term (>10 5 yr time scale) erosion rates for the granitic core of the Front Range by measuring the concentration of 10 Be and 26 Al produced mainly by muon interactions at depths 1.7–10 m below the surface. Concentrations imply erosion rates of 9–31 mm k.y. –1 , similar to shorter-term erosion rates inferred from alluvial sediment. The spatial distribution of erosion rates and stratigraphic evidence imply that relief in the southern Rocky Mountains increased in the late Cenozoic; modern relief probably dates from post-middle Miocene time.

EROSIONAL RESPONSE TO NORTHWARD-PROPAGATING CRUSTAL THICKENING IN THE COASTAL RANGES OF THE U.S. PACIFIC NORTHWEST

We measured basin-scale erosion rates, using cosmogenic 10Be concentrations in quartz, from fluvial sediment in rivers draining the coastal mountain ranges of the U.S. Pacific Northwest between 40° and 47° N. Apparent erosion rates are 0.1 to 0.2 mm yr−1 throughout the Oregon Coast Ranges north of 43° N, and increase to the south to 0.6 to 1.1 mm yr−1 in the northern California coast ranges near 40° N. We propose that these observations display the erosional response to northward-migrating crustal thickening associated with subduction of the Mendocino Triple Junction. North-south variations in erosion rate, range elevation, and metrics of landscape relief and steepness are consistent with the hypotheses that i) their primary cause is northward-migrating crustal thickening; ii) erosion rates are strongly controlled by topographic relief and weakly, if at all, controlled by climate; and iii) the dependence of erosion on relief is nonlinear and obeys a threshold-relief relationship.