Radiocarbon (14C) ages cannot provide absolutely dated chronologies for archaeological or paleoenvironmental studies directly but must be converted to calendar age equivalents using a calibration curve compensating for fluctuations in atmospheric 14C concentration. Although calibration curves are constructed from independently dated archives, they invariably require revision as new data become available and our understanding of the Earth system improves. In this volume the international 14C calibration curves for both the Northern and Southern Hemispheres, as well as for the ocean surface layer, have been updated to include a wealth of new data and extended to 55,000 cal BP. Based on tree rings, IntCal20 now extends as a fully atmospheric record to ca. 13,900 cal BP. For the older part of the timescale, IntCal20 comprises statistically integrated evidence from floating tree-ring chronologies, lacustrine and marine sediments, speleothems, and corals. We utilized improved evaluation of the timescales and location variable 14C offsets from the atmosphere (reservoir age, dead carbon fraction) for each dataset. New statistical methods have refined the structure of the calibration curves while maintaining a robust treatment of uncertainties in the 14C ages, the calendar ages and other corrections. The inclusion of modeled marine reservoir ages derived from a three-dimensional ocean circulation model has allowed us to apply more appropriate reservoir corrections to the marine 14C data rather than the previous use of constant regional offsets from the atmosphere. Here we provide an overview of the new and revised datasets and the associated methods used for the construction of the IntCal20 curve and explore potential regional offsets for tree-ring data. We discuss the main differences with respect to the previous calibration curve, IntCal13, and some of the implications for archaeology and geosciences ranging from the recent past to the time of the extinction of the Neanderthals.

https://dspace.stir.ac.uk/bitstream/1893/30981/1/intcal20_northern_hemisphere_radiocarbon_age_calibration_curve_055_cal_kbp.pdf

The IntCal20 Northern Hemisphere Radiocarbon Age Calibration Curve (0-55 cal kBP)

Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed. Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation. The impacts of anthropogenic climate change so far include decreased ocean productivity, altered food web dynamics, reduced abundance of habitat-forming species, shifting species distributions, and a greater incidence of disease. Although there is considerable uncertainty about the spatial and temporal details, climate change is clearly and fundamentally altering ocean ecosystems. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.

http://science.sciencemag.org/content/sci/328/5985/1523.full.pdf

The impact of climate change on the world's marine ecosystems.

Imaging Spectroscopy and the Airborne Visible/Infrared Imaging Spectrometer (AVIRIS)

Zircon and rutile are common accessory minerals whose essential structural constituents, Zr, Ti, and Si can replace one another to a limited extent. Here we present the combined results of high pressure–temperature experiments and analyses of natural zircons and rutile crystals that reveal systematic changes with temperature in the uptake of Ti in zircon and Zr in rutile. Detailed calibrations of the temperature dependencies are presented as two geothermometers—Ti content of zircon and Zr content of rutile—that may find wide application in crustal petrology. Synthetic zircons were crystallized in the presence of rutile at 1–2 GPa and 1,025–1,450°C from both silicate melts and hydrothermal solutions, and the resulting crystals were analyzed for Ti by electron microprobe (EMP). To augment and extend the experimental results, zircons hosted by five natural rocks of well-constrained but diverse origin (0.7–3 GPa; 580–1,070°C) were analyzed for Ti, in most cases by ion microprobe (IMP). The combined experimental and natural results define a log-linear dependence of equilibrium Ti content (expressed in ppm by weight) upon reciprocal temperature:

 $$\log ({\text{Ti}}_{{{\text{zircon}}}}) = (6.01 \pm 0.03) - \frac{{5080 \pm 30}}{{T\;(\hbox{K})}}.$$
 In a strategy similar to that used for zircon, rutile crystals were grown in the presence of zircon and quartz (or hydrous silicic melt) at 1–1.4 GPa and 675–1,450°C and analyzed for Zr by EMP. The experimental results were complemented by EMP analyses of rutile grains from six natural rocks of diverse origin spanning 0.35–3 GPa and 470–1,070°C. The concentration of Zr (ppm by weight) in the synthetic and natural rutiles also varies in log-linear fashion with T
 −1:

 $$\log ({\text{Zr}}_{{{\text{rutile}}}}) = (7.36 \pm 0.10) - \frac{{4470 \pm 120}}{{T\;(\hbox{K})}}.$$
 The zircon and rutile calibrations are consistent with one another across both the synthetic and natural samples, and are relatively insensitive to changes in pressure, particularly in the case of Ti in zircon. Applied to natural zircons and rutiles of unknown provenance and/or growth conditions, the thermometers have the potential to return temperatures with an estimated uncertainty of ±10 ° or better in the case of zircon and ±20° or better in the case of rutile over most of the temperature range of interest (∼400–1,000°C). Estimates of relative temperature or changes in temperature (e.g., from zoning profiles in a single mineral grain) made with these thermometers are subject to analytical uncertainty only, which can be better than ±5° depending on Ti or Zr concentration (i.e., temperature), and also upon the analytical instrument (e.g., IMP or EMP) and operating conditions.

Crystallization thermometers for zircon and rutile

Polar temperatures over the last several million years have, at times, been slightly warmer than today, yet global mean sea level has been 6-9 metres higher as recently as the Last Interglacial (130,000 to 115,000 years ago) and possibly higher during the Pliocene epoch (about three million years ago). In both cases the Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability. Here we use a model coupling ice sheet and climate dynamics-including previously underappreciated processes linking atmospheric warming with hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs-that is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios. Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years.

Contribution of Antarctica to past and future sea-level rise

Generation of rhyolitic melt in an artificial magma: Implications for fractional crystallization processes in natural magmas

High precision 40Ar/39Ar dates on a variety of volcanic materials from the AND-1B drill core provide pinning points for defining the chronostratigraphy for the core. The volcanic materials dated include 1) felsic and basaltic tephra, 2) interior of a ~ 3 m thick intermediate submarine lava flow, and 3) felsic and basaltic volcanic clasts. In the upper 600 m of the core, two felsic tephra, two basaltic tephra and the intermediate laval flow yield precise and depositional ages, with further maximum age constraints from volcanic clasts. Below 600 m in the core, tephric intervals are significantly altered and maximum age constraints only are available from volcanic clasts. The ages for eight stratigraphic intervals are 1) 17.17–17.18 mbsf, basaltic clast (maximum depositional age 0.310 ± 0.039 Ma, all errors quoted at 2σ), 2) 52.80–52.82 mbsf, three basaltic clasts (maximum depositional age 0.726 ± 0.052 Ma), 3) 85.27–85.87 mbsf felsic tephra (1.014 ± 0.008 Ma), 4) ~ 112–145 mbsf sequence of basaltic tephra (1.633 ± 0.057 to 1.683 ± 0.055 Ma), 5) 480.97–481.96 mbsf pumice-rich mudstone (4.800 ± 0.076 Ma), 6) 646.30–649.34 mbsf intermediate lava flow (6.48 ± 0.13 Ma), 7) 822.78 mbsf kaersutite phenocrysts from volcanic clasts (maximum depositional age 8.53 ± 0.53 Ma) and 8) ~ 1280 mbsf, three volcanic clasts (maximum depositional age 13.57 ± 0.13 Ma). Minimum average sediment accumulation rates or 102 and 87 m/Ma for the upper and lower 650 m of core, respectively were calculated using the 40Ar/39Ar analyses. The volcanic material recovered from AND-1B also reveals a general northward progression of volcanism in Southern McMurdo Sound.

Development of a precise and accurate age–depth model based on 40Ar/39Ar dating of volcanic material in the ANDRILL (1B) drill core, Southern McMurdo Sound, Antarctica

Volcanic and Glacial Geology of the Miocene Minna Bluff Volcanic Complex, Antarctica

In situ vitrification (ISV) is an environmental remediation technology used to melt contaminated soil sites into more stable configurations. The behavior of water and other volatile constituents in the soil-melt system is important to the overall performance of the ISV technology. Mass and volume balance constraints are used to derive a method to indirectly estimate the volume of: (1) soil that dehydrates and releases water vapor to the off-gas, and (2) outside air pulled into the off-gas treatment system. These constraints allow us to speculate on whether some water may remain in the soil rather than being completely transported into the off-gas system. The method is tested with data from a field-scale test. Results suggest that, contrary to previous conceptual models, not all water that is vaporized reaches the surface and captured by the off-gas treatment system. It is probable that some percentage remains within the soil beneath and around the molten ISV mass.

Constraints on mass balance of soil moisture during in situ vitrification

We present new tephrochronologic and radiometric data that constrain the ages of terraces along the Rio Ojo Caliente. Draining the southern Tusas Mountains, the Rio Ojo Caliente is a tributary of the Rio Chama in the northwestern Española Basin. The highest terrace, labeled Qtoc1, has a strath located 107-134 m above the modern river. Qtoc1 contains ash and pumice-lapilli tephra probably correlative to the Guaje Pumice Bed of the Otowi Member of the Bandelier Tuff (1.61-1.63 Ma), although correlation to the Tsankawi Pumice Bed of the Tshirege Member of the Bandelier Tuff (1.22-1.26 Ma) is permissible. The second highest terrace, labeled Qtoc2, is 95-103 m above the modern river and correlates to terraces along the Rio Chama that contain the Lava Creek B ash (640 ka). The strath of the most laterally extensive terrace lies 19-30 m above the modern river. Called the 20-40 m terrace (labeled Qtoc6 in this study), it is older than 103 ka and probably younger than 210 ka based on stratigraphic relations and three U-series ages of an adjacent travertine mound complex. Both the Qtoc6 terrace and the next higher terrace level (Qtoc5) are characterized by having two straths (erosional terrace bases) separated by 2-7 m. Quartzite-rich gravel that overlies these strath couplets is generally 3-6 m-thick. Overlying these basal gravels is a 10-18 m-thick sequence of alluvium with higher sand content and higher amounts of locally derived alluvium. This sedimentologic trend indicates that fol$ % & '* ' $ + % / % ' & : % +% < ' ' $ % * = by input of detritus derived from nearby hillslopes and low order drainages; this locally derived alluvium was deposited as allu& % '* > % & '* ? '* V X/ % 'Z ' % ' % ' * % also occurred. Using this comparison, we agree with previous interpretations along the lower Rio Chama that locally derived, sand-dominated alluvium in the middle and upper parts of the terraces, best exposed in Qtoc6 and Qtoc5, were also deposited in interglacial climates or at the glacial/interglacial transition, and that strath formation and deposition of the quartzite-rich, basal gravel occurred largely during glacial climates (possibly at the glacial/interglacial transition). We use this process-response model to infer a preferred age of 130-160 ka for the Qtoc6 terrace deposit and its lowest strath. Consideration of terrace ages and their relative heights indicates an increase in incision rates after 640 ka, consistent with earlier interpretations.

/pdf/terrace-stratigraphy-ages-and-incision-rates-along-the-rio-32t6zyfcus.pdf

Nelia W. Dunbar

Papers

Generation of rhyolitic melt in an artificial magma: Implications for fractional crystallization processes in natural magmas

Development of a precise and accurate age–depth model based on 40Ar/39Ar dating of volcanic material in the ANDRILL (1B) drill core, Southern McMurdo Sound, Antarctica

Volcanic and Glacial Geology of the Miocene Minna Bluff Volcanic Complex, Antarctica

Constraints on mass balance of soil moisture during in situ vitrification

Terrace stratigraphy, ages, and incision rates along the Rio Ojo Caliente, north-central New Mexico