The first magnetic chart

Almost 300 years to the day since Edmond Halley's first magnetic chart, the RAS held a Discussion Meeting to commemorate this achievement and to consider Halley's work in navigation and geophysics. David Barraclough and Toby Clark report on the "300th anniversary of the first magnetic chart: Edmond Halley's work in geophysics and navigation".     

Improving in Situ Cosmogenic Chronometers

New radiocarbon ages for Sierra Nevada deglaciation, the first ¹⁰ Be measurements from the Laurentide terminal moraine, and calculations based on paleomagnetic field strength have the potential to substantially improve the accuracy of cosmogenic age estimates. Specifically, three new constraints apply to the interpretation of measured abundances of in situ produced cosmogenic ¹⁰Be and ²⁶Al: (1) A suite of minimum-limiting radiocarbon dates indicates that the Sierra Nevada was deglaciated at least several thousand years earlier than assumed when Nishiizumi et al. (1989) first calibrated ¹⁰Be and ²⁶ Al production rates based on polished bedrock surfaces in the range, with retreat beginning by 18,000 cal yr B.P. and completed by 13,000 cal yr B.P. (2) Concentrations of ¹⁰Be in moraine boulders and glacier-polished bedrock in New Jersey show little variance (10%, 1σ) and can be used to calculate a preliminary ¹⁰Be production rate (integrated over the past 21,000-22,000 cal yr B.P. at 41°, 200-300 m altitude) that is about 20% lower than currently accepted. (3) Calculations of the effect of past geomagnetic field-strength variations on production rates suggest that the use of temporally averaged production rates may generate age errors of >20%; however, cosmogenic exposure ages can be corrected for this effect, although the corrections currently are imprecise. Many previously reported late-Pleistocene ¹⁰Be and ²⁶Al exposure ages are probably too young and are less accurate and less precise than implied by reported uncertainties. The discrepancy between accepted production rates and those calculated from Laurentide exposures, when considered together with the Sierran deglacial chronology and the model results, suggest that correlations between cosmogenic and other numerical ages, especially for brief events like the Younger Dryas and Heinrich events, will not be robust until temporal variations and the altitude/latitude scaling of production rates are fully understood and quantified at levels comparable to current analytic uncertainties (3%).     

Integrating climatic change and forests: Economic and ecologic assessments

Effective policies for dealing with anticipated climatic changes must reflect the two-way interactions between climate, forests and society. Considerable analysis has focused on one aspect of forests - timber production - at a local and regional scale, but no fully integrated global studies have been conducted. The appropriate ecological and economic models appear to be available to do so. Nontimber aspects of forests dominate the social values provided by many forests, especially remote or unmanaged lands where the impacts of climatic change are apt to be most significant. Policy questions related to these issues and lands are much less well understood. Policy options related to afforestation are well studied, but other ways the forest sector can help ameliorate climatic change merit more extensive analysis. Promising possibilities include carbon taxes to influence the management of extant forests, and materials policies to lengthen the life of wood products or to encourage the substitution of CO₂-fixing wood products for ones manufactured from less benign materials.     

K-Ar geochronology of the late cenozoic volcanic rocks of the Cordillera Occidental, southernmost Peru

Twenty-four K-Ar radiometric ages are presented for late Cenozoic continental volcanic rocks of the Cordillera Occidental of southernmost Perú (lat. 16° 57′–17° 36′S). Rhyodacitic ignimbrite eruptions began in this transect during the Late Oligocene and continued episodically through the Miocene. The development of andesitic-dacitic strato volcanoes was initiated in the Pliocene and continues to the present.The earliest ignimbrite flows (25.3–22.7 Ma) are intercalated in the upper, coarsely-elastic member of the Moquegua Formation and demonstrate that this sedimentary unit accumulated in a trough, parallel to Andean tectonic trends, largely in the Oligocene. More voluminous ash-flow eruptions prevailed in the Early Miocene (22.8–17.6 Ma) and formed the extensively preserved Huaylillas Formation. This episode was coeval with a major phase of Andean uplift, and the pyroclastics overlie an erosional surface of regional extent incised into a Paleogene volcano-plutonic arc terrain. An age span of 14.2–8.9 Ma (mid-Late Miocene) is indicated for the younger Chuntacala Formation, which again comprises felsic ignimbrite flows, largely restricted to valleys incised into the pre-Huaylillas Formation lithologies, and, at lower altitudes, an extensive aggradational elastic facies. The youngest areally extensive ignimbrites, constituting the Sencca Formation, were extruded during the Late Miocene.In the earliest Pliocene, the ignimbrites were succeeded by more voluminous calcalkaline, intermediate flows which generated numerous large and small stratovolcanoes; these range in age from 5.3 to 1.6 Ma. Present-day, or Holocene, volcanism is restricted to several large stratovolcanoes which had begun their development during the Pleistocene (by 0.7 Ma).The late Oligocene/Early Miocene (ca. 22–23 Ma) reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the Cordilleras Occidental and Oriental of the Central Andes.