⁴⁰Ar/³⁹Ar, K–Ar and ²³⁰Th–²³⁸U dating of the Laschamp excursion: A radioisotopic tie-point for ice core and climate chronologies

Abstract

A brief period of enhanced ¹⁰Be flux that straddles the interstadial warm period known as Dansgaard–Oeschger event 10 in Greenland and its counterpart in Antarctica, the Antarctic Isotope Maximum 10 is but one consequence of the weakening of Earth's magnetic field associated with the Laschamp excursion. This ¹⁰Be peak measured in the GRIP ice core is dated at 41,250 y b2k (= before year 2000 AD) in the most recent GICC05 age model obtained from the NorthGRIP core via multi-parameter counting of annual layers. Uncertainty in the age of the ¹⁰Be peak is, however, no better than ± 1630 y at the 95% confidence level, reflecting accumulated error in identifying annual layers. The age of the Laschamp excursion [Guillou, H., Singer, B.S., Laj, C., Kissel, C., Scaillet, S., Jicha, B., 2004. On the age of the Laschamp geomagnetic excursion. Earth Planet. Sci. Lett. 227, 331-343.] is revised on the basis of new ⁴⁰Ar/³⁹Ar, unspiked K–Ar and ²³⁸U–²³⁰Th data from three lava flows in the Massif Central, France, together with the ⁴⁰Ar/³⁹Ar age of a transitionally magnetized lava flow at Auckland, New Zealand. Combined, these data yield an age of 40,700 ± 950 y b2k, where the uncertainty includes both analytical and systematic (⁴⁰K and ²³⁰Th decay constant) errors. Taking the radioisotopic age as a calibration tie point suggests that the layer-counting chronologies for the NorthGRIP and GISP2 ice cores are more accurate and precise than previously thought at depths corresponding to the Laschamp excursion.

Introduction

Accurate and precise geochronology is fundamental to understanding the past 50,000 years of Earth history during which climate frequently oscillated on a millenial time scale while the major transition into and out of the last global glacial maximum took place. The main chronometer for this period is radiocarbon (¹⁴C), but because the ¹⁴C/¹²C ratio in the atmosphere varies over time, radiocarbon dates depend upon a precise knowledge of this temporal variation (Bard et al., 2004). The calibration curve most widely adopted by the radiocarbon community – known as INTCAL04 (Reimer et al., 2004) – uses ¹⁴C dates of rings in fossil trees, extended by way of paired ²³⁰Th and ¹⁴C dates from tropical corals to 26,000 y BP (= before year 1950 AD; radiocarbon convention). However, between 26,000 and 50,000 y BP coral data are sparse and there is no consensus curve (Reimer et al., 2006). Several approaches, including varved lake sediments dated by ²³⁰Th (Kitigawa and van der Plicht, 1998), a set of carefully selected, pristine corals that have been dated using ²³⁰Th and ¹⁴C methods (Fairbanks et al., 2005), and the “stratigraphic” method of tuning climate-sensitive signals in ¹⁴C-dated marine sediment cores to the δ¹⁸O stratigraphy of the Greenland Ice Sheet Project II (GISP2) ice core (Hughen et al., 2004), yield different curves from which the calibration of a single ¹⁴C age can vary by more than ± 16% in calendar age (e.g., Bard et al., 2004). A more recent approach by Hughen et al. (2006) to reduce uncertainty in the ¹⁴C calibration curve ties the marine record in the Cariaco Basin to the ²³⁰Th chronology of the Hulu Cave speleothem record (Wang et al., 2001).

On one hand, the stratigraphic method in which the chronology is based on the counting of annual layers in ice offers perhaps the most continuous and straightforward template for calibrating ¹⁴C ages between 26,000 and 50,000 y b2k. On the other hand, it has proven difficult to establish a precise chronology for the GISP2 and Greenland Ice core Project (GRIP) ice cores using annual layer counting beyond the Holocene (Southon, 2004). For example, uncertainties in the most widely used timescale for the GISP2 ice core rise dramatically from ± 700 y to ± 1700 y in the period between about 40 and 50 ka (Alley et al., 1997, Meese et al., 1997). No single or consensus chronology arose until the recent multi-parameter counting of annual layers, including visual stratigraphy, conductivity of ice and melt-water and concentrations of Na⁺, Ca²⁺, SO₄²⁻, NO₃⁻, and NH₄⁺ at the NorthGRIP core site correlated to previous results from the GRIP ice core, resulting in the Greenland Ice Core Chronology 2005 or GICC05 (Andersen et al., 2006). Although the GICC05 timescale is a major step forward, 2σ precision estimated from layer counting degrades with depth from < 1% at 15 ka to > 4%, or more than ± 1600 y, at 40 ka, near the useful limit of ¹⁴C (Andersen et al., 2006, Svensson et al., 2006). Many types of proxy records of climate depend on correlation to the GISP2 or GRIP ice cores. For example, interpretations of high-resolution ²³⁰Th-dated speleothem records of climate and rainfall in temperate regions far removed from the polar ice caps rely on matching δ¹⁸O records of polar ice and cave carbonate that, particularly for the period of marine isotope stage 3 between 35 and 50 ka, would benefit from independently-dated tie points (Wang et al., 2001, Genty et al., 2003, Yuan et al., 2004). Thus, it remains desirable to find an alternative, independent method with which to calibrate the ice core chronologies, rather than relying on the ice cores themselves to calibrate the ¹⁴C chronometer (Southon, 2004).

The production of radiocarbon and other cosmogenic nuclides such as ¹⁰Be and ³⁶Cl in the atmosphere is strongly modulated by changes in the Earth's magnetic field (Wagner et al., 2000, Laj et al., 2002, Muscheler et al., 2004). The most significant fluctuation of the past 50,000 years occurred during the Laschamp excursion as the strength of the magnetic field recorded by lava flows and marine sediments dropped to less than about 10% of its prior value (Laj et al., 2002, Laj et al., 2004, Channell, 2006). Cosmogenic nuclide flux into polar ice and marine sediments increased significantly at this time (Raisbeck et al., 1992, Yiou et al., 1997, CiniCastagnoli et al., 1995), and several cooling lava flows captured “snapshots” of the changing field geometry and intensity (Roperch et al., 1988, Chauvin et al., 1989, Guillou et al., 2004, Cassata et al., 2008). We report new ⁴⁰Ar/³⁹Ar data from the Laschamp lava flow, ⁴⁰Ar/³⁹Ar and unspiked K–Ar data from the la Louchadière flow, and ²³⁰Th–²³⁸U data from the Olby flow that had been previously dated using combined ⁴⁰Ar/³⁹Ar and unspiked K–Ar methods (Guillou et al., 2004). Moreover, we re-calculate the ⁴⁰Ar/³⁹Ar ages obtained previously from the Laschamp and Olby flows (Guillou et al., 2004) and from a transitionally magnetized flow in New Zealand (Cassata et al., 2008), using a newly proposed standard age in order to combine these data into a single, robust, accurate date for the Laschamp excursion. Given the effect of the changes in geomagnetic dipolar intensity on the production of cosmogenic nuclides, including ¹⁰Be and ³⁶Cl, our new radioisotopic age determination provides a tie-point that can be used to constrain correlations between proxy climate records from speleothems (e.g., Wang et al., 2001, Genty et al., 2003) or marine sediments (e.g., Hughen et al., 2004), and the ice cores from Greenland and Antarctica.