⁴⁰Ar/³⁹Ar chronology of Late Pliocene and Early Pleistocene geomagnetic and glacial events in southern Argentina

Abstract

K-Ar dating and paleomagnetic directions from the lava sequence atop Cerro delFraile, Argentina, contributed to the nascent Geomagnetic Polarity Time Scale (GPTS), recording the Reunion event, and the Olduvai and Jaramillo subchrons [Fleck et al., 1972]. New stratigraphy, paleomagnetic analyses, 40 Ar/ 39 Ar incremental heating ages, and unspiked K-Ar dating of 10 lava flows on Cerro del Fraile place these eruptions between 2.181±0.097 and 1.073±0.036 Ma and enhance this unique record, which includes seven tills interbedded with the lavas. The Reunion event is recorded by three lavas with transitional, normal, and reversed polarity that yielded identical 40 Ar/ 39 Ar isochron ages and a weighted mean age of 2.136±0.019 Ma. When com bined with 40 Ar/ 39 Ar ages from lavas on Reunion Island and a normal tuff in the Massif Central, the age of the Reunion event is 2.137±0.016 Ma and is older by ~50 kyr than the 2.086±0.016 Ma Huckleberry Ridge event. The onset and termination of the Olduvai are similarly constrained to 1.922±0.066 Ma and 1.775±0.015 Ma, whereas the onset of the Jaramillo occurred 1.069±0.011 Ma. A discordant age spec trum from another transitional lava gave a total fusion age of 1.61 Ma and an unspiked K-Ar age of 1.43 Ma. It is uncertain whether this corresponds to the Gilsa, Gardar, Stage 54, or Sangiran events, or represents an unrecognized period of geomagnetic instability. Deposition of till on the piedmont surface prior to 2.186 Ma and six sub sequent tills between 2.186 Ma and ~1.073 Ma mark frequent glaciations of south ern South America during marine oxygen isotope stages 82 to 48.

Precise ages of the Réunion event and Huckleberry Ridge excursion: Episodic clustering of geomagnetic instabilities and the dynamics of flow within the outer core

“…Although the geochronologic control is poor, the sedimentary records from the Beaver basin and Death Valley are remarkably consistent with our interpretation of a short Réunion event, followed by a longer (Feni) normal subchron, then deposition several thousand years later of a singular, transitionally magnetized, HRT (Holt and Kirshvink, 1995). Singer et al (2008aSinger et al ( , 2008b proposed that there are two periods during the Brunhes chron in which the dynamo is relatively weak, each lasting about 200 kyr and encompassing at least five excursions. These two periods are separated by ∼300 kyr during which there is little compelling evidence for excursional behavior (Singer, 2014).…”

“…These two periods are separated by ∼300 kyr during which there is little compelling evidence for excursional behavior (Singer, 2014). Singer et al (2008aSinger et al ( , 2008b hypothesized that the pattern of fluid flow within the outer core may be such that there is a 200-300 kyr oscillation between stable states of the main dipole and weaker states during which excursions or reversals can take place. The possibility that the dynamo was relatively weak and therefore unstable between 2.20 and 2.07 Ma may explain the occurrence of a short-lived event, two successive polarity reversals bounding a subchron, and another short-lived excursion during this interval.…”

“…The timing of these phenomena, particularly the number excursions and events has led to a nascent geomagnetic instability time scale (GITS) for the Quaternary (Singer et al, 1999;Singer, 2014). Notably, this chronology has revealed a temporal clustering of excursions during the early (730-530 ka) and late (212-17 ka) portions of the Brunhes chron (Singer et al, 2008a(Singer et al, , 2008bSinger, 2014). Here, we present new geochronologic data from volcanic rocks that, together with published data from marine sediments, record a sequence of excursions, reversals, and events during 125 kyr of the early portion of the Matuyama chron.…”

Chronology of Lake El'gygytgyn sediments – a combined magnetostratigraphic, palaeoclimatic and orbital tuning study based on multi-parameter analyses

“…Besides these, there are three further, much shorter intervals of normal polarity, not listed in Lisiecki and Raymo (2005): the Réunion subchron (Chamalaun and McDougall, 1966;McDougall and Watkins, 1973), an Olduvai precursor (Channell et al, 2003), and the Cobb Mountain subchron (Mankinen et al, 1978). The ICDP Site 5011-1 derived age range for the Réunion subchron of 2.1216-2.1384 Ma is in good agreement with the GPTSs (Table 1) and radiometric dating results of 2.14 ± 0.03 Ma by Baksi et al (1993), 2.137 ± 0.016 Ma by Singer et al (2004), or 2.15 ± 0.02 Ma by Quidelleur et al (2010). The results are also consistent with magnetostratigraphic data from North Atlantic ODP Site 981 (Feni Drift;Channell et al, 2003, Fig.…”

A glacial geomorphological map of the southernmost ice lobes of Patagonia: the Bahía Inútil – San Sebastián, Magellan, Otway, Skyring and Río Gallegos lobes

“…Recent cosmogenic dating of outwash depth-profiles at Lago Pueyrredón, east of the North Patagonian icefield, further supports these old ages (Hein, Dunai, Hulton, & Xu, 2011). The inner nested limits have been attributed to advances during intervening glaciations prior to the LGM, largely using cosmogenic dating (Hein et al, 2009;Hein et al, 2010;Hein et al, 2011;Glasser et al, 2011;Kaplan, Douglass, Singer, & Caffee, 2005;Singer et al, 2004a;Singer et al, 2004b;Wenzens, 2006).…”