Investigating late Holocene variations in hydroclimate and the stable isotope composition of precipitation using southern South American peatlands: an hypothesis

Abstract

Abstract. Ombrotrophic raised peatlands provide an ideal archive for integrating late Holocene records of variations in hydroclimate and the estimated stable isotope composition of precipitation with recent instrumental measurements. Modern measurements of mean monthly surface air temperature, precipitation, and δD and δ 18 O-values in precipitation from the late twentieth and early twenty-first centuries provide a short but invaluable record with which to investigate modern relationships between these variables, thereby enabling improved interpretation of the peatland palaeodata. Stable isotope data from two stations in the Global Network for Isotopes in Precipitation (GNIP) from southern South America (Punta Arenas, Chile and Ushuaia, Argentina) were analysed for the period 1982 to 2008 and compared with longer-term meteorological data from the same locations (1890 to present and 1931 to present, respectively). δD and δ 18 O-values in precipitation have exhibited quite different trends in response to local surface air temperature and precipitation amount. At Punta Arenas, there has been a marked increase in the seasonal difference between summer and winter δ 18 O-values. A decline in the deuterium excess of summer precipitation at this station was associated with a general increase in relative humidity at 1000 mb over the surface of the Southeast Pacific Ocean, believed to be the major vapour source for the local precipitation. At Ushuaia, a fall in δ 18 O-values was associated with an increase in the mean annual amount of precipitation. Both records are consistent with a southward retraction and increase in zonal wind speed of the austral westerly wind belt. These regional differences, observed in response to a known driver, should be detectable in peatland sites close to the GNIP stations. Currently, insufficient data with suitable temporal resolution are available to test for these regional differences over the last 3000 yr. Existing peatland palaeoclimate data from two sites near Ushuaia, however, provide evidence for changes in the late Holocene that are consistent with the pattern observed in modern observations.

Retroceso glacial reciente en el Campo de Hielo Norte, región de Aysén, Chile: relación con variaciones climáticas

“…El calentamiento del clima patagónico cuyo efecto más directo es el retroceso generalizado de los glaciares al S de los 45° S de latitud y específicamente en el CHN, ha sido relacionado con un desplazamiento persistente al S de los vientos del W durante el verano y el otoño austral, asociado con una fase positiva del Modo Anular del Sur, posterior a la PEH (Daley et al, 2012;Moreno et al, 2014); con el aumento significativo de las temperaturas mínimas a partir de la segunda mitad del siglo XX, particularmente después del cambio de régimen del Indice de Oscilación del Sur en 1976 (Rosenbluth et al, 1997) y con los cambios en la Oscilación Decadal del Pacífico, que influyen desde 1976 en el clima patagónico, elevando las temperaturas medias de verano, particularmente en las altas latitudes (Villalba et al, 2003).…”

“…La tendencia general en el sector es a evolucionar hacia periodos más secos, al igual que en el resto de Patagonia (Rasmussen et al, 2007;Masiokas et al, 2008;Neukom et al, 2010;Daley et al, 2012). Durante la primera parte del siglo XX, las precipitaciones están por sobre la media culminando en 1950, luego disminuyen las precipitaciones de invierno (Figura N° 14a).…”

The ‘Little Ice Age’ in the Southern Hemisphere in the context of the last 3000 years: Peat-based proxy-climate data from Tierra del Fuego

“…They have only sparse data points for southernmost South America, whereas our reconstructions extend the evidence for (late) 17 th Century dryness, further south into Tierra del Fuego. Daley et al (2012) noted that Ushuaia (see Fig. 1) has in recent decades been getting wetter over the instrumental period in response to intensification and southward movement of the Westerlies.…”

“…Nevertheless, the datasets reported here indicate that the greatest inferred climatic deviations fit well with climate excursions elsewhere, even though it is assumed that the principal change in the mire relates to bog surface wetness, rather than to temperature. Further work might be directed to investigating deuterium excess, as suggested in Daley et al (2012), which could help to show whether shifting Westerlies were indeed the mechanism for inferred changes in bog surface wetness at this site.…”

Late-Holocene climate dynamics recorded in the peat bogs of Tierra del Fuego, South America

“…Moreover, rather than a north-south orientation, the Andean divide in Tierra del Fuego has a northwest-southeast orientation which may attenuate the rain shadow effect to the northeast. Considering the orographic complexity of southern Patagonia, Daley et al (2012) suggested relatively important intra-regional differences in moisture source for precipitation between Punta Arenas and Ushuaia at decadal timescales.…”