Tree-ring isotopes from Araucaria araucana as useful proxies for climate reconstructions
Tree-ring width (TRW) chronologies have been widely and long-time used to reconstruct past climate variations in the Andes in South America. The use of tree-ring isotopic chronologies is still not widespread in this region although they have proved to be very efficient climate proxies. Araucaria araucana (Molina) K. Koch is a conifer tree species with some multi-century-old individuals that offers an excellent opportunity to measure stable carbon (δ13C) and oxygen (δ18O) isotopes in cellulose from long tree-ring records. Here, we explore whether current or stored carbohydrates are used for A. araucana radial growth and we assess the potential of a tree-ring isotopic record to study past climate variability. Eleven A. araucana cores from a dry and high-elevation forest at the northern border of Patagonia, Argentina (38◦55’S, 70◦44’W) were selected for stable isotopes analyses. The strong correlation between the isotopic composition of the first and second parts of the same ring, but also the strong relationships between δ13C and δ18O records with climate parameters of the current growing season such as temperature, show that tree-rings are built mostly with carbohydrates produced during the current growing season with little or no supply from storage or reserves. This finding leads to reconsidering the interpretation of the legacy effect (i.e. ecological memory effects) based on the previously described strong negative correlation between A. araucana TRW chronologies and previous growing season temperature and suggests a dependence of radial tree growth on the level of development of organs. Regarding climate sensitivity, the A. araucana tree-ring δ13C chronology is strongly related to current summer temperature (r = 0.82, p < 0.001), vapour pressure deficit (VPD; r = 0.79, p < 0.001), precipitation (r = -0.53, p < 0.001) and SPEI2 (r = - 0.73, p < 0.001) (Figure 1). These strong relationships support the use of δ13C of A. araucana tree-ring cellulose to reconstruct past temperature variations at regional scale in relation with large-atmospheric drivers of climate variability such as the Southern Annular Mode. The A. araucana tree-ring δ18O chronology is also correlated with temperature (r = 0.42, p < 0.01) and VPD (r = 0.45, p < 0.01) of the winter preceding the growing season. This suggests that trees are using water from precipitation infiltrated in the soil during the previous recharge period (autumn-winter). The weak correlations of δ18O with current summer atmospheric conditions and the decoupling between δ18O and δ13C, may be due to a high rate of oxygen exchange between sugars and xylem water (Pex) during cellulose synthesis, which dampens evaporative isotopic fractionation.
Figure 1: Comparisons of temporal variations in A. araucana δ13C (solid black line) at Sainuco, northern Patagonia with (upper) regional mean (December-January) maximum temperature (Tmax), (middle) mean (December-January) maximum vapor pressure deficit (VPDmax) and (bottom) January Standardized Precipitation-Evapotranspiration Index (SPEI2, calculated over December- January corresponding to January) over the common interval 1974–2014 (41 years. The correlation coefficients between records are indicated.
Reference: Penchenat T., Daux V., Mundo I., Pierre M., Stievenard M., Srur A., Andreu-Hayles L., Villalba R. Dendrochronologia 74, 2022 125979.
Multi-millennial human impacts and climate change during the Maya early Anthropocene: implications on hydro-sedimentary dynamics and socio-environmental trajectories (Naachtun, Guatemala)
During the Maya early Anthropocene (2000 BCE - 1000 CE) in Mesoamerica, socio-environmental interactions contributed to the rise and decline of the ancient Maya civilization. At the scale of the exploitation territories of the Maya cities, the temporal variations of hydrological and sedimentary dynamics in response to anthropogenic and climate drivers are still poorly known.
This manuscript analyses and presents a regional comparison of the dynamics of one of the most transformed hydrosystems and morpho-sedimentary systems by the societies of the Southern Maya Lowlands, during the second half of the Holocene. It focuses on the lake basin of the polje named El Infierno bajo and its watershed, which was the main water storage area for the Maya city of Naachtun —a large regional capital between 150 and 950 CE —, and which contains many remains of hydraulic and agrarian structures. This integrated palaeolimnological, geoarchaeological and hydrological approach, based on the analyses of morpho-sedimentary archives, LiDAR altimetry data, hydrological and geochemical data, resulted in the construction of hydro-sedimentary baselines (pre- syn- and post-ancient Maya anthropogenic impacts).
During the past 5500 years, hydrosedimentary fluctuations were marked by the alternation of seven main hydrological periods, characterized by high and low lake levels (alternately perennial, intermittent and dry lake) and six main erosion and sediment transfer periods, marked by strong and low alluvial and colluvial detrital inputs in the lowlands. Anthropogenic and climate forcings have independently or jointly controlled the hydrologic and sedimentary budgets of the lake basin. Lithofacies, depositional processes, accumulation rates and drivers of the anthropogenic detrital inputs — the so-called “Maya clays” —, are analyzed and quantified from ~1500 BCE to ~1150 CE. It thus reveals one of the longest periods of occupation and exploitation of natural resources, for over 2500 years during the Preclassic, Classic and Postclassic Maya periods. The hydro-sedimentary dynamics in the bajos of the Elevated Interior Region, such as El Infierno, enabled the long-term exploitation of water and soil resources for agrarian purposes, thanks to the construction of hydraulic and agrarian palimpsest landscapes shaped by the socio-ecosystems.
Référence : Cyril Castanet, Louise Purdue, Marc Testé, Aline Garnier, Anne-Lise Develle-Vincent, Fatima Mokadem, Christine Hatté, Caroline Gauthier, Philippe Lanos, Philippe Dufresne, Eva Lemonnier, Lydie Dussol, Julien Hiquet, Philippe Nondédé. Multi-millennial human impacts and climate change during the Maya early Anthropocene: implications on hydro-sedimentary dynamics and socio-environmental trajectories (Naachtun, Guatemala). Quaternary Sciences Reviews (2022) 283, 107458 - doi: 10.1016/j.quascirev.2022.107458
Synthesis of the hydrological and sedimentary dynamics of the El Infierno bajo lake basin during the past 5500 years, in responses to anthropogenic and climate drivers.
