Quantifying climate change in Huelmo mire (Chile, Northwestern Patagonia) during the Last Glacial Termination using a newly developed chironomid-based temperature model
Introduction
Patagonia is an important area for paleoclimate studies in southern South America because it is significant in understanding climate synchronization between the North and South Hemispheres. However, views are still polarized concerning climate dynamics during key periods of large-scale climate fluctuations (e.g. the Lateglacial/Holocene transition) (Whitlock et al., 2006, Rojas et al., 2009, Killian and Lamy, 2012). Despite efforts to identify climate fluctuations after the Last Glacial Maximum (LGM), the timing and extent of a cold reversal contemporaneous to the Younger Dryas (YD) is still in unresolved. Divergent evidence comes from paleoclimate studies based mainly on terrestrial records from the Andean Patagonian forest of Chile and Argentina. These studies show either i) a cooling pattern synchronous with the YD (12,500 to 11,200 cal yr BP) (Heusser et al., 1996, Ariztegui et al., 1997, Moreno, 1997, Moreno, 2004, Moreno et al., 2001, Massaferro and Brooks, 2002); ii) a cooling pattern synchronous with the Antarctic Cold Reversal (ACR, 14,500 to 13,000 cal yr BP) (Lamy et al., 2004, Moreno et al., 2009); or iii) an intermediate YD/ACR climate signal called Huelmo–Mascardi Cold Reversal (HMCR, 13,500 to 11,600 cal yr BP) (Hajdas et al., 2003, Bertrand et al., 2008b Massaferro et al., 2009). Differences between these records may be attributed to the individual response of proxies, chronological control, sampling resolution, individual site characteristics, the differential influence of the Southern Westerly Winds (SWW) on the east or west side of the Andes or merely because the signature of the cold event in the southern hemisphere is weak. In New Zealand, similar discrepancies were apparent during the Late Glacial/Holocene transition (Denton and Hendy, 1994, Newnham et al., 2003, Turney et al., 2003, McGlone et al., 2004). Resolving this problem is important for understanding the intra- and inter-hemispheric modes of millennial-scale climate changes during the Last Glacial Termination, and for determining the climatic mechanisms involved in their initiation and propagation. Production of quantitative, high resolution reconstructions of summer temperature will be particularly important in this respect. To contribute to the resolution of this problem we have used chironomids to obtain a summer temperature reconstruction, the first of its kind in northern Patagonia, from a Late Glacial lake sediment sequence.
Summer temperature is one of the major controls over the chironomid life cycle (Rossaro, 1991, Brodersen and Lindegaard, 1999) and many models (transfer functions) have been developed in the Northern Hemisphere to quantify summer temperature (e.g. Walker et al., 1991, Olander et al., 1999, Brooks and Birks, 2001, Larocque et al., 2001, Larocque et al., 2006, Self et al., 2011, Eggermont and Heiri, 2012). In the southern Hemisphere, transfer functions have been developed for southern Patagonia (Massaferro and Larocque, 2013), northern Chile (Araneda et al., in prep), Tasmania (Rees et al., 2008) and New Zealand (Dieffenbacher-Krall et al., 2007, Woodward and Shulmeister, 2007) showing the potential of using chironomids in this region for temperature reconstruction. Even though northern Patagonian paleoenvironmental investigations using chironomids are not numerous, there are several qualitative and semiquantitative records indicating that changes have occurred in this faunal community during the Late Glacial period (Massaferro and Brooks, 2002, Massaferro et al., 2009). A recent quantitative reconstruction at Potrok Aike, in southern Patagonia, confirms the potential of chironomids to help in understanding the complex climate fluctuations in southern South America (Massaferro and Larocque, 2013).
In this paper, we present the first quantitative chironomid-inferred temperature model for Northern Patagonia (Argentina and Chile) and use it to reconstruct temperatures during the Late Glacial from a fossil chironomid record from Huelmo mire, located in Chilean Patagonia at 41° S. An earlier qualitative analysis of chironomids and pollen from Huelmo indicated temperature and precipitation changes between ca. 20 and 10 cal kyr BP (Massaferro et al., 2009). The chironomid and pollen records from Huelmo indicated step-wise deglacial warming beginning at ca. 18,000 cal yr BP, in agreement with other paleoclimate records from northwestern Patagonia, and ice core records from Antarctica (Pedro et al., 2011). Isotopic signals from Antarctic ice cores indicate relatively warm conditions between ~ 15,000 and 14,000 cal yr BP, followed by a reversal in trend with cooling pulses at ~ 14,000 and 13,500 cal yr BP, and warming at the beginning of the Holocene (Jouzel et al., 2003) Peak warmth during the Last Glacial Termination was achieved during ~ 14,500 cal yr BP, followed by a cooling trend that commenced during the ACR (~ 14,000 cal yr BP), which later intensified and persisted during the so-called Huelmo–Mascardi Cold Reversal (HMCR) (Hajdas et al., 2003, Massaferro et al., 2009). A reconstruction of temperature using chironomids will quantify the warmer and colder periods suggested by the previous pollen and chironomid records.
Section snippets
Site location
The Huelmo mire (41°31′ S, 73°00′ W) is located in the lowlands of the southern Chilean Lake District, on the western side of Seno Reloncaví (Fig. 1). The area is characterized by a mix of Valdivian, North Patagonian and temperate rainforest vegetation. The area that surrounds Huelmo has been altered by human activities. Current vegetation close to the mire is heavily influenced by clearance, leaving grassland for grazing. Quaternary glacial, volcanic, eolian and alluvial–colluvial deposits
Sampling, stratigraphy and chronology
Several overlapping sediment cores from the center of the Huelmo mire were extracted using a square-rod Livingstone corer. For this study, cores 601A, 990-1A and 990-1B were combined in a 421 cm long composite sequence spanning the time interval between 19,600 and 10,000 cal yr BP (1071–650 cm). Stratigraphic correlation between these cores was achieved using loss-on-ignition records and two prominent tephra layers (Moreno and Leon, 2003). The stratigraphy of the composite core consisted of a floor
Exploratory analysis
After removing the taxa present in only one lake at percentages lower than 3%, 49 taxa were retained in the dataset of 46 sampled lakes. The length of the first DCA gradient was 3.872, suggesting a unimodal distribution of taxa. WMM, secchi depth and conductivity were the three variables with the highest and significant scores of variability explained. The first significant axis of the CCA explained 36.3% of the variance while CCA axis 2 explained only 12.8% of the variance (Fig. 3). Alone, WMM
Chironomids, temperature and other environmental variables
Since the early work of Walker et al. (1995), temperature has been shown to be one of the most important variables affecting the distribution of chironomids in lakes distributed along a large temperature gradient (e.g. Lotter et al., 1997, Olander et al., 1999, Brooks and Birks, 2001, Larocque et al., 2001, Larocque et al., 2006, Massaferro and Larocque, 2013). Other environmental variables influencing chironomids are nutrients (Lotter et al., 1998), depth (Korhola et al., 2000), conductivity (
Conclusions
Quantitative temperature reconstructions from the Southern Hemisphere are extremely valuable to clarify climate relationships between both hemispheres. This investigation shows that chironomid distribution and abundance in the 46 Patagonian lakes surveyed were strongly influenced by WMM. Furthermore, the good performance statistics of a chironomid-based WMM inference model and the fact that summer temperature has a direct impact through life history effects on chironomid distribution and
Acknowledgments
We thank Sarah Gilchrist for collecting some of the samples and for preliminary chironomid analysis. JM acknowledges G. Denton and COMER Science Foundation for providing financial support and M. Colombres for lab assistance.
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