Quantifying late-Holocene climate in the Ecuadorian Andes using a chironomid-based temperature inference model
Matthews-Bird, F., Brooks, S.J., Holden, P.B., Montoya, E. & Gosling, W.D.
Presented here is the first chironomid calibration dataset for tropical South America. Surface sediments were collected from 59 lakes across Bolivia (15 lakes), Peru (32 lakes) and Ecuador (12 lakes) between 2004 and 2013 over an altitudinal gradient from 150 m above sea level (a.s.l) to 4655 m a.s.l, between 0-17°S and 64-78°W. The study sites cover a mean annual temperature (MAT) gradient of 25°C. In total, 55 chironomid taxa were identified in the 59 calibration data-set lakes. When used as a single explanatory variable, MAT explains 12.9% of the variance (λ1/λ2= 1.431). Two inference models were developed using weighted averaging and Bayesian methods. The best performing model using conventional statistical methods was a WA (inverse) model (R2jack= 0.890, RMSEPjack= 2.404, Mean biasjack= -0.017, Max biasjack= 4.665). The Bayesian method produced a model with R2jack= 0.909, RMSEPjack= 2.373, Mean biasjack= 0.598, Max biasjack= 3.158. Both models were used to infer past temperatures from a c. 3000 yr record from the tropical Andes of Ecuador, Laguna Pindo. Inferred temperatures fluctuated around modern day conditions but showed significant departures at certain intervals (c. 1600 cal yr BP; c. 3000-2500 cal yr BP). Both methods (WA/Bayesian) showed similar patterns of temperature variability; however, the magnitude of fluctuations differed. In general the WA method was more variable often inferring unrealistically cold temperatures (c. -7±2.5°C relative to the modern). The Bayesian method provided temperature anomaly estimates for cool periods that lay within the expected range of the Holocene (c. -3±3.4°C). The chironomid-based MAT recon struction from the Laguna Pindo fossil record suggests that periods of low solar output not only affect the tropics through changes in precipitation, but also directly affect tropical temperatures. Inferred temperatures were 2-3°C colder relative to the modern during the widely recognised 3500-2500 cal yr BP cooling event. Long-term cooling during the late-Holocene culminating in the Little Ice Age (LIA) is not apparent in the Laguna Pindo record. A cooling by 1-2°C relative to the modern during the LIA is recorded in a single fossil sample.