Climate reconstructions based on postglacial macrofossil assemblages from four river systems in southwestern Alberta

Following floods experienced in southwestern Alberta in June 2013, 36 palaeoenvironmental sediment samples were collected from the banks of four affected streams: the Kananaskis River, Highwood River, Tongue Creek and Jumping-pound Creek. The sampled layers were deposited during the Holocene, and provide evidence of riparian ecology in the region since the end of the last glacial interval. The samples were processed to extract sub-fossil macroremains (including seeds, fruits, and aquatic and terrestrial mollusc shells). Macrofossils are particularly useful for creating fine-resolution (site-specific) reconstructions. An innovative aspect of this study was the derivation of regional bioclimatic trends through time. This was achieved by applying a dual-layered weighted calibration function which incorporated the relative productivity of indicator taxa, their modelled climatic optima, and the influence of climate (temperature and precipitation) on their ecological niche. Optima and climatic influence values were calculated using the machine-learning maximum entropy environmental niche modelling tool, MAXENT, trained on global occurrence records for the taxa. This study aimed to address gaps in prior palaeoenvironmental research, allowing for a spatially explicit quantitative reconstruction of climate in the transitional foothill region near Calgary. Results indicated warm conditions immediately following glacial retreat, followed by an early to mid-Holocene cooling which coincided with the driest interval. Temperature and moisture regimes oscillated in the mid-Holocene, exhibiting the highest values for both parameters. Increased moisture was at least in part prompted by increased input from summer glacial meltwater (the result of higher summer temperatures) which added to the riparian water budget. The mid-to late Holocene, and up to modern times, showed a gradual decrease in both moisture and temperature, until stabilizing near the Holocene average.