Multi-proxy evidence for climatic and environmental change during the late glacial and Holocene at Kelly Lake, Kenai Peninsula, Alaska

• Main Author: Wrobleski, Emily Amory
• Format: Thesis
• Language: English
• Published: 2021
• Subjects: QH Natural history; Pacific; aleutian low; Alaska
• Online Access: https://openknowledge.nau.edu/id/eprint/5706/; https://openknowledge.nau.edu/id/eprint/5706/1/Wrobleski_2021_multi-proxy_evidence_climatic_environmental_change_duri.pdf; https://openknowledge.nau.edu/id/eprint/5706/2/Wrobleski_Thesis_2021_SupplementaryFile.xlsx

Past climate reconstructions using multiple proxies from lake sediments are crucial to developing our understanding of environmental and landscape response to climate change. Previous research on hydroclimatic change, particularly changes in the Aleutian Low (AL) pressure system in the North Pacific region, and the effects on aquatic and terrestrial ecosystems in south-central Alaska during the last deglacial and Holocene is extensive, but our understanding of the changing influence of groundwater on lake ecosystems is lacking. Although they are rare, freshwater calcium carbonate (marl) deposits in lakes on the Kenai Peninsula, Alaska, provide the opportunity to reconstruct past changes in hydroclimate and groundwater influence on the lake. A 6.2-m-long sediment core from Kelly Lake (KLY18-4; 4 m depth; 60.514°N, 150.374°W) was dated using 14C and analyzed for multiple proxy types in both the inorganic and organic matter including: isotopes of carbon and oxygen in marl calcite (δ13Cmarl and δ18Omarl), and isotopes of carbon (ẟ13COM) and abundances of C and N in organic matter. Bulk sediment analyses include organic matter (OM) and calcium carbonate (CaCO3) contents, along with visual stratigraphy and sediment flux. The results show that Kelly Lake was deglaciated prior to 14.6 ka (14,600 cal yr BP). By 14.0 ka, marl deposition dominated the core site, with CaCO3 precipitation driven by groundwater solute input and mediated by shallow-water charophytes. The Younger Dryas climate reversal is not well expressed in the proxy data from Kelly Lake. However, marl deposition decreased as organic and clastic matter increased between around 12.2 and 11.5 ka. This shift, together with an increase in both δ13Cmarl and δ18Omarl values and a decrease in CaCO3 content during this interval indicate an increase in the influence of meteoric water on the Kelly Lake hydrologic budget under wet conditions, possibly driven by a strengthened AL. A shift to lower δ13Cmarl and δ18Omarl values at around 11.5 ka is attributed to an increase in the proportion of groundwater relative to meteoric water in the lake, driven by drier conditions. Fluctuations in δ13COM values parallel those in δ13Cmarl prior to ~11.0 ka, indicating that lacustrine primary producers, likely charophytes, were utilizing the same carbon species as was consumed during CaCO3 precipitation. After ~11.0 ka, these datasets diverge and δ13COM values remain very low (~ –38.0‰) for the remainder of the early Holocene, indicating methanogenesis and methanotrophy under relatively warm conditions, an increase in soil-derived dissolved inorganic carbon from the increasingly vegetated catchment area, or a combination of both. Geological methane could be an additional influence. Beginning around 9 ka, δ13COM values increase, indicating that algal photosynthesis shifted toward using CO2 as the availability of dissolved atmospheric CO2 increased. The proportion of meteoric to groundwater input continued to increase, the surrounding coniferous forest became established, and by 8 ka, CaCO3 deposition ceased. Two millennial-scale increases in sediment flux during the middle and late Holocene reflect increases in runoff driven by strengthening of the AL. This study has broader implications for understanding groundwater influence on lake ecosystems in the face of climate change. Additionally, this study provides new insights into marl lake systems and the environmental conditions under which marl was deposited during the late glacial and early Holocene.