Reconstructing postglacial hydrologic and environmental change in the eastern Kenai Peninsula lowlands using proxy data and mass balance modeling

Abstract

Despite extensive paleoenvironmental research on the postglacial history of the Kenai Peninsula, Alaska, uncertainties remain regarding the region's deglaciation, vegetation development, and past hydroclimate. To elucidate this complex environmental history, we present new proxy datasets from Hidden and Kelly lakes, located in the eastern Kenai lowlands at the foot of the Kenai Mountains, including sedimentological properties (magnetic susceptibility, organic matter, grain size, and biogenic silica), pollen and macrofossils, diatom assemblages, and diatom oxygen isotopes. We use a simple hydrologic and isotope mass balance model to constrain interpretations of the diatom oxygen isotope data. Results reveal that glacier ice retreated from Hidden Lake's headwaters by ca. 13.1 cal ka BP, and that groundwater was an important component of Kelly Lake's hydrologic budget in the Early Holocene. As the forest developed and the climate became wetter in the Middle to Late Holocene, Kelly Lake reached or exceeded its modern level. In the last ca. 75 years, rising temperature caused rapid changes in biogenic silica content and diatom oxygen isotope values. Our findings demonstrate the utility of mass balance modeling to constrain interpretations of paleolimnologic oxygen isotope data, and that groundwater can exert a strong influence on lake water isotopes, potentially confounding interpretations of regional climate.

Postglacial environmental change inferred from carbonate‐ and organic‐rich sediments of groundwater‐fed Kelly Lake, Kenai Peninsula, south‐central Alaska

“…5 and supplementary table 2 in Broadman et al, 2020a). The ẟ 18 O value of Kelly Lake water is similar to the average of regional ground and river waters (–15.8‰, n = 8; Broadman et al, 2020a) and slightly higher than the volume‐weighted annual ẟ 18 O of precipitation measured in Anchorage by Bailey et al (2019), as illustrated in Figure 2A of Broadman et al (2022). Kelly Lake lacks a perennial surface inflow, and therefore might be expected to be more strongly influenced by evaporative effects that would increase lake water ẟ 18 O.…”

“…In this study, we apply these multiple geochemical proxies to investigate past environmental change at groundwater‐fed Kelly Lake on Alaska's Kenai Peninsula. We focus on a core from shallow water (4 m below the lake level), which goes back to nearly 15 ka, thereby extending the sedimentary record by almost 4000 years compared with the recent study of a core from the deepest part of the lake (13 m depth), which did not recover the older sediment (Broadman et al, 2022). In the previous study, Broadman et al (2022) used sedimentary diatom oxygen isotopes and mass balance modeling to show that Kelly Lake received a substantial portion of groundwater derived from melting glacier ice from ~10 to 8 ka, and that the hydrologic budget became dominated by precipitation during the middle and late Holocene.…”

“…In the previous study, Broadman et al (2022) used sedimentary diatom oxygen isotopes and mass balance modeling to show that Kelly Lake received a substantial portion of groundwater derived from melting glacier ice from ~10 to 8 ka, and that the hydrologic budget became dominated by precipitation during the middle and late Holocene. In this study, we extend the analysis presented in Broadman et al (2022) earlier into the Holocene and late Pleistocene; we compare previously published oxygen isotopes in diatoms (Broadman et al, 2022) with those of marl (this study) from the same lake, which has seldom been done (e.g. Leng et al, 2010; Lamb et al, 2005; Dean et al, 2013), and has not been done in this region.…”

“…1B). In June 2021, pH and alkalinity of subsurface lake water were 8.0 and 135 mg L –1 , respectively, and total hardness was 62 mg L –1 (Broadman et al, 2022).…”

“…The low ẟ 18 O value probably reflects the high elevation of the confined aquifer recharge, which derives from abundant snowmelt with lower ẟ 18 O values relative to that of rainwater at lower elevations. Alternatively or additionally, the low ẟ 18 O values might reflect a contribution of glacier meltwater into the surficial aquifer, although the physical connection between the surficial aquifer and the Kelly Lake subaqueous springs remains uncertain (Broadman et al, 2022).…”