A Reconstruction of Southcentral Alaska Late Holocene Hydroclimate from Peatland Cellulose Oxygen Isotopes
Abstract
The use of peat cellulose oxygen isotopes as a proxy for hydroclimate has commonly been applied to a single species in Sphagnum-dominated bogs; however, recent advances in the application of this proxy to wetlands with diverse species, such as fens, broadens the scope of their utility. Here we present a late-Holocene record of species-specific cellulose oxygen isotopes (δ18Ocellulose) from a 7-m peat core collected from a fen located on the Kenai Peninsula in Southcentral Alaska. Plant macrofossils were categorized (herbaceous, bryophytic, ligneous) and tallied (e.g., seeds, leaves) for the size fraction >250 µm, and select specimens were picked for 14C dating to generate a chronology in conjunction with 210Pb. Oxygen isotopic analysis was performed on alpha cellulose extracted from moss (Sphagnum and brown mosses) and sedge (Carex spp.) macrofossils that were separated prior to cellulose extraction to account for unique fractionation factors resulting from differences in plant physiology. Previous analyses revealed that the moss δ18Ocellulose values were ~2 ‰ lighter on average than sedge at the same site. Fen surface water δ18O values fall on the global meteoric water line, indicating they accurately reflect precipitation. The average isotopic difference between modern plant cellulose and peatland water (Δδ18Ocellulose-water) is 33.9 ± 1.2 ‰. Lacustrine sediments with aquatic macrofossils (Chara oospores, Daphnia ephippia, and chironomids) comprise the lower half of the core, indicating a shallow pond persisted from ~10 ka to ~6 ka, when the pond terrestrialized to a peatland. Between ~6 to 4 ka, sedge macrofossils dominated, followed by a period when the system fluctuated between sedge and Sphagnum, before transitioning back to sedge by ~2 ka. An abrupt transition to Sphagnum peat occurred in the last several decades and persists to present. The δ18Ocellulose values during the past 3 kyr range from ~17 ‰ to ~21.5 ‰, with higher values at the core top. The inferred bog water values between -12.5 ‰ and -17 ‰ are consistent with modern precipitation values and indicate that this record can be placed into the context of regional synoptic-scale hydroclimate changes throughout the late-Holocene, including variability in the strength and position of the Aleutian Low.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMPP0030003N
- Keywords:
-
- 3305 Climate change and variability;
- ATMOSPHERIC PROCESSES;
- 3344 Paleoclimatology;
- ATMOSPHERIC PROCESSES;
- 1041 Stable isotope geochemistry;
- GEOCHEMISTRY;
- 1655 Water cycles;
- GLOBAL CHANGE