Centennial-scale age offsets of plant wax n-alkanes in Adirondack lake sediments

摘要

Lacustrine sediments have the potential to preserve paleoenvironmental proxies such as terrestrial plant waxes at high temporal resolution. The temporal fidelity of plant wax records, however, can be up to several thousand years older than the corresponding sediment, based on compound-specific radiocarbon (C-14) analysis. Unfortunately, the factors controlling C-14 age offsets between plant waxes and sediments are not well understood. The aims of this study are to quantify and compare plant wax n-alkane C-14 age offsets among lakes within the same region, and to determine if drainage and lake basin differences (e.g., size, relief, drainage basin morphology, fluvial inputs) influence these offsets. We collected sediments from five lakes in the Adirondack Mountains, NY, USA that lack petrogenic sources of n-alkanes, and range in drainage basin characteristics. Sediment age models were constructed for each lake using a combination of sediment 210Pb, 137Cs, and macrofossil C-14 ages, and compared to C-14 ages of long-chain ( n-C-27, n-C-29, n-C-31) alkanes sampled from Holocene and Late Glacial aged sediments. The n-alkane C-14 age(initial) is a measure of the age offset in n-alkanes relative to the sediment they are deposited in. Across all lakes, the n-alkane C-14 age(initial) ranged from penecontemporaneous to 2440 C-14 yr. This oldest n-alkane C-14 age(initial) is significantly outside the distribution of all other samples; when excluded, the overall mean n-alkane C-14 age(initial) was 400 +/- 255 14C yr (n = 23). The largest n-alkane C-14 age(initial) values were observed in the last 1 cal kyr BP (565 +/- 180 C-14 yr (1r), n = 12)) compared to sediments deposited between 1 and 13 cal kyr BP (220 +/- 195 C-14 yr (1 sigma), n = 11)). These age offsets develop early after initial lake basin sedimentation and follow a similar pattern in all of the lakes despite differences in lake basin and catchment size, relief, and geomorphology. We speculate that C-14 age offsets occur due to reworking of older n-alkanes stored in drainage basin sediments and soils and this mechanism is most effective in the last similar to 1 kyr. Regardless, lakes in temperate forested metamorphic bedrock terrains have relatively small and stable n-alkane age offsets compared with lakes with extensive catchment soils or anthropogenic disturbance. By targeting lakes with minimal catchment soils, age offsets may be reduced, thereby minimizing attenuation or age distortion of climate signals in plant wax isotopic records. (C) 2021 Elsevier Ltd. All rights reserved.