Quantifying marine reservoir effect variability along the Northwest Coast of North America

Abstract

Recognition of marine reservoir effect (MRE) spatial and temporal variability must be accounted for in any radiocarbon-based paleoclimate, geomorphological, or archaeological reconstruction in a coastal setting. ΔR values from 37 shell-wood pairs across southern Southeast Alaska provide a robust local evaluation of the MRE, reporting a local Early Holocene weighted ΔR average of 265 ± 205, with a significantly higher ΔR average of 410 ± 60 for samples near limestone karst. Integration with our synthesis of extant MRE calibrations for the Northwest Coast of North America suggests that despite local variability, regional ΔR averages echo proxies for coastal upwelling: regional weighted averages were at their highest in the Bølling-Allerød interstade (575 ± 165) and their lowest in the Younger Dryas stade (−55 ± 110). Weighted ΔR averages across the Northwest Coast rose to a Holocene high during the Early Holocene warm period (245 ± 200) before settling into a stable Holocene average ΔR of 145 ± 165, which persisted until the late Holocene. Our quantification of local and regional shifts in the MRE shines a light on present methodological issues involved in MRE corrections in mixed-feeder, diet-based calibrations of archaeological and paleontological specimens.

Late Pleistocene and early Holocene sea-level history and glacial retreat interpreted from shell-bearing marine deposits of southeastern Alaska, USA

“…Samples were prepared for accelerator mass spectrometry measurements at Beta Analytic, Miami, Florida, USA, or the U.S. Geological Survey 14 C Laboratory, Reston, Virginia, USA, and 14 C measurements were completed at Beta Analytic or Lawrence Livermore National Laboratory (Livermore, California, USA). The ages were calibrated in OxCal version 4.3 (Bronk Ramsey, 2009) using the IntCal20 (Reimer et al, 2020) and Marine20 (Heaton et al, 2020) calibration curves and locally derived marine reservoir corrections (Schmuck et al, 2021;see below). Calibrated ages are reported as the upper and lower limits of the entire calibrated age range at 2σ uncertainty.…”

“…Calibrated ages are reported as the upper and lower limits of the entire calibrated age range at 2σ uncertainty. Schmuck et al (2021) aggregated shell-wood sample pairs from publications across the northwestern North American Pacific Coast to identify variation in the marine reservoir effect between samples of similar ages, with significant regional shifts in the reservoir effect through time. One of their most significant findings was the identification of a dramatic decrease in the reservoir age [R(t)] from the Bølling-Allerød interstade [R(t) = 1100 ± 170 yr, ΔR = 575 ± 165] to roughly the global average in the Younger Dryas stade [R(t) = ± 200 yr, ΔR = −55 ± 110].…”

“…Six distinct terraces can be seen below the erosion of the glacial and fluvial features. Carlson and Baichtal (2015) attempted to define the timing and extent of these terraces to the location of early Holocene archaeological sites using the occurrences of shell-bearing strata; >30 sites have been found using this methodology (Carlson, 2017;Schmuck, 2021). However, the oldest shell deposits predate the oldest known cultural deposits by ~40-700 yr. We interpret the age of these oldest deposits as a minimum-limiting age for the transgression associated with the collapse of the forebulge.…”

“…yr B.P. (Schmuck et al, 2021). However, the establishment of the willow-sedge and pine parklands on land (Ager, 2019) was tempered by intense volcanism; the Mount Edgecumbe volcanic field alone was responsible for 22 individual eruptions in central southeastern Alaska, 19 within a peak interval between 14,600 and 13,100 cal.…”

“…Coupled with the sampling of marine basins, targeted sampling of Younger-Dryas and Bølling-Allerød marine deposits for shell-wood pairs is necessary for further refining the marine-reservoir-effect calibrations used to date shellfish, our proxies for sea-level change, in these critical periods. At present, the calibration curve used here is a regional synthesis of pairs from across the Northwest Coast (Schmuck et al, 2021). Further sample pairs from southeastern Alaska would provide a means to account for regional variation driven by the "hard water effect" of karst groundwaters.…”