Challenging the hypothesis of an Arctic Ocean lake during recent glacial episodes

Abstract

The Arctic Ocean is one of the last frontiers on Earth with many unknowns about its geological and climate history and considerable speculation on its role in the Earth's climate and ocean system. It has been proposed recently that it was occupied by a freshwater body of more than 9.5 × 10 6 km 3 underneath a thick ice mass during part of glacial isotopic stages 6 and 4. We argue that such a dramatic scenario, implying replacement of marine waters by freshwater throughout the entire Arctic Ocean and Nordic Seas, is physically implausible. The very low 230 Th excesses ( 230 Th xs ) observed in sediments from these intervals were used as evidence for the presence of a U-depleted overlying freshwater column. We show here that they may simply result from short, sporadic sedimentary pulses, below multiyear sea ice or ice shelves, linked to deglacial ice streaming and surging events interrupting long-duration sedimentary gaps. Due to this sporadic sedimentation regime, interpolating time from 230 Th xs data or between benchmark ages in sedimentary sequences would simply be erroneous.

Challenging Radiocarbon Chronostratigraphies in Central Arctic Ocean Sediment

“…Such features were also previously inferred from lithostratigraphic studies (e.g., Nørgaard‐Perdersen et al., 2003). Unfortunately, the precise duration of these sedimentary gaps, both during the LGM and other glacial intervals (Geibert et al., 2021; Hillaire‐Marcel et al., 2022), especially at sites close to or below thick ice‐shelves (∼780–1,250 m; Schlager et al., 2022), may vary from site to site.…”

“…However, deeper downcore, the apparent 14 C ages of foraminifer assemblages ranging 10–25 kyr are unlikely recording high productivity events during the Last Glacial Maximum (LGM) and early deglaciation, as the Arctic Ocean was mostly glaciated (Xiao et al., 2015) and its shelves largely emerged due to the low sea‐level then prevailing (Lambeck et al., 2014). Moreover, other tracers rather illustrate sedimentary hiatuses in many central Arctic Ocean sequences during the LGM associated with a perennial ice cover or the development of ice shelves (e.g., Hillaire‐Marcel et al., 2022; Not & Hillaire‐Marcel, 2010). Such features were also previously inferred from lithostratigraphic studies (e.g., Nørgaard‐Perdersen et al., 2003).…”

“…Such features were also previously inferred from lithostratigraphic studies (e.g., Nørgaard-Perdersen et al, 2003). Unfortunately, the precise duration of these sedimentary gaps, both during the LGM and other glacial intervals (Geibert et al, 2021;Hillaire-Marcel et al, 2022), especially at sites close to or below thick ice-shelves (∼780-1,250 m; Schlager et al, 2022), may vary from site to site. As documented in Purcell et al (2022), the central Arctic Ocean mostly recorded millennial-scale sedimentary pulses: either during glacial advances and retreats (lake drainage events, possibly ice surging events during stadials), or during "warm" time windows of interstadial and interglacial stages, through sea-ice rafting deposition, under maximum sea-level, maximum summer season insolation conditions, leading to the development of sea-ice factories over shelves (see Hillaire-Marcel et al, 2021).…”

“…The principal debated issue about the 14 C approach has been the assessment of the apparent age of the marine dissolved inorganic carbon at present and through time, thus the Δ R value to use for converting radiocarbon ages into calibrated ages (e.g., Hanslik et al., 2010; Pearce et al., 2017). The highly variable ventilation rate of the Arctic Ocean waters now and back in time (e.g., Hillaire‐Marcel et al., 2022; Schlosser et al., 1994), presents difficulties for estimating reliable Δ R values, not to mention the issue of the calibration curve to use in the Arctic Ocean for the conversion of conventional 14 C‐ages into calibrated ages (Heaton et al., 2020). Another critical problem for low 14 C activity samples should also be mentioned.…”

A 600-kyr reconstruction of deep Arctic seawater δ18O from benthic foraminiferal δ18O and ostracode Mg/Ca paleothermometry

“…The "Fresh Arctic" hypothesis (Geibert et al, 2021) has generated significant discussion about both the interpretation of excess 230 Th and cosmogenic 10 Be in Arctic sediments (Hillaire-Marcel, 2022;Geibert et al, 2022a) and the evidence supporting or refuting this hypothesis from outside the Arctic Ocean (Spielhagen et al, 2022;Geibert et al, 2022b).…”