Stable isotope signatures of Holocene syngenetic permafrost trace seabird presence in the Thule District (NW Greenland)

Abstract

Abstract. Holocene permafrost from ice wedge polygons in the vicinity of large seabird breeding colonies in the Thule District, NW Greenland, was drilled to explore the relation between permafrost aggradation and seabird presence. The latter is reliant on the presence of the North Water (NOW) polynya in the northern Baffin Bay. The onset of peat accumulation associated with the arrival of little auks (Alle alle) in a breeding colony at Annikitisoq north of Cape York is radiocarbon-dated to 4400 cal yr BP. A thick-billed murre (Uria lomvia) colony on Appat (Saunders Ø) in the mouth of the Wolstenholme Fjord started 5650 cal yr BP. Both species provide marine-derived nutrients (MDNs) that fertilise vegetation and promote peat growth. The geochemical signature of organic matter left by the birds is traceable in the frozen Holocene peat. The peat accumulation rates at both sites are highest after the onset and decrease over time and were about two-times faster at the little auk site than at the thick-billed murre site. High accumulation rates induce shorter periods of organic matter (OM) decomposition before it enters the perennially frozen state. This is seen in comparably high C / N and less depleted δ13C, pointing to a lower degree of OM decomposition at the little auk site, while the opposite pattern can be discerned at the thick-billed murre site. Peat accumulation rates correspond to δ15N trends, where decreasing accumulation led to increasing depletion in δ15N as seen in the little-auk related data. In contrast, the more decomposed OM of the thick-billed murre site shows almost stable δ15N. Late Holocene wedge ice fed by cold season precipitation was studied at the little auk site and provides the first such stable-water isotopic records from Greenland with mean δ18O of −18.0 ± 0.8 ‰, mean δD of −136.2 ± 5.7 ‰, mean d excess of 7.7 ± 0.7 ‰, and a δ18O-δD slope of 7.27, which is close to those of the modern Thule Meteoric Water Line. The syngenetic ice wedge polygon development is mirrored in testacean records of the little auk site and delineates polygon low-centre, dry-out and polygon-high-centre stages. The syngenetic permafrost formation directly depending on peat growth (controlled by bird activity) falls within the period of Neoglacial cooling and the establishment of the NOW polynya, thus indirectly follows the Holocene climate trends.

Vulnerability of the North Water ecosystem to climate change

“…Here, the arrival of little auks, and with them large quantities of MDN, transformed nutrient availability, prompting a period of exceptionally high terrestrial and aquatic biological productivity. This is demonstrated by the fact that 2 m of peat accumulated in the catchment adjacent to the lake in just 1000 years, following little auk colonisation 65 , and the extremely high LOI values recorded in the lake (75% instead of <10% as expected for a High Arctic lake). Therefore, in addition to the large input of marine-derived cholesterol, there was also the input of cholesterol of terrestrial and freshwater origin, which has a higher proportion of β-sitosterol.…”

Responses from the authors to comments of anonymous Referee #1 - bg-2019-71

“…The radiocarbon dating and age-depth modeling could be describe more detail. I know, that it was a topic of paper by Davidson et al 2018, but still I have some doubts: Some samples for dating were collected from significantly thick core segments (even 10 cm long for topmost part of SI 1). Do author consider depth uncertinity in age-depth models?…”

“…Therefore depth uncertainty in the age-depth models is of minor relevance for the present core records. The complete presentation of the agedepth models from Davidson et al (2018) can be found as Supplementary Material here: https://static-content.springer.com/esm/art%3A10.1007%2Fs13280-018-1031-1/MediaObjects/13280_2018_1031_MOESM1_ESM.pdf Please give some details on geological settings. According to my knowlage, at least in Saunders Is there is a lot of carbonate rocks.…”

“…How did the authors did overcome this issue? REPLY: Indeed, according to Kurtz (1950) Saunders Island is mainly composed of siltstones, shales and dolomite of the Narssârssuk formation, which is described by Dawes (2006) to be composed of pale carbonates and siliciclastic redbeds of the Narssârssuk Group. Although carbonatic rock is present in the study area we leave out of account its potential impact on the selected peat mosses, which were radiocarbondated in the present study, as described in detail by Davidson et al (2018).…”