Greenhouse gas production in degrading ice-rich permafrost deposits in northeastern Siberia

Josefine Walz,Ronja Tigges,Thomas Opel,Lutz Schirrmeister,Eva-Maria Pfeiffer,Christian Knoblauch

doi:10.5194/bg-15-5423-2018

Abstract

Abstract. Permafrost deposits have been a sink for atmospheric carbon for millennia. Thaw-erosional processes, however, can lead to rapid degradation of ice-rich permafrost and the release of substantial amounts of organic carbon (OC). The amount of the OC stored in these deposits and their potential to be microbially decomposed to the greenhouse gases carbon dioxide (CO2) and methane (CH4) depends on climatic and environmental conditions during deposition and the decomposition history before incorporation into the permafrost. Here, we examine potential greenhouse gas production as a result of degrading ice-rich permafrost deposits from three locations in the northeastern Siberian Laptev Sea region. The deposits span a period of about 55 kyr from the last glacial period and Holocene interglacial. Samples from all three locations were incubated under aerobic and anaerobic conditions for 134 days at 4 ∘C. Greenhouse gas production was generally higher in deposits from glacial periods, where 0.2 %–6.1 % of the initially available OC was decomposed to CO2. In contrast, only 0.1 %–4.0 % of initial OC was decomposed in permafrost deposits from the Holocene and the late glacial transition. Within the deposits from the Kargin interstadial period (Marine Isotope Stage 3), local depositional environments, especially soil moisture, also affected the preservation of OC. Sediments deposited under wet conditions contained more labile OC and thus produced more greenhouse gases than sediments deposited under drier conditions. To assess the greenhouse gas production potentials over longer periods, deposits from two locations were incubated for a total of 785 days. However, more than 50 % of total CO2 production over 785 days occurred within the first 134 days under aerobic conditions, while 80 % were produced over the same period under anaerobic conditions, which emphasizes the nonlinearity of the OC decomposition processes. Methanogenesis was generally observed in active layer samples but only sporadically in permafrost samples and was several orders of magnitude smaller than CO2 production.

Highlights

Permafrost, i.e., ground that is at or below 0 ◦C for at least two consecutive years, may preserve organic matter (OM) for millennia (Ping et al, 2015)
Total carbon (TC) and total nitrogen (TN) contents were measured with an element analyzer (VarioMAX cube, Elementar Analysensysteme GmbH, Hanau, Germany), while total OC (TOC) contents were measured with a liquiTOC II coupled to a solids module (Elementar Analysensysteme GmbH, Hanau, Germany)
Deposits from the uppermost section between 0.5 and 2.4 m b.s. were classified as Holocene deposits from the Marine Isotope Stages (MIS) 1 and deposits from the late glacial to early Holocene transition, confirmed by radiocarbon ages of 7.5 and 13.2 ka before present (BP) for samples at 1.3 and 2.4 m b.s., respectively

Summary

Introduction

Permafrost, i.e., ground that is at or below 0 ◦C for at least two consecutive years (van Everdingen, 2005), may preserve organic matter (OM) for millennia (Ping et al, 2015). The current organic carbon (OC) pool of soils, refrozen thermokarst, and Holocene cover deposits in the top 3 m as well as sediments and deltaic deposits below 3 m in permafrost landscapes is estimated to be about 1300 Pg, of which about 800 Pg are perennially frozen (Hugelius et al, 2014). Warming-induced environmental changes and permafrost degradation could lead to rapid thawing of substantial amounts of currently frozen OM, microbial decomposition of the thawed OM, and rising greenhouse gas fluxes to the atmosphere (Natali et al, 2015; Schuur et al, 2015). The effects of elevated atmospheric greenhouse gas concentrations and temperatures on processes in soils and sediments are expected to be most pronounced in nearsurface layers (Schneider von Deimling et al, 2012). Walz et al.: Greenhouse gas production in degrading ice-rich permafrost deposits ever, thermo-erosion of ice-rich permafrost, i.e., permafrost with more than 20 vol % ice (Brown et al, 1998), enables deep thawing of several tens of meters (Schneider von Deimling et al, 2015)

Methods

Results

Discussion

Conclusion