Effect of permafrost thawing on organic carbon and trace element colloidal speciation in the thermokarst lakes of western Siberia

Abstract

To examine the mechanisms of carbon mobilization and biodegradation during permafrost thawing and to establish a link between organic carbon (OC) and other chemical and microbiological parameters in forming thermokarst (thaw) lakes, we studied the biogeochemistry of OC and trace elements (TEs) in a chronosequence of small lakes that are being formed due to permafrost thawing in the northern part of western Siberia. Twenty lakes and small ponds of various sizes and ages were sampled for dissolved and colloidal organic carbon, metals and culturable heterotrophic bacterial cell number. We observed a sequence of ecosystems from peat thawing and palsa degradation due to permafrost subsidence in small ponds to large, km-size lakes that are subject to drainage to, finally, the khasyrey (drained lake) formation. There is a systematic evolution of both total dissolved and colloidal concentration of OC and TEs in the lake water along with the chronosequence of lake development that may be directly linked to the microbial mineralization of dissolved organic matter and the liberation of the inorganic components (Fe, Al, and TEs) from the organo-mineral colloids. <br><br> In this chronosequence of lake development, we observed an apparent decrease in the relative proportion of low molecular weight <1 kDa (1 kDa ~ 1 nm) OC concentration along with a decrease in the concentration of total dissolved (<0.45 μm) OC. This decrease was accompanied by an increase in the small size organic ligands (probably autochthonous exometabolites produced by the phytoplankton) and a simultaneous decrease in the proportion of large-size organic (humic) complexes of allochthonous (soil) origin. This evolution may be due to the activity of heterotrophic bacterioplankton that use allochthonous organic matter and dissolved nutrients originating from peat lixiviation. Most insoluble TEs demonstrate a systematic decrease in concentration during filtration (5 μm, 0.45 μm) exhibiting a similar pattern among different samples. At the same time, there is an increase in the relative proportion of large size particles over the <1 kDa fraction for most insoluble elements along the chronosequence of lake evolution. TEs are likely to be bound to colloidal OC and coprecipitate with the mineral (Fe, Al) part of the colloids. Upon progressive consumption of dissolved OC by the heterotrophic bacteria, there is liberation of Fe, Al, and insoluble TEs in the water column that may be subjected to coagulation in the form of particles or large-size mineral colloids

Spatial distribution of small thermokarst lakes of Western Siberia on imagies KH-7 and GeoEye-1

“…Мерзлота, являясь хранилищем законсервированного углерода в обширных мерзлых торфяных болотах севера Евразии и Америки, при потеплении климата может стать источ-ником возникновения еще большего потепления при высвобождении парниковых газов [1]. По данным [2], наиболее активным источником эмиссии метана и углекислого газа в аркти-ческих и субарктических районах Западной Сибири являются малые термокарстовые озера с площадью менее 0,1-0,2 га.…”

“…Мерзлота, являясь хранилищем законсервированного углерода в обширных мерзлых торфяных болотах севера Евразии и Америки, при потеплении климата может стать источ-ником возникновения еще большего потепления при высвобождении парниковых газов [1]. По данным [2], наиболее активным источником эмиссии метана и углекислого газа в аркти-ческих и субарктических районах Западной Сибири являются малые термокарстовые озера с площадью менее 0,1-0,2 га.Из-за своих малых размеров такие озера, как правило, не учитываются в прогнозных оценках их вклада в общий парниковый эффект. В этих условиях получение обоснованных прогнозов вклада эмиссии метана и углекислого газа из малых озер в зоне вечной мерзлоты под действием климатических изменений является важной задачей, решение которой должно быть основано на использовании экспериментальных данных о пространственном распреде-лении термокарстовых озер по их размерам.…”

The relevance of the contemporary landscape-ecological and biogeochemical studies of the Ob floodplain

“…The Ob and most floodplain lakes exhibited the pH above 7; it is most likely linked to groundwater feeding. Because of a clear trend of DOC concentration decrease with pH increase in the pH -DOC plot, the DOC can be used as the best indicator of different water objects and the main governing factor of trace elements migration [5,73].…”

“…At different times the study of hydrology and river run-off in the region was conducted by PS Kuzin, DA Burakov, LK Malik, AN Antipov, SP Nikitin, VA Zemtsov and others [27,[69][70][71][72][73][74]. A hydro-chemical study of the Ob's middle course, first order tributaries, temporary and persisting lakes, the large flood zone determined first-order factors controlling the dissolved organic carbon and related metal sources and sinks.…”

“…Animals and birds were studied by AM Adam, SS Moskvitin and AA Maksimov [51][52][53][54][55][56][57][58]. Many scientists (NF Vyltsan, GD Dymina, AV Ogorodnikova, AI Boynov, VN Tyurin and PS Kuzin) studied the floodplain resources and hydrological peculiarities and problems [59][60][61][62][63][64][65][66][67][68][69][70][71][72][73][74].…”