Abstract
Arctic organic carbon (OC) stores are substantial and have accumulated over millennia as a function of changes in climate and terrestrial vegetation. Arctic lakes are also important components of the regional C-cycle as they are sites of OC production and CO2 emissions but also store large amounts of OC in their sediments. This sediment OC pool is a mixture derived from terrestrial and aquatic sources, and sediment cores can therefore provide a long-term record of the changing interactions between lakes and their catchments in terms of nutrient and C transfer. Sediment carbon isotope composition (δ13C), C/N ratio and organic C accumulation rates (C AR) of 14C-dated cores covering the last ∼10,000 years from six lakes close to Sisimiut (SW Greenland) are used to determine the extent to which OC dynamics reflect climate relative to lake or catchment characteristics. Sediment δ13C ranges from −19 to −32‰ across all lakes, while C/N ratios are <8 to >20 (mean = 12), values that indicate a high proportion of the organic matter is from autochthonous production but with a variable terrestrial component. Temporal trends in δ13C are variable among lakes, with neighbouring lakes showing contrasting profiles, indicative of site-specific OC processing. The response of an individual lake reflects its morphometry (which influences benthic primary production), the catchment:lake ratio, and catchment relief, lakes with steeper catchments sequester more carbon. The multi-site, landscape approach used here highlights the complex response of individual lakes to climate and catchment disturbance, but broad generalisations are possible. Regional Neoglacial cooling (from ∼5000 cal yr BP) influenced the lateral transfer of terrestrial OC to lakes, with three lakes showing clear increases in OC accumulation rate. The lakes likely switched from being autotrophic (i.e. net ecosystem production > ecosystem respiration) in the early Holocene to being heterotrophic after 5000 cal yr BP as terrestrial OC transfer increased.
Highlights
The Arctic is a major organic carbon (OC) store (~1600 Pg) (Tarnocai et al, 2009), most of it contained in frozen soils
Input of terrestrial dissolved organic matter (DOM) to lakes has a number of process implications for lake functioning/lake metabolism, supporting a microbial loop, as well as stimulating primary production, because of the nutrients (N, P) that are transferred together with the C
Four lakes are located within 10 km of each other, whereas SS49 is situated on a broad peninsula some 50 km to the south-east of the other lakes (Fig. 1), the maritime influence is still pronounced here (Anderson et al, 2001)
Summary
The Arctic is a major organic carbon (OC) store (~1600 Pg) (Tarnocai et al, 2009), most of it contained in frozen soils. N.J. Anderson et al / Quaternary Science Reviews 202 (2018) 98e108 rates as the active layer deepens and permafrost thaws (Schuur et al, 2015). Anderson et al / Quaternary Science Reviews 202 (2018) 98e108 rates as the active layer deepens and permafrost thaws (Schuur et al, 2015) The fate of this terrestrial C pool is, ambiguous, because some C may be oxidised as soils become warmer, there is considerable lateral transfer into streams, lakes and the Arctic Ocean (McClelland et al, 2016). Input of terrestrial dissolved organic matter (DOM) to lakes has a number of process implications for lake functioning/lake metabolism (the auto-heterotrophic balance), supporting a microbial loop, as well as stimulating primary production, because of the nutrients (N, P) that are transferred together with the C. Terrestrial C input can restrict aquatic production, because of its influence on in-lake light climate (Karlsson et al, 2009; Seekell et al, 2015)
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