1 Spatial variation in concentration and sources of organic carbon in the Lena 2 River, Siberia 3 4 Liselott Kutscher1,2*, Carl-Magnus Mörth1, Don Porcelli3, Catherine Hirst1,2, Trofim C. 5 Maximov4,5, Roman E. Petrov 4,5, Per S. Andersson2 6 1Department of Geological Sciences, Stockholm University, SE-10691 Stockholm, Sweden. 7 2Department of Geosciences, Swedish Museum of Natural History, SE-10405 Stockholm, Sweden 8 3Department of Earth Sciences, University of Oxford, Oxford, UK 9 4Institute for Biological Problems of Cryolithozone of Siberian Branch of Russian Academy of Science 10 5Institute for Natural Sciences of North-Eastern Federal University, Yakutsk, Russia 11 *Corresponding author: Liselott Kutscher, Deperatment of Geological Sciences, Stockholm University, 12 SE-106 91 Stockholm, Sweden ([email protected]) 13 14 Key points: 15 Spatial differences in OC concentrations and export due to catchment topography and 16 regional hydroclimate in Lena River basin 17 Sources of DOM were fresh plant material and SOM with seasonal patterns reflecting 18 differences in water flow pathways 19 Sources of POM were aquatic primary production and SOM, and were related to 20 topography 1 21 Abstract 22 Global warming in permafrost areas is expected to change fluxes of riverine organic carbon (OC) 23 to the Arctic Ocean. Here OC concentrations, stable carbon isotope signatures (δ13C) and carbon- 24 nitrogen ratios (C/N) are presented from 22 sampling stations in the Lena River and 40 of its 25 tributaries. Sampling was conducted during two expeditions: the first in July 2012 in the south 26 and southeastern region and the second in June 2013 in the northern region of the Lena basin. 27 The data showed significant spatial differences in concentrations and major sources of OC. Mean 28 sub-catchment slopes were correlated with OC concentrations, implying that mountainous areas 29 in general had lower concentrations than lowland areas. δ13C and C/N data from tributaries 30 originating in mountainous areas indicated that both dissolved and particulate OC (DOC and 31 POC) were mainly derived from soil organic matter (SOM). In contrast, tributaries originating in 32 lowland areas had larger contributions from fresh vegetation to DOC, while aquatically produced 33 OC was the major source of POC. We suggest that these differences in dominant sources 34 indicated differences in dominant flow pathways. Tributaries with larger influence of fresh 35 vegetation probably had surficial flow pathways, while tributaries with more SOM influence had 36 deeper water flow pathways. Thus, the future export of OC to the Arctic Ocean will likely be 37 controlled by changes in spatial patterns in hydroclimatology and the depth of the active layers 38 influencing the dominant water flow pathways in Arctic river basins. 39 2 40 1 Introduction 41 Permafrost soils are considered to be among the largest terrestrial reservoirs of carbon [Hugelius 42 et al., 2014]. During the last century, large areas underlain by permafrost in the Northern 43 Hemisphere have experienced environmental changes due to climate warming. Permafrost 44 temperatures have risen [Romanovsky et al., 2010], the active layer has become deeper during 45 summer months [Frey and McClelland, 2009; Zhang et al., 2005], and freshwater runoff from 46 rivers to the Arctic Ocean has increased [Costard et al., 2007; Peterson et al., 2002; Rachold et 47 al., 1996; Shiklomanov and Lammers, 2009]. Some of the largest Arctic Rivers have also had 48 increased sediment export [Rachold et al., 1996]. According to climate models, climate warming 49 in permafrost regions will continue during the next century and the Arctic is predicted to 50 experience one of the largest temperature increases in the world [Collins and Krinner, 2013]. It is 51 expected that permafrost thawing will change fluxes of natural organic matter (NOM) from the 52 terrestrial environment to freshwater and marine systems due to changes in discharge, transport 53 pathways, residence times in soils and water, plant species composition, and increased erosion 54 [Costard et al., 2007; Frey and Smith, 2005; Frey and McClelland, 2009; Guo et al., 2007; Neff 55 and Hooper, 2002; Striegl et al., 2005]. However, there is no consensus regarding the direction of 56 these changes [Frey and McClelland, 2009]. Whatever the direction of changes, it will ultimately 57 have implications for the carbon cycling and ecosystem functioning in Arctic freshwater systems 58 as well as in the Arctic Ocean [Frey and McClelland, 2009; Keller et al., 2010], since NOM 59 plays an important role in the aquatic food web as nutrient and energy source as well as transport 60 media of essential metals and dissolved nutrients [Jansson et al., 2007; Tipping, 2002]. 3 61 Riverine NOM is a terrestrially (allochtonous) or aquatically (autochthonous) produced 62 heterogeneous mixture of organic compounds with a generally complex structure containing 63 carbon as its major constituent [Finlay and Kendall, 2008; Lam et al., 2007]. Due to the high 64 content of carbon, NOM is often equated with organic carbon (OC) and can be divided into two 65 size fractions: dissolved organic carbon (DOC) and particulate organic carbon (POC). DOC in 66 stream ecosystems is generally derived from terrestrial sources, e.g. litterfall and soil leachate, 67 while POC primarily is derived from eroded soil and sedimentary rocks as well as from 68 autochthonous sources, e.g. macrophytes, algae and heterotrophic bacteria [Galy et al., 2015; 69 Meybeck, 1993]. However, the division between DOC and POC is arbitrary, since DOC can 70 flocculate and form POC; also, both DOC and POC can be mineralized along river networks [del 71 Giorgio and Pace, 2008; Drake et al., 2015; Kerner et al., 2003; Meybeck, 1993]. 72 Several different factors controlling OC concentrations in rivers have been suggested. Mean OC 73 concentrations have been correlated to spatial variables such as topography and landscape 74 characteristics, e.g. amount of upstream wetland cover, lake cover, and average channel slope 75 [Eckhardt and Moore, 1990; Mulholland, 1997; Mulholland, 2003]. Factors controlling temporal 76 DOC dynamics have been shown to include discharge [Eckhardt and Moore, 1990; Winterdahl et 77 al., 2014], soil temperature, local climate (air temperature and precipitation) and antecedent flow 78 [Winterdahl et al., 2011]. OC concentrations in rivers often peak in connection with high 79 discharge events; this is commonly explained to be an effect of shifting flow pathways from 80 deeper soil horizons to shallower organic-rich soil layers [Mulholland, 2003]. For example, in 81 rivers in permafrost areas and in many boreal rivers, OC concentrations often increase during 82 snowmelt and decrease with decreasing flow so that the lowest concentrations commonly occur 83 during base flow [Holmes et al., 2012; Le Fouest et al., 2013; Raymond et al., 2007; Winterdahl 4 84 et al., 2014]. A warming climate is expected to shift dominating flow paths to deeper soil layers 85 [Frey and McClelland, 2009]. However, the subsurface hydrology in permafrost is not fully 86 understood [Watson et al., 2013; Woo et al., 2008] and limits our understanding of element 87 transport from areas underlain by permafrost. 88 For a better understanding of the influence of permafrost thawing on export and fluxes of riverine 89 carbon it is necessary to unravel the main contributing areas and sources of OC in permafrost 90 regions. Among the world’s deepest layers of permafrost are found in the Lena River drainage 91 basin, in central Siberia, where the major part of the basin is underlain by continuous permafrost 92 [Chevychelov and Bosikov, 2010]. It is the ninth largest drainage basin in the world and has the 93 largest transport of OC to the Arctic Ocean [Holmes et al., 2012; Le Fouest et al., 2013]. Holmes 94 et al. [2012] estimates that the annual export from the six largest Arctic rivers draining directly 95 into the Arctic Ocean is 25 Tg DOC year-1, of which the Lena River contributes approximately 96 5.8 Tg DOC year-1 and 0.8 Tg POC year-1 [Holmes et al., 2012; Le Fouest et al., 2013; 97 McClelland et al., 2016; Raymond et al., 2007]. The export of OC from the Lena varies 98 seasonally and OC concentrations peak during snowmelt in the beginning of June and then 99 decrease to reach annual lows during winter baseflow [Amon et al., 2012; Holmes et al., 2012; Le 100 Fouest et al., 2013; McClelland et al., 2016; Raymond et al., 2007]. Previous studies in the Lena 101 River indicate that the dominant sources of DOC in the main channel are relatively young soil 102 and plant material [Amon et al., 2012; Lara et al., 1998; Lobbes et al., 2000]. POC on the other 103 hand is suggested to be older degraded soil organic matter (SOM) [Lobbes et al., 2000; 104 McClelland et al., 2016]. The amount of aquatically produced OC in the Lena River is believed 105 to be low [Lobbes et al., 2000; McClelland et al., 2016; Sorokin and Sorokin, 1996]. The 106 contribution of old, highly biologically available OC from thawing permafrost to fluvial networks 5 107 increase aquatic microbial metabolism and CO2 emissions within Arctic drainage basins [Mann et 108 al., 2015]. This permafrost-derived OC is also suggested to result in loss of ancient OC in river 109 networks and a selective export of younger OC to the oceans [Mann et al., 2015; Vonk et al., 110 2010]. 111 Stable carbon isotopes values (δ13C) and carbon/nitrogen atomic ratios (C/N) can be used to 112 identify sources of freshwater OC, since these could be indicative of different sources [Cleveland 113 and Liptzin, 2007; Elser et al., 2000; Finlay and Kendall, 2008; McGroddy et al., 2004; Sterner 114 and Elser, 2002].