Tank et al. Dissolved C flux from circumboreal catchments
                                                                           
Study regions
Yukon River basin 
The 42 sub-watersheds surveyed in Dornblaser and Halm [2006] and 
Halm and Dornblaser [2007] cover a combined drainage area of approximately 
80% of the full Yukon basin, which drains 854,700 km^2 of terrain across 
Alaska and northwestern Canada.  The larger basin lies almost entirely 
within the boreal, with some transition to polar tundra at higher elevations 
[Fig. 1; Food and Agriculture Organization of the United Natins (FAO), 2001].  
Based on the lithological classification of Durr et al. 
[2005], the larger basin is composed approximately of 5% volcanic basalts 
and 8% carbonate rocks, with a further 35% of the basin underlain by 
sedimentary rocks with moderate (mixed sedimentary, 30-70%) to low 
(siliciclastic sedimentary and unconsolidated sedimentary rocks; < 20%) 
carbonate content (S. E. Tank et al., A land-to-ocean perspective on the magnitude, 
source and implication of DIC flux from major Arctic rivers to the Arctic Ocean, 
submitted Global Biogeochemical Cycles, 2012).  The Yukon River basin is almost 
entirely underlain by various classes of permafrost, with coverage ranging 
from continuous (23% of total basin area) across the northern Brooks Range 
and coastal subarctic plains, to discontinuous (66%) throughout much of the 
central and southern basin, to intermittent/sporadic (10%) in the eastern 
Pelly and Boreal Mountains and Southern Yukon Lakes [Brown et al., 1998].  
Significant glaciers along the southern edge of the basin [Brabets et al., 
2000] notably increase dissolved and particulate inorganic carbon flux from 
the basins in which they occur [Striegl et al., 2007]. 

West Siberia
The 98 sub-watersheds surveyed by Frey [Frey and Smith, 2005; Frey 
et al., 2007] lie within the Ob', Nadym, and Pur watersheds of western 
Siberia, and encompass roughly 10^6 km^2 of the 2.6 x 106 km^2 west Siberian 
landmass [Frey et al., 2007].  The study region terrain is largely boreal,
with some of the northern-most catchments lying in a transition to polar 
tundra, and some of the southern-most catchments lying in a transition to 
temperate forest [Fig. 1; FAO, 2001].  Permafrost ranges from continuous for 
the northern-most sub-catchments, to absent for the 55 southern-most sub-
catchments of the dataset [Brown et al., 1998; Frey et al., 2007].  The 
larger Ob' watershed is carbonate and basalt-poor, with less than 1% 
underlain by carbonate or basaltic rocks, 2% underlain by sedimentary rocks 
of moderate carbonate content, 26% underlain by sedimentary rocks of low 
carbonate content, and the remainder of the watershed underlain by other 
rock types [Durr et al., 2005; Tank et al., submitted manuscript, 2012].  
The Pur and Nadym watersheds are devoid of carbonates and basalts 
[Durr et al., 2005]. Notably, western Siberia also contains the world's most 
extensive peatlands [Sheng et al., 2004] and peatland extent is strongly 
positively correlated to stream DOC concentration in this region 
[Frey and Smith, 2005].

Mackenzie River basin
The data extracted from Millot et al. [2003] comprise sub-watersheds 
that drain 25% of the total Mackenzie basin (total area of 1.78 x 10^6 km^2) 
and lie almost entirely within the boreal, with some transition to polar 
tundra in the north-east, and temperate mountains in the south-west [Fig. 1; 
FAO, 2001].  The larger Mackenzie basin is carbonate rock rich, with 15% of 
the catchment underlain by carbonate rock and a further 48% underlain by 
sedimentary rocks of moderate or low carbonate content; however, the 
eastern portion of the basin is underlain by the Precambrian basement of 
the Canadian Shield [Durr et al., 2005].  Within the sub-catchment dataset, 
permafrost ranges from discontinuous to entirely absent, although permafrost 
is continuous in the northern reaches of the greater Mackenzie basin 
[Brown et al., 1998].  

East Siberia
The 70 sub-watersheds extracted from the data sets of Huh [Huh et 
al., 1998; Huh and Edmond, 1999] lie within the Lena, Anabar, and Olenek 
basins, and encompass ~1.2 x 10^6 km^2 of this 2.8 x 10^6 km^2 region.  The 
terrain of the larger watersheds is almost entirely boreal, with transition 
to tundra in the northernmost reaches of each of the catchments [Fig. 1; 
FAO, 2001].  Two primary lithologies are present in the region.  The 
Siberian Platform, which comprises most of the central and northern study 
area, is a craton overlain by several km of sedimentary deposits.  In 
contrast, the basement terrain of the Aldan and Anabar Shields and Trans 
Baikal Highlands are exposed in the southernmost reaches of the Lena 
watershed, and in the Anabar watershed to the north [Huh et al., 1998; Huh 
and Edmond, 1999].  Because the Siberian Platform covers much of this 
region, the larger Lena basin is carbonate rich, with 26% of the catchment 
underlain by carbonate rocks, and 36% underlain by sedimentary rocks of 
moderate to low carbonate content [Durr et al., 2005; Tank et al., 
submitted manuscript, 2012]. Permafrost extent within the extracted 
sub-watersheds ranges from sporadic to continuous [Brown et al., 1998].

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