Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO 2 is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15 1C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO 2 with radiocarbon ( 14 C) ages ranging between $ 21 and 24 ka BP was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q 10 value of 1.9 AE 0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of D 14 CO 2 values in respiration CO 2 and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO 2 in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5 1C, about 1 Pg C will be initially released from labile C pools, followed by respiration of $ 40 Pg C to the atmosphere over a period of four decades.

[1] The magnitude of future CO 2 -induced climate warming is difficult to predict because of uncertainties in the role of ecosystems and oceans as CO 2 sources and sinks. Siberia has extensive areas (1 Â 10 6 km 2 ) of deep (up to 90 m) deposits of organic-rich frozen loess (wind-blown silt) that accumulated during the Pleistocene but have not been considered in most global carbon (C) inventories. Similar deposits occur less extensively in Alaska. Recent warming at high latitudes causes this permafrost (permanently frozen ground) to thaw, raising questions about the fate of C in thawing permafrost. Here we show that Siberian loess permafrost contains a large organic C pool ($450 GT-more than half the quantity in the current atmosphere) that decomposes quickly when thawed, and could act as a positive feedback to climate warming.

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Apparent marine radiocarbon ages are reported for the northern Indian Ocean region for the pre-nuclear period, based on measurements made in seven mollusk shells collected between 1930 and 1954. The conventional 14C ages of these shells range from 693 ± 44 to 434 ± 51 BP in the Arabian Sea and 511 ± 34 to 408 ± 51 BP in the Bay of Bengal. These ages correspond to mean ΔR correction values of 163 ± 30 yr for the northern Arabian Sea, 11 ± 35 yr for the eastern Bay of Bengal (Andaman Sea) and 32 ± 20 yr for the southern Bay of Bengal. Contrasting reservoir ages for these two basins are most likely due to differences in their thermocline ventilation rates.

The amount of soil organic carbon (SOC) in stable, slow-turnover pools is likely to change in response to climate warming because processes mediating soil C balance (net primary production and decomposition) vary with environmental conditions. This is important to consider in boreal forests, which constitute one of the world's largest stocks of SOC. We investigated changes in soil C stabilization along four replicate gradients of black spruce productivity and soil temperature in interior Alaska to develop empirical relationships between SOC and stand and physiographic features. Total SOC harbored in mineral soil horizons decreased by 4.4 g C·m–2 for every degree-day increase in heat sum within the organic soil across all sites. Furthermore, the proportion of relatively labile light-fraction (density <1.6 g·cm–3) soil organic matter decreased significantly with increased stand productivity and soil temperature. Mean residence times of SOC (as determined by Δ14C) in dense-fraction (>1.6 g·cm–3) mineral soil ranged from 282 to 672 years. The oldest SOC occurred in the coolest sites, which also harbored the most C and had the lowest rates of stand production. These results suggest that temperature sensitivities of organic matter within discrete soil pools, and not just total soil C stocks, need to be examined to project the effects of changing climate and primary production on soil C balance.