Bröder Lisa, Andersson August, Tesi Tommaso, Semiletov Igor, Gustafsson Örjan
Department of Environmental Science and Analytical Chemistry Stockholm University Stockholm Sweden.
Bolin Centre for Climate Research Stockholm University Stockholm Sweden.
Global Biogeochem Cycles. 2019 Jan;33(1):85-99. doi: 10.1029/2018GB005967. Epub 2019 Jan 28.
Ongoing permafrost thaw in the Arctic may remobilize large amounts of old organic matter. Upon transport to the Siberian shelf seas, this material may be degraded and released to the atmosphere, exported off-shelf, or buried in the sediments. While our understanding of the fate of permafrost-derived organic matter in shelf waters is improving, poor constraints remain regarding degradation in sediments. Here we use an extensive data set of organic carbon concentrations and isotopes ( = 109) to inventory terrigenous organic carbon (terrOC) in surficial sediments of the Laptev and East Siberian Seas (LS + ESS). Of these ~2.7 Tg terrOC about 55% appear resistant to degradation on a millennial timescale. A first-order degradation rate constant of 1.5 kyr is derived by combining a previously established relationship between water depth and cross-shelf sediment-terrOC transport time with mineral-associated terrOC loadings. This yields a terrOC degradation flux of ~1.7 Gg/year from surficial sediments during cross-shelf transport, which is orders of magnitude lower than earlier estimates for degradation fluxes of dissolved and particulate terrOC in the water column of the LS + ESS. The difference is mainly due to the low degradation rate constant of sedimentary terrOC, likely caused by a combination of factors: (i) the lower availability of oxygen in the sediments compared to fully oxygenated waters, (ii) the stabilizing role of terrOC-mineral associations, and (iii) the higher proportion of material that is intrinsically recalcitrant due to its chemical/molecular structure in sediments. Sequestration of permafrost-released terrOC in shelf sediments may thereby attenuate the otherwise expected permafrost carbon-climate feedback.
北极地区正在进行的永久冻土融化可能会使大量古老的有机物质重新活动起来。这些物质被输送到西伯利亚陆架海域后，可能会被降解并释放到大气中，或者被输送到陆架之外，又或者被埋藏在沉积物中。虽然我们对陆架水域中源自永久冻土的有机物质的归宿的理解正在不断完善，但对于沉积物中的降解情况，仍然存在很大的不确定性。在这里，我们使用了一个包含有机碳浓度和同位素的广泛数据集（n = 109），来清查拉普捷夫海和东西伯利亚海（LS + ESS）表层沉积物中的陆源有机碳（terrOC）。在这约2.7 Tg的陆源有机碳中，约55%在千年时间尺度上似乎对降解具有抗性。通过将先前建立的水深与跨陆架沉积物—陆源有机碳输运时间之间的关系与与矿物相关的陆源有机碳负荷相结合，得出了1.5 kyr的一级降解速率常数。这产生了在跨陆架输运过程中表层沉积物中陆源有机碳的降解通量约为1.7 Gg/年，这比之前对LS + ESS水柱中溶解态和颗粒态陆源有机碳降解通量的估计值低几个数量级。差异主要是由于沉积态陆源有机碳的降解速率常数较低，这可能是由多种因素共同作用导致的：（i）与完全含氧的水体相比，沉积物中的氧气含量较低；（ii）陆源有机碳—矿物结合的稳定作用；（iii）由于沉积物中物质的化学/分子结构，其内在难降解的比例较高。因此，陆架沉积物中封存源自永久冻土的陆源有机碳可能会减弱原本预期的永久冻土碳—气候反馈。
