Interannual variability in the summer dissolved organic matter inventory of the North Sea: implications for the continental shelf pump
Interannual variability in the summer dissolved organic matter inventory of the North Sea: implications for the continental shelf pump
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DOI：
10.5194/bg-16-1073-2019
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发表时间：
2019-03-18
期刊：
影响因子：
4.9
通讯作者：
Johnson, Martin
中科院分区：
文献类型：
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作者：
Chaichana, Saisiri;Jickells, Tim;Johnson, Martin
我们给出了北海两个夏季(2011年8月和2012年8月)溶解有机物(DOM)的分布和C：N化学计量比，并提供了介于其间的冬季(2012年1月)的支持数据。这些数据表明，局部变异性叠加在DOM随距离陆地增加而减少的一般模式上，这表明DOM浓度在大的空间尺度上是通过开放的北大西洋与河流来源或由河流营养物质排放驱动的近岸沿海水域的高DOM生产力混合来控制的。考虑到北海水的大尺寸和长停留时间，我们发现浓度通常是由两个端元之间简单的保守混合而改变的。我们观察到两个夏季溶解有机碳(DOC)和溶解有机氮(DON)浓度以及陆地-海洋梯度的差异，导致两年间DOC清单估计存在10-20Tg的差异，这与北海系统对大气CO2的年吸收具有相同的量级，因此对北海的碳收支具有重要意义。这一差异与额外的陆地负荷不一致，更有可能是由于整个北海的混合和原地生产和损失过程的平衡。差异在北海北部季节性分层的底层尤为明显，2011年的DOC和C/N比高于2012年。利用其他数据，我们考虑了DOM浓度和C/N比的这些差异在多大程度上可能是由于这两年生物地球化学或物理循环的变化，或两者的结合。我们掌握的证据与2011/12年冬季的一次冲刷事件相一致，该事件交换了富含DOM的高C V N陆架水域，这些水域可能积累了超过1年的时间，与DOC较低、DON略高的北大西洋深海水域交换。我们讨论了这些观察结果对陆架海碳泵和富碳有机质从陆架输出的影响，并假设温带陆架系统的间歇性冲洗可能是通过积累和随后输出富碳DOM来维持大陆架泵的关键机制。
We present the distribution and C : N stoichiometry of dissolved organic matter (DOM) in the North Sea in two summers (August 2011 and August 2012), with supporting data from the intervening winter (January 2012). These data demonstrate local variability superimposed on a general pattern of decreasing DOM with increasing distance from land, suggesting concentrations of DOM are controlled on large spatial scales by mixing between the open North Atlantic and either riverine sources or high DOM productivity in nearshore coastal waters driven by riverine nutrient discharge. Given the large size and long residence time of water in the North Sea, we find concentrations are commonly modified from simple conservative mixing between two endmembers. We observe differences in dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) concentrations and land-ocean gradients between the two summers, leading to an estimated 10-20 Tg difference in the DOC inventory between the two years, which is of the same order of magnitude as the annual uptake of atmospheric CO2 by the North Sea system, and thus significant for the carbon budget of the North Sea. This difference is not consistent with additional terrestrial loading and is more likely to be due to balancing of mixing and in situ production and loss processes across the North Sea. Differences were particularly pronounced in the bottom layer of the seasonally stratifying northern North Sea, with higher DOC and C : N ratio in 2011 than in 2012. Using other data, we consider the extent to which these differences in the concentrations and C : N ratio of DOM could be due to changes in the biogeochemistry or physical circulation in the two years, or a combination of both. The evidence we have is consistent with a flushing event in winter 2011/12 exchanging DOM-rich, high C V N shelf waters, which may have accumulated over more than 1 year, with deep North Atlantic waters with lower DOC and marginally higher DON. We discuss the implications of these observations for the shelf sea carbon pump and the export of carbon-rich organic matter off the shelf and hypothesise that intermittent flushing of temperate shelf systems may be a key mechanism in the maintenance of the continental shelf pump, via the accumulation and subsequent export of carbon-rich DOM.