冰期旋回中热带西太平洋碳源汇过程、机理与效应

Carbon sources and sinks in the western tropical Pacific during glacial cycles:Processes,mechanisms and effects

冰期旋回是第四纪地球气候变化最显著的特征,热带西太平洋通过水文循环控制全球气候演化,这是“气候演变低纬驱动”理论的核心内容.由于现代热带西太平洋CO_(2)总体上处于海-气平衡,因此基于“将今论古”的思想,长期以来学术界认为热带西太平洋相对于南大洋等高纬海区在大气CO_(2)分压(pCO_(2))冰期旋回中的作用并没那么重要,从而低估了热带西太平洋通过碳源汇演替调控全球气候的作用.在综合分析上新世以来冰期旋回中热带西太平洋碳源汇过程与机制的基础上,总结出了其气候效应模式,发现冰期时海平面降低导致更多的河流营养物进入海洋,致使近海表层有机碳生产和海底有机碳埋藏加强;同时,远洋为了“补偿”陆架珊瑚礁等钙质生物碳酸盐生产的减少,引起其深部CO32和碱度增加,致使远洋深部碳库增强;另外,开放洋区大型硅藻勃发等有机碳生产演化事件在大洋深部形成呼吸碳库,进一步“放大”了上述增强的深部碳库.这些碳循环过程表明冰期热带西太平洋扮演碳汇角色.间冰期时上述碳循环过程发生的方向或强度与冰期相反,但目前还没有证据证明间冰期热带西太平洋是否扮演碳源角色;尽管冰消期,其确实能向大气释放CO_(2).此外,叠加在冰期旋回之上的长尺度记录以及跨越冰期旋回的重大事件记录均显示,热带西太平洋可以通过全球的遥相关或洋际交换,在深部“被动”形成碳库.但这种“被动”形成的深部碳库是否也发生在冰期旋回中还有待于进一步证实.总之,热带西太平洋碳循环也同水文循环一样,通过碳源汇演替对冰期旋回中大气pCO_(2)和全球气候产生重要影响,从而丰富了“气候演变低纬驱动”理论的内涵.

Glacial cycles are the primary feature of Earth’s climate during the Quaternary.According to the core of the“low-latitude forcing of climate change”theory,the western tropical Pacific(WTP)could control global climate evolution through changes in regional hydrological processes such as the Asian Monsoon,El Niño-Southern Oscillation,and the West Pacific Warm Pool during glacial cycles.Considering that the surface waters of the modern WTP are close to equilibrium with atmospheric CO_(2),academics have long believed that the role of the WTP in glacial-interglacial changes in the partial pressure of atmospheric CO_(2)(pCO_(2))is not as important as for other regions of the global ocean.However,some recent studies have implied enhanced oceanic carbon storage in the WTP during at least the last three glacial periods.Furthermore,the WTP have experienced dramatic shifts in the ocean carbonate system during major climate transitions such as the Northern Hemisphere glaciation and the mid-Pleistocene Climate Transition.Thus,it appears that the carbon cycling processes in the WTP may also have had an important influence on atmospheric pCO_(2)and global climate on glacial-interglacial time scales.The oceanic carbon source and sink processes operate in two ways:Surface carbon fixation and deep carbon storage.This review summarizes a pattern of the WTP carbon cycle based on a comprehensive analysis of these two types of processes in the WTP during glacial cycles since the Pliocene.According to this pattern,during glacial periods,falling sea-level brings more riverine nutrients into the ocean,resulting in increased surface organic carbon production and organic carbon burial offshore,and thus a more efficient organic carbon pump.Deep ocean carbonate ion concentration and alkalinity increase owing to a shift in carbonate deposition from shelves to the deep ocean due to a global sea-level drop,consistent with the“coral reef hypothesis”.An additional alkalinity input from the weathering of exposed carbonate contributes to the observed glacial pCO_(2)drawdown.In addition,pelagic organic carbon production events,such as giant diatom blooms due to increased glacial aeolian nutrient supply,generate more respired CO_(2)in the deep ocean,which further increases deep oceanic carbon storage.These aforementioned carbon cycle mechanisms,including carbon fixation and storage,ultimately sequester atmospheric CO_(2)in the form of dissolved inorganic carbon and organic carbon in the ocean,thereby contributing to the drawdown of glacial atmospheric pCO_(2).During interglacial periods,these processes have operated in the opposite direction or intensity.However,there is currently no evidence that the interglacial WTP acts as a carbon source.Furthermore,during the long-term trends superimposed on glacial cycles and major climate events,the WTP can passively form carbon pools in the deep through the basin-to-basin carbon transfer via the global thermohaline circulation.However,it remains to be confirmed whether such passive formation of deep carbon storage also occurred during glacial cycles.Nevertheless,we suggest that,similar to hydrological processes,carbon cycling in the WTP has an important impact on glacial-interglacial changes in atmospheric pCO_(2)and global climate,thereby improving our understanding of the“lowlatitude forcing of climate change”theory.This review introduces a general framework for the WTP carbon sources and sinks during glacial cycles,however,a set of challenges need to be addressed.Future research endeavors should focus on the following aspects:Development of reliable proxies for direct or indirect estimation of past seawater carbonate system parameters,establishment of carbon cycle stack records based on extensive data collection,and development of model simulations to validate mechanisms and hypotheses of carbon cycle evolution.