Carbon sources and sinks in the western tropical Pacific during glacial cycles:Processes,mechanisms and effects
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 Nino-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 CO2,academics have long believed that the role of the WTP in glacial-interglacial changes in the partial pressure of atmospheric CO2(pCO2)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 pCO2 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 pCO2 drawdown.In addition,pelagic organic carbon production events,such as giant diatom blooms due to increased glacial aeolian nutrient supply,generate more respired CO2 in the deep ocean,which further increases deep oceanic carbon storage.These aforementioned carbon cycle mechanisms,including carbon fixation and storage,ultimately sequester atmospheric CO2 in the form of dissolved inorganic carbon and organic carbon in the ocean,thereby contributing to the drawdown of glacial atmospheric pCO2.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 pCO2 and global climate,thereby improving our understanding of the"low-latitude 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.
surface-ocean carbon fixationdeep-ocean carbon storagesea levelaeolian dustcarbon cyclesince the Pliocene
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