From the East China Sea to Global Continental Margins: Their Role in
the Global Biogeochemical Cycle in the Past, Present and Future
KON-KEE LIU
Institute of Hydrological and Oceanic Sciences, National Central University, Jungli, Taoyuan
320, Taiwan; kkliu@ncu.edu.twContinental margins are an important yet poorly understood component in theEarth’s biogeochemical system. However, we do know that they serve as efficient
CO2 sequestration machinery in the ocean, known as the continental shelf pump. The
estimated net CO2 uptake by continental margins could be as much as 0.3 Pg C/yr,representing ca 20% of the net CO2 uptake (1.6 PgC/yr) in the global ocean. They
also account for more than 3/4 of oceanic denitrification (including anoxicammonium oxidation) and probably nitrogen fixation, too. It is remarkable that many
continental margins that receive high loads of carbon discharged by rivers can stilltake up atmospheric CO2. In fact, they could be a net source of CO2 in pre-industrial
era, but how they have evolved to become a net sink at present and how they willchange in the future are critical issues we need to grapple with. The future change
will likely have a significant impact on the global carbon cycle, while the pastevolution may be the key to future prediction.
The biogeochemical system of the East China Sea (ECS), which firstinspired Professor Tsunogai to coin the term, continental shelf pump, may serve as amodel system for us to examine the critical biogeochemical processes in continental
margins. It is intriguing that the ECS can be so efficient in absorbing atmospheric
CO2 in spite of a huge riverine load of organic and inorganic carbon discharged fromsome of the largest rivers in the world. There have to be very effective processes intransporting the received carbon to the deep ocean. In recent years, it has beenrecognized that a suite of physical processes, including some rarely known ones, e.g.,dense water cascading, fluid mud flow, tide-induced solitons, etc., facilitate thecross-shelf exchange. The fates of the carbon input follow three pathways ofcomparable importance, namely, burial in shelf sediments, export as organic carbon
and export as dissolved inorganic carbon. Most of the buried carbon is probably themore refractory terrigenous organic carbon and carbonates. The export of organiccarbon from the shelf may be facilitated by sediments, which serve as the carrier andprotector. Dissolved organic carbon is another important form for export.
Traditionally, heterotrophy (respiration exceeding photosynthesis) in continental
shelves was equated to outgasing of CO2. However, recent studies show that thestratified water column in continental margins, resulting mainly from freshwater2runoff, often serves as an insulator to prevent escaping of CO2 from respired organiccarbon and allows export of the produced CO2 to the deep ocean.
As we have gained better knowledge of how the biogeochemical system
operates currently in continental margins, we are still far from understanding thechanges of air-sea CO2 fluxes since the industrial revolution, let alone predicting thefuture. Nevertheless, emerging new findings shed light on these crucial issues. A
recent assessment indicates that the riverine loading of nutrients have increased
significantly by human activities. The extra nutrient loads may have contributed tothe sequestration of atmospheric CO2. On the other hand, negative impacts of humanactivities on riverine loads of Si may reduce the abundance of diatoms, an efficientCO2 fixer, in the river plume. It is highly desirable to develop coupled physicalbiogeochemical models to examine how the natural and human-induced changes inphysical-chemical conditions that have driven and will continue to drive thebiogeochemical changes in continental margins in the Anthropocene.