As a follow-up to the meeting of experts convened at the International Atomic Energy Agency (IAEA) in February 1989, and the International 14C Workshop held in Glasgow in September 1989, the 14C Quality Assurance Program was formulated. In a joint effort of several radiocarbon teams and IAEA staff, we have prepared a set of five new intercomparison materials. These are natural materials frequently used by radiocarbon laboratories. The materials were distributed to 137 laboratories in May 1990. In February 1991, a meeting of experts was convened in Vienna to evaluate the results, to determine the radiocarbon activity of the five samples expressed in % Modern (pMC) terms and to define the 13C/12C ratio, and to make recommendations on further use of these materials. We present here the results of the exercise and the agreed consensus values for each of the five materials and discuss the different analyses that were undertaken.

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River systems draining peaty catchments are considered a source of atmospheric CO2,thus understanding the behavior of the dissolved inorganic carbon pool (DIC) is valuable. The carbon isotopic composition, delta13C(DIC), and concentration, [DIC], of fluvial samples collected diurnally, over 14 months, reveal the DIC pools to be dynamic in range (-22 to -4.9% per hundred, 0.012 to 0.468 mmol L(-1) C), responding predictably to environmental influences such as changing hydrologic conditions or increased levels of primary production. delta(18)O of dissolved oxygen (DO) corroborates the delta(13)C(DIC) interpretation. A nested catchment sampling matrix reveals that similar processes affect the DIC pool and thus delta(13)C(DIC) across catchment sizes. Not so with [DIC]: at high flow, the DIC export converges across catchment size, but at low flow catchments diverge in their DIC load. Contextualizing delta(13)C with discharge reveals that organic soil-waters and groundwaters comprise end-member sources, which in varying proportions constitute the fluvial DIC pool. Discharge and pH describe well [DIC] and delta(13)C(DIC), allowing carbon to be apportioned to each end-member from continuous profiles, demonstrated here for the hydrological year 2003-2004. This approach is powerful for assessing whether the dynamic response exhibited here is ubiquitous in other fluvial systems at the terrestrial-aquatic interface or in larger catchments.