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Rationale The precision obtained in routine isotope analysis of water (δ17O, δ18O, δ2H, 17O‐excess and d‐excess values) by Cavity Ring‐Down Spectroscopy is usually below the instrument specifications provided by the manufacturer. This study aimed at reducing this discrepancy, with particular attention paid to mitigating the memory effect (ME). Methods We used a Picarro L2140i analyzer coupled with a high‐precision A0211 vaporizer and an A0325 autosampler. The magnitude and duration of the ME were estimated using 24 series of 50 successive injections of samples with contrasting compositions. Four memory correction methods were compared, and the instrument performance was evaluated over a 17‐month period of routine analysis, using two different run architectures. Results The ME remains detectable after the 30th injection, implying that common corrections procedures only based on the last preceding sample need to be revised. We developed a new ME correction based on the composition of several successive samples, and designed a run architecture to minimize the magnitude of the ME. The standard deviation obtained from routine measurement of a QA water over a 7 months‐period was 0.015‰ for δ17O, 0.023‰ for δ18O, 0.078‰ for δ2H, 0.006‰ for 17O‐excess and 0.173‰ for d‐excess. In addition, we provided the first δ17O and 17O‐excess values for the GRESP certified reference material. Conclusions This study demonstrates the long‐term persistence of the ME, which is often overlooked in routine analysis of natural samples. As already evidenced when measuring labelled water, it calls for consideration of the compositions of several previous samples to obtain an appropriate correction, a prerequisite to achieve high precision data.

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Past atmospheric methane-concentration oscillations recorded in polar ice cores vary together with rapid global climatic changes during the last glacial episode. In the ``clathrate gun hypothesis,'' massive releases of deep-sea methane from marine gas-hydrate dissociation led to these well known, global, abrupt warmings in the past. If evidence for such releases in the water column exists, however, the mechanism and eventual transfer to the atmosphere has not yet been documented clearly. Here we describe a high-resolution marine-sediment record of stable carbon isotopic changes from the Papua Gulf, off Papua New Guinea, which exhibits two extremely depleted excursions (down to -9parts per thousand) at approximate to39,000 and approximate to55,000 years. Morphological, isotopic, and trace metal evidence dismisses authigenic calcite as the main source of depleted carbon. Massive methane release associated with deep-sea gas-hydrate dissociation is the most likely cause for such large depletions of delta(13)C. The absence of a delta(13)C gradient in the water column during these events implies that the methane rose through the entire water column, reaching the sea-air interface and thus the atmosphere. Foraminiferal delta(18)O composition suggests that the rise of the methane in the water column created an upwelling flow. These inferred emission events suggest that during the last glacial episode, this process was likely widespread, including tropical regions. Thus, the release of methane from the ocean floor into the atmosphere cannot be dismissed as a strong positive feedback in climate dynamics processes.

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