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Oxygen isotopes in biogenic silica (δ18OBSi) from lake sediments allow for quantitative reconstruction of past hydroclimate and proxy-model comparison in terrestrial environments. The signals of individual records have been attributed to different factors, such as air temperature (Tair), atmospheric circulation patterns, hydrological changes, and lake evaporation. While every lake has its own local set of drivers of δ18O variability, here we explore the extent to which regional or even global signals emerge from a series of paleoenvironmental records. This study provides a comprehensive compilation and combined statistical evaluation of the existing lake sediment δ18OBSi records, largely missing in other summary publications (i.e. PAGES network). For this purpose, we have identified and compiled 71 down-core records published to date and complemented these datasets with additional lake basin parameters (e.g. lake water residence time and catchment size) to best characterize the signal properties. Records feature widely different temporal coverage and resolution, ranging from decadal-scale records covering the past 150 years to records with multi-millennial-scale resolution spanning glacial–interglacial cycles. The best coverage in number of records (N = 37) and data points (N = 2112) is available for Northern Hemispheric (NH) extratropical regions throughout the Holocene (roughly corresponding to Marine Isotope Stage 1; MIS 1). To address the different variabilities and temporal offsets, records were brought to a common temporal resolution by binning and subsequently filtered for hydrologically open lakes with lake water residence times < 100 years. For mid- to high-latitude (> 45° N) lakes, we find common δ18OBSi patterns among the lake records during both the Holocene and Common Era (CE). These include maxima and minima corresponding to known climate episodes, such as the Holocene Thermal Maximum (HTM), Neoglacial Cooling, Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). These patterns are in line with long-term air temperature changes supported by previously published climate reconstructions from other archives, as well as Holocene summer insolation changes. In conclusion, oxygen isotope records from NH extratropical lake sediments feature a common climate signal at centennial (for CE) and millennial (for Holocene) timescales despite stemming from different lakes in different geographic locations and hence constitute a valuable proxy for past climate reconstructions. [ABSTRACT FROM AUTHOR]

Oxygen isotopes in biogenic silica (δ18OBSi) from lake sediments allow for quantitative reconstruction of past hydroclimate and proxy-model comparison in terrestrial environments. The signals of individual records have been attributed to different factors, such as air temperature (Tair), atmospheric circulation patterns, hydrological changes and lake evaporation. While every lake will have its own set of drivers of d18O, here we explore the extent to which regional or even global signals emerge from a series of palaeoenvironmental records. For this purpose, we have identified and compiled 71 down-core records published to date and complemented these datasets with additional lake basin parameters (e.g. lake water residence time and catchment size) to best characterize the signal properties. Records feature widely different temporal coverage and resolution ranging from decadal-scale records covering the last 150 years to records with multi-millennial scale resolution spanning glacial-interglacial cycles. Best coverage in number of records (N=37) and datapoints (N=2112) is available for northern hemispheric (NH) extra-tropic regions throughout the Holocene (corresponding to Marine Isotope Stage 1; MIS 1). To address the different variabilities and temporal offsets, records were brought to a common temporal resolution by binning and subsequently filtered for hydrologically open lakes with lake water residence times <100 yrs. For mid- to high-latitude (>45° N) lakes, we find common δ18OBSi patterns during both the Holocene and the Common Era and maxima and minima corresponding to known climate episodes such as the Holocene Thermal Maximum (HTM), Neoglacial Cooling, Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). These patterns are in line with long-term Tair changes supported by previously published climate reconstructions from other archives as well as Holocene summer insolation changes. In conclusion, oxygen isotope records from NH extratopic lake sediments feature a common climate signal at centennial (for CE) and millennial (for Holocene) time scales despite stemming from different lakes in different geographic locations and constitute a valuable proxy for past climate reconstructions. [ABSTRACT FROM AUTHOR]

The triple oxygen isotope composition of phytoliths (17O-excessphyto) can provide key information on past atmospheric relative humidity (RH) over land. Here, we examined how leaf-to-air temperature gradients and changes in the silica polymerization rate in response to stomatal conductance influence the interpretation of 17O-excessphyto in terms of RH. Further, we assessed the reliability of a theoretical isotope model of leaf water evaporation to predict the triple oxygen isotope composition of leaf water on diurnal and seasonal scale. For this purpose, we monitored a grass plot within a natural Mediterranean woodland for one year. We measured in particular the isotope composition of atmospheric water vapor and plot-scale grass leaf temperatures - two variables that are often only estimated. Grass leaf blades were collected in different seasons and over a 24-hour period for leaf water and phytolith isotope analysis. We found that the steady state model reliably predicts the triple oxygen isotope composition of leaf water during daytime but remains sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented by the non-steady state model. In our study, the 17O-excessphyto best reflects average daytime RH over the growth period, rather than daily RH. Average daytime leaf-to-air temperature gradients of less than 2 °C introduce an insignificant bias to the RH estimate. The results also confirm the established triple oxygen isotope fractionation factors between phytoliths and leaf water. The findings of this study help to better understand how to interpret 17O-excessphyto of fossil phytolith assemblages in terms of past RH. [ABSTRACT FROM AUTHOR]

An IR-laser fluorination technique is reported here for analyzing the oxygen isotope composition (b180) of microscopic biogenic silica grains (phytoliths and diatoms). Performed after a controlled isotopic exchanged (CIE) procedure, the laser fluorination technique that allows one to visually check the success of the fluorination reaction is faster than the conventional fluorination technique and allows analyzing [delta][sup.18]O of small to minute samples (1.6-0.3 mg) as required for high-resolution paleoenvironmental reconstructions. The long-term reproducibility achieved with the IR laser-heating fluorination/[O.sub.2] [delta][sup.18]O analysis is lower than or equal to [+ or -] 0.26 [per thousand] (1 SD; n = 99) for phytoliths and [+ or -] 0.17%0 (1 SD; n = 47) for diatoms. When several CIE are taken into account in the SD calculation, the resulting reproducibility is lower than or equal to +0.51 [per thousand] for phytoliths (1 SD; n = 99; CIE > 5) and [+ or -] 0.54 [per thousand] (1 SD; n = 47; CIE = 13) for diatoms. A minimum reproducibility of [+ or -] 0.5 [per thousand] leads to an estimated uncertainty on [delta][sup.18][O.sub.silica] close to [+ or -] 0.5 [per thousand]. Resulting uncertainties on reconstructed temperature and [delta][sup.18][O.sub.forming] water are, respectively, [+ or -] 2 [degrees] C and [+ or -] 0.5 [per thousand] and fit in the precisions required for intertropical paleoenvironmental reconstructions. Several methodological points such as optimal extraction protocols and the necessity or not of performing two CIE prior to oxygen extraction are assessed.

Rationale: Producing robust high‐frequency time series of raw atmospheric water vapor isotope data using laser spectrometry requires accurate calibration. In particular, the chemical composition of the analyzed sample gas can cause isotope bias. This study assesses the matrix effect on calibrated δ17O, δ18O, δ2H, 17O‐excess, and d‐excess values of atmospheric water vapor. Methods: A Picarro L2140‐i cavity ring‐down spectrometer with an autosampler and a vaporizer is used to analyze δ17O, δ18O, δ2H, 17O‐excess, and d‐excess of two water standards. Isotope data obtained using synthetic air and dry ambient air as carrier gas at water mixing ratios ranging from 2000 to 30 000 ppmv are compared. Based on the results, atmospheric water vapor measurements are calibrated. The expected precision is estimated by Monte Carlo simulation. Results: The dry air source strongly impacts raw isotope values of the two water standards but has no effect on the mixing ratio dependency functions. When synthetic air is used, δ17O, δ18O, and 17O‐excess of calibrated atmospheric water vapor are overestimated by 0.6‰, 0.7‰, and 217 per meg, respectively, whereas δ2H and d‐excess are underestimated by 1.5‰ and 7.3‰. Optimum precisions for the calibrated δ17O, δ18O, δ2H, 17O‐excess, and d‐excess values and 12 min integration time are 0.02‰, 0.03‰, 0.4‰, 14 per meg, and 0.4‰, respectively. Conclusions: Regarding the obtained results, recommendations for the calibration of atmospheric water vapor isotope measurements are presented. The necessity to use dry ambient air as dry air source when running the standards for calibration is pointed out as a prerequisite for accurate atmospheric water vapor 17O‐excess and d‐excess measurements. [ABSTRACT FROM AUTHOR]