A comparison of isotope ratio mass spectrometry and cavity ring-down spectroscopy techniques for isotope analysis of fluid inclusion water.

RATIONALE: Online oxygen (δ18 O) and hydrogen (δ2 H) isotope analysis of fluid inclusion water entrapped in minerals is widely applied in paleo-fluid studies. In the state of the art of fluid inclusion isotope research, however, there is a scarcity of reported inter-technique comparisons to account for possible analytical offsets. Along with improving analytical precisions and sample size limitations, interlaboratory comparisons can lead to a more robust application of fluid inclusion isotope records. METHODS: Mineral samples-including speleothem, travertine, and vein material-were analyzed on two newly setup systems for fluid inclusion isotope analysis to provide an inter-platform comparison. One setup uses a crusher unit connected online to a continuous-flow pyrolysis furnace and an isotope ratio mass spectrometry (IRMS) instrument. In the other setup, a crusher unit is lined up with a cavity ring-down spectroscopy (CRDS) system, and water samples are analyzed on a continuous standard water background to achieve precisions on water injections better than 0.1‰ for δ18 O values and 0.4‰ for δ2 H values for amounts down to 0.2 µL. RESULTS: Fluid inclusion isotope analyses on the IRMS setup have an average 1σ reproducibility of 0.4‰ and 2.0‰ for δ18 O and δ2 H values, respectively. The CRDS setup has a better 1σ reproducibility (0.3‰ for δ18 O values and 1.1‰ for δ2 H values) and also a more rapid sample throughput (<30 min per sample). Fluid inclusion isotope analyses are reproducible at these uncertainties for water amounts down to 0.1 µL on both setups. Fluid inclusion isotope data show no systematic offsets between the setups. CONCLUSIONS: The close match in fluid inclusion isotope results between the two setups demonstrates the high accuracy of the presented continuous-flow techniques for fluid inclusion isotope analysis. Ideally, experiments such as the one presented in this study will lead to further interlaboratory comparison efforts and the selection of suitable reference materials for fluid inclusion isotopes studies.

Triple oxygen isotope reveals insolation-forced tropical moisture cycles.

Tropical oceans are the main global water vapor and latent heat sources, but their responses to radiative forcing remain unclear. Here, we investigate oceanic moisture dynamics of the western tropical Pacific (WTP) over the past 210,000 years through an approach of planktonic foraminiferal triple oxygen isotope (Δ'17O). The Δ'17O record is dominated by the precession cycles (~23,000 years), with lower values reflecting higher humidity in concert with higher Northern Hemisphere summer insolation. Our empirical and modeling results, combined with other geological archives, suggest that the enhanced moisture convergence over the WTP largely intensifies changes in the meridional and zonal hydrological cycles, affecting rainfall patterns in East Asia and northern South America. We propose that the insolation-driven WTP moisture dynamics play a pivotal role in regulating tropical hydroclimate.

The spatiotemporal extent of the Green Sahara during the last glacial period.

The Sahara Desert, one of today's most inhospitable environments, has known periods of enhanced precipitation that supported pre-historic humans. However, the Green Sahara timing and moisture sources are not well known due to limited paleoclimate information. Here, we present a multi-proxy (δ18O, δ13C, Δ17O, and trace elements) speleothem-based climate record from Northwest (NW) Africa. Our data document two Green Sahara periods during Marine Isotope Stage (MIS) 5a and the Early to Mid-Holocene. Consistency with paleoclimate records across North Africa highlights the east-west geographical extent of the Green Sahara, whereas millennial-scale North Atlantic cooling (Heinrich) events consistently resulted in drier conditions. We demonstrate that an increase in westerly-originating winter precipitation during MIS5a resulted in favorable environmental conditions. The comparison of paleoclimate data with local archaeological sequences highlights the abrupt climate deterioration and the decline in human density in NW Africa during the MIS5-4 transition, which suggests climate-forced dispersals of populations, with possible implications for pathways into Eurasia.

Holocene climate change in southern Oman deciphered by speleothem records and climate model simulations.

Qunf Cave oxygen isotope (δ18Oc) record from southern Oman is one of the most significant of few Holocene Indian summer monsoon cave records. However, the interpretation of the Qunf δ18Oc remains in dispute. Here we provide a multi-proxy record from Qunf Cave and climate model simulations to reconstruct the Holocene local and regional hydroclimate changes. The results indicate that besides the Indian summer monsoon, the North African summer monsoon also contributes water vapor to southern Oman during the early to middle Holocene. In principle, Qunf δ18Oc values reflect integrated oxygen-isotope fractionations over a broad moisture transport swath from moisture sources to the cave site, rather than local precipitation amount alone, and thus the Qunf δ18Oc record characterizes primary changes in the Afro-Asian monsoon regime across the Holocene. In contrast, local climate proxies appear to suggest an overall slightly increased or unchanged wetness over the Holocene at the cave site.

Publisher Correction: Multi-decadal to centennial hydro-climate variability and linkage to solar forcing in the Western Mediterranean during the last 1000 years.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Multi-decadal to centennial hydro-climate variability and linkage to solar forcing in the Western Mediterranean during the last 1000 years.

Here we present a new composite record from two well-dated speleothem records from two caves in Northern Morocco. The high-resolution record covers the last millennium allowing to detect multi-decadal to centennial periodicities. Over the industrial period, δ18O values of our speleothems are shown to be dominated by the main mode of decadal variability in the North Atlantic region: the North Atlantic Oscillation (NAO). Statistical analyses confirm the previously reported multi-decadal variability related to the influence of the Atlantic Multidecadal Oscillation (AMO) in the region. High power and persistent centennial-scale periodicities, similar to the Vries-Suess 200-year solar cycle, are observed as well. Indeed, comparison between solar activity reconstructions and our record confirms the in-phase relationship on centennial time-scales. Low δ18O values, and hence negative phases of NAO that bring precipitation towards the Western Mediterranean, are observed during well-known solar minima periods. The results are consistent with previous models which describe low irradiance as a  trigger for southward shifts of precipitation-bearing westerlies during winter.

Climate variations of Central Asia on orbital to millennial timescales.

The extent to which climate variability in Central Asia is causally linked to large-scale changes in the Asian monsoon on varying timescales remains a longstanding question. Here we present precisely dated high-resolution speleothem oxygen-carbon isotope and trace element records of Central Asia's hydroclimate variability from Tonnel'naya cave, Uzbekistan, and Kesang cave, western China. On orbital timescales, the supra-regional climate variance, inferred from our oxygen isotope records, exhibits a precessional rhythm, punctuated by millennial-scale abrupt climate events, suggesting a close coupling with the Asian monsoon. However, the local hydroclimatic variability at both cave sites, inferred from carbon isotope and trace element records, shows climate variations that are distinctly different from their supra-regional modes. Particularly, hydroclimatic changes in both Tonnel'naya and Kesang areas during the Holocene lag behind the supra-regional climate variability by several thousand years. These observations may reconcile the apparent out-of-phase hydroclimatic variability, inferred from the Holocene lake proxy records, between Westerly Central Asia and Monsoon Asia.