High-precision Measurement Of Triple Oxygen Isotope Of Carbonates And Its Application In The Speleothem Paleoclimate Research

During the past few decades,speleothemδ¹⁸O records have provided important insights for the research of global climate change.However,there are still some critical scientific issues to resolve mainly due to various influencing factors in theδ¹⁸O proxy studies.Meanwhile,continuous breakthroughs of high-precision analysis techniques of triple oxygen isotope(¹⁶O-¹⁷O-¹⁸O)provide a new potential proxy to study geochemical and hydroclimatic processes.In this work,firstly,we established a high-precision triple oxygen isotope analysis method for carbonate samples;secondly,based on the method,we measured triple oxygen isotope compositions of three international carbonate standards NBS18,NBS19 and IAEA-603,as well as speleothem carbonate samples for the first time;and finally,we used primary speleothem triple oxygen isotope data as possible constraints for understanding hydroclimate dynamics in some important climate systems.This work creates a new research frontier in paleoclimate field,which is important to further understand the regional/global moisture source and recycle history and the underlying mechanisms.The main results are as follows:1.Following a recently developed method,the triple oxygen isotope for CO₂ were obtained by measuring O₂ equilibrated with CO₂,instead of measuring CO₂ directly.The experimental protocols for precisely measurements of triple oxygen isotopes of carbonate have been established in the Isotope Lab in Xi’an Jiaotong University,including a system for the routine-offline phosphoric acid digestion of carbonate,a system for the graphite combustion,and a reaction system for O₂-CO₂ Pt-catalyzed oxygen isotope equilibrium.One of critical steps of the method is the Pt-catalyzed oxygen isotope equilibrium between O₂ and CO₂ under high temperature condition.The major procedures are described as follows:（1）The classical Mc Crea method was employed to extract CO₂ gas from carbonates by using concentrated phosphoric acid（H₃PO₄,1.92 g/ml,～104%）at 25°C.（2）The CO₂（～50–70μmol from carbonate digestion）was equilibrated with equal amount of the O₂gas for 30 minutes under 750oC condition to reach a Pt-catalyzed equilibrium.（3）The two post equilibration gases were separated from each other cryogenically:the CO₂was frozen in a cold trap using liquid nitrogen.Then,the O₂ was transferred to a sample tube containing a single pellet of molecular sieves cooled by liquid nitrogen.The resultant O₂ and CO₂ were analyzed by a Thermo Scientific MAT 253 mass spectrometer.The long-term external reproducibility is 9 per meg(1 per meg=10^-6=10^-3‰)（1σSD）or better for our new speleothemΔ¹⁷O(carbonate ¹⁷O anomaly)measurements,which can detect the small natual changes inΔ¹⁷O value of speleothem carbonates,and thus applicable for resolvinge the variations caused by many various natural hydroclimatic changes signals.2.Based on O₂-CO₂ Pt-catalyzed oxygen isotope equilibrium method,we obtained a set of the triple oxygen isotope data from speleothems collected from regions influenced predominantly by the Westerlies,the Mediterranean climate,and monsoonal climates in Asia and South America for some of key climatic periods,such as glacial and interglacial periods.（1）In this work,triple oxygen isotope data over the past 135 kyr BP were obtained from Tonnel’naya（TON）cave,the Central Asia in the Westerlies domained,which provided new information that characterizes the regional hydroclimate history on orbital-millennial timescales.Our speleothemΔ¹⁷O data reveal a～20 per meg difference on precession timescales,suggesting large changes in moisture source and/or fractionation history at the precession cycle.In contrast,it does not appear to have a significant difference in theΔ¹⁷O values between the glacial-interglacial samples from Kesang cave,western China in the Westerlies domained,indicating that the water vapor sources might be consistent across the glacial-interglacial cycle,which is different from Central Asia.（2）There is a～16 per meg difference between mid-Holocene and Last Glacial Maximum speleothem samples from the eastern Mediterranean,suggesting their considerably different moisture sources and/or fractionation history from glacial to interglacial.（3）There are no measurableΔ¹⁷O differences between glacial and interglacial samples from both the South American（western Amazon）and Asian（southern China）monsoon domains,implying consistent moisture-source conditions across glacial and interglacial cycles.3.The fractionation factors of triple oxygen isotopes were measured precusley between the paired modern speleothem carbonates and cave drip water,which are prerequisites to calculate triple oxygen isotopic compositions of the meteoric water at cave sites via measuring speleothem carbonates.Two sets of paired cave carbonate-water samples,both collected at cave temperature of 17±1°C,yield identical oxygen isotope fractionation factors（exponents）.Using these exponents and estimated temperatures,triple oxygen isotope compositions of the parent water were calculated using the measured triple oxygen isotope data of speleothem calcites.The calculated results closely track the Global Meteoric Water Line（θ=0.528）,providing a primary validation test on our measurements and calculations.Additionally,the calculated ¹⁷O-excess values of cave parent water fall within the range of modern meteoric water and constitute a valuable complementary proxy to the existing speleothemδ¹⁸O records that may potentially improve the interpretation of hydroclimatic dynamics in various climate systems.Moreover,the relative humidity values inferred from the analysis of modern speleothem carbonate from Lianhuan and Furong caves in southwestern China are 63±8%and 71±5%,consistent broadly with the observed relative humidity at their preconceived moisture source regions,providing a validation test on the paleoclimatic application.Based on aforementioned work,we attribute to first order for the observed variations in parent water¹⁷O-excess values(inferred from the spleothemΔ¹⁷O data)to relative humidity of moisture source condition in multiple climatic regions as follows:（1）In the Westerlies-dominated region,a～19%decrease in relative humidity from the MIS-5d（82±4%;115 kyr BP）to MIS-5e（63±5%;126 kyr BP）（1σSD）over the moisture source regions for the TON cave site.（2）TheΔ¹⁷O data from Jeita cave in the eastern Mediterranean area revealed a shift in relative humidity of moisture source from 38±4%at～16 kyr BP in the Last Glacial to 59±4%at～6kyr BP in the mid-Holocene（1σSD）.（3）Speleothem data from southern China in the Asian monsoon region suggest that the moisture sources are similar in terms of relative humidity during the penultimate glacial at～133 kyr BP（MIS-6）and the last interglacial at～126 kyr BP（MIS-5e）:85±6%and 86±13%,respectively（1σSD）.（4）In the western South American monsoon,data from El Condor cave,western Amazonia in northern Peru show that variations of reconstructed relative humidity in the moisture source regions are similar within uncertainty across glacial-interglacial cycle:73±3%at～40 kyr BP（MIS 3,1σSD）and 68±3%at～6 kyr BP（the mid-Holocene,1σSD）without the dramatic shift as observed from theirδ¹⁸O records.