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Laboratory O2 clumped-isotopic composition data (as Δ36 values) for air occluded in ice core spanning gas ages of 8000-18000 ky BP. O2 clumped isotopic composition data was generated between 2017-2022 at Rice University, Houston, TX, using a Nu Perspective Isotope Ratio Mass Spectrometer (IRMS). Reported data was measured in an Antarctic ice core: West Antarctic Ice Sheet Divide Ice Core (WDC06A) . The chronology and gas ages for the core were obtained from Sigl et al., 2016 (doi:10.5194/cp-12-769-2016) and Buizert et al., 2015 (doi:10.5194/cp-11-153-2015). In addition to O2 clumped isotope data, measured δ18Ο data are also reported. Gas loss corrections to generated Δ36 data are made using previously reported raw δ18O data in Seltzer et al., 2017 (doi:10.5194/cp-13-1323-2017) and established gas loss corrections in Yeung et al., 2012 and Banerjee et al. 2022. 

This file includes the clumped-isotope composition (18O18O) molecular oxygen (O2) that is in the trapped air from the S27 ice core collected in Allan Hills Blue Ice Area. 

The history of atmospheric oxygen ( P O 2 ) and the processes that act to regulate it remain enigmatic because of difficulties in quantitative reconstructions using indirect proxies. Here, we extend the ice-core record of P O 2 using 1.5-million-year-old (Ma) discontinuous ice samples drilled from Allan Hills Blue Ice Area, East Antarctica. No statistically significant difference exists in P O 2 between samples at 1.5 Ma and 810 thousand years (ka), suggesting that the Late-Pleistocene imbalance in O 2 sources and sinks began around the time of the transition from 40- to 100-ka glacial cycles in the Mid-Pleistocene between ~1.2 Ma and 700 ka. The absence of a coeval secular increase in atmospheric CO 2 over the past ~1 Ma requires negative feedback mechanisms such as P co 2 -dependent silicate weathering. Fast processes must also act to suppress the immediate P co 2 increase because of the imbalance in O 2 sinks over sources beginning in the Mid-Pleistocene. 

Abstract. The S27 ice core, drilled in the Allan Hills Blue IceArea of East Antarctica, is located in southern Victoria Land, ∼80 km away from the present-day northern edge of the RossIce Shelf. Here, we utilize the reconstructed accumulation rate of S27covering the Last Interglacial (LIG) period between 129 ka and 116 ka (where ka indicates thousands of years before present) to infer moisture transport into the region. Theaccumulation rate is based on the ice-age–gas-age differences calculatedfrom the ice chronology, which is constrained by the stable water isotopesof the ice, and an improved gas chronology based on measurements of oxygenisotopes of O2 in the trapped gases. The peak accumulation rate in S27occurred at 128.2 ka, near the peak LIG warming in Antarctica. Even the mostconservative estimate yields an order-of-magnitude increase in theaccumulation rate during the LIG maximum, whereas other Antarctic ice coresare typically characterized by a glacial–interglacial difference of a factorof 2 to 3. While part of the increase in S27 accumulation rates mustoriginate from changes in the large-scale atmospheric circulation,additional mechanisms are needed to explain the large changes. Wehypothesize that the exceptionally high snow accumulation recorded in S27reflects open-ocean conditions in the Ross Sea, created by reduced sea iceextent and increased polynya size and perhaps by a southward retreat of theRoss Ice Shelf relative to its present-day position near the onset of the LIG.The proposed ice shelf retreat would also be compatible with a sea-levelhigh stand around 129 ka significantly sourced from West Antarctica. Thepeak in S27 accumulation rates is transient, suggesting that if the Ross IceShelf had indeed retreated during the early LIG, it would have re-advancedby 125 ka. 

Abstract The history of tropospheric O₃, an important atmospheric oxidant, is poorly constrained because of uncertainties in its historical budget and a dearth of independent records. Here, we estimate the mean tropospheric O₃burden during the Last Interglacial period (LIG; 115 to 130 thousand years ago) using a record of the clumped isotopic composition of O₂(i.e., Δ₃₆values) preserved in Antarctic ice. The measured LIG Δ₃₆value is 0.03 ± 0.02‰ (95% CI) higher than the late pre‐industrial Holocene (PI; 1,590–1,850 CE) value and corresponds to a modeled 9% reduction in LIG tropospheric O₃burden (95% CI: 3%–15%), caused in part by a substantial reduction in biomass burning emissions during the LIG relative to the PI. These results are consistent with the hypothesis that late‐Pleistocene megafaunal extinctions caused woody and grassy fuels to accumulate on land, leading to enhanced biomass burning in the preindustrial Holocene. 

Abstract Tropospheric¹⁸O¹⁸O is an emerging proxy for past tropospheric ozone and free‐tropospheric temperatures. The basis of these applications is the idea that isotope‐exchange reactions in the atmosphere drive¹⁸O¹⁸O abundances toward isotopic equilibrium. However, previous work used an offline box‐model framework to explain the¹⁸O¹⁸O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the¹⁸O¹⁸O proxy, we developed a scheme to simulate atmospheric¹⁸O¹⁸O abundances (quantified as ∆₃₆values) online within the GEOS‐Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O₂isotopic equilibration scheme within GEOS‐Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆₃₆values are too high elsewhere. Investigations using a comprehensive model of the O‐O₂‐O₃isotopic photochemical system and proof‐of‐principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆₃₆values: the anomalously efficient titration of¹⁸O¹⁸O to form ozone. Incorporating these effects into the online ∆₃₆calculation scheme in GEOS‐Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of¹⁸O¹⁸O in O₂, the modeled change in the mean tropospheric ∆₃₆value since 1850 CE is only slightly altered; it is still quantitatively consistent with the ice‐core ∆₃₆record, implying that the tropospheric ozone burden increased less than 40% over the twentieth century.