The isotopic composition of dissolved oxygen offers a family of potentially unique tracers of respiration and transport in the subsurface ocean. Uncertainties in transport parameters and isotopic fractionation factors, however, have limited the strength of the constraints offered by 18O/16O and 17O/16O ratios in dissolved oxygen. In particular, puzzlingly low 17O/16O ratios observed for some low-oxygen samples have been difficult to explain. To improve our understanding of oxygen cycling in the ocean’s interior, we investigated the systematics of oxygen isotopologues in the subsurface Pacific using new data and a 2-D isotopologue-enabled isopycnal reaction-transport model. We measured 18O/16O and 17O/16O ratios, as well as the “clumped” 18O18O isotopologue in the northeast Pacific, and compared the results to previously published data. We find that transport and respiration rates constrained by O2 concentrations in the oligotrophic Pacific yield good measurement-model agreement across all O2 isotopologues only when using a recently reported set of respiratory isotopologue fractionation factors that differ from those most often used for oxygen cycling in the ocean. These fractionation factors imply that an elevated proportion of 17O compared to 18O in dissolved oxygen―i.e., its triple-oxygen isotope composition―does not uniquely reflect gross primary productivity and mixing. For all oxygen isotopologues, transport, respiration, and photosynthesis comprise important parts of their respective budgets. Mechanisms of oxygen removal in the subsurface ocean are discussed.

Tropospheric 18O18O 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 drive 18O18O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the 18O18O 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 the 18O18O proxy, we developed a scheme to simulate atmospheric 18O18O abundances (quantified as ∆36 values) 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 O2 isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36 values are too high elsewhere. Investigations using a comprehensive model of the O-O2-O3 isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36 values: the anomalously efficient titration of 18O18O to form ozone. Incorporating these effects into the online ∆36 calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of 18O18O in O2, the modeled change in the mean tropospheric ∆36 value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆36 record, implying that the tropospheric ozone burden increased less than ~40% over the twentieth century.

The oxygen and hydrogen isotopic composition in snow and ice have long been utilized to reconstruct past temperatures of polar regions, under the assumption that post-depositional processes such as sublimation do not fractionate snow. In low-accumulation (<0.01 m yr-1) areas near the McMurdo Dry Valleys in Antarctica surface snow and ice samples have negative deuterium excess values (δD - 8*δ18O). This unique phenomenon, only observed near the Dry Valleys, is not fully understood. Here we use both an isotope-enabled general circulation model and an ice physics model and establish that negative deuterium excess values can only arise from precipitation if the majority of the moisture is sourced from the Southern Ocean. However, the model results show that moisture sourced from oceans north of 55°S contributes significantly (>50%) to precipitation in Antarctica today. We thus propose that sublimation must have occurred to yield the negative deuterium excess values in snow observed in and near the Dry Valleys and that solid-phase-diffusion in ice grains is sufficiently fast to allow Rayleigh-like isotopic fractionation in similar environments. We calculate that under present-day conditions at the Allan Hills outside the Dry Valleys, 3 to 24% of the surface snow is lost due to sublimation. Because a higher fraction of snow is expected to be sublimed when accumulation rates are lower, the magnitude of δ18O and δD enrichment due to sublimation will be higher during past cold periods than at present, altering the relationship between the snow isotopic composition and polar temperatures.

Ice cores and other paleotemperature proxies, together with general circulation models, have provided information on past surface temperatures and the atmosphere’s composition in different climates. Little is known, however, about past temperatures at high altitudes, which play a crucial role in Earth’s radiative energy budget. Paleoclimate records at high-altitude sites are sparse, and the few that are available show poor agreement with climate model predictions. These disagreements could be due to insufficient spatial coverage, spatiotemporal biases, or model physics; new records that can mitigate or avoid these uncertainties are needed. Here, we constrain the change in upper-tropospheric temperature at the global scale during the Last Glacial Maximum (LGM) using the clumped-isotope composition of molecular oxygen trapped in polar ice cores. Aided by global three-dimensional chemical transport modeling, we exploit the intrinsic temperature sensitivity of the clumped-isotope composition of atmospheric oxygen to infer that the upper troposphere (5 – 15 km altitude, effective mean 10 – 11 km) was 4 – 10°C cooler during the LGM than during the late preindustrial Holocene. These results support a minor or negligible steepening of atmospheric lapse rates during the LGM, which is consistent with a range of climate model simulations. Proxy-model disagreements with other high-altitude records may stem from inaccuracies in regional hydroclimate simulation, possibly related to land-atmosphere feedbacks.

Climate and carbon cycling during the Eocene were complex, as inferred from records of stable isotopes and carbonate accumulation in marine sediment sections. Following a now well-documented early Eocene interval characterized by extreme global warmth and numerous short-term C-cycle perturbations documented in many sediment sections across the world, the ‘warmhouse’ climate state of the middle-late Eocene remains far less studied. In particular, the middle Eocene was punctuated by an event of significant global warming and seafloor carbonate dissolution (Middle Eocene Climate Optimum or MECO, ca. ~40.5 Ma). Over the last decade, studies from multiple sites in the Atlantic have suggested another abrupt and transient (and potentially a hyperthermal) warming event seemingly associated with a C-cycle perturbation at ~41.5 Ma, referred to as the Late Lutetian Thermal Maximum (LLTM). While both MECO and LLTM punctuate the post-EECO long-term cooling, their isotopic expression and duration are fundamentally different. At present, a dearth of continuous middle Eocene chemostratigraphic records limits our understanding of warming events like MECO and especially, the global extent of the LLTM. In this study, we develop an isotope stratigraphy of Lutetian-Bartonian age sediments from three different sites in the southwest Pacific region, two of which were drilled during IODP Expedition 371 in the Tasman Sea and a third that derives from field work in New Caledonia. We identify long-term changes in carbon and oxygen isotope records, possibly related to 405 Ky eccentricity cycles, and identify the stratigraphic expression of the LLTM and MECO. These new middle Eocene chemostratigraphic records from the southwest Pacific help to establish the global nature and relevance of multiple warming events that occurred during the ‘warmhouse’ climatic conditions of middle Eocene and highlight the utility of sedimentary carbon isotopes as a tool for chemostratigraphy and deciphering causes of past global changes.

Carbon isotope (δ13C) records from marine sediments have been extensively used in Cenozoic chemostratigraphy. The early Paleogene interval in particular has received exceptional attention because negative carbon isotope excursions (CIEs) documented in the sedimentary record, e.g. at the Paleocene Eocene Thermal Maximum (PETM), ca ~56 Ma, are believed to reflect significant global carbon cycle perturbations during the warmest interval of the Cenozoic era. However, while bulk-carbonate δ13C values exhibit robust correlations across widely separated marine sedimentary sections, their absolute values and magnitude of CIEs vary spatially. Moreover, bulk-carbonates in open-marine environments are an ensemble of different components, each with a distinct isotopic composition. Consequently, a complete interpretation of the bulk δ13C record requires an understanding of co-evolution of these components. In this study, we dissect sediments, from early Paleogene interval, at ODP Site 1209, Shatsky Rise, Pacific Ocean to investigate how an evolving bulk-carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for δ13C and δ18O compositions, as bulk and individual size fractions. We find a significant increase in coarse-fraction abundance across PETM, driven by a changing community structure of calcifiers, modulating the size of CIE at Site 1209 and thus making it distinct from those recorded at other open-marine sites. These results highlight the importance of biogeography in marine stable-isotope record, especially when isotopic excursions are driven by climate- and/or carbon-cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as conventional proxy for carbonate dissolution.