Khatiwala
Samar
Khatiwala
Samar
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ArticleGlobal ocean carbon uptake : magnitude, variability and trends(Copernicus Publications on behalf of the European Geosciences Union, 2013-03-22) Wanninkhof, Rik ; Park, Geun-Ha ; Takahashi, Taro ; Sweeney, Colm ; Feely, Richard A. ; Nojiri, Yukihiro ; Gruber, Nicolas ; Doney, Scott C. ; McKinley, Galen A. ; Lenton, Andrew ; Le Quere, Corinne ; Heinze, Christoph ; Schwinger, Jorg ; Graven, Heather ; Khatiwala, SamarThe globally integrated sea–air anthropogenic carbon dioxide (CO2) flux from 1990 to 2009 is determined from models and data-based approaches as part of the Regional Carbon Cycle Assessment and Processes (RECCAP) project. Numerical methods include ocean inverse models, atmospheric inverse models, and ocean general circulation models with parameterized biogeochemistry (OBGCMs). The median value of different approaches shows good agreement in average uptake. The best estimate of anthropogenic CO2 uptake for the time period based on a compilation of approaches is −2.0 Pg C yr−1. The interannual variability in the sea–air flux is largely driven by large-scale climate re-organizations and is estimated at 0.2 Pg C yr−1 for the two decades with some systematic differences between approaches. The largest differences between approaches are seen in the decadal trends. The trends range from −0.13 (Pg C yr−1) decade−1 to −0.50 (Pg C yr−1) decade−1 for the two decades under investigation. The OBGCMs and the data-based sea–air CO2 flux estimates show appreciably smaller decadal trends than estimates based on changes in carbon inventory suggesting that methods capable of resolving shorter timescales are showing a slowing of the rate of ocean CO2 uptake. RECCAP model outputs for five decades show similar differences in trends between approaches.
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DatasetElemental and isotopic noble gas ratios from the Bermuda Atlantic Time-series (BATS) on cruise 10391 on R/V Atlantic Explorer (AE2208) from 30 April 2022 to 05 May 2022(Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2023-02-24) Seltzer, Alan M. ; Barry, Peter ; Jenkins, William J. ; Khatiwala, Samar ; Nicholson, David P. ; Smethie Jr., William M. ; Stanley, Rachel ; Stute, MartinThis dataset includes new observations of heavy noble gas ratios (elemental and isotopic ratios) from the Bermuda Atlantic Time-series (BATS) on cruise 10391 on R/V Atlantic Explorer (AE2208) from 30 April 2022 - 05 May 2022. These data were used, along with measurements of Kr/Ar and N2/Ar ratios in stored dissolved gas samples from the Transient Tracers in the Ocean (TTO) program, to model simulations of these tracers using the Transport Matrix Method (TMM). Together these new measurements and model simulations provide insight into physical processes governing gas exchange in the high-latitude regions of North Atlantic Deep Water formation, and a comparison of physical simulations of N2/Ar ratios to observations in TTO samples reveals excess N2 that arises from benthic denitrification in the deep North Atlantic. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/890342
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DatasetMeasurements of Kr/Ar and N2/Ar ratios in stored dissolved gas samples collected in 1981 through the Transient Tracers in the Ocean (TTO) program North Atlantic Survey (NAS)(Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2023-02-24) Seltzer, Alan M. ; Barry, Peter ; Jenkins, William J. ; Khatiwala, Samar ; Nicholson, David P. ; Smethie Jr., William M. ; Stanley, Rachel ; Stute, MartinThis dataset includes measurements of Kr/Ar and N2/Ar ratios in stored dissolved gas samples collected in 1981 through the Transient Tracers in the Ocean (TTO) program. These data were used, along with new observations of heavy noble gas ratios (elemental and isotopic ratios) from the Bermuda Atlantic Time-series (BATS) on cruise 10391 (30 April 2022 - 05 May 2022), to model simulations of these tracers using the Transport Matrix Method (TMM). Together these new measurements and model simulations provide insight into physical processes governing gas exchange in the high-latitude regions of North Atlantic Deep Water formation, and a comparison of physical simulations of N2/Ar ratios to observations in TTO samples reveals excess N2 that arises from benthic denitrification in the deep North Atlantic. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/890427
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DatasetModel simulations of elemental and isotopic heavy noble gas ratios using the Transport Matrix Method (TMM)(Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2023-02-24) Seltzer, Alan M. ; Barry, Peter ; Jenkins, William J. ; Khatiwala, Samar ; Nicholson, David P. ; Smethie Jr., William M. ; Stanley, Rachel ; Stute, MartinThis dataset includes model simulations of the following tracers using the Transport Matrix Method (TMM): (i) new observations of heavy noble gas ratios (elemental and isotopic ratios) from the Bermuda Atlantic Time-series (BATS) on cruise 10391 (30 April 2022 - 05 May 2022), and (ii) measurements of Kr/Ar and N2/Ar ratios in stored dissolved gas samples collected in 1981 through the Transient Tracers in the Ocean (TTO) program. Together these measurements and model simulations provide insight into physical processes governing gas exchange in the high-latitude regions of North Atlantic Deep Water formation, and a comparison of physical simulations of N2/Ar ratios to observations in TTO samples reveals excess N2 that arises from benthic denitrification in the deep North Atlantic. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/890293
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ArticleDissolved gases in the deep North Atlantic track ocean ventilation processes(National Academy of Sciences, 2023-03-14) Seltzer, Alan M. ; Nicholson, David P. ; Smethie, William M. ; Tyne, Rebecca L. ; Le Roy, Emilie ; Stanley, Rachel H. R. ; Stute, Martin ; Barry, Peter H. ; McPaul, Katelyn ; Davidson, Perrin W. ; Chang, Bonnie X. ; Rafter, Patrick A. ; Lethaby, Paul ; Johnson, Rod J. ; Khatiwala, Samar ; Jenkins, William J.Gas exchange between the atmosphere and ocean interior profoundly impacts global climate and biogeochemistry. However, our understanding of the relevant physical processes remains limited by a scarcity of direct observations. Dissolved noble gases in the deep ocean are powerful tracers of physical air-sea interaction due to their chemical and biological inertness, yet their isotope ratios have remained underexplored. Here, we present high-precision noble gas isotope and elemental ratios from the deep North Atlantic (~32°N, 64°W) to evaluate gas exchange parameterizations using an ocean circulation model. The unprecedented precision of these data reveal deep-ocean undersaturation of heavy noble gases and isotopes resulting from cooling-driven air-to-sea gas transport associated with deep convection in the northern high latitudes. Our data also imply an underappreciated and large role for bubble-mediated gas exchange in the global air-sea transfer of sparingly soluble gases, including O, N, and SF. Using noble gases to validate the physical representation of air-sea gas exchange in a model also provides a unique opportunity to distinguish physical from biogeochemical signals. As a case study, we compare dissolved N/Ar measurements in the deep North Atlantic to physics-only model predictions, revealing excess N from benthic denitrification in older deep waters (below 2.9 km). These data indicate that the rate of fixed N removal in the deep Northeastern Atlantic is at least three times higher than the global deep-ocean mean, suggesting tight coupling with organic carbon export and raising potential future implications for the marine N cycle.
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PresentationTemporal and spatial perspectives on the fate of anthropogenic carbon : a carbon cycle slide deck for broad audiences(Ocean Carbon & Biogeochemistry Program, 2015-12-08) Khatiwala, Samar ; DeVries, Timothy ; Cook, Jack ; McKinley, Galen A. ; Carlson, Craig A. ; Benway, Heather M.This slide deck was developed to inform broader scientific, as well as general audiences about the role of the ocean in the global carbon cycle, including key sinks and sources of anthropogenic carbon and how they have evolved through time and space.
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ArticleDecomposing the oxygen signal in the ocean interior: beyond decomposing organic matter(American Geophysical Union, 2021-09-13) Cassar, Nicolas ; Nicholson, David P. ; Khatiwala, Samar ; Cliff, EllenIn the subsurface ocean, O2 depleted because of organic matter remineralization is generally estimated based on apparent oxygen utilization (AOU). However, AOU is an imperfect measure of oxygen utilization because of O2 air-sea disequilibrium at the site of deepwater formation. Recent methodological and instrumental advances have paved the way to further deconvolve the processes driving the O2 signature. Using numerical model simulations of the global ocean, we show that the measurements of the dissolved O2/Ar ratio, which so far have been confined to the ocean surface, can provide improved estimates of oxygen utilization, especially in regions where the disequilibrium at the site of deepwater formation is associated with physical processes. We discuss applications of this new approach and implications for the current tracers relying on O2 such as remineralization ratios, respiratory quotients, and preformed nutrients. Finally, we propose a new composite geochemical tracer, [O2]bio combining dissolved O2/Ar and phosphate concentration. Being insensitive to photosynthesis and respiration, the change in this new tracer reflects gas exchange at the air-sea interface at the sites of deepwater formation.
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ArticleGlobal ocean cooling of 2.3°C during the last glacial maximum(American Geophysical Union, 2024-05-08) Seltzer, Alan M. ; Davidson, Perrin W. ; Shackleton, Sarah A. ; Nicholson, David P. ; Khatiwala, SamarQuantitative constraints on past mean ocean temperature (MOT) critically inform our historical understanding of Earth's energy balance. A recently developed MOT proxy based on paleoatmospheric Xe, Kr, and N2 ratios in ice core air bubbles is a promising tool rooted in the temperature dependences of gas solubilities. However, these inert gases are systematically undersaturated in the modern ocean interior, and it remains unclear how air-sea disequilibrium may have changed in the past. Here, we carry out 30 tracer-enabled model simulations under varying circulation, sea ice cover, and wind stress regimes to evaluate air-sea disequilibrium in the Last Glacial Maximum (LGM) ocean. We find that undersaturation of all three gases was likely reduced, primarily due to strengthened high-latitude winds, biasing reconstructed MOT by −0.38 ± 0.37°C (1σ). Accounting for air-sea disequilibrium, paleoatmospheric inert gases indicate that LGM MOT was 2.27 ± 0.46°C (1σ) colder than the pre-industrial era.