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1. A Next Generation Ocean Carbon Isotope Model for Climate Studies I: Steady State Controls on Ocean 13 C

   https://doi.org/10.1029/2020GB006757  

   Claret, Mariona ; Sonnerup, Rolf E. ; Quay, Paul D. ( April 2021 , Global Biogeochemical Cycles)

   The¹³C/¹²C of dissolved inorganic carbon (δ¹³C_DIC) carries valuable information on ocean biological C‐cycling, air‐sea CO₂exchange, and circulation. Paleo‐reconstructions of oceanic¹³C from sediment cores provide key insights into past as changes in these three drivers. As a step toward full inclusion of¹³C in the next generation of Earth system models, we implemented¹³C‐cycling in a 1° lateral resolution ocean‐ice‐biogeochemistry Geophysical Fluid Dynamics Laboratory (GFDL) model driven by Common Ocean Reference Experiment perpetual year forcing. The model improved the mean of modernδ¹³C_DICover coarser resolution GFDL‐model implementations, capturing the Southern Ocean decline in surfaceδ¹³C_DICthat propagates to the deep sea via deep water formation. Controls onδ¹³C_DICof the deep‐sea are quantified using both observations and model output. The biological control is estimated from the relationship between deep‐sea Pacificδ¹³C_DICand phosphate (PO₄). Theδ¹³C_DIC:PO₄slope from observations is revised to a value of 1.01 ± 0.02‰ (μmol kg⁻¹)⁻¹, consistent with a carbon to phosphate ratio of organic matter (C:P_org) of 124 ± 10. Model output yields a lowerδ¹³C_DIC:PO₄than observed due to too low C:P_org. The ocean circulation impacts deep modernδ¹³C_DICin two ways, via the relative proportion of Southern Ocean and North Atlantic deep water masses, and via the preindustrialδ¹³C_DICof these water mass endmembers. Theδ¹³C_DICof the endmembers ventilating the deep sea are shown to be highly sensitive to the wind speed dependence of air‐sea CO₂gas exchange. Reducing the coefficient for air‐sea gas exchange following OMIP‐CMIP6 protocols improves significantly surfaceδ¹³C_DICrelative to previous gas exchange parameterizations.

    

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2. Probing the Ecology and Climate of the Eocene Southern Ocean With Sand Tiger Sharks Striatolamia macrota

   https://doi.org/10.1029/2020PA003997  

   Kim, Sora L. ; Zeichner, Sarah S. ; Colman, Albert S. ; Scher, Howie D. ; Kriwet, Jürgen ; Mörs, Thomas ; Huber, Matthew ( December 2020 , Paleoceanography and Paleoclimatology)

   Many explanations for Eocene climate change focus on the Southern Ocean—where tectonics influenced oceanic gateways, ocean circulation reduced heat transport, and greenhouse gas declines prompted glaciation. To date, few studies focus on marine vertebrates at high latitudes to discern paleoecological and paleoenvironmental impacts of this climate transition. The Tertiary Eocene La Meseta (TELM) Formation has a rich fossil assemblage to characterize these impacts;Striatolamia macrota, an extinct (†) sand tiger shark, is abundant throughout the La Meseta Formation. Body size is often tracked to characterize and integrate across multiple ecological dimensions. †S. macrotabody size distributions indicate limited changes during TELMs 2–5 based on anterior tooth crown height (n = 450, mean = 19.6 ± 6.4 mm). Similarly, environmental conditions remained stable through this period based on δ¹⁸O_PO4values from tooth enameloid (n = 42; 21.5 ± 1.6‰), which corresponds to a mean temperature of 22.0 ± 4.0°C. Our preliminaryε_Nd(n = 4) results indicate an early Drake Passage opening with Pacific inputs during TELM 2–3 (45–43 Ma) based on single unit variation with an overall radiogenic trend. Two possible hypotheses to explain these observations are (1) †S. macrotamodified its migration behavior to ameliorate environmental changes related to the Drake Passage opening, or (2) the local climate change was small and gateway opening had little impact. While we cannot rule out an ecological explanation, a comparison with climate model results suggests that increased CO₂produces warm conditions that also parsimoniously explain the observations.

    

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3. Glacial–interglacial Nd isotope variability of North Atlantic Deep Water modulated by North American ice sheet

   https://doi.org/10.1038/s41467-019-13707-z  

   Zhao, Ning ; Oppo, Delia W. ; Huang, Kuo-Fang ; Howe, Jacob N. W. ; Blusztajn, Jerzy ; Keigwin, Lloyd D. ( December 2019 , Nature Communications)

   The Nd isotope composition of seawater has been used to reconstruct past changes in the contribution of different water masses to the deep ocean. In the absence of contrary information, the Nd isotope compositions of endmember water masses are usually assumed constant during the Quaternary. Here we show that the Nd isotope composition of North Atlantic Deep Water (NADW), a major component of the global overturning ocean circulation, was significantly more radiogenic than modern during the Last Glacial Maximum (LGM), and shifted towards modern values during the deglaciation. We propose that weathering contributions of unradiogenic Nd modulated by the North American Ice Sheet dominated the evolution of the NADW Nd isotope endmember. If water mass mixing dominated the distribution of deep glacial Atlantic Nd isotopes, our results would imply a larger fraction of NADW in the deep Atlantic during the LGM and deglaciation than reconstructed with a constant northern endmember.

    

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4. The Connected Isotopic Water Cycle in the Community Earth System Model Version 1

   https://doi.org/10.1029/2019MS001663  

   Brady, E. ; Stevenson, S. ; Bailey, D. ; Liu, Z. ; Noone, D. ; Nusbaumer, J. ; Otto‐Bliesner, B. L. ; Tabor, C. ; Tomas, R. ; Wong, T. ; et al ( August 2019 , Journal of Advances in Modeling Earth Systems)

   Because of the pervasive role of water in the Earth system, the relative abundances of stable isotopologues of water are valuable for understanding atmospheric, oceanic, and biospheric processes, and for interpreting paleoclimate proxy reconstructions. Isotopologues are transported by both large‐scale and turbulent flows, and the ratio of heavy to light isotopologues changes due to fractionation that can accompany condensation and evaporation processes. Correctly predicting the isotopic distributions requires resolving the relationships between large‐scale ocean and atmospheric circulation and smaller‐scale hydrological processes, which can be accomplished within a coupled climate modeling framework. Here we present the water isotope‐enabled version of the Community Earth System Model version 1 (iCESM1), which simulates global variations in water isotopic ratios in the atmosphere, land, ocean, and sea ice. In a transient Last Millennium simulation covering the 850–2005 period, iCESM1 correctly captures the late‐twentieth‐century structure of δ¹⁸O and δD over the global oceans, with more limited accuracy over land. The relationship between salinity and seawater δ¹⁸O is also well represented over the observational period, including interbasin variations. We illustrate the utility of coupled, isotope‐enabled simulations using both Last Millennium simulations and freshwater hosing experiments with iCESM1. Closing the isotopic mass balance between all components of the coupled model provides new confidence in the underlying depiction of the water cycle in CESM, while also highlighting areas where the underlying hydrologic balance can be improved. The iCESM1 is poised to be a vital community resource for ongoing model development with both modern and paleoclimate applications.

    

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5. Enhanced vertical mixing in the glacial ocean inferred from sedimentary carbon isotopes

   https://doi.org/10.1038/s43247-021-00239-y  

   Wilmes, Sophie-Berenice ; Green, J. A. Mattias ; Schmittner, Andreas ( August 2021 , Communications Earth & Environment)

   Reconstructing the circulation, mixing and carbon content of the Last Glacial Maximum ocean remains challenging. Recent hypotheses suggest that a shoaled Atlantic meridional overturning circulation or increased stratification would have reduced vertical mixing, isolated the abyssal ocean and increased carbon storage, thus contributing to lower atmospheric CO₂concentrations. Here, using an ensemble of ocean simulations, we evaluate impacts of changes in tidal energy dissipation due to lower sea levels on ocean mixing, circulation, and carbon isotope distributions. We find that increased tidal mixing strengthens deep ocean flow rates and decreases vertical gradients of radiocarbon andδ¹³C in the deep Atlantic. Simulations with a shallower overturning circulation and more vigorous mixing fit sediment isotope data best. Our results, which are conservative, provide observational support that vertical mixing in the glacial Atlantic may have been enhanced due to more vigorous tidal dissipation, despite shoaling of the overturning circulation and increases in stratification.

    

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