Zhang Jiaxu

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Zhang
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Jiaxu
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  • Preprint
    Asynchronous warming and δ18O evolution of deep Atlantic water masses during the last deglaciation
    ( 2017-08-21) Zhang, Jiaxu ; Liu, Zhengyu ; Brady, Esther C. ; Oppo, Delia W. ; Clark, Peter U. ; Jahn, Alexandra ; Marcott, Shaun A. ; Lindsay, Keith
    The large-scale reorganization of deep-ocean circulation in the Atlantic involving changes in North Atlantic Deep Water (NADW) and Antarctic Bottom Water (AABW) played a critical role in regulating hemispheric and global climate during the last deglaciation. However, changes in the relative contributions of NADW and AABW and their properties are poorly constrained by marine records, including δ18O of benthic foraminiferal calcite (δ18Oc). Here we use an isotope-enabled ocean general circulation model with realistic geometry and forcing conditions to simulate the deglacial water mass and δ18O evolution. Model results suggest that in response to North Atlantic freshwater forcing during the early phase of the last deglaciation, NADW nearly collapses while AABW mildly weakens. Rather than reflecting changes in NADW or AABW properties due to freshwater input as suggested previously, the observed phasing difference of deep δ18Oc likely reflects early warming of the deep northern North Atlantic by ~1.4°C while deep Southern Ocean temperature remains largely unchanged. We propose a thermodynamic mechanism to explain the early warming in the North Atlantic, featuring a strong mid-depth warming and enhanced downward heat flux via vertical mixing. Our results emphasize that the way ocean circulation affects heat, a dynamic tracer, is considerably different than how it affects passive tracers like δ18O, and call for caution when inferring water mass changes from δ18Oc records while assuming uniform changes in deep temperatures.
  • Article
    Remineralization dominating the δ13 C decrease in the mid-depth Atlantic during the last deglaciation
    (Elsevier, 2021-07-20) Gu, Sifan ; Liu, Zhengyu ; Oppo, Delia W. ; Lynch-Stieglitz, Jean ; Jahn, Alexandra ; Zhang, Jiaxu ; Lindsay, Keith ; Wu, Lixin
    δ 13 C records from the mid-depth Atlantic show a pronounced decrease during the Heinrich Stadial 1 (HS1), a deglacial episode of dramatically weakened Atlantic Meridional Ocean Circulation (AMOC). Proposed explanations for this mid-depth decrease include a greater fraction of δ 13 C -depleted southern sourced water (SSW), a δ 13 C decrease in the North Atlantic Deep Water (NADW) end-member, and accumulation of the respired organic carbon. However, the relative importance of these proposed mechanisms cannot be quantitatively constrained from current available observations alone. Here we diagnose the individual contributions to the deglacial Atlantic mid-depth δ 13 C change from these mechanisms using a transient simulation with carbon isotopes and idealized tracers. We find that although the fraction of the low- δ 13 C SSW increases in response to a weaker AMOC during HS1, the water mass mixture change only plays a minor role in the mid-depth Atlantic δ 13 C decrease. Instead, increased remineralization due to the AMOC-induced mid-depth ocean ventilation decrease is the dominant cause. In this study, we differentiate between the deep end-members, which are assigned to deep water regions used in previous paleoceanography studies, and the surface end-members, which are from the near-surface water defined from the physical origin of deep water masses. We find that the deep NADW end-member includes additional remineralized material accumulated when sinking from the surface (surface NADW end-member). Therefore, the surface end-members should be used in diagnosing mechanisms of changes. Furthermore, our results suggest that remineralization in the surface end-member is more critical than the remineralization along the transport pathway from the near-surface formation region to the deep ocean, especially during the early deglaciation.
  • Article
    Coherent response of Antarctic Intermediate Water and Atlantic meridional overturning circulation during the last deglaciation : reconciling contrasting neodymium isotope reconstructions from the tropical Atlantic
    (John Wiley & Sons, 2017-10-24) Gu, Sifan ; Liu, Zhengyu ; Zhang, Jiaxu ; Rempfer, Johannes ; Joos, Fortunat ; Oppo, Delia W.
    Antarctic Intermediate Water (AAIW) plays important roles in the global climate system and the global ocean nutrient and carbon cycles. However, it is unclear how AAIW responds to global climate changes. In particular, neodymium isotopic composition (εNd) reconstructions from different locations from the tropical Atlantic have led to a debate on the relationship between northward penetration of AAIW into the tropical Atlantic and the Atlantic meridional overturning circulation (AMOC) variability during the last deglaciation. We resolve this controversy by studying the transient oceanic evolution during the last deglaciation using a neodymium-enabled ocean model. Our results suggest a coherent response of AAIW and AMOC: when AMOC weakens, the northward penetration and transport of AAIW decrease while its depth and thickness increase. Our study highlights that as part of the return flow of the North Atlantic Deep Water, the northward penetration of AAIW in the Atlantic is determined predominately by AMOC intensity. Moreover, the inconsistency among different tropical Atlantic εNd reconstructions is reconciled by considering their corresponding core locations and depths, which were influenced by different water masses in the past. The very radiogenic water from the bottom of the Gulf of Mexico and the Caribbean Sea, which was previously overlooked in the interpretations of deglacial εNd variability, can be transported to shallow layers during active AMOC and modulates εNd in the tropical Atlantic. Changes in the AAIW core depth must also be considered. Thus, interpretation of εNd reconstructions from the tropical Atlantic is more complicated than suggested in previous studies.
  • Article
    Assessing the potential capability of reconstructing glacial Atlantic water masses and AMOC using multiple proxies in CESM
    (Elsevier, 2020-05-06) Gu, Sifan ; Liu, Zhengyu ; Oppo, Delia W. ; Lynch-Stieglitz, Jean ; Jahn, Alexandra ; Zhang, Jiaxu ; Wu, Lixin
    Reconstructing the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) is essential for understanding glacial-interglacial climate change and the carbon cycle. However, despite many previous studies, uncertainties remain regarding the glacial water mass distributions in the Atlantic and the AMOC intensity. Here we use an isotope enabled ocean model with multiple geotracers (δ 13 C,E Νd,231 Pa/ 230Th,δ 18 Ο and Δ 14 C) and idealized water tracers to study the potential constraints on LGM ocean circulation from multiple proxies. Our model suggests that the glacial Atlantic water mass distribution can be accurately constrained by the air-sea gas exchange signature of water masses (δ13 C AS), but E Nd might overestimate the North Atlantic Deep Water (NADW) percentage in the deep Atlantic probably because of the boundary source of Nd. A sensitivity experiment with an AMOC of similar geometry but much weaker strength suggests that the correct AMOC geometry is more important than the AMOC strength for simulating the observed glacial δ13 C AS and E Nd and distributions. The kinematic tracer 231Pa/230Th is sensitive to AMOC intensity, but the interpretation might be complicated by the AMOC geometry and AABW transport changes during the LGM. δ 18 Ο in the benthic foraminifera (δ 18 Οc) from the Florida Straits provides a consistent measure of the upper ocean boundary current in the model, which potentially provides an unambiguous method to reconstruct glacial AMOC intensity. Finally, we propose that the moderate difference between AMOC intensity at LGM and PD, if any, is caused by the competition of the responses to CO2 forcing and continental ice sheet forcing.