Southern Ocean controls of the vertical marine δ13C gradient – a modelling study

δ 13 C, the standardised 13C/12C ratio expressed in per mille, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO2, and the biological carbon pump on timescales ranging from decades to tens of millions of years. δ13C data derived from ocean sediment core analysis provide information on δ 13 C of dissolved inorganic carbon and the vertical δ13C gradient (i.e. Dδ13C) in past oceans. In order to correctly interpret δ13C and Dδ13C variations, a good under standing is needed of the influence from ocean circulation, air–sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that Dδ13C in all ocean basins is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct a set of idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the Southern and Global Ocean on atmospheric δ13C, pCO2, and marine δ13C and Dδ13C. The experiments cover changes in air–sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency in an unchanged ocean circulation field. Our experiments show that global mean Dδ13C varies by up to about ±0.35 ‰ around the pre-industrial model reference (1.2 ‰) in response to biogeochemical change. The amplitude of this sensitivity can be larger at smaller scales, as seen from a maximum sensitivity of about −0.6 ‰ on ocean basin scale. The ocean’s oldest water (North Pacific) responds most to biological changes, the young deep water (North Atlantic) responds strongly to air–sea gas exchange changes, and the vertically well-mixed water (SO) has a low or even reversed Dδ13C sensitivity compared to the other basins. This local Dδ13C sensitivity depends on the local thermodynamic dis- equilibrium and the Dδ13C sensitivity to local POC export production changes. The direction of both glacial (intensi- fication of Dδ13C) and interglacial (weakening of Dδ13C) Dδ13C change matches the direction of the sensitivity of bio-geochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Dδ13C, in addition to changes in ocean circulation.publishedVersio

Impact of deoxygenation and warming on global marine species in the 21st century

Ocean temperature and dissolved oxygen shape marine habitats in interplay with species&rsquo; physiological characteristics. Therefore, the observed and projected warming and deoxygenation in the 21st century of the world&rsquo;s oceans may strongly affect species&rsquo; habitats. Here, we implement an extended version of the Aerobic Growth Index (AGI), which quantifies whether a viable population of a species can be sustained in a particular location. We assess the impact of projected deoxygenation and warming on the contemporary habitat of 47 representative marine species covering the epipelagic, mesopelagic/bathypelagic, and demersal realms. AGI is calculated for these species for the historical period and into the 21st century using bias-corrected environmental data from six comprehensive Earth System Models. While habitat viability decreases nearly everywhere with global warming, impact of this decrease is strongly species-dependent. Most species lose less than 5 % of their contemporary habitat volume over the 21st century even at 3 &deg;C of global warming relative to preindustrial, although some individual species are projected to incur losses 2&ndash;3 times greater than that. We find that the contemporary spatiotemporal variability of O2 and temperature (and hence AGI) provides a quantifiable measure of a species&rsquo; vulnerability to change. Species&rsquo; vulnerability is the most important indicator for large (&gt;5 %) potential habitat losses &ndash; not relative or absolute changes in habitat viability (i.e., AGIrel or &Delta;AGI), temperature or O2. Loss of contemporary habitat is for most epipelagic species driven by warming of ocean water and is therefore expanded with increased levels of global warming. In the mesopelagic/bathypelagic and demersal realms habitat loss is also affected by pO2 decrease for some species. Our analysis is constrained by the uncertainties involved in species-specific critical thresholds, which we quantify, by data limitations on 3D species distributions as well as by high uncertainty in model O2 projections in equatorial regions. Focus on these topics in future research will strengthen our confidence in assessing climate-change driven losses of contemporary habitat across the global oceans.</p

Ocean biogeochemistry in the Norwegian Earth System Model version 2 (NorESM2)

The ocean carbon cycle is a key player in the climate system through its role in regulating the atmospheric carbon dioxide concentration and other processes that alter the Earth's radiative balance. In the second version of the Norwegian Earth System Model (NorESM2), the oceanic carbon cycle component has gone through numerous updates that include, amongst others, improved process representations, increased interactions with the atmosphere, and additional new tracers. Oceanic dimethyl sulfide (DMS) is now prognostically simulated and its fluxes are directly coupled with the atmospheric component, leading to a direct feedback to the climate. Atmospheric nitrogen deposition and additional riverine inputs of other biogeochemical tracers have recently been included in the model. The implementation of new tracers such as “preformed” and “natural” tracers enables a separation of physical from biogeochemical drivers as well as of internal from external forcings and hence a better diagnostic of the simulated biogeochemical variability. Carbon isotope tracers have been implemented and will be relevant for studying long-term past climate changes. Here, we describe these new model implementations and present an evaluation of the model's performance in simulating the observed climatological states of water-column biogeochemistry and in simulating transient evolution over the historical period. Compared to its predecessor NorESM1, the new model's performance has improved considerably in many aspects. In the interior, the observed spatial patterns of nutrients, oxygen, and carbon chemistry are better reproduced, reducing the overall model biases. A new set of ecosystem parameters and improved mixed layer dynamics improve the representation of upper-ocean processes (biological production and air–sea CO2 fluxes) at seasonal timescale. Transient warming and air–sea CO2 fluxes over the historical period are also in good agreement with observation-based estimates. NorESM2 participates in the Coupled Model Intercomparison Project phase 6 (CMIP6) through DECK (Diagnostic, Evaluation and Characterization of Klima) and several endorsed MIP simulations.publishedVersio

The role of the Southern Ocean in past global biogeochemical cycling

This thesis describes several recent advances on the role of the Southern Ocean in past global biogeochemical cycling. We focus on the ocean of the Last Glacial Maximum (LGM) and the Pleistocene epoch and apply proxies of long-term climate variability (particularly the deep-sea sediment records of δ$^{13}$C and δ$^{18}$O). Specifically, we aim to explore how the physical and biogeochemical state of the Southern Ocean influenced past global marine tracer distributions, such that we can better interpret proxy data and improve our understanding of the drivers of long-term climate variability. The focus on the ocean realm is motivated by the large carbon reservoir in the (deep) ocean, which is able to interact with the atmosphere and govern atmospheric pCO$_2$ on millennial timescales – particularly through Southern Ocean processes. The LGM and Pleistocene represent the most recent glacial extreme and glacial-interglacial cycles, respectively. Therefore, relatively many proxy data are available, and their recorded climate variability is likely indicative of long-term natural climate variability. We applied global ocean models of different complexities (NorESM-OC, HAMOCC2s and TMI) to study the drivers that shape the benthic δ$^{13}$C and δ$^{18}$O records. Applying these, we studied the role of the Southern Ocean in shaping vertical marine δ$^{13}$C gradients (Paper I), as well as its contribution to the δ$^{18}$O archive of glacial-interglacial cycles (Paper IV) using idealized model experiments. Besides these, the LGM ocean and its circulation and biological changes are studied to reveal and explore their relative importance in a more complex model setup (NorESM-OC; Papers II and III). A central conclusion of this thesis is that knowledge of the relevant water mass end-member characteristics is fundamental for interpretation of the benthic δ$^{13}$C and δ$^{18}$O records. We show that Southern source waters (waters originating in the Southern Ocean) have a particularly large potential to influence the records of both δ$^{13}$C and δ$^{18}$O, through changes in the biogeochemical or physical state of the Southern Ocean. We find that biogeochemical changes in the Southern Ocean (of particularly air-sea gas exchange and nutrient utilisation) have the potential to affect δ$^{13}$C globally and with a magnitude relevant for global δ$^{13}$C deep-sea records (Paper I). Such major biogeochemical changes are indeed implied by the estimated near doubling of the global mean biological carbon pump efficiency required to satisfy LGM proxy records of δ$^{13}$C, besides the changes in ocean circulation (Papers II and III). Last, Southern source water characteristics are also highly relevant for deep-sea δ$^{18}$O records – and have likely been incompletely archived due to interference with out-of-phase cyclic signals from Northern source water during the Early Pleistocene (Paper IV). These findings have direct implications for the interpretation of δ$^{13}$C and δ$^{18}$O records, as well as for (paleo)-modelling efforts of global climate. Regarding the first, we see the need for increased (interdisciplinary) efforts to constrain the drivers of long-term end-member variability (Papers I-IV) as well as the need for an improved understanding of what part of the end-member signal is recorded (Paper IV). Regarding (paleo)-modelling, we anticipate that only models that contain the processes and/or components that realistically change both ocean circulation and biogeochemistry will be able to simulate long-term climate variability in satisfactory agreement with (proxy) data. Furthermore, we note that these processes and/or components are currently not (fully) represented in Earth System Models

The role of the Southern Ocean in past global biogeochemical cycling

This thesis describes several recent advances on the role of the Southern Ocean in past global biogeochemical cycling. We focus on the ocean of the Last Glacial Maximum (LGM) and the Pleistocene epoch and apply proxies of long-term climate variability (particularly the deep-sea sediment records of δ$^{13}$C and δ$^{18}$O). Specifically, we aim to explore how the physical and biogeochemical state of the Southern Ocean influenced past global marine tracer distributions, such that we can better interpret proxy data and improve our understanding of the drivers of long-term climate variability. The focus on the ocean realm is motivated by the large carbon reservoir in the (deep) ocean, which is able to interact with the atmosphere and govern atmospheric pCO$_2$ on millennial timescales – particularly through Southern Ocean processes. The LGM and Pleistocene represent the most recent glacial extreme and glacial-interglacial cycles, respectively. Therefore, relatively many proxy data are available, and their recorded climate variability is likely indicative of long-term natural climate variability. We applied global ocean models of different complexities (NorESM-OC, HAMOCC2s and TMI) to study the drivers that shape the benthic δ$^{13}$C and δ$^{18}$O records. Applying these, we studied the role of the Southern Ocean in shaping vertical marine δ$^{13}$C gradients (Paper I), as well as its contribution to the δ$^{18}$O archive of glacial-interglacial cycles (Paper IV) using idealized model experiments. Besides these, the LGM ocean and its circulation and biological changes are studied to reveal and explore their relative importance in a more complex model setup (NorESM-OC; Papers II and III). A central conclusion of this thesis is that knowledge of the relevant water mass end-member characteristics is fundamental for interpretation of the benthic δ$^{13}$C and δ$^{18}$O records. We show that Southern source waters (waters originating in the Southern Ocean) have a particularly large potential to influence the records of both δ$^{13}$C and δ$^{18}$O, through changes in the biogeochemical or physical state of the Southern Ocean. We find that biogeochemical changes in the Southern Ocean (of particularly air-sea gas exchange and nutrient utilisation) have the potential to affect δ$^{13}$C globally and with a magnitude relevant for global δ$^{13}$C deep-sea records (Paper I). Such major biogeochemical changes are indeed implied by the estimated near doubling of the global mean biological carbon pump efficiency required to satisfy LGM proxy records of δ$^{13}$C, besides the changes in ocean circulation (Papers II and III). Last, Southern source water characteristics are also highly relevant for deep-sea δ$^{18}$O records – and have likely been incompletely archived due to interference with out-of-phase cyclic signals from Northern source water during the Early Pleistocene (Paper IV). These findings have direct implications for the interpretation of δ$^{13}$C and δ$^{18}$O records, as well as for (paleo)-modelling efforts of global climate. Regarding the first, we see the need for increased (interdisciplinary) efforts to constrain the drivers of long-term end-member variability (Papers I-IV) as well as the need for an improved understanding of what part of the end-member signal is recorded (Paper IV). Regarding (paleo)-modelling, we anticipate that only models that contain the processes and/or components that realistically change both ocean circulation and biogeochemistry will be able to simulate long-term climate variability in satisfactory agreement with (proxy) data. Furthermore, we note that these processes and/or components are currently not (fully) represented in Earth System Models

A Last Glacial Maximum forcing dataset for ocean modelling

Model simulations of the Last Glacial Maximum (LGM; ∼ 21 000 years before present) can aid the interpretation of proxy records, can help to gain an improved mechanistic understanding of the LGM climate system, and are valuable for the evaluation of model performance in a different climate state. Ocean-ice only model configurations forced by prescribed atmospheric data (referred to as “forced ocean models”) drastically reduce the computational cost of palaeoclimate modelling compared to fully coupled model frameworks. While feedbacks between the atmosphere and ocean and sea-ice compartments of the Earth system are not present in such model configurations, many scientific questions can be addressed with models of this type. Our dataset supports simulations of the LGM in a forced ocean model set-up while still taking advantage of the complexity of fully coupled model set-ups. The data presented here are derived from fully coupled palaeoclimate simulations of the Palaeoclimate Modelling Intercomparison Project phase 3 (PMIP3). The data are publicly accessible at the National Infrastructure for Research Data (NIRD) Research Data Archive at https://doi.org/10.11582/2020.00052 (Morée and Schwinger, 2020). They consist of 2-D anomaly forcing fields suitable for use in ocean models that employ a bulk forcing approach and are optimized for use with CORE forcing fields. The data include specific humidity, downwelling long-wave and short-wave radiation, precipitation, wind (v and u components), temperature, and sea surface salinity (SSS). All fields are provided as climatological mean anomalies between LGM and pre-industrial (PI) simulations. These anomaly data can therefore be added to any pre-industrial ocean forcing dataset in order to obtain forcing fields representative of LGM conditions as simulated by PMIP3 models. Furthermore, the dataset can be easily updated to reflect results from upcoming and future palaeo-model intercomparison activities

Southern Ocean controls of the vertical marine δ13C gradient – a modelling study

δ 13 C, the standardised 13C/12C ratio expressed in per mille, is a widely used ocean tracer to study changes in ocean circulation, water mass ventilation, atmospheric pCO2, and the biological carbon pump on timescales ranging from decades to tens of millions of years. δ13C data derived from ocean sediment core analysis provide information on δ 13 C of dissolved inorganic carbon and the vertical δ13C gradient (i.e. Dδ13C) in past oceans. In order to correctly interpret δ13C and Dδ13C variations, a good under standing is needed of the influence from ocean circulation, air–sea gas exchange and biological productivity on these variations. The Southern Ocean is a key region for these processes, and we show here that Dδ13C in all ocean basins is sensitive to changes in the biogeochemical state of the Southern Ocean. We conduct a set of idealised sensitivity experiments with the ocean biogeochemistry general circulation model HAMOCC2s to explore the effect of biogeochemical state changes of the Southern and Global Ocean on atmospheric δ13C, pCO2, and marine δ13C and Dδ13C. The experiments cover changes in air–sea gas exchange rates, particulate organic carbon sinking rates, sea ice cover, and nutrient uptake efficiency in an unchanged ocean circulation field. Our experiments show that global mean Dδ13C varies by up to about ±0.35 ‰ around the pre-industrial model reference (1.2 ‰) in response to biogeochemical change. The amplitude of this sensitivity can be larger at smaller scales, as seen from a maximum sensitivity of about −0.6 ‰ on ocean basin scale. The ocean’s oldest water (North Pacific) responds most to biological changes, the young deep water (North Atlantic) responds strongly to air–sea gas exchange changes, and the vertically well-mixed water (SO) has a low or even reversed Dδ13C sensitivity compared to the other basins. This local Dδ13C sensitivity depends on the local thermodynamic dis- equilibrium and the Dδ13C sensitivity to local POC export production changes. The direction of both glacial (intensi- fication of Dδ13C) and interglacial (weakening of Dδ13C) Dδ13C change matches the direction of the sensitivity of bio-geochemical processes associated with these periods. This supports the idea that biogeochemistry likely explains part of the reconstructed variations in Dδ13C, in addition to changes in ocean circulation

Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using d13C

Although both physical and biological marine changes are required to explain the 100 ppm lower atmospheric pCO2 of the Last Glacial Maximum (LGM, ∼21 ka) as compared to preindustrial (PI) times, their exact contributions are debated. Proxies of past marine carbon cycling (such as δ13C) document these changes and thus provide constraints for quantifying the drivers of long-term carbon cycle variability. This modeling study discusses the physical and biological changes in the ocean needed to simulate an LGM ocean in satisfactory agreement with proxy data, here focusing especially on δ13C. We prepared a PI and LGM equilibrium simulation using the ocean model NorESM-OC with full biogeochemistry (including the carbon isotopes δ13C and radiocarbon) and dynamic sea ice. The modeled LGM–PI differences are evaluated against a wide range of physical and biogeochemical proxy data and show agreement for key aspects of the physical ocean state within the data uncertainties. However, the lack of a simulated increase of regenerated nutrients for the LGM indicates that additional biogeochemical changes are required to simulate an LGM ocean in agreement with proxy data. In order to examine these changes, we explore the potential effects of different global mean biological pump efficiencies on the simulated marine biogeochemical tracer distributions. Through estimating which biological pump efficiency reduces LGM model–proxy biases the most, we estimate that the global mean biological pump efficiency increased from 38 % (PI) to up to 75 % (LGM). The drivers of such an increase in the biological pump efficiency may be both biological and related to circulation changes that are incompletely captured by our model – such as stronger isolation of Southern Source Water. Finally, even after considering a 75 % biological pump efficiency in the LGM ocean, a remaining model–proxy error in δ13C exists that is 0.07 ‰ larger than the 0.19 ‰ data uncertainty. This error indicates that additional changes in ocean dynamics are needed to simulate an LGM ocean in agreement with proxy data.publishedVersio

Evaluating the biological pump efficiency of the Last Glacial Maximum ocean using δ13C

Although both physical and biological marine changes are required to explain the 100 ppm lower atmospheric pCO2 of the Last Glacial Maximum (LGM, ∼21 ka) as compared to preindustrial (PI) times, their exact contributions are debated. Proxies of past marine carbon cycling (such as δ13C) document these changes and thus provide constraints for quantifying the drivers of long-term carbon cycle variability. This modeling study discusses the physical and biological changes in the ocean needed to simulate an LGM ocean in satisfactory agreement with proxy data, here focusing especially on δ13C. We prepared a PI and LGM equilibrium simulation using the ocean model NorESM-OC with full biogeochemistry (including the carbon isotopes δ13C and radiocarbon) and dynamic sea ice. The modeled LGM–PI differences are evaluated against a wide range of physical and biogeochemical proxy data and show agreement for key aspects of the physical ocean state within the data uncertainties. However, the lack of a simulated increase of regenerated nutrients for the LGM indicates that additional biogeochemical changes are required to simulate an LGM ocean in agreement with proxy data. In order to examine these changes, we explore the potential effects of different global mean biological pump efficiencies on the simulated marine biogeochemical tracer distributions. Through estimating which biological pump efficiency reduces LGM model–proxy biases the most, we estimate that the global mean biological pump efficiency increased from 38 % (PI) to up to 75 % (LGM). The drivers of such an increase in the biological pump efficiency may be both biological and related to circulation changes that are incompletely captured by our model – such as stronger isolation of Southern Source Water. Finally, even after considering a 75 % biological pump efficiency in the LGM ocean, a remaining model–proxy error in δ13C exists that is 0.07 ‰ larger than the 0.19 ‰ data uncertainty. This error indicates that additional changes in ocean dynamics are needed to simulate an LGM ocean in agreement with proxy data

Cancellation of the precessional cycle in δ18O records during the Early Pleistocene

The dominant pacing of glacial‐interglacial cycles in deep‐ocean δ18O records changed substantially during the Mid‐Pleistocene Transition. The precessional cycle (∼23 ky) is absent during the Early Pleistocene, which we show can be explained by cancellation of the hemispherically anti‐phased precessional cycle in the Early Pleistocene interior ocean. Such cancellation develops due to mixing of North Atlantic and Southern Ocean δ18O signals at depth, and shows characteristic spatial patterns. We explore the cancellation potential for different North Atlantic and Southern Ocean deep‐water source δ18O values using a tracer transport ocean model. Cancellation of precession occurs for all signal strengths and is widespread for a signal strength typical for the Early Pleistocene. Early Pleistocene precessional power is therefore likely incompletely archived in deep‐sea δ18O records, concealing the true periodicity of the glacial cycles in the two hemispheres