ORCID Profile
0000-0001-7142-882X
Current Organisation
University of Tasmania
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Publisher: Springer Science and Business Media LLC
Date: 10-10-2019
DOI: 10.1038/S41467-019-12549-Z
Abstract: Roughly a third (~30 ppm) of the carbon dioxide (CO 2 ) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO 2 is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained. We show that an Fe-induced stimulation of dinitrogen (N 2 ) fixation can induce a low latitude drawdown of 7–16 ppm CO 2 . This mechanism involves a closer coupling between N 2 fixers and denitrifiers that alleviates widespread nitrate limitation. Consequently, phosphate utilisation and carbon export increase near upwelling zones, causing deoxygenation and deeper carbon injection. Furthermore, this low latitude mechanism reproduces the regional patterns of organic δ 15 N deposited in glacial sediments. The positive response of marine N 2 fixation to dusty ice age conditions, first proposed twenty years ago, therefore compliments high latitude changes to lify CO 2 drawdown.
Publisher: Springer Science and Business Media LLC
Date: 09-10-2022
DOI: 10.1007/S13280-021-01635-6
Abstract: Nitrogen stable isotopes (δ 15 N) are used to study food web and foraging dynamics due to the step-wise enrichment of tissues with increasing trophic level, but they rely on the isoscape baseline that varies markedly in the Arctic due to the interplay between Atlantic- and Pacific-origin waters. Using a hierarchy of simulations with a state-of-the-art ocean-biogeochemical model, we demonstrate that the canonical isotopic gradient of 2–3‰ between the Pacific and Atlantic sectors of the Arctic Ocean has grown to 3–4‰ and will continue to expand under a high emissions climate change scenario by the end of the twenty-first century. δ 15 N increases in the Pacific-influenced high Arctic due to increased primary production, while Atlantic sector decreases result from the integrated effects of Atlantic inflow and anthropogenic inputs. While these trends will complicate longitudinal food web studies using δ 15 N, they may aid those focussed on movement as the Arctic isoscape becomes more regionally distinct.
Publisher: Springer Science and Business Media LLC
Date: 13-06-2015
Publisher: Copernicus GmbH
Date: 14-02-2019
Publisher: Copernicus GmbH
Date: 07-07-2019
Publisher: Copernicus GmbH
Date: 17-03-2009
Abstract: Abstract. The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial–interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate. Here we present simulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL (Carbon–Ocean–Atmosphere–Land) earth system model to test the contribution of physical and biogeochemical processes to ocean carbon storage. For the LGM simulation, we find a significant global cooling of the surface ocean (3.2 °C) and the expansion of both minimum and maximum sea ice cover broadly consistent with proxy reconstructions. The glacial ocean stores an additional 267 Pg C in the deep ocean relative to the pre-industrial (PI) simulation due to stronger Antarctic Bottom Water formation. However, 889 Pg C is lost from the upper ocean via equilibration with a lower atmospheric CO2 concentration and a global decrease in export production, causing a net loss of carbon relative to the PI ocean. The LGM deep ocean also experiences an oxygenation ( 100 mmol O2 m−3) and deepening of the calcite saturation horizon (exceeds the ocean bottom) at odds with proxy reconstructions. With modifications to key biogeochemical processes, which include an increased export of organic matter due to a simulated release from iron limitation, a deepening of remineralisation and decreased inorganic carbon export driven by cooler temperatures, we find that the carbon content of the glacial ocean can be sufficiently increased (317 Pg C) to explain the reduction in atmospheric and terrestrial carbon at the LGM (194 ± 2 and 330 ± 400 Pg C, respectively). Assuming an LGM–PI difference of 95 ppm pCO2, we find that 55 ppm can be attributed to the biological pump, 28 ppm to circulation changes and the remaining 12 ppm to solubility. The biogeochemical modifications also improve model–proxy agreement in export production, carbonate chemistry and dissolved oxygen fields. Thus, we find strong evidence that variations in the oceanic biological pump exert a primary control on the climate.
Publisher: American Geophysical Union (AGU)
Date: 25-10-2021
DOI: 10.1029/2021GL094797
Abstract: Ocean deoxygenation is an emerging hazard for marine ecosystems and a fingerprint of anthropogenic change. Interior ocean oxygen concentrations respond to changes in ventilation that supply oxygen and the demand of biogeochemical processes that consume oxygen. A better understanding of their regional importance would improve confidence in Earth System Model projections, which underpin ecosystem risk and vulnerability assessments. Using a hindcast reanalysis simulation, we find that oxygen trends between 1975 and 2014 in low‐oxygen zones along eastern margins are strongly affected by biogeochemical processes that alter oxygen demand, while oxygen‐rich regions of the open ocean are driven by ventilation changes. A similar regional distinction emerges among CMIP6 Earth System Models. Therefore, while biogeochemical functioning is an important source of uncertainty in low‐latitude, low‐oxygen regions, uncertainty in global trends is due to insufficient physical supply changes in Earth System Models, which is confirmed using repeat hydrographic section data in the southern mid‐latitudes.
Publisher: Wiley
Date: 26-07-2023
DOI: 10.1111/GCB.16891
Abstract: Climate change is one of the top three global threats to seabirds, particularly species that visit polar regions. Arctic terns migrate between both polar regions annually and rely on productive marine areas to forage, on sea ice for rest and foraging, and prevailing winds during flight. Here, we report 21st‐century trends in environmental variables affecting arctic terns at key locations along their Atlantic/Indian Ocean migratory flyway during the non‐breeding seasons, identified through tracking data. End‐of‐century climate change projections were derived from Earth System Models and multi‐model means calculated in two Shared Socioeconomic Pathways: ‘middle‐of‐the‐road’ and ‘fossil‐fuelled development’ scenarios. Declines in North Atlantic primary production emerge as a major impact to arctic terns likely to affect their foraging during the 21st century under a ‘fossil‐fuelled development’ scenario. Minimal changes are, however, projected at three other key regions visited by arctic terns (Benguela Upwelling, Subantarctic Indian Ocean and the Southern Ocean). Southern Ocean sea ice extent is likely to decline, but the magnitude of change and potential impacts on tern survival are uncertain. Small changes ( m s −1 ) in winds are projected in both scenarios, but with minimal likely impacts on migration routes and duration. However, Southern Ocean westerlies are likely to strengthen and contract closer to the continent, which may require arctic terns to shift routes or flight strategies. Overall, we find minor effects of climate change on the migration of arctic terns, with the exception of poorer foraging in the North Atlantic. However, given that arctic terns travel over huge spatial scales and live for decades, they integrate minor changes in conditions along their migration routes such that the sum effect may be greater than the parts. Meeting carbon emission targets is vital to slow these end‐of‐century climatic changes and minimise extinction risk for a suite of polar species.
Publisher: Copernicus GmbH
Date: 23-03-2020
DOI: 10.5194/EGUSPHERE-EGU2020-19451
Abstract: & & Southern Ocean mode and intermediate waters supply nitrate-rich but silicate-poor waters to the lower latitudes, impeding diatom growth throughout the extra-polar ocean and weakening the ocean& #8217 s ability to absorb carbon dioxide from the atmosphere. This silicate deficiency is widely attributed to high silicate to nitrate uptake by iron-limited diatoms. Here, we show that nitrification, by rapidly regenerating nitrate in shallow waters, drives the silicate deficiency. Measurements of nitrate dual isotopes and complementary modelling independently suggest that 15-35% of the nitrate within mode waters is generated by nitrification. Our results reveal that without nitrification, the silicate deficiency would disappear, which would allow the diatomaceous niche to expand. Nitrification therefore provides a key buffering service that mitigates against change in the silicate deficit and subsequently restricts diatom dominance to the polar ocean. This insight highlights the critical importance for understanding Southern Ocean processes, such that the large-scale effects of ongoing environmental change may be realised.& &
Publisher: Springer Science and Business Media LLC
Date: 28-10-2021
DOI: 10.1038/S41467-021-26552-W
Abstract: The open ocean nitrogen cycle is being altered by increases in anthropogenic atmospheric nitrogen deposition and climate change. How the nitrogen cycle responds will determine long-term trends in net primary production (NPP) in the nitrogen-limited low latitude ocean, but is poorly constrained by uncertainty in how the source-sink balance will evolve. Here we show that intensifying nitrogen limitation of phytoplankton, associated with near-term reductions in NPP, causes detectable declines in nitrogen isotopes (δ 15 N) and constitutes the primary perturbation of the 21 st century nitrogen cycle. Model experiments show that ~75% of the low latitude twilight zone develops anomalously low δ 15 N by 2060, predominantly due to the effects of climate change that alter ocean circulation, with implications for the nitrogen source-sink balance. Our results highlight that δ 15 N changes in the low latitude twilight zone may provide a useful constraint on emerging changes to nitrogen limitation and NPP over the 21 st century.
Publisher: Copernicus GmbH
Date: 15-02-2019
Publisher: Wiley
Date: 03-2022
DOI: 10.1111/GCB.16138
Abstract: Multiple environmental forcings, such as warming and changes in ocean circulation and nutrient supply, are affecting the base of Arctic marine ecosystems, with cascading effects on the entire food web through bottom‐up control. Stable nitrogen isotopes (δ 15 N) can be used to detect and unravel the impact of these forcings on this unique ecosystem, if the many processes that affect the δ 15 N values are constrained. Combining unique 60‐year records from compound specific δ 15 N biomarkers on harp seal teeth alongside state‐of‐the‐art ocean modelling, we observed a significant decline in the δ 15 N values at the base of the Barents Sea food web from 1951 to 2012. This strong and persistent decadal trend emerges due to the combination of anthropogenic atmospheric nitrogen deposition in the Atlantic, increased northward transport of Atlantic water through Arctic gateways and local feedbacks from increasing Arctic primary production. Our results suggest that the Arctic ecosystem has been responding to anthropogenically induced local and remote drivers, linked to changing ocean biology, chemistry and physics, for at least 60 years. Accounting for these trends in δ 15 N values at the base of the food web is essential to accurately detect ecosystem restructuring in this rapidly changing environment.
Publisher: Copernicus GmbH
Date: 14-02-2019
Publisher: Copernicus GmbH
Date: 20-02-2019
Publisher: Copernicus GmbH
Date: 27-11-2018
DOI: 10.5194/GMD-2018-225
Abstract: Abstract. The isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ13C and δ15N in the ocean component of an Earth system model. We evaluate our simulated δ13C and δ15N against contemporary measurements, place the model's performance alongside other isotope enabled models, and document the response of δ13C and δ15N to changes in ecosystem functioning. The model combines the Commonwealth Scientific and Industrial Research Organisation Mark 3L (CSIRO Mk3L) climate system model with the Carbon of the Ocean, Atmosphere and Land (COAL) biogeochemical model. The oceanic component of CSIRO Mk3L-COAL has a resolution of 1.6° latitude × 2.8° longitude and resolves multi-millennial timescales, running at a rate of ∼400 years per day. We show that this coarse resolution, computationally efficient model adequately reproduces water column and coretop δ13C and δ15N measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCO3 production based on calcite saturation state, and varying N2 fixation via iron limitation. We find that large changes in CaCO3 production have little effect on δ13C and δ15N, while changes in N2 fixation and phytoplankton stoichiometry have substantial and complex effects. Interpretations of palaeoceanographic records are therefore open to multiple lines of interpretation where multiple processes imprint on the isotopic signature, such as in the tropics where denitrification, N2 fixation and nutrient utilisation influence δ15N. Hence, there is significant scope for isotope enabled models to provide more robust interpretations of the proxy records.
Publisher: American Geophysical Union (AGU)
Date: 04-2018
DOI: 10.1002/2017GB005753
Abstract: The biogeochemistry of the ocean exerts a strong influence on the climate by modulating atmospheric greenhouse gases. In turn, ocean biogeochemistry depends on numerous physical and biological processes that change over space and time. Accurately simulating these processes is fundamental for accurately simulating the ocean's role within the climate. However, our simulation of these processes is often simplistic, despite a growing understanding of underlying biological dynamics. Here we explore how new parameterizations of biological processes affect simulated biogeochemical properties in a global ocean model. We combine 6 different physical realizations with 6 different biogeochemical parameterizations (36 unique ocean states). The biogeochemical parameterizations, all previously published, aim to more accurately represent the response of ocean biology to changing physical conditions. We make three major findings. First, oxygen, carbon, alkalinity, and phosphate fields are more sensitive to changes in the ocean's physical state. Only nitrate is more sensitive to changes in biological processes, and we suggest that assessment protocols for ocean biogeochemical models formally include the marine nitrogen cycle to assess their performance. Second, we show that dynamic variations in the production, remineralization, and stoichiometry of organic matter in response to changing environmental conditions benefit the simulation of ocean biogeochemistry. Third, dynamic biological functioning reduces the sensitivity of biogeochemical properties to physical change. Carbon and nitrogen inventories were 50% and 20% less sensitive to physical changes, respectively, in simulations that incorporated dynamic biological functioning. These results highlight the importance of a dynamic biology for ocean properties and climate.
Publisher: American Geophysical Union (AGU)
Date: 09-2021
DOI: 10.1029/2021GB006961
Abstract: The hydrography of the Arctic Seas is being altered by ongoing climate change, with knock‐on effects to nutrient dynamics and primary production. As the major pathway of exchange between the Arctic and the Atlantic, the Fram Strait hosts two distinct water masses in the upper water column, northward flowing warm and saline Atlantic Waters in the east, and southward flowing cold and fresh Polar Surface Water in the west. Here, we assess how physical processes control nutrient dynamics in the Fram Strait using nitrogen isotope data collected during 2016 and 2018. In Atlantic Waters, a weakly stratified water column and a shallow nitracline reduce nitrogen limitation. To the west, in Polar Surface Water, nitrogen limitation is greater because stronger stratification inhibits nutrient resupply from deeper water and lateral nitrate supply from central Arctic waters is low. A historical hindcast simulation of ocean biogeochemistry from 1970 to 2019 corroborates these findings and highlights a strong link between nitrate supply to Atlantic Waters and the depth of winter mixing, which shoaled during the simulation in response to a local reduction in sea‐ice formation. Overall, we find that while the eastern Fram Strait currently experiences seasonal nutrient replenishment and high primary production, the loss of winter sea ice and continued atmospheric warming has the potential to inhibit deep winter mixing and limit primary production in the future.
Publisher: Cold Spring Harbor Laboratory
Date: 22-02-2023
DOI: 10.1101/2023.02.22.529547
Abstract: Anoxic marine zones (AMZs) are host to anaerobic metabolisms that drive losses of bioavailable nitrogen from the ocean. The discovery of active nitrite-oxidising bacteria (NOB), long thought to be obligately aerobic, in AMZs has altered our perception of how nitrogen cycles in these oxygen-deficient waters. Yet, why NOB succeed in AMZs remains unclear. Here, we show that obligately aerobic NOB can thrive alongside aerobic microheterotrophs in AMZs via infrequent intrusions of oxygen. Ecological theory, biogeochemical modelling and metagenome-based maximum growth rate estimates suggest that NOB are opportunists that take advantage of periodic oxygen intrusions to rapidly accumulate biomass. Rather than harsh, AMZs prone to oxygen intrusions appear optimal for NOB, whose abundance and activity peaks in a ‘goldilocks’ zone of periodic oxygen and high nitrite supply. Our results recast the intermediate disturbance hypothesis to AMZs and highlight how the nitrogen cycle relies on dynamic coexistence of aerobic and anaerobic metabolisms.
Publisher: Springer Science and Business Media LLC
Date: 22-06-2014
Publisher: Copernicus GmbH
Date: 11-07-2016
DOI: 10.5194/CP-2016-73
Abstract: Abstract. The ocean's ability to store large quantities of carbon, combined with the millennial longevity over which this reservoir is overturned, has implicated the ocean as a key driver of glacial-interglacial climates. However, the combination of processes that cause an accumulation of carbon within the ocean during glacial periods is still under debate. Here we present simulations of the Last Glacial Maximum (LGM) using the CSIRO Mk3L-COAL Earth System Model to test the contribution of physical and biogeochemical processes to ocean carbon storage. For the LGM simulation, we find a significant global cooling of the surface ocean (3.2 °C) and the expansion of both minimum (Northern Hemisphere: 105 % Southern Hemisphere: 225 %) and maximum (Northern Hemisphere: 145 % Southern Hemisphere: 120 %) sea ice cover broadly consistent with proxy reconstructions. Within the ocean, a significant reorganisation of the large-scale circulation and biogeochemical fields occurs. The LGM simulation stores an additional 322 Pg C in the deep ocean relative to the Pre-Industrial (PI) simulation, particularly due to a strengthening in Antarctic Bottom Water circulation. However, 839 Pg C is lost from the upper ocean via equilibration with a lower atmospheric CO2 concentration, causing a net loss of 517 Pg C relative to the PI simulation. The LGM deep ocean also experiences an oxygenation ( 100 mmol O2 m−3) and deepening of the aragonite saturation depth ( 2000 m deeper) at odds with proxy reconstructions. Hence, physical changes cannot in isolation produce plausible biogeochemistry nor the required drawdown of atmospheric CO2 of 80–100 ppm at the LGM. With modifications to key biogeochemical processes, which include an increased export of organic matter due to a simulated release from iron limitation, a deepening of remineralisation and decreased inorganic carbon export driven by cooler temperatures, we find that the carbon content in the glacial oceanic reservoir can be increased (326 Pg C) to a level that is sufficient to explain the reduction in atmospheric and terrestrial carbon at the LGM (520 ± 00 Pg C). These modifications also go some way to reconcile simulated export production, aragonite saturation state and oxygen fields with those that have been reconstructed by proxy measurements, thereby implicating changes in ocean biogeochemistry as an essential driver of the climate system.
Publisher: Proceedings of the National Academy of Sciences
Date: 04-01-2022
Abstract: Humans are exposed to toxic methylmercury mainly by consuming marine fish. New environmental policies under the Minamata Convention rely on a yet-poorly-known understanding of how mercury emissions translate into fish methylmercury levels. Here, we provide the first detailed map of mercury concentrations from skipjack tuna across the Pacific. Our study shows that the natural functioning of the global ocean has an important influence on tuna mercury concentrations, specifically in relation to the depth at which methylmercury concentrations peak in the water column. However, mercury inputs originating from anthropogenic sources are also detectable, leading to enhanced tuna mercury levels in the northwestern Pacific Ocean that cannot be explained solely by oceanic processes.
Publisher: Authorea, Inc.
Date: 09-12-2022
DOI: 10.22541/ESSOAR.167058932.27589471/V1
Abstract: Oceanic emissions of nitrous oxide (N2O) account for roughly one-third of all natural sources to the atmosphere. Hot-spots of N2O outgassing occur over oxygen minimum zones (OMZs), where the presence of steep oxygen gradients surrounding anoxic waters leads to enhanced N2O production from both nitrification and denitrification. However, the relative contributions from these pathways to N2O production and outgassing in these regions remains poorly constrained, in part due to shared intermediary nitrogen tracers, and the tight coupling of denitrification sources and sinks. To shed light on this problem, we embed a new, mechanistic model of the OMZ nitrogen cycle within a three-dimensional eddy-resolving physical-biogeochemical model of the ETSP, tracking contributions from remote advection, atmospheric exchange, and local nitrification and denitrification. Our results indicate that net N2O production from denitrification is approximately one order of magnitude greater than nitrification within the ETSP OMZ. However, only ~30% of denitrification-derived N2O production ultimately outgasses to the atmosphere in this region (contributing ~34% of the air-sea N2O flux on an annual basis), while the remaining is exported out of the domain. Instead, remotely-produced N2O advected into the OMZ region accounts for roughly half (~56%) of the total N2O outgassing, with smaller contributions from nitrification (~7%). Our results suggests that, together with enhanced production by denitrification, upwelling of remotely-derived N2O (likely produced via nitrification in the oxygenated ocean) contributes the most to N2O outgassing over the ETSP OMZ.
Publisher: Copernicus GmbH
Date: 16-04-2019
Abstract: Abstract. The isotopes of carbon (δ13C) and nitrogen (δ15N) are commonly used proxies for understanding the ocean. When used in tandem, they provide powerful insight into physical and biogeochemical processes. Here, we detail the implementation of δ13C and δ15N in the ocean component of an Earth system model. We evaluate our simulated δ13C and δ15N against contemporary measurements, place the model's performance alongside other isotope-enabled models and document the response of δ13C and δ15N to changes in ecosystem functioning. The model combines the Commonwealth Scientific and Industrial Research Organisation Mark 3L (CSIRO Mk3L) climate system model with the Carbon of the Ocean, Atmosphere and Land (COAL) biogeochemical model. The oceanic component of CSIRO Mk3L-COAL has a resolution of 1.6∘ latitude × 2.8∘ longitude and resolves multimillennial timescales, running at a rate of ∼400 years per day. We show that this coarse-resolution, computationally efficient model adequately reproduces water column and core-top δ13C and δ15N measurements, making it a useful tool for palaeoceanographic research. Changes to ecosystem function involve varying phytoplankton stoichiometry, varying CaCO3 production based on calcite saturation state and varying N2 fixation via iron limitation. We find that large changes in CaCO3 production have little effect on δ13C and δ15N, while changes in N2 fixation and phytoplankton stoichiometry have substantial and complex effects. Interpretations of palaeoceanographic records are therefore open to multiple lines of interpretation where multiple processes imprint on the isotopic signature, such as in the tropics, where denitrification, N2 fixation and nutrient utilisation influence δ15N. Hence, there is significant scope for isotope-enabled models to provide more robust interpretations of the proxy records.
Publisher: Copernicus GmbH
Date: 11-02-2018
Publisher: Copernicus GmbH
Date: 13-10-2016
Publisher: Copernicus GmbH
Date: 14-10-2016
Publisher: Copernicus GmbH
Date: 13-10-2016
Publisher: Copernicus GmbH
Date: 14-10-2016
Location: United Kingdom of Great Britain and Northern Ireland
No related grants have been discovered for Pearse James Buchanan.