Nutrient Controls on Export Production in the Southern Ocean

Abstract. We conduct a model–data analysis of the marine carbon cycle to understand and quantify the drivers of atmospheric CO2 concentration during the last glacial–interglacial cycle. We use a carbon cycle box model, “SCP-M”, combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and Southern Ocean biological export productivity across marine isotope stages spanning 130 000 years ago to the present. The model is constrained by proxy data associated with a range of environmental conditions including sea surface temperature, salinity, ocean volume, sea-ice cover and shallow-water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimized in each marine isotope stage against proxy data for atmospheric CO2, δ13C and Δ14C and deep-ocean δ13C, Δ14C and CO32-. Our model–data results suggest that global overturning circulation weakened during Marine Isotope Stage 5d, coincident with a ∼ 25 ppm fall in atmospheric CO2 from the last interglacial period. There was a transient slowdown in Atlantic meridional overturning circulation during Marine Isotope Stage 5b, followed by a more pronounced slowdown and enhanced Southern Ocean biological export productivity during Marine Isotope Stage 4 (∼ −30 ppm). In this model, the Last Glacial Maximum was characterized by relatively weak global ocean and Atlantic meridional overturning circulation and increased Southern Ocean biological export productivity (∼ −20 ppm during MIS 3 and MIS 2). Ocean circulation and Southern Ocean biological export productivity returned to modern values by the Holocene period. The terrestrial biosphere decreased by 385 Pg C in the lead-up to the Last Glacial Maximum, followed by a period of intense regrowth during the last glacial termination and the Holocene (∼ 600 Pg C). Slowing ocean circulation, a colder ocean and to a lesser extent shallow carbonate dissolution contributed ∼ −70 ppm to atmospheric CO2 in the ∼ 100 000-year lead-up to the Last Glacial Maximum, with a further ∼ −15 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological export productivity was one of the ingredients required to achieve the Last Glacial Maximum atmospheric CO2 level. We find that the incorporation of glacial–interglacial proxy data into a simple quantitative ocean transport model provides useful insights into the timing of past changes in ocean processes, enhancing our understanding of the carbon cycle during the last glacial–interglacial period.

show abstract

Section: Last Glacial-interglacial Cyclesupporting

confidence: 88%

Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study

et al. 2021

View full text Add to dashboard Cite

show abstract

“…8), falls within modern observations for the Southern Ocean of 0.5-6 mol C m −2 yr −1 (e.g. Lourey and Trull, 2001;Weeding and Trull, 2004;Ebersbach et al, 2011;Jacquet et al, 2011;Cassar et al, 2015;Arteaga et al, 2019). Our model-data experiment results reproduce values that fall within one standard deviation of the mean value in each model box, for all of the 10 atmosphere and ocean proxies in each MIS ( Figs.…”

supporting

confidence: 87%

Sequential changes in ocean circulation and biological export productivity during the last glacial cycle: a model-data study

O'Neill

Hogg

Ellwood

et al. 2019

Preprint

View full text Add to dashboard Cite

We conduct a model-data analysis of the ocean, atmosphere and terrestrial carbon system to understand their effects on atmospheric CO 2 during the last glacial cycle. We use a carbon cycle box model "SCP-M", combined with multiple proxy data for the atmosphere and ocean, to test for variations in ocean circulation and biological productivity across marine isotope stages spanning 130 thousand years ago to the present. The model is constrained by proxy data associated with a range of 5 environmental conditions including sea surface temperature, salinity, ocean volume, sea ice cover and shallow water carbonate production. Model parameters for global ocean circulation, Atlantic meridional overturning circulation and Southern Ocean biological export productivity are optimised in each marine isotope stage, against proxy data for atmospheric CO 2 , δ 13 C and ∆ 14 C and deep ocean δ 13 C, ∆ 14 C and carbonate ion. Our model-data results suggest that global overturning circulation weakened at marine isotope stage 5d, coincident with a ∼25 ppm fall in atmospheric CO 2 from the penultimate interglacial 10 level. This change was followed by a further slowdown in Atlantic meridional overturning circulation and enhanced Southern Ocean biological export productivity at marine isotope stage 4 (∼-30 ppm). There was also a transient slowdown in Atlantic meridional overturning circulation at MIS 5b. In this model, the last glacial maximum was characterised by relatively weak global ocean and Atlantic meridional overturning circulation, and increased Southern Ocean biological export productivity (∼-20 ppm during MIS 2-4). Ocean circulation and Southern Ocean biology rebounded to modern values by the Holocene period. 15The terrestrial biosphere decreased by ∼500 Pg C in the lead up to the last glacial maximum, followed by a period of intense regrowth during the Holocene (∼750 Pg C). Slowing ocean circulation, a cooler ocean and, to a lesser extent, shallow carbonate dissolution, contributed ∼-75 ppm to atmospheric CO 2 in the ∼100 thousand-year lead-up to the last glacial maximum, with a further ∼-10 ppm contributed during the glacial maximum. Our model results also suggest that an increase in Southern Ocean biological productivity was one of the ingredients required to achieve the last glacial maximum atmospheric CO 2 level. The 20 incorporation of longer-timescale data into quantitative ocean transport models, provides useful insights into the timing of changes in ocean processes, enhancing our understanding of the last glacial maximum and Holocene carbon cycle transition.Large and regular fluctuations in atmospheric CO 2 and ocean proxy signals for carbon isotopes and carbonate ion concentration, over the last 800 kyr, are preserved in ice and marine core records. The most obvious of these fluctuations is the repeated oscillation of atmospheric CO 2 over the range of ∼180-280 ppm every ∼100 kyr. The magnitude and regularity of these oscillations in atmospheric CO 2 , combined with proxy observations for carbon isotopes, point to ...

show abstract

“…Southern Ocean phytoplankton are essential for Antarctic food-webs and the regulation of global climate through the marine carbon cycle (Deppeler and Davidson, 2017). Here, phytoplankton blooms are dominated by large silica-forming diatoms which have unique physiology that is adapted to the low iron, light and temperature conditions they live in (Cermeño and Falkowski, 2009;Soppa et al, 2014;Arteaga et al, 2018Arteaga et al, , 2019Tréguer et al, 2018;Schallenberg et al, 2019;Strzepek et al, 2019). Residing in the mixed layer of the surface ocean, phytoplankton stocks can be observed from space using satellites due to the visibility of chlorophyll-a (McClain, 2009;Johnson et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

“…Southern Ocean SCMs are often located at or deeper than the pycnocline, rather than above it, making them distinguishable from those of the tropics (Quilty et al, 1985;Bathmann et al, 1997;Fiala et al, 1998;Kopczynska et al, 2001;Parslow et al, 2001;Garibotti et al, 2003;Armand et al, 2008a;Gomi et al, 2010;Westwood et al, 2011;Demidov et al, 2013;Tripathy et al, 2015). The deeper SCM coupled with the macro-nutrient replete surface ocean, suggest that Southern Ocean SCMs are different to that of the tropics (Figure 1; Louanchi and Najjar, 2000;Moore et al, 2001;Arteaga et al, 2019). These deeper SCMs are characterized by a marked community shift toward large diatoms across the pycnocline and are referred to as "diatom SCMs" herein (Bathmann et al, 1997;Cailliau et al, 1997;Kopczynska et al, 2001;Parslow et al, 2001;Quéguiner, 2001;Armand et al, 2008a;Gomi et al, 2010;Westwood et al, 2011;Tripathy et al, 2015).…”

Section: Introductionmentioning

confidence: 99%

Subsurface Chlorophyll-a Maxima in the Southern Ocean

et al. 2020

View full text Add to dashboard Cite

Our review of the literature has revealed Southern Ocean subsurface chlorophyll-a maxima (SCMs) to be an annually recurrent feature throughout the basin. Most of these SCMs are different to the "typical" SCMs observed in the tropics, which are maintained by the nutrient-light co-limitation of phytoplankton growth. Rather, we have found that SCMs are formed by other processes including diatom aggregation, sea-ice retreat, eddies, subduction events and photo-acclimation. At a local scale, these SCMs can facilitate increased downward carbon export, primary production and food availability for higher trophic levels. A large proportion of Southern Ocean SCMs appear to be sustained by aggregates of large diatoms that form under severe iron limitation in the seasonal mixed layer. The ability of large diatoms to regulate their buoyancy must play a role in the development of these SCMs as they appear to increase buoyancy at the SCM and thus avoid further sinking with the decline of the spring bloom or naturally iron fertilized blooms. These SCMs remain largely unobserved by satellites and it seems that ship-based sampling may not be able to fully capture their biomass. In the context of the Marine Ecosystem Assessment of the Southern Ocean it is important to consider that this phenomenon is missing in our current understanding of Southern Ocean ecology and future climate scenarios. The broader implications of SCMs for Southern Ocean ecology will only be revealed through basin-wide observations. This can only be achieved through an integrated observation system that is able to harness the detailed information encapsulated in ship-based sampling, with the increased observational capacity of fluorometers on autonomous platforms such as those in the biogeochemical Argo (BGC-Argo) and the Marine Mammals Exploring the Ocean Pole to pole (MEOP) programs. The main challenge toward achieving this is the uncertainties associated with translating fluorescence to chlorophyll-a concentrations. Until this translation is resolved, the reporting of subsurface fluorescence maxima (SFMs) in place of SCMs could still yield valuable insights with careful interpretation.

show abstract

Nutrient Controls on Export Production in the Southern Ocean

Cited by 47 publications

References 66 publications

Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study

Sequential changes in ocean circulation and biological export productivity during the last glacial–interglacial cycle: a model–data study

Sequential changes in ocean circulation and biological export productivity during the last glacial cycle: a model-data study

Subsurface Chlorophyll-a Maxima in the Southern Ocean

Contact Info

Product

Resources

About