Subantarctic Southern Ocean Research Articles

Patagonian dust, Agulhas Current, and Antarctic ice-rafted debris contributions to the South Atlantic Ocean over the past 150,000 years

Disentangling inputs of aeolian dust, ice-rafted debris (IRD), and eroded continental detritus delivered by ocean currents to marine sediments provide important insights into Earth System processes and climate. This study uses Sr-Nd-Pb isotope ratios of the continent-derived (lithogenic) fraction in deep-sea core TN057-6 from the subantarctic Southern Ocean southwest of Africa over the past 150,000 y to identify source regions and quantify their relative contributions and fluxes utilizing a mixing model set in a Bayesian framework. The data are compared with proxies from parallel core Ocean Drilling Program Site 1090 and newly presented data from potential South America aeolian dust source areas (PSAs), allowing for an integrated investigation into atmospheric, oceanic, and cryospheric dynamics. PSA inputs varied on glacial/interglacial timescales, with southern South American sources dominating up to 88% of the lithogenic fraction (mainly Patagonia, which provided up to 68%) during cold periods, while southern African sources were more important during interglacials. During the warmer Marine Isotope Stage (MIS) 3 of the last glacial period, lithogenic fluxes were twice that of colder MIS2 and MIS4 at times, and showed unique isotope ratios best explained by Antarctic-derived IRD, likely from the Weddell Sea. The IRD intrusions contributed up to 41% at times and followed Antarctic millennial warming events that raised temperatures, causing instability of icesheet margins. High IRD was synchronous with increased bioavailable iron, nutrient utilization, high biological productivity, and decreased atmospheric CO2. Overall, TN057-6 sediments record systematic Southern Hemisphere climate shifts and cryospheric changes that impacted biogeochemical cycling on both glacial/interglacial and subglacial timescales.

Response of the coccolithophore Calcidiscus leptoporus to environmental change during the industrial era in the Subantarctic Southern Ocean.

Response of the coccolithophore Calcidiscus leptoporus to environmental change during the industrial era in the Subantarctic Southern Ocean.

Environmental Drivers of Coccolithophore Growth in the Pacific Sector of the Southern Ocean

AbstractThe Great Calcite Belt (GCB) is a band of high concentrations of suspended particulate inorganic carbon (PIC) spanning the subantarctic Southern Ocean and plays an important role in the global carbon cycle. The key limiting factors controlling coccolithophore growth supporting this high PIC have not yet been well‐characterized in the remote Pacific sector, the lowest PIC but largest area of the GCB. Here, we present in situ physical and biogeochemical measurements along 150°W from January to February 2021, where a coccolithophore bloom occurred. In both months, PIC was elevated in the Subantarctic Zone (SAZ), where nitrate was >1 μM and temperatures were ∼13°C in January and ∼14°C in February, consistent with conditions previously associated with optimal coccolithophore growth potential. The highest PIC was associated with a relatively narrow temperature range that increased about 1°C between occupations. A fresher water mass had been transported to the 150°W meridian between occupations, and altimetry‐informed Lagrangian backtracking estimates show that most of this water was likely transported from the southeast within the SAZ. Applying the observations in a coccolithophore growth model for both January and February, we show that the ∼1.7°C increase in temperature can explain the rise in PIC between occupations.

Reduction in size of the calcifying phytoplankton Calcidiscus leptoporus to environmental changes between the Holocene and modern Subantarctic Southern Ocean

The Subantarctic Zone of the Southern Ocean plays a disproportionally large role on the Earth system. Model projections predict rapid environmental change in the coming decades, including ocean acidification, warming, and changes in nutrient supply which pose a serious risk for marine ecosystems. Yet despite the importance of the Subantarctic Zone, annual and inter-annual time series are extremely rare, leading to important uncertainties about the current state of its ecosystems and hindering predictions of future response to climate change. Moreover, as the longest observational time series available are only a few decades long, it remains unknown whether marine pelagic ecosystems have already responded to ongoing environmental change during the industrial era. Here, we take advantage of multiple sampling efforts – monitoring of surface layer water properties together with sediment trap, seafloor sediment and sediment core sampling – to reconstruct the modern and pre-industrial state of the keystone calcifying phytoplanktonCalcidiscus leptoporus, central to the global marine carbonate cycle. Morphometric measurements reveal that modernC. leptoporuscoccoliths are 15% lighter and 25% smaller than those preserved in the underlying Holocene-aged sediments. The cumulative effect of multiple environmental factors appears responsible for the coccolith size variations since the Last Deglaciation, with warming and ocean acidification most likely playing a predominant role during the industrial era. Notably, extrapolation of our results suggests a future reduction in cell and coccolith size which will have a negative impact on the efficiency of the biological pump in the Southern Ocean through a reduction of carbonate ballasting. Lastly, our results tentatively suggest thatC. leptoporuscoccolith size could be used as a palaeo-proxy for growth rate. Future culture experiments will be needed to test this hypothesis.

Bacteria and Archaea Regulate Particulate Organic Matter Export in Suspended and Sinking Marine Particle Fractions.

The biological carbon pump (BCP) in the Southern Ocean is driven by phytoplankton productivity and is a significant organic matter sink. However, the role of particle-attached (PA) and free-living (FL) prokaryotes (bacteria and archaea) and their diversity in influencing the efficiency of the BCP is still unclear. To investigate this, we analyzed the metagenomes linked to suspended and sinking marine particles from the Sub-Antarctic Southern Ocean Time Series (SOTS) by deploying a Marine Snow Catcher (MSC), obtaining suspended and sinking particulate material, determining organic carbon and nitrogen flux, and constructing metagenome-assembled genomes (MAGs). The suspended and sinking particle-pools were dominated by bacteria with the potential to degrade organic carbon. Bacterial communities associated with the sinking fraction had more genes related to the degradation of complex organic carbon than those in the suspended fraction. Archaea had the potential to drive nitrogen metabolism via nitrite and ammonia oxidation, altering organic nitrogen concentration. The data revealed several pathways for chemoautotrophy and the secretion of recalcitrant dissolved organic carbon (RDOC) from CO2, with bacteria and archaea potentially sequestering particulate organic matter (POM) via the production of RDOC. These findings provide insights into the diversity and function of prokaryotes in suspended and sinking particles and their role in organic carbon/nitrogen export in the Southern Ocean. IMPORTANCE The biological carbon pump is crucial for the export of particulate organic matter in the ocean. Recent studies on marine microbes have shown the profound influence of bacteria and archaea as regulators of particulate organic matter export. Yet, despite the importance of the Southern Ocean as a carbon sink, we lack comparable insights regarding microbial contributions. This study provides the first insights regarding prokaryotic contributions to particulate organic matter export in the Southern Ocean. We reveal evidence that prokaryotic communities in suspended and sinking particle fractions harbor widespread genomic potential for mediating particulate organic matter export. The results substantially enhance our understanding of the role played by microorganisms in regulating particulate organic matter export in suspended and sinking marine fractions in the Southern Ocean.

Giant Virus Infection Signatures Are Modulated by Euphotic Zone Depth Strata and Iron Regimes of the Subantarctic Southern Ocean.

Viruses can alter the abundance, evolution, and metabolism of microorganisms in the ocean, playing a key role in water column biogeochemistry and global carbon cycles. Large efforts to measure the contribution of eukaryotic microorganisms (e.g., protists) to the marine food web have been made, yet the in situ activities of the ecologically relevant viruses that infect these organisms are not well characterized. Viruses within the phylum Nucleocytoviricota ("giant viruses") are known to infect a diverse range of ecologically relevant marine protists, yet how these viruses are influenced by environmental conditions remains under-characterized. By employing metatranscriptomic analyses of in situ microbial communities along a temporal and depth-resolved gradient, we describe the diversity of giant viruses at the Southern Ocean Time Series (SOTS), a site within the subpolar Southern Ocean. Using a phylogeny-guided taxonomic assessment of detected giant virus genomes and metagenome-assembled genomes, we observed depth-dependent structuring of divergent giant virus families mirroring dynamic physicochemical gradients in the stratified euphotic zone. Analyses of transcribed metabolic genes from giant viruses suggest viral metabolic reprogramming of hosts from the surface to a 200-m depth. Lastly, using on-deck incubations reflecting a gradient of iron availability, we show that modulating iron regimes influences the activity of giant viruses in the field. Specifically, we show enhanced infection signatures of giant viruses under both iron-replete and iron-limited conditions. Collectively, these results expand our understanding of how the water column's vertical biogeography and chemical surroundings affect an important group of viruses within the Southern Ocean. IMPORTANCE The biology and ecology of marine microbial eukaryotes is known to be constrained by oceanic conditions. In contrast, how viruses that infect this important group of organisms respond to environmental change is less well known, despite viruses being recognized as key microbial community members. Here, we address this gap in our understanding by characterizing the diversity and activity of "giant" viruses within an important region in the sub-Antarctic Southern Ocean. Giant viruses are double-stranded DNA (dsDNA) viruses of the phylum Nucleocytoviricota and are known to infect a wide range of eukaryotic hosts. By employing a metatranscriptomics approach using both in situ samples and microcosm manipulations, we illuminated both the vertical biogeography and how changing iron availability affects this primarily uncultivated group of protist-infecting viruses. These results serve as a foundation for our understanding of how the open ocean water column structures the viral community, which can be used to guide models of the viral impact on marine and global biogeochemical cycling.

Ecology and management of invasive plants in the sub-Antarctic and Antarctic regions: evidence and synthesis from Macquarie Island

ABSTRACT Background The Antarctic is an extreme environment for plants. Several invasive plant species, however, have invaded the sub-Antarctic Southern Ocean islands and increasingly threaten the vulnerability of maritime and continental Antarctica, particularly with changes in climate. Aims We provide an overview of issues to consider with regard to the impact, ecology and management of non-native plants in the Antarctic, focusing on knowledge gained and recent research results from the sub-Antarctic with possible application to Antarctica. Methods We provide a brief review of literature and bring together experience and previously unpublished research with Poa annua and Stellaria media on Macquarie Island. Results While no one set of biological characteristics predicts plant invasiveness in the sub-Antarctic, all are adapted to survive extreme cold conditions and persistence is enabled through large and long-lived soil seed banks. Ecological drivers for invasion include accidental human introductions and continuing movement, along with animal and other disturbance to soil. Conclusion The invasive cold-tolerant plant species now prevalent on sub-Antarctic islands also pose a threat to Antarctica due to a warming climate and so efforts should continue to prevent further spread, while developing effective, low-impact control and eradication options to protect these high-value extreme ecosystems.

Deglacial Subantarctic CO2 outgassing driven by a weakened solubility pump

The Subantarctic Southern Ocean has long been thought to be an important contributor to increases in atmospheric carbon dioxide partial pressure (pCO2) during glacial-interglacial transitions. Extensive studies suggest that a weakened biological pump, a process associated with nutrient utilization efficiency, drove up surface-water pCO2 in this region during deglaciations. By contrast, regional influences of the solubility pump, a process mainly linked to temperature variations, have been largely overlooked. Here, we evaluate relative roles of the biological and solubility pumps in determining surface-water pCO2 variabilities in the Subantarctic Southern Ocean during the last deglaciation, based on paired reconstructions of surface-water pCO2, temperature, and nutrient utilization efficiency. We show that compared to the biological pump, the solubility pump imposed a strong impact on deglacial Subantarctic surface-water pCO2 variabilities. Our findings therefore reveal a previously underappreciated role of the solubility pump in modulating deglacial Subantarctic CO2 release and possibly past atmospheric pCO2 fluctuations.

Lithogenic Particle Flux to the Subantarctic Southern Ocean: A Multi‐Tracer Estimate Using Sediment Trap Samples

AbstractMineral dust is a key source of essential micronutrients, particularly iron (Fe), for phytoplankton in the Southern Ocean. However, observations of dust deposition over the Southern Ocean are sparse, hindering assessments of its influence on marine biogeochemistry. We present a time series (2010–2019) of lithogenic particle flux estimates using sediment trap samples collected at 1,000 m depth at the subantarctic Southern Ocean Time Series (SOTS) site. Lithogenic flux was estimated using individual Fe, aluminium (Al), titanium, and thorium concentrations in sediment trap particles less than 1 mm in size. These tracers showed good agreement with one another, and their average was investigated as a proxy for mineral dust deposition. This multi‐tracer average lithogenic flux exhibited strong seasonality, peaking in late spring and summer. No significant Fe enrichment was observed compared to the average upper continental crust, indicating that lithogenic material dominates particulate Fe flux at SOTS. Similar Fe:Al ratios in our samples compared to those reported in marine aerosols off southern Australia, coupled with particle trajectory analysis, suggested Australian dust constitutes the primary lithogenic source to SOTS sinking particles. Lead enrichment in our samples also highlighted an anthropogenic contribution to sinking particles, which might represent an additional aeolian source of more bio‐available Fe to subantarctic waters. This study contributes a new long‐term estimate of lithogenic particle fluxes and aeolian deposition over the subantarctic Southern Ocean. These estimates may enhance model representation of trace metal contribution to biogeochemical processes in the Southern Ocean.

Influence of environmental variability and Emiliania huxleyi ecotypes on alkenone-derived temperature reconstructions in the subantarctic Southern Ocean

Long-chain unsaturated alkenones produced by haptophyte algae are widely used as paleotemperature indicators. The unsaturation relationship to temperature is linear at mid-latitudes, however, non-linear responses detected in subpolar regions of both hemispheres have suggested complicating factors in these environments. To assess the influence of biotic and abiotic factors in alkenone production and preservation in the Subantarctic Zone, alkenone fluxes were quantified in three vertically-moored sediment traps deployed at the SOTS observatory (140°E, 47°S) during a year. Alkenone fluxes were compared with coccolithophore assemblages, satellite measurements and surface-water properties obtained by sensors at SOTS. Alkenone-based temperature reconstructions generally mirrored the seasonal variations of SSTs, except for late winter when significant deviations were observed (3–10 °C). Annual flux-weighted averages in the 3800 m trap returned alkenone-derived temperatures ~1.5 °C warmer than those derived from the 1000 m trap, a distortion attributed to surface production and signal preservation during its transit through the water column. Notably, changes in the relative abundance of E. huxleyi var. huxleyi were positively correlated with temperature deviations between the alkenone-derived temperatures and in situ SSTs (r = 0.6 and 0.7 at 1000 and 2000 m, respectively), while E. huxleyi var. aurorae, displayed an opposite trend. Our results suggest that E. huxleyi var. aurorae produces a higher proportion of C37:3 relative to C37:2 compared to its counterparts. Therefore, the dominance of var. aurorae south of the Subtropical Front could be at least partially responsible for the less accurate alkenone-based SST reconstructions in the Southern Ocean using global calibrations. However, the observed correlations were largely influenced by the samples collected during winter, a period characterized by low particle fluxes and slow sinking rates. Thus, it is likely that other factors such as selective degradation of the most unsaturated alkenones could also account for the deviations of the alkenone paleothermometer.

Southern Ocean carbonate dissolution paced by Antarctic Ice-Sheet expansion in the early Miocene

The preservation of calcium carbonate deposits on the seafloor is one of the factors that regulates Earth's carbon cycle over longer timescales. Fluctuations in the carbonate compensation depth (CCD), the depth below which calcium carbonate is completely dissolved, can therefore significantly alter the ocean's capacity to sequester or release atmospheric carbon. High-latitude deep oceans are particularly important to the carbon cycle given their large capacity to store carbon dioxide. Here we present carbonate content results from an abyssal Subantarctic record (Site U1514) that spans the early Miocene and the onset of the Miocene Climatic Optimum. We show that carbonate preservation was highly variable from 19.9 to 17 Ma, with multiple episodes of total carbonate dissolution. Carbonate preservation then improved gradually after 17 Ma following the onset of the Miocene Climatic Optimum. Episodes of carbonate dissolution occurred preferentially at long eccentricity minima while periods of long eccentricity maxima improved carbonate preservation. These patterns of carbonate accumulation are in opposite phase to lower latitude Atlantic and Pacific records, which suggests that different mechanisms of carbonate accumulation/preservation were in place in high and low-latitude oceans. Comparison of Site U1514 with Antarctic records indicates an influence of ice sheet oscillations on carbonate preservation at the Subantarctic seafloor. We propose that carbonate dissolution occurred via the northward expansion of southern-derived undersaturated waters during periods of Antarctic ice sheet expansion. These findings indicate that the Subantarctic Southern Ocean was highly sensitive to Antarctic ice sheet fluctuations during the early Miocene, consistent with observations from Antarctica that show intensified polar amplification under a warmer climate.

Particle Fluxes at the Australian Southern Ocean Time Series (SOTS) Achieve Organic Carbon Sequestration at Rates Close to the Global Median, Are Dominated by Biogenic Carbonates, and Show No Temporal Trends Over 20-Years

Particle fluxes at the Southern Ocean time series (SOTS) site in the Subantarctic zone (SAZ) south of Australia (~47°S, 142°E, 4600m water depth) were collected from 1997 – 2017 using moored sediment traps at nominal depths of 1000m, 2000m and 3800m. Annually integrated mass fluxes showed moderate variability of 14 ± 6 g m-2 yr-1 at 1000m, 20 ± 6 g m-2 yr-1 at 2000m and 21 ± 4 g m-2 yr-1 at 3800m. Particulate organic carbon (POC) fluxes were similar to the global median, indicating that the Subantarctic Southern Ocean exports considerable amounts of carbon to the deep sea despite its high-nutrient, low chlorophyll characteristics. The interannual flux variations were larger than those of net primary productivity as estimated from satellite observations. Particle compositions were dominated by carbonate minerals (> 60% at all depths), opal (~10% at all depths), and particulate organic matter (~17% at 1000m, decreasing to ~10% at 3800m), with seasonal and interannual variability much smaller than for their flux magnitudes. The carbonate counter-pump effect reduced carbon sequestration by ~8±2%. The average seasonal cycle at 1000m had a two-peak structure, with a larger early spring peak (October/November) and a smaller late summer (January/February) peak. At the two deeper traps, these peaks became less distinct with a greater proportion of the fluxes arriving in autumn. Singular value decomposition (SVD) shows that this temperate seasonal structure accounts for ~80% of the total variance (SVD Mode 1), but also that its influence varies significantly relative to Modes 2 and 3 which describe changes in seasonal timings. This occurrence of significant interannual variability in seasonality yet relatively constant annual fluxes, is likely to be useful in selecting appropriate models for the simulation of environmental-ecological coupling and its role in controlling the biological carbon pump. No temporal trends were detected in the mass or component fluxes, or in the time series of the SVD Modes. The SOTS observations provide an important baseline for future changes expected to result from warming, stratification, and acidification in this globally significant region.

Limited variability in the phytoplankton Emiliania huxleyi since the pre-industrial era in the Subantarctic Southern Ocean

The Southern Ocean is warming faster than the average global ocean and is particularly vulnerable to ocean acidification due to its low temperatures and moderate alkalinity. Coccolithophores are the most productive calcifying phytoplankton and an important component of Southern Ocean ecosystems. Laboratory observations on the most abundant coccolithophore, Emiliania huxleyi, suggest that this species is susceptible to variations in seawater carbonate chemistry, with consequent impacts in the carbon cycle. Whether anthropogenic environmental change during the industrial era has modified coccolithophore populations in the Southern Ocean, however, remains uncertain. This study analysed the coccolithophore assemblage composition and morphometric parameters of E. huxleyi coccoliths of a suite of Holocene-aged sediment samples from south of Tasmania. The analysis suggests that dissolution diminished the mass and length of E. huxleyi coccoliths in the sediments, but the thickness of the coccoliths was decoupled from dissolution allowing direct comparison of samples with different degree of preservation. The latitudinal distribution pattern of coccolith thickness mirrors the latitudinal environmental gradient in the surface layer, highlighting the importance of the geographic distribution of E. huxleyi morphotypes on the control of coccolith morphometrics. Additionally, comparison of the E. huxleyi coccolith assemblages in the sediments with those of annual subantarctic sediment trap records found that modern E. huxleyi coccoliths are ∼2% thinner than those from the pre-industrial era. The subtle variation in coccolith thickness contrasts sharply with earlier work that documented a pronounced reduction in shell calcification and consequent shell-weight decrease of ∼30-35% on the planktonic foraminifera Globigerina bulloides induced by ocean acidification. Results of this study underscore the varying sensitivity of different marine calcifying plankton groups to ongoing environmental change.

Coccolithophore biodiversity controls carbonate export in the Southern Ocean

Abstract. Southern Ocean waters are projected to undergo profound changes in their physical and chemical properties in the coming decades. Coccolithophore blooms in the Southern Ocean are thought to account for a major fraction of the global marine calcium carbonate (CaCO3) production and export to the deep sea. Therefore, changes in the composition and abundance of Southern Ocean coccolithophore populations are likely to alter the marine carbon cycle, with feedbacks to the rate of global climate change. However, the contribution of coccolithophores to CaCO3 export in the Southern Ocean is uncertain, particularly in the circumpolar subantarctic zone that represents about half of the areal extent of the Southern Ocean and where coccolithophores are most abundant. Here, we present measurements of annual CaCO3 flux and quantitatively partition them amongst coccolithophore species and heterotrophic calcifiers at two sites representative of a large portion of the subantarctic zone. We find that coccolithophores account for a major fraction of the annual CaCO3 export, with the highest contributions in waters with low algal biomass accumulations. Notably, our analysis reveals that although Emiliania huxleyi is an important vector for CaCO3 export to the deep sea, less abundant but larger species account for most of the annual coccolithophore CaCO3 flux. This observation contrasts with the generally accepted notion that high particulate inorganic carbon accumulations during the austral summer in the subantarctic Southern Ocean are mainly caused by E. huxleyi blooms. It appears likely that the climate-induced migration of oceanic fronts will initially result in the poleward expansion of large coccolithophore species increasing CaCO3 production. However, subantarctic coccolithophore populations will eventually diminish as acidification overwhelms those changes. Overall, our analysis emphasizes the need for species-centred studies to improve our ability to project future changes in phytoplankton communities and their influence on marine biogeochemical cycles.

Comparison Between Early and Late 21stC Phytoplankton Biomass and Dimethylsulfide Flux in the Subantarctic Southern Ocean

Time-series of chlorophyll-a (CHL), a proxy for phytoplankton biomass, and various satellite-derived climate indicators are compared in a region of the Subantarctic Southern Ocean (40°−60°S, 110°−140°E) for years 2012−2014. CHL reached a minimum in winter (June) and a maximum in late summer (early February). Zonal mean CHL decreased towards the south. Mean sea surface temperature (SST) ranged between 8℃ and 15℃ and peaked in late February. CHL and SST were positively correlated from March to June, negatively correlated from July to September. CHL and wind speed (WIND) were negatively correlated with peak WIND occurred in winter. Wind direction (WIRD) was mostly in the southwest to westerly direction. The Antarctic Oscillation index (AAO) and CHL were negatively correlated (R = −0.58), indicating that as synoptic wind systems move southwards, CHL increases, and conversely when wind systems move northwards, CHL decreases. A genetic algorithm is used to calibrate the biogeochemical DMS model’s key parameters. Under 4 × CO2 (after year 2100) Regional mean SST increases 12%−17%, WIND increases 1.2 m s−1, Cloud Cover increases 4.8% and mixed layer depth (MLD) decreases 48m. The annual CHL increases 6.3%. The annual mean DMS flux increase 25.2%, increases 37% from day 1 to day 280 and decrease 3% from day 288 to day 360. The general increase of DMS flux under 4 × CO2 conditions indicates the Subantarctic regional climate would be affected by changes in the DMS flux, with the potential for a cooling effect in the austral summer and autumn.

Surface heat budget in the Southern Ocean from 42°S to the Antarctic marginal ice zone: four atmospheric reanalyses versus icebreaker Aurora Australis measurements

Surface heat fluxes from four atmospheric reanalyses in the Southern Ocean are evaluated using air–sea measurements obtained from the Aurora Australis during off-winter seasons in 2010–12. The icebreaker tracked between Hobart, Tasmania (ca. 42°S), and the Antarctic continent, providing in situ benchmarks for the surface energy budget change in the Subantarctic Southern Ocean (58–42°S) and the eastern Antarctic marginal ice zone (MIZ, 68–58°S). We find that the reanalyses show a high-level agreement among themselves, but this agreement reflects a universal bias, not a “truth.” Downward shortwave radiation (SW↓) is overestimated (warm biased) and downward longwave radiation (LW↓) is underestimated (cold biased), an indication that the cloud amount in all models is too low. The ocean surface in both regimes shows a heat gain from the atmosphere when averaged over the seven months (October–April). However, the ocean heat gain in reanalyses is overestimated by 10–36 W m−2 (80–220%) in the MIZ but underestimated by 6–20 W m−2 (7–25%) in the Subantarctic. The biases in SW↓ and LW↓ cancel out each other in the MIZ, causing the surface heat budget to be dictated by the underestimation bias in sensible heat loss. These reanalyses biases affect the surface energy budget in the Southern Ocean by meaningfully affecting the timing of the seasonal transition from net heat gain to net heat loss at the surface and the relative strength of SW↓ at different regimes in summer, when the length-of-day effect can lead to increased SW↓ at high latitudes.

Predicting near-term variability in ocean carbon uptake

Abstract. Interannual variations in air–sea fluxes of carbon dioxide (CO2) impact the global carbon cycle and climate system, and previous studies suggest that these variations may be predictable in the near term (from a year to a decade in advance). Here, we quantify and understand the sources of near-term predictability and predictive skill in air–sea CO2 flux on global and regional scales by analyzing output from a novel set of retrospective decadal forecasts of an Earth system model. These forecasts exhibit the potential to predict year-to-year variations in the globally integrated air–sea CO2 flux several years in advance, as indicated by the high correlation of the forecasts with a model reconstruction of past CO2 flux evolution. This potential predictability exceeds that obtained solely from foreknowledge of variations in external forcing or a simple persistence forecast, with the longest-lasting forecast enhancement in the subantarctic Southern Ocean and the northern North Atlantic. Potential predictability in CO2 flux variations is largely driven by predictability in the surface ocean partial pressure of CO2, which itself is a function of predictability in surface ocean dissolved inorganic carbon and alkalinity. The potential predictability, however, is not realized as predictive skill, as indicated by the moderate to low correlation of the forecasts with an observationally based CO2 flux product. Nevertheless, our results suggest that year-to-year variations in ocean carbon uptake have the potential to be predicted well in advance and establish a precedent for forecasting air–sea CO2 flux in the near future.

Effect of anthropogenic warming on microbial respiration and particulate organic carbon export rates in the sub-Antarctic Southern Ocean

ABSTRACT: Microbial respiration of particulate organic carbon (POC) is one of the key processes controlling the magnitude of POC export from the surface ocean and its storage on long timescales in the deep. Metabolic processes are a function of temperature, such that warming sea temperatures should increase microbial respiration, potentially reducing POC export. To investigate this in the Southern Ocean, we measured microbial oxygen consumption of large particles over a 10°C temperature range (summer maximum +8°C) to then estimate the decrease in export by 2100. Our results showed that POC-normalised respiration increased with warming. We estimate that POC export (scaled to primary production) could decrease by 17 ± 7% (SE) by 2100, using projected regional warming (+1.9°C) from the IPCC RCP 8.5 (‘business-as-usual’ scenario) for our sub-Antarctic site. Increased microbial respiration is one of many processes that will be altered by future climate change, which could all modify carbon storage in the future. Our estimate of the potential decline in carbon sequestration is within previous estimates from lab and field experiments, but higher than simple mechanistic models. To explore our results further, we used the metabolic theory of ecology (MTE) to determine the activation energy of microbial respiration, which was 0.9 eV. This is higher than classical MTE (0.6-0.7 eV), suggesting that sub-Antarctic microbes are particularly sensitive to temperature change. Such regional characteristics in the response of organisms to increased temperatures should be accounted for in large-scale or global model analyses to ensure that the results do not underestimate microbial responses to warming.

Highly bioavailable dust-borne iron delivered to the Southern Ocean during glacial periods

Changes in bioavailable dust-borne iron (Fe) supply to the iron-limited Southern Ocean may influence climate by modulating phytoplankton growth and CO2 fixation into organic matter that is exported to the deep ocean. The chemical form (speciation) of Fe impacts its bioavailability, and glacial weathering produces highly labile and bioavailable Fe minerals in modern dust sources. However, the speciation of dust-borne Fe reaching the iron-limited Southern Ocean on glacial-interglacial timescales is unknown, and its impact on the bioavailable iron supply over geologic time has not been quantified. Here we use X-ray absorption spectroscopy on subantarctic South Atlantic and South Pacific marine sediments to reconstruct dust-borne Fe speciation over the last glacial cycle, and determine the impact of glacial activity and glaciogenic dust sources on bioavailable Fe supply. We show that the Fe(II) content, as a percentage of total dust-borne Fe, increases from ∼5 to 10% in interglacial periods to ∼25 to 45% in glacial periods. Consequently, the highly bioavailable Fe(II) flux increases by a factor of ∼15 to 20 in glacial periods compared with the current interglacial, whereas the total Fe flux increases only by a factor of ∼3 to 5. The change in Fe speciation is dominated by primary Fe(II) silicates characteristic of glaciogenic dust. Our results suggest that glacial physical weathering increases the proportion of highly bioavailable Fe(II) in dust that reaches the subantarctic Southern Ocean in glacial periods, which represents a positive feedback between glacial activity and cold glacial temperatures.

The plumbing of the global biological pump: Efficiency control through leaks, pathways, and time scales

AbstractWe systematically quantify the pathways and time scales that set the efficiency, , of the global biological pump by applying Green‐function‐based diagnostics to a data‐assimilated phosphorus cycle embedded in a jointly assimilated ocean circulation. We consider “bio pipes” that consist of phosphorus paths that connect specified regions of last biological utilization with regions where regenerated phosphate first reemerges into the euphotic zone. The bio pipes that contribute most to Ebio connect the Eastern Equatorial Pacific (EEqP) and Equatorial Atlantic to the Southern Ocean ((21 ± 3)% of ), as well as the Southern Ocean to itself ((15 ± 3)% of ). The bio pipes with the largest phosphorus flow rates connect the EEqP to itself and the subantarctic Southern Ocean to itself. The global mean sequestration time of the biological pump is 130 ± 70 years, while the sequestration time of the bio pipe from anywhere to the Antarctic region of the Southern Ocean is 430 ± 30 years. The distribution of phosphorus flowing within a given bio pipe is quantified by its transit‐time partitioned path density. For the largest bio pipes, ∼1/7 of their phosphorus is carried by thermocline paths with transit times less than ∼300–400 years, while ∼4/7 of their phosphorus is carried by abyssal paths with transit times exceeding ∼700 years. The path density reveals that Antarctic Intermediate Water carries about a third of the regenerated phosphate last utilized in the EEqP that is destined for the Southern Ocean euphotic zone. The Southern Ocean is where (62 ± 2)% of the regenerated inventory and (69 ± 1)% of the preformed inventory first reemerge into the euphotic zone.