Oceanic Uptake Of Atmospheric CO2 Research Articles

Modeling Phytoplankton Blooms and Inorganic Carbon Responses to Sea‐Ice Variability in the West Antarctic Peninsula

AbstractThe ocean coastal‐shelf‐slope ecosystem west of the Antarctic Peninsula (WAP) is a biologically productive region that could potentially act as a large sink of atmospheric carbon dioxide. The duration of the sea‐ice season in the WAP shows large interannual variability. However, quantifying the mechanisms by which sea ice impacts biological productivity and surface dissolved inorganic carbon (DIC) remains a challenge due to the lack of data early in the phytoplankton growth season. In this study, we implemented a circulation, sea‐ice, and biogeochemistry model (MITgcm‐REcoM2) to study the effect of sea ice on phytoplankton blooms and surface DIC. Results were compared with satellite sea‐ice and ocean color, and research ship surveys from the Palmer Long‐Term Ecological Research (LTER) program. The simulations suggest that the annual sea‐ice cycle has an important role in the seasonal DIC drawdown. In years of early sea‐ice retreat, there is a longer growth season leading to larger seasonally integrated net primary production (NPP). Part of the biological uptake of DIC by phytoplankton, however, is counteracted by increased oceanic uptake of atmospheric CO2. Despite lower seasonal NPP, years of late sea‐ice retreat show larger DIC drawdown, attributed to lower air‐sea CO2 fluxes and increased dilution by sea‐ice melt. The role of dissolved iron and iron limitation on WAP phytoplankton also remains a challenge due to the lack of data. The model results suggest sediments and glacial meltwater are the main sources in the coastal and shelf regions, with sediments being more influential in the northern coast.

River-dominated pCO2 dynamics in the northern South China Sea during summer: A modeling study

River-dominated ocean margins (RiOMars), characterized by river plumes and abundant riverine nutrient inputs, are especially critical in determining the oceanic uptake of atmospheric CO2. Using a well validated three-dimensional, coupled physical-biogeochemical model, we examined the dynamics of the carbonate system in the Pearl River Plume (PRP) during summer over a typical RiOMar in the northern South China Sea (NSCS). Sea surface pCO2 in the PRP was mainly influenced by a combination of physical processes, air-sea exchange, and biological activity. The interplay between these complex processes differed spatially and temporally depending on the evolution of the PRP. The latter was divided into three sub-regions: near-, mid- and far-field. In the near-field PRP, the evolution of surface pCO2 was primarily influenced by biological activity. Surface pCO2 decreased substantially at the initial stage as a result of phytoplankton blooms, and then increased due to the reduction in the phytoplankton and increase of zooplankton and detritus. In the mid-field, surface pCO2 was initially dominated by air-sea exchange. Subsequently, the rates of biological processes exceeded the rate of air-sea exchange, resulting in a strong drawdown of surface pCO2 or a strong sink for atmospheric CO2. The far-field of the PRP acted as a weak CO2 sink, where surface pCO2 was dominated by air-sea exchange as biological processes were fairly weak. In addition, given that the air-sea CO2 equilibrium time is much longer than water residence time of the PRP, the biologically-mediated low pCO2 surface water was enabled to be transported far away from estuary. Taken together, the combined effect of enhanced primary production, strong plume current and strong seawater carbonate buffering capacity were responsible for maintaining low surface pCO2 levels in this subtropical RiOMar system.

Insights into the bioavailability of oceanic dissolved Fe from phytoplankton uptake kinetics

Phytoplankton growth in large parts of the world ocean is limited by low availability of dissolved iron (dFe), restricting oceanic uptake of atmospheric CO2. The bioavailability of dFe in seawater is however difficult to appraise since it is bound by a variety of poorly characterized organic ligands. Here, we propose a new approach for evaluating seawater dFe bioavailability based on its uptake rate constant by Fe-limited cultured phytoplankton. We utilized seven phytoplankton species of diverse classes, sizes, and provenances to probe for dFe bioavailability in 12 seawater samples from several ocean basins and depths. All tested phytoplankton acquired organically bound Fe in any given sample at similar rates (after normalizing to cellular surface area), confirming that multiple, Fe-limited phytoplankton species can be used to probe dFe bioavailability in seawater. These phytoplankton-based uptake rate constants allowed us to compare water types, and obtain a grand average estimate of seawater dFe bioavailability. Among water types, dFe bioavailability varied by approximately four-fold, and did not clearly correlate with Fe concentrations or any of the measured Fe speciation parameters. Compared with well-studied Fe complexes, seawater dFe is more available than model siderophore Fe, but less available than inorganic Fe. Exposure of seawater to sunlight, however, significantly enhanced dFe bioavailability. The rate constants established in this work, not only facilitate comparison between water types, but also allow calculation of Fe uptake rates by phytoplankton in the ocean based on measured dFe concentrations. The approach established and verified in this study, opens a new way for determining dFe bioavailability in samples across the ocean, and enables modeling of in situ Fe uptake rates by phytoplankton using dFe concentrations from GEOTRACES datasets.

CO2-driven ocean acidification weakens mussel shell defense capacity and induces global molecular compensatory responses.

Oceanic uptake of atmospheric CO2 is reducing seawater pH and shifting carbonate chemistry within, a process termed as ocean acidification (OA). Marine mussels are a family of ecologically and economically significant bivalves that are widely distributed along coastal areas worldwide. Studies have demonstrated that OA greatly disrupts mussels' physiological functions. However, the underlying molecular responses (e.g., whether there were any molecular compensation mechanisms) and the extent to which OA affects mussel shell defense capacity remain largely unknown. In this study, the thick shell mussels Mytilus coruscus were exposed to the ambient pH (8.1) or one of two lowered pH levels (7.8 and 7.4) for 40 days. The results suggest that future OA will damage shell structure and weaken shell strength and shell closure strength, ultimately reducing mussel shell defense capacity. In addition, future OA will also disrupt haemolymph pH and Ca2+ homeostasis, leading to extracellular acidosis and Ca2+ deficiency. Mantle transcriptome analyses indicate that mussels will adopt a series of molecular compensatory responses to mitigate these adverse effects; nevertheless, weakened shell defense capacity will increase mussels' susceptibility to predators, parasites and pathogens, and thereby reduce their fitness. Overall, the findings of this study have significant ecological and economic implications, and will enhance our understanding of the future of the mussel aquaculture industry and coastal ecosystems.

The role of biological rates in the simulated warming effect on oceanic CO2 uptake

AbstractMarine biology plays an important role in the ocean carbon cycle. However, the effect of warming‐induced changes in biological rates on oceanic CO2 uptake has been largely overlooked. We use an Earth system model of intermediate complexity to investigate the effect of temperature‐induced changes in biological rates on oceanic uptake of atmospheric CO2 and compare it with the effects from warming‐induced changes in CO2 solubility and ocean mixing and circulation. Under the representative CO2 concentration pathway RCP 8.5 and its extension, by year 2500, relative to the simulation without warming effect on the ocean carbon cycle, CO2‐induced warming reduces cumulative oceanic CO2 uptake by 469 Pg C, of which about 20% is associated with the warming‐induced change in marine biological rates. In our simulations, the bulk effect of biological‐mediated changes on CO2 uptake is smaller than that mediated by changes in CO2 solubility and ocean mixing and circulation. However, warming‐induced changes in individual biological rates, including phytoplankton growth, phytoplankton mortality, and detritus remineralization, are found to affect oceanic CO2 uptake by an amount greater than or comparable to that caused by changes in CO2 solubility and ocean physics. Our simulations, which include only a few temperature‐dependent biological processes, demonstrate the important role of biological rates in the oceanic CO2 uptake. In reality, many more complicated biological processes are sensitive to temperature change, and their responses to warming could substantially affect oceanic uptake of atmospheric CO2.

Global variability and changes in ocean total alkalinity from Aquarius satellite data

AbstractThis work demonstrates how large‐scale Aquarius satellite salinity data have provided an unprecedented opportunity when combined with total alkalinity (TA) equations as a function of salinity and temperature to examine global changes in the CO2 system. Alkalinity is a gauge on the ability of seawater to neutralize acids. TA correlates strongly with salinity. Spatial variability in alkalinity and salinity exceed temporal variability. Northern Hemisphere has more spatial variability in TA and salinity, while less variability in Southern Ocean TA is due to less salinity variability and upwelling of waters enriched in alkalinity. For the first time it is shown that TA in subtropical regions has increased as compared with climatological data; this is reflective of large‐scale changes in the global water cycle. Thus, as temperature and salinity increase in subtropical regions, the resultant increase in TA and ocean acidification is reinforcing that from oceanic uptake of atmospheric CO2.

South Africa in the Antarctic Circumnavigation Expedition: A multi-institutional and interdisciplinary scientific project

The polar regions are more critically affected by climate change than any other region on our planet. On the Antarctic continent and in its surrounding oceans, the effects of climate change are likely to be dramatic, and include largescale catastrophic ice melt, loss of habitat and biodiversity, and global sea level rise. The 'Southern Ocean' refers to the region where Atlantic, Indian and Pacific Ocean waters come together to encircle Antarctica. These waters connect the different ocean basins by linking the shallow and deep limbs of the global ocean current system ('overturning circulation') and play a critical role in storing and distributing heat and carbon dioxide (CO2). The Southern Ocean thus regulates not only the climate of the Antarctic, but of the entire earth system. By extension, the capacity of the global ocean to ameliorate earth's changing climate is strongly controlled by the Southern Ocean. Marine phytoplankton (microscopic plants inhabiting the sunlit upper ocean) convert CO2 (an inorganic form of carbon) dissolved in surface waters into organic carbon through photosynthesis. This organic carbon fuels upper trophic levels such as fish, mammals and birds, and a portion sinks into the deep ocean where it remains stored for hundreds to thousands of years. This mechanism, which lowers the atmospheric concentration of CO2, is termed the 'biological pump'. The efficiency of the global ocean's biological pump is currently limited by the Southern Ocean, where the macronutrients (nitrate and phosphate) required for photosynthesis are never fully consumed in surface waters. In theory, increased consumption of these nutrients could drive higher organic carbon removal to the deep ocean, enhancing the oceanic uptake of atmospheric CO2. Indeed, more complete consumption of Southern Ocean nutrients is a leading hypothesis for the decrease in atmospheric CO2 that characterised the ice ages. Despite the global importance of the Southern Ocean, knowledge of the controls on and interactions among the physical, chemical and biological processes operating in Antarctic ecosystems is limited, largely because of a scarcity of in-situ observational data, compounded by the challenge of integrating siloed scientific fields. Given predictions that diverse aspects of Southern Ocean physics and carbon biogeochemistry are likely to change in the coming decades, a transdisciplinary approach to studying Antarctic systems is critical.

An organic carbon budget for coastal Southern California determined by estimates of vertical nutrient flux, net community production and export

Organic carbon export and burial in coastal upwelling regions is an important mechanism for oceanic uptake of atmospheric CO2. In order to understand how these complex systems will respond to future climate forcing, further studies of nutrient input, biological production and export are needed. Using a 7Be-based approach, we produced an 18-month record of upwelling velocity estimates at the San Pedro Ocean Time-series (SPOT), Southern California Bight. These upwelling rates and vertical nutrient distributions have been combined to make estimates of potential new production (PNP), which are compared to estimates of net community oxygen production (NOP) made using a one-dimensional, two-box non-steady state model of euphotic zone biological oxygen supersaturation. NOP agrees within uncertainty with PNP, suggesting that upwelling is the dominant mechanism for supplying the ecosystem with new nutrients in the spring season, but negligible in the fall and winter. Combining this data set with estimates of sinking particulate organic carbon (POC) flux from water column 234Th:238U disequilibrium and sediment trap deployments, and an estimate of the ratio of dissolved organic carbon (DOC):POC consumption rates, we construct a simple box model of organic carbon in the upper 200m of our study site. This box model (with uncertainties of ±50%) suggests that in spring, ~28% of net production leaves the euphotic zone as DOC, of this, ~12% as horizontal export and ~16% via downward mixing. The remaining ~72% of net organic carbon export exits as sinking POC, with only ~10% of euphotic zone export reaching 200m. We find the metabolic requirement for the local heterotrophic community below the euphotic zone, but above 200m, is ~105±50mmolCm−2d−1, or ~80% of net euphotic zone production in spring.

Historical reconstruction of ocean acidification in the Australian region

Abstract. The ocean has become more acidic over the last 200 years in response increasing atmospheric carbon dioxide (CO2) levels. Documenting how the ocean has changed is critical for assessing how these changes impact marine ecosystems and for the management of marine resources. Here we use present-day ocean carbon observations, from shelf and offshore waters around Australia, combined with neural network mapping of CO2, sea surface temperature, and salinity to estimate the current seasonal and regional distributions of carbonate chemistry (pH and aragonite saturation state). The observed changes in atmospheric CO2 and sea surface temperature (SST) and climatological salinity are then used to reconstruct pH and aragonite saturation state changes over the last 140 years (1870–2013). The comparison with data collected at Integrated Marine Observing System National Reference Station sites located on the shelf around Australia shows that both the mean state and seasonality in the present day are well represented, with the exception of sites such as the Great Barrier Reef. Our reconstruction predicts that since 1870 decrease in aragonite saturation state of 0.48 and of 0.09 in pH has occurred in response to increasing oceanic uptake of atmospheric CO2. Large seasonal variability in pH and aragonite saturation state occur in southwestern Australia driven by ocean dynamics (mixing) and in the Tasman Sea by seasonal warming (in the case of the aragonite saturation state). The seasonal and historical changes in aragonite saturation state and pH have different spatial patterns and suggest that the biological responses to ocean acidification are likely to be non-uniform depending on the relative sensitivity of organisms to shifts in pH and saturation state. This new historical reconstruction provides an important link to biological observations that will help to elucidate the consequences of ocean acidification.

Control of the glacial carbon budget by topographically induced mixing

AbstractEvidence for the oceanic uptake of atmospheric CO2 during glaciations suggests that there was less production of southern origin deep water but, paradoxically, a larger volume of southern origin water than today. Here we demonstrate, using a theoretical box model, that the inverse relationship between volume and production rate of this water mass can be explained by invoking mixing rates in the deep ocean that are proportional to topographic outcropping area scaled with ocean floor slope. Furthermore, we show that the resulting profile, of a near‐linear decrease in mixing intensity away from the bottom, generates a positive feedback on CO2 uptake that can initiate a glacial cycle. The results point to the importance of using topography‐dependent mixing when studying the large‐scale ocean circulation, especially in the paleo‐intercomparison models that have failed to produce the weaker and more voluminous bottom water of the Last Glacial Maximum.

A multi-variable box model approach to the soft tissue carbon pump

Abstract. The canonical question of which physical, chemical or biological mechanisms were responsible for oceanic uptake of atmospheric CO2 during the last glacial is yet unanswered. Insight from paleo-proxies has led to a multitude of hypotheses but none so far have been convincingly supported in three dimensional numerical modelling experiments. The processes that influence the CO2 uptake and export production are inter-related and too complex to solve conceptually while complex numerical models are time consuming and expensive to run which severely limits the combinations of mechanisms that can be explored. Instead, an intermediate inverse box model approach of the soft tissue pump is used here in which the whole parameter space is explored. The glacial circulation and biological production states are derived from these using proxies of glacial export production and the need to draw down CO2 into the ocean. We find that circulation patterns which explain glacial observations include reduced Antarctic Bottom Water formation and high latitude upwelling and mixing of deep water and to a lesser extent reduced equatorial upwelling. The proposed mechanism of CO2 uptake by an increase of eddies in the Southern Ocean, leading to a reduced residual circulation, is not supported. Regarding biological mechanisms, an increase in the nutrient utilization in either the equatorial regions or the northern polar latitudes can reduce atmospheric CO2 and satisfy proxies of glacial export production. Consistent with previous studies, CO2 is drawn down more easily through increased productivity in the Antarctic region than the sub-Antarctic, but that violates observations of lower export production there. The glacial states are more sensitive to changes in the circulation and less sensitive to changes in nutrient utilization rates than the interglacial states.

Quantifying biological carbon export for the northwest North Atlantic continental shelves

It has been suggested that continental shelf systems contribute disproportionately to the oceanic uptake of atmospheric CO2, but the magnitude of this flux and the relative contributions of different underlying mechanisms are poorly quantified. A biological continental shelf pump mechanism has been implied; however, the magnitude of this export depends on advective transport of carbon‐rich water off the shelf, a process that is difficult to observe directly. Here we use a physical‐biogeochemical model for the northeastern North American continental margin to estimate the uptake of atmospheric CO2, the fraction of this uptake that results from biological processes, and the transport of organic carbon off the shelf. Our results suggest that there is no systematic difference in the area‐normalized CO2 uptake between the shelf regions and the adjacent deep ocean. The advective transport of carbon‐rich water off the shelf is insufficient to drive a Continental Shelf Pump in this region.

Net Loss of CaCO<sub>3</sub> from a subtropical calcifying community due to seawater acidification: mesocosm-scale experimental evidence

Abstract. Acidification of seawater owing to oceanic uptake of atmospheric CO2 originating from human activities such as burning of fossil fuels and land-use changes has raised serious concerns regarding its adverse effects on corals and calcifying communities. Here we demonstrate a net loss of calcium carbonate (CaCO3) material as a result of decreased calcification and increased carbonate dissolution from replicated subtropical coral reef communities (n=3) incubated in continuous-flow mesocosms subject to future seawater conditions. The calcifying community was dominated by the coral Montipora capitata. Daily average community calcification or Net Ecosystem Calcification (NEC=CaCO3 production – dissolution) was positive at 3.3 mmol CaCO3 m−2 h−1 under ambient seawater pCO2 conditions as opposed to negative at −0.04 mmol CaCO3 m−2 h−1 under seawater conditions of double the ambient pCO2. These experimental results provide support for the conclusion that some net calcifying communities could become subject to net dissolution in response to anthropogenic ocean acidification within this century. Nevertheless, individual corals remained healthy, actively calcified (albeit slower than at present rates), and deposited significant amounts of CaCO3 under the prevailing experimental seawater conditions of elevated pCO2.

Poorly cemented coral reefs of the eastern tropical Pacific: Possible insights into reef development in a high-CO 2 world

Ocean acidification describes the progressive, global reduction in seawater pH that is currently underway because of the accelerating oceanic uptake of atmospheric CO(2). Acidification is expected to reduce coral reef calcification and increase reef dissolution. Inorganic cementation in reefs describes the precipitation of CaCO(3) that acts to bind framework components and occlude porosity. Little is known about the effects of ocean acidification on reef cementation and whether changes in cementation rates will affect reef resistance to erosion. Coral reefs of the eastern tropical Pacific (ETP) are poorly developed and subject to rapid bioerosion. Upwelling processes mix cool, subthermocline waters with elevated pCO(2) (the partial pressure of CO(2)) and nutrients into the surface layers throughout the ETP. Concerns about ocean acidification have led to the suggestion that this region of naturally low pH waters may serve as a model of coral reef development in a high-CO(2) world. We analyzed seawater chemistry and reef framework samples from multiple reef sites in the ETP and found that a low carbonate saturation state (Omega) and trace abundances of cement are characteristic of these reefs. These low cement abundances may be a factor in the high bioerosion rates previously reported for ETP reefs, although elevated nutrients in upwelled waters may also be limiting cementation and/or stimulating bioerosion. ETP reefs represent a real-world example of coral reef growth in low-Omega waters that provide insights into how the biological-geological interface of coral reef ecosystems will change in a high-CO(2) world.

Iron in the Ross Sea: 2. Impact of discrete iron addition strategies

Presented are results of a regional‐scale numerical investigation into the effectiveness of Fe fertilization as a means to increase the efficiency of the biological pump in Fe‐limited waters of the Ross Sea, Antarctica. This investigation was conducted using a modified version of the Coupled Ice And Ocean (CIAO) ecosystem model of the Ross Sea sector of the Southern Ocean. Four sets of experiments were performed, investigating the impacts of differences in (1) timing of fertilization, (2) duration of fertilization, (3) amount of Fe added, and (4) size of the fertilized patch. Results show that the stimulation of air‐sea CO2 exchange (FCO2) depends primarily on the timing of fertilization, regardless of the amount of Fe added. When Fe was added at the optimal time of year, FCO2 from the atmosphere into the Ross Sea was increased by 3–22%, depending on fertilization strategy. Increasing patch size produced the largest response, and increasing initial Fe concentration produced the smallest. In all cases, as the intensity of Fe fertilization increased, the fertilization efficiency (increase in CO2 uptake per unit added Fe) dropped. Strategies that maximized the fertilization efficiency resulted in relatively little additional CO2 being drawn out of the atmosphere. To markedly increase oceanic uptake of atmospheric CO2 would require the addition of large amounts of Fe due to the low fertilization efficiencies associated with maximum air‐sea CO2 exchange. Our results also show that differences in the fertilization strategy should be kept in mind when comparing the results of different Fe fertilization experiments.

Possible Effects of Ozone Depletion on the Global Carbon Cycle

The increase of UV-B radiation resulting from ozone depletion is considered to have damaging effects on marine ecosystems. A cutback of marine productivity would tend to reduce the oceanic uptake of atmospheric CO2. Box models of the global oceans based on the distribution of bomb-produced 14C are used to evaluate the possible effects of ozone depletion on the atmospheric CO2 concentration. The maximum effect presumably takes place if the ozone hole reduces the marine productivity to zero in the Antarctic Ocean. In a business-as-usual scenario of future CO2 emissions, the atmospheric CO2 partial pressure (pCO2) would increase by an additional 37 μatm over the course of the next century. This increase corresponds to 4.6% of the projected atmospheric pCO2 in the year 2090. However, if the damaging effect caused by the destruction of the stratospheric ozone layer is assumed to lower the productivity over the Antarctic Ocean by 10%, the atmospheric pCO2 would rise by less than 3 μatm over the expected atmospheric level in the next century.

Enhanced particle fluxes in Bay of Bengal induced by injection of fresh water

THE melting of ice sheets during deglaciation results in the injection of large amounts of fresh water into the oceans1. To investigate how such injections might influence particle fluxes in the ocean, and hence the uptake of atmospheric CO2, we deployed three sediment-trap moorings (two traps in each mooring) in the northern, central and southern parts of the Bay of Bengal, respectively. The Bay of Bengal is suitable for such a study, because some of the world's largest rivers2 supply pulses of fresh water and sediment to the bay, resulting in large seasonal changes in surface salinity3. We find that the maximum river discharge, which occurs during the southwest monsoon, coincides with the maximum observed flux of participate matter. From north to south, the carbonate flux increases, whereas fluxes of opal, organic carbon and particulate matter decrease. The overall flux pattern seems to be controlled by the seasonally varying input from the rivers and the accompanying shift in marine biogenic production. We conclude that fresh-water pulses during deglaciation may therefore have caused similar shifts in marine biogenic production, resulting in short-term episodes of increased oceanic uptake of atmospheric CO2.