Calcification-driven CO2 emissions exceed “Blue Carbon” sequestration in a carbonate seagrass meadow

Abstract Long-term “blue carbon” burial in seagrass meadows is complicated by other carbon and alkalinity exchanges that shape net carbon sequestration. We measured a suite of such processes, including denitrification, sulfur, and inorganic carbon cycling, and assessed their impact on air-water carbon dioxide exchange in a typical seagrass meadow underlain by carbonate sediments. Contrary to the prevailing concept of seagrass meadows acting as carbon sinks, eddy covariance measurements reveal this ecosystem as a consistent source of carbon dioxide to the atmosphere, at an average rate of 610 ± 990 µmol m-2 hr-1 during our study and 700 ± 660 µmol m-2 hr-1 over an annual cycle. A robust mass-balance shows that net alkalinity consumption by ecosystem calcification explains >95% of the observed carbon dioxide emissions, far exceeding alkalinity generated by net reduced sulfur, iron and organic carbon burial. Isotope geochemistry of porewaters suggests substantial dissolution and re-crystallization of more stable carbonates mediated by sulfide oxidation-induced acidification, enhancing long-term carbonate burial and ultimate carbon dioxide production. We show that the “blue carbon” sequestration potential of calcifying seagrass meadows has been over-estimated, and that in-situ organic carbon burial only offsets a small fraction (<5%) of calcification-induced CO2 emissions. Ocean-based climate change mitigation activities in such calcifying regions should be approached with caution and an understanding that net carbon sequestration may not be possible.

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Long-term Aptian marine osmium isotopic record of Ontong Java Nui activity

Geology ◽

10.1130/g48863.1 ◽

2021 ◽

Author(s):

Hironao Matsumoto ◽

Rodolfo Coccioni ◽

Fabrizio Frontalini ◽

Kotaro Shirai ◽

Luigi Jovane ◽

...

Keyword(s):

Organic Carbon ◽

Volcanic Eruptions ◽

Hydrothermal Activity ◽

Published Data ◽

Isotopic Data ◽

Oceanic Plateaus ◽

Carbon Burial ◽

Organic Carbon Burial ◽

Isotopic Records

The early to mid-Aptian was punctuated by episodic phases of organic-carbon burial in various oceanographic settings, which are possibly related to massive volcanism associated with the emplacement of the Ontong Java, Manihiki, and Hikurangi oceanic plateaus in the southwestern Pacific Ocean, inferred to have formed a single plateau called Ontong Java Nui. Sedimentary osmium (Os) isotopic compositions are one of the best proxies for determining the timing of voluminous submarine volcanic episodes. However, available Os isotopic records during the age are limited to a narrow interval in the earliest Aptian, which is insufficient for the reconstruction of long-term hydrothermal activity. We document the early to mid-Aptian Os isotopic record using pelagic Tethyan sediments deposited in the Poggio le Guaine (Umbria-Marche Basin, Italy) to precisely constrain the timing of massive volcanic episodes and to assess their impact on the marine environment. Our new Os isotopic data reveal three shifts to unradiogenic values, two of which correspond to black shale horizons in the lower to mid-Aptian, namely the Wezel (herein named) and Fallot Levels. These Os isotopic excursions are ascribed to massive inputs of unradiogenic Os to the ocean through hydrothermal activity. Combining the new Os isotopic record with published data from the lowermost Aptian organic-rich interval in the Gorgo a Cerbara section of the Umbria-Marche Basin, it can be inferred that Ontong Java Nui volcanic eruptions persisted for ~5 m.y. during the early to mid-Aptian.

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Increasing salinization and organic carbon burial rates in seagrass meadows from an anthropogenically-modified coastal lagoon in southern Gulf of Mexico

Estuarine Coastal and Shelf Science ◽

10.1016/j.ecss.2020.106843 ◽

2020 ◽

Vol 242 ◽

pp. 106843 ◽

Cited By ~ 2

Author(s):

Ana Carolina Ruiz-Fernández ◽

Joan-Albert Sanchez-Cabeza ◽

Tomasa Cuéllar-Martínez ◽

Libia Hascibe Pérez-Bernal ◽

Vladislav Carnero-Bravo ◽

...

Keyword(s):

Organic Carbon ◽

Gulf Of Mexico ◽

Coastal Lagoon ◽

Seagrass Meadows ◽

Carbon Burial ◽

Organic Carbon Burial ◽

Southern Gulf Of Mexico ◽

Burial Rates

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Earliest land plants created modern levels of atmospheric oxygen

Proceedings of the National Academy of Sciences ◽

10.1073/pnas.1604787113 ◽

2016 ◽

Vol 113 (35) ◽

pp. 9704-9709 ◽

Cited By ~ 99

Author(s):

Timothy M. Lenton ◽

Tais W. Dahl ◽

Stuart J. Daines ◽

Benjamin J. W. Mills ◽

Kazumi Ozaki ◽

...

Keyword(s):

Organic Carbon ◽

Land Surface ◽

Atmospheric Oxygen ◽

Geochemical Data ◽

Regulatory Regime ◽

Carbon Burial ◽

C:P Ratio ◽

Marine Biomass ◽

Organic Carbon Burial

The progressive oxygenation of the Earth’s atmosphere was pivotal to the evolution of life, but the puzzle of when and how atmospheric oxygen (O2) first approached modern levels (∼21%) remains unresolved. Redox proxy data indicate the deep oceans were oxygenated during 435–392 Ma, and the appearance of fossil charcoal indicates O2 >15–17% by 420–400 Ma. However, existing models have failed to predict oxygenation at this time. Here we show that the earliest plants, which colonized the land surface from ∼470 Ma onward, were responsible for this mid-Paleozoic oxygenation event, through greatly increasing global organic carbon burial—the net long-term source of O2. We use a trait-based ecophysiological model to predict that cryptogamic vegetation cover could have achieved ∼30% of today’s global terrestrial net primary productivity by ∼445 Ma. Data from modern bryophytes suggests this plentiful early plant material had a much higher molar C:P ratio (∼2,000) than marine biomass (∼100), such that a given weathering flux of phosphorus could support more organic carbon burial. Furthermore, recent experiments suggest that early plants selectively increased the flux of phosphorus (relative to alkalinity) weathered from rocks. Combining these effects in a model of long-term biogeochemical cycling, we reproduce a sustained +2‰ increase in the carbonate carbon isotope (δ13C) record by ∼445 Ma, and predict a corresponding rise in O2 to present levels by 420–400 Ma, consistent with geochemical data. This oxygen rise represents a permanent shift in regulatory regime to one where fire-mediated negative feedbacks stabilize high O2 levels.

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Exploring the Growing Role of Terrestrial Carbon Across North Atlantic Fjords

10.5194/egusphere-egu2020-9393 ◽

2020 ◽

Author(s):

Craig Smeaton ◽

William Austin

Keyword(s):

Organic Carbon ◽

Marine Environment ◽

North Atlantic ◽

Planetary Science ◽

Terrestrial Environment ◽

Carbon Burial ◽

Organic Carbon Burial ◽

Fjord Sediments

Fjords are recognized as globally significant hotspots for the burial (Smith et al., 2015) and long-term storage (Smeaton et al., 2017) of marine and terrestrially derived organic carbon (OC). By trapping and locking away OC over geological timescales, fjord sediments provide a potentially important yet largely overlooked climate regulation service. The proximity of fjords to the terrestrial environment in combination with their geomorphology and hydrography results in the fjordic sediments being subsidized with organic carbon (OC) from the terrestrial environment. This terrestrial OC (OCterr) transferred to the marine environment has traditionally be considered lost to the atmosphere in the form of CO2 in most carbon (C) accounting schemes yet globally it is estimated that 55% of OC trapped in fjord sediments is derived from terrestrial sources (Cui et al., 2016). So is this terrestrial OC truly lost? Here, we estimate the quantity of OCterr held within North Atlantic fjords with the aim of better understanding the recent and long-term role of the terrestrial environment in the evolution of these globally significant sedimentary OC stores. By understanding this subsidy of OC from the terrestrial to the marine environment we can take the first steps in quantifying the terrestrial OC stored in fjords and the wider coastal marine environment.Cui, X., Bianchi, T.S., Savage, C. and Smith, R.W., 2016. Organic carbon burial in fjords: Terrestrial versus marine inputs.&#160;Earth and Planetary Science Letters,&#160;451, pp.41-50.Smeaton, C., Austin, W.E., Davies, A., Baltzer, A., Howe, J.A. and Baxter, J.M., 2017. Scotland's forgotten carbon: a national assessment of mid-latitude fjord sedimentary stocks.&#160;Biogeosciences.Smith, R.W., Bianchi, T.S., Allison, M., Savage, C. and Galy, V., 2015. High rates of organic carbon burial in fjord sediments globally.&#160;Nature Geoscience,&#160;8(6), p.450.&#160;

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Blue carbon sequestration dynamics within tropical seagrass sediments: Long-term incubations for changes over climatic scales

10.1101/604587 ◽

2019 ◽

Author(s):

Chuan Chee Hoe ◽

John Barry Gallagher ◽

Chew Swee Theng ◽

Norlaila Binti Mohd. Zanuri

Keyword(s):

Carbon Sequestration ◽

Organic Carbon ◽

Loss Rate ◽

Blue Carbon ◽

Tight Coupling ◽

Before And After ◽

Reactivity Continuum ◽

Particulate Inorganic Carbon

AbstractDetermination of blue carbon sequestration in seagrass sediments over climatic time scales relies on several assumptions, such as no loss of particulate organic carbon (POC) after one or two years, tight coupling between POC loss and CO2emissions, no dissolution of carbonates and removal of the stable black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition/mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49% and 51% after 100 years of burial and 20–22 cm horizons, respectively. However, the loss rate of POC was far greater than the release rate of CO2, both before and after accounting for CO2from anoxic particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2fixation. Instead, evidence based on δ13CO2, acidity and lack of sulphate reduction suggested methanogenesis. The results indicate the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

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When “Blue Carbon” turns white: Isotopic evidence of calcification-driven CO2 emissions in a carbonate seagrass meadow

10.5194/egusphere-egu21-4539 ◽

2021 ◽

Author(s):

Mary Zeller ◽

Bryce Van Dam ◽

Christian Lopes ◽

Ashley Smyth ◽

Michael Böttcher ◽

...

Keyword(s):

Seagrass Meadow ◽

Solid Phase ◽

Blue Carbon ◽

Total Alkalinity ◽

Carbonate Content ◽

Carbonate Precipitation ◽

Seagrass Meadows ◽

Organic Carbon Burial ◽

Seagrass Density

Seagrasses are often considered important players in the global carbon cycle, due to their role in sequestering and protecting sedimentary organic matter as &#8220;Blue Carbon&#8221;.&#160; However, in shallow calcifying systems the ultimate role of seagrass meadows as a sink or source of atmospheric CO2 is complicated by carbonate precipitation and dissolution processes, which produce and consume CO2, respectively.&#160; In general, microbial sulfate, iron, and nitrate reduction produce total alkalinity (TA), and the reverse reaction, the re-oxidation of the reduced species, consumes TA. Therefore, net production of TA only occurs when these reduced species are protected from re-oxidation, for example through the burial of FeSx or the escape of N2.&#160; Seagrasses also affect benthic biogeochemistry by pumping O2 into the rhizosphere, which for example may allow for direct H2S oxidation.Our study investigated the role of these factors and processes (seagrass density, sediment biogeochemistry, carbonate precipitation/dissolution, and ultimately air-sea CO2 exchange), on CO2 source-sink behavior in a shallow calcifying (carbonate content ~90%) seagrass meadow (Florida Bay, USA), dominated by Thalassia testudinum. We collected sediment cores from high and low seagrass density areas for flow through core incubations (N2, O2, DI13C, sulfide, DO13C flux), solid phase chemistry (metals, PO13C, Ca13C18O3, AVS: FeS + H2S, CRS: FeS2 + S0), and porewater chemistry (major cations, DI13C, sulfide, 34S18O4). An exciting aspect of this study is that it was conducted inside the footprint of an Eddy Covariance tower (air-sea CO2 exchange), allowing us to directly link benthic processes with CO2 sink-source dynamics.During the course of our week long study, the seagrass meadow was a consistent source of CO2 to the atmosphere (610 &#177; 990 &#181;mol&#183;m-2&#183;hr-1).&#160; Elevated porewater DIC near 15 cmbsf suggests rhizosphere O2 induced carbonate dissolution, while consumption of DIC in the top 5-10 cm suggests reprecipitation.&#160; With high seagrass density, enriched &#948;13CDIC in the DIC maximum zone (10-25 cm) suggests continual reworking of the carbonates through dissolution/precipitation processes towards more stable PIC, indicating that seagrasses can promote long-term stability of PIC.&#160; We constructed a simple elemental budget, which suggests that net alkalinity consumption by ecosystem calcification explains >95% of the observed CO2 emissions.&#160; Net alkalinity production through net denitrification (and loss of N2) and net sulfate reduction (and subsequent burial of FeS2 + S0), as well as observed organic carbon burial, could only minimally offset ecosystem calcification. &#160;&#160;

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A tentative attempt to better trace the late Pleistocene oxygen cycle

10.5194/egusphere-egu21-12623 ◽

2021 ◽

Author(s):

Ji-Woong Yang ◽

Thomas Extier ◽

Martin Kölling ◽

Amaëlle Landais ◽

Gaëlle Leloup ◽

...

Keyword(s):

Organic Carbon ◽

Late Pleistocene ◽

Global Ocean ◽

Geological Time Scale ◽

Sources And Sinks ◽

The Earth ◽

Carbon Burial ◽

Organic Carbon Burial ◽

Burial Flux

Atmospheric abundance of oxygen (O2) has been co-evolved with different aspects of the Earth system since appearance of oxygenic photosynthesis by cyanobacteria around 2.4 109 years before present (Ga). Therefore, much attention has been paid to understand the changes in O2 and the underlying mechanisms over the Earth&#8217;s history. The pioneering work by Stolper et al. (2016) revealed the long-term decreasing trend of O2 mixing ratios over the last 800,000 years using the ice-core composite record of molar ratios of O2 and nitrogen (&#948;(O2/N2)), implying a slight imbalance between sources and sinks. Over geological time scale, O2 is mainly controlled by burial and oxidation of organic carbon and pyrite, but also by oxidation of volcanic gases and sedimentary rocks. Nevertheless, the O2 cycle of the late Pleistocene has not been well understood, partly due to the lack of knowledge about the individual sources and sinks. Since then, K&#246;lling et al. (2019) proposed a simple model to estimate the O2 release/uptake fluxes due to the pyrite burial/oxidation that predicts up to ~70% of the O2 decrease of the last 800,000 years could be explained by pyrite burial/oxidation.Building on this, we present here our preliminary, tentative attempt for reconstruction of the net organic carbon burial flux over the last 800,000 years by combining available information (including new &#948;(O2/N2) data) and assuming constant O2 fluxes associated with volcanic outgassing and rock weathering. The long-term organic carbon burial flux trend obtained with our new calculations is similar to the global ocean &#948;13C records but also to simulations using a conceptual carbon cycle model (Paillard, 2017). These results partly support the geomorphological hypothesis that the major sea-level drops during the earlier period of the last 800,000 years lead to enhanced organic carbon burial, and that significant changes in the net organic carbon happen around Marine Isotopic Stage (MIS) 13. In addition, we present the long-term decreasing trend of the global biosphere productivity, or gross photosynthetic O2 flux, reconstructed from new measurements of triple-isotope composition of atmospheric O2 trapped in ice cores. As the largest O2 flux, the observed decrease in gross photosynthesis requires to be compensated by parallel reduction of global ecosystem respiration.

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Sediment Organic Carbon Sequestration of Balkhash Lake in Central Asia

Sustainability ◽

10.3390/su13179958 ◽

2021 ◽

Vol 13 (17) ◽

pp. 9958

Author(s):

Wen Liu ◽

Long Ma ◽

Jilili Abuduwaili ◽

Gulnura Issanova ◽

Galymzhan Saparov

Keyword(s):

Carbon Sequestration ◽

Organic Carbon ◽

The Past ◽

Burial Rate ◽

Carbon Burial ◽

Sediment Organic Carbon ◽

Organic Carbon Burial ◽

Organic Carbon Burial Rate ◽

Organic Carbon Sequestration ◽

Global Carbon

As an important part of the global carbon pool, lake carbon is of great significance in the global carbon cycle. Based on a study of the sedimentary proxies of Balkhash Lake, Central Asia’s largest lake, changes in the organic carbon sequestration in the lake sediments and their possible influence over the past 150 years were studied. The results suggested that the organic carbon in the sediments of Lake Balkhash comes mainly from aquatic plants. The organic carbon burial rate fluctuated from 8.16 to 30.04 g·m−2·a−1 and the minimum appeared at the top of the core. The organic carbon burial rate continues to decline as it has over the past 150 years. Global warming, higher hydrodynamic force, and low terrestrial input have not been conducive to the improvement of organic carbon sequestration in Balkhash Lake; the construction of a large reservoir had a greater impact on the sedimentary proxy of total organic carbon content, which could lead to a large deviation for environmental reconstruction. This is the first study to assess the sediment organic carbon sequestration using the modern sediments of Central Asia’s largest lake, which is of great scientific significance. The results contribute to an understanding of organic carbon sequestration in Central Asia and may provide a scientific basis for carbon balance assessment in regional and global scales.

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Blue carbon sequestration dynamics within tropical seagrass sediments: long-term incubations for changes over climatic scales

Marine and Freshwater Research ◽

10.1071/mf19119 ◽

2020 ◽

Vol 71 (8) ◽

pp. 892 ◽

Cited By ~ 2

Author(s):

Chee Hoe Chuan ◽

John Barry Gallagher ◽

Swee Theng Chew ◽

M. Zanuri Norlaila Binti

Keyword(s):

Carbon Sequestration ◽

Organic Carbon ◽

Co2 Fixation ◽

Blue Carbon ◽

Tight Coupling ◽

Reactivity Continuum ◽

Dark Co2 Fixation ◽

Particulate Inorganic Carbon

Determination of blue carbon sequestration in seagrass sediments over climatic time scales (>100 years) relies on several assumptions, including no loss of particulate organic carbon (POC) after 1–2 years, tight coupling between POC loss and CO2 emissions, no dissolution of carbonates, and removal of the recalcitrant black carbon (BC) contribution. We tested these assumptions via 500-day anoxic decomposition and mineralisation experiments to capture centennial parameter decay dynamics from two sediment horizons robustly dated as 2 and 18 years old. No loss of BC was detected, and decay of POC was best described for both horizons by near-identical reactivity continuum models. The models predicted average losses of 49 and 51% after 100 years of burial for the surface and 20–22-cm horizons respectively. However, the loss rate of POC was far greater than the release rate of CO2, even after accounting for CO2 from particulate inorganic carbon (PIC) production, possibly as siderite. The deficit could not be attributed to dissolved organic carbon or dark CO2 fixation. Instead, evidence based on δ13CO2, acidity and lack of sulfate reduction suggested methanogenesis. The results indicated the importance of centennial losses of POC and PIC precipitation and possibly methanogenesis in estimating carbon sequestration rates.

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Past climate variations recorded in needle-like aragonites correlate with organic carbon burial efficiency as revealed by lake sediments in Croatia

Scientific Reports ◽

10.1038/s41598-021-87166-2 ◽

2021 ◽

Vol 11 (1) ◽

Author(s):

Ivan Razum ◽

Petra Bajo ◽

Dea Brunović ◽

Nikolina Ilijanić ◽

Ozren Hasan ◽

...

Keyword(s):

Organic Carbon ◽

Stable Carbon Isotope ◽

Climate Variations ◽

Before Present ◽

Stable Carbon ◽

Climate Trends ◽

Geochemical Proxies ◽

Carbon Burial ◽

Organic Carbon Burial ◽

Burial Efficiency

AbstractThe drivers of organic carbon (OC) burial efficiency are still poorly understood despite their key role in reliable projections of future climate trends. Here, we provide insights on this issue by presenting a paleoclimate time series of sediments, including the OC contents, from Lake Veliko jezero, Croatia. The Sr/Ca ratios of the bulk sediment are mainly derived from the strontium (Sr) and calcium (Ca) concentrations of needle-like aragonite in Core M1-A and used as paleotemperature and paleohydrology indicators. Four major and six minor cold and dry events were detected in the interval from 8.3 to 2.6 calibrated kilo anno before present (cal ka BP). The combined assessment of Sr/Ca ratios, OC content, carbon/nitrogen (C/N) ratios, stable carbon isotope (δ13C) ratios, and modeled geochemical proxies for paleoredox conditions and aeolian input revealed that cold and dry climate states promoted anoxic conditions in the lake, thereby enhancing organic matter preservation and increasing the OC burial efficiency. Our study shows that the projected future increase in temperature might play an important role in the OC burial efficiency of meromictic lakes.

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