Factors regulating carbon sinks in mangrove ecosystems

Mangroves are recognized as one of the richest carbon storage systems. However, the factors regulating carbon sinks in mangrove ecosystems are still unclear, particularly in the subtropical mangroves. The biomass, production, litterfall, detrital export and decomposition of the dominant mangrove vegetation in subtropical (Kandelia obovata) and tropical (Avicennia marina) Taiwan were quantified from October 2011 to July 2014 to construct the carbon budgets. Despite the different tree species, a principal component analysis revealed the site or environmental conditions had a greater influence than the tree species on the carbon processes. For both species, the net production (NP) rates ranged from 10.86 to 27.64 Mg C ha^-1  year^-1 and were higher than the global average rate due to the high tree density. While most of the litterfall remained on the ground, a high percentage (72%-91%) of the ground litter decomposed within 1 year and fluxed out of the mangroves. However, human activities might cause a carbon flux into the mangroves and a lower NP rate. The rates of the organic carbon export and soil heterotrophic respiration were greater than the global mean values and those at other locations. Only a small percentage (3%-12%) of the NP was stored in the sediment. The carbon burial rates were much lower than the global average rate due to their faster decomposition, indicating that decomposition played a critical role in determining the burial rate in the sediment. The summation of the organic and inorganic carbon fluxes and soil heterotrophic respiration well exceeded the amount of litter decomposition, indicating an additional source of organic carbon that was unaccounted for by decomposition in the sediment. Sediment-stable isotope analyses further suggest that the trapping of organic matter from upstream rivers or adjacent waters contributed more to the mangrove carbon sinks than the actual production of the mangrove trees.

Widespread global peatland establishment and persistence over the last 130,000 y

During the Holocene (11,600 y ago to present), northern peatlands accumulated significant C stocks over millennia. However, virtually nothing is known about peatlands that are no longer in the landscape, including ones formed prior to the Holocene: Where were they, when did they form, and why did they disappear? We used records of peatlands buried by mineral sediments for a reconstruction of peat-forming wetlands for the past 130,000 y. Northern peatlands expanded across high latitudes during warm periods and were buried during periods of glacial advance in northern latitudes. Thus, peat accumulation and burial represent a key long-term C storage mechanism in the Earth system.

Glacial−interglacial variations in CO₂ and methane in polar ice cores have been attributed, in part, to changes in global wetland extent, but the wetland distribution before the Last Glacial Maximum (LGM, 21 ka to 18 ka) remains virtually unknown. We present a study of global peatland extent and carbon (C) stocks through the last glacial cycle (130 ka to present) using a newly compiled database of 1,063 detailed stratigraphic records of peat deposits buried by mineral sediments, as well as a global peatland model. Quantitative agreement between modeling and observations shows extensive peat accumulation before the LGM in northern latitudes (>40°N), particularly during warmer periods including the last interglacial (130 ka to 116 ka, MIS 5e) and the interstadial (57 ka to 29 ka, MIS 3). During cooling periods of glacial advance and permafrost formation, the burial of northern peatlands by glaciers and mineral sediments decreased active peatland extent, thickness, and modeled C stocks by 70 to 90% from warmer times. Tropical peatland extent and C stocks show little temporal variation throughout the study period. While the increased burial of northern peats was correlated with cooling periods, the burial of tropical peat was predominately driven by changes in sea level and regional hydrology. Peat burial by mineral sediments represents a mechanism for long-term terrestrial C storage in the Earth system. These results show that northern peatlands accumulate significant C stocks during warmer times, indicating their potential for C sequestration during the warming Anthropocene.

Stocks and losses of soil organic carbon from Chinese vegetated coastal habitats

Global vegetated coastal habitats (VCHs) represent a large sink for organic carbon (OC) stored within their soils. The regional patterns and causes of spatial variation, however, remain uncertain. The sparsity and regional bias of studies on soil OC stocks from Chinese VCHs have limited the reliable estimation of their capacity as regional and global OC sinks. Here, we use field and published data from 262 sampled soil cores and 181 surface soils to report estimates of soil OC stocks, burial rates and losses of VCHs in China. We find that Chinese mangrove, salt marsh and seagrass habitats have relatively low OC stocks, storing 6.3 ± 0.6, 7.5 ± 0.6, and 1.6 ± 0.6 Tg C (±95% confidence interval) in the top meter of the soil profile with burial rates of 44 ± 17, 159 ± 57, and 6 ± 45 Gg C/year, respectively. The variability in the soil OC stocks is linked to biogeographic factors but is mostly impacted by sedimentary processes and anthropic activities. All habitats have experienced significant losses, resulting in estimated emissions of 94.2-395.4 Tg CO₂ e (carbon dioxide equivalent) over the past 70 years. Reversing this trend through conservation and restoration measures has, therefore, great potential in contributing to the mitigation of climate change while providing additional benefits. This assessment, on a national scale from highly sedimentary environments under intensive anthropogenic pressures, provides important insights into blue carbon sink mechanism and sequestration capacities, thus contributing to the synchronous progression of global blue carbon management.

Seeking sunlight: rapid phototactic motility of filamentous mat-forming cyanobacteria optimize photosynthesis and enhance carbon burial in Lake Huron's submerged sinkholes

We studied the motility of filamentous mat-forming cyanobacteria consisting primarily of Oscillatoria-like cells growing under low-light, low-oxygen, and high-sulfur conditions in Lake Huron's submerged sinkholes using in situ observations, in vitro measurements and time-lapse microscopy. Gliding movement of the cyanobacterial trichomes (100-10,000 μm long filaments, composed of cells ∼10 μm wide and ∼3 μm tall) revealed individual as well as group-coordinated motility. When placed in a petri dish and dispersed in ground water from the sinkhole, filaments re-aggregated into defined colonies within minutes, then dispersed again. Speed of individual filaments increased with temperature from ∼50 μm min(-1) or ∼15 body lengths min(-1) at 10°C to ∼215 μm min(-1) or ∼70 body lengths min(-1) at 35°C - rates that are rapid relative to non-flagellated/ciliated microbes. Filaments exhibited precise and coordinated positive phototaxis toward pinpoints of light and congregated under the light of foil cutouts. Such light-responsive clusters showed an increase in photosynthetic yield - suggesting phototactic motility aids in light acquisition as well as photosynthesis. Once light source was removed, filaments slowly spread out evenly and re-aggregated, demonstrating coordinated movement through inter-filament communication regardless of light. Pebbles and pieces of broken shells placed upon intact mat were quickly covered by vertically motile filaments within hours and became fully buried in the anoxic sediments over 3-4 diurnal cycles - likely facilitating the preservation of falling debris. Coordinated horizontal and vertical filament motility optimize mat cohesion and dynamics, photosynthetic efficiency and sedimentary carbon burial in modern-day sinkhole habitats that resemble the shallow seas in Earth's early history. Analogous cyanobacterial motility may have played a key role in the oxygenation of the planet by optimizing photosynthesis while favoring carbon burial.

Carbon burial over the last four millennia is regulated by both climatic and land use change

Carbon sequestration by sediments and vegetated marine systems contributes to atmospheric carbon drawdown, but little empirical evidence is available to help separate the effects of climate change and other anthropogenic activities on carbon burial over centennial timescales. We used marine sediment organic carbon to determine the role of historic climate variability and human habitation in carbon burial over the past 5,071 years. There was centennial-scale sensitivity of carbon supply and burial to climatic variability, with Little Ice Age cooling causing an abrupt ecosystem shift and an increase in marine carbon contributions compared to terrestrial carbon. Although land use changes during the late 1800s did not cause marked alteration in average carbon burial, they did lead to marked increases in the spatial variability of carbon burial. Thus, while carbon burial by vegetated systems is expected to increase with projected climate warming over the coming century, ecosystem restructuring caused by abrupt climate change may produce unexpected change in carbon burial whose variability is also modulated by land use change.

Deep soil flipping increases carbon stocks of New Zealand grasslands

Sequestration of soil organic carbon (SOC) has been recognized as an opportunity to off-set global carbon dioxide (CO₂ ) emissions. Flipping (full inversion to 1-3 m) is a practice used on New Zealand's South Island West Coast to eliminate water-logging in highly podzolized sandy soils. Flipping results in burial of SOC formed in surface soil horizons into the subsoil and the transfer of subsoil material low in SOC to the "new" topsoil. The aims of this study were to quantify changes in the storage and stability of SOC over a 20-year period following flipping of high-productive pasture grassland. Topsoils (0-30 cm) from sites representing a chronosequence of flipping (3-20 years old) were sampled (2005/07) and re-sampled (2017) to assess changes in topsoil carbon stocks. Deeper samples (30-150 cm) were also collected (2017) to evaluate the changes in stocks of SOC previously buried by flipping. Density fractionation was used to determine SOC stability in recent and buried topsoils. Total SOC stocks (0-150 cm) increased significantly by 69 ± 15% (179 ± 40 Mg SOC ha^-1 ) over 20 years following flipping. Topsoil burial caused a one-time sequestration of 160 ± 14 Mg SOC ha^-1 (30-150 cm). The top 0-30 cm accumulated 3.6 Mg SOC ha^-1  year^-1 . The chronosequence and re-sampling revealed SOC accumulation rates of 1.2-1.8 Mg SOC ha^-1  year^-1 in the new surface soil (0-15 cm) and a SOC deficit of 36 ± 5% after 20 years. Flipped subsoils contained up to 32% labile SOC (compared to <1% in un-flipped subsoils) thus buried SOC was preserved. This study confirms that burial of SOC and the exposure of SOC depleted subsoil results in an overall increase of SOC stocks of the whole soil profile and long-term SOC preservation.

Foraminifera-derived carbon contribution to sedimentary inorganic carbon pool: A case study from three Norwegian fjords

Norwegian fjords have been recently recognized as hot spots for carbon burial due to the large amounts of terrestrial organic matter delivered to fjord sediments, as well as the high sediment accumulation rates. Here, we present the first data on the contribution of benthic foraminiferal inorganic carbon to the sediments of three Norwegian fjords. Our study shows that calcareous foraminifera, which are among the most abundant calcifying organisms in the modern global oceans, can constitute between 15% and 33% of inorganic carbon accumulated in the sediments of the two studied southern Norwegian fjords (Raunefjorden and Hjeltefjorden). In a northern Norwegian fjord (Balsfjorden), the contribution of calcareous foraminifera to the inorganic carbon pool is smaller (<1%) than the one observed in southern fjords. We also found that the amount of foraminifera-derived carbon is primarily dependent on the species composition of the foraminifera community. Large calcareous foraminifera species, despite a lower number of individuals, constitute, on average, 13%-29% of the inorganic carbon in the two southern Norwegian fjords, while the contribution of small, highly abundant species does not exceed 4% of the inorganic carbon pools in the sediments. Calcareous foraminifera species that are indicative of dysoxic conditions have been found to have low inorganic carbon contents per specimen compared to other analysed similar-sized calcareous foraminifera species. This relationship most likely exists due to the thin test walls of these foraminifera species, which may facilitate gas exchange. The results of our case study suggest that the climate-driven formation of near-bottom low-oxygen zones may lead to the dominance of foraminifera associated with dysoxic conditions and, in consequence, to the decrease of foraminifera-derived inorganic carbon. However, to properly analyse the contribution of carbon from thin-walled foraminifera to the sedimentary carbon pool, further studies analysing a broader range of these species is needed.

Closing the geologic carbon cycle

Estimates of sedimentary organic carbon burial fluxes based on inventory and isotope mass balance methods have been divergent. A new calculation of the isotope mass balance using a revised assessment of the inputs to the ocean-atmosphere system resolves the apparent discrepancy. Inputs include weathering of carbonate and old kerogen, geogenic methane oxidation, and volcanic and metamorphic degassing. Volcanic and metamorphic degassing comprise ≈23% of the total C input. Inputs from isotopically light OCpetro and CH4-geo drive the mean δ13C of the input to =-8.0 ± 1.9‰, notably lower than the commonly assumed volcanic degassing value. The isotope mass balance model yields a modern burial flux =15.9 ± 6.6 Tmol y-1. The impact of the mid-Miocene Climatic Optimum isotope anomaly is an integrated excess deposition ≈ 4.3 × 106 Tmol between 18 and 11 Ma, which is both longer and larger than estimates for the total degassing by the Columbia River Basalt eruptions, implying a complex carbon system response to large eruptive events. Monte Carlo evaluation finds that late Cenozoic net growth of the carbonate reservoir is very likely while net growth of the Corg reservoir is less certain but more likely than not. At present, subduction does not appear to keep up with net sedimentation and the overall masses of sedimentary carbonate and organic carbon are likely increasing. Growth in the sedimentary Corg reservoir implies oxidation of the surface environment and likely increases in atmospheric pO2.

Fischer W. Reconciling Archean organic-rich mudrocks with low primary productivity before the Great Oxygenation Event

The organic carbon content of ancient rocks provides a fundamental record of the biosphere on early Earth. For over 50 y, the high organic content of Archean (>2.5 Ga) mudrocks has puzzled geologists and evolutionary biologists, because high biological primary productivity was unexpected for the nascent biosphere before the rise of O2. Here, we took a different approach to resolve this apparent paradox, by studying the accumulation rates of Archean organic-rich mudrocks. We evaluated the sedimentation rates of three sections of the Mount McRae Shale and Jeerinah Formation (2.68 to 2.48 Ga, Pilbara Craton, Australia) with new and recently published U-Pb zircon ages from intraformational ash beds. For comparison, we compiled Phanerozoic (<500 Ma) data from comparable depositional settings and developed an idealized model that considers the sedimentation rates for predicting rock organic content. We found that organic-rich Archean mudrocks were deposited under exceptionally low sedimentation rates (~1 m/Ma), in sharp contrast to organic-rich rocks from the Phanerozoic Eon (10 to 100 m/Ma). Constrained by observations, model results indicated that the Archean data reflect low primary productivity (~100-fold lower than during the Phanerozoic) and enhanced preservation under anoxic conditions, with the principal control on organic carbon content provided by dilution with inorganic sediment. Thus, the high organic carbon content which is typically attributed to high productivity instead reflects slow accumulation, high preservation, and minimal inorganic dilution-reconciling the geological evidence with a slow carbon cycle cadence during Archean time.