Carbon Accumulation In Sediments Research Articles

Pathways of carbonate sediment accumulation in a large, shallow lake

Since carbonate formation is an important process linking inorganic and biological components of freshwater ecosystems, we characterized the formation of modern carbonate sediments in a large, shallow, calcareous lake (Lake Balaton in Hungary). We measured the amount of allochtonous mineral particles delivered to the lake by tributaries and through the atmosphere over a 2-year period, and estimated the mass of carbonate minerals that precipitated from lakewater. Chemical and structural features of mineral particles from various sources were also studied. Both the mineralogical character and the amount of particles delivered by streams and through the atmosphere were similar, and formed a minor fraction of the annual sediment increment (∼5%–6% by mass). Since the watercourses feeding the lake had high concentrations of Ca2+, Mg2+, and HCO3− (with a Mg/Ca mol ratio ranging from 1 to 4), Mg-bearing calcite (with 2–17 mol% MgCO3) was found to continually precipitate in the lake. According to X-ray powder diffraction measurements, the Mg content of calcite increased from West to East, in parallel with changes in water chemistry. Dolomite was detected as a minor phase, and in the eastern part of the lake it typically produced a split 104 peak in X-ray diffractograms, suggesting two distinct sources: stoichiometric dolomite was allochtonous, whereas a Ca-rich protodolomite fraction formed in the lake. Mg-bearing calcite precipitating in the lake was found by far the largest contributor to sediment formation, with an estimated annual accumulation of about 0.75–0.9 mm consolidated sediment; thus, ∼89% of the currently forming sediment consists of autochtonous carbonate. In addition to providing new estimates for the rates of accumulation of distinct sediment fractions, our results also provide a baseline for further studies on the retention and release of nutrients by sediment minerals.

Limestones of the Uk Formation (Upper Riphean, the Southern Urals): Effect of contamination and diagenetic fluids on the distribution of rare-earth elements and yttrium

Research subject. A study was conducted into the geochemical features of various lithotypes of carbonate rocks (granular and clastic limestones, limestones with planar, planar-columnar and columnar stromatolites) from the upper subformation of the Uk Formation. This formation crowns the Upper Riphean Karatau Group on the Western slope of the Southern Urals (Bashkir meganticlinorium). Material and methods. The research material was two collections of carbonate rocks (11 and 32 samples), the general geochemical features of which were previously considered in publications in 2019. The main research method was the analysis of the correlation relationships of the main rock-forming components, trace elements and their indicator relationships. Research results. It was established that all lithotypes are characterized by a significant content of both the main rock-forming components and the trace elements that make up the insoluble residue/fine-grained terrigenous admixture. This suggests that the area of accumulation of carbonate sediments was located near the continent, which served as a source of a significant amount of fine-grained/clay suspension. Conclusions. A strong positive correlation was observed between K2O and REE in the carbonate rocks of the Upper Uk Subformation, and a strong negative correlation was observed between REE and CaO. This suggests that the REE content in carbonate rocks is controlled mainly by terrigenous/clay admixture. A positive correlation is also observed between REE and Th, Zr, and Al2O3. All this shows that REE in carbonate rocks are mainly present in clay admixtures. The correlation coefficient between Ce/Ce* and (Dy/Sm)N is 0.20, while this coefficient for Eu/Eu* and (Pr/Sm)N equals 0.12. Thus, a definite conclusion about the effect of diagenetic fluids on the distribution of REE cannot be made. The data obtained show that the distribution of REE and Y in our collection of carbonate rocks of the Upper subformation of the Uk Formation of the Southern Urals is controlled mainly by clay admixtures.

Carbon budgets of two typical polyculture pond systems in coastal China and their potential roles in the global carbon cycle

The role of aquaculture systems in the global carbon cycle is poorly understood to date. We investigated the carbon budgets in 2 polyculture systems (PM: swimming crab Portunus trituberculatus with kuruma shrimp Marsupenaeus japonicus; and PMR: crab, shrimp and short-necked clam Ruditapes philippinarum) during the farming season (125 d in total). The main carbon input occurred via water (PM: 55.06%; PMR: 62.50%), followed by that via feed. PM absorbed 21.55 g C m-2 across the air-water interface, accounting for 13.27% of the total input. The carbon output via water was the main output in both PM (89.17%) and PMR (46.43%). PMR emitted 53.00 g C m-2 into the atmosphere during the farming season, accounting for 32.53% of the total output. The carbon output by harvested animals in PMR accounted for 19.48% of the total output, which was much higher than that in PM (5.71%). Carbon accumulation in the sediment was significantly higher in PM than in PMR (p < 0.05), but the value of both represented a small portion of the total output. Clam farming in the polyculture system profoundly altered the carbon budgets by changing the CO2 flux direction, reducing carbon accumulation in water and sediments and increasing the carbon output by harvested animals. In terms of the global warming potential, which was calculated from the results of CO2 and CH4 fluxes across the air-water interface, PM had the potential for mitigating the greenhouse effect on the 20 yr horizon, whereas PMR exacerbated global warming. Our results suggest that PM acted as a net carbon sink in the global carbon cycle, while PMR acted as a carbon source.

A review of carbon sink or source effect on artificial reservoirs

The inland water system is a key conduit in transporting carbon from land to ocean, but this conduit is not a neutral pipeline because it releases greenhouse gases into the atmosphere and also stores carbon in sediment. Currently, the role of inland waters in the global carbon cycle is modified and complicated by the huge number of reservoirs that are associated with climatic alteration due to their carbon sequestration and greenhouse gas emission functions. Moreover, reservoirs are distinct from natural rivers and lakes in various ways, promoting the emission of greenhouse gases from their surfaces and carbon accumulation in sediment through high sedimentation rates and larger ratios of watershed areas to reservoir areas. Consequently, their contributions to the global carbon cycle will increase in the coming decade as a result of an increase in the number of reservoirs. This assessment (1) demonstrates that reservoirs play a significant role in carbon burial and greenhouse gas emissions, (2) defines factors that influence the carbon budget of reservoirs, and (3) explains how reservoirs affect global climatic change. According to previous findings, reservoirs could serve as carbon sinks or sources, depending on the age, location, and climate of the given reservoir. Further, the magnitude of carbon burial and greenhouse gas emissions in reservoirs depends on productivity, land use, geology, water body type, and watershed morphometry. So far, few studies have examined both carbon storage and greenhouse gas emissions in reservoirs, and many efforts to quantify burial and emissions have been compromised by limited data availability. In this review, we mainly focus on recent literature on carbon burial and greenhouse gas emissions in reservoirs and global estimates of terrestrial carbon input to inland waters.

Variability in the organic carbon stocks, sources, and accumulation rates of Indonesian mangrove ecosystems

Mangrove ecosystems are an important natural carbon sink that accumulate and store large amounts of organic carbon (Corg), in particular in the sediment. However, the magnitude of carbon stocks and the rate of carbon accumulation (CAR) vary geographically due to a large variation of local factors. In order to better understand the blue carbon sink of mangrove ecosystems, we measured organic carbon stocks, sources and accumulation rates in three Indonesian mangrove ecosystems with different environmental settings and conditions; (i) a degraded estuarine mangrove forest in the Segara Anakan Lagoon (SAL), Central Java, (ii) an undegraded estuarine mangrove forest in Berau region, East Kalimantan, and (iii) a pristine marine mangrove forest on Kongsi Island, Thousand Islands, Jakarta. In general, Corg stocks were higher in estuarine than in marine mangroves, although a large variation was observed among the estuarine mangroves. The mean total Corg stock in Berau (615 ± 181 Mg C ha−1) is twice as high as that in SAL (298 ± 181 Mg C ha−1). However, the Segara Anakan Lagoon displayed large within-system variation with a much higher Corg stock in the eastern (483 ± 124 Mg C ha−1) than in the central lagoon (167 ± 36 Mg C ha−1). The predominant accumulation of autochthonous mangrove organic matter likely contributed to the higher Corg stocks in Berau and the eastern SAL. Interestingly, the CAR distribution pattern in SAL is opposite to that of its Corg stocks. The central SAL that receives high sediment inputs from the hinterland has a much higher CAR than the eastern SAL (658 ± 311 g C m−2 yr−1 and 194 ± 46 g C m−2 yr−1, respectively), while Berau has one of the highest CAR (1722 ± 183 g C m−2 yr−1) ever measured. It appears that these large differences are driven by the environmental setting and conditions, mainly sediment dynamics and hydrodynamics, landform, and vegetation conditions. It is inferred that quantifying carbon accumulation in sediments is a useful tool in estimating the present-day carbon storage of mangrove ecosystems. This is a precondition for taking measures under REDD+ (Reducing Emissions from Deforestation and Forest Degradation and the role of conservation, sustainable management of forests and enhancement of forest carbon stocks in developing countries) schemes.

Latitudinal trends in stable isotope signatures and carbon-concentrating mechanisms of northeast Atlantic rhodoliths

Abstract. Rhodoliths are free-living calcifying red algae that form extensive beds in shallow marine benthic environments (<250 m), which provide important habitats and nurseries for marine organisms and contribute to carbonate sediment accumulation. There is growing concern that these organisms are sensitive to global climate change, yet little is known about their physiology. Considering their broad distribution along most continental coastlines, their potential sensitivity to global change could have important consequences for the productivity and diversity of benthic coastal environments. The goal of this study was to determine the plasticity of carbon-concentrating mechanisms (CCMs) of rhodoliths along a latitudinal gradient in the northeast Atlantic using natural stable isotope signatures. The δ13C signature of macroalgae can be used to provide an indication of the preferred inorganic carbon source (CO2 vs. HCO3-). Here we present the total (δ13CT) and organic (δ13Corg) δ13C signatures of northeast Atlantic rhodoliths with respect to changing environmental conditions along a latitudinal gradient from the Canary Islands to Spitsbergen. The δ13CT signatures (−11.9 to −0.89) of rhodoliths analyzed in this study were generally higher than the δ13Corg signatures, which ranged from −25.7 to −2.8. We observed a decreasing trend in δ13CT signatures with increasing latitude and temperature, while δ13Corg signatures were only significantly correlated to dissolved inorganic carbon. These data suggest that high-latitude rhodoliths rely more on CO2 as an inorganic carbon source, while low-latitude rhodoliths likely take up HCO3- directly, but none of our specimens had ∂13Corg signatures less than −30, suggesting that none of them relied solely on diffusive CO2 uptake. However, depth also has a significant effect on both skeletal and organic δ13C signatures, suggesting that both local and latitudinal trends influence the plasticity of rhodolith inorganic carbon acquisition and assimilation. Our results show that many species, particularly those at lower latitudes, have CCMs that facilitate HCO3- use for photosynthesis. This is an important adaptation for marine macroalgae, because HCO3- is available at higher concentrations than CO2 in seawater, and this becomes even more extreme with increasing temperature. The flexibility of CCMs in northeast Atlantic rhodoliths observed in our study may provide a key physiological mechanism for potential adaptation of rhodoliths to future global climate change.

Fractionation of Carbonate Carbon (Ccarb) Accumulation between Continents and Oceans in the Late Mesozoic–Cenozoic

The Ccarb masses per time unit was determined for separate oceanic basins and for the entire World Ocean using lithological–facies mapping of the Neo– and Eopleistocene age sections of the Pleistocene pelagic zones in the World Ocean. These parameters are compared with those of continents, continental shelves and slopes, and oceans, which were recalculated using data by Ronov (1993) for the Upper Jurassic–Pliocene. At the Mesozoic–Cenozoic boundary, carbonate accumulation was shifted from continents to oceans. The accumulation of carbonate sediments on continents is determined by areas of epicontinental seas. Significant role in the history of oceanic carbonate sedimentation is played by the nutrient fluxes from continents into the World Ocean. Subduction and evolution of the carbonate compensation depth (CCD) play significant role in calculating the quantitative parameters of carbonate accumulation in ocean.

Sediment Carbon Accumulation in Southern Latitude Saltmarsh Communities of Tasmania, Australia.

Carbon sequestration values of wetlands are greatest in their sediments. Northern hemisphere research dominates the earlier saltmarsh carbon sequestration literature, recently augmented by analyses across mainland Australia where species assemblages, catchment histories and environmental settings differ. No previous assessment has been made for Tasmania. Carbon stores and accumulation rates in saltmarsh sediments of the Rubicon estuary, Tasmania, were investigated. Carbon was determined from sediment cores by Elemental Analyser, combined with analysis of organic content and bulk density. Carbon accumulation was determined using short-term and long-term sediment accretion indicators. Results showed carbon densities to be lower than global averages, with variation found between carbon stores of native and introduced species zones. Cores from introduced Spartina anglica indicated a trend of higher sediment carbon percentages relative to cores from native saltmarsh Juncus kraussii and Sarcocornia quinqueflora, and in finer grain sizes. Sediment carbon stock of 30 cm depths was 49.5 Mg C ha−1 for native saltmarsh and 55.5 Mg C ha−1 for Spartina. Carbon percentages were low owing to high catchment inorganic sediment yields, however carbon accumulation rates were similar to global averages, particularly under Spartina. Covering 85% of saltmarsh area in the estuary, Spartina contributes the majority to carbon stores, potentially indicating a previously unrecognized value for this invasive species in Australia.

Sedimentary carbon forms in relation to climate and phytoplankton biomass in a large, shallow, hard-water boreal lake

Carbon storage in lakes can have huge implications for the global carbon cycle, as lakes annually accumulate up to one half the amount of organic carbon buried in marine sediments. Yet little is known of the effect of recent climate change on carbon storage in lakes. We analyzed century-scale time series of climate variables (precipitation, temperature, NAO winter index) and profiles of sediment characteristics in a dated sediment core from shallow, eutrophic Lake Vortsjarv, south Estonia. We used path analysis to evaluate the effect of climate conditions on phytoplankton biomass in the lake and accumulation of organic and inorganic carbon in the sediment. Changes in winter and spring climate influenced the lake’s phytoplankton growth significantly. Carbon pathways in hard-water Lake Vortsjarv were influenced by both hydrological (most significant in colder periods) and biogeochemical processes. Increased nutrient and water input to Lake Vortsjarv, anticipated with projected climate warming, favours greater in-lake productivity, larger accumulation of inorganic carbon in sediments, and an increase in organic carbon mineralisation, which fuels atmospheric greenhouse gas emissions from the lake.

Global, regional and local controls on the development of a Triassic carbonate ramp system, Western Balkanides, Bulgaria

AbstractThe Early to Late Triassic development of a carbonate ramp system in the subtropical belt of the NW Tethys was controlled by the interplay of several global and regional factors: geotectonic setting (slow continuous subsidence on a passive continental margin), antecedent topography (low-gradient relief inherited from preceding depositional regime), climate and oceanography (warm and dry climatic conditions, storm influence), relative sea-level changes (Olenekian to Anisian eustatic rise, middle Anisian to early Carnian sea-level fall), lack of frame-builders (favouring the maintenance of ramp morphology), and carbonate production (abundant formation of lime mud, non-skeletal grains and marine cements, development of diverse biota controlled by biological evolution and environmental conditions). Elevated palaeorelief affected the ramp initialization on a local scale, while autogenic processes largely controlled the formation of peritidal cyclicity during the early stage of ramp retrogradation. Probably fault-driven differential subsidence caused a local distal steepening of the ramp profile in middle–late Anisian time. The generally favourable conditions promoted long-term maintenance of homoclinal ramp morphology and accumulation of carbonate sediments having great maximum thickness (~500 m). Shutdown of the carbonate factory and demise of the ramp system in the early Carnian resulted from relative sea-level fall and subsequent emergence. After a period of subaerial exposure with minor karstification, the deposition of continental quartz arenites suggests the possible effect of the Carnian Pluvial Episode.

Global Carbon Cycle and Organic Matter Accumulation in the Earth Crust

Global Carbon Cycle and Organic Matter Accumulation in the Earth Crust The natural process of organic carbon accumulation in sediments is considered in the light of recently suggested model of global redox carbon cycle. It is inferred that in geologic time the process was uneven and some periods were favorable for the increased burial organic carbon rate. Organic carbon accumulation occurred in parallel with oxygen growth in the atmosphere. According to the model, the accumulation process is a cyclic one.

A fundamental paradigm for coral reef carbonate sediment dissolution

The long-term success of coral reefs depends on a positive balance of calcium carbonate production exceeding dissolution, erosion, and material export. As a result of ocean acidification, coral reefs could transition from net accretion to net erosion owing to decreasing rates of calcification and increasing rates of chemical dissolution and bioerosion. Here, I present a fundamental paradigm that aims to explain the main driver of carbonate sediment dissolution on coral reefs based on theory and a new empirical dataset of pore water carbonate chemistry from the Bermuda coral reef platform. The paradigm shows that carbonate sediment dissolution is most strongly controlled by the extent of organic matter decomposition in the sediments, but that the magnitude of dissolution is influenced by how much decomposition is required to reach pore water undersaturation with respect to the most soluble bulk carbonate mineral phase present in the sediments, a condition defined as the Carbonate Critical Threshold (CCT). Decomposition of organic matter beyond the CCT under aerobic conditions results in stoichiometric proportional dissolution of carbonate sediments. As ocean acidification proceeds over the next several decades, the extent of organic matter decomposition required to reach the CCT will decrease, carbonate dissolution will increase, and subsequently the accumulation of carbonate sediments will decrease. Since drastic reductions in anthropogenic CO2 emission are unlikely in the foreseeable future, the paradigm shows that active controls and reduction of organic matter input to coral reefs at the local scale might be an effective mitigation strategy to prevent or delay coral reefs transitioning to a state of net dissolution.

Unravelling the influence of water depth and wave energy on the facies diversity of shelf carbonates

AbstractCarbonate sequence stratigraphy is founded on the principle that changes in relative sea‐level are recorded in the rock record by the accumulation of sediment with relative water depth‐dependent attributes. While at the scale of a shelf to basin transect, facies clearly arrange by water depth, the relation blurs for depths <40 m, the most vigorous zone of carbonate production. The reason for this change in behaviour is two‐fold. Firstly, in shallow water, the intrinsic processes of storm and wave reworking influence the seabed through submarine erosion and sediment redistribution. Secondly, facies diversity tends to be greater in shallow water than deep water because of a greater diversity in grain producers. Remote sensing imagery, field observations and hydrodynamic models for two reef‐rimmed shore‐attached carbonate platforms in the Red Sea show neither water depth nor energy regime to be reliable indicators of facies type when considered in isolation. Considered together, however, the predictive power of the two variables rises significantly. The results demonstrate it to be an oversimplification to assume a direct link between palaeo‐water depth and depositional diversity of subtidal lithofacies, while highlighting the importance of hydrodynamics in directing the accumulation of carbonate sediments in the shallow photic zone. While the size distributions of facies extents in the two focus areas follow power laws, no direct relation between the lateral continuity of the facies belts and water depth or wave height is reported. The work is relevant for the interpretation of metre‐scale subtidal carbonate cycles throughout the geological record by demonstrating how caution must be applied when inferring palaeo‐water depths from depositional facies.

Denitrification in a Laurentian Great Lakes coastal wetland invaded by hybrid cattail (Typha × glauca)

Wetland ecosystems maintain and improve water quality through the process of denitrification, an increasingly important ecosystem service due to global N pollution. Invasive plants have the potential to disrupt denitrification by altering the environmental conditions that facilitate this process. Great Lakes coastal wetlands are experiencing widespread invasion by highly productive hybrid cattail with largely uncertain biogeochemical effects. Through field and controlled mesocosm studies, we sought to determine the effects of cattail invasion through time on denitrification rates and associated environmental factors in a Great Lakes coastal wetland. In the field, we found that cattail density correlated with increased denitrification and a suite of environmental and plant community characteristics and denitrification rates were positively correlated with NH4 +, sediment organic matter, reduced water levels, and cattail stand age. Through our controlled mesocosm study, we documented conditions 1- and 5-year following invasion and found that denitrification rates and soil organic matter increased in year 5, and cattail and year-since-invasion altered plant communities and soil NH4 +. Only a weak correlation between denitrification rates and cattail treatments was noted, however, owing to high replicate variability. Our results indicate that with increasing cattail residence time, one ecosystem service, biodiversity, was negatively impacted, while two other services, denitrification and sediment carbon accumulation, were enhanced. Thus, this highly invaded wetland still provides valuable services to aquatic ecosystems and to society. A holistic perspective is therefore critical when evaluating invasive species impacts in which negative impacts are weighed against other ecosystem services, which may be stimulated.

Comparing carbon sequestration and stand structure of monoculture and mixed mangrove plantations of Sonneratia caseolaris and S. apetala in Southern China

Mangroves are one of the most carbon-rich tropical ecosystems. Two fast-growing mangrove species of the genus Sonneratia, the native S. caseolaris and non-native S. apetala, have been widely used for mangrove reforestation in China; however their ability to sequester carbon is still unclear. The present study aimed to estimate the growth, carbon accumulation in biomass and carbon sequestration in sediments of these two species, in both mixed (50% S. caseolaris; 50% S. apetala) and monoculture plantations, in the intertidal zones of Shenzhen Bay, Guangdong Province, China. Twenty-five months of observation showed strong competition between the two species in the mixed plantation, with the native S. caseolaris outcompeting the non-native S. apetala due to a faster growth rate. Although S. caseolaris in the mixed plantation had lower carbon storage in biomass than in monoculture, carbon accumulation in sediment was higher in the mixed plantation. Thus, the relative advantage of the mixed plantation was in sequestering more carbon in sediment, as opposed to high carbon accumulation to biomass. These results indicated that the mixed plantation could be a good option for mangrove restoration and carbon sequestration of sediment. However, the two and a half years of this study may not indicate long-term trends, so more research on long-term species performance is essential for successful mangrove reforestation.

Permian lysocline shoaling and ocean acidification along NW Pangea led to carbonate eradication and chert expansion

The Upper Carboniferous to Upper Permian deposits of the Sverdrup Basin (Arctic Canada) record a transition from carbonate to silica dominated shallow shelf ecosystems. Chert expansion started during the latest Carboniferous in distal deep-water slope environments when prolific warm-water, photozoan carbonate factories developed on the adjacent shallow shelves. A shift to heterozoan factories during the Sakmarian–Artinskian was caused by the introduction of cooler water in response to paleoceanographic changes along NW Pangea. By Kungurian time, heterozoan carbonate factories narrowed substantially while hyalosponge factories had expanded well into the mid shelves. A brief return to widespread heterozoan carbonate sedimentation occurred during the Wordian, followed by further encroachment of shallow shelves by biosiliceous factories during the Capitanian and Wuchiapingian. A major drop in relative rates of carbonate sediment accumulation with time suggests both carbonate and biosiliceous factories were under considerable stress prior to the Late Permian Extinction event. It is proposed that Sverdrup Basin waters became progressively more acidic in response to build up of atmospheric CO2 throughout the Permian which led to a gradual shoaling of the lysocline and the calcite compensation depth, which was amplified by upwelling along the northwestern margin of Pangea. Ocean acidification initiated in response to amalgamation of the Pangea supercontinent. This inhibited the silicate weathering-response through development of thick protective soil blankets. A temperature shock associated with the extinction event led to a major fall of the lysocline allowing carbonates to resume accumulation in spite of low pH conditions. The subsequent die off of terrestrial vegetation, stripping of soil cover, widespread continental erosion and repeated exposures of fresh rock surfaces associated with a major base level shift allowed silicate weathering to restart and drive oceans back to carbonate saturation.

Controls and predictability of carbonate facies architecture in a Lower Jurassic three‐dimensional barrier‐shoal complex (Djebel Bou Dahar, High Atlas, Morocco)

AbstractSpatial information on lithofacies from outcrops is paramount for understanding the internal dynamics, external controls and degree of predictability of the facies architecture of shallow‐water carbonate‐platform tops. To quantify the spatial distribution and vertical stacking of lithofacies within an outer‐platform shoal‐barrier complex, integrated facies analysis and digital field technologies have been applied to a high‐relief carbonate platform exposed in the Djebel Bou Dahar (Lower Jurassic, High Atlas, Morocco). The outer platform is characterized by subtidal, cross‐bedded, coarse grainstone to rudstone grading into supratidal, pisoidal packstone‐rudstone with tepees that together formed a 350 to 420 m wide shoal‐barrier belt parallel to the margin. This belt acted as a topographic high separating a restricted lagoon from the subtidal, open marine region. Low‐energy tidal flats developed on the protected flank of the barrier facing the lagoon. Lithofacies patterns were captured quantitatively from outcrop and integrated into a digital outcrop model. The outcrop model enabled rapid visualization of field data and efficient extraction of quantitative data such as widths of facies belts. In addition, the spatial heterogeneity was captured in multiple time slices, i.e. during different phases of cyclic base‐level fluctuations. In general, the lateral continuity of lithofacies is highest when relative water depth increased during flooding of the platform top, establishing low‐energy subtidal conditions across the whole platform, and when the accommodation space was filled with tidal flat facies. Heterogeneity increased during deposition of the relief‐building bar facies that promoted spatial diversification of depositional environments during the initial phases of accommodation space creation. Cycles commonly are composed of a thin transgressive tidal flat unit, followed by coated‐grain rudstone bar facies. Lateral to the bar facies, pisoidal‐grainstone beach deposits accumulated. These bar and beach deposits were overlain by subtidal lagoonal facies or would grow through the maximum flooding and highstand. There the bars either graded into supratidal pisoidal facies with tepees (when accommodation space was filled) or were capped by subaerial exposure (due to a sea‐level fall). Modified embedded Markov analysis was used to test the presence of common ordering in vertical lithofacies stacking in a stationary interval (constant depositional mode). Analysis of individual sections did not reveal any ordering, which may be related to the limited thickness of these sections. Composite sections, however, rejected the null hypothesis of randomness. The addition of stratigraphically significant information to the Markov analysis, such as exposure surfaces and lateral dimensions of facies bodies, strengthens the verdict of unambiguous preferential ordering. Through careful quantitative reconstruction of stratal geometry and facies relationships in fully integrated digital outcrop models, accurate depositional models could be established that enhanced the predictability of carbonate sediment accumulation.

Demise of reef-flat carbonate accumulation with late Holocene sea-level fall: evidence from Molokai, Hawaii

Twelve cores from the protected reef-flat of Molokai revealed that carbonate sediment accumulation, ranging from 3 mm year−1 to less than 1 mm year−1, ended on average 2,500 years ago. Modern sediment is present as a mobile surface veneer but is not trapped within the reef framework. This finding is consistent with the arrest of deposition at the end of the mid-Holocene highstand, known locally as the “Kapapa Stand of the Sea,” ~2 m above the present datum ca. 3,500 years ago in the main Hawaiian Islands. Subsequent erosion, non-deposition, and/or a lack of rigid binding were probable factors leading to the lack of reef-flat accumulation during the late Holocene sea-level fall. Given anticipated climate changes, increased sedimentation of reef-flat environments is to be expected as a consequence of higher sea level.

Recent sediment accumulation and carbon burial in the East China Sea

Sediment and carbon budgets were constructed for the East China Sea (ECS) Shelf based on up‐to‐date data. The total annual sediment flux in the ECS Shelf is approximately 96% of the terrestrial input. There is high retention of sediment on the ECS Shelf owing to the broad nature of the shelf edge (i.e., 500–600 km) and because of an along‐shore sediment dispersal path driven by the East China Sea Coastal Current (ECSCC) and the Taiwan Warm Current (TWC). Organic carbon (OC) burial was also observed, based on the results for sediment accumulation and the OC proportion in surface sediment. A total of 7.4 × 106 t of OC is preserved in shelf sediments. Source discrimination of sedimentary OC revealed that the percentage of OC burial for both terrigenous and marine OC sources was higher than the global mean value, at 10% and 5.5%, respectively, presumably owing to high sediment accumulation. High OC flux to sediment in the estuary and shelf break suggests that riverine discharge and upwelling of the Kuroshio intrusion are key factors regulating OC burial over the shelf. Estimated using a mass balance model, 15–20% of the carbon inventory is buried in the shelf sediment, while >80% of the carbon inventory must be transported out of shelf area in the form of dissolved carbon (organic and inorganic). The accumulation of carbon in sediments and the outflow of dissolved carbon sustain a high carbon sink in the ECS.

Reserves growth in a mature oil field: The Devonian Leduc Formation at Innisfail field, south-central Alberta, Canada

Oil has been continuously produced from the Devonian Leduc Formation at Innisfail field since its discovery in 1957. To date, cumulative production at Innisfail is 98.9% of recoverable oil in place (84.3 MMBO) and indicates that the Leduc is near the end of its productive life. A sequence-stratigraphic interpretation produced from core, well logs, and three-dimensional (3-D) reflection seismic data, however, suggests that production may be extended through the development of two previously undetected scenarios of bypassed oil entrapment: (1) attic oil accumulations associated with small buildups atop the isolated Innisfail platform and (2) backstep-edge accumulations located in positions structurally low to currently producing wells. In both cases, oil accumulations are related to a Leduc depositional history that was characterized by rates of long-term carbonate sediment accumulation exceeded by the rate of sea level rise. Positive bathymetric relief created at the time of deposition and, subsequently, present-day stratigraphic traps were produced during the final phase of stratal retrogradation that immediately preceded the drowning of the isolated Innisfail platform. High-quality 3-D seismic data are essential in the exploration for both attic and backstep-edge oil accumulations at Innisfail. Integration of 3-D seismic data, well-log, and core data indicates that although depositional relief on both types of features may be low, they are nonetheless seismically resolvable. The small platform-top buildups are up to 0.16 km2 (40 ac) in diameter and have up to 10 m (33 ft) of independent closure above spill point. Backstep-edge oil accumulations occur within the structurally highest parts of the Leduc high-frequency sequence G downlap limit. Depositional relief along the downlap limit and associated trap closure ranges from approximately 5 to 15 m (16 to 49 ft). During 2003 and 2004, two platform-top buildups were directionally drilled, and one existing well located near a backstep-edge position and suspended since 1988 was reactivated. All three wells are successful and, to date, have cumulative production of 71 thousand bbl of oil. A total of 17 platform-top buildups and backstep-edge development drilling prospects exist across Innisfail and have expected-case recoverable reserves of approximately 960 thousand bbl of oil. These reserve additions suggest the potential for 1.14% field growth at Innisfail and are representative of the potential value that may yet exist within all mature Leduc fields in western Canada.