gC Yr Research Articles

Enhanced terrestrial carbon preservation promoted by reactive iron in deltaic sediments

AbstractWe examined the role of reactive iron (FeR) in preserving organic carbon (OC) across a subaerial chronosequence of the Wax Lake Delta, a prograding delta within the Mississippi River Delta complex. We found that ~15.0% of the OC was bound to FeR, and the dominant binding mechanisms varied from adsorption in the youngest subaerial region to coprecipitation at the older, vegetated sites. The δ13C of the iron‐associated OC was more negative than the total OC (mean = −2.6‰), indicating greater preference for terrestrial material and/or compounds with more negative δ13C values. However, only the adsorbed OC displayed preferential binding of lignin phenols. We estimate that ~8% of the OC initially deposited in deltaic systems is bound to FeR (equivalent to 6 × 1012 gC yr−1), and this percentage increases postdepositionally, as coprecipitation of FeR and OC allows for an even greater amount of OC to be bound to FeR.

Spatial and temporal variability in the net primary production of alpine grassland on the Tibetan Plateau since 1982

Based on the GIMMS AVHRR NDVI data (8 km spatial resolution) for 1982-2000, the SPOT VEGETATION NDVI data (1 km spatial resolution) for 1998-2009, and observational plant biomass data, the CASA model was used to model changes in alpine grassland net primary production (NPP) on the Tibetan Plateau (TP). This study will help to evaluate the health conditions of the alpine grassland ecosystem, and is of great importance to the promotion of sustainable development of plateau pasture and to the understanding of the function of the national ecological security shelter on the TP. The spatio-temporal characteristics of NPP change were investigated using spatial statistical analysis, separately on the basis of physico-geographical factors (natural zone, altitude, latitude and longitude), river basin, and county-level administrative area. Data processing was carried out using an ENVI 4.8 platform, while an ArcGIS 9.3 and ANUSPLIN platform was used to conduct the spatial analysis and mapping. The primary results are as follows: (1) The NPP of alpine grassland on the TP gradually decreases from the southeast to the northwest, which corresponds to gradients in precipitation and temperature. From 1982 to 2009, the average annual total NPP in the TP alpine grassland was 177.2x10(12) gC yr(-1)(yr represents year), while the average annual NPP was 120.8 gC m(-2) yr(-1). (2) The annual NPP in alpine grassland on the TP fluctuates from year to year but shows an overall positive trend ranging from 114.7 gC m(-2) yr(-1) in 1982 to 129.9 gC m(-2) yr(-1) in 2009, with an overall increase of 13.3%; 32.56% of the total alpine grassland on the TP showed a significant increase in NPP, while only 5.55% showed a significant decrease over this 28-year period. (3) Spatio-temporal characteristics are an important control on annual NPP in alpine grassland: a) NPP increased in most of the natural zones on the TP, only showing a slight decrease in the Ngari montane desert-steppe and desert zone. The positive trend in NPP in the high-cold shrub-meadow zone, high-cold meadow steppe zone and high-cold steppe zone is more significant than that of the high-cold desert zone; b) with increasing altitude, the percentage area with a positive trend in annual NPP follows a trend of "increasing-stable-decreasing", while the percentage area with a negative trend in annual NPP follows a trend of "decreasing-stable-increasing", with increasing altitude; c) the variation in annual NPP with latitude and longitude co-varies with the vegetation distribution; d) the variation in annual NPP within the major river basins has a generally positive trend, of which the growth in NPP in the Yellow River Basin is most significant. Results show that, based on changes in NPP trends, vegetation coverage and phonological phenomenon with time, NPP has been declining in certain places successively, while the overall health of the alpine grassland on the TP is improving.

Benthic ecology from space: optics and net primary production in seagrass and benthic algae across the Great Bahama Bank

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 411:1-15 (2010) - DOI: https://doi.org/10.3354/meps08665 FEATURE ARTICLE Benthic ecology from space: optics and net primary production in seagrass and benthic algae across the Great Bahama Bank Heidi M. Dierssen1,*, Richard C. Zimmerman2, Lisa A. Drake2,3, David Burdige2 1Department of Marine Sciences/Geography, University of Connecticut, Groton, Connecticut 06340, USA 2Department of Ocean, Earth and Atmospheric Sciences, Old Dominion University, 4600 Elkhorn Avenue, Norfolk, Virginia 23529, USA 3Present address: Science Applications International Corporation, c/o Naval Research Laboratory, Key West, Florida 33041, USA *Email: heidi.dierssen@uconn.edu ABSTRACT: Development of repeatable and quantitative tools are necessary for determining the abundance and distribution of different types of benthic habitats, detecting changes to these ecosystems, and determining their role in the global carbon cycle. Here we used ocean color remote sensing techniques to map different major groups of primary producers and estimate net primary productivity (NPP) across Great Bahama Bank (GBB). Field investigations on the northern portion of the GBB in 2004 revealed 3 dominant types of benthic primary producers: seagrass, benthic macroalgae, and microalgae attached to sediment. Laboratory measurements of NPP ranged from barely net autotrophic for grapestone sediment with thin microalgal biofilm to highly productive for dense accumulations of brown macroalgae. A logarithmic relationship between NPP and green seafloor reflectance described the general trend in NPP across various benthic constituents. Using a radiative transfer-based approach, satellite-derived estimates of NPP for the region totaled ~2 × 1013 gC yr–1 across the GBB. The prevailing benthic habitat was mapped as sediment with little to no microalgal biofilm. Moderate to dense seagrass meadows of Thalassia testudinum were the dominant primary producers and contributed over 80% of NPP in the region. If the vast majority of seagrass leaves decompose in the primarily carbonate sediments, carbonate dissolution processes associated with this decomposition may result in sequestration of seagrass above- and below-ground carbon into the bicarbonate pool (2.4 × 1013 gC yr–1), where it has a residence time on the order of tens of thousands of years. KEY WORDS: Primary productivity · Remote sensing · Ocean color · Inherent optical properties · Benthic reflectance · Bahamas · Grapestone sediment Full text in pdf format Information about this Feature Article NextCite this article as: Dierssen HM, Zimmerman RC, Drake LA, Burdige D (2010) Benthic ecology from space: optics and net primary production in seagrass and benthic algae across the Great Bahama Bank. Mar Ecol Prog Ser 411:1-15. https://doi.org/10.3354/meps08665 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 411. Online publication date: July 29, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Mesoscale and diel to monthly variability of CO2 and carbon fluxes at the ocean surface in the northeastern Atlantic

Four CARIOCA Lagrangian buoys were deployed in the northeast Atlantic Ocean as part of the Programme Océan Multidisciplinaire Méso Echelle (POMME) dedicated to the study of the role of mesoscale eddies in biological production, the carbon budget, and the subduction of mode waters. An extensive set of hourly surface measurements of temperature, salinity, and carbon dioxide fugacity (fCO2) was collected from February to August 2001. The high‐frequency spatial and temporal variability observed in surface fCO2 and in the derived dissolved inorganic carbon (DIC) concentrations suggests that abrupt changes of the carbon variables are being generated along frontal patterns and filaments by submesoscale and mesoscale eddy‐eddy interactions, especially in winter. On the basis of a 1‐D model of the diurnal mixed layer along the buoy trajectory, we show that under certain conditions the diel cycle of DIC is driven by the daily cycle of photosynthesis and metabolic CO2 release at the ocean surface. This information is quantitatively used to derive in‐situ primary production, carbon gross, and net community production. The analysis of 107 observed diel cycles of DIC shows that episodic biological production processes are triggered by the mesoscale activity of surface eddies. Over the sampled period, the POMME area is a sink for atmospheric CO2 with an estimated flux of −4 mmol m−2 day−1. The calculated amount of anthropogenic carbon transported into the ocean interior by subduction of subpolar mode water is equal to 2.8 1013 gC yr−1.

Export Production in the Equatorial and North Pacific Derived from Dissolved Oxygen, Nutrient and Carbon Data

A global ocean inverse model that includes the 3D ocean circulation as well as the production, sinking and remineralization of biogenic particulate matter is used to estimate the carbon export flux in the Pacific, north of 10°S. The model exploits the existing large datasets for hydrographic parameters, dissolved oxygen, nutrients and carbon, and determines optimal export production rates by fitting the model to the observed water column distributions by means of the “adjoint method”. In the model, the observations can be explained satisfactorily with an integrated carbon export production of about 3 Gt C yr−1 (equivalent to 3⋅1015 gC yr−1) for the considered zone of the Pacific Ocean. This amounts to about a third of the global ocean carbon export of 9.6 Gt C yr−1 in the model. The highest export fluxes occur in the coastal upwelling region off northwestern America and in the tropical eastern Pacific. Due to the large surface area, the open-ocean, oligotrophic region in the central North Pacific also contributes significantly to the total North Pacific export flux (0.45 Gt C yr−1), despite the rather small average flux densities in this region (13 gC m−2yr−1). Model e-ratios (calculated here as ratios of model export production to primary production, as inferred from satellite observations) range from as high a value as 0.4 in the tropical Pacific to 0.17 in the oligotrophic central north Pacific. Model e-ratios in the northeastern Pacific upwelling regions amount to about 0.3 and are lower than previous estimates.

Riverine inorganic carbon flux and rate of biological uptake in the Mississippi River plume

Inorganic carbon parameters were studied for the first time in the Mississippi River estuary and plume. Area‐integrated biological uptake rates (1.5–3 gC m−2 d−1) derived from riverine total dissolved inorganic carbon (DIC) and total alkalinity (TAlk) fluxes as well as that based on the consumptions of DIC and TAlk in the plume are comparable to earlier results of 14C incubation in individual water samples. This rate is among the highest in the estuaries and plumes of the world's largest rivers. Fluvial DIC flux (13.5 × 1012 gC yr−1) indicates a 16% increase in the inorganic carbon flux over the earlier estimate and perhaps an increase in the weathering rate in the Mississippi River basin over the past four decades.

Dissolved organic carbon in the Mid-Atlantic Bight

The Mid-Atlantic Bight (MAB) is a highly productive continental shelf region on the eastern North American coast and a potential sight for the export of organic carbon to the open ocean. The dynamics of dissolved organic carbon (DOC) in the MAB were studied during three cruises in April 1994, March 1996 and August 1996 in order to characterize DOC distributions on the shelf, estimate inventories of DOC on the shelf and quantify DOC transport. There were clear spatial and temporal trends in DOC distributions. Concentrations of DOC were greater inshore than offshore and increased southward along the shelf. The total DOC inventory on the shelf during the March and April cruises was estimated at B5.88 � 10 12 g C and this increased by 0.4 � 10 12 g C (7%) in August. A simple mass balance of DOC input and export in the MAB resulted in total export of 18.7–19.6 � 10 12 gC yr � 1 . Up to 67% of this carbon is recycled oceanic DOC that is refractory with respect to MAB seawater residence times, B4% is introduced through estuaries, sediments and rainwater and at least 29% of the exported shelf DOC (4.1–7.4 � 10 12 gC yr � 1 ) is produced on the shelf. Net ecosystem production of DOC in the MAB is estimated to be between 10% and 18% of MAB primary productivity estimates. Our data supports the hypothesis that the MAB as a whole is net autotrophic for the periods of study and that DOC is an important component in ocean margin carbon budgets. r 2002 Elsevier Science Ltd. All rights reserved.

Vegetation/Ecosystem Modeling and Analysis Project:Comparing biogeography and biogeochemistry models in a continental‐scale study of terrestrial ecosystem responses to climate change and CO2 doubling

We compare the simulations of three biogeography models (BIOME2, Dynamic Global Phytogeography Model (DOLY), and Mapped Atmosphere‐Plant Soil System (MAPSS)) and three biogeochemistry models (BIOME‐BGC (BioGeochemistry Cycles), CENTURY, and Terrestrial Ecosystem Model (TEM)) for the conterminous United States under contemporary conditions of atmospheric CO2 and climate. We also compare the simulations of these models under doubled CO2 and a range of climate scenarios. For contemporary conditions, the biogeography models successfully simulate the geographic distribution of major vegetation types and have similar estimates of area for forests (42 to 46% of the conterminous United States), grasslands (17 to 27%), savannas (15 to 25%), and shrublands (14 to 18%). The biogeochemistry models estimate similar continental‐scale net primary production (NPP; 3125 to 3772 × 1012 gC yr−1) and total carbon storage (108 to 118 × 1015 gC) for contemporary conditions. Among the scenarios of doubled CO2 and associated equilibrium climates produced by the three general circulation models (Oregon State University (OSU), Geophysical Fluid Dynamics Laboratory (GFDL), and United Kingdom Meteorological Office (UKMO)), all three biogeography models show both gains and losses of total forest area depending on the scenario (between 38 and 53% of conterminous United States area). The only consistent gains in forest area with all three models (BIOME2, DOLY, and MAPSS) were under the GFDL scenario due to large increases in precipitation. MAPSS lost forest area under UKMO, DOLY under OSU, and BIOME2 under both UKMO and OSU. The variability in forest area estimates occurs because the hydrologic cycles of the biogeography models have different sensitivities to increases in temperature and CO2. However, in general, the biogeography models produced broadly similar results when incorporating both climate change and elevated CO2 concentrations. For these scenarios, the NPP estimated by the biogeochemistry models increases between 2% (BIOME‐BGC with UKMO climate) and 35% (TEM with UKMO climate). Changes in total carbon storage range from losses of 33% (BIOME‐BGC with UKMO climate) to gains of 16% (TEM with OSU climate). The CENTURY responses of NPP and carbon storage are positive and intermediate to the responses of BIOME‐BGC and TEM. The variability in carbon cycle responses occurs because the hydrologic and nitrogen cycles of the biogeochemistry models have different sensitivities to increases in temperature and CO2. When the biogeochemistry models are run with the vegetation distributions of the biogeography models, NPP ranges from no response (BIOME‐BGC with all three biogeography model vegetations for UKMO climate) to increases of 40% (TEM with MAPSS vegetation for OSU climate). The total carbon storage response ranges from a decrease of 39% (BIOME‐BGC with MAPSS vegetation for UKMO climate) to an increase of 32% (TEM with MAPSS vegetation for OSU and GFDL climates). The UKMO responses of BIOME‐BGC with MAPSS vegetation are primarily caused by decreases in forested area and temperature‐induced water stress. The OSU and GFDL responses of TEM with MAPSS vegetations are primarily caused by forest expansion and temperature‐enhanced nitrogen cycling.

Sedimentary organic matter preservation: an assessment and speculative synthesis

Throughout Earth history, almost all preserved organic matter has been incorporated in marine sediments deposited under oxygenated waters along continental margins. Given modern oceanic productivity and sediment burial rates of 50 × 1015 and 0.16 × 1015 gC yr−1, respectively, organic preservation in the marine environment is < 0.5% efficient. Although correlative information is often used to suggest that productivity, sediment accumulation rate, bottom water oxicity, and organic matter source are key variables, the mechanisms governing sedimentary organic matter preservation have remained unclear. The factors which directly determine preservation vary with depositional regime, but have in common a critical interaction between organic and inorganic materials over locally variable time scales. More than 90% of total sedimentary organic matter from a wide variety of marine depositional environments cannot be physically separated from its mineral matrix. This strongly associated organic component varies directly in concentration with sediment surface area and thus appears to be sorbed to mineral grains. Sediments accumulating outside deltas along continental shelves and upper slopes characteristically exhibit mineral surface area loadings approximately equivalent to a single molecular covering. These monolayer-equivalent coatings include a fraction of reversibly bound organic molecules that are intrinsically labile, but resist appreciable mineralization as they pass rapidly through oxygenated surface sediments and are preserved within underlying anoxic deposits. The delivery of mineral surface area is the primary control on organic matter preservation within these expansive coastal margin regions where roughly 45% of all organic carbon accumulates. Deltaic sediments account for roughly another 45% of global carbon burial, but often exhibit much less than monolayer-equivalent organic coatings. This pattern is seen in periodically oxygenated sediments off the mouth of the Amazon River, even though the component clastic minerals are discharged by the river with monolayer coatings. Comparably extensive losses of organic matter, including distinct particles such as pollen grains, occur in the surfaces of deep-sea turbidites in which long term reaction with O2 is clearly the causative factor. Sub-monolayer organic coatings also are observed in continental rise and abyssal plain sediments where slower accumulation rates and deeper O2 penetration depths result in increased oxygen exposure times and little (~ 5% of the global total) organic matter preservation. A transition zone between monolayer and sub-monolayer organic coatings apparently occurs on lower continental slopes, and is marked along the Washington coast by parallel offshore decreases in total organic matter and pollen between 2000–3000 m water depth. Sediments underlying highly productive, low-oxygen coastal waters such as off Peru and western Mexico are characteristically rich in organic matter, but account for only ~ 5% of total organic carbon burial. These sediments show a direct relationship between organic matter content and mineral surface area, but at organic loadings 2–5 times a monolayer equivalent. Organic materials sorbed in excess of a monolayer thus also may be partially protected. Such high sedimentary organic contents may result from equilibration with DOM-rich porewaters, or very brief O2 exposure times which allow preservation of extremely oxygen-sensitive organic materials such as pigments and unsaturated lipids. Thus organic matter preservation throughout much of the ocean may be controlled largely by competition between sorption at different protective thresholds and oxic degradation. Future research strategies should be specifically directed at delineating the mechanisms for organic matter preservation in marine sediments. In particular, special effort is needed to determine the amounts and types of sorbed organic materials and the nature of their bonding to mineral surfaces. The extent and dynamics with which organic molecules are partitioned between porewaters and solid phases also should be determined, as well as the effects of these phase associations on their reactivities toward chemical and biological agents. In addition, processes for slow oxic (and suboxic) degradation of organic materials bear investigation in deep-sea sediments, as well as in other extreme environments such as oxidizing turbidites, weathering shales, and soils. Such studies should include characterizations of hydrolysis-resistant organic materials and emphasize the complementary use of biochemical compositions with readily separable particles such as pollen to calibrate and typify the mechanisms and stages of sedimentary organic degradation.

Methane in groundwater used for Japanese agriculture: Its relationship to other physico‐chemical properties and possible tropospheric source strength

The concentration of CH4 in 131 groundwater samples used for agriculture in Aichi Prefecture, central Japan, averaged 1.58 mgC l−1 for those water samples above detection (&gt;0.006 mgC l−1), with the highest value of 18.4 mgC l−1. Methane was detected in more than half of the groundwater samples. The amount of CH4 released to the atmosphere because of agricultural groundwater use was estimated to be 2.00 × 107 gC yr−1 for a cultivated area of 8.61 × 104 ha, or about 1.4% of the CH4 production in paddy fields derived from soil organic matter in the same geographic area. Distribution of measurements of redox potential (Eh), chemical oxygen demand (COD), Fe, Mn, NH4‐N, and NO3‐N was clearly different between the CH4‐detected and undetected samples; Eh values and NO3‐N concentrations were lower while the other four factors were higher in the CH4‐detected samples.

Role of coral reefs in global ocean production

Coral reefs cover some 600 × 103 km−2 of the earth’s surface (0.17% of the ocean surface). First-order estimates show coral reefs to contribute about 0.85% of the estimated net CO2 fixation rate of the global oceans. Gross CO2 fixation is relatively high (about 700 × 1012 gC yr−1), but most of this material is recycled within the reefs. Excess (net) production of organic material (E) is much smaller, about 20 × 1012 gC yr−1. We estimate that 3 × 1012 gC yr−1 (15% of E) is buried in reef structure; 2 × 1012 gC yr−1 (10% of E) for sustained human harvest, and the remaining 75% of E for export from coral reefs to adjacent areas. Comparison of estimates for net production by reefs and their surrounding oceans indicates that the excess production by coral reefs is similar to new production in the photic zone of the oligotrophic oceans. Consequently, estimates for global ocean production should include as a first approximation areas of reefs with the surrounding ocean when assigning average net production rates. While there are significant uncertainties in these numbers, we conclude that organic production by reefs plays a relatively minor role in the global scale fluxes and storage of elements. In comparison, the companion process of biologically-mediated inorganic carbon precipitation represents a major role for reefs. While reef production does respond on local scales to variation in ocean climate, neither the absolute rates nor the amount accumulated into organic pools are either sensitive indicators or accurate recorders of climatic change in most reef systems. Similarly, the productivity of most reefs should be little affected by the environmental changes currently predicted from the Greenhouse Effect.

The role of lake and reservoir sediments as sinks in the perturbed global carbon cycle

We estimate that recent increases in organic carbon accumulation in the sediments of lakes and reservoirs amount to about 0.02 m? 10 15 gC yr -1 and 0.2 m? 10 15 gC yr -1 , respectively. The reservoir accumulation represents a small but significant fraction of the carbon missing in current global budgets. DOI: 10.1111/j.2153-3490.1982.tb01837.x

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m -2 yr -1 ). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10 12 ± 5 × 10 12 gC yr -1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 10 12 gC yr -1 , slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years. DOI: 10.1111/j.2153-3490.1982.tb01805.x

Transport of organic carbon to the oceans by rivers of North America: a synthesis of existing data

Transport of organic carbon in rivers of North America to the oceans was assessed by compiling and analyzing data from 82 North American rivers draining 60% of Canada and the United States. These data were collected by the U.S. Geological Survey and the Water Resources Branch of Canada's Inland Waters Directorate. Mean annual concentrations of total organic carbon showed considerably less variation than did annual specific export (gC m^-2 yr-1). Regional variation in annual specific export of total organic carbon was attributed primarily to differences in annual runoff. Transport of total organic carbon to coastal regions by North American rivers totalled about 35 × 10¹² ± 5 × 10¹² gC yr^-1 in 1977 and 1978. The total organic carbon flux to the oceans associated with the long-term average annual flow in North American rivers was computed to be about 40 × 1012 gC yr^-1, slightly larger than that in 1977 and 1978 because of the lower-than-average runoff in some regions those years.

Transport of organic carbon in the world's rivers

Total transport of organic carbon to the world's oceans in dissolved and particulate form in rivers is estimated as 0.37 × 10 15 gC yr -1 by an inventory and extrapolation of data on loss of carbon per unit volume of river discharge from twelve intermediate and large rivers. An alternative estimate of 0.41 × 10 15 gC yr -1 is derived from measurements of the fluvial loss of organic carbon per unit area of land in various ecosystem types. Rates of loss range from 1 gC m -2 yr -1 in grassland systems to 10 gC m -2 yr -1 in some forests. Organic carbon transport in rivers is a small flux in the global carbon cycle, but our present understanding is limited by inadequate sampling and few data from major world rivers. DOI: 10.1111/j.2153-3490.1981.tb01742.x

A comparison of several models of carbon turnover in the ocean with respect to their distributions of transit time and age, and responses to atmospheric CO&lt;sub&gt;2&lt;/sub&gt; and &lt;sup&gt;14&lt;/sup&gt;C

Five models of carbon depth distribution in the world ocean are compared with respect to carbon transit-time distribution, age distribution, and integrated responses to histories of fossil CO2 and weapons 14 C. Two models represent the ocean as two well-mixed layers and differ only in the relative sizes of these compartments. The remaining three models consider 19 well-mixed layers and differ in the patterns of carbon transfer among the layers. All five models are required to be compatible with a preindustrial depth profile of natural 14 C, which is used to fix the values of transfer coefficients. Within this constraint the five models exhibit significantly different responses to fossil CO2 and weapons 14 C regimes. Of the estimated 134 × 10 15 g of fossil carbon produced in the period 1860–1975, the models estimate a net uptake range of 24 × 10 15 to 66 × 10 15 gC by the ocean. For weapons 14 C the range is 6.0 × 10 2 to 1.0 × 10 3 kg 14 C. Each of these models shows shifts in the implied role of the terrestrial biota between net carbon source and sink, with the latter role dominating in the last years of the simulated period. The largest estimate of source strength for the five models was 0.74 × 10 15 gC yr -1 , in contrast to the fossil CO2 production rate of ˜5 × 10 15 gC yr -1 in 1975. The five models were also used to predict atmospheric CO2 levels during the period 1975–2300 for a logistic source that would ultimately release 5000 × 10 15 gC. DOI: 10.1111/j.2153-3490.1981.tb01751.x