Marine Particulate Organic Carbon Research Articles

Terrestrial and marine POC export fluxes estimated by 234Th–238U disequilibrium and δ13C measurements in the East China Sea shelf

The use of 234Th–238U disequilibrium has been widely employed to estimate the sinking flux of particulate organic carbon (POC) from the upper sea and ocean. Here, the deficits of 234Th relative to 238U in the water column and the carbon isotope signature (δ13C) of POC in the East China Sea (ECS) Shelf were measured, which was used to distinguish the fraction of marine and terrestrial POC export fluxes. In the ECS Shelf, very strong deficits of 234Th relative to 238U were observed throughout the water column, with 234Th/238U activity ratios ranging from 0.158 ± 0.045 to 0.904 ± 0.068 (averaging 0.426 ± 0.159). The residence times of particle reactive radionuclide 234Th (τTh–T) in the ECS shelf water varied between 9 and 44 days, which is significantly shorter than that in the continental slope area or the basin area. This phenomenon indicates that there is a more rapid particle scavenging process in the ECS shelf water compared to the continental slope and basin upper water. By applying a two-end-member mixing model based on the δ13C, the fraction of terrestrial POC was estimated to be 0 to 74% (mean: 30 ± 22%) and the fraction of marine POC was in the range of 25% to 100% (mean: 70 ± 22%). Fluxes of marine and terrestrial POC settling to the seafloor exhibited significant spatial differences among different stations, ranging from 11 to 129 mmol C/m2/day and from 2.6 to 38 mmol C/m2/day, respectively. The averaged terrestrial POC fluxes in the southern and northern ECS Shelf were similar (~ 21 to 24 mmol C/m2/day), while the marine POC fluxes in the north (86 ± 37 mmol C/m2/day) were approximately four times higher than those in the south (26 ± 20 mmol C/m2/day). Interestingly, the estimated export flux of both marine and terrestrial POC were approximately one order of magnitude higher than the previously reported burial fluxes of POC (ranging from 1.1 ± 0.1 to 11.4 ± 1.1 mmol C/m2/day) in the underlying bottom sediments, indicating that the majority (> 90%) of both terrestrial and marine POC exported from the upper water column are degraded in the sediments of the ECS Shelf. This “carbon missing” phenomenon can greatly be attributed to rapid decomposition by other processes (including microbial reworking, cross-shelf transport, and possible consumption by benthic organisms). Our findings highlight the dynamic nature of carbon cycling in the continental shelf and the need for further research to understand these processes and improve carbon budget assessments.

Variable carbon isotope fractionation of photosynthetic communities over depth in an open-ocean euphotic zone

Marine particulate organic carbon (POC) contributes to carbon export, food webs, and sediments, but uncertainties remain in its origins. Globally, variations in stable carbon isotope ratios (δ13C values) of POC between the upper and lower euphotic zones (LEZ) indicate either varying aspects of photosynthetic communities or degradative alteration of POC. During summertime in the subtropical north Atlantic Ocean, we find that δ13C values of the photosynthetic product phytol decreased by 6.3‰ and photosynthetic carbon isotope fractionation (εp) increased by 5.6‰ between the surface and the LEZ-variation as large as that found in the geologic record during major carbon cycle perturbations, but here reflecting vertical variation in δ13C values of photosynthetic communities. We find that simultaneous variations in light intensity and phytoplankton community composition over depth may be important factors not fully accounted for in common models of photosynthetic carbon isotope fractionation. Using additional isotopic and cell count data, we estimate that photosynthetic and non-photosynthetic material (heterotrophs or detritus) contribute relatively constant proportions of POC throughout the euphotic zone but are isotopically more distinct in the LEZ. As a result, the large vertical differences in εp result in significant, but smaller, differences in the δ13C values of total POC across the same depths (2.7‰). Vertical structuring of photosynthetic communities and export potential from the LEZ may vary across current and past ocean ecosystems; thus, LEZ photosynthesis may influence the exported and/or sedimentary δ13C values of both phytol and total organic carbon and affect interpretations of εp over geologic time.

Nanoplanktonic diatom rapidly alters sinking velocity via regulating lipid content and composition in response to changing nutrient concentrations

Diatom sinking plays a crucial role in the global carbon cycle, accounting for approximately 40% of marine particulate organic carbon export. While oceanic models typically represent diatoms as microphytoplankton (> 20 μm), it is important to recognize that many diatoms fall into the categories of nanophytoplankton (2-20 μm) and picophytoplankton (< 2 μm). These smaller diatoms have also been found to significantly contribute to carbon export. However, our understanding of their sinking behavior and buoyancy regulation mechanisms remains limited. In this study, we investigate the sinking behavior of a nanoplanktonic diatom, Phaeodactylum tricornutum (P. tricornutum), which exhibits rapid changes in sinking behavior in response to varying nutrient concentrations. Our results demonstrate that a higher sinking rate is observed under phosphate limitation and depletion. Notably, in phosphate depletion, the sinking rate of P. tricornutum was 0.79 ± 0.03 m d-1, nearly three times that of the previously reported sinking rates for Skeletonema costatum, Ditylum brightwellii, and Chaetoceros gracile. Furthermore, during the first 6 h of phosphate spike, the sinking rate of P. tricornutum remained consistently high. After 12 h of phosphate spike, the sinking rate decreased to match that of the phosphate repletion phase, only to increase again over the next 12 hours due to phosphate depletion. This rapid sinking behavior contributes to carbon export and potentially allows diatoms to exploit nutrient-rich patches when encountering increased nutrient concentrations. We also observed a significant positive correlation (P< 0.001) between sinking rate and lipid content (R = 0.91) during the phosphate depletion and spike experiment. It appears that P. tricornutum regulates its sinking rate by increasing intracellular lipid content, particularly digalactosyldiacylglycerol, hexosyl ceramide, monogalactosyldiacylglycerol, and triglycerides. Additionally, P. tricornutum replaces phospholipids with more dense membrane sulfolipids, such as sulfoquinovosyldiacylglycerol under phosphate shortage. These findings shed light on the intricate relationship between nutrient availability, sinking behavior, and lipid composition in diatoms, providing insights into their adaptive strategies for carbon export and nutrient utilization.

Remote Sensing Estimates of Particulate Organic Carbon Sources in the Zhanjiang Bay Using Sentinel-2 Data and Carbon Isotopes

The source information of coastal particulate organic carbon (POC) with high spatial and temporal resolution is of great significance for the study of marine carbon cycles and marine biogeochemical processes. Over the past decade, satellite ocean color remote sensing has greatly improved our understanding of the spatiotemporal dynamics of ocean particulate organic carbon concentrations. However, due to the complexity of coastal POC sources, remote sensing methods for coastal POC sources have not yet been established. With an attempt to fill the gap, this study developed an algorithm for retrieving coastal POC sources using remote sensing and geochemical isotope technology. The isotope end-member mixing model was used to calculate the proportion of POC sources, and the response relationship between POC source information and in situ remote sensing reflectance (Rrs) was established to develop a retrieval algorithm for POC sources with the following four bands: (Rrs(443)/Rrs(492)) × (Rrs(704)/Rrs(665)). The results showed that the four-band algorithm performed well with R2, mean absolute percentage error (MAPE) and root mean square error (RMSE) values of 0.78, 33.57% and 13.74%, respectively. Validation against in situ data showed that the four-band algorithm derived calculated the proportion of marine POC accurately, with an MAPE and RMSE of 27.49% and 13.58%, respectively. The accuracy of the algorithm was verified based on the Sentinel-2 data, with an MAPE and RMSE of 28.02% and 15.72%, respectively. Additionally, we found that the proportion of marine POC sources was higher outside the Zhanjiang Bay than inside it using in situ survey data, which was consistent with the retrieved results. Influencing factors of POC sources may be due to the occurrence of phytoplankton blooms outside the bay and the impact of terrestrial inputs inside the bay. Remote sensing in combination with carbon isotopes provides important technical assistance in comprehending the biogeochemical process of POC and uncovering spatiotemporal variations in POC sources and their underlying causes.

Source, composition, and reactivity of particulate organic matter along the Changjiang Estuary salinity gradient and adjacent sea

The sources, distribution, and fate of particulate organic matter (POM) in estuaries are dynamic and complex, influenced by highly intensive human activities and high productivity. In this study, water samples were collected along the Changjiang Estuary salinity gradient and adjacent sea (CEAS) in February and May 2017. Particulate organic carbon (POC), particulate nitrogen (PN), the δ13C isotope values and major biochemical constituent (total particulate amino acids, TPAA) were measured. The concentrations of POC, PN, and TPAA showed an overall decreasing trend from the river end-member to the open sea; however, their maximum always occurred around the turbidity maximum zone (TMZ). Concentrations of POC and TPAA showed a negative correlation with salinity and a positive correlation with chlorophyll a, indicating that the variation in POM concentrations and composition was mainly controlled by both terrigenous input and in situ phytoplankton production. The δ13C values gradually increased from the river mouth to the open sea in both winter and spring, in contrast to the molar C/N, reflecting the transition from terrestrial POC to phytoplankton-derived fresh POC with increasing salinity. Major biochemical indicators of TPAA/POC (%) and the degradation index (DI), showed a gradual shift towards more bioactive POM with increasing salinity in spring, although low TPAA/POC (%) values appeared within the TMZ. In spring, POC reactivity was higher than in winter. The proportions of glycine (Gly) and serine (Ser) were higher in winter, indicating that POM had suffered extensive degradation. Based on a two end-member mixing model, the contribution of marine POC in spring (53 ± 14%) was significantly greater than in winter (39 ± 19%), indicating that phytoplankton-derived POM was dominant in spring, associated with the increase in phytoplankton biomass from winter to spring. Based on mass balance, a box model showed evidence of a net POC sink over the Changjiang estuary and its adjacent East China Sea shelf in both winter and spring, with a net POC budget of 20.49 ± 7.01 and 15.87 ± 6.57 kmol s−1, respectively. Results illustrate that the spatio-temporal distribution of POM varies distinctively and will further affect the variability in its composition and reactivity in the CEAS.

Assessing the sources and dynamics of organic matter in a high human impact bay in the northern Beibu Gulf: Insights from stable isotopes and optical properties

Severe human activities in coastal areas have greatly impacted the sources and biogeochemical behaviors of organic matter (OM), including particulate OM (POM) and dissolved OM (DOM). However, few studies have incorporated the indices of POM and DOM to address this issue. Here, a dataset of the combination of stable isotopes of carbon and nitrogen in POM and the optical properties of DOM was presented in Xi Bay, a semi-enclosed bay with a highly developing industrial port in Beibu Gulf, South China, to reveal the origin, distribution, and fate of OM during the rainy season. In the upper bay, depleted δ13C suggested that particulate organic carbon (POC) mainly originated from terrestrial sources. However, the negative relationship between chromophoric DOM (CDOM) and particulate nitrogen (PN) suggested that bacterial-mediated decomposition of POM may be the primary source of CDOM. The negative correlation between humic-like fluorescent components (C1 and C2) and salinity suggested that those two components were mainly affected by terrestrial input. The significant correlation between the protein-like component (C3) and Chl a suggested that C3 was mainly derived from phytoplankton production in the upper bay. In the lower bay, the increase of δ13C values indicated an increased contribution of marine POC. The high levels of CDOM may be due to the decomposition of marine (fresh) POM. However, the low levels of C1 and C2 might be affected by dilution with seawater, and the increased levels of the protein-like C3 were due to enhanced primary production. In addition, the enhancement of δ15N values in both the upper and lower bays indicated serious nitrogen pollution in the bay. This study highlights that biological production fueled by excess nutrients is the dominant OM dynamic process in the bay with high human impact in Beibu Gulf.

Carbon Isotopic Constraints on Basin‐Scale Vertical and Lateral Particulate Organic Carbon Dynamics in the Northern South China Sea

AbstractLateral particle transport in shelf/slope settings of marginal sea systems may potentially supply allochthonous particulate organic carbon (POC) to the ocean interior. The magnitude and prevalence of such processes has implications for our understanding of the functioning and efficiency of modern biological carbon pump processes, as well as for interpretation of deep ocean sediment records. Yet, ages and relative contribution of POC from different sources to deep ocean basins are much less understood. Here, based on radiocarbon (Fm, fraction modern), stable carbon (δ13Corg), and other geochemical analysis of particles intercepted by time‐series sediment traps deployed at three different depths (1,000, 2,150, and 3,200 m), we constrain the source of the sinking POC in the deep basin of northern South China Sea (SCS). The results show that the modern POC derived from surface ocean productivity, acting as the vertical vector, accounts for 87% ± 4% of sinking POC on average, while the lateral vector (aged POC hosted on resuspended sediment) comprises the remainder (13%). The contribution of laterally supplied POC increases with depth and is composed of aged POC derived from several sources. The majority stems from sediment resuspension of the northeastern SCS slope, that entrains aged marine POC (dominant), fossil OC, and soil OC (least significant). The diminished proportion of fossil OC in deep basin sinking POC relative to Taiwan‐proximal areas is likely due to its dispersion via resuspension–deposition loops and dilution by POC stemming from modern surface ocean productivity during the delivery.

Description of a global marine particulate organic carbon-13 isotope data set

Abstract. Marine particulate organic carbon stable isotope ratios (δ13CPOC) provide insights into understanding carbon cycling through the atmosphere, ocean and biosphere. They have for example been used to trace the input of anthropogenic carbon in the marine ecosystem due to the distinct isotopically light signature of anthropogenic emissions. However, δ13CPOC is also significantly altered during photosynthesis by phytoplankton, which complicates its interpretation. For such purposes, robust spatio-temporal coverage of δ13CPOC observations is essential. We collected all such available data sets and merged and homogenized them to provide the largest available marine δ13CPOC data set (https://doi.org/10.1594/PANGAEA.929931; Verwega et al., 2021). The data set consists of 4732 data points covering all major ocean basins beginning in the 1960s. We describe the compiled raw data, compare different observational methods, and provide key insights in the temporal and spatial distribution that is consistent with previously observed large-scale patterns. The main different sample collection methods (bottle, intake, net, trap) are generally consistent with each other when comparing within regions. An analysis of 1990s median δ13CPOC values in a meridional section across the best-covered Atlantic Ocean shows relatively high values (≥-22 ‰) in the low latitudes (<30∘) trending towards lower values in the Arctic Ocean (∼-24 ‰) and Southern Ocean (≤-28 ‰). The temporal trend since the 1960s shows a decrease in the median δ13CPOC by more than 3 ‰ in all basins except for the Southern Ocean, which shows a weaker trend but contains relatively poor multi-decadal coverage.

Sediment and particulate organic carbon budgets of a subarctic estuarine fjard: Lake Melville, Labrador

Sediment and particulate organic carbon (POC) budgets constructed for oceanic regions provide a means to quantify the burial of POC, providing a baseline against which future change can be measured. Here we developed the first mass balance budgets of sediment and POC (terrestrial and marine) for the Lake Melville system (LMS; Goose Bay and Lake Melville) in central Labrador using previously published data (210Pb-based mass accumulation rates, river discharge, and total suspended solids) together with measurements of %OC and δ13C signatures within dated sediment cores collected from the LMS basins and water from main tributaries and Groswater Bay. A two end-member mixing model was used to establish fractions of terrigenous and marine POC. Overall, our mass balance budgets show that the Churchill River is the major supply of sediment (16.7 ± 7.8 × 108 kg y−1) and POCterr (18.4 ± 6.6 × 106 kg y−1) to the LMS, accounting for 47% and 56% of the total input to the system, respectively. Although the Churchill River drains directly into Goose Bay along with the smaller Goose River, the majority of sediment (12.9 ± 7.8 × 108 kg y−1) and POCterr (11.8 ± 7.0 × 106 kg y−1) is transported eastward through the surface water plume into Lake Melville. Inputs of POCmar from autochthonous primary production, estimated by applying an empirical relationship between sediment-surface flux of POCmar and water depth, to Goose Bay (1.8 × 106 kg y−1) and Lake Melville (98.3 ± 4.7 × 106 kg y−1) increase with increasing distance from the Churchill River. To investigate the potential changes to the sediment load of the Churchill River to the LMS following impoundment at Muskrat Falls, we calculated the sediment increase using the median reservoir shoreline erosion potential. This approach showed that the delivery of sediment and POCterr to the LMS by the Churchill River may double during the two years following impoundment of the lower Churchill River.

Late-Holocene subtropical mangrove dynamics in response to climate change during the last millennium

This work aims to study the impacts of climate change in the subtropical mangroves during the late-Holocene on a southern Brazilian coastal plain and discuss the environmental conditions to the mangrove establishment near the austral limit of South America mangroves. Samples were collected to study palynological groups, sedimentary facies, and geochemistry analyses (δ13C, δ15N, TOC, TN, C:N ratio, TS, and C:S ratio), synchronized with four radiocarbon ages. The main result of this research was the mangrove succession, divided into three palynological zones composing two facies association: (1) herbaceous tidal flat and (2) mangrove tidal flat. The first zone between at least ~1815 and ~1629 cal. yr BP was marked by the mangrove absence with marine particulate organic carbon, C3 terrestrial plants, and macrophytes organic matter influence with δ13C values between −26.7‰ and −20‰, δ15N values x̅ = 3.5‰ and C:N around 21.8. The second zone between ~1629 and ~853 cal. yr BP was marked by the mangrove establishment represented only by Laguncularia pollen under influence of estuarine organic matter (δ13C x̅ = −26.5‰; δ15N x̅ = 3.2‰, and C:N around 16.4). The third zone reveals an increase of Laguncularia and presence of Avicennia pollen since ~853 cal. yr BP. Near the surface (<10 cm) occurs Rhizophora pollen, indicating the establishment of this genus during the past decades, under the modern environmental condition (δ13C x̅ = −27.02‰; δ15N x̅ = 3.12‰, and C:N around 17.42). This mangrove vertical succession may be associated with the low-temperature tolerance, where the sequence Rhizophora, Avicennia, and Laguncularia occurs from the northern to the southern limits of Santa Catarina coast, respectively, reflecting the temperature gradient. Therefore, probably, the establishment of the mangrove assemblage identified along the studied pollen profile was influenced by a temperature increase of air and water during the late-Holocene and, considering the Rhizophora genus, only during the past decades.

Contrasting Fates of Petrogenic and Biospheric Carbon in the South China Sea

AbstractA synthesis of published and newly acquired stable and radiocarbon isotope data from soil, river, and marine particulate organic carbon (OC) from the South China Sea drainage and sedimentary basin reveals that OC derived from bedrock‐erosion (petrogenic OC) and marine productivity comprises the major contributors to bulk OC in particulate matter reaching abyssal depths, while soil‐derived OC appears negligible. Aluminum‐radiocarbon relationships of sediments suggest that soil OC initially associated with detrital terrestrial minerals is lost and replaced by marine OC during transport beyond the continental shelf. We estimate that petrogenic OC sinking to a ~30,000 km2 region of the deep northeastern South China Sea accounts for 0.6% of global petrogenic OC burial. The basin‐wide OC isotope patterns coupled with sediment trap observations highlight both the spatial variabilities of OC components as they propagate from source to sedimentary sink and the significance of petrogenic OC to deep ocean sediments.

The effect of sample drying temperature on marine particulate organic carbon composition

AbstractCompositional changes in marine particulate organic carbon (POC) throughout the water column trace important processes that underlie the biological pump's efficiency. While labor‐intensive, particle sampling efforts offer potential to expand the empirical POC archive at different stages in the water column, provided that organic composition is sufficiently preserved between sampling and analysis. The standard procedure for preserving organic matter composition in marine samples is to immediately store particles at −80°C to −20°C until they can be freeze‐dried for analysis. This report investigates the effect of warmer drying and storage temperatures on POC composition, which applies to the majority of POC samples collected in the field without intention for organic analysis. Particle samples collected off Woods Hole, Massachusetts were immediately dried at 56°C, at room temperature, or stored in a freezer until freeze‐drying. Results show that oven‐ and air‐drying did not shift the bulk composition (i.e., carbon and nitrogen content and stable isotope composition) of POC in the samples relative to freeze‐drying. Similarly, warmer drying temperatures did not affect POC thermal stability, as inferred by ramped pyrolysis/oxidation, a growing technique that uses a continuous temperature ramp to differentiate components of organic carbon by their decomposition temperature. Oven‐ and air‐drying did depress lipid abundances relative to freeze‐drying, the extent of which depended on compound size and structure. The data suggest that field samples dried at room temperatures and 56°C are appropriate for assessing bulk POC composition and thermal stability, but physical mechanisms such as molecular volatilization bias their lipid composition.

Seasonal carbon cycling in a Greenlandic fjord: an integrated pelagic and benthic study

Climate change is expected to have a pronounced effect on biogeochemical cycling in Arctic fjords, but current insight on the biogeochemical functioning of these systems is limited. Here, we present seasonal data on primary production, export of particulate organic carbon (POC), and the coupling to benthic biogeochemistry in Kobbefjord (SW Greenland). Primary production and associated POC export from the photic zone showed marked seasonality, with annual integrated values of 7.2 and 19.9 mol C m-2 yr-1, respectively. This discrepancy, the isotopic signature, and C:N ratio of the sedimentating material suggested substantial import of marine POC from outside the fjord. At least 52% of the POC export reached the sediment, but the seasonality in pelagic productivity was not reflected in the sediment biogeochemistry, showing only moderate variation. Benthic mineralization and burial of organic carbon amounted to 3.2 and 5.3 mol C m-2 yr-1, respectively. Sulfate reduction was the most prominent mineralization pathway, accounting for 69% of the benthic mineralization, while denitrification accounted for 2%. Overall, the carbon mineralization and burial in Kobbefjord were significantly higher than previously observed in other more northerly Arctic fjords. Data compilation from Arctic fjords suggests proportional increases in surface production, POC export, benthic mineralization and burial of organic material with increasing duration of the ice-free period. Thus, the projected decline in ice coverage in higher Arctic Greenlandic fjords will, as a first approximation, entail proportional increases in productivity, mineralization, and burial of organic carbon in the fjords, which will thus become similar to present-day southerly systems.

Evidence for permafrost thaw and transport from an Alaskan North Slope watershed

Burial of organic carbon (OC) in marine sediments is one of the most important linkages between the short-term biologic carbon cycle and the long-term geologic carbon cycle. Yet much is still unknown about the fate of terrigenous OC in marine coastal margins. Here the delivery of particulate OC (POC) to the Colville River deltaic region in the Alaskan Beaufort Sea by particulates of varying densities is studied through the use of ramped temperature pyrolysis and radiocarbon analyses. The Colville River is the largest river in North America whose watershed is underlain completely by high Arctic permafrost tundra. A variety of sources of POC are considered, including terrestrial soils, Pleistocene-aged yedoma-like sediments, coastal peat erosion, and marine POC. We provide the first evidence that riverine POC from the Colville River contains old (Pleistocene-sourced) OC, suggesting ongoing thaw and mobilization of yedoma-like permafrost OC from this northern Alaskan watershed. Additionally, much of this OC appears to be fairly labile and therefore could be readily oxidized and returned to the atmosphere.

Sources of terrigenous inputs to surface sediments of the Colville River Delta and Simpson's Lagoon, Beaufort Sea, Alaska

AbstractThe provenance of sediments and particulate organic carbon (POC) in the Colville River delta and adjacent Simpson's Lagoon, in the Alaskan Beaufort Sea, was investigated using a variety of bulk and molecular techniques, including stable and radiocarbon isotopes, neodymium isotopes, algal pigments, and lignin‐phenols. Additionally, stable carbon isotopes and lignin‐phenols were analyzed on four different density fractions from sediments. The Colville River, the largest river in North America with a watershed exclusively located in the high‐Arctic tundra, was an important source of terrestrial POC to the western edge of the Lagoon, shown by extremely old radiocarbon ages (fraction modern of 0.165 ± 0.001 close to the river mouth up to 0.418 ± 0.002 farther away). Stations without northern protective barrier islands had large amounts of marine POC input, and evidence of benthic microalgae was found in one area of the Lagoon (chlorophyll‐a concentration 35.0 µg gOC−1 in the high‐algal biomass area compared to 1 to 7 µg gOC−1 outside of it). Stations in the middle and eastern end of the Lagoon showed significant sediment input from coastal erosion (input of peat‐indicating non‐lignin‐phenols 0.079 mg (100mgOC)−1 in the eastern lagoon compared to 0.022 mg (100mgOC)−1 near the Colville delta), and on the eastern end of the Lagoon there was evidence of input of Mackenzie River POC, shown with neodymium isotopes and also COP. POC inputs derived from rivers, coastal erosion, and marine productivity were quantified using a ternary mixing model and showed that fluvial supply and coastal erosion were the dominant carbon sources to the Lagoon. This constitutes the first study of POC delivery by the Colville River into a nearshore region and illustrates that continued warming of the high Arctic tundra will likely lead to increased riverine POC delivery to this region of the world.

Photooxidation of particulate organic matter, carbon/oxygen stoichiometry, and related photoreactions

We investigated photoredox transformations of oxygen, carbon, peroxides, and iron that accompany “photodissolution” of suspended marine particulate organic carbon (POC), a sunlight-induced process that transfers POC to the dissolved organic carbon (DOC) pool. During 18- to 24-hour photodissolution experiments with POC of varying composition, about 0.28 mol of O 2 was consumed per mole POC photodissolved. Mean dissolved inorganic carbon (DIC) production was 6% of initial POC in suspended river delta sediments and 1% in algal membrane detritus. The mean O 2 loss:DIC production ratio was − 1.3:1 in sediment suspensions, which slightly exceeds the typical range reported for DOC. The O 2 loss:DIC production ratio was − 7.7:1 in suspensions of algal detritus, which implies significant oxygen incorporation into (oxygenation of) organic matter. Irradiated sediment suspensions rapidly achieved low, steady-state peroxide concentrations but rose more slowly with algal detritus. Elevated iron concentrations in the 0.7–8.0 μm particle size fraction after 24 h of irradiation are consistent with photoredox cycling of metals and/or with physical disintegration of organic-mineral aggregates driven by organic matter dissolution. These oxidation and oxygenation results differ from analogous reactions previously found for marine DOC, and estimates of DIC production in particle-rich environments will require incorporation of POC-specific information.

Black carbon in marine particulate organic carbon: Inputs and cycling of highly recalcitrant organic carbon in the Gulf of Maine

To increase our understanding of the roles of black carbon (BC), a highly sorptive and recalcitrant material, we measured BC concentrations and fluxes in marine particulate organic carbon (POC) out of the water column in the Gulf of Maine (GoM), a representative coastal area downwind of important BC sources of the Northeastern United States. Concentrations ranged from < 0.1 to 16 μg/L in the spring and late summer, typically contributing between 1 and 20% of the POC. Water-column export fluxes were near 10 g BC/m 2∙yr. These observations suggest that (a) up to 50% of the “molecularly uncharacterized” POC in this region's seawater is combustion-derived BC, and (b) the “bioavailabilities” of hydrophobic pollutants like polycyclic aromatic hydrocarbons (PAHs) would be influenced substantially by sorption to BC. The observed BC spatial distributions imply that a large part of the BC was carried offshore by wind and that much of it is accumulated in the coastal sediments. On a global scale, these results suggest the GoM and other coastal areas with similar BC loadings accumulate significant amounts of highly recalcitrant organic carbon that remineralizes on geological time scales in the world's oceans.

Terrestrial and marine POC fluxes derived from 234Th distributions and δ13C measurements on the Mackenzie Shelf

Water column deficits of 234Th relative to 238U in the Mackenzie Shelf, Cape Bathurst Polynya, and Amundsen Gulf were used to estimate sinking fluxes of particulate organic carbon (POC) in these areas. The 234Th fluxes were converted to marine and terrestrial POC fluxes using the POC/Th ratio on filterable particles &gt;70 μm and δ13C measurements to determine the fraction of marine and terrestrial POC. In June/July 2004, the greatest 234Th deficits (0–100 m: 56–95 dpm m−2) were observed in the Mackenzie outer shelf. Deficits in the upper 100 m ranged from 3 to 59 dpm m−2 in the Cape Bathurst Polynya. The δ13C values of POC in the &gt;70‐μm particles filtered in situ pumps ranged from −25.1‰ to −28‰. Using a two‐end‐member mixing model with marine POC = −21.4‰ and terrestrial POC = −28‰ shows that terrestrial POC is most evident at the Mackenzie Shelf stations but is present throughout the region. The fraction of marine POC ranged from 0 to 59% in the area in June/July 2004, with highest values in the Cape Bathurst Polynya. Fluxes of marine POC in the polynya average ∼5 mmol C m−2 d−1 at 50 m in June 2004 and increase to ∼12 mmol C m−2 d−1 in July. Comparable fluxes are observed at 100 m in June, but values decrease to ∼6 mmol C m−2 d−1 at 100 m in July. These fluxes are greater than estimates of organic carbon remineralization and burial in sediments of the polynya (∼3 mmol m−2 d−1), suggesting that POC may be exported out of the area, effectively remineralized by microbial activity in the twilight zone or incorporated into biomass.

Satellite estimation of marine particulate organic carbon in waters dominated by different phytoplankton taxa

The relationship between in situ remote sensing reflectance (Rrs) and particulate organic carbon (POC) was assessed for the continental shelf waters of the southwestern Ross Sea, Antarctica. This sector of the Southern Ocean supports two distinct phytoplankton communities, one dominated by diatoms and the other dominated by the haptophyte Phaeocystis antarctica. In both communities, Rrs exhibited a statistically significant (p &lt; 0.05) positive linear correlation with POC concentration, although the slope of the regression was over twofold higher in waters dominated by P. antarctica. These distinct relationships between POC and Rrs are likely due to differences in the cell size and composition, as well as detrital degradation products, of the dominant taxa within each of these phytoplankton communities. Taxon‐specific POC‐Rrs relationships were translated into a bio‐optical algorithm for estimation of surface POC concentration from remotely sensed estimates of Rrs. Applying this algorithm to satellite‐derived Rrs from the Ross Sea, where distributions of P. antarctica and diatoms are reasonably well known, yields distributions of POC and POC:chlorophyll ratios that are in good agreement with field measurements. Performance of the algorithm can be improved by in situ characterization of the detrital contribution to total particle backscatter.

Comments on the importance of black carbon in the global carbon cycle

Comments on the importance of black carbon in the global carbon cycle