Particulate Organic Carbon Flux Research Articles

Do Indus Delta mangroves and Indus River contribute to organic carbon in deltaic creeks and coastal waters (Northwest Indian Ocean, Pakistan)?

The biogeochemistry of organic carbon (OC) is of great significance to regional and global biogeochemical cycles. Studies on dissolved organic carbon (DOC) and particulate organic carbon (POC) concentrations and their fluxes from the Indus River, Indus Delta and coastal waters of the shelf area off the Indus Delta are limited. Major creeks of the Indus Delta (Waddi Khuddi Creek (WK), Dabbo Creek (DC), Hajamro Creek (HC) and Khobar Creek (KC)), along with the Indus River and the adjacent offshore area of the Arabian Sea, were sampled in the winter, summer, and post and pre-monsoon periods to investigate the fate, transformation, and source of OC. In this study, a high concentration (3.1 mg L-1) of POC was found in KC (the main river channel), which is directly influenced by runoff. The POC concentration varied seasonally in the creeks (WK, HC and DC). The POC% decreased as the total suspended matter (TSM) increased, and this correlation strengthens during the southwest monsoon (SW monsoon). The concentrations of DOC and POC have decreased (by 19.3 and 22.7%, respectively) relative to values reported in previous studies from the Indus River. The source of the DOC and POC was mainly the mangrove litter and benthic microalgae. Stable carbon (δ13C) was used as a proxy to trace the autochthonous and allochthonous sources of DOC and POC in the creeks. The contribution of POC to TOC in WK, DC, HC, and KC was 19.2 ± 7.5%, 26.5 ± 10.7%, 27.1 ± 8.7%, and 61.3%, respectively, increasing from west to east in the creeks; the contribution of POC to TOC in the offshore region was 22.3 ± 17.2%. There was clear evidence through the isotopic studies that the mangroves contribute significantly to the DOC and POC in the Indus Deltaic creeks. Towards to the Arabian Sea, high fluxes of TSM (5.43 × 1013 kg year-1), were recorded during the SW monsoon period when there were rains and floods. In comparison with the previous studies, POC and DOC flux is reduced from 1.30 × 109 to 2.42 × 105 kg year-1 and 3.83 × 109 to 2.89 × 105 kg year-1, respectively. In comparison with the Himalayan rivers (Brahmaputra and Ganges), the reduction in POC and DOC flux is also observed due to the low discharge of Indus River. There is still need of the study for the discharge and fluxes (TSM, DOC and POC) of the creeks of eastern and western side of Indus River for the complete transfer of OC through the creeks of Indus Delta to the world oceans.

Fluvial organic carbon cycling regulated by sediment transit time and mineral protection

Rivers transfer terrestrial organic carbon (OC) from mountains to ocean basins, playing a key role in the global carbon cycle. During fluvial transit, OC may be oxidized and emitted to the atmosphere as CO2 or preserved and transported to downstream depositional sinks. The balance between oxidation and preservation determines the amount of particulate OC (POC) that can be buried long term, but the factors regulating this balance are poorly constrained. Here, we quantify the effects of fluvial transit on POC fluxes along an ~1,300 km lowland channel with no tributaries. We show that sediment transit time and mineral protection regulate the magnitude and rate of POC oxidation, respectively. Using a simple turnover model, we estimate that annual POC oxidation is a small percentage of the POC delivered to the river. Modelling shows that lateral erosion into POC-rich floodplains can increase POC fluxes to downstream basins, thereby offsetting POC oxidation. Consequently, rivers with high channel mobility can enhance CO2 drawdown while management practices that stabilize river channels may reduce the potential for CO2 drawdown.

Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

A first assessment of particle flux over the South Brazil Bight continental slope

The vertical export of particulate organic carbon (POC) is a key component of the oceanic biological pump, which regulates the distribution of elements associated with primary production in the upper euphotic zone. To better understand how oceanic biogeochemical processes will respond in climate change scenarios, it is necessary to comprehend the mechanisms that regulate the magnitude and variations of POC flux among distinct oceanic domains, including the boundaries between continental margins and oceanic basins. This work is a first assessment of particle flux over the South Brazil Bight continental slope, a tropical western boundary system dominated by the Brazil Current. Total mass flux, and specific carbon, nitrogen and biogenic silica fluxes were measured periodically by two sediment traps installed at 450 m deep over the 900 m isobath, and at 750 m deep over the 2000 m isobath, from November 2016 to September 2017. Due to strong contribution of advected particles, high and extremely irregular fluxes were registered at the 900 m isobath site (mean = 1.1 g m-2 d− 1, stand. dev. ± 2.9 g m-2 d-1). The captured material suggests this sediment trap was affected by turbidity flows, which precludes the estimation of the biological pump contribution over this isobath. Over the 2000 m isobath, POC flux (3.7 mg m-2 d-1; ±2.5 mg m-2 d− 1) followed the seasonal pattern of primary production measured by satellite. POC flux was strongly correlated with biogenic silica content (r = 0.79; p < 0.01). This ballast mineral was more efficient than CaCO3 in transferring carbon from the base of the euphotic zone to 750 m. We suggest intra-seasonal variations found in the POC flux over the 2000 m isobath to be related to Brazil Current instabilities. Measurements of δ15N suggest that higher POC flux occurred during subsurface nitrate-based production regimes. A regional exponential export model is proposed, with a mean exponent (b value) of 1.4 (±0.3). Although initial, the results of this work allow us to estimate that 2.2 × 1012 g C is exported annually over the South Brazil Bight continental slope.

Diel metabolic patterns in a migratory oceanic copepod

Diel vertical migration of zooplankton profoundly impacts the transport of nutrients and carbon through the water column. Despite the acknowledged importance of this active flux to ocean biogeochemistry, these contributions remain poorly constrained, in part because daily variations in metabolic rates are not considered or are modeled as simple functions of temperature. To address this uncertainty, we sampled the subtropical copepod Pleuromamma xiphias at 4- to 7-h intervals throughout the daily migration and measured rates of oxygen consumption, ammonium excretion, fecal pellet production and metabolic enzyme activity. No significant patterns were detected in rates of oxygen consumption or ammonium excretion for freshly caught animals over the diel cycle. Fecal pellet production was highest during mid-night, consistent with several hours of feeding near the surface. Surface feeding resulted in fecal pellet production at depth in the morning, providing direct evidence that active flux of particulate organic carbon occurs in this region. Electron transport system activity was highest during the afternoon, contrary to our prediction of reduced daytime metabolism. Activity of both glutamate dehydrogenase and citrate synthase increased during early night, reflecting higher capacity for excretion and aerobic respiration, respectively. Overall, these results show that activities of metabolic enzymes vary during diel vertical migration. The surprising observation of elevated afternoon enzyme activity coupled with daytime fecal pellet and ammonium production suggests that additional characterization of the daytime activity of migratory zooplankton is warranted.

Biogeochemical responses to internal-wave impacts in the continental margin off Dongsha Atoll in the Northern South China Sea

This study aimed to understand the effects of nonlinear internal waves (NLIWs) on nutrient-involved biogeochemical processes and the consequences in the continental margin off Dongsha Atoll (DA) in the northern South China Sea (NSCS). Shipboard and mooring observations were conducted to determine hydrological and biogeochemical conditions mainly from two transects covering the DA-associated upper slope, shelf break, and shelf zones. A dense array of thermistor moorings and a temperature-fluorescence-sediment trap array were deployed on the same transect to determine the short-period variability of transport and shoaling of NLIWs across the shelf and to link physical variability to biogeochemical consequences induced by NLIW events. The shoaling of NLIWs appeared to uplift cooler and nutrient-rich subsurface water into the nutrient-deplete surface in shelf and nearshore environments as well as increase nutrient concentrations and biological production. We observed shoaling events that induced a notable range of upward fluxes in nitrate and nitrite (0.42–2.92 mmol N m−2 d−1) onto the euphotic zone to enhance and sustain new production, depending on the strength of shoaling activity in various observations. The shoaling-derived new production may fully elucidate the amount of sinking fluxes of particulate organic carbon measured from the shoaling field. According to short-time sequential observations during the passage of a single NLIW, the depressed waves may play a crucial role in enhancing the vertical transfers of surface-replete dissolved gases, carbon, or suspended constituents through the surface layers of stratified waters. This NLIW forcing appears to be effective in supplying cooler and nutrient-rich water to the DA-associated coral community, and this may be vital for improving the future development and conservation of the coral ecosystem.

A Visual Tour of Carbon Export by Sinking Particles.

To better quantify the ocean's biological carbon pump, we resolved the diversity of sinking particles that transport carbon into the ocean's interior, their contribution to carbon export, and their attenuation with depth. Sinking particles collected in sediment trap gel layers from four distinct ocean ecosystems were imaged, measured, and classified. The size and identity of particles was used to model their contribution to particulate organic carbon (POC) flux. Measured POC fluxes were reasonably predicted by particle images. Nine particle types were identified, and most of the compositional variability was driven by the relative contribution of aggregates, long cylindrical fecal pellets, and salp fecal pellets. While particle composition varied across locations and seasons, the entire range of compositions was measured at a single well‐observed location in the subarctic North Pacific over one month, across 500 m of depth. The magnitude of POC flux was not consistently associated with a dominant particle class, but particle classes did influence flux attenuation. Long fecal pellets attenuated most rapidly with depth whereas certain other classes attenuated little or not at all with depth. Small particles (<100 μm) consistently contributed ∼5% to total POC flux in samples with higher magnitude fluxes. The relative importance of these small particle classes (spherical mini pellets, short oval fecal pellets, and dense detritus) increased in low flux environments (up to 46% of total POC flux). Imaging approaches that resolve large variations in particle composition across ocean basins, depth, and time will help to better parameterize biological carbon pump models.

Impact of mesoscale eddies on particulate organic carbon flux in the western subarctic North Pacific

Impact of mesoscale eddies on particulate organic carbon flux in the western subarctic North Pacific

Influence of Riverine Discharge and Timing of Ice Retreat on Particle Sedimentation Patterns on the Laptev Sea Shelf

AbstractWe studied the influence of river discharge and timing of ice retreat on sinking fluxes of total particulate matter and its components in the Laptev Sea in August–September 2017 and 2018. Vertical fluxes were measured with short‐term sediment traps located on the shelf adjusted to the Lena and Khatanga Rivers inflows (four shallow stations in 2017) and in Vilkitski Strait/Trough area (two deeper stations) in 2018. One station outside the Khatanga plume was repeated in both years. The effect of river discharge manifested by high total particle flux (1–1.6 g m−2 d−1), high Si (0.3 g m−2 d−1) and Al (0.1 g m−2 d−1) fluxes, a high proportion of terrigenous organic carbon (10.7%–27.5% total organic carbon) was clearly recognizable only in the shelf area closest to the Lena inflow. Large contribution of the lithogenic matter to the flux on the Laptev shelf is suggested to result also from sediment resuspension (caused by the tidal mixing and upwelling events), lateral offshore transport of sediment just above the seabed, and coastal abrasion. The magnitude of vertical fluxes of total particulate matter and organic carbon was two to three times higher in 2017 (late ice retreat) than in 2018 (early ice retreat). Centric diatoms dominated protist flux in 2017 while in 2018, dinoflagellates and tintinnids dominated. The contribution of protists to the biogenic organic carbon flux reached 62% in 2018 versus &lt;1% in 2017. Zooplankton fecal pellet flux was not influenced by river discharge but was higher soon after ice melt.

Carbon and nitrogen isotope constraints on source and variation of particulate organic matter in high-latitude agricultural rivers, Northeast China

The release of biospheric organic matter (OM) stored in the humus-rich black soils of Northeast China is highly susceptible to permafrost thaw and agriculturally induced erosion processes, and plays a disproportionate role in the global carbon cycle. However, the perturbations of global change and anthropogenic activities on riverine export of particulate organic carbon (POC) in the high-latitude agricultural areas remain poorly constrained. In the present study, this knowledge gap was filled by identifying the provenances of particulate organic matter (POM) and the erosional flux of POC from the rivers draining Northeast China. The elemental (POC%, PON% and C/N ratio) and isotopic (δ13C and δ15N) compositions of POM in suspended particulate matter (SPM) from the rivers in Northeast China (the Songhua River (SRB) and Liao River basins (LRB)) were analyzed. The results showed that during the wet season, the δ13C and δ15N values of POM in SPM were −25.8 ± 1.2‰ and 6.6 ± 2.2‰ respectively. Elemental and isotopic characteristics of the SPM samples and potential sources indicated that the sources of POM differed significantly among different sampling periods and sites. During the wet season, Soil OM was the primary POM contributor. Compared to the SRB, autochthonous OM made relatively larger contributions to POM in the LRB. During the dry season, for the SPM samples mainly from the LRB, the δ13C and δ15N values of POM were −22.7 ± 4.3‰ and 3.1 ± 2.4% respectively, suggesting that effluent detritus was the predominant source. The flux and yield of SPM revealed the seasonal and spatial variations of physical erosion rate that controls POC export. Seasonally, up to 61% of the annual SPM load and 58% of the annual POC flux of the SRB occurred in May and August due to thawing processes and increasing precipitation. Spatially, the lower reaches of the SRB had the highest SPM yield and contributed more than half of the annual SPM load of the SRB, likely attributable to the high intensity of agricultural activities. Globally, the POC% in SPM and POC yield of the SRB were larger than a series of the world's largest rivers under a given SPM condition, suggesting the high sensitivity of terrestrial OM export to erosion. These findings highlight the perturbation and mobilization of terrestrial OM in Northeast China under intensive agricultural activities and ongoing global climate change.

The Wind Effect on Biogeochemistry in Eddy Cores in the Northern South China Sea

Cyclonic and anticyclonic eddies are usually characterized by upwelling and downwelling, respectively, which are induced by eddy pumping near their core. Using a repeated expendable bathythermograph transect (XBT) and Argo floats, and by cruise experiments, we determined that not all eddies in the northern South China Sea (NSCS) were accompanied by eddy pumping. The weakening of background thermocline was attributed to the strengthening of eddy pumping, affected by (1) wind-induced meridional Sverdrup transports and (2) Kuroshio intrusion into the NSCS. Higher particulate organic carbon (POC) fluxes (&amp;gt; 100 mg-C m−2 day−1) were found near the eddy cores with significant eddy pumping (defined by a depth change of 22°C isotherm near the thermocline for over 10 m), although the satellite-estimated POC fluxes were inconsistent with the in-situ POC fluxes. nitrogen limitation transition and high POC flux were even found near the core of a smaller mesoscale (diameter &amp;lt; 100 km) cyclonic eddy in May 2014, during the weakening of the background thermocline in the NSCS. This finding provides evidence that small mesoscale eddies can efficiently provide nutrients to the subsurface, and that they can remove carbon from the euphotic zone. This is important for global warming, which generally strengthens upper ocean stratification.

Impact of a warm anomaly in the Pacific Arctic region derived from time-series export fluxes.

Unusually warm conditions recently observed in the Pacific Arctic region included a dramatic loss of sea ice cover and an enhanced inflow of warmer Pacific-derived waters. Moored sediment traps deployed at three biological hotspots of the Distributed Biological Observatory (DBO) during this anomalously warm period collected sinking particles nearly continuously from June 2017 to July 2019 in the northern Bering Sea (DBO2) and in the southern Chukchi Sea (DBO3), and from August 2018 to July 2019 in the northern Chukchi Sea (DBO4). Fluxes of living algal cells, chlorophyll a (chl a), total particulate matter (TPM), particulate organic carbon (POC), and zooplankton fecal pellets, along with zooplankton and meroplankton collected in the traps, were used to evaluate spatial and temporal variations in the development and composition of the phytoplankton and zooplankton communities in relation to sea ice cover and water temperature. The unprecedented sea ice loss of 2018 in the northern Bering Sea led to the export of a large bloom dominated by the exclusively pelagic diatoms Chaetoceros spp. at DBO2. Despite this intense bloom, early sea ice breakup resulted in shorter periods of enhanced chl a and diatom fluxes at all DBO sites, suggesting a weaker biological pump under reduced ice cover in the Pacific Arctic region, while the coincident increase or decrease in TPM and POC fluxes likely reflected variations in resuspension events. Meanwhile, the highest transport of warm Pacific waters during 2017–2018 led to a dominance of the small copepods Pseudocalanus at all sites. Whereas the export of ice-associated diatoms during 2019 suggested a return to more typical conditions in the northern Bering Sea, the impact on copepods persisted under the continuously enhanced transport of warm Pacific waters. Regardless, the biological pump remained strong on the shallow Pacific Arctic shelves.

Microscopy and DNA‐based characterization of sinking particles at the Bermuda Atlantic Time‐series Study station point to zooplankton mediation of particle flux

AbstractPlankton‐derived, microscopic, and macroscopic sinking aggregates constitute most of the particulate organic carbon (POC) flux in the oceans. While the flux of particulate organic matter and associated elements has been quantified at the Bermuda Atlantic Time‐series Study (BATS) station for several decades, we lack an understanding of the source and composition of sinking particles, as well as the fate of predominant phytoplankton taxa. We determined the composition of individual sinking particles and their microbial communities in the upper 300 m depth at the BATS station in fall 2017 and spring 2018 by image analysis and V4 amplicon sequencing of the 16S and 18S rRNA genes. The sinking particles were primarily composed of phytodetrital aggregates, fecal aggregates, and fecal pellets. In the fall, phytodetrital aggregates were numerically dominant and drove the majority of the POC flux; however, in the spring, particle flux of all particle categories declined below 150 m, and the POC flux at 200 m shifted to one driven by fecal aggregates. The relative composition of the microbial communities associated with phytodetrital and fecal aggregates were statistically indistinguishable in both seasons, and prokaryotic taxa known to be associated with the gut microbiomes of zooplankton were indicators of the sinking particles. Our results point to the utilization and modification of sinking particles by resident midwater zooplankton populations, and to fecal pellets as the predominant mechanism transporting picophytoplankton to depth.

Oxygen Isotope Offsets in Deep-Water Benthic Foraminifera

ABSTRACT Despite the extensive use of the benthic foraminiferal oxygen isotope composition (δ18O) as a proxy for paleoclimatic reconstructions, uncertainties remain regarding the consistency of interspecies offsets and the environmental factors controlling 18O fractionation. We investigated δ18O offsets of some frequently used Uvigerina, Bulimina, and Cibicidoides species in core top samples from different hydrographic and sedimentary regimes in the South China Sea, Makassar Strait, and Timor Strait/Eastern Indian Ocean. The δ18O values of the epifaunal taxa Cibicidoides mundulus and Cibicidoides wuellerstorfi showed no significant offset in all investigated regions, whereas shallow infaunal Cibicidoides species exhibited higher variability and were less reliable. We found no offsets between species of Uvigerina and Bulimina and assume that these genera can be measured together and/or substituted. Our results show that epifaunal taxa are close to equilibrium with ambient seawater and thus provide more reliable records of past ice volume and/or bottom water temperature variations than infaunal taxa. Offsets among equilibrium calcite, epifaunal taxa, and infaunal taxa are not constant “vital effects” but are influenced by changing gradients in bottom to pore water pH and carbonate ion concentrations that depend on deep-water ventilation and export flux of particulate carbonate and organic carbon. Offsets between epifaunal and infaunal taxa varied between 0.58 and 0.73‰, depending on regional bottom and pore water conditions. Our findings highlight the importance of regional and temporal variations in organic carbon flux/degradation and dissolution of calcite that may lead to slight under- or overestimates of the amplitude of δ18O fluctuations, especially during times of rapidly changing calcite-saturation of bottom and pore water.

210Po and 210Pb as Tracers of Particle Cycling and Export in the Western Arctic Ocean

The distribution and vertical fluxes of particulate organic carbon and other key elements in the Arctic Ocean are primarily governed by the spatial and seasonal changes in primary productivity, areal extent of ice cover, and lateral exchange between the shelves and interior basins. The Arctic Ocean has undergone rapid increase in primary productivity and drastic decrease in the areal extent of seasonal sea ice in the last two decades. These changes can greatly influence the biological pump as well as associated carbon export and key element fluxes. Here, we report the export of particulate organic and inorganic carbon, particulate nitrogen and biogenic silica using 210Po and 210Pb as tracers for the seasonal vertical fluxes. Samples were collected as a part of US GEOTRACES Arctic transect from western Arctic Basin in 2015. The total activities of 210Po and 210Pb in the upper 300 m water column ranged from 0.46 to 16.6 dpm 100L–1 and 1.17 to 32.5 dpm 100L–1, respectively. The 210Pb and 210Po fluxes varied between 5.04–6.20 dpm m–2 d–1 and 8.26–21.02 dpm m–2 d–1, respectively. The corresponding particulate organic carbon (POC) and particulate nitrogen (PN) fluxes ranged between 0.75–7.43 mg C m–2 d–1 and 0.08–0.78 mg N m–2 d–1, respectively, with highest fluxes observed in the northern ice-covered stations. The particulate inorganic carbon (PIC) and biogenic silica (bSi) fluxes were extremely low ranging from 0 to 0.14 mg C m–2 d–1 and 0.14 to 2.88 mg Si m–2 d–1, respectively, at all stations suggesting absence of ballast elements in facilitating the biological pump. The variability in POC fluxes with depth suggest prominent influence of lateral transport to downward fluxes across the region. The results provide a better understanding of the spatial variability in the vertical fluxes POC, PN, bSi, and PIC in the western Arctic which is currently undergoing dramatic changes.

Causal analysis of the temperature impact on deep-sea biodiversity.

The deep sea comprises more than 90% of the ocean; therefore, understanding the controlling factors of biodiversity in the deep sea is of great importance for predicting future changes in the functioning of the ocean system. Consensus has recently been increasing on two plausible factors that have often been discussed as the drivers of deep-sea species richness in the contexts of the species-energy and physiological tolerance hypotheses: (i) seafloor particulate organic carbon (POC) derived from primary production in the euphotic zone and (ii) temperature. Nonetheless, factors that drive deep-sea biodiversity are still actively debated potentially owing to a mirage of correlations (sign and magnitude are generally time dependent), which are often found in nonlinear, complex ecological systems, making the characterization of causalities difficult. Here, we tested the causal influences of POC flux and temperature on species richness using long-term palaeoecological datasets derived from sediment core samples and convergent cross mapping, a numerical method for characterizing causal relationships in complex systems. The results showed that temperature, but not POC flux, influenced species richness over 103-104-year time scales. The temperature-richness relationship in the deep sea suggests that human-induced future climate change may, under some conditions, affect deep-sea ecosystems through deep-water circulation changes rather than surface productivity changes.

Effects of river damming and delta erosion on organic carbon burial in the Changjiang Estuary and adjacent East China Sea inner shelf

River damming reduces sediment load in rivers, leading to instability and erosion of coastal deltas; this is a global issue in many large-river delta-front estuaries (LDEs). The total organic carbon, stable carbon isotope, total nitrogen, lignin phenols in suspended particulate matter, and three sediment cores collected from the Changjiang LDE were analyzed to study the effects of river damming and delta erosion on the redistribution and burial of organic carbon (OC) in the coastal margin. The OC budgets that include the delta erosion process were established to better constrain the burial fluxes of OC in the Changjiang River system before and after river damming. We used a three-end-member mixing model with Monte Carlo simulation and found that the ratio of deltaic OC to riverine OC significantly increased in estuarine SPM and sediment cores in the Changjiang LDE after the operation of the Three Gorges Dam (TGD), reflective of reduced sediment load and enhanced delta erosion. Our budget showed that, compared with the pre-dam period (1953–1985), the burial of terrestrial OC in the Changjiang LDE sediments had been reduced by ~97.3%, on the basis of 72.7% and 82.7% reductions in sediment load and particulate OC flux, respectively. Although trapping of terrestrial OC in reservoirs is the key driver in these reductions, re-exposure and decomposition of OC from eroding delta sediments also contribute significantly to OC losses in coastal sediments. On the contrary, the ratio of marine OC to riverine OC increased in sediment cores after TGD building, due in part to more inputs of marine OC because of serious eutrophication in recent decades. As continued dam construction and sea-level rise change the spatial and temporal dynamics of carbon sequestration in the coastal zones of LDEs, further consideration of alterations in these carbon burial “hotspots” is needed in global carbon models.

Carbonate fluxes by coccolithophore species between NW Africa and the Caribbean: Implications for the biological carbon pump

Coccolithophores are among the most important calcifying pelagic organisms. To assess how coccolithophore species with different coccolith‐carbonate mass and distinct ecological resilience to ocean warming will influence the “rain ratio” and the “biological carbon pump”, 1 yr of species‐specific coccolith‐carbonate export fluxes were quantified using sediment traps moored at four sites between NW Africa and the Caribbean (i.e., CB‐20°N/21°W, at 1214 m; M1‐12°N/23°W, at 1150 m; M2‐14°N/37°W, at 1235 m; M4‐12°N/49°W, at 1130 m). Highest coccolith‐CaCO3 fluxes at the westernmost site M4, where the nutricline is deepest along the tropical North Atlantic, were dominated by deep‐dwelling small‐sized coccolith species Florisphaera profunda and Gladiolithus flabellatus. Total coccolith‐CaCO3 fluxes of 371 mg m−2 yr−1 at M4 were followed by 165 mg m−2 yr−1 at the north‐easternmost CB, 130 mg m−2 yr−1 at M1, and 114 mg m−2 yr−1 at M2 in between. Coccoliths accounted for nearly half of the total carbonate flux at M4 (45%), much higher compared to 23% at M2 and 15% at M1 and CB. At site M4, highest ratios of coccolith‐CaCO3 to particulate organic carbon fluxes and weak correlations between the carbonate of deep‐dwelling species and particulate organic carbon suggest that increasing productivity in the lower photic zone in response to ocean warming might enhance the rain ratio and reduce the coccolith‐ballasting efficiency. The resulting weakened biological carbon pump could, however, be counterbalanced by increasing frequency of Saharan dust outbreaks across the tropical Atlantic, providing mineral ballast as well as nutrients to fuel fast‐blooming and ballast‐efficient coccolithophore species.

Molecular Level Characterization of Diatom and Coccolithophore-Associated Biopolymers That Are Binding 210Pb and 210Po in Seawater

Through a combination of selective extractions and molecular characterization techniques including Isoelectric Focusing Chromatography and Electrospray Ionization Fourier-Transform Ion Cyclotron Resonance Mass spectrometry, molecular structures of diatom (Phaeodactylum tricornutum) and coccolithophore (Emiliania huxleyi)-associated biopolymers that are responsible for the distinct partitioning behavior between 210Pb and 210Po were determined. Our results show that diatom-derived biopolymers have distinctive elemental grouping distributions as compared to those excreted by the coccolithophore, with the former consisting of more heterogeneous elements (i.e., nitrogen, sulfur and phosphorus-containing organic compounds). For the coccolithophore culture, two 210Pb-enriched biopolymers (non-attached exopolymeric substances and coccosphere shell-associated biopolymers) have a higher abundance of CHO-type compounds, suggesting CHO-only-type compounds as the main binding moieties for 210Pb. In contrast, such association was not evident in the diatom culture. Different with 210Pb, 210Po enrichment in coccolithophore-derived attached exopolymeric substances and Fe-Mn-associated metabolites coincided with the higher abundance of nitrogen/sulfur-containing organic compounds in these two biopolymer fractions, suggesting the strong parallel of Po with the production of nitrogen-rich organic matter as well as sulfur-containing amino acids. These different associations between 210Pb/210Po and organic functional groups were further explored by separating 210Pb or 210Po-labeled coccolithophore-derived biopolymers via isoelectric focusing. This technique suggests that phosphate group-containing molecules but not the other molecules that contain heterogeneous elements (e.g., CHONS, CHON, and CHOS) as the strongest binding agents for 210Pb, while the more hydrophobic (high protein to carbohydrate ratio) nitrogen/sulfur-enriched organic moieties acted as the main 210Po-binding ligands. It is concluded that the deficiency of 210Po with respect to 210Pb can be influenced by the relative abundance of nitrogen/sulfur-enriched organic moieties to the nitrogen/sulfur-depleted organic compounds in the water column. This behavior constrains the application of 210Po-210Pb approach to quantify the particulate organic carbon (POC) export flux in the ocean. It also explains that differences in chemical binding of the 210Po as compared to those of other radionuclides (e.g., thorium-234) as the main factor. That suggests that differences in decay half-lives or physical factors are less important when these nuclides are applied to estimate the POC flux in the ocean.

Biogenic sinking particle fluxes and sediment trap collection efficiency at Ocean Station Papa

Comprehensive field observations characterizing the biological carbon pump (BCP) provide the foundation needed to constrain mechanistic models of downward particulate organic carbon (POC) flux in the ocean. Sediment traps were deployed three times during the EXport Processes in the Ocean from RemoTe Sensing campaign at Ocean Station Papa in August–September 2018. We propose a new method to correct sediment trap sample contamination by zooplankton “swimmers.” We consider the advantages of polyacrylamide gel collectors to constrain swimmer influence and estimate the magnitude of possible trap biases. Measured sediment trap fluxes of thorium-234 are compared to water column measurements to assess trap performance and estimate the possible magnitude of fluxes by vertically migrating zooplankton that bypassed traps. We found generally low fluxes of sinking POC (1.38 ± 0.77 mmol C m–2 d–1 at 100 m, n = 9) that included high and variable contributions by rare, large particles. Sinking particle sizes generally decreased between 100 and 335 m. Measured 234Th fluxes were smaller than water column 234Th fluxes by a factor of approximately 3. Much of this difference was consistent with trap undersampling of both small (&amp;lt;32 μm) and rare, large particles (&amp;gt;1 mm) and with zooplankton active migrant fluxes. The fraction of net primary production exported below the euphotic zone (0.1% light level; Ez-ratio = 0.10 ± 0.06; ratio uncertainties are propagated from measurements with n = 7–9) was consistent with prior, late summer studies at Station P, as was the fraction of material exported to 100 m below the base of the euphotic zone (T100, 0.55 ± 0.35). While both the Ez-ratio and T100 parameters varied weekly, their product, which we interpret as overall BCP efficiency, was remarkably stable (0.055 ± 0.010), suggesting a tight coupling between production and recycling at Station P.