Particulate Organic Carbon Export Rates Research Articles

Export of particulate organic carbon by the mountainous tropical rivers of Western Ghats, India: Variations and controls

Transport of organic carbon by small mountainous rivers is essential, but the poorly constrained component of the global carbon cycle. In the current research, we sampled and analyzed particulate organic carbon (POC) contents from 70 sizeable tropical coastal rivers, draining the Western Ghats (WG) of India. This study aimed to investigate the spatiotemporal variability in POC contents, to estimate flux and to identify environmental controls on POC sources and transport characteristics across the region. The averaged value of organic carbon (OC) in the particulate samples is 3.24%, and the mean POC concentration is 2.86 mg l−1. We classified the samples based on total suspended matter (TSM) classes for source appropriation. Litter/riparian (42.5%) pools are the largest source of organic matter, followed by autochthonous (36%) and soil (21.5%) for the WG region. However, locally autochthonous sources contribute exceptionally to POC pools, indicating a favorable environmental condition for the growth of algae and phytoplankton. Land-use & land-cover, climate, topography, and sediment erosion seems to be determining the local variability in sources to POC pools and fluxes. The POC export rates suggest that within the region, the POC yields of the Deccan Trap (DT) and the Western Dharwar Craton (WDC) blocks are about two times higher than that of the Southern Granulite Terrain (SGT) region. With POC yield of 7.0 g m−2 yr−1, this region exports 0.79 Tg C (~ 0.5% of the global POC) to the Arabian Sea annually. The POC flux of the WG region (covering 0.25% of Asia's land area) is approximately 1.0% of Asia's riverine POC flux to the ocean.

Effect of anthropogenic warming on microbial respiration and particulate organic carbon export rates in the sub-Antarctic Southern Ocean

ABSTRACT: Microbial respiration of particulate organic carbon (POC) is one of the key processes controlling the magnitude of POC export from the surface ocean and its storage on long timescales in the deep. Metabolic processes are a function of temperature, such that warming sea temperatures should increase microbial respiration, potentially reducing POC export. To investigate this in the Southern Ocean, we measured microbial oxygen consumption of large particles over a 10°C temperature range (summer maximum +8°C) to then estimate the decrease in export by 2100. Our results showed that POC-normalised respiration increased with warming. We estimate that POC export (scaled to primary production) could decrease by 17 ± 7% (SE) by 2100, using projected regional warming (+1.9°C) from the IPCC RCP 8.5 (‘business-as-usual’ scenario) for our sub-Antarctic site. Increased microbial respiration is one of many processes that will be altered by future climate change, which could all modify carbon storage in the future. Our estimate of the potential decline in carbon sequestration is within previous estimates from lab and field experiments, but higher than simple mechanistic models. To explore our results further, we used the metabolic theory of ecology (MTE) to determine the activation energy of microbial respiration, which was 0.9 eV. This is higher than classical MTE (0.6-0.7 eV), suggesting that sub-Antarctic microbes are particularly sensitive to temperature change. Such regional characteristics in the response of organisms to increased temperatures should be accounted for in large-scale or global model analyses to ensure that the results do not underestimate microbial responses to warming.

A synthesis of upper ocean carbon and dissolved iron budgets for Southern Ocean natural iron fertilisation studies

For over a decade the relationship between iron addition and carbon export has been an active topic of Southern Ocean biogeochemical investigation. To study the iron-carbon interaction, a number of natural iron sources within the Antarctic Circumpolar Current have been targeted to quantify the resultant particulate organic carbon (POC) export. Three studies in particular near the Crozet Islands, Kerguelen Plateau, and Antarctic Peninsula have produced estimates of both dissolved iron (DFe) supply and POC export. For each area, we present a detailed synthesis of 234Thorium-derived POC export and of the DFe budgets that fuel the substantial phytoplankton blooms observed in these areas. Furthermore, we discuss the nuances of calculating seasonal POC export, which is required to estimate the seasonal response of POC export to DFe fertilisation. To conclude, we review the relationship between DFe supply and POC export (C:Fe ratio) for these areas, which provides an estimate of POC export efficiency. Daily rates of POC export from the mixed layer of naturally Fe-enriched Southern Ocean blooms range from 15 to 32mmol Cm−2d−1, with associated control sites typically exporting ∼3-times less POC, (5 to 12mmol Cm−2d−1). Within each project, the 3-fold trend is also observed in estimates of seasonal POC export; however, variation between study regions shows up to a 4-fold difference (range 1.4–5.0molm−2). DFe supply is dominated by horizontal processes and spans an order of magnitude depending on the location (190–2700nmolm−2d−1). Where the calculation of seasonal C:Fe ratios is possible, almost an order of magnitude variation (16,790–154,000) is observed between different Southern Ocean regions.

A carbon budget for a naturally iron fertilized bloom in the Southern Ocean

Subantarctic islands in the high-nutrient, low-chlorophyll (HNLC) Southern Ocean are natural sources of iron and stimulate blooms in their proximity, such as the one observed close to the Crozet Islands (52°E, 46°S). During 2004/2005, particulate organic carbon (POC) export was measured using the 234Th technique in the Crozet bloom and compared with an HNLC control region. Initial results showed that iron release had no effect on daily POC export rates, thus any iron-driven enhancement in POC export was due to a longer export phase in the bloom region when compared to the control region. The duration of the export event was empirically estimated by closing the silicon budget, thus allowing seasonal POC export to be calculated by applying the export duration to the daily rates of POC export. This yields a seasonal estimate of POC export that is 3.6 times larger (range 1.9–7.1) in the iron-fertilized region than in the HNLC control region. These estimates of POC export were then compared to independent estimates of organic matter storage in the upper ocean, which are significant in both the HNLC and control regions. Overall, integrated POC export was significantly (approximately 50%) lower than estimated seasonal new production, the fraction of production that is supported by inputs of new nutrients. Finally, the sequestration efficiency, the numerical relationship between the supply of the limiting nutrient, iron, and the key ecosystem function of POC export at 100 m, is estimated to be 16,790 mol:mol.

234Th/238U disequilibrium and particulate organic carbon export in the northern South China Sea

We utilized 234Th, a naturally occurring radionuclide, to quantify the particulate organic carbon (POC) export rates in the northern South China Sea (SCS) based on data collected in July 2000 (summer), May 2001 (spring) and November 2002 (autumn). Th-234 deficit was enhanced with depth in the euphotic zone, reaching a subsurface maximum at the Chl-a maximum in most cases, as commonly observed in many oceanic regimes. Th-234 was in general in equilibrium with 238U at a depth of ∼100 m, the bottom of the euphotic zone. In this study the 234Th deficit appeared to be less significant in November than in July and May. A surface excess of 234Th relative to 238U was found in the summer over the shelf of the northern SCS, most likely due to the accumulation of suspended particles entrapped by a salinity front. Comparison of the 234Th fluxes from the upper 10 m water column between 2-D and traditional 1-D models revealed agreement within the errors of estimation, suggesting the applicability of the 1-D model to this particular shelf region. 1-D model-based 234Th fluxes were converted to POC export rates using the ratios of bottle POC to 234Th. The values ranged from 5.3 to 26.6 mmol C m−2d−1 and were slightly higher than those in the southern SCS and other oligotrophic areas. POC export overall showed larger values in spring and summer than in autumn, the seasonality of which was, however, not significant. The highest POC export rate (26.6 mmol C m−2d−1) appeared at the shelf break in spring (May), when Chl-a increased and the community structure changed from pico-phytoplankton (<2 µm) dominated to nano-phytoplankton (2–20 µm) and micro-phytoplankton (20–200 µm) dominated.

Fluvial organic carbon flux from an eroding peatland catchment, southern Pennines, UK

Abstract. This study investigates for the first time the relative importance of dissolved organic carbon (DOC) and particulate organic carbon (POC) in the fluvial carbon flux from an actively eroding peatland catchment in the southern Pennines, UK. Event scale variability in DOC and POC was examined and the annual flux of fluvial organic carbon was estimated for the catchment. At the event scale, both DOC and POC were found to increase with discharge, with event based POC export accounting for 95% of flux in only 8% of the time. On an annual cycle, exports of 35.14 t organic carbon (OC) are estimated from the catchment, which represents an areal value of 92.47 g C m−2 a−1. POC was the most significant form of organic carbon export, accounting for 80% of the estimated flux. This suggests that more research is required on both the fate of POC and the rates of POC export in eroding peatland catchments.

How accurate are 234Th measurements in seawater based on the MnO2‐impregnated cartridge technique?

In the past decade, a MnO2‐impregnated cartridge technique has been widely used to extract 234Th from seawater. One of the inherent assumptions associated with this technique is that all Th species in the dissolved phase are subject to extraction by the MnO2 cartridges. In this study, an intercalibration between the cartridge technique and a small‐volume MnO2 coprecipitation technique was carried out to test this assumption. We demonstrated that the collection efficiency for 234Th could be substantially overestimated by the MnO2 cartridge technique. This may be the result of organic complexation of a significant portion of 234Th in seawater, causing this fraction of Th to pass through the MnO2 cartridges. The overestimate in collection efficiency may explain the deep‐water 234Th deficit observed in some oceanographic settings (e.g., the Gulf of Mexico, the Middle Atlantic Bight, and the Gulf of Maine). Sensitivity tests show that using the cartridge technique can yield 234Th‐based particulate organic carbon export rates that are overestimated by factors of up to 10. Furthermore, the frequent observed disagreements between Th fluxes recorded by shallow sediment traps and estimated using the cartridge method may be ascribed, at least partially, to this methodological issue.

Upper ocean export of particulate organic carbon in the Arabian Sea derived from thorium-234

Thorium-234 is used in the Arabian Sea as a tracer of sinking particle fluxes. Samples were collected from January to August 1995 on four cruises during the Northeast Monsoon, the Spring Intermonsoon and the mid- and late-Southwest Monsoon periods. In this study, 234Th activity distributions are used to quantify the 234Th flux on sinking particles, and the measured ratio of particulate organic carbon (POC) to particulate 234Th is used to convert from 234Th to POC export at 100 m. The calculated POC fluxes range from <1 to >25 mmols C m -2 d -1, and strong seasonal and spatial gradients are observed. The single largest feature is a basinwide export maximum associated with the late-SW Monsoon cruise when POC export rates are 17–28% of the observed primary production rates along the southern sampling line. During all other cruises, this export ratio is <2–10%, with an increase near shore where POC fluxes are generally elevated. Also, during the Spring Intermonsoon, a POC export maximum is observed along the northern sampling line. Both this Spring export feature and late-SW Monsoon flux maximum appear to be associated with a phytoplankton community structure dominated by diatoms. The timing of the late-SW Monsoon flux peak agrees with the observed flux maximum in the deep moored time-series sediment traps (Honjo et al., 1998). This dramatic increase in export between the mid- and late-SW Monsoon also corresponds to measured decreases in the stocks of total organic C in the upper 150 m (Hansell and Peltzer, 1998) and a sharp decline in surface water Al and Fe (Measures and Vink, 1998). These 100 m flux results, plus a series of POC flux profiles, allow for a more complete understanding of the magnitude and timing of sub-euphotic zone export in the Arabian Sea.

Exports of organic carbon in two river systems in NE Scotland

Riverine fluxes of dissolved organic carbon (DOC) and particulate organic carbon (POC) were estimated at 23 sites on two major river systems in NE Scotland, from fortnightly measurements of concentration and flow. The annual flux of DOC and POC increased cumulatively downstream in both the R. Dee and the R. Don. At the lowest, non-tidal site the estimated annual DOC export (95% confidence limits) was 4.1 (±2.1)×106kg per year for the R. Dee and 2.3 (±1.3)×106kg per year for the R. Don. POC exports in the two rivers comprised about 10% of the total organic carbon flux in the R. Dee at 0.3 (±0.2)×106kg per year and 25% in the R. Don at 0.7 (±0.5)×106kg per year. When expressed on a unit area basis, DOC flux estimates were approximately 20kgha−1 per year and POC exports were approximately 1–5kgha−1 per year at main stem sites on both rivers. However, there were considerable variations in fluxes between tributary and headwater sites on the R. Dee. DOC exports ranged from 13.2 to 115.0kgha−1 per year and POC exports were between 6.0 and 85.3kgha−1 per year. In a sample of 11 R. Dee sub-catchments, annual fluxes of DOC and POC both showed a strong positive correlation with the cover of hill peat in the catchment area, which indicates that areas of hill peat are a major source of organic carbon to the river. In the lower parts of the R. Dee and R. Don catchments, POC export rates during non-storm flow conditions also appeared to be related to the extent of arable land in the catchment. © 1997 Elsevier Science B.V.