Laodong Guo

Sensitivity of the carbon cycle in the Arctic to climate change

The recent warming in the Arctic is affecting a broad spectrum of physical, ecological, and human/cultural systems that may be irreversible on century time scales and have the potential to cause rapid changes in the earth system. The response of the carbon cycle of the Arctic to changes in climate is a major issue of global concern, yet there has not been a comprehensive review of the status of the contemporary carbon cycle of the Arctic and its response to climate change. This review is designed to clarify key uncertainties and vulnerabilities in the response of the carbon cycle of the Arctic to ongoing climatic change. While it is clear that there are substantial stocks of carbon in the Arctic, there are also significant uncertainties associated with the magnitude of organic matter stocks contained in permafrost and the storage of methane hydrates beneath both subterranean and submerged permafrost of the Arctic. In the context of the global carbon cycle, this review demonstrates that the Arctic plays an important role in the global dynamics of both CO2 and CH4. Studies suggest that the Arctic has been a sink for atmospheric CO2 of between 0 and 0.8 Pg C/yr in recent decades, which is between 0% and 25% of the global net land/ocean flux during the 1990s. The Arctic is a substantial source of CH4 to the atmosphere (between 32 and 112 Tg CH4/yr), primarily because of the large area of wetlands throughout the region. Analyses to date indicate that the sensitivity of the carbon cycle of the Arctic during the remainder of the 21st century is highly uncertain. To improve the capability to assess the sensitivity of the carbon cycle of the Arctic to projected climate change, we recommend that (1) integrated regional studies be conducted to link observations of carbon dynamics to the processes that are likely to influence those dynamics, and (2) the understanding gained from these integrated studies be incorporated into both uncoupled and fully coupled carbon-climate modeling efforts. (Less)

The distribution of colloidal and dissolved organic carbon in the Gulf of Mexico

Cross-flow ultrafiltration techniques have been used to extract colloidal organic carbon (COC) from seawater and to investigate different molecular weight fractions of dissolved organic carbon (DOC). Using a high-temperature catalytic oxidation (HTCO) method, DOC and COC of seawater in the Gulf of Mexico were measured during a R/V Gyre cruise in June 1992. DOC concentrations in surface water varied from 131 μM at a near-shore station (water depth ∼ 20 m) to 83 μM at an off-shore station (water depth ∼ 1550 m). DOC concentrations show statistically significant correlations with apparent oxygen utilization (AOU), as well as with temperature. However, as a upper limit, only 20–30% of the oxygen consumption could be due to dissolved organic carbon oxidation. Furthermore, a good correlation between DOC and AOU existed only in the upper water column across the pycnocline, which we ascribed to lateral exchange processes. Water mixing can be quite important in controlling the distribution of DOC and the relationship between DOC and AOU in the water column. Concentrations of COC > 1000 Dalton ranged from 20 to 69 μM, while COC > 10,000 Dalton ranged from 4 to 16 μM in the study area. On average, COC (> 1000 Dalton) comprised about 45% of the initial DOC, and the mass concentration of colloids was > 1 mg l−1. This was one order of magnitude higher than the concentration of suspended particulate matter, and indicates that COC may be an important component of the carbon cycle in the ocean. The relative abundance of COC (both > 1000 and > 10,000 Dalton) decreased from surface water to deep water, not only in terms of concentration but also relative to total DOC. The measurement of molecular weight distributions indicated that ∼ 35% of the initial DOC was in the 1000 –10,000 Dalton fraction, while only about 10% was in the > 10,000 Dalton fraction, leaving approximately 55% in the truly dissolved fraction (i.e. < 1000 Dalton).

Isotopic evidence for the contemporary origin of high-molecular weight organic matter in oceanic environments

Abstract Previous work has suggested that apparent old 14C ages for oceanic DOC are the result of mixing of different organic carbon fractions. This report provides direct evidence for a contemporary 14C age of a high-molecular-weight (HMW) fraction of colloidal organic carbon (≥10 kD). Colloidal organic matter, COM10 (from 10 kDaltons (kD) to 0.2 μm), isolated from the upper water column of the Gulf of Mexico and the Middle Atlantic Bight (MAB) region, generally has a contemporary age (i.e., younger than a few decades), while COM1 (from 1 kD to 0.2 μm), is apparently old: 380–4500 y b P. Thus, BMW COM10 (3–5% of DOC) from the upper water column is derived from living particulate organic matter (POM) and cycles rapidly, while a significant fraction of low-molecular-weight (≤1 kD) DOM is likely more refractory, and cycles on much longer time scales. The presence of pigment biomarker compounds in COM1 from the upper water column points to selected phytoplankton species as one of the sources of COM. Terrestrial carbon as another source of COM is suggested from the inverse correlation between Δ14C and δ13C values, as well as the increasing δ13C values with increasing salinity. 234Th-derived turnover times of COM10 and COM1 from both the Gulf of Mexico and MAB are consistently short, 1–20 and 3–30 days, respectively. These short residence times support the hypothesis that 14C ages of colloidal fractions of DOC are the result of COM fractions being a mixture of several endmembers with fast and slow turnover rates.

A critical evaluation of the cross-flow ultrafiltration technique for sampling colloidal organic carbon in seawater

The retention characteristics, integrity and performance of cross-flow ultrafilters [Amicon S10N1 with 1-kilodalton (kDa) cutoff] were examined in the laboratory. In addition, the effects of concentration factors and sample storage on dissolved organic carbon (DOC) mass balance and size fractionation were investigated using natural seawater and macromolecular solutions containing compounds of different molecular weights (MWs). The concentration of DOC in the permeate and the percentage of colloidal organic carbon (COC) retained by the ultrafilter change with the concentration factor during the ultrafiltration process. Thus, the DOC concentration in the entire permeate is necessary to reliably evaluate the DOC mass balance during ultrafiltration and for the calculation of the COC fraction. Replicate ultrafiltration experiments show that reproducible results can be achieved (±2%), and that losses or contamination problems during ultrafiltration were negligible. However, reproducible and accurate results for size fractionation of DOC require rigorous cleaning and strict sampling protocols. Short-term storage of coastal seawater samples (salinity ~ 28; DOC ~ 170 μM) did not significantly affect the overall results of DOC size fractionation but could have severe effects on minor constituents. A certain percentage of lower-MW ( 1 kDa) can pass through the ultrafilter, depending on the concentration factor and bulk DOC concentrations. Inter-molecular interactions and/or steric effects of low-MW compounds and the slow breakthrough of high-MW macromolecules are likely the major processes responsible for these observations. Ultrafiltration of DOC in seawater can be characterized by a permeation model with a constant permeation coefficient of < 1. A high concentration factor is suggested for size fractionation of DOC in order to minimize the retention of low-MW DOC. In addition, results from isotopic (13C and 14C) and elemental (C and N) characterization of colloidal samples are presented.

Characterization of Siberian Arctic coastal sediments: Implications for terrestrial organic carbon export

Surface sediments were collected during the 2000 TransArctic Expedition along the Siberian Arctic coastline, including the Ob, Yenisey, Khatanga, Lena, and Indigirka estuaries. Sediments were chara ...

Composition and cycling of colloids in marine environments

Colloidal (COM) or macromolecular organic matter makes up a significant portion of the bulk dissolved organic matter (DOM) pool in aquatic environments. Because of their high specific surface areas and complexation capacities, marine colloids are of great importance not only in the global carbon cycle but also in the biogeochemical cycling of many particle-reactive nuclides and trace elements in the ocean. However, the colloidal pool as a whole is still poorly understood and largely uncharacterized. Recently, cross-flow ultrafiltration and other separation techniques, which have been successfully used to isolate marine colloids, combined with a multitracer approach, have greatly advanced our understanding of the cycling of COM and its associated trace elements in marine environments. In this paper we focus on recent developments on isotopic and elemental composition of colloids which allow organic matter cycling in marine environments to be constrained. Major sections review sampling techniques for aquatic colloids, concentrations and distribution of COM, biochemical and elemental (organic and inorganic) characterization, and stable isotopic (13C and 15N) and radioisotopic (14C and 234Th) characterization of marine colloids. We discuss sources and turnover rates of organic matter in the ocean, importance of benthic boundary layer processes in the cycling of DOM, changes in the paradigms of marine organic matter cycling, and research needs for a better understanding of the biogeochemistry of marine colloids.

Isotopic and elemental characterization of colloidal organic matter from the Chesapeake Bay and Galveston Bay

In order to investigate sources and turnover rates of dissolved organic matter from Chesapeake Bay and Galveston Bay, colloidal organic matter (COM) was isolated using cross-flow ultrafiltration and subsequently characterized for its elemental (C, N, and S) and isotopic (13C and 14C) composition. Distributions of dissolved organic carbon (DOC) in Chesapeake Bay showed a non-systematic variation, while in Galveston Bay, a non-conservative behavior of DOC with source inputs in the low salinity region was observed. Results of size fractionation of total organic carbon (TOC) revealed that, on average, paniculate organic carbon (POC) comprised ~ 12% and 39% of the TOC pool in Galveston Bay and the Chesapeake Bay, respectively. Colloidal organic carbon (COC) between 1 kDa and 0.2 μm (COC1) constituted ~ 53% and 35%, respectively, with 6–7% of TOC in the HMW fraction (10 kDa to 0.2 μm, COC10), and only ~ 34% and 25%, respectively, of the TOC in the < 1 kDa dissolved fraction. Values of Δ14C and CN ratios of COM, in general, decreased from river to coastal seawater whereas δ13C values increased with increasing salinity, indicating that organic carbon sources changed from more terrestrial components to phytoplankton-derived sources during estuarine mixing. The distinct isotopic signature and elemental composition of riverine and estuarine COM also suggest that most riverine HMW COM could be removed or decomposed rapidly within the estuary. The fact that values of CN ratios increased from particulate to HMW to medium MW COM suggest that reactivities of organic matter decrease with reducing size. While Δ14C values of COM1 were generally equivalent to contemporary ages, they were consistently lower for the COM10. Lower Δ14C values and lower CN ratios in the COM10 than in the COM1 suggest that most of the estuarine HMW COM is from older and more proteinaceous sources within the estuaries. We hypothesize that resuspended sedimentary organic matter or recycled older DOM is likely the source for COM10.

234Th scavenging and its relationship to acid polysaccharide abundance in the Gulf of Mexico

Size-fractionated particulate 234 Th and acid polysaccharides (APS) were collected from stations along a transect in the Gulf of Mexico, in order to examine the role of APS content in controlling the extent and rates of 234 Th scavenging in the ocean and to explore, for the first time, the relationship between Th scavenging and biochemical composition of particulate matter. Oceanographically consistent vertical profiles of dissolved and particulate 234 Th concentrations were observed, with a considerable 234 Th deficit relative to 238 U in the upper water column and in benthic nepheloid layers, but reaching secular equilibria between 234 Th and 238 U in intermediate waters. Within the total particulate 234 Th pool (>0.5 Am), the 10–53 Am fraction had the largest share of 234 Th (37–57%), followed by the >53 Am (13–36%), the 1–10 Am (10–21%), and the 0.5–1 Am (8–17%) fractions, resulting in a decrease of POC/ 234 Th ratios with increasing particle size. Residence times of 234 Th in size-fractionated particles, calculated with a serial multi-box model, were, as expected, consistently shorter than those for total particulate 234 Th, with the shortest residence times ( 53 Am. These results suggest that submicron and micronsized particles are the most important intermediary in the Th scavenging and that 234 Th on smaller particles ( 53 Am were also significantly and inversely correlated with uronic acid (URA, a fraction of total APS) concentrations, indicating that the APS content controls not only rates and amounts of 234 Th sorption, but also rates of coagulation of particles. Thus, the biochemical composition of marine particles needs to be considered in improved Th(IV) scavenging models. D 2002 Elsevier Science B.V. All rights reserved.

Distributions of carbohydrate species in the Gulf of Mexico

In order to study the role of polysaccharides in the cycling of marine organic matter and transparent exopolymeric particles (TEP), the concentrations of total carbohydrates (p-TCHO), total uronic acids (URA) and total acid polysaccharides (APS) in suspended and sinking particles, as well as carbohydrates in the filter-passing “dissolved” phase (d-TCHO), were measured in vertical profiles along a N–S transect in the Gulf of Mexico, across a cold core (CCR) and a warm core (WCR) ring (eddy) during both July 2000 and May 2001. The concentrations of d-TCHO in 2000 ranged from 4 to 22 μM C, with a subsurface maximum, which was located slightly above the depth of chl a maximum, amounting to, on average, 34% of DOC in the CCR, and 13% in the WCR. The concentration of particulate carbohydrates (p-TCHO) in different size fractions (0.7–10, 10–53, and >53 μm) ranged from 0.04 to 1.1, 0.005 to 0.40, and 0.006 to 0.26 μM C, respectively, indicating that carbohydrates are mostly concentrated in small particles (0.7–10 μm). URA and APS were similarly concentrated in small particles, in which, on average, URA accounted for 87% and 57% of total URA, and APS for 92% and 88% of total APS in 2000 and 2001, respectively. URA accounted for 3–9% of carbohydrates in suspended particles, suggesting that URA are a minor component of the p-TCHO pool. Due to its surface-reactive nature, URA could play a major role in the coagulation of particles and macromolecules despite its relatively low abundance. While, on average, p-TCHO and total APS were more enriched in suspended particles than in sinking particles in both 2000 and 2001, the opposite was true for URA in both years. The greater contents of URA that are present in settling particles compared to suspended particles could indicate a mass flow in the direction of sinking particles, either caused by coagulation, by bacterial reworking of particulate and colloidal organic matter, or by their more refractory nature.

Boundary exchange and scavenging of radionuclides in continental margin waters of the Middle Atlantic Bight: implications for organic carbon fluxes

Continental margins are sites where energy dissipation from physical (e.g., waves currents and tides) and biochemical process (e.g., rates of photosynthesis and decomposition) are highest. As a consequence, continental margins have been considered as possibly important sites of organic matter production and export to the ocean interior. As part of the Ocean Margins Program, vertical profiles of 234Th, 210Pb, particulate organic carbon, suspended particulate matter concentrations, vertical fluxes of sinking particles and associated natural radionuclides (e.g., 234Th, 210Pb, 14C), and elemental composition (e.g., C, N) were measured. Boundary scavenging and exchange processes of natural radionuclides and organic matter were investigated across the continental margin in the Middle Atlantic Bight (MAB). Large deficiencies of 214Th with respect to production from 218U decay in the MAB down to 2300 m water depth imply rapid benthic nepheloid layer exchange processes not only in deeper water, but also in the upper water column of the continental slope. Calculated lateral fluxes of organic carbon, likely of episodic nature and related to the relative position of the Gulf Stream, are up to an order of magnitude larger than long-term vertical deposition rates in the sediments.