Dennis A. Hansell

Microbial production of recalcitrant dissolved organic matter: long-term carbon storage in the global ocean

The biological pump is a process whereby CO2 in the upper ocean is fixed by primary producers and transported to the deep ocean as sinking biogenic particles or as dissolved organic matter. The fate of most of this exported material is remineralization to CO2, which accumulates in deep waters until it is eventually ventilated again at the sea surface. However, a proportion of the fixed carbon is not mineralized but is instead stored for millennia as recalcitrant dissolved organic matter. The processes and mechanisms involved in the generation of this large carbon reservoir are poorly understood. Here, we propose the microbial carbon pump as a conceptual framework to address this important, multifaceted biogeochemical problem.

Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms

Episodic eddy-driven upwelling may supply a significant fraction of the nutrients required to sustain primary productivity of the subtropical ocean. New observations in the northwest Atlantic reveal that, although plankton blooms occur in both cyclones and mode-water eddies, the biological responses differ. Mode-water eddies can generate extraordinary diatom biomass and primary production at depth, relative to the time series near Bermuda. These blooms are sustained by eddy/wind interactions, which amplify the eddy-induced upwelling. In contrast, eddy/wind interactions dampen eddy-induced upwelling in cyclones. Carbon export inferred from oxygen anomalies in eddy cores is one to three times as much as annual new production for the region.

Deep-ocean gradients in the concentration of dissolved organic carbon

There is as much carbon in dissolved organic material in the oceans as there is CO2 in the atmosphere, but the role of dissolved organic carbon (DOC) in the global carbon cycle is poorly understood. DOC in the deep ocean has long been considered to be uniformly distributed, and hence largely refractory to biological decay. But the turnover of DOC, and therefore its contribution to the carbon cycle, has been evident from radiocarbon dating studies,. Here we report the results of a global survey of deep-ocean DOC concentrations, including the region of deep-water formation in the North Atlantic Ocean, the Circumpolar Current of the Southern Ocean, and the Indian and Pacific oceans. DOC concentrations decreased by 14 micromolar from the northern North Atlantic Ocean to the northern North Pacific Ocean, representing a 29% reduction in concentration. We evaluate the spatial patterns in terms of source/sink processes. Inputs of DOC to the deep ocean are identifiable in the mid-latitudes of the Southern Hemisphere, but the mechanisms have not been identified with certainty.

Recalcitrant dissolved organic carbon fractions.

Marine dissolved organic carbon (DOC) exhibits a spectrum of reactivity, from very fast turnover of the most bioavailable forms in the surface ocean to long-lived materials circulating within the ocean abyss. These disparate reactivities group DOC by fractions with distinctive functions in the cycling of carbon, ranging from support of the microbial loop to involvement in the biological pump to a hypothesized major source/sink of atmospheric CO(2) driving paleoclimate variability. Here, the major fractions constituting the global oceans recalcitrant DOC pool are quantitatively and qualitatively characterized with reference to their roles in carbon biogeochemistry. A nomenclature for the fractions is proposed based on those roles.

Net community production of dissolved organic carbon

Each ycar large amounts of carbon, with a residence time of months, accumulate in the surface layer of the ocean as semilabile dissolved organic carbon (DOC). This material is transported long distances, contributing to the interhemispheric transfer and deep ocean export of carbon. The fraction of net community production resulting in the accumulation of semilabile DOC is estimated here by mass balance during periods of net phytoplankton production in three diverse environments: the Ross Sea polynya, the Equatorial Pacific Ocean, and the Sargasso Sea. In the eutrophic systems of the Ross Sea and the Equatorial Pacific, net DOC production generally fell between 10 and 20% of net community production. For the 1995 spring bloom in the Sargasso Sea. net DOC production was 59-70% of the net community production. Net DOC production was maximal during the period of deep convective overturn of the water column, indicating linkage between the processes. Following the Sargasso Sea spring bloom and into the summer period, net DOC production was nil over the upper 250 m so that net DOC production was reduced to ∼8% of net community production on a seasonal timescale. Consideration of the various types of production regimes in the ocean indicates that the global net production of semilabile DOC is ∼17% of global new production. Regions of the worlds oceans with the greatest contributions to global net community production, such as equatorial and coastal upwelling areas, contribute most to the global production of semilabile DOC.

Degradation of Terrigenous Dissolved Organic Carbon in the Western Arctic Ocean

The largest flux of terrigenous organic carbon into the ocean occurs in dissolved form by way of rivers. The fate of this material is enigmatic; there are numerous reports of conservative behavior over continental shelves, but the only knowledge we have about removal is that it occurs on long unknown time scales in the deep ocean. To investigate the removal process, we evaluated terrigenous dissolved organic carbon concentration gradients in the Beaufort Gyre of the western Arctic Ocean, which allowed us to observe the carbons slow degradation. Using isotopic tracers of water-mass age, we determined that terrigenous dissolved organic carbon is mineralized with a half-life of 7.1 ± 3.0 years, thus allowing only 21 to 32% of it to be exported to the North Atlantic Ocean.

Chapter 15 – DOC in the Global Ocean Carbon Cycle

Dissolved organic carbon (DOC) makes up the second largest of the bio-reactive pools of carbon in the ocean (second to the very large pool of dissolved inorganic carbon). The size of the reservoir as well as its positions as a sink for auto-trophically fixed carbon and as a source of substrate to microbial heterotrophs indicates that DOC plays a central role in the ocean carbon cycle. But what is this role, how is it realized, and what are its mechanisms and controls. The fundamentals of these questions have remained unchanged over the past 40 years and continue to challenge the ocean carbon research community today. A considerable amount of financial and intellectual capital has been expended and significant progress has been made over the past decade. In this chapter, the role of DOC in the ocean carbon cycle is considered in its broadest temporal and spatial scales. The chapter evaluates the spatial distribution of DOC at the regional and basin scales in both the surface and deep ocean.

Distribution of CO2 species, estimates of net community production, and air-sea CO2 exchange in the Ross Sea polynya

Measurements of surface total carbon dioxide (TCO2), alkalinity, and calculated pCO2, along with water column nutrients and hydrography, were made on two cruises to the Ross Sea polynya (NBP 94-6, November-December 1994 and NBP 95-8, December 1995 to January 1996). The polynya experiences an intense phytoplankton bloom during a short period of open water conditions from mid-December to mid-February each year. Our biogeochemical observations were used to determine the temporal variability of CO2, fluxes of carbon within the ocean, and rates of air-sea exchange of CO2. Depletions of TCO2, pCO2, and nitrate+nitrite were considerable (?70–150 ?mol kg?1, 80–150 ?atm, and 10–20 ?mol kg?1, respectively) and associated primarily with biological uptake during Phaeocystis and diatom blooms. Alkalinity was a conservative tracer of salinity and nitrate+nitrite. Surface ?CO2 was undersaturated by ?50–150 ?atm, and air-sea gas exchange of CO2 during open water conditions was directed from atmosphere to ocean. Observed surface stoichiometric C:N ratios were 6.66: 1 and 6.77:1 for the 2 years, consistent with global “Redfield” ratios, while C:P and N:P ratios were variable (75–141:1, 12–18:1). Estimates of net community production (NCP) rates were made using in situ changes in TCO2 and nitrate+nitrite across repeated transects along 76°30?S. Mean NCP rates across the polynya ranged from 0.86 to 0.98 g C m?2 d?1. These values may be underestimated by 5–25% because of the contribution of atmospheric CO2 to the surface layer through gas exchange. Export of carbon from the surface to depth was at least 55–60% of NCP rates.

Glucose fluxes and concentrations of dissolved combined neutral sugars (polysaccharides) in the Ross Sea and Polar Front Zone, Antarctica

We hypothesized that dissolved carbohydrates would be large components of the labile dissolved organic carbon (DOC) pool and would support much bacterial growth in Antarctic waters, especially the Ross Sea, since previous work had observed extensive phytoplankton blooms with potentially high production rates of carbohydrates in Antarctic seas. These hypotheses were tested on cruises in the Ross Sea and Antarctic Polar Front Zone as part of the US JGOFS program. Concentrations and fluxes of free glucose (the only free sugar detected) were very low, but dissolved polysaccharides appeared to be important components of the DOC pool. Concentrations of dissolved combined neutral sugars increased >3-fold during the phytoplankton bloom in the Ross Sea and were a large fraction (ca. 50%) of the semi-labile fraction of DOC. The relatively high concentrations of dissolved combined neutral sugars, which are thought to be quite labile, appear to explain why DOC accumulated during the phytoplankton bloom was degraded so quickly once the bloom ended. Some of the polysaccharides appeared to be more refractory, however, since dissolved combined neutral sugars were observed in deep waters (>550 m) and in early spring (October) in the Ross Sea, apparently having survived degradation for >8 months. The molecular composition of these refractory polysaccharides differed from that of polysaccharides sampled during the phytoplankton bloom. Fluxes of DOC were low in the Ross Sea compared to standing stocks and fluxes of particulate material,

Organic carbon and apparent oxygen utilization in the western South Pacific and the central Indian Oceans

Samples for total organic carbon (TOC) analysis were collected on WOCE Line P15S (0° to 67°S along 170°W) and from 53° to 67°S along 170°E in the western South Pacific, and on Line I8 (5°N to 43°S along 80°/90°E) in the central Indian Ocean. TOC concentrations in the upper ocean varied greatly between the regions studied. Highest surface TOC concentrations (81-85 μM C and 68-73 μM C) were observed in the warmest waters (> 27°C) of the western South Pacific and central Indian Oceans, respectively. Lowest surface TOC concentrations (45-65 μM C) were recorded in the southernmost waters occupied (> 50°S along 170°W and 170°E). Deep water (> 1000 m) TOC concentrations were uniform across all regions analyzed, averaging between 42.3 and 43 μM C (SD: ±0.9 μM C). Mixing between TOC-rich surface waters and TOC-poor deep waters was indicated by the strong correlations between TOC and temperature (r 2 > 0.80, north of 45°S) and TOC and density (r 2 > 0.50, southernmost regions). TOC was inversely correlated with apparent oxygen utilization (AOU) along isopycnal surfaces north of the Polar Frontal Zone (PFZ) and at depths < 500 m. The TOC:AOU molar ratios at densities of σ 1 23-27 ranged from - 0.15 to -0.34 in the South Pacific and from - 0.13 to - 0.31 in the Indian Ocean. These ratios indicate that TOC oxidation was responsible for 21%-47% and 18%-43% of oxygen consumption in the upper South Pacific and Indian Oceans, respectively. At greater depths, TOC did not contribute to the development of AOU, There was no evidence for significant export of dissolved and suspended organic carbon along isopycnal surfaces that ventilate near the PFZ.