Ellen R. M. Druffel

Cycling of dissolved and particulate organic matter in the open ocean

Author(s): Druffel, Ellen R. M; Williams, Peter M; Bauer, James E; Ertel, John R | Abstract: Radiocarbon (Δ14C), δ13C, bulk carbon and organic constituent concentration measurements are presented for dissolved and particulate carbon pools from the North Central Pacific Ocean (NCP) and the Sargasso Sea (SS). We operationally define three overlapping pools of dissolved organic carbon (DOC): (1) DOC that is oxidizable by UV radiation (DOCuv); (2) “extra” DOC measured by Co/CoO flow-through high-temperature catalytic oxidation (DOCFt-htc), which also has low Δ14C values like DOCuv (Bauer et al., 1992a); and (3) a potential residual DOC fraction that is the difference between DOC measured by discrete-injection high-temperature catalytic oxidation (DOChtc) and DOCFt-htc, and which has unknown Δ14C signature. The distribution of a large fraction of DOC appears to be controlled by circulation of deep ocean waters between major oceans. The DOC in the SS is slightly younger than would be expected if circulation was the sole process controlling DOC cycling. We propose that there is more bomb 14C in the deep SS DOC to account for this difference. The Δ14C values of suspended, and to a lesser extent sinking particulate organic carbon (POC), decrease with depth, with the suspended POC displaying a much steeper gradient in the SS than in the NCP. These data reflect the incorporation of low-activity organic matter into the POC pool, possibly through incorporation of DOC by physical adsorption and/or biological heterotrophy.

A Large Drop in Atmospheric 14C/12C and Reduced Melting in the Younger Dryas, Documented with 230Th Ages of Corals.

Paired carbon-14 (14C) and thorium-230(230Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14C and 230Th ages suggests that the atmospheric 14C/12C ratio dropped by 15 percent during the latter part of and after the Younger Dryas (YD). This prominent drop coincides with greatly reduced rates of sea-level rise. Reduction of melting because of cooler conditions during the YD may have caused an increase in the rate of ocean ventilation, which caused the atmospheric 14C/12C ratio to fall. The record of sea-level rise also shows that globally averaged rates of melting were relatively high at the beginning of the YD. Thus, these measurements satisfy one of the conditions required by the hypothesis that the diversion of meltwater from the Mississippi to the St. Lawrence River triggered the YD event.

Ocean margins as a significant source of organic matter to the deep open ocean

Continental shelves and slopes comprise less than 20% of the world ocean area, yet they are proposed to be quantitatively important sources of the organic matter that fuels respiration in the open oceans interior,. At least certain regions of the coastal ocean produce more organic carbon than they respire, suggesting that some fraction of this non-respired, unburied organic carbon is available for export from the coastal to the open ocean. Previous studies of carbon fluxes in ocean margins,, have not considered the potential roles of dissolved organic carbon (DOC) and suspended particulate organic carbon (POCsusp), even though both pools are quantitatively far larger than sinking POC. Here we report natural radiocarbon (14C) abundance measurements that reveal continental slope and rise waters to contain both DOC and POCsusp that are concurrently older and in higher concentrations than DOC and POCsusp from the adjacent North Atlantic and North Pacific central gyres. Mass-balance calculations suggest that DOC and POCsusp inputs from ocean margins to the open ocean interior may be more than an order of magnitude greater than inputs of recently produced organic carbon derived from the surface ocean. Inputs from ocean margins may thus be one of the factors contributing to the old apparent age of organic carbon observed in the deep North Atlantic and Pacific central gyres.

Carbon isotope geochemistry of the Santa Clara River

The Santa Clara River is a prototypical small mountainous river, with a headwater height greater than 1000 m and a basin area smaller than 10,000 m 2. Although individual small mountainous rivers export trivial amounts of sediment and carbon to the ocean, as a group these rivers may export a major fraction (as much as 50%) of the total global river sediment flux [Milliman and Syvitski, 1992], making their geochemistry relevant the study of the oceans carbon cycle. In addition, many small rivers export sediment in a few high flux events, causing massive, sporadic discharge of carbon onto coastal shelves, discharge conditions very different from those of large rivers. This class of rivers is an end-member of the river-ocean carbon exchange system,. opposite the Earths largest river, the Amazon. The carbon mass and isotopic properties of the Santa Clara River are significantly different from previously studied large rivers. During the 1997–1998 winter, all Santa Clara carbon pools were old, with flux-weighted average Δl4C values of−428±76‰ for particulate organic carbon, −73±31‰ for dissolved organic carbon, and−644±58‰ for black carbon. The age of exported carbon is primarily due to the deep erosion of old soils and not to inclusion of fossil fuel carbon. Additionally, the δ13C signatures of exported carbon pools were high relative to terrestrial carbon, bearing a signature quite similar to marine carbon (average particulate organic carbon (POC) δ13C = −22.2±0.8‰). The Santa Claras estuary is small and drains onto the narrow eastern Pacific coastal margin, exporting this old soil organic matter directly into the ocean. If the Santa Clara export patterns are representative of this class of rivers, they may be a significant source of refractory terrestrial carbon to the ocean.

Variable ageing and storage of dissolved organic components in the open ocean

Seawater dissolved organic matter (DOM) is the largest reservoir of exchangeable organic carbon in the ocean, comparable in quantity to atmospheric carbon dioxide. The composition, turnover times and fate of all but a few planktonic constituents of this material are, however, largely unknown. Models of ocean carbon cycling are thus limited by the need for information on temporal scales of carbon storage in DOM subcomponents, produced via the ‘biological pump’, relative to their recycling by bacteria. Here we show that carbohydrate- and protein-like substances in the open Atlantic and Pacific oceans, though often significantly aged, comprise younger fractions of the DOM, whereas dissolved lipophilic material exhibits up to ∼90 per cent fossil character. In contrast to the millennial mean ages of DOM observed throughout the water column, weighted mean turnover times of DOM in the surface ocean are only decadal in magnitude. An observed size–age continuum further demonstrates that small dissolved molecules are the most highly aged forms of organic matter, cycling much more slowly than larger, younger dissolved and particulate precursors, and directly links oceanic organic matter age and size with reactivity.

Bomb radiocarbon in the Pacific: Annual and seasonal timescale variations

Author(s): Druffel, ER; Druffel, ER | Abstract: Banded corals are used as proxy recorders of bomb radiocarbon in the surface of the Pacific Ocean. Bomb radiocarbon levels appeared to still be rising in the tropical Pacific by 1982, in contrast to temperate locations that peaked in the early 1970s. This is representative of the geostrophic transport of bomb-laden waters from higher latitudes toward the equator. The seasonal radiocarbon signal at Canton Island (3S, 172W) during the early 1970s was twice the amplitude of that at Fanning Island (4N, 159W), and the radiocarbon minima at these locations were offset by several months. The phase lag is caused primarily by the seasonally variant transequatorial Ekman transport, which funnels more upwelled, 14C-poor water from summer to winter hemisphere. The seasonal variation of Δ14C at Canton is larger because the peak input of 14C-poor South Equatorial Current water is coincident with the period of greatest transequatorial Ekman transport to this region. Whereas, at the Fanning site, these inputs reach their maxima during opposing seasons, which causes a damping of the seasonal Δ14C signal. A time-stepped, multi-box model calculation is made to describe bomb radiocarbon distributions in the tropical Pacific, and hence to determine the influence of the South Equatorial Current flow on the chemistry of the central equatorial Pacific. The annual model results show that radiocarbon is influenced to a minor extent by this lateral flow, in agreement with previous studies. However, the seasonal version of the model reveals that the South Equatorial Current flow varies by a factor of 2–3 in order to explain the seasonal variations in bomb radiocarbon. Meridional geostrophic convergence and transequatorial Ekman transport from summer to winter hemisphere alone are not sufficient for defining the observed seasonal signals.

Large variations of surface ocean radiocarbon: Evidence of circulation changes in the southwestern Pacific

Radiocarbon (Δ14C) and stable isotope (δ18O and δ13C) records are presented for biannual samples from a 323-year banded coral series collected from the southern Great Barrier Reef, Australia. The high-precision Δ14C record contains variations on an interannual timescale, that are particularly large between A.D. 1680 and 1730. By comparison with tree ring Δ14C records [Stuiver and Quay, 1980; M. Stuiver, personal communication, 1992), it is clear that these shifts were not caused by changes in the Δ14C of atmospheric CO2. Changes in vertical mixing and large scale advective changes involving source waters to the western Coral Sea region are likely processes that could account for these large Δ14C variations. Most low Δ14C values for the period A.D. 1635-1875 coincide with El Nino/Southern Oscillation (ENSO) events as reported by Quinn et al. [1987] for the eastern tropical Pacific. However, ENSO does not explain all of the variations, especially during 1875–1920 when Δ14C values remained high. Cross-spectral analysis of the early half of the Δ14C and δ18O records (A.D. 1635–1795) reveals that the 6-year period is coherent; this coherency is not present in the latter half (A.D. 1797–1957) of the isotope records. These data support the concept of century timescale changes in the nature of ENSO, as it is manifest in the southwestern Pacific. Our coral record shows no evidence of a Suess effect, the lowering of Δ14C from late 1800s through 1955 due mainly to CO2 input from fossil fuel burning. This is coincident with the change we observe in the nature of ENSO and is further evidence that a long-term change in mixing of upper waters occurred in this region.

Aged black carbon identified in marine dissolved organic carbon

GEOPHYSICAL RESEARCH LETTERS, VOL. 37, L16601, doi:10.1029/2010GL043963, 2010 Aged black carbon identified in marine dissolved organic carbon L. A. Ziolkowski 1,2 and E. R. M. Druffel 1 Received 13 May 2010; revised 17 June 2010; accepted 1 July 2010; published 17 August 2010. [ 1 ] Produced on land by incomplete combustion of organic matter, black carbon (BC) enters the ocean by aerosol and river deposition. It has been postulated that BC resides in the marine dissolved organic carbon (DOC) pool before sedimentary deposition and may attribute to its great 14 C age (1500–6500 14 C years). Here we report the first radiocarbon measurements of BC in high molecular weight DOC (UDOM). BC exported from rivers is highly aromatic and 50,000 years (the detection limit). In contrast, BC recently produced from biomass burning has a C content equal to that in the contemporary biosphere C (D 14 C = 0 to 200‰). Here we present the distribution of BPCAs in conjunction with radiocarbon measurements of Auxiliary materials are available in the HTML. doi:10.1029/ 2010GL043963. L16601 1 of 4

The Keck Carbon Cycle AMS laboratory, University of California, Irvine: Initial operation and a background surprise

A new radiocarbon accelerator mass spectrometry (AMS) laboratory for carbon cycle studies has been established at the University of California, Irvine. The 0.5MV AMS system was installed in mid-2002 and has operated routinely since October of that year. This paper briefly describes the spectrometer and summarizes lessons learned during the first year of operation. In the process of setting up the system, we identified and largely suppressed a previously unreported (super 14) C AMS background: charge exchange tails from (super 14) N beams derived from nitrogen-containing molecular ions produced near the entrance of the accelerator.

Seasonal variability of particulate organic radiocarbon in the northeast Pacific Ocean

We present Δ14C measurements of particulate organic carbon (POC) collected on four cruises at our time series site (station M) in the northeast Pacific Ocean. We observe a large gradient with depth in the suspended POC Δ14C values (124–160‰). These profiles display lower Δ14C values (by 20–30‰) in samples between 2500 m and the bottom during June 1992 and July 1993 than those during February and October 1992. Values of Δ14C in sinking POC from deep-moored sediment trap collections suggest a semiannual trend that displays lower overall Δ14C in material collected during periods of high flux. A limited number of Δ14C measurements of small swimmers picked from the trap 650 m above bottom are similar to surface Δ14C measurements of dissolved inorganic carbon (DIC) and suspended POC, indicating a surface carbon source. Overall, we postulate that the major process causing lower Δ14C values of deep suspended and sinking POC is sorption (or biological incorporation) of “old” DOC onto particulate matter. There appears to be a higher ratio of DOC sorbed to sinking particulate matter at times of high flux (late spring and early fall) that can be thought of as a “stripping out” of DOC from the water column. The DIC Δ14C display a small seasonal variation in the surface waters and is not the sole source of the observed seasonality in the POC Δ14C signals.