James E. Bauer

Riverine export of aged terrestrial organic matter to the North Atlantic Ocean.

Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00–6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural 14C and 13C data from four rivers that discharge to the western North Atlantic Ocean and find that these rivers are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in 14C from nuclear testing, and hence newly produced. We also find that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans.

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.

The changing carbon cycle of the coastal ocean

The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal oceans carbon budget.

Use of 14C and 13C natural abundances for evaluating riverine, estuarine, and coastal DOC and POC sources and cycling : a review and synthesis

Though not typically regarded as “biomarkers” in the traditional sense of the word, the radioactive and stable isotopes of carbon (14C and 13C, respectively) can serve as powerful tools for identifying sources and estimating turnover times of organic matter in aquatic systems. Paired 14C and 13C measurements of carbon pools can provide an additional degree of specificity for studies of organic matter cycling as a result of: (1) the lower susceptibility of natural isotopes to diagenetic effects that can alter organic biomolecules; (2) the “dual” isotopic nature of the approach; (3) the unique input functions for each isotope; and (4) the greater dynamic range in Δ14C (−1000 to ∼+200‰) compared to δ13C (∼−32 to −12‰). Relatively few geochemical studies in rivers, estuaries and the coastal ocean waters have employed 14C and 13C analyses of organic matter. In this paper we summarize the available data on 14C and 13C measurements in dissolved and particulate organic carbon (DOC and POC, respectively) in these systems. A brief review is presented of current methods for the separation and oxidation of DOC and POC from water samples, for subsequent Δ14C and δ13C analyses. We also compile the existing datasets on paired 14C and 13C measurements across the riverine to coastal marine continuum in order to elucidate sources, ages, and transformations of organic matter within each system, and during transport from rivers to the coastal ocean. The natural range in the Δ14C values of both DOC and POC across similar system types was 500 and 1000‰, respectively. In general, riverine DOC was enriched in 14C relative to POC in rivers and estuaries, but the opposite generally held for coastal marine waters. This is indicative of the different sources and transport mechanisms for DOC and POC within and across these three general types of systems. During river and estuarine transport, DOC generally becomes enriched in 13C and depleted in 14C due to simultaneous additions from autochthonous production and removals from heterotrophic bacteria and abiotic processes. Bacterial utilization experiments indicate that bacteria preferentially utilize a 14C enriched (i.e. young) DOC fraction and, therefore, DOC utilization is a partial explanation for the 14C-depeleted riverine and estuarine DOC. It is concluded that through the use of paired 14C and 13C measurements in DOC and POC, a more robust interpretation of sources, sinks, and residence times of organic matter may be attained than by using either isotope separately.

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.

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.

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.

Colloidal trace metals, organic carbon and nitrogen in a southeastern U.S. estuary

We have used cross-flow ultrafiltration on eleven 0.45-μm-filtered samples taken across the salinity gradient in the Ochlockonee Estuary. We sequentially ultrafiltered the samples using 10- and 1-kD filters (1 kD = 1000 daltons). The results indicate that total Fe and Mn behave nonconservatively in the estuary and that the removal is from the high-molecular-weight (HMW; > 10 kD) fraction although Mn is removed at lower salinity than Fe. For Ni, Cu and Cd, the HMW fraction is very important in the river but these elements are quickly converted from HMW to low-molecular-weight (LMW) species with increasing salinity. Carbon in the HMW fraction is strongly correlated with Fe but only weakly correlated with Fe in the smaller size fractions. The two important processes controlling the behavior of metals, carbon and nitrogen in the estuary are colloid aggregation and desorption or dissociation.

Millennial-aged organic carbon subsidies to a modern river food web

Recent studies indicate that highly aged material is a major component of organic matter transported by most rivers. However, few studies have used natural 14C to trace the potential entry of this aged material into modern river food webs. Here we use natural abundance 14C, 13C, and deuterium (2H) to trace the contribution of aged and contemporary organic matter to an important group of consumers, crustacean zooplankton, in a large temperate river (the Hudson River, New York, USA). Zooplankton were highly 14C depleted (mean delta14C = -240 per thousand) compared to modern primary production in the river or its watershed (delta14C = -60 per thousand to +50 per thousand). In order to account for the observed 14C depletion, zooplankton must be subsidized by highly aged particulate organic carbon. IsoSource modeling suggests that the range of the aged dietary subsidy is between approximately 57%, if the aged organic matter source was produced 3400 years ago, and approximately 21%, if the organic carbon used is > or = 50 000 years in age, including fossil material that is millions of years in age. The magnitude of this aged carbon subsidy to river zooplankton suggests that modern river food webs may in some cases be buffered from the limitations set by present-day primary production.

Distribution of particulate organic carbon and radiocarbon in the water column from the upper slope to the abyssal NE Pacific Ocean

We report profiles of concentrations and radiocarbon contents of suspended particulate organic carbon (POCsusp) and sedimentary organic carbon from an abyssal site (Stn M) in the northeast (NE) Pacific collected in September 1994 (a period of very high flux of particulate carbon in the deep sea) and June 1995, as well as from stations on the continental rise and slope off the coast of California in June 1995. We show that during a period of anomalously high sinking POC flux to the deep sea (September 1994), Δ14C of suspended POC did not decrease detectably between 85 and 1600 m depth. This is in contrast to depth profiles during low and moderate fluxes of sinking POC at this station where Δ14C-POCsusp decreased 50–60‰ in this depth range. One explanation for the constant Δ14C values of POCsusp between 85 and 1600 m is that large quantities of sinking POC could continuously release labile, 14C-enriched POCsusp during biological and chemical alteration of the sinking POC. The radiocarbon evidence further suggests that resuspension of organic carbon from the sediment surface, either locally or laterally transported from the slope to the deep sea, is likely, but is probably limited to depths within a few hundred meters of the bottom. Sorption of ‘old’ DOC by suspended particulate matter in the water column is also possible, especially at shallower depths (<3500 m), though proof of this mechanism cannot be demonstrated at this time.