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.

No abstract

An ammonia-oxidizing, carbon-fixing archaeon, Candidatus ''Nitrosopumilus maritimus,'' recently was isolated from a salt-water aquarium, definitively confirming that chemoautotrophy exists among the marine archaea. However, in other incubation studies, pelagic archaea also were capable of using organic carbon. It has remained unknown what fraction of the total marine archaeal community is autotrophic in situ. If archaea live primarily as autotrophs in the natural environment, a large ammonia-oxidizing population would play a significant role in marine nitrification. Here we use the natural distribution of radiocarbon in archaeal membrane lipids to quantify the bulk carbon metabolism of archaea at two depths in the subtropical North Pacific gyre. Our compound-specific radiocarbon data show that the archaea in surface waters incorporate modern carbon into their membrane lipids, and archaea at 670 m incorporate carbon that is slightly more isotopically enriched than inorganic carbon at the same depth. An isotopic mass balance model shows that the dominant metabolism at depth indeed is autotrophy (83%), whereas heterotrophic consumption of modern organic carbon accounts for the remainder of archaeal biomass. These results reflect the in situ production of the total community that produces tetraether lipids and are not subject to biases associated with incubation and͞or culture experiments. The data suggest either that the marine archaeal community includes both autotrophs and heterotrophs or is a single population with a uniformly mixotrophic metabolism. The metabolic and phylogenetic diversity of the marine archaea warrants further exploration; these organisms may play a major role in the marine cycles of nitrogen and carbon.biomarkers ͉ carbon isotopes ͉ microbial ecology ͉ nitrogen cycle ͉ oceanography N onthermophilic archaea represent up to 40% of the free-living prokaryotic community in the water column of the world's oceans (1-6), but until recently there has been limited information about the sources of carbon and energy that fuel these organisms (7-11). The characteristic membrane lipids of planktonic archaea include glycerol dialkyl glycerol tetraethers (GDGTs) (12). These compounds are ubiquitous in marine sediments and ocean water (12-15). The relative abundance of individual GDGTs recovered from sediments is used to reconstruct sea-surface temperatures (16)(17)(18). This distribution, known as TEX 86 , has been shown through experimental manipulation of surface-water mesocosm experiments to respond to changes in incubation temperature (18). In addition, the ␦ 13 C values of GDGTs display a remarkably constant offset from ␦ 13 C values of dissolved inorganic carbon (DIC) over a range of settings and geologic time (13,(19)(20)(21). The collective metabolic activities of the numerous archaea in the ocean are likely to play a significant role in the cycling of organic carbon (OC) and nutrients, and their membrane lipids show significant utility for paleoceanography. However, neither the metabolic requiremen...

Black carbon (BC) enters the ocean through aerosol and river deposition. BC makes up 12 to 31 percent of the sedimentary organic carbon (SOC) at two deep ocean sites, and it is 2400 to 13,900 carbon-14 years older than non-BC SOC deposited concurrently. BC is likely older because it is stored in an intermediate reservoir before

Paired carbon-14 ( 14 C) and thorium-230( 230 Th) ages were determined on fossil corals from the Huon Peninsula, Papua New Guinea. The ages were used to calibrate part of the 14 C time scale and to estimate rates of sea-level rise during the last deglaciation. An abrupt offset between the 14 C and 230 Th ages suggests that the atmospheric 14 C/ 12 C 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 14 C/ 12 C 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.

Corals o!er a rich archive of past climate variability in tropical ocean regions where instrumental data are limited and where our knowledge of multi-decadal climate sensitivity is incomplete. In the eastern equatorial Paci"c, coral isotopic records track variations in ENSO-related changes in sea-surface temperature; further west, corals record variability in sea-surface temperature and rainfall that accompanies zonal displacement of the Indonesian Low during ENSO events. These multi-century records reveal previously unrecognised ENSO variability on time scales of decades to centuries. Outside the ENSO-sensitive equatorial Paci"c, long-term trends towards recent warmer/wetter conditions suggest the tropics respond to global forcings. New coral paleothermometers indicate that surface-ocean temperatures in the tropical southwestern Paci"c were depressed by 4}63C during the Younger Dryas climatic event and rose episodically during the next 4000 yr. High temporal-resolution measurements of Sr/Ca and O in corals provide information about the surface-ocean hydrologic balance and can resolve the seasonal balance between precipitation and evaporation. Radiocarbon measurements in corals, coupled with ocean circulation models, may be used to reconstruct near-surface ocean circulation, past mixing rates, and the distribution of fossil fuel CO in the upper ocean. Most recently, seasonal to interannual variations in the radiocarbon of corals from the equatorial Paci"c have been linked to the redistribution of surface waters associated with the ENSO.

This paper describes the application of a novel, practical approach for isolation of individual compounds from complex organic matrices for natural abundance radiocarbon measurement. This is achieved through the use of automated preparative capillary gas chromatography (PCGC) to separate and recover sufficient quantities of individual target compounds for 14 C analysis by accelerator mass spectrometry (AMS). We developed and tested this approach using a suite of samples (plant lipids, petroleums) whose ages spanned the 14 C time scale and which contained a variety of compound types (fatty acids, sterols, hydrocarbons). Comparison of individual compound and bulk radiocarbon signatures for the isotopically homogeneous samples studied revealed that ∆ 14 C values generally agreed well ((10%). Background contamination was assessed at each stage of the isolation procedure, and incomplete solvent removal prior to combustion was the only significant source of additional carbon. Isotope fractionation was addressed through compound-specific stable carbon isotopic analyses. Fractionation of isotopes during isolation of individual compounds was minimal (<5‰ for δ 13 C), provided the entire peak was collected during PCGC. Trapping of partially coeluting peaks did cause errors, and these results highlight the importance of conducting stable carbon isotopic measurements of each trapped compound in concert with AMS for reliable radiocarbon measurements. The addition of carbon accompanying derivatization of functionalized compounds (e.g., fatty acids and sterols) prior to chromatographic separation represents a further source of potential error. This contribution can be removed using a simple isotopic mass balance approach. Based on these preliminary results, the PCGC-based approach holds promise for accurately determining 14 C ages on compounds specific to a given source within complex, heterogeneous samples.

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 14C age (1500–6500 14C years). Here we report the first radiocarbon measurements of BC in high molecular weight DOC (UDOM). BC exported from rivers is highly aromatic and <500 14C years old, while open ocean samples contain less aromatic BC and have an age of 18,000 ± 3,000 14C years. The low abundance of BC in UDOM (0.5–3.5%) suggests that it is more labile than presently believed and/or the low molecular weight DOC contains a larger proportion of aged BC.